Letter to the editor

On the mechanism of hot deformation

The deformation behavior of a radially forged 2A50 aluminum alloy was investigated by thermal compression test at high-temperature solid (300–500 °C) and semi-solid (565–585 °C) states with strain rates of 0.01–0.5 s^–1. A strain-compensated Arrhenius constitutive equation was established to describe the relationship between flow stress and compression parameters. The inhomogeneous deformation and microstructure evolution of the specimen were explored. Results reveal that the strain-compensated Arrhenius model can accurately predict experimental flow stress in both two temperature states. In the high-temperature solid state, the equilibrium between the softening effect (dynamic recovery and dynamic recrystallization) and work hardening leads to a steady stage of the flow curves after peak. In the semi-solid temperature, the softening mechanisms mainly include the high level of dynamic recrystallization (DRX), lubrication of liquid phases, and partial collapse of solid-phase skeleton, contributing to a declining stage of flow stress after peak. Strain and temperature profoundly influence DRX. At high-temperature solid state, coarse and elongated grains predominate in the regions with small strain, whereas DRX and numerous subgrains massively occur in the regions with large strain. The entire sample compressed in semi-solid temperature is subjected to DRX, attributed to the elevated temperature compared to the high-temperature solid state. Fine DRX grains are concentrated at the center of the large deformation zone (with large strain) due to the frequent DRX. Elevating semi-solid temperature leads to the formation of large equiaxed grains. DRX facilitates the weakening of deformed textures, demonstrated by the presence of deformed textures (<111> and <100>//X0 fiber textures, <111>//Z0 fiber texture, and {110}<111> texture) in the sample at high-temperature solid state, contrasted with the absence of pronounced preferred orientation in semi-solid compressed sample.

This study investigates the hot deformation behavior of the GH99 alloy for hypersonic missile airfoils at 1020–1170 °C and 0.001–1 s⁻¹ by single-pass hot compression experiments. Based on the establishment of the constitutive equation, thermal processing map, and dynamic recrystallization kinetic model, it elucidates the influence of the second phase strengthening on dynamic recrystallization (DRX). It determines the optimal hot deformation parameters of the GH99 alloy. Moreover, through designing different multi-pass hot compression experiments, we investigated the relaxation characteristics of the organization properties during the water quenching-cooling-holding process in the interpass and the influence of the interpass retention-relaxation behavior on the DRX mechanism. The results show that the water quenching process increases the rheological stress of the second pass compression, and the nucleation of discontinuously dominates the organization dynamically recrystallized (DDRX), supplemented by continuously dynamically recrystallized (CDRX); the static recrystallization (SRX) occurring in the high-temperature retention stage of the interpass can effectively reduce the rheological stress. The organization is characterized by the growth of recrystallized grains during the subsequent deformation process. With the prolongation of the high-temperature retention time, the number of recrystallized grains and the dislocation density increase.

Controlling the dynamic recrystallization of the hot deformation process to optimize the grain structure has been a long-standing research area. However, studies on the microstress-induced strengthening and grain refinement caused by different dynamic recrystallization modes after solid-solution treatment and artificial aging are limited. This paper examines the effect of deformation-induced precipitation on the dynamic recrystallization structure using a Gleblee-1500 machine and elucidates the dynamic recrystallization mechanism of crossover alloys. Under a hot deformation temperature of 400 °C, strain rate of 10 s⁻¹, and deformation amount of 70 %, the alloy exhibits complete dynamic recrystallization owing to the presence of multiscale T-phase Mg₃₂(Al Zn Cu)₄₉. Calculations and experiments confirm that the increase in the strength of hot-compressed samples after subsequent solid-solution treatment and artificial aging is mainly due to microstress-induced strengthening and grain refinement. The results of this study provide valuable guidelines for designing a novel generation of high-strength alloys.

The deformation behavior and microstructure evolution of a novel Mn-containing HEA were explored during the deformation temperature ranges of 800∼1100 °C, and strain rate ranges of 0.001∼1s⁻¹. The study results indicate that the true stress-strain curves after work hardening exhibit continuous softening type at low strain rate ranges of 0.001–0.01 s⁻¹, while softening then steady-state type at high strain rate ranges of 0.1∼1s⁻¹, respectively. The activation energy (Q) and stress exponent (n) are calculated to as 404.6 kJ/mol and 4.9016, respectively. As the strain increases, the flow instability is prone to expand towards the regions of lower strain rate and higher deformation temperature, and the optical processing window is determined at 1050–1100 °C and 0.001–0.002 s⁻¹. The strain and stress field distribution of the FEM simulations reveal the inhomogeneous deformation and the damage field distribution predict the crack region of deformation specimen. The critical (ε_c)/peak strain (ε_p) for DRX do not display a proportional relationship with strain rate and an inversely proportional relationship with deformation temperature, and the relationship between ε_c and ε_p can be expressed as: ε_c = (0.3–0.5) ε_p. Only the discontinuous dynamic recrystallization (DDRX) mechanism is activated in the whole range of deformation conditions, but it is not fully carried out. Because of the inhomogeneous stress distribution, the dynamic recrystallized grains show bimodal grain size. Furthermore, the recrystallized grains do not grow significantly due to the synergistic contribution of sluggish diffusion effect, high solution hardening effect and the trimodal B2 precipitate phase distribution.

The cutting process of titanium alloy Ti6Al4V is accompanied by high strain, high strain rate, and high-temperature thermal-force coupled deformation. A suitable constitutive model is critical for whether the finite element simulation can accurately reflect the elastic-plastic deformation behavior during the actual machining process. To solve the problem that the conventional Johnson-Cook constitutive model cannot accurately describe the material strain softening effect when turning Ti6Al4V, and the applicability of the model included in the material library of commercial ABAQUS finite element software is limited, a simulation research of turning Ti6Al4V based on the TANH constitutive model was conducted. Firstly, the secondary development subroutine of the TANH constitutive model is written by combining the traditional elastic-plastic theory, Newton's iterative method, and the damage evolution criterion based on equivalent plastic displacement and embedded into the two-dimensional orthogonal cutting finite element model. Then the material properties were defined using the J-C constitutive and self-programmed TANH constitutive models, respectively. The experimental data of cutting force and chip morphology were compared. The TANH constitutive model subroutine simulation results were more accurate than those using the J-C constitutive model. The average cutting force error was improved by 3.8 %. The error of chip serration degree was 17.4 %, indicating that the TANH constitutive model subroutine can better describe the material deformation behavior during cutting Ti6Al4V titanium alloy. In addition, the effects of TANH constitutive model parameters and damage evolution characteristics parameters on cutting simulation are also analyzed based on the constructed finite element model, which can provide a reference for improving the accuracy of metal cutting finite element simulation and exploring the mechanism of serrated chip formation.

Refractory high-entropy alloys (RHEAs) are promising candidates for those applications requiring of strong materials at high temperatures with elevated thermal stability and excellent oxidation, irradiation, and corrosion resistance. Particularly, RHEAs synthesized using mechanical alloying (MA) followed by spark plasma sintering (SPS) has proven to be a successful path to produce stronger alloys than those produced by casting techniques. This superior behavior, at both room and high temperature, can be attributed to the microstructural features resultant from this powder metallurgy route, that include the presence of homogeneously distributed non-metallic particles, fine- and ultrafine-grained microstructures, and higher content of interstitial solutes. Nevertheless, the powder metallurgy fabrication relies over a complex balance of several operational variables, and the process is no exempt of certain challenges, such as contamination or the presence of pores in the bulk parts. This review aims to cover all the peculiarities of the MA + SPS route, the resultant microstructures, their mechanical properties, and the strengthening and deformation mechanisms behind their superior performance, as well as a brief description of their oxidation resistance.