Decreasing Deformation Temperature Research Articles

The influence of V microalloying on the hot deformation behavior and microstructure evolution of high-manganese steel for LNG storage tanks.

Herein, the high-temperature hot deformation and dynamic recrystallization (DRX) behavior of high-manganese steel were investigated through the measurement of true stress-strain curves, the establishment of constitutive equations, the construction of recrystallization and hot processing maps, and the characterization of microstructure evolution. Furthermore, the influence of V on the DRX behavior of high-manganese steel during the hot deformation process, particularly for LNG storage tanks, was systematically evaluated. The results indicate that both the critical stress (σc) and peak stress (σp) of the tested steels increase as the deformation temperature decreases and the strain rate increases. Additionally, a higher V content leads to an increase in σc and σp, thereby suppressing the occurrence of DRX. Compared to 0.1V steel, 0.2V steel precipitates a greater quantity of carbides at the grain boundaries (GBs), which effectively stabilizes the GBs and inhibits the growth of DRX grains. In contrast, 0.1V steel achieves DRX over a wider range of Z parameters (at lower temperatures and higher strain rates). The activation energies for hot deformation (Q) of the two tested steels are 479.100 kJ/mol and 438.274 kJ/mol, respectively. The hot processing maps indicate that an increase in V content reduces the hot workability of high-manganese steel. The optimal temperature and strain rate for hot processing of 0.1V steel are 1035–1095°C and 0.01–0.0451 s⁻1, respectively. For 0.2V steel, the optimal hot processing parameters are a temperature of 1065–1100°C and a strain rate of 0.01–0.07 s⁻1.

The hot deformation behavior and processing map analysis of a high-nitrogen austenitic stainless steel

Understanding the hot deformation behavior of steel and establishing corresponding hot processing maps are crucial methods for determining appropriate hot processing parameters. In this article, the hot deformation behavior of a high-nitrogen austenitic stainless steel was investigated in the temperature range of 900–1150 °C and the strain rate range of 0.01–10 s−1 on a Gleeble-3500 thermo-mechanical simulator. The results show that the flow stress of experimental steel increases with decreasing deformation temperature and increasing strain rate. Based on the Arrhenius model, a constitutive equation was developed for the experimental steel, with a hot deformation activation energy of 603.347 kJ/mol. The hot processing maps were created employing the dynamic material model to determine the optimum hot processing conditions for the experimental steel at different strain levels.

Flow softening and recrystallization accompanied flow in AZ61 magnesium alloy during thermomechanical processing

The present work deals with characterizing the recrystallization accompanied flow and high temperature softening behavior of an extruded AZ61 magnesium alloy. This was supported by conducting a set of hot compression tests at temperatures in the range of 250–450 °C under the strain rate ranging from 0.001 to 0.1s−1. The flow curves at all thermomechanical conditions indicated high fractional softening representing the domination of dynamic recrystallization mechanism. Through a new quantitative approach, “Arrhenius type model”, “modified Avrami equations” and Poliak and Jonas method were simultaneously employed to investigate the kinetic of dynamic recrystallization. It was revealed that the strain required for the same amount of recrystallization fraction increased with decreasing deformation temperature, and at a specified temperature, the required strain increases with increasing strain rate. Interestingly, an anomaly was found at 400 °C under the strain rate of 0.001s−1, where the recrystallization kinetic was faster than that of what was recorded at 450 °C. This anomaly was discussed relying on the nanoprecipitation of γ-phase at the prior boundaries and sub-boundaries which prohibited the grain boundary migration and also the rotation and coalescence of adjacent sub-grains.

Low temperature tensile behavior and microstructure analysis of copper containing antibacterial stainless steel

Using Shimadzu AGS-100KN universal tensile testing machine, tensile tests were conducted on copper containing antibacterial stainless steel at 25 °C, −20 °C, −60 °C, −100 °C, and −140 °C. The fracture morphology and microstructure evolution of the material were then analyzed using various techniques including scanning electron microscopy (SEM), electron backscatter diffraction (EBSD), X-ray diffraction (XRD), and transmission electron microscopy (TEM) techniques. The experimental findings showed that with the decrease of deformation temperature, the ultimate tensile strength(UTS) increased by about 49.7 %, the yield strength (YS) increased by about 35.5 %, and the elongation(EI) decreased by about 27.3 %, showed obvious low temperature strengthening effect. The High-angle grain boundaries (HAGBs) increased with decreasing temperature, while low-angle grain boundaries (LAGBs) decreased. The plasticity gradually decreased and the strength gradually increased, resulting in an increase in its ability to resist deformation. The number of precipitated phases gradually increased, and the size also gradually becomes larger, and the interaction between the diffusely distributed precipitated phases and dislocations led to precipitation strengthening. With the decrease of temperature, the stacking fault energy gradually decreased, the SFE was 21.26 mJ/m2 at the temperature of −140 °C. The main plastic deformation mechanism of copper-containing austenitic stainless steel in the low-temperature tensile process was transformed from deformation twins to strain-induced martensitic phase transformation.

Hot deformation mechanism of a rapidly-solidified Al–Zn–Mg–Cu–Zr alloy

This study systematically investigates the flow behavior of 7050 aluminum (Al) alloy produced via spray forming and subjected to a proposed pretreatment through thermal compression tests. The experimental findings reveal that the peak stress increased with decreasing deformation temperature or increasing strain rates. The parameters in the Arrhenius constitutive model were determined and verified by experimental results for spray formed (SFed) 7050 Al alloy. Hot deformation activation energy of the SFed 7050 Al alloy (147.38 kJ/mol) is notably lower than that of cast Al–Zn–Mg–Cu–Zr alloys, which is attributed to factors such as fine grains, low segregation, pores, and dispersed Al3Zr induced by pretreatment. Furthermore, processing maps have been constructed at various strains, revealing optimal stable deformation conditions at 300–450 °C/0.001–0.15s−1 and 425–450 °C/0.2–1s−1. Microstructural analysis of deformed samples highlights the critical role of dynamic recrystallization in influencing the stable and unstable areas of the alloy, which may be activated by high temperature and slow strain rate. The hot deformation mechanism of the SFed Al alloy is governed by interactions among η-Mg(Zn,Al,Cu)2 precipitates, dispersed Al3Zr particles, dislocation structures, and (sub-)grain boundaries.

Microstructure evolution and work hardening behavior of medium/high carbon Fe–Mn–C TWIP steel during tensile deformation at 298 K and 223 K

In this paper, a comparative investigation was conducted on the tensile properties, microstructure evolution, and work hardening behavior of Fe18Mn0.6C and Fe18Mn1.0C TWIP steels at 298 K and 223 K by tensile tests and multi-scale microscopic characterization. With decreasing deformation temperature, two steels exhibit increased yield strength and ultimate tensile strength. However, the total elongation decreases for both, with Fe18Mn0.6C steel showing a more pronounced decrease. The work hardening behavior of two steels at 298 K and 223 K was analyzed. It is observed that the work hardening rate of both steels is higher at 223 K compared to 298 K, attributed to enhanced dislocation accumulation and the formation of deformation twins or martensite at lower temperatures. Particularly, Fe18Mn1.0C steel exhibits a continuous upward stage in work hardening at any temperature, driven by its higher dislocation density. On the other hand, Fe18Mn0.6C steel at 233 K shows a relatively stable trend in work hardening rate, attributed to the presence of martensite generated during deformation, which enhances its work hardening ability. Furthermore, the contribution of different strengthening mechanisms to the total flow stress was evaluated. The calculation results highlight dislocation strengthening as the primary source of flow stress in two steels at different temperatures. These findings provide theoretical support for understanding the mechanical properties of Fe–Mn–C TWIP steel at low temperature and optimizing the material design.

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.