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TEM bright-field images of the HPT-processed samples with different shear strains: (a) 5.55; (b) 11.1; (c) 22.2; (d) 44.4, corresponding to 1/4 (90°), 1/2 (180°), 1 (360°) and 2 (720°) rotations.
Source publication
Metastable austenite in a Fe-24Ni-0.3C (wt.%) alloy was processed by high-pressure torsion and subsequently heat-treated. The HPT-processed material had lamellae structures composed of highly deformed austenite and deformation-induced martensite. The reverse transformation of the deformation-induced martensite and recovery/recrystallization of the...
Contexts in source publication
Context 1
... the HPT processing, the material was refined and martensitic transformation was also induced during the deformation. After HPT processing with a shear strain of 5.55, microstructure consists of lamellar structure, as shown in Figure 1(a). The lamellae are elongated along the shear direction and have a spacing of around 50 nm. ...
Context 2
... lamellae are elongated along the shear direction and have a spacing of around 50 nm. When the shear strain increased to 11.1, the lamellae spacing decreased to about 20 nm (Figure 1b). With further increase in shear strain, the lamellar boundaries became ambiguous in Figure 1(c, d). ...
Context 3
... the shear strain increased to 11.1, the lamellae spacing decreased to about 20 nm (Figure 1b). With further increase in shear strain, the lamellar boundaries became ambiguous in Figure 1(c, d). Even after HPT processing with a much higher shear strain of 222, the material still had a lamellar structure without significant change from a shear strain of 22.2 (360°). ...
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Citations
... Severe plastic deformation processing such as HPT can be used to impose very high equivalent strains of the order of 5000% or more on such materials. Due to the presence of such high strains, extreme dislocation densities of the order of 10 16 m −2 and grain refinement of the order of hundreds of nanometers can be achieved [5,[21][22][23]. HPT allows for higher strain application because the force used for the processing is primarily in shear accompanied by hydrostatic pressure. ...
High-pressure-torsion (HPT) processing introduces a large density of dislocations that form sub-grain boundaries within the refined nano-scale structure, leading to changes in precipitate morphology compared to hot-rolled maraging steels. The impact of such nanostructuring on the deformation and fracture micro-mechanisms is being reported for the first time usingin situcharacterization techniques along with transmission electron microscopy and atom probe tomography analysis, in this study. Digital image correlation has been used to quantify the full field strain maps in regions of severe strain localization as well as to determine the fracture toughness through critical crack tip opening displacements. It is seen that the phenomenon of planar slip leads to strain softening under uniaxial deformation and to crack branching under a triaxial stress state in hot rolled maraging steels. On the other hand, nano-structuring after HPT processing creates a large number of high angle grain boundaries as dislocation barriers, leading to strain hardening under uniaxial tension and nearly straight crack path with catastrophic fracture under triaxial stress state. Upon overaging, the hot-rolled sample shows signature of transformation induced plasticity under uniaxial tension, which is absent in the HPT processed overaged samples, owing to the finer reverted austenite grains containing higher Ni concentration in the latter. In the overaged fracture test samples of both the hot-rolled and HPT conditions, crack tips show a signature of strain induced transformation of the reverted austenite to martensite, due to the accompanying severe strain gradients. This leads to a higher fracture toughness even while achieving high strengths in the overaged conditions of the nanocrystalline HPT overaged samples. The results presented here will aid in design of suitable heat treatment or microstructure engineering of interface dominated nano-scale maraging steels with improved damage tolerance.
... Additionally, an annealing treatment after SPD processing may produce small austenite grains in the a 0martensitic microstructure such that a mixture of high strength and fine ductility becomes feasible [29,30]. A combination of HPT processing and subsequent annealing has a great impact in introducing a newly formed austenite with a smaller grain size even at higher temperatures by comparison with annealing without pre-deformation [31]. It was reported that, following a high temperature annealing treatment of the HPT-processed austenitic stainless steel, a reverse transformation of a 0 -martensite to the g-austenite (reverse transformation) yielded a fully austenitic microstructure with a small grain size of around 200 nm which gave remarkably high strength with a good ductility compared with the initial steel [32]. ...
Research was conducted to investigate the effects of high-pressure torsion (HPT) and post-deformation annealing (PDA) on the microstructure evolution and mechanical behavior of an Fe-9.6Ni-7.1Mn (at.%) steel with an initial lath martensitic microstructure. The experimental results showed that HPT processing led to the formation of an ultrafine grain martensitic microstructure accompanied by small amounts of strain-induced austenite. Phase analysis and microstructural examination confirmed that during PDA at 600 °C a large fraction of fine and coaxial austenite grains was introduced in the microstructure by diffusionless shear mechanism whereas its volume fraction at ambient temperature was drastically decreased by increasing the annealing time. Also, the grain size was reduced from a value of about 5.2 μm in the solution-treated specimen to ultrafine values of about 570 and 280 nm for the martensite and austenitic phases, respectively, after PDA for 7.2 ks. PDA yielded an outstandingly good combination of an ultimate tensile strength (∼1340 MPa) and fracture strain (∼11.9%) in comparison to the solution-annealed condition which can be attributed to the finer grain size and the presence of shear-formed austenite in the microstructure. In addition, the fracture mode changed from a fully ductile nature in the solution-treated specimen to a combination of ductile and brittle nature after applying the HPT and then returned again to a ductile behavior after PDA.
... It can be observed that the start temperature of austenite formation is decreased about 35°C after the HPT processing (Fig. 7). Large fractions of crystalline defects may also promote the reverse transformation of martensite to austenite by acting as nucleation sites [39]. ...
Metastable austenitic steels having ultrafine-grained (UFG) microstructures can be fabricated by conventional cold rolling and annealing processes by utilizing the deformation-induced martensitic transformation during cold rolling and its reverse transformation to austenite upon annealing. However, such processes are not applicable when the austenite has high mechanical stability against deformation-induced martensitic transformation, since there is no sufficient amount of martensite formed during cold rolling. In the present study, a two-step cold rolling and annealing process was applied to an Fe-24Ni-0.3C metastable austenitic steel having high mechanical stability. Prior to the cold rolling, a repetitive subzero treatment and reverse annealing treatment were applied. Such a treatment dramatically decreased the mechanical stability of the austenite and greatly accelerated the formation of deformation-induced martensite during the following cold rolling processes. As a result, the grain refinement was significantly promoted, and a fully recrystallized specimen with a mean austenite grain size of 0.5 μm was successfully fabricated, which exhibited both high strength and high ductility.
