Characterization of hot deformation behavior of Al0.3CoCrFeNi high-entropy alloy and development of processing map

Abstract This study presents the characteristics of hot deformation behavior of a Al0.3CoCrFeNi high–entropy alloy in the temperature and strain rate ranges of 1023–1423 K and 10–3–10 s–1, respectively. The constitutive flow behavior was modeled based on the hyperbolic–sinusoidal Arrhenius–type equations and a mathematical relation was used to observe the influence of true strain on material constants. To define the hot workability of the alloy, a processing map was developed based on the principles of the dynamic materials model. Accordingly, a dynamic recrystallization (DRX) domain was identified as prudent for processing in the temperature and strain rate ranges of 1273–1423 K and 10–2–2 × 10–1 s–1 respectively, with a peak efficiency of ~45% at 1423 K/6 × 10–2 s–1. At lower temperatures (1048–1148 K) and strain rates (10–3–3 × 10–3 s–1), a dynamic recovery (DRV) domain was identified with a peak efficiency of 38% at 1123 K/10–3 s–1. A large instability regime occurred above 3 × 10–1 s–1 with an increased tendency of adiabatic shear bands. It extended to lower strain rates 10–2–10–1 s–1 at temperatures < 1123 K, manifested by localized shear bands and grain boundary cracking. At low strain rates (5 × 10–3–10–3 s–1) and temperatures (1148–1298 K), particle stimulated nucleation of new DRX grains occurred at B2 precipitates, though the efficiency of power dissipation dropped sharply to ∼9%

Characteristics of dynamic softening during high temperature deformation of CoCrFeMnNi high-entropy alloy and its correlation with the evolving microstructure and micro-texture

Abstract The characteristics of dynamic recrystallization (DRX) of a CoCrFeMnNi high–entropy alloy (HEA) was investigated via hot compression testing in the temperature range 950–1100 °C and at true strain rates of 10–2 and 10–1 s-1. The discontinuous DRX was found to be the dominant mechanism corroborating the microstructural evolution. The progress of the initiation of DRX was investigated in terms of critical strain/stress required using the Poliak–Jonas analytical criterion. Consequently, a new kinetic model based on Avrami–type function was established for the HEA to predict the DRX fractional recrystallization. It was revealed that the volume fraction of DRX grains increased with increasing strain. In the case of 10–2 s-1, steady–state flow was achieved after the completion of one DRX process cycle resulting in further straining, leading to the occurrence of dynamic restoration processes involving formation of substructures and generation and annihilation of dislocations inside the DRX grains which effectively increased the fraction of partially deformed DRX (substructured) grains. A good agreement between the proposed DRX kinetics model and microstructure observation results validated the accuracy of DRX kinetics model for CoCrFeMnNi HEA. The preferred orientation of the non–recrystallized grains was towards the formation of <101> fiber texture, whereas a random micro–texture is revealed in the recrystallized grains

Dynamic softening kinetics of Al0.3CoCrFeNi high-entropy alloy during high temperature compression and its correlation with the evolving microstructure and micro-texture

Abstract To establish the characteristics and kinetics of dynamic softening in a Al0.3CoCrFeNi high–entropy alloy (HEA), isothermal compression tests were carried out in a suitable temperature range of 1273–1423 K at 10-2 and 10-1 s-1 in accord with our previous study. It was found that the discontinuous dynamic recrystallization (DRX) was the dominant microstructural reconstitution mechanism. The conditions of critical stress/strain for the onset of dynamic recrystallization were determined using the Poliak–Jonas analytical criterion. Further, a kinetic model was established based on the Avrami-type function in order to be able to predict the volume fraction of DRX. The DRX volume fraction expectedly increased with strain. The microstructural investigation of the isothermally compressed specimens revealed a good agreement with the proposed DRX kinetics model and validated its accuracy. Additionally, the evolution of DRX with strain was characterized by interrupting the test carried out at 1323 K/10-1 s-1 at different strains. The progress of DRX evolving as increased formation of new recrystallized grains further corroborated the predictions of the kinetic model. The micro-texture analysis revealed random texture in the recrystallized grains, whereas the unrecrystallized grains had shown their preferred orientation towards the <101> fiber texture

Constitutive modelling of hot deformation behaviour of a CoCrFeMnNi high-entropy alloy

Abstract Models describing the constitutive flow behaviour of a metallic material are desired for appropriate process design and realization of defect-free components. In this study, constitutive equations based on the hyperbolic-sinusoidal Arrhenius-type model have been developed to define the hot deformation characteristics of a CoCrFeMnNi high-entropy alloy. The experimental true stress-true strain data were generated over a wide temperature (1023–1423 K) and strain rates (10−3–10 s−1) ranges. The impact of strain rate and temperature on deformation behaviour was further characterized through a temperature compensated strain rate parameter, i.e. Zener-Hollomon parameter. Additionally, a mathematical relation was employed to express the influence of various material constants on true-strain ranging from 0.2 to 0.75. Typical third order polynomial relations were found to be appropriate to fit the true-strain dependency of these material constants. The accuracy of the developed constitutive equations was evaluated by using the average absolute relative error (AARE) and correlation coefficient (R); the obtained values were 7.63% and 0.9858, respectively, suggesting reasonable predictions. These results demonstrate that the developed constitutive equations can predict the flow stress behaviour of the alloy with a good accuracy over a wide range of temperature and strain rate conditions and for large strains

Processing map for controlling microstructure and unraveling various deformation mechanisms during hot working of CoCrFeMnNi high entropy alloy

Abstract In the current study, the hot deformation characteristics and workability of a CoCrFeMnNi high entropy alloy was characterized using processing maps developed on the basis of dynamic materials model in the temperature range 1023–1423 K and strain rate range 10⁻³–10s⁻¹. The processing map delineated various deterministic domains including those of cracking processes and unstable flow, thus enabling identification of a ‘safe’ processing window for the hot working of the alloy. Accordingly, a deterministic domain in the temperature and strain rate ranges of 1223–1373 K and 10⁻²–5 × 10⁻¹s⁻¹, respectively, was identified to be the domain of dynamic recrystallization (DRX) with a peak efficiency of the order of ~34% at 1293 K and 3 × 10⁻²s⁻¹ and these were considered to be the optimum parameters for hot deformation. The DRX grain size was dependent on the deformation temperature and strain rate, increasing with the increase in temperature and decrease in strain rate, whereas DRX volume increased with the strain rate. At still higher temperatures (1403–1423 K) and lower strain rates (10⁻³–3 × 10⁻³s⁻¹), there was a sharp decrease in efficiency values from 27% to 5% thus forming a trough and the microstructure was characterized with coarse grains. In the instability regime, grain boundary cracking/sliding and localized shear bands manifested at temperatures <1223 K and strain rates <10⁻²s⁻¹. The increase in strain rate resulted in an intense adiabatic shear banding along with formation of voids. At 10s⁻¹ and temperatures >1398 K, microstructural reconstitution occurred in the shear bands leading to the formation of fine grains, presumably as a consequence of continuous recrystallization

Press hardening of zinc-coated boron steels:role of steel composition in the development of phase structures within coating and interface regions

Abstract Zn and ZnFe coated 22MnB5 and 34MnB5 steels were subjected to the direct press hardening process in order to investigate the influence of steel composition on the resulting phase structures. Microstructures were characterized using advanced methods of microscopy. In addition, X-ray diffraction, glow discharge optical emission spectroscopy and thermodynamic calculations with Thermo-Calc® were carried out to support the analysis. The results indicate that the steel composition has a clear effect on the phase development within coating and interface regions. Whereas the behavior of the 22MnB5 was comparable to earlier investigations, a clearly non-conventional behavior of the 34MnB5 was observed: the formation of martensitic micro constituents, designated here as α′-Fe(Zn), were identified after die-quenching. The regions of the α′-Fe(Zn) formed mainly in vicinity of steel/coating interface and were emerged into the steel by sharing martensitic morphology with the base steel. The thermodynamic calculations suggest that carbon is effective in stabilizing the γ-Fe(Zn) phase, which enables the formation of the α′-Fe(Zn) in fast cooling. Therefore, the higher initial C content of the 34MnB5 may result in the kinetic stabilization of the γ-Fe(Zn) as the inter-diffusion between Zn and Fe occurs during annealing. Simultaneously occurring carbon partitioning from α-Fe(Zn) to γ-Fe(Zn) could explain a clearly increased C content of the coating/steel interface as well as higher Zn contents in the α′-Fe(Zn) phase compared to 22MnB5. Actually, the present study shows that the same phenomenon occurs also in 22MnB5 steels, but in a much smaller scale. In Zn and ZnFe coated 34MnB5, the thickness of the α′-Fe(Zn) layer was increased with longer annealing times and at higher temperatures. The morphology of the α′-Fe(Zn) layer resembled plate-like martensite and can be assumed to be brittle. Regarding this, the formation of α′-Fe(Zn) interface layer needs to be taken into account in press hardening of 34MnB5 steels

Constitutive modeling and hot deformation processing map of a new biomaterial Ti–14Cr alloy

Abstract A new biomaterial Ti–14Cr alloy was designed for biomedical applications. The manufacturing process of Ti alloys through hot deformation is crucial for controlling the grain structure and the mechanical performance of the alloy. In the present study, several compression tests at elevated temperatures (1123–1273 K) and strain rate ranges of 0.01–10 s−1 were conducted using a Gleeble-3800 thermomechanical simulator. A processing map of the studied alloy was constructed using the principles of the dynamic material model to evaluate the hot workability and deformation mechanisms at different ranges of temperature and strain rate. The resulting grain structure was correlated with the processing map. The processing map showed that adiabatic shear bands are expected to form at low temperatures (1123–1223 K) and moderate to high strain rates (1–10 s−1), whereas the nucleation of wedge cracks is likely to develop at the grain boundary at high temperatures and low strain rates (1248–1273/0.01 s−1). Consequently, a deterministic domain in the temperature and strain rate ranges of 1148–1273 K and 0.01–0.1 s−1, respectively, was identified as the domain of dynamic recrystallization with a peak efficiency of the order of ∼70% at 1173 K/0.01 s−1, and these were considered to be the optimum parameters for hot deformation. The constitutive flow behavior was modeled based on the hyperbolic–sinusoidal Arrhenius-type equations, and a mathematical relation was used to elucidate the influence of true strain on material constants