Fig 1 - uploaded by Lijia Zhao
Content may be subject to copyright.
TEM microstructures observed from three orthogonal directions of a pure aluminum (99.99 mass% purity) specimen deformed by ECAP process up to 4 passes at room temperature. The route Bc was taken in the ECAP process using a 90° die. 8)
Source publication
Severe plastic deformation (SPD) have had a revolutionary impact in fabricating ultrafine grained (UFG) or nanostructured metallic materials with bulky dimensions. It is, however, difficult to understand from a viewpoint of conventional metallurgy why UFG microstructures form in the as-deformed (as-SPD-processed) state without annealing process. Th...
Contexts in source publication
Context 1
... However, SPD processes are carried out at relatively low temperatures below recrystallization temperature of the materials in order to effectively accumulate lattice defects introduced by plastic deformation, and UFG microstructures are obtained in the as-SPDprocessed state (or the as-deformed state) without annealing treatments. Figure 1 shows transmission electron microscopy (TEM) microstructures of a pure aluminum with 99.99 mass% purity (4N-Al) processed by equal-channel angular pressing (ECAP) at room temperature (RT), reported by Iwahashi et al. 8) In the as-deformed state of 4-pass ECAP using a 90° angle die, an UFG microstructure with equiaxed morphology is clearly observed. The question is how and by what mechanism this UFG structure was formed. ...
Context 2
... TEM observations with Kikuch-line analysis revealed that the number and fraction of HAGBs increased with increasing the total strain (the number of ARB cycle) applied. TEM image and corresponding boundary misorientation map of the specimen ARB processed by 7 cycles (¾ = 5.6) are shown in Fig. 4. 14) The microstructure became more homogeneous throughout the specimen, and grains elongated to RD were observed (Fig. 4(a)). Grain boundaries surrounding elongated grains looked sharp, and the mean interval of boundaries along ND (the mean grain thickness) was 0.20 µm. The misorientation map (Fig. 4(b)) indicates that most of the ...
Context 3
... Gholizadeh et al. 30) have carried out systematic investigations of continuous SPD for various kinds of metallic materials at different temperatures and strain rates by the use of a simple torsion machine equipped with induction heating system. Figure 10 shows the result of such torsion experiments, where IF steel specimens were continuously deformed to high strains up to von Mises equivalent strain (¾) of 7.0 at various constant temperatures ranging from 400°C to 850°C and various constant strain rates from 0.01 s ¹1 to 1 s ¹1 . The IF steel specimens were quenched into water immediately after the torsion deformation, and frozen microstructures were analyzed in detail. ...
Context 4
... IF steel specimens were quenched into water immediately after the torsion deformation, and frozen microstructures were analyzed in detail. 30) Figure 10(a) exhibits relationships between the obtained grain/sub-grain sizes and the flow stress in the torsion deformation. The grain/sub-grain sizes showed certain correlations with the flow stress, but there were three different regions in the change of the grain size determined as the interval of HAGBs. ...
Context 5
... the obtained grain/sub-grain sizes and the flow stress in the torsion deformation. The grain/sub-grain sizes showed certain correlations with the flow stress, but there were three different regions in the change of the grain size determined as the interval of HAGBs. Typical microstructures (EBSD grain boundary maps) in three regions are shown in Fig. 10(b), (c), and (d). In the region of high flow stresses, ultrafine lamellar structures (type I) were mainly observed. This type of microstructure is quite similar to the lamellar UFG structures formed by SPD (ARB) of IF steel shown in Fig. 5(d) and Fig. 9(a), which suggests that the type I structures (Fig. 10(b)) are formed by the grain ...
Context 6
... maps) in three regions are shown in Fig. 10(b), (c), and (d). In the region of high flow stresses, ultrafine lamellar structures (type I) were mainly observed. This type of microstructure is quite similar to the lamellar UFG structures formed by SPD (ARB) of IF steel shown in Fig. 5(d) and Fig. 9(a), which suggests that the type I structures (Fig. 10(b)) are formed by the grain subdivision mechanism. In fact, the sizes of sub-grains (a kind of deformed microstructures formed by plastic deformation) were on the identical straight line of the grain size determined by HAGBs in the high-stress region, suggesting the same formation mechanism. In the transition region under medium flow ...
Context 7
... formed by plastic deformation) were on the identical straight line of the grain size determined by HAGBs in the high-stress region, suggesting the same formation mechanism. In the transition region under medium flow stresses, morphologies of fine grains changed into more equiaxed ones and the grain size increased with decreasing the flow stress (Fig. 10(c): type II). In the low flow-stress region, heterogeneous microstructures composed of some coarse grains surrounded by HAGBs within regions of subgrains with LAGBs were observed ( Fig. 10(d): type III). Such microstructures were similar to those formed by dynamic recrystallization (DRX; recrystallization during deformation). The flow ...
Context 8
... region under medium flow stresses, morphologies of fine grains changed into more equiaxed ones and the grain size increased with decreasing the flow stress (Fig. 10(c): type II). In the low flow-stress region, heterogeneous microstructures composed of some coarse grains surrounded by HAGBs within regions of subgrains with LAGBs were observed ( Fig. 10(d): type III). Such microstructures were similar to those formed by dynamic recrystallization (DRX; recrystallization during deformation). The flow stress dependence of the grain size determined by HAGBs in this region had nearly the same slope as that for the sub-grain size, but the grain sizes were much coarser than the sub-grain sizes ...
Context 9
... from the high flow-stress region). The results can be summarized in the following way: when the sub-grain size formed by deformation is maintained up to high strains (under high stress conditions at low temperatures or/and at high strain rates), the grain subdivision mechanism would dominate the formation process of UFG microstructures (type I in Fig. 10(b)). In deformation under low flow stress (at high temperatures or/and at low strain rates), recovery inhibits the development of HAGBs by the grain subdivision mechanism, leading to heterogeneous distribution of the regions surrounded by HAGBs (type III in Fig. 10(d)). The regions surrounded by HAGBs heterogeneously distributed within ...
Context 10
... mechanism would dominate the formation process of UFG microstructures (type I in Fig. 10(b)). In deformation under low flow stress (at high temperatures or/and at low strain rates), recovery inhibits the development of HAGBs by the grain subdivision mechanism, leading to heterogeneous distribution of the regions surrounded by HAGBs (type III in Fig. 10(d)). The regions surrounded by HAGBs heterogeneously distributed within sub-grains with LAGBs would grow due to the high mobility of HAGBs. Consequently the microstructures can be recognized as typical (discontinuous) DRX structures characterized by nucleation and growth of recrystallized regions. When the DRX characterized by grain ...
Context 11
... by lattice diffusion is inhibited rather than that under the low flow stress, so that the microstructure evolution is still dominated by the grain subdivision. However, migration of HAGBs controlled by grain boundary diffusion can happen under this condition, resulting in the relatively fine grains with equiaxed morphologies (type II in Fig. 10(c)). The increase of the grain size from that expected as the result of the grain subdivision mechanism (sub-grain sizes in Fig. 10(a)) is also ...
Context 12
... dominated by the grain subdivision. However, migration of HAGBs controlled by grain boundary diffusion can happen under this condition, resulting in the relatively fine grains with equiaxed morphologies (type II in Fig. 10(c)). The increase of the grain size from that expected as the result of the grain subdivision mechanism (sub-grain sizes in Fig. 10(a)) is also ...
Context 13
... on the understanding, let us look back the equiaxed UFG structures observed in the pure aluminum SPD (ECAP) processed at RT (Fig. 1). It should be noted firstly that the material used was a high-purity aluminum (4N-Al). For aluminum of which melting point temperature (T m ) is 660.3°C (937.3 K), ambient temperature (25°C = 298 K) is relatively a high temperature (³0.32T m ). High purity of the material and adiabatic heating during heavy deformation in a small ...
Context 14
... temperature (25°C = 298 K) is relatively a high temperature (³0.32T m ). High purity of the material and adiabatic heating during heavy deformation in a small deformation zone at the corner of ECAP die would enhance grain boundary migration (of HAGBs) formed by the deformation. It can be concluded that the equiaxed finegrained structure shown in Fig. 1 corresponds to the type II microstructure (Fig. 10(c)) in the continuous torsion deformation under medium flow stress conditions. High density of HAGBs in such a type II microstructure is resulted from the grain subdivision, and grain boundary migration changes morphologies of grains from lamellar one to equiaxed one and somewhat ...
Context 15
... temperature (³0.32T m ). High purity of the material and adiabatic heating during heavy deformation in a small deformation zone at the corner of ECAP die would enhance grain boundary migration (of HAGBs) formed by the deformation. It can be concluded that the equiaxed finegrained structure shown in Fig. 1 corresponds to the type II microstructure (Fig. 10(c)) in the continuous torsion deformation under medium flow stress conditions. High density of HAGBs in such a type II microstructure is resulted from the grain subdivision, and grain boundary migration changes morphologies of grains from lamellar one to equiaxed one and somewhat coarsens the grain size. The combinations of different ...
Context 16
... may point out the effect of strain paths on morphologies and sizes of UFG microstructures formed by SPD, since the equiaxed UFG structure shown in Fig. 1 was fabricated by the route-B C ECAP process where the barspecimen was rotated by 90° around the bar axis after every A d v a n c e V i e w pass of the ECAP. 8) In fact, a number of papers reported that different strain paths in SPD changed morphologies and formation speeds of UFG microstructures. The effect of redundant shear strain ...
Context 17
... to the same amount of plastic strains. It is, on the other hand, noteworthy that when high-purity Al (4N-Al with 99.99 mass% purity) was heavily deformed, equiaxed fine grains formed even in case of monotonic HPT. 37) The equiaxed fine-grain structures they reported are quite similar to that obtained by the route-B C ECAP process shown in Fig. 1. Zhang et al. 39,40) studied the microstructural evolution during monotonic HPT at RT for nickel (Ni) having higher melting point and therefore recovery and grain boundary migration are more difficult at RT compared to Al. The Ni specimens showed typical deformation microstructures that can be explained by the grain subdivision ...
Context 18
... fairly high plastic strains are necessary to form homogeneous UFG and nanostructures. Kamikawa et al. 41) assessed the development of UFG structures in a commercial purity aluminum (2N-Al) ARB processed under unlubricated condition at RT. Figure 11 shows their results, where the average spacing of HAGBs (a) and the fraction of HAGBs (b) in the material are plotted as a function of the total equivalent strain. 41) Since it had been clarified that the redundant shear strain in the unlubricated ARB process accelerate the formation of UFGs, 2729) the equivalent strain in Fig. 11 took into account of the redundant shear strain in addition to the rolling strain. ...
Context 19
... at RT. Figure 11 shows their results, where the average spacing of HAGBs (a) and the fraction of HAGBs (b) in the material are plotted as a function of the total equivalent strain. 41) Since it had been clarified that the redundant shear strain in the unlubricated ARB process accelerate the formation of UFGs, 2729) the equivalent strain in Fig. 11 took into account of the redundant shear strain in addition to the rolling strain. The spacing of HAGBs (grain size) quickly decreases and the fraction of HAGBs quickly increases with increasing the equivalent strain applied by the ARB, but both parameters eventually saturate to show constant values. The graphs clearly show that the ...
Context 20
... as-quenched martensite of a 0.13 mass% C steel was simply cold-rolled only by 50% reduction in thickness and then annealed at relatively low temperatures where only recovery occurs usually. Figure 12 shows a TEM image and corresponding grain boundary misorientation map obtained by TEM Kikuchi-line analysis of the 0.13%C steel annealed at 500°C for 1.8 ks after 50% cold-rolling of martensite starting microstructure. Nearly equiaxed UFGs with an average grain size of 180 nm were observed (Fig. 12(a)). ...
Context 21
... where only recovery occurs usually. Figure 12 shows a TEM image and corresponding grain boundary misorientation map obtained by TEM Kikuchi-line analysis of the 0.13%C steel annealed at 500°C for 1.8 ks after 50% cold-rolling of martensite starting microstructure. Nearly equiaxed UFGs with an average grain size of 180 nm were observed (Fig. 12(a)). The misorientation map (Fig. 12(b)) clearly indicated that many UFGs were surrounded by HAGBs. Since martensite of carbon steels are supersaturated solid solution of carbon, fine carbides near-homogeneously precipitated within the matrix. Such carbides inhibited grain growth of the ferrite matrix to maintain UFG microstructures, and ...
Context 22
... usually. Figure 12 shows a TEM image and corresponding grain boundary misorientation map obtained by TEM Kikuchi-line analysis of the 0.13%C steel annealed at 500°C for 1.8 ks after 50% cold-rolling of martensite starting microstructure. Nearly equiaxed UFGs with an average grain size of 180 nm were observed (Fig. 12(a)). The misorientation map (Fig. 12(b)) clearly indicated that many UFGs were surrounded by HAGBs. Since martensite of carbon steels are supersaturated solid solution of carbon, fine carbides near-homogeneously precipitated within the matrix. Such carbides inhibited grain growth of the ferrite matrix to maintain UFG microstructures, and also enhanced strain-hardening in ...
Context 23
... a result, martensite microstructure is a kind of fine-grained microstructures even in the as-transformed state. Figure 13(a) shows an EBSD orientation map of an as-quenched martensite in a low-C steel (0.2 mass% C steel). 45) The white lines were grain boundaries surrounding a prior austenite grain reconstructed from the crystallographic analysis. ...
Context 24
... were grain boundaries surrounding a prior austenite grain reconstructed from the crystallographic analysis. It could be clearly seen that, within the single austenite grain, a number of different variants of martensite (painted in different colors and indicated as "V2", "V14", etc.) formed. A grain boundary map corresponding to the area shown in Fig. 13(a) is represented in Fig. 13(b). Boundaries existing in the microstructure are categorized into three kinds depending on their misorientation angles and drawn in different colors. It was recognized that the original austenite grains were finely subdivided by the martensitic transformation. The effective grain size of this martensite ...
Context 25
... a prior austenite grain reconstructed from the crystallographic analysis. It could be clearly seen that, within the single austenite grain, a number of different variants of martensite (painted in different colors and indicated as "V2", "V14", etc.) formed. A grain boundary map corresponding to the area shown in Fig. 13(a) is represented in Fig. 13(b). Boundaries existing in the microstructure are categorized into three kinds depending on their misorientation angles and drawn in different colors. It was recognized that the original austenite grains were finely subdivided by the martensitic transformation. The effective grain size of this martensite microstructure estimated from ...
Context 26
... was accelerated due to the constraint of grain boundaries. It should be also noted that the as-quenched martensite involved a high density of dislocations. These were probably the reason why only 50% cold-rolling and subsequent recovery annealing could produce the UFG microstructure. The TEM microstructure of the 50% cold-rolled specimen (Fig. 13(c)) revealed a complicated deformation microstructure. Ultrafine lamellar structures were observed in most areas, and they had wavy shapes due to additional shear localization. Selected area diffraction patterns suggested the existence of local misorientations. That is, the finely subdivided structure comparable to those formed after SPD ...
Context 27
... coldrolling of martensite. This is an example of the simple processes to fabricate UFG microstructures without SPD. Firstly, the phase transformation (martensitic transformation) made the appropriate starting microstructure, and then plastic deformation (conventional cold-rolling) and subsequent annealing realized the UFG microstructure shown in Fig. 12. Okitsu et al. 46) showed another example of the simple processes, using a 0.10C1.98Mn0.018Nb0.0015B (mass%) steel. They firstly made a dual phase microstructure composed of ferrite (F) and martensite (M) shown in Fig. 14(b) through conventional hot-rolling, air cooling, and quenching from ferrite + austenite two-phase temperatures ...
Context 28
... and then plastic deformation (conventional cold-rolling) and subsequent annealing realized the UFG microstructure shown in Fig. 12. Okitsu et al. 46) showed another example of the simple processes, using a 0.10C1.98Mn0.018Nb0.0015B (mass%) steel. They firstly made a dual phase microstructure composed of ferrite (F) and martensite (M) shown in Fig. 14(b) through conventional hot-rolling, air cooling, and quenching from ferrite + austenite two-phase temperatures ( Fig. 14(a)). Figure 14(c) represents a SEM image observed from TD of the as 91% cold-rolled specimen having A d v a n c e V i e w the dual phase starting microstructure. Since martensite was much harder than ferrite, plastic ...
Context 29
... Fig. 12. Okitsu et al. 46) showed another example of the simple processes, using a 0.10C1.98Mn0.018Nb0.0015B (mass%) steel. They firstly made a dual phase microstructure composed of ferrite (F) and martensite (M) shown in Fig. 14(b) through conventional hot-rolling, air cooling, and quenching from ferrite + austenite two-phase temperatures ( Fig. 14(a)). Figure 14(c) represents a SEM image observed from TD of the as 91% cold-rolled specimen having A d v a n c e V i e w the dual phase starting microstructure. Since martensite was much harder than ferrite, plastic deformation was concentrated in soft ferrite, and hard martensite was not heavily deformed and showed an island morphology ...
Context 30
... firstly made a dual phase microstructure composed of ferrite (F) and martensite (M) shown in Fig. 14(b) through conventional hot-rolling, air cooling, and quenching from ferrite + austenite two-phase temperatures ( Fig. 14(a)). Figure 14(c) represents a SEM image observed from TD of the as 91% cold-rolled specimen having A d v a n c e V i e w the dual phase starting microstructure. Since martensite was much harder than ferrite, plastic deformation was concentrated in soft ferrite, and hard martensite was not heavily deformed and showed an island morphology with diamond shapes. ...
Context 31
... microstructures comparable to SPD processed ones (like Figs. 4 and 9), even though the total rolling reduction was not huge (91%, ¾ = 2.8). Although the degree of deformation in martensite was much smaller than that in ferrite, it was shown by Ueji et al. 34,35) that smaller amount of strain was enough to finely subdivide as-quenched martensite (Fig. 13). As a result, homogeneous UFG structures having equiaxed morphologies like Fig. 14(d) could be obtained after annealing the 91% cold-rolled sheet under appropriate conditions. The equiaxed microstructure having a mean grain size of 490 nm shown in Fig. 14(d) is considered to be a kind of statically recrystallized microstructure similar ...
Context 32
... the total rolling reduction was not huge (91%, ¾ = 2.8). Although the degree of deformation in martensite was much smaller than that in ferrite, it was shown by Ueji et al. 34,35) that smaller amount of strain was enough to finely subdivide as-quenched martensite (Fig. 13). As a result, homogeneous UFG structures having equiaxed morphologies like Fig. 14(d) could be obtained after annealing the 91% cold-rolled sheet under appropriate conditions. The equiaxed microstructure having a mean grain size of 490 nm shown in Fig. 14(d) is considered to be a kind of statically recrystallized microstructure similar to the type II microstructure shown in Fig. 10(c). In this case, again, the first ...
Context 33
... 34,35) that smaller amount of strain was enough to finely subdivide as-quenched martensite (Fig. 13). As a result, homogeneous UFG structures having equiaxed morphologies like Fig. 14(d) could be obtained after annealing the 91% cold-rolled sheet under appropriate conditions. The equiaxed microstructure having a mean grain size of 490 nm shown in Fig. 14(d) is considered to be a kind of statically recrystallized microstructure similar to the type II microstructure shown in Fig. 10(c). In this case, again, the first transformation produced the appropriate starting microstructure (the dual phase microstructure of ferrite and martensite), and subsequent cold-rolling and annealing realized ...
Context 34
... structures having equiaxed morphologies like Fig. 14(d) could be obtained after annealing the 91% cold-rolled sheet under appropriate conditions. The equiaxed microstructure having a mean grain size of 490 nm shown in Fig. 14(d) is considered to be a kind of statically recrystallized microstructure similar to the type II microstructure shown in Fig. 10(c). In this case, again, the first transformation produced the appropriate starting microstructure (the dual phase microstructure of ferrite and martensite), and subsequent cold-rolling and annealing realized the UFG microstructure without SPD. Combining two processes in the sequence of plastic deformation first and then phase ...
Context 35
... Recently, Zhao et al. 50,56) have found that two different mechanisms happen sequentially in certain materials and hot-deformation conditions, resulting to form UFG microstructures. Figure 15(a) schematically illustrates the mechanisms. 56,57) When austenite (£) is hotdeformed at supercooled temperatures under certain deformation conditions, DT to ferrite (¡) occurs to form relatively fine ferrite microstructures. ...
Context 36
... ferrite (¡) occurs to form relatively fine ferrite microstructures. Although the DT ferrite grow after nucleation, they then show DRX during continuous deformation. After such understanding, a thermo-mechanically controlled process under designed multi-step conditions realized UFG ferrite microstructures in a 10Ni0.1C (mass%) steel, as shown in Fig. 15(b) and (c). Figure 15(b) and (c) are EBSD grain boundary map and TEM image of the obtained UFG microstructure, respectively. The GB map clearly showed an uniform UFG structure composed of ultrafine ferrite grains with nearly equiaxed morphologies. The average grain size of the ultrafine ferrite was 0.55 µm. The TEM image confirmed the ...
Context 37
... steel, as shown in Fig. 15(b) and (c). Figure 15(b) and (c) are EBSD grain boundary map and TEM image of the obtained UFG microstructure, respectively. The GB map clearly showed an uniform UFG structure composed of ultrafine ferrite grains with nearly equiaxed morphologies. ...
Context 38
... high strength and large ductility probably caused by deformation twinning. 69) Then, Tian et al. 61) applied this finding to highstrength TWIP steel (Fe22Mn0.6C; mass%), and succeeded in obtaining fully recrystallized UFG microstructures in this steel, too. An example of the fully recrystallized UFG structures in the 22Mn0.6C steel is shown in Fig. 16. Figure 16(a) and (b) are EBSD orientation color map and grain boundary map of the same area of the fully recrystallized UFG structure. The grain sizes were fairly uniform, and the average grain size of this microstructure was 580 nm. Table 1 summarizes fully recrystallized UFG microstructures in several kinds of alloys reported by ...
Context 39
... is shown in Fig. 16. Figure 16(a) and (b) are EBSD orientation color map and grain boundary map of the same area of the fully recrystallized UFG structure. The grain sizes were fairly uniform, and the average grain size of this microstructure was 580 nm. ...
Context 40
... should be emphasized that all these materials having fully recrystallized UFG structures managed both high strength and large tensile ductility. Figure 16(c) shows engineering stress-strain curves of the 22Mn0.6C steel having two different average grain sizes, 21 µm and 0.58 µm (580 nm). ...
Context 41
... However, SPD processes are carried out at relatively low temperatures below recrystallization temperature of the materials in order to effectively accumulate lattice defects introduced by plastic deformation, and UFG microstructures are obtained in the as-SPDprocessed state (or the as-deformed state) without annealing treatments. Figure 1 shows transmission electron microscopy (TEM) microstructures of a pure aluminum with 99.99 mass% purity (4N-Al) processed by equal-channel angular pressing (ECAP) at room temperature (RT), reported by Iwahashi et al. 8) In the as-deformed state of 4-pass ECAP using a 90° angle die, an UFG microstructure with equiaxed morphology is clearly observed. The question is how and by what mechanism this UFG structure was formed. ...
Context 42
... TEM observations with Kikuch-line analysis revealed that the number and fraction of HAGBs increased with increasing the total strain (the number of ARB cycle) applied. TEM image and corresponding boundary misorientation map of the specimen ARB processed by 7 cycles (¾ = 5.6) are shown in Fig. 4. 14) The microstructure became more homogeneous throughout the specimen, and grains elongated to RD were observed (Fig. 4(a)). Grain boundaries surrounding elongated grains looked sharp, and the mean interval of boundaries along ND (the mean grain thickness) was 0.20 µm. The misorientation map (Fig. 4(b)) indicates that most of the ...
Context 43
... Gholizadeh et al. 30) have carried out systematic investigations of continuous SPD for various kinds of metallic materials at different temperatures and strain rates by the use of a simple torsion machine equipped with induction heating system. Figure 10 shows the result of such torsion experiments, where IF steel specimens were continuously deformed to high strains up to von Mises equivalent strain (¾) of 7.0 at various constant temperatures ranging from 400°C to 850°C and various constant strain rates from 0.01 s ¹1 to 1 s ¹1 . The IF steel specimens were quenched into water immediately after the torsion deformation, and frozen microstructures were analyzed in detail. ...
Context 44
... IF steel specimens were quenched into water immediately after the torsion deformation, and frozen microstructures were analyzed in detail. 30) Figure 10(a) exhibits relationships between the obtained grain/sub-grain sizes and the flow stress in the torsion deformation. The grain/sub-grain sizes showed certain correlations with the flow stress, but there were three different regions in the change of the grain size determined as the interval of HAGBs. ...
Context 45
... the obtained grain/sub-grain sizes and the flow stress in the torsion deformation. The grain/sub-grain sizes showed certain correlations with the flow stress, but there were three different regions in the change of the grain size determined as the interval of HAGBs. Typical microstructures (EBSD grain boundary maps) in three regions are shown in Fig. 10(b), (c), and (d). In the region of high flow stresses, ultrafine lamellar structures (type I) were mainly observed. This type of microstructure is quite similar to the lamellar UFG structures formed by SPD (ARB) of IF steel shown in Fig. 5(d) and Fig. 9(a), which suggests that the type I structures (Fig. 10(b)) are formed by the grain ...
Context 46
... maps) in three regions are shown in Fig. 10(b), (c), and (d). In the region of high flow stresses, ultrafine lamellar structures (type I) were mainly observed. This type of microstructure is quite similar to the lamellar UFG structures formed by SPD (ARB) of IF steel shown in Fig. 5(d) and Fig. 9(a), which suggests that the type I structures (Fig. 10(b)) are formed by the grain subdivision mechanism. In fact, the sizes of sub-grains (a kind of deformed microstructures formed by plastic deformation) were on the identical straight line of the grain size determined by HAGBs in the high-stress region, suggesting the same formation mechanism. In the transition region under medium flow ...
Context 47
... formed by plastic deformation) were on the identical straight line of the grain size determined by HAGBs in the high-stress region, suggesting the same formation mechanism. In the transition region under medium flow stresses, morphologies of fine grains changed into more equiaxed ones and the grain size increased with decreasing the flow stress (Fig. 10(c): type II). In the low flow-stress region, heterogeneous microstructures composed of some coarse grains surrounded by HAGBs within regions of subgrains with LAGBs were observed ( Fig. 10(d): type III). Such microstructures were similar to those formed by dynamic recrystallization (DRX; recrystallization during deformation). The flow ...
Context 48
... region under medium flow stresses, morphologies of fine grains changed into more equiaxed ones and the grain size increased with decreasing the flow stress (Fig. 10(c): type II). In the low flow-stress region, heterogeneous microstructures composed of some coarse grains surrounded by HAGBs within regions of subgrains with LAGBs were observed ( Fig. 10(d): type III). Such microstructures were similar to those formed by dynamic recrystallization (DRX; recrystallization during deformation). The flow stress dependence of the grain size determined by HAGBs in this region had nearly the same slope as that for the sub-grain size, but the grain sizes were much coarser than the sub-grain sizes ...
Context 49
... from the high flow-stress region). The results can be summarized in the following way: when the sub-grain size formed by deformation is maintained up to high strains (under high stress conditions at low temperatures or/and at high strain rates), the grain subdivision mechanism would dominate the formation process of UFG microstructures (type I in Fig. 10(b)). In deformation under low flow stress (at high temperatures or/and at low strain rates), recovery inhibits the development of HAGBs by the grain subdivision mechanism, leading to heterogeneous distribution of the regions surrounded by HAGBs (type III in Fig. 10(d)). The regions surrounded by HAGBs heterogeneously distributed within ...
Context 50
... mechanism would dominate the formation process of UFG microstructures (type I in Fig. 10(b)). In deformation under low flow stress (at high temperatures or/and at low strain rates), recovery inhibits the development of HAGBs by the grain subdivision mechanism, leading to heterogeneous distribution of the regions surrounded by HAGBs (type III in Fig. 10(d)). The regions surrounded by HAGBs heterogeneously distributed within sub-grains with LAGBs would grow due to the high mobility of HAGBs. Consequently the microstructures can be recognized as typical (discontinuous) DRX structures characterized by nucleation and growth of recrystallized regions. When the DRX characterized by grain ...
Context 51
... by lattice diffusion is inhibited rather than that under the low flow stress, so that the microstructure evolution is still dominated by the grain subdivision. However, migration of HAGBs controlled by grain boundary diffusion can happen under this condition, resulting in the relatively fine grains with equiaxed morphologies (type II in Fig. 10(c)). The increase of the grain size from that expected as the result of the grain subdivision mechanism (sub-grain sizes in Fig. 10(a)) is also ...
Context 52
... dominated by the grain subdivision. However, migration of HAGBs controlled by grain boundary diffusion can happen under this condition, resulting in the relatively fine grains with equiaxed morphologies (type II in Fig. 10(c)). The increase of the grain size from that expected as the result of the grain subdivision mechanism (sub-grain sizes in Fig. 10(a)) is also ...
Context 53
... on the understanding, let us look back the equiaxed UFG structures observed in the pure aluminum SPD (ECAP) processed at RT (Fig. 1). It should be noted firstly that the material used was a high-purity aluminum (4N-Al). For aluminum of which melting point temperature (T m ) is 660.3°C (937.3 K), ambient temperature (25°C = 298 K) is relatively a high temperature (³0.32T m ). High purity of the material and adiabatic heating during heavy deformation in a small ...
Context 54
... temperature (25°C = 298 K) is relatively a high temperature (³0.32T m ). High purity of the material and adiabatic heating during heavy deformation in a small deformation zone at the corner of ECAP die would enhance grain boundary migration (of HAGBs) formed by the deformation. It can be concluded that the equiaxed finegrained structure shown in Fig. 1 corresponds to the type II microstructure (Fig. 10(c)) in the continuous torsion deformation under medium flow stress conditions. High density of HAGBs in such a type II microstructure is resulted from the grain subdivision, and grain boundary migration changes morphologies of grains from lamellar one to equiaxed one and somewhat ...
Context 55
... temperature (³0.32T m ). High purity of the material and adiabatic heating during heavy deformation in a small deformation zone at the corner of ECAP die would enhance grain boundary migration (of HAGBs) formed by the deformation. It can be concluded that the equiaxed finegrained structure shown in Fig. 1 corresponds to the type II microstructure (Fig. 10(c)) in the continuous torsion deformation under medium flow stress conditions. High density of HAGBs in such a type II microstructure is resulted from the grain subdivision, and grain boundary migration changes morphologies of grains from lamellar one to equiaxed one and somewhat coarsens the grain size. The combinations of different ...
Context 56
... may point out the effect of strain paths on morphologies and sizes of UFG microstructures formed by SPD, since the equiaxed UFG structure shown in Fig. 1 was fabricated by the route-B C ECAP process where the barspecimen was rotated by 90° around the bar axis after every A d v a n c e V i e w pass of the ECAP. 8) In fact, a number of papers reported that different strain paths in SPD changed morphologies and formation speeds of UFG microstructures. The effect of redundant shear strain ...
Context 57
... to the same amount of plastic strains. It is, on the other hand, noteworthy that when high-purity Al (4N-Al with 99.99 mass% purity) was heavily deformed, equiaxed fine grains formed even in case of monotonic HPT. 37) The equiaxed fine-grain structures they reported are quite similar to that obtained by the route-B C ECAP process shown in Fig. 1. Zhang et al. 39,40) studied the microstructural evolution during monotonic HPT at RT for nickel (Ni) having higher melting point and therefore recovery and grain boundary migration are more difficult at RT compared to Al. The Ni specimens showed typical deformation microstructures that can be explained by the grain subdivision ...
Context 58
... fairly high plastic strains are necessary to form homogeneous UFG and nanostructures. Kamikawa et al. 41) assessed the development of UFG structures in a commercial purity aluminum (2N-Al) ARB processed under unlubricated condition at RT. Figure 11 shows their results, where the average spacing of HAGBs (a) and the fraction of HAGBs (b) in the material are plotted as a function of the total equivalent strain. 41) Since it had been clarified that the redundant shear strain in the unlubricated ARB process accelerate the formation of UFGs, 2729) the equivalent strain in Fig. 11 took into account of the redundant shear strain in addition to the rolling strain. ...
Context 59
... at RT. Figure 11 shows their results, where the average spacing of HAGBs (a) and the fraction of HAGBs (b) in the material are plotted as a function of the total equivalent strain. 41) Since it had been clarified that the redundant shear strain in the unlubricated ARB process accelerate the formation of UFGs, 2729) the equivalent strain in Fig. 11 took into account of the redundant shear strain in addition to the rolling strain. The spacing of HAGBs (grain size) quickly decreases and the fraction of HAGBs quickly increases with increasing the equivalent strain applied by the ARB, but both parameters eventually saturate to show constant values. The graphs clearly show that the ...
Context 60
... as-quenched martensite of a 0.13 mass% C steel was simply cold-rolled only by 50% reduction in thickness and then annealed at relatively low temperatures where only recovery occurs usually. Figure 12 shows a TEM image and corresponding grain boundary misorientation map obtained by TEM Kikuchi-line analysis of the 0.13%C steel annealed at 500°C for 1.8 ks after 50% cold-rolling of martensite starting microstructure. Nearly equiaxed UFGs with an average grain size of 180 nm were observed (Fig. 12(a)). ...
Context 61
... where only recovery occurs usually. Figure 12 shows a TEM image and corresponding grain boundary misorientation map obtained by TEM Kikuchi-line analysis of the 0.13%C steel annealed at 500°C for 1.8 ks after 50% cold-rolling of martensite starting microstructure. Nearly equiaxed UFGs with an average grain size of 180 nm were observed (Fig. 12(a)). The misorientation map (Fig. 12(b)) clearly indicated that many UFGs were surrounded by HAGBs. Since martensite of carbon steels are supersaturated solid solution of carbon, fine carbides near-homogeneously precipitated within the matrix. Such carbides inhibited grain growth of the ferrite matrix to maintain UFG microstructures, and ...
Context 62
... usually. Figure 12 shows a TEM image and corresponding grain boundary misorientation map obtained by TEM Kikuchi-line analysis of the 0.13%C steel annealed at 500°C for 1.8 ks after 50% cold-rolling of martensite starting microstructure. Nearly equiaxed UFGs with an average grain size of 180 nm were observed (Fig. 12(a)). The misorientation map (Fig. 12(b)) clearly indicated that many UFGs were surrounded by HAGBs. Since martensite of carbon steels are supersaturated solid solution of carbon, fine carbides near-homogeneously precipitated within the matrix. Such carbides inhibited grain growth of the ferrite matrix to maintain UFG microstructures, and also enhanced strain-hardening in ...
Context 63
... a result, martensite microstructure is a kind of fine-grained microstructures even in the as-transformed state. Figure 13(a) shows an EBSD orientation map of an as-quenched martensite in a low-C steel (0.2 mass% C steel). 45) The white lines were grain boundaries surrounding a prior austenite grain reconstructed from the crystallographic analysis. ...
Context 64
... were grain boundaries surrounding a prior austenite grain reconstructed from the crystallographic analysis. It could be clearly seen that, within the single austenite grain, a number of different variants of martensite (painted in different colors and indicated as "V2", "V14", etc.) formed. A grain boundary map corresponding to the area shown in Fig. 13(a) is represented in Fig. 13(b). Boundaries existing in the microstructure are categorized into three kinds depending on their misorientation angles and drawn in different colors. It was recognized that the original austenite grains were finely subdivided by the martensitic transformation. The effective grain size of this martensite ...
Context 65
... a prior austenite grain reconstructed from the crystallographic analysis. It could be clearly seen that, within the single austenite grain, a number of different variants of martensite (painted in different colors and indicated as "V2", "V14", etc.) formed. A grain boundary map corresponding to the area shown in Fig. 13(a) is represented in Fig. 13(b). Boundaries existing in the microstructure are categorized into three kinds depending on their misorientation angles and drawn in different colors. It was recognized that the original austenite grains were finely subdivided by the martensitic transformation. The effective grain size of this martensite microstructure estimated from ...
Context 66
... was accelerated due to the constraint of grain boundaries. It should be also noted that the as-quenched martensite involved a high density of dislocations. These were probably the reason why only 50% cold-rolling and subsequent recovery annealing could produce the UFG microstructure. The TEM microstructure of the 50% cold-rolled specimen (Fig. 13(c)) revealed a complicated deformation microstructure. Ultrafine lamellar structures were observed in most areas, and they had wavy shapes due to additional shear localization. Selected area diffraction patterns suggested the existence of local misorientations. That is, the finely subdivided structure comparable to those formed after SPD ...
Context 67
... coldrolling of martensite. This is an example of the simple processes to fabricate UFG microstructures without SPD. Firstly, the phase transformation (martensitic transformation) made the appropriate starting microstructure, and then plastic deformation (conventional cold-rolling) and subsequent annealing realized the UFG microstructure shown in Fig. 12. Okitsu et al. 46) showed another example of the simple processes, using a 0.10C1.98Mn0.018Nb0.0015B (mass%) steel. They firstly made a dual phase microstructure composed of ferrite (F) and martensite (M) shown in Fig. 14(b) through conventional hot-rolling, air cooling, and quenching from ferrite + austenite two-phase temperatures ...
Context 68
... and then plastic deformation (conventional cold-rolling) and subsequent annealing realized the UFG microstructure shown in Fig. 12. Okitsu et al. 46) showed another example of the simple processes, using a 0.10C1.98Mn0.018Nb0.0015B (mass%) steel. They firstly made a dual phase microstructure composed of ferrite (F) and martensite (M) shown in Fig. 14(b) through conventional hot-rolling, air cooling, and quenching from ferrite + austenite two-phase temperatures ( Fig. 14(a)). Figure 14(c) represents a SEM image observed from TD of the as 91% cold-rolled specimen having A d v a n c e V i e w the dual phase starting microstructure. Since martensite was much harder than ferrite, plastic ...
Context 69
... Fig. 12. Okitsu et al. 46) showed another example of the simple processes, using a 0.10C1.98Mn0.018Nb0.0015B (mass%) steel. They firstly made a dual phase microstructure composed of ferrite (F) and martensite (M) shown in Fig. 14(b) through conventional hot-rolling, air cooling, and quenching from ferrite + austenite two-phase temperatures ( Fig. 14(a)). Figure 14(c) represents a SEM image observed from TD of the as 91% cold-rolled specimen having A d v a n c e V i e w the dual phase starting microstructure. Since martensite was much harder than ferrite, plastic deformation was concentrated in soft ferrite, and hard martensite was not heavily deformed and showed an island morphology ...
Context 70
... firstly made a dual phase microstructure composed of ferrite (F) and martensite (M) shown in Fig. 14(b) through conventional hot-rolling, air cooling, and quenching from ferrite + austenite two-phase temperatures ( Fig. 14(a)). Figure 14(c) represents a SEM image observed from TD of the as 91% cold-rolled specimen having A d v a n c e V i e w the dual phase starting microstructure. Since martensite was much harder than ferrite, plastic deformation was concentrated in soft ferrite, and hard martensite was not heavily deformed and showed an island morphology with diamond shapes. ...
Context 71
... microstructures comparable to SPD processed ones (like Figs. 4 and 9), even though the total rolling reduction was not huge (91%, ¾ = 2.8). Although the degree of deformation in martensite was much smaller than that in ferrite, it was shown by Ueji et al. 34,35) that smaller amount of strain was enough to finely subdivide as-quenched martensite (Fig. 13). As a result, homogeneous UFG structures having equiaxed morphologies like Fig. 14(d) could be obtained after annealing the 91% cold-rolled sheet under appropriate conditions. The equiaxed microstructure having a mean grain size of 490 nm shown in Fig. 14(d) is considered to be a kind of statically recrystallized microstructure similar ...
Context 72
... the total rolling reduction was not huge (91%, ¾ = 2.8). Although the degree of deformation in martensite was much smaller than that in ferrite, it was shown by Ueji et al. 34,35) that smaller amount of strain was enough to finely subdivide as-quenched martensite (Fig. 13). As a result, homogeneous UFG structures having equiaxed morphologies like Fig. 14(d) could be obtained after annealing the 91% cold-rolled sheet under appropriate conditions. The equiaxed microstructure having a mean grain size of 490 nm shown in Fig. 14(d) is considered to be a kind of statically recrystallized microstructure similar to the type II microstructure shown in Fig. 10(c). In this case, again, the first ...
Context 73
... 34,35) that smaller amount of strain was enough to finely subdivide as-quenched martensite (Fig. 13). As a result, homogeneous UFG structures having equiaxed morphologies like Fig. 14(d) could be obtained after annealing the 91% cold-rolled sheet under appropriate conditions. The equiaxed microstructure having a mean grain size of 490 nm shown in Fig. 14(d) is considered to be a kind of statically recrystallized microstructure similar to the type II microstructure shown in Fig. 10(c). In this case, again, the first transformation produced the appropriate starting microstructure (the dual phase microstructure of ferrite and martensite), and subsequent cold-rolling and annealing realized ...
Context 74
... structures having equiaxed morphologies like Fig. 14(d) could be obtained after annealing the 91% cold-rolled sheet under appropriate conditions. The equiaxed microstructure having a mean grain size of 490 nm shown in Fig. 14(d) is considered to be a kind of statically recrystallized microstructure similar to the type II microstructure shown in Fig. 10(c). In this case, again, the first transformation produced the appropriate starting microstructure (the dual phase microstructure of ferrite and martensite), and subsequent cold-rolling and annealing realized the UFG microstructure without SPD. Combining two processes in the sequence of plastic deformation first and then phase ...
Context 75
... Recently, Zhao et al. 50,56) have found that two different mechanisms happen sequentially in certain materials and hot-deformation conditions, resulting to form UFG microstructures. Figure 15(a) schematically illustrates the mechanisms. 56,57) When austenite (£) is hotdeformed at supercooled temperatures under certain deformation conditions, DT to ferrite (¡) occurs to form relatively fine ferrite microstructures. ...
Context 76
... ferrite (¡) occurs to form relatively fine ferrite microstructures. Although the DT ferrite grow after nucleation, they then show DRX during continuous deformation. After such understanding, a thermo-mechanically controlled process under designed multi-step conditions realized UFG ferrite microstructures in a 10Ni0.1C (mass%) steel, as shown in Fig. 15(b) and (c). Figure 15(b) and (c) are EBSD grain boundary map and TEM image of the obtained UFG microstructure, respectively. The GB map clearly showed an uniform UFG structure composed of ultrafine ferrite grains with nearly equiaxed morphologies. The average grain size of the ultrafine ferrite was 0.55 µm. The TEM image confirmed the ...
Context 77
... steel, as shown in Fig. 15(b) and (c). Figure 15(b) and (c) are EBSD grain boundary map and TEM image of the obtained UFG microstructure, respectively. The GB map clearly showed an uniform UFG structure composed of ultrafine ferrite grains with nearly equiaxed morphologies. ...
Context 78
... high strength and large ductility probably caused by deformation twinning. 69) Then, Tian et al. 61) applied this finding to highstrength TWIP steel (Fe22Mn0.6C; mass%), and succeeded in obtaining fully recrystallized UFG microstructures in this steel, too. An example of the fully recrystallized UFG structures in the 22Mn0.6C steel is shown in Fig. 16. Figure 16(a) and (b) are EBSD orientation color map and grain boundary map of the same area of the fully recrystallized UFG structure. The grain sizes were fairly uniform, and the average grain size of this microstructure was 580 nm. Table 1 summarizes fully recrystallized UFG microstructures in several kinds of alloys reported by ...
Context 79
... is shown in Fig. 16. Figure 16(a) and (b) are EBSD orientation color map and grain boundary map of the same area of the fully recrystallized UFG structure. The grain sizes were fairly uniform, and the average grain size of this microstructure was 580 nm. ...
Context 80
... should be emphasized that all these materials having fully recrystallized UFG structures managed both high strength and large tensile ductility. Figure 16(c) shows engineering stress-strain curves of the 22Mn0.6C steel having two different average grain sizes, 21 µm and 0.58 µm (580 nm). ...
Citations
... By employing low-cost thermo-mechanical approaches such as warm rolling (WR) [9-11] and accumulative roll bonding (ARB) [12-13], high-strength ultra-fine grained (UFG, with grain sizes ≤ 1 μm) bcc steels have been designed. Regrettably, UFG steels, despite their desirable DBTT, show compromised ductility, toughness, and low work-hardening capabilities at ambient temperatures [9][10][11][12][13]. To achieve high strength with reduced DBTT, Kimura et al. combined grain refinement with CRediT authorship contribution state ment ...
... Figure 2a-c displays the HPT-processed microstructures of NoSc, LoSc and HiSc alloys acquired by TEM-OIM technique. The microstructural features for the three alloys represented characteristic severely deformed microstructures [34]. The strain gradients within the grains were observed in the case of NoSc and LoSc alloys but not in the case of HiSc alloy. ...
The effect of Sc addition to AA2195 (NoSc) alloy by 0.025 wt% (LoSc) and 0.25 wt% (HiSc) on the evolution of microstructure, texture and mechanical property in AA2195 alloy during room temperature HPT processing was studied by electron microscopy, X-ray diffraction and Vicker’s microhardness test, respectively. Higher amount of Sc addition increased the volume fraction of precipitates formed after HPT processing by 5 rotations, as inferred from both STEM-HAADF imaging and high-resolution X-rays diffraction pattern analysis. This was primarily attributed to the size, distribution and morphology of the precipitates. Increased Sc content resulted in the decrease in solid solubility of Cu in the Al matrix, thereby causing higher precipitation of Cu containing precipitates. The increase in Sc content resulted in decreased intensities of the A1∗\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$A_{1}^{*}$$\end{document} and A2∗\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$A_{2}^{*}$$\end{document} ideal shear texture components and an increase in the intensity of the C components ({100} < 011 >), resulting from the easier dynamic recovery and recrystallization that occurs with the presence of larger precipitates. As a result, the highest microhardness was achieved in the LoSc alloy due to the formation of a nanocrystalline microstructure along with a homogeneous distribution of nanoscaled precipitates. This fine distribution of precipitates in LoSc alloy retained the smallest crystallite size and highest dislocation density at the disk periphery around a strain of 30. These enhanced properties in LoSc were attributed to the nearly homogeneous formation of fine precipitates ranging between 15 and 25 nm within the grains and < 100 nm at the grain boundaries during HPT processing.
... Within the last two decades, Equal-Channel Angu lar Pressing (ECAP) has been intensively studied as an effective technique of severe plastic deforma tion (SPD) designed to produce ultrafine or even nanograin me tals and alloys with considerably improved mechanical properties combining both strength and ductility [1]. Severe plastic deformation uses ECAP to produce large bulk samples free from residual stress where the samples are pressed to the die with an angled cavity witho ut any change in the cross-sectional dimen sions [2]. ...
Equal-Channel Angular Pressing (ECAP) has become an effective technique of severe plastic deformation designed to produce ultrafine grain metals with improved mechanical properties, such as a good combination of strength and ductility. A report on the effect of ECAP routes on the mechanical and microstructure of commercial 5052 aluminum alloy needs also to be included. This work has been undertaken, in order to obtain the results. In this work, several deformation routes were used to process the Al – Mg (5052) alloy, namely A , Ba , Bc and C . Deformation route A involved repeatedly pushing the sample into the ECAP die without rotation, route Ba was performed by rotating the sample through 90° in alternate directions between each pass, route Bc by rotating the sample 90° in the same sense between each pass and route C by rotating the sample 180° between passes. The addition of the pass number decreases the grain size of ECAP-processed samples when compared to the as-annealed sample. It also confirmed that the microstructure of the 8-pass samples shows a finer grain size than the as-annealed sample. Furthermore, the Bc route (samples rotated in the same sense by 90° between each pass) has been proven to be the most effective deformation route, in order to obtain equiaxed ultrafine grain structure when compared to other deformation routes. This phenomenon takes place due to the continuous deformation in all cubic planes. The restoration after the 4-pass number will lead to the rapid evolution of sub-grains to high-angle grain boundaries, forming equiaxed grains. The characterization of the hardness number also shows that the addition of the ECAP pass number increases the hardness number of 5052 aluminum alloy, where samples processed with the Bc route indicate the highest hardness number at 168.4 HB. Moreover, a similar phenomenon also suggests that the tensile strength of all ECAP deformation routes has comparable values. The effect of heat treatment for samples with the Bc route also shows that 200 °C annealed samples have the highest hardness number and tensile strength when compared to other samples.
... Severe Plastic Deformation (SPD) provides bulk ultrafinegrained (UFG) or nanostructured (NS) materials with enhanced mechanical, chemical and physical properties, due to significant grain refinement and high density of lattice defects. 920) High-Pressure Torsion (HPT), 11,12) Equal-Channel Angular Pressing (ECAP) 13) and Accumulative Roll-Bonding (ARB) 14) are among the most studied techniques of SPD. Other SPD methods such as High-Pressure Sliding (HPS), 21) ECAP with Parallel Channels (ECAP-PC) 22) Rotating Shear Plane (ECAP-R), 23) Repetitive Corrugation and Straightening (RCS), 24) Constrained Studded Pressing (CSP) 25) and High-Pressure Torsion Extrusion (HPTE) 2628) have been developed to expand the sample size for industrial application. ...
... SPD methods provide an effective path for fine-structuring a wide range of materials including the capability for powder consolidation and alloy design with multiple applications in several research fields and industry. 30,118,119) Electrical and mechanical properties of Cu and Al alloys can be balanced by factors such as thermal-mechanical processing, strengthening mechanisms, 120) alloy thermodynamics (precipitation of second phases accelerate grain refinement), 14,122) grain boundary phenomena 30,103,120133) and microstructural evolution. 14,30,128) All these factors can be partially controlled by SPD processing complemented with aging treatments, or even, dynamic aging to produce high conductive and strengthened bulk-nanostructured alloy. ...
... 30,118,119) Electrical and mechanical properties of Cu and Al alloys can be balanced by factors such as thermal-mechanical processing, strengthening mechanisms, 120) alloy thermodynamics (precipitation of second phases accelerate grain refinement), 14,122) grain boundary phenomena 30,103,120133) and microstructural evolution. 14,30,128) All these factors can be partially controlled by SPD processing complemented with aging treatments, or even, dynamic aging to produce high conductive and strengthened bulk-nanostructured alloy. 30,134) However, the potential application for commercialization of UFG materials is limited by the fabrication capabilities and the sample size. ...
In recent years, the severe plastic deformation community has developed an interest for metallic materials with high strength and high electrical conductivity, with special focus in Cu- and Al-based alloys, including composite materials. Several processing and metallurgical strategies have been applied to control the influence of microstructure features such as grain refinement, grain boundary condition, defect structures and segregation of secondary phases, over the electrical and mechanical properties. This work summarizes an important body of literature where several strengthening mechanisms and methods to restore the electrical conductivity have been applied to produce ultrafine-grained or nanostructured Cu and Al alloys, mainly by intense imposed strain. A wide variety of alloy systems were studied for their industrial applications in the electrical and electronic market. It can be concluded that the balanced combination of alloying element selection and processing route (mainly attainable under high hydrostatic conditions) could provide high strength with high conductivity and thermal stability materials.
Fullsize Image
... The SPD field experienced significant progress in the past three decades, as discussed in several review papers [124][125][126][127][128][129], and more recently in a special issue in 2019 [130], which gathered overviews on both historical developments [131] and recent advancements [132]. A survey of these overviews indicates that despite significant progress on theoretical aspects [133,134], mechanisms [135,136], processing [137][138][139][140][141][142][143][144], microstructure [145][146][147][148][149], and mechanical properties [150][151][152][153][154][155] of metallic materials, there is a recent tendency to apply SPD to a wider range of materials (oxides [156], semiconductors [157], carbon polymorphs [158], glasses [159], and polymers [160]) to control phase transformations [161] and solid-state reaction [162][163][164] for achieving advanced functional properties [165][166][167][168][169][170][171][172]. CO2 conversion is perhaps the newest application of SPD to functional materials, which expanded the synthesis capability of SPD from metallic materials to ceramics [37,38]. ...
Excessive CO2 emission from fossil fuel usage has resulted in global warming and environmental crises. To solve this problem, the photocatalytic conversion of CO2 to CO or useful components is a new strategy that has received significant attention. The main challenge in this regard is exploring photocatalysts with high efficiency for CO2 photoreduction. Severe plastic deformation (SPD) through the high-pressure torsion (HPT) process has been effectively used in recent years to develop novel active catalysts for CO2 conversion. These active photocatalysts have been designed based on four main strategies: (i) oxygen vacancy and strain engineering, (ii) stabilization of high-pressure phases, (iii) synthesis of defective high-entropy oxides, and (iv) synthesis of low-bandgap high-entropy oxynitrides. These strategies can enhance the photocatalytic efficiency compared with conventional and benchmark photocatalysts by improving CO2 adsorption, increasing light absorbance, aligning the band structure, narrowing the bandgap, accelerating the charge carrier migration, suppressing the recombination rate of electrons and holes, and providing active sites for photocatalytic reactions. This article reviews recent progress in the application of SPD to develop functional ceramics for photocatalytic CO2 conversion.
... The SPD field experienced significant progress in the past three decades, as discussed in several review papers [124][125][126][127][128][129], and more recently in a special issue in 2019 [130], which gathered overviews on both historical developments [131] and recent advancements [132]. A survey of these overviews indicates that despite significant progress on theoretical aspects [133,134], mechanisms [135,136], processing [137][138][139][140][141][142][143][144], microstructure [145][146][147][148][149], and mechanical properties [150][151][152][153][154][155] of metallic materials, there is a recent tendency to apply SPD to a wider range of materials (oxides [156], semiconductors [157], carbon polymorphs [158], glasses [159], and polymers [160]) to control phase transformations [161] and solid-state reaction [162][163][164] for achieving advanced functional properties [165][166][167][168][169][170][171][172]. CO2 conversion is perhaps the newest application of SPD to functional materials, which expanded the synthesis capability of SPD from metallic materials to ceramics [37,38]. ...
Excessive CO2 emission from fossil fuel usage has resulted in global warming and environmental crises. To solve this problem, photocatalytic conversion of CO2 to CO or useful components is a new strategy that has received significant attention. The main challenge in this regard is exploring photocatalysts with high activity for CO2 photoreduction. Severe plastic deformation (SPD) through the high-pressure torsion (HPT) process has been effectively used in recent years to develop novel active catalysts for CO2 conversion. These active photocatalysts have been designed based on four main strategies (i) oxygen vacancy and strain engineering, (ii) stabilization of high-pressure phases, (iii) synthesis of defective high-entropy oxides, and (iv) synthesis of low-bandgap high-entropy oxynitrides. These strategies can enhance the photocatalytic efficiency compared to conventional and benchmark photocatalysts by improving CO2 adsorption, increasing light absorbance, aligning the band structure, narrowing the bandgap, accelerating the charge carrier migration, suppressing the recombination rate of electrons and holes, and providing active sites for photocatalytic reactions. This article reviews recent progress in the application of SPD to develop functional ceramics for photocatalytic CO2 conversion.
... However, the high strength is generally obtained at the sacrifice of ductility due to the excess defects [2]. Among various successful strategies [2][3][4][5], one kind of ultrafine-grained (UFG) materials with recrystallized structure have been developed recently and drawn much attention [6][7][8]. The ultrafine grains raise the yield strength and the well-annealed structure provides the potential strain-hardening capability, facilitating the simultaneous achievement of strength and ductility [6][7][8]. ...
... Among various successful strategies [2][3][4][5], one kind of ultrafine-grained (UFG) materials with recrystallized structure have been developed recently and drawn much attention [6][7][8]. The ultrafine grains raise the yield strength and the well-annealed structure provides the potential strain-hardening capability, facilitating the simultaneous achievement of strength and ductility [6][7][8]. ...
The yield strength of 316LN stainless steel is always insufficient due to the single-phase austenitic structure. We processed bulky specimens with recrystallized ultrafine-grained microstructure by cold rolling and annealing treatments. The specimens are strong and ductile at 293 K as a result of dislocation-dominated plastic deformation process. In contrast, the yield strength was doubled, and the ductility was further enhanced due to the sequentially activated deformation modes at 77 K. The yielding behavior and plastic flow were discussed based on the dramatic change of deformation mechanisms at 293 K and 77 K.
... As bulky samples can be fabricated even in the laboratory, ARB is beneficial for studying microstructure evolution and mechanical properties of nanostructured materials systematically. Careful observations on materials processed by ARB to various strains have clarified that the formation of UFG microstructures during SPD can be understood in terms of grain subdivision [176][177][178], as will be described in Section 6.1. Figure 15(c) indicates the effect of equivalent strain applied by ARB on the average spacing of high-angle grain boundaries and their fraction in an ultra-low carbon IF steel [169]. ...
... From such studies, it was clearly shown that UFG metals exhibit very high strength 3 ∼ 4 times higher than the same materials having conventionally coarse grain sizes [181], but their tensile ductility (especially uniform elongation) is limited due to early plastic instability [179]. After this understanding, various kinds of nanostructured metallic materials managing both high strength and large tensile ductility have been found [178,181,182]. Fully recrystallized nanostructured metals [176] are typical example of such new nanostructured metals. ...
... It is well established that such a grain refinement finally saturates to a steady-state level at large strains. Understanding the mechanism of grain refinement as well as the physical phenomena underlying the saturation of grain refinement are still of critical significance in the NanoSPD field [178,472]. In addition to the formation of ultrafine grains with large fraction of high-angle grain boundaries, various kinds of defects such as twins, dislocations, stacking faults and vacancies [452,473] as well as some unique microstructural features including grain boundary segregation, precipitate formation/dissolution [474] and texture development [475,476], are characteristics of SPD processing. ...
Severe plastic deformation (SPD) is effective in producing bulk ultrafine-grained and nanostructured materials with large densities of lattice defects. This field, also known as NanoSPD, experienced a significant progress within the past two decades. Beside classic SPD methods such as high-pressure torsion, equal-channel angular pressing, accumulative roll-bonding, twist extrusion, and multi-directional forging, various continuous techniques were introduced to produce upscaled samples. Moreover, numerous alloys, glasses, semiconductors, ceramics, polymers, and their composites were processed. The SPD methods were used to synthesize new materials or to stabilize metastable phases with advanced mechanical and functional properties. High strength combined with high ductility, low/room-temperature superplasticity, creep resistance, hydrogen storage, photocatalytic hydrogen production, photocatalytic CO2 conversion, superconductivity, thermoelectric performance, radiation resistance, corrosion resistance, and biocompatibility are some highlighted properties of SPD-processed materials. This article reviews recent advances in the NanoSPD field and provides a brief history regarding its progress from the ancient times to modernity.
Abbreviations: ARB: Accumulative Roll-Bonding; BCC: Body-Centered Cubic; DAC: Diamond Anvil Cell; EBSD: Electron Backscatter Diffraction; ECAP: Equal-Channel Angular Pressing (Extrusion); FCC: Face-Centered Cubic; FEM: Finite Element Method; FSP: Friction Stir Processing; HCP: Hexagonal Close-Packed; HPT: High-Pressure Torsion; HPTT: High-Pressure Tube Twisting; MDF: Multi-Directional (-Axial) Forging; NanoSPD: Nanomaterials by Severe Plastic Deformation; SDAC: Shear (Rotational) Diamond Anvil Cell; SEM: Scanning Electron Microscopy; SMAT: Surface Mechanical Attrition Treatment; SPD: Severe Plastic Deformation; TE: Twist Extrusion; TEM: Transmission Electron Microscopy; UFG: Ultrafine Grained
... The solution to the limitations of classic methods of metal strengthening is, e.g., cold plastic processing of metal or more effective metal forming processes with severe plastic deformation (SPD) [3] In these processes, the increase in mechanical properties is achieved by controlled grain refinement at low temperatures [4,5]. The SPD process allows for obtaining high grain refinement to a nanometric level (where the average grain size is below 100 nm). ...
The paper presents the microstructural investigation of a friction-welded joint made of 316L stainless steel with an ultrafine-grained structure obtained by hydrostatic extrusion (HE). Such a plastically deformed material is characterized by a metastable state of energy equilibrium, increasing, among others, its sensitivity to high temperatures. This feature makes it difficult to weld ultra- fine-grained metals without losing their high mechanical properties. The use of high-speed friction welding and a friction time of <1 s reduced the scale of the weakening of the friction joint in relation to result obtained in conventional rotary friction welding. The study of changes in the microstructure of individual zones of the friction joint was carried out on an optical microscope (OM), scanning electronmicroscope(SEM),transmissionelectronmicroscope(TEM)andelectronbackscattered diffraction (EBSD) analysis system. The correlation between the microstructure and hardness of the friction joint is also presented. The heat released during the high-speed friction welding initiated the process of dynamic recrystallization (DRX) of single grains in the heat-affected zone (HAZ). The additional occurrence of strong plastic deformations (in HAZ) during flash formation and internal friction (in the friction weld and high-temperature HAZ) contributed to the formation of a highly deformed microstructure with numerous sub-grains. The zones with a microstructure other than the base material were characterized by lower hardness. Due to the complexity of the microstructure and its multifactorial impact on the properties of the friction-welded joint, strength should be the criterion for assessing the properties of the joint.
... The microstructures after low values of imposed plastic strain usually contain cells and subgrains with LAGBs. However, in the case that extremely large deformation is imposed on the material, the transformation of CG microstructure to ultrafine-grained (UFG) one (grain size 0.1-1 µm) occurs [16]. Thus SPD method provides a unique opportunity to study creep in the microstructures containing either subgrain or grain microstructure predominantly. ...
