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Tensile curves (engineering stress and strain) for model predictions and experimental measurements for the six specimens deformed along the last rolling direction.
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This work analyzes the role of both the grain size distribution and the crystallographic texture on the tensile behavior of commercially pure titanium. Specimens with different microstructures, especially with several mean grain sizes, were specifically prepared for that purpose. It is observed that the yield stress depends on the grain size follow...
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... can be noticed from the tensile experimental curves (Figures 1 and 4) that the tendency to form an initial plateau becomes more and more pronounced with decreasing mean grain size. This general trend is in agreement with other experiments in titanium [50] and was also characterized as such in copper [9] for instance. The model reproduces very well this trend (Figure 4). Moreover, the evolution of the work hardening rate θ = dσ/dε P for all the simulations of RR type is plotted in Figure 10. In agreement with the present experiments and previous tensile tests on cp Ti [17,19,29,30], a three-stage work hardening behavior is obtained, where an initial fall of θ is followed by an increase and a final progressive decrease. More importantly, the model predicts a growing well depth with decreasing grain size, or, in other words, a transient low work hardening rate with smaller and smaller value as the grain size decreases. In Figure 10, it is also noteworthy that the predicted work hardening rates of all the specimens converge at high strains, meaning that the model correctly predicts an insensitivity to grain size at high strains [9]. The reason is that the mobile dislocation density reaches a saturation value (see further Figure 11). The low point of the work hardening rate precisely corresponds to the moment when the activity of the 1st order pyramidal <c+a> systems approaches its maximum value, around 0.5% of plastic strain ( Figure 9). Hence, the appearance of a low point in the work hardening rate evolution seems to be related to the start of multiple slip within grains, or to a kind of balance between <a> and <c+a> slip activities. Under such conditions, plastic strain incompatibilities and the induced intergranular stresses are strongly decreased. Along with the fast multiplication of mobile dislocations (Equation (11)), this gives rise to an initial state with very weak hardening. In agreement with previous experiments, when the tensile axis is parallel to the transverse direction (comparisons of samples RT 9.8 and RR 9.8), the plateau effect is even more pronounced due to an even more balanced slip activity distribution (Figure 9). This texture effect is very well reproduced by the model ( Figure ...
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... can be noticed from the tensile experimental curves (Figures 1 and 4) that the tendency to form an initial plateau becomes more and more pronounced with decreasing mean grain size. This general trend is in agreement with other experiments in titanium [50] and was also characterized as such in copper [9] for instance. The model reproduces very well this trend (Figure 4). Moreover, the evolution of the work hardening rate θ = dσ/dε P for all the simulations of RR type is plotted in Figure 10. In agreement with the present experiments and previous tensile tests on cp Ti [17,19,29,30], a three-stage work hardening behavior is obtained, where an initial fall of θ is followed by an increase and a final progressive decrease. More importantly, the model predicts a growing well depth with decreasing grain size, or, in other words, a transient low work hardening rate with smaller and smaller value as the grain size decreases. In Figure 10, it is also noteworthy that the predicted work hardening rates of all the specimens converge at high strains, meaning that the model correctly predicts an insensitivity to grain size at high strains [9]. The reason is that the mobile dislocation density reaches a saturation value (see further Figure 11). The low point of the work hardening rate precisely corresponds to the moment when the activity of the 1st order pyramidal <c+a> systems approaches its maximum value, around 0.5% of plastic strain ( Figure 9). Hence, the appearance of a low point in the work hardening rate evolution seems to be related to the start of multiple slip within grains, or to a kind of balance between <a> and <c+a> slip activities. Under such conditions, plastic strain incompatibilities and the induced intergranular stresses are strongly decreased. Along with the fast multiplication of mobile dislocations (Equation (11)), this gives rise to an initial state with very weak hardening. In agreement with previous experiments, when the tensile axis is parallel to the transverse direction (comparisons of samples RT 9.8 and RR 9.8), the plateau effect is even more pronounced due to an even more balanced slip activity distribution (Figure 9). This texture effect is very well reproduced by the model ( Figure ...
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... tensile curves of the seven specimens were simulated using the same set of model parameters. The specific set of grain orientations, volume fractions and equivalent diameters was used for each specimen. The numbers of grains considered in the simulations were the numbers of grains of the EBSD maps indicated in Table 1. Figures 4 and 5 show the simulated curves and their equivalent experimental counterparts. In order to characterize further the mechanical behavior, several quantities were collected from the tensile ...
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... can be noticed from the tensile experimental curves (Figures 1 and 4) that the tendency to form an initial plateau becomes more and more pronounced with decreasing mean grain size. This general trend is in agreement with other experiments in titanium [50] and was also characterized as such in copper [9] for instance. The model reproduces very well this trend (Figure 4). Moreover, the evolution of the work hardening rate θ = dσ/dε P for all the simulations of RR type is plotted in Figure 10. In agreement with the present experiments and previous tensile tests on cp Ti [17,19,29,30], a three-stage work hardening behavior is obtained, where an initial fall of θ is followed by an increase and a final progressive decrease. More importantly, the model predicts a growing well depth with decreasing grain size, or, in other words, a transient low work hardening rate with smaller and smaller value as the grain size decreases. In Figure 10, it is also noteworthy that the predicted work hardening rates of all the specimens converge at high strains, meaning that the model correctly predicts an insensitivity to grain size at high strains [9]. The reason is that the mobile dislocation density reaches a saturation value (see further Figure 11). The low point of the work hardening rate precisely corresponds to the moment when the activity of the 1st order pyramidal <c+a> systems approaches its maximum value, around 0.5% of plastic strain ( Figure 9). Hence, the appearance of a low point in the work hardening rate evolution seems to be related to the start of multiple slip within grains, or to a kind of balance between <a> and <c+a> slip activities. Under such conditions, plastic strain incompatibilities and the induced intergranular stresses are strongly decreased. Along with the fast multiplication of mobile dislocations (Equation (11)), this gives rise to an initial state with very weak hardening. In agreement with previous experiments, when the tensile axis is parallel to the transverse direction (comparisons of samples RT 9.8 and RR 9.8), the plateau effect is even more pronounced due to an even more balanced slip activity distribution (Figure 9). This texture effect is very well reproduced by the model ( Figure ...
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... can be noticed from the tensile experimental curves (Figures 1 and 4) that the tendency to form an initial plateau becomes more and more pronounced with decreasing mean grain size. This general trend is in agreement with other experiments in titanium [50] and was also characterized as such in copper [9] for instance. The model reproduces very well this trend (Figure 4). Moreover, the evolution of the work hardening rate θ = dσ/dε P for all the simulations of RR type is plotted in Figure 10. In agreement with the present experiments and previous tensile tests on cp Ti [17,19,29,30], a three-stage work hardening behavior is obtained, where an initial fall of θ is followed by an increase and a final progressive decrease. More importantly, the model predicts a growing well depth with decreasing grain size, or, in other words, a transient low work hardening rate with smaller and smaller value as the grain size decreases. In Figure 10, it is also noteworthy that the predicted work hardening rates of all the specimens converge at high strains, meaning that the model correctly predicts an insensitivity to grain size at high strains [9]. The reason is that the mobile dislocation density reaches a saturation value (see further Figure 11). The low point of the work hardening rate precisely corresponds to the moment when the activity of the 1st order pyramidal <c+a> systems approaches its maximum value, around 0.5% of plastic strain ( Figure 9). Hence, the appearance of a low point in the work hardening rate evolution seems to be related to the start of multiple slip within grains, or to a kind of balance between <a> and <c+a> slip activities. Under such conditions, plastic strain incompatibilities and the induced intergranular stresses are strongly decreased. Along with the fast multiplication of mobile dislocations (Equation (11)), this gives rise to an initial state with very weak hardening. In agreement with previous experiments, when the tensile axis is parallel to the transverse direction (comparisons of samples RT 9.8 and RR 9.8), the plateau effect is even more pronounced due to an even more balanced slip activity distribution (Figure 9). This texture effect is very well reproduced by the model ( Figure ...
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... tensile curves of the seven specimens were simulated using the same set of model parameters. The specific set of grain orientations, volume fractions and equivalent diameters was used for each specimen. The numbers of grains considered in the simulations were the numbers of grains of the EBSD maps indicated in Table 1. Figures 4 and 5 show the simulated curves and their equivalent experimental counterparts. In order to characterize further the mechanical behavior, several quantities were collected from the tensile ...
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Citations
... Titanium alloys are widely used in aerospace fields because of their high specific strength and corrosion resistance. Grain size significantly affects most mechanical properties of titanium alloys, such as tensile strength, yield stress, flow stress, and hardness, which are important parameters to characterize the properties of polycrystalline materials [1][2][3]. The conventional method of grain size characterization mainly includes metallographic observation and Electron Backscattering Diffraction (EBSD) [4][5][6][7]. ...
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... The UFG structure formation may also be accompanied by the formation of multi-component metallographic and crystallographic textures. Heavily textured materials often exhibit the anisotropy of mechanical properties [17][18][19][20][21]. For example, according to [21], a highly textured material from the Ti6/4 alloy exhibited essentially different strength characteristics depending on the test direction with respect to the predominant texture, where loading in the transverse direction, perpendicular to the basal planes, contributed to a high ultimate tensile strength (UTS) and yield strength (YS), while loading under cyclic loads in the longitudinal orientation provided a high fatigue endurance limit. ...
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... Except for the recent work of Richeton et al. [48] , c + a slip is generally regarded as more difficult than a slip mechanisms, which can result in twinning to accommodate strain along the caxis. Slip along first-order pyramidal planes ( P y r 1 st c + a ) is the main slip mechanism for accommodating strain along the c -axis. ...
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Near β titanium alloys can now compete with quasi-α or α/β titanium alloys for airframe forging applications. The body-centered cubic β-phase can represent up to 40% of the volume. However, the way that its elastic anisotropy impacts the mechanical behavior remains an open question. In the present work, an advanced elasto-viscoplastic self-consistent model is used to investigate the tensile behavior at different applied strain rates of a fully β-phase Ti alloy taken as a model material. The model considers crystalline anisotropic elasticity and plasticity. It is first shown that two sets of elastic constants taken from the literature can be used to well reproduce the experimental elasto-viscoplastic transition, but lead to scattered mechanical behaviors at the grain scale. Incompatibility stresses and strains are found to increase in magnitude with the elastic anisotropy factor. The highest local stresses are obtained toward the end of the elastic regime for grains oriented with their <111> direction parallel to the tensile axis. Finally, as a major result, it is shown that the elastic anisotropy of the β-phase can affect the distribution of slip activities. In contrast with the isotropic elastic case, it is predicted that {112} <111> slip systems become predominant at the onset of plastic deformation when elastic anisotropy is considered in the micromechanical model.
The wide use of two-phase titanium alloys in aircraft engine building, as well as the intense development of this industry, stipulate more and more stringent requirements to structural materials and the enhancement of their reliability, strength and performance characteristics. The formation of an ultrafine-grained (UFG) state in metals and alloys using severe plastic deformation (SPD) processing enables achieving high strength properties. However, an important aspect of UFG materials is their structural and textural effects which may lead to a strong anisotropy of their properties. In this respect, the authors studied the effect of microstructural features on the mechanical properties and impact toughness of the VT6 alloy after equal-channel angular pressing (ECAP) and subsequent deformation by upsetting, imitating die forging. The study showed that the formation of a UFG structure in the VT6 titanium alloy with a grain size of about 0.4 µm allows increasing the ultimate tensile strength up to 1250 MPa. The additional upsetting of the UFG alloy at T=750 °C leads to grain growth up to 0.5–1 µm and a decline in strength to 1090 MPa as a result of the recovery and recrystallization processes. Impact toughness tests were conducted on specimens with a V-shaped stress raiser at room temperature, showing that the impact toughness of the UFG VT6 alloy was 0.41 MJ/m2. The tests revealed the anisotropy of impact toughness in the UFG VT6 alloy after equal-channel angular pressing and additional upsetting due to the metallographic and crystallographic texture formed as the result of deformation treatment. In test direction No. 1, the impact toughness value is the lowest and equals 0.31 MJ/m2.
