Decreasing Deformation Temperature Research Articles

Study on interfacial healing mechanism of a Ni-Co base dual-phase superalloy during hot-compression bonding

Hot-compression bonding (HCB) as an advanced technique for preparation of large-size homogenized components is widely applied. Despite the fact that interfacial healing mechanisms during HCB have been extensively investigated, the interfacial healing mechanisms in duplex superalloys still need to be clarified. This article investigates effect of deformation temperature on multiple interface (γ′-γ′, γ-γ, γ-γ′) healing mechanism of dual-phase superalloys. The healing mechanism of γ-γ interface at different temperatures involves a necklace-like distribution of discontinuous dynamic recrystallization (DDRX) grains occupying original interface. With increasing deformation temperature, the healing mechanism of γ′-γ′ interface transforms from DDRX to diffusion bonding due to weakening of strain concentration. Simultaneously, as deformation temperature increases, the deformation mechanism of primary γ′ phase transforms from shearing by stacking fault to dislocation pairs. At deformation temperature 1100 °C, the healing mechanism of γ′-γ interface is heterogeneous epitaxial recrystallization (HERX) induced by primary γ′ phase driven by diffusion of Cr element in dislocation pairs. However, as deformation temperature decreases, healing mechanism in γ′-γ interface transforms into DDRX. Although interfacial healing mechanism is transformed at different deformation temperatures, the synergistic action of different recrystallization mechanisms ensures that mechanical properties of joints reached the same level as those of base material.

Study of flow stress in Mg-Gd-Y-Nd-Zr alloys based on IWOA-BPNN model

This study investigates the compression deformation behavior of an Mg-Gd-Y-Nd-Zr alloy at temperatures ranging from 293 K to 573 K and strain rates ranging from 293K to 573K and strain rates ranging from 1000 s−1 to 2100 s−1 using a split Hopkinson pressure bar. A modified Johnson-Cook (JC) constitutive model and a backpropagation neural network (BPNN) model based on the improved whale optimization algorithm (IWOA) are established. Four statistical metrics, including correlation coefficient (R), mean absolute error (MAE), mean absolute percentage error (MAPE), and root mean square error (RMSE), are employed to evaluate the predictive accuracy of the two models. The findings indicate that the flow stress of the alloy is sensitive to strain, strain rate, and temperature. Increasing strain and strain rate or decreasing deformation temperature results in higher flow stress. Error calculations revealed that the modified Johnson-Cook constitutive model has an R of 0.98731, a MAPE of 7.3653%, a MAE of 19.6305 MPa, and a RMSE of 26.5704 MPa. In contrast, the model established using the IWOA-BPNN has an R of 0.99996, a MAPE of 0.58894%, a MAE of 1.2671 MPa, and a RMSE of 1.7709 MPa. The IWOA-BPNN model demonstrates higher accuracy and accurately predicts the flow stress of the alloy.

Dual heterogeneous structure enabled ultrahigh strength and ductility across a broad temperature range in CrCoNi-based medium-entropy alloy

Developing alloys with exceptional strength-ductility combinations across a broad temperature range is crucial for advanced structural applications. The emerging face-centered cubic medium-entropy alloys (MEAs) demonstrate outstanding mechanical properties at both ambient and cryogenic temperatures. They are anticipated to extend their applicability to elevated temperatures, owing to their inherent advantages in leveraging multiple strengthening and deformation mechanisms. Here, a dual heterostructure, comprising of heterogeneous grain structure with heterogeneous distribution of the micro-scale Nb-rich Laves phases, is introduced in a CrCoNi-based MEA through thermo-mechanical processing. Additionally, a high-density nano-coherent γ' phase is introduced within the grains through isothermal aging treatments. The superior thermal stability of the heterogeneously distributed precipitates enables the dual heterostructure to persist at temperatures up to 1073 K, allowing the MEA to maintain excellent mechanical properties across a wide temperature range. The yield strength of the dual-heterogeneous-structured MEA reaches up to 1.2 GPa, 1.1 GPa, 0.8 GPa, and 0.6 GPa, coupled with total elongation values of 28.6 %, 28.4 %, 12.6 %, and 6.1 % at 93 K, 298 K, 873 K, and 1073 K, respectively. The high yield strength primarily stems from precipitation strengthening and hetero-deformation-induced strengthening. The high flow stress and low stacking fault energy of the dual-heterogeneous-structured MEA promote the formation of high-density stacking faults and nanotwins during deformation from 93 K to 1073 K, and their density increase with decreasing deformation temperature. This greatly contributes to the enhanced strain-hardening capability and ductility across a wide temperature range. This study offers a practical solution for designing dual-heterogeneous-structured MEAs with both high yield strength and large ductility across a wide temperature range.

Microstructural characteristics and recrystallization mechanism of Ti-6.5Al–2Zr–1Mo–1V alloy during two-stage hot deformation

In this study, we investigated the two-stage deformation behavior of near α Ti-6.5Al–2Zr–1Mo–1V alloy at various temperatures (915 °C, 950 °C, 1020 °C) and strain rates (0.01 s−1, 0.1 s−1). The alloy underwent differing holding periods (500s, 1000s) between deformation stages. The results demonstrated an increase in flow stress with decreasing deformation temperature and increasing strain rate. Work hardening of flow stress was observed during the two-stage thermomechanical processing within the α+β two-phase region. Microstructural evolution was characterized by the formation of α lamellar kink bands, the separation and migration of grain boundaries, and grain spheroidization. An increase in deformation temperature and a decrease in strain rate enhanced grain refinement, accelerated structural evolution, and intensified the degree of spheroidization. Dynamic recrystallization (DRX) was more prevalent at higher temperatures and lower strain rates, as the critical nucleation rate for DRX was more readily achieved. The increased interval between thermomechanical processing stages led to a transition from low-angle to high-angle grain boundaries, and a reduction in dislocation density due to static recovery occurred during the holding period. At 915 °C, hot deformation induced the formation of {10-10} twinning, altering grain orientation and reducing stress. The Schmid factor for {10-10}<1120> slip was higher at lower temperatures, which reduced the critical resolved shear stress necessary for yielding in the α phase of the Ti-6.5Al–2Zr–1Mo–1V alloy, thereby facilitating further plastic deformation.

Hot Deformation Behavior of a Marine Engineering Steel for Novel 980 MPa Grade Extra‐Thick Plate

The hot deformation behavior of a marine engineering steel for a novel 980 MPa grade extra‐thick plate is investigated through hot compression tests at temperatures of 850–1200 °C and strain rates of 0.01–10 s−1. The flow stress curves are corrected to eliminate the effects of friction on the flow stress, and a modified constitutive model is established to accurately quantify the flow behaviors of the steel. Hot working maps of the steel are derived by using a dynamic materials model and the Prasad instability criterion. The effects of the deformation parameters on the dynamic recrystallization (DRX) nucleation mechanisms are evaluated using scanning electron microscopy and electron backscatter diffraction. The flow stress and peak strain increased with decreasing deformation temperature and increasing strain rate, respectively. The activation energy for hot deformation after friction correction is 380.49 kJ mol−1. The DRX mechanism involves the migration of subgrains. The deformation parameters are optimized by combining the degree of DRX and the size of recrystallized grain. The optimum hot working window is determined as 1100–1200 °C and 0.1–10 s−1.

Hot deformation behavior and hot working window of a solid solution strengthened Ni–Cr–Fe-based superalloy

The hot deformation behavior of Ni–Cr–Fe-based ChS57 alloy was investigated by isothermal compression tests in the temperature range of 1000–1200 °C with strain rates covering 0.001–10 s−1. Based upon the hyperbolic sine function and dynamic material model, the constitutive model and processing maps of the alloy were established, the microstructure evolution was systematically analyzed and the optimization of the hot working window was completed. The results indicate that the flow stress increases remarkably with increasing strain rate and decreasing deformation temperature, while the amount of strain required to reach peak stress also gradually increases. The hot deformation process of this alloy is the result of the competition between dynamic softening and work hardening, with a deformation activation energy of 451.34 kJ/mol; dynamic recrystallization (DRX) is the main softening mechanism, while both continuous dynamic recrystallization (CDRX) and discontinuous dynamic recrystallization (DDRX) exist; the low temperatures and medium strain rates (1000–1025 °C, 0.045–0.37s−1) are the instability zone parameters of the alloy when flow localization and deformation bands are the microstructural manifestations of unstable hot working. The optimum hot working window for ChS57 alloy is the medium-low temperatures and low strain rates（1050–1100 °C, 0.001–0.003s−1）and the medium-high temperatures and high rates（1125–1175 °C, 1-10s−1 or 1100 °C/10s−1）, where the microstructure appears as randomly oriented and uniformly distributed full DRX grains.

Mechanical properties and rate-sensitive deformation of AA6063 aluminum alloys at 298 K, 78 K, and 4 K

The mechanical properties and rate-sensitive deformation of T4 and T6 AA6063 aluminum alloys have been analyzed at 298 K, 78 K, and 4 K. The strength, ductility, and work hardening capacity of two alloys increase with decreasing deformation temperature. Plastic deformation of AA6063-T4 is accompanied by adiabatic flow stress instabilities at 4 K and the Portevin-Le Chatelier (PLC) effect at 298 K. The alloy shows a better synergy of flow stress and work hardening than the T6 condition, determined by the underlying microstructure. The limited ability of dislocation storage in T6 alloy arises from shearing β″ particles and the enhanced dislocation annihilation occurring in pure Al matrix. The failure of the alloys is promoted by void nucleation at intermetallic particles before the Considére criterion is fulfilled. AA6063-T6 exhibits positive strain rate sensitivity (SRS) at three temperatures. Dislocations-β″ particle interactions determine its SRS and are a rate-controlling process at 298 K. AA6063-T4 shows negative strain rate sensitivity (nSRS) at 298 K controlled by dislocation-solute interactions. Data for SRS and thermodynamic deformation parameters characterizing different mechanisms in T4 and T6 alloys between 4 K and 298 K are discussed. The results indicate that plastic deformation produces vacancies by jogs' dragging at 298 K, but this process does not occur at 78 K and 4 K.

Investigation of the Hot Deformation Behavior and Mechanism of a Medium-Entropy CoCr0.4NiSi0.3 Alloy

The CoCrNi-based medium-entropy alloys (MEA) have been extensively investigated due to their exceptional mechanical properties at both room and cryogenic temperatures. To investigate the hot deformation behavior and the recrystallization mechanism of the CoCr0.4NiSi0.3 medium-entropy alloy, a series of deformation tests was conducted using the MMS-100 thermal simulation tester, with deformation conditions of 0.001–1 s−1/850–1150 °C. During the hot deformation process, the flow stress initially increases up to its peak value before gradually decreasing towards a steady state level. Higher flow stress levels are observed with increasing strain rate and decreasing deformation temperature. The estimated activation energy for hot deformation of this alloy is approximately 423.6602 kJ/mol. The Arrhenius-type constitutive equation is utilized to establish a modified model while incorporating power dissipation theory and the instability criterion of a dynamic material model to construct power dissipation maps and instability maps. By superimposing these maps, hot processing maps with strains of 0.4, 0.5, and 0.7 are derived. In this investigation, it is observed that regions of instability exclusively occur when the true strain exceeds 0.4. These regions of instability on the hot processing map align well with experimental findings. The suitable range of parameters for hot-working decreases as the true strain increases. The microstructure was analyzed using electron backscatter diffraction and transmission electron microscopy (TEM) techniques. The volume fraction of dynamic recrystallization (DRX) decreases with increasing strain rate but diminishes with rising temperature. The TEM characterization elucidated the mechanism of DRX in this MEA. The presence of the long-period stacking ordered (LPSO) phase was observed in both the face-centered cubic matrix and hexagonal close-packed recrystallized grains under different deformation conditions. The LPSO phase originates from the matrix at a low strain rate, whereas it is generated during recrystallization at a high strain rate. The observed increase in flow stress of the as-cast MEA is primarily attributed to the synergistic effects arising from the interaction of the dislocation with twins and the second phase. The onset of instability is effectively suppressed within a limited range through the formation of coherent second phases such as L12, LPSO, and superlattice structures resulting from phase transitions. These second phases serve as nucleation sites for recrystallization and contribute to the strengthening of dispersion. Furthermore, their interaction with dislocations and twins significantly influences both flow stress behavior and recrystallization kinetics under hot deformation. These findings not only deepen our understanding of the underlying deformation mechanisms governing MEA but also offer valuable insights for designing CoCrNi-based alloys with improved mechanical properties at elevated temperatures.

Deformation twinning and feature size mediated strain hardening behavior in a medium-entropy alloy at the mesoscopic scale

MP35N medium-entropy alloy with low stacking fault energy (SFE) is widely used in the biomedical field due to its excellent biocompatibility and mechanical property matching. With the miniaturization of products in the biomedical field, the deformation behavior of MP35N alloy has changed, which is different from the traditional plastic deformation at the macroscopic scale. In the present paper, tensile experiments of MP35N alloy at the deformation of 293 K and 93 K at the mesoscopic scale were carried out to analyze its mechanical properties and deformation mechanism. The results show that the reduction of the deformation temperature leads to a synergistic increase in the strength and plasticity of MP35N alloy. The feature size (ratio of sample thickness to grain size, t/d) and deformation twins are decisive in the mesoscopic deformation process. At the same deformation temperature, the reduction of the feature size makes the deformation twin occupy a dominant position in the competition with dislocations. At the deformation temperature of 93 K, the strain corresponding to the first inflection point decreases from 0.18 to 0.12 as t/d decreases from 8.21 to 1.52. However, the alloy with t/d of 8.21 preferentially produces the second inflection point. The decrease in deformation temperature drives the inflection point down to a larger strain for the same feature size. The reduction of deformation temperature gives dislocations an advantage in competition with deformation twins in the initial deformation stage. The decrease in deformation temperature promotes the emergence of multi-level deformation twin structures in the later deformation stage, causing the second inflection point to move toward larger strains.

Study on thermal deformation constitutive model of NiCoFeAlCrMo high-entropy alloy with FCC / L12 coherent structure

Due to its multi-principal component characteristics, high-entropy alloys exhibit excellent comprehensive mechanical properties and become a new generation of engineering structure alternative materials. At present, a single constitutive model is used to describe the change of flow stress in the study of hot deformation behavior of high-entropy alloys, and there are some problems such as inaccurate prediction accuracy or inapplicability of the model in some hot processing intervals. In this paper, three different constitutive models are combined to fully consider the applicability of the model in different temperature ranges, and the change of flow stress behavior is accurately described, which is of great significance for subsequent deformation mechanism research, hot processing process optimization, numerical simulation and so on. Thermal compression experiments on cast Ni28.5Co18.6Fe18.2Al16.3Cr10.5Mo4 Ti2.3Nb0.6W0.9C0.2 high-entropy alloy were carried out using a Gleeble-3800 thermal simulation tester with deformation temperatures in the range from 900 ℃ to 1150 ℃ and strain rates from 0.001 s−1 to 1 s−1. The constitutive relationships between true stress and strain, temperature and strain rate were established, while the heat deformation process of this alloy under different deformation conditions by Arrhennius, Modified Johnson-Cook and Modified Fields-Backofen models were characterized. The results indicate that Arrenius model describe the rheological behavior of the alloy better in a wider temperature range and at a wider strain rate range. Comparatively, the model shows the highest prediction accuracy in the high-temperature region and slightly lower accuracy in the mesothermal and low-temperature regions. The modified J-C and F-B models have better prediction accuracy in the low-temperature and medium-temperature regions, respectively. Due to the different deformation conditions, dislocation configurations such as dislocation tangles, dislocation networks, and subcrystalline junctions can be observed in the plastic deformation region. The dislocation density of this alloy increases with increasing strain rate and decreasing deformation temperature after deformation.

Hot deformation behavior and mechanism of cold deformed CoCrCu1.2FeNi high entropy alloy

The high temperature deformation behavior and mechanisms of the cold-deformed CoCrCu1.2FeNi high-entropy alloy (HEA) were investigated by optical microscope (OM), scanning electron microscope (SEM) and electron backscattered diffraction (EBSD) at temperatures and strain rates of 950–1100 °C and 0.001–1 s−1, respectively. The results show that the flow stress increases as the deformation temperature decreases and the strain rate increases, and there is significant dynamic recrystallization (DRX) during the deformation process. 1050–1090 °C/0.02–1 s−1 and 975–1050 °C/0.002–0.2 s−1 are the optional hot temperature deformation regions. Discontinuous dynamic recrystallization (DDRX) occurs at different temperatures when the strain rate is 0.001 s−1 with a maximum recrystallization percentage of 90.28 %. The DRX degree gradually increases as the temperature rises from 950 °C to 1000 °C and from 1050 °C to 1100 °C. The maximum polar density of texture sharply increases to 11.78 at 1050 °C. The deformed microstructure leads to an increase in dislocation density, which could be effectively reduced by DRX.

Temperature dependence of hydrogen-assisted damage evolution and fracture behavior in TRIP-aided bainitic ferrite steel

We systematically investigated the effects of hydrogen and deformation temperature on the micro-damage evolution and fracture behavior of the TRIP-aided bainitic ferrite steel using slow strain rate tensile tests and microstructure examinations. For hydrogen-uncharged condition, the micro-damage evolution behavior was not dependent on temperature, except for −100 °C. The exceptional behavior at −100 °C was attributed to the occurrence of brittle fracture. Hydrogen uptake significantly increased the number density of micro-damage in all deformation temperatures. The fracture mode of hydrogen-charged specimens changed from quasi-cleavage and ductile fractures to cleavage, quasi-cleavage, and intergranular fractures with decreasing deformation temperature. Quasi-cleavage fracture features were attributed to the crack growth process consisting of repetition of small crack initiation/blunting (or void initiation) and crack coalescence, whereas the cleavage and intergranular surfaces with smooth facets were due to the occurrence of hydrogen-reduction cohesive strength of cleavage planes and prior austenite grain boundaries.

Dynamic recrystallization behavior and microstructure evolution of high-strength low-alloy steel during hot deformation

The dynamic recrystallization (DRX) behavior and microstructure evolution of the high-strength low-alloy (HSLA) steel were studied by isothermal compression experiments at temperatures ranging from 850 to 1150 °C and strain rates ranging from 0.01 to 10 s−1 with a true strain of 0.8. The true stress-strain curves show that the flow stress increases with increasing strain rate and decreasing deformation temperature. The constitutive equation of the HSLA steel was established based on the peak stress, and the deformation activation energy was calculated to be 414.263 kJ/mol. Processing maps corresponding to different true strains were constructed based on the dynamic material model. High strain rates and low deformation temperatures are more likely to lead to flow instability. The microstructure deformed at low temperatures and high strain rates mainly consists of elongated initial grains. The DRX nucleation mechanism of the HSLA steel was explained. Many fine grains nucleate at the initial grain boundaries, forming a necklace structure. The serrated or bulged grain boundaries are the beginning of DRX. In addition, the DRX volume fraction model was established, and the DRX grain size was analyzed. High temperatures and low strain rates promote the nucleation and growth of the DRX.

Multiple effects of forced cooling on joint quality in coolant-assisted friction stir welding

To understand the multiple effects of forced cooling on the joint quality in coolant-assisted friction stir welding and the underlying mechanism, commercial pure aluminum AA1050 was welded by liquid–CO2–assisted friction stir welding (FSW) within a large process parameter range. The cooling effects on the weld formation, microstructure, and mechanical properties were investigated by comparison with conventional FSW. The microstructure evolution during the liquid–CO2–assisted FSW process was revealed by EBSD characterization together with thermal cycle measurement and short-time annealing. The results show that the process parameter window was narrowed by the liquid CO2 cooling. Forced cooling led to weld surface grooves and interior cavities under “too cold” welding conditions. The grains in the weld zone were refined and the areas of the weld cross-section were reduced by the forced cooling, resulting in the tensile strength of the joint being improved but the elongation being reduced. The thermal cycle shows that the liquid CO2 cooling compressed the welding temperature field by decreasing the heating rate and increasing the cooling rate, giving rise to the decrease in deformation temperature at the stirring stage. The low deformation temperature promotes dynamic recrystallization by changing the way of recrystallization and thus refines the grains. Further, the liquid CO2 cooling suppresses the grain growth which should have happened in natural cooling. Combining the above two aspects, the grain in the weld zone was refined and the tensile properties of the joints were changed. Nevertheless, the contribution of decreasing the deformation temperature to the grain refinement is more significant than that of increasing the cooling rate.

Enhanced mechanical properties of a near-α titanium alloy by tailoring the silicide precipitation behavior through severe plastic deformation

The microstructure evolution, tensile properties, and strengthening mechanisms of a near-α high-temperature titanium alloy with high Zr and Si content during isothermal multi-dimensional forging (IMDF) at the β-phase region were systematically investigated. The results show that the alloys exhibit a basket-weave microstructure after IMDF, with the silicides diffusely distributed in the matrix. As the forging temperature decreased, the intragranular primary α laths became more refined and the volume fraction of silicides increased, while the average size of silicides gradually decreased. The dynamic spheroidization of α laths occurs preferentially at the prior β grain boundary and extends into the grains as the deformation temperature decreases. The IMDF-1100 alloy possesses excellent high-temperature tensile properties and moderate room-temperature tensile properties, with ultimate tensile strengths (UTS) of 738.4 MPa and 1124.8 MPa at 650 °C and room temperature, respectively, which makes the alloy have tremendous potential for aerospace applications. The strengthening of the alloy at room temperature is primarily attributed to microstructure refinement and the thermal mismatch between silicide and matrix. However, owing to the weakening of grain boundaries at 650 °C, grain refinement leads to a decrease in tensile strengths. When tensile at 650 °C, the fragmentation and decohesion of large-size silicides tend to cause premature fracture of the alloy, and the microstructure with small-size silicide distribution presents better ductility.

The behaviour and deformation mechanisms for 316L stainless steel deformed at cryogenic temperatures

The microstructural evolution and deformation mechanisms of 316L stainless steel (SS) have been investigated at 297, 173, 50 and 15 K by in situ neutron diffraction during tensile loading and correlative transmission electron microscopy (TEM). The yield strength and ultimate tensile strength of 316L stainless steel are significantly improved at cryogenic temperatures. In contrast to room temperature, deformation-induced martensitic transformation was observed at all cryogenic temperatures. The γ-austenite (FCC) content decreases and α′-martensite (BCC) increases with increasing strain, a fraction of this γ to α′ transformation is accompanied by the transient appearance of ε-martensite (HCP) as an intermediate phase. The maximum volume fraction of ε-martensite increases with decreasing deformation temperature and reaches 13% for deformation at 15 K. TEM results confirm that γ → ε → α′ and γ → α′ martensitic transformations occur during cryogenic deformation, while mechanical twins were observed only at 173 K. The evolution of lattice strain, phase volume fraction, stacking fault probability (SFP) and stacking fault energy (SFE) were quantified to investigate the correlation between deformation mechanisms and mechanical behaviour of 316L SS as a function of deformation temperature.

Hot deformation behavior and dynamic recrystallization mechanism of Ti2ZrTa0.75 refractory complex concentrated alloy

Hot-deformation behavior and dynamic recrystallization (DRX) mechanism of Ti2ZrTa0.75 refractory complex concentrated alloy were systematically studied via uniaxial compression tests at various deformation temperatures from 800 °C to 1100 °C and different strain rates from 10−3 s−1 to 1 s−1. Stress-strain curves showed an increase in flow stress with decreasing deformation temperature and/or increasing strain rate, and there was continuous strain softening after the peak stress. Microstructure analysis based on electron backscattering diffraction (EBSD) indicated that deformation bands occurred in the hot deformation microstructure with a strain rate of 1 s−1, which is the appropriate location for DRX. Microstructure analysis revealed that DRX occurred during the hot deformation process and DRX fraction and grain size increase with increasing deformation temperature and/or decreasing strain rate. The grain boundary bulging nucleation mechanism and necklace-like structure of DRX grains demonstrate a typical discontinuous dynamic recrystallization (DDRX) process. Cumulative misorientation analysis of the initial grain interior and new grains formed without relying on the initial grain boundaries proved the occurrence of continuous dynamic recrystallization (CDRX).

A study on hot deformation behavior of 40Cr/Q345B bimetal

The hot deformation behavior of 40Cr/Q345B bimetal during hot compression bonding was studied using the isothermal compression method. A true stress–strain curve of the bimetal was obtained and modified to eliminate the effects of friction. The results show that the flow stress increases with decreasing deformation temperature and increasing strain rate. By analyzing the relationship of stress, deformation temperature, and strain rate, the constitutive equation of 40Cr/Q345B bimetal was obtained. The hot processing maps of 40Cr/Q345B bimetal under different strains are drawn. Through microstructure characterization, 40Cr/Q345B bimetal formed metallurgical bonding under the deformation process of this study. Combined with the hot processing map, microstructure, prior austenite grain, and microhardness distribution, it is proposed that the optimal deformation parameters of 40Cr/Q345B bimetal is 1050–1150°C and 0.02–0.4 s−1.

Microstructure and twinning behavior of b.c.c tantalum under dynamic plastic deformation

In this paper, dynamic plastic deformation (DPD) of tantalum (Ta) was achieved by split Hopkinson pressure bar and a variety of DPD conditions were set to systematically investigate the microstructure of the deformed samples, focusing on the twinning behavior. Results showed that the deformation conditions have a significant effect on the number and distribution of {112}<111>twins. Compared with {100} and {111} grains, {110} grains are more prone to twinning due to the fewer slip systems. As the strain rate increases or the deformation temperature decreases, the number of twins increases dramatically. In addition, the texture of Ta samples gradually changes from {110} to mixed {100} and {111} textures with the increasing deformation, and the number of twins first increases and then decreases. After calculation, it is found that the selection of twin variants in Ta under DPD not only follows Schmid law but is also influenced by strain coordination. The research in this paper deepens the understanding of tantalum as a material for armour-piercing projectile.

Softening mechanisms and microstructure evolution of 42CrMo steel during hot compressive deformation

Hot compression experiments were conducted on 42CrMo steel under different conditions using a Gleeble-3500 thermomechanical simulator. Systematic studies on the microstructure evolution and flow behavior under different deformation conditions were conducted. The results demonstrate that the flow curves exhibit significant peaks at low strain rates and high temperatures, and that the peak stress increases with decreasing deformation temperature and increasing strain rate. The dynamic recrystallization (DRX) critical points under various deformation conditions were derived from the work-hardening curves. The relationship between the peak (εp) and critical (εc) strains satisfies εc = 0.65εp. The value of εc decreased as the strain rate decreased and deformation temperature increased. The softening mechanism under different deformation conditions was determined in conjunction with the high-temperature microstructure and critical strains. When deformed at low temperature, the softening mechanism was a combination of dynamic recovery (DRV) and DRX; when deformed at high temperature, the softening mechanism was mainly DRX, and at low strain rates, the deformed DRX grains coarsened. Analysis of the room-temperature microstructure revealed that the substructure introduced during hot compression can affect the phase transition. Electron backscatter diffraction results show that room-temperature microstructure have a higher proportion of medium orientation angle boundaries and greater average local strain when DRX is incomplete. The higher dislocation density in the tissue with incomplete DRX disrupts the martensitic multilevel structure, whereas with complete DRX, it tends to form a typical martensitic multilevel structure.