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A computational thermodynamics approach has been employed to design CoCrFeMnNi-based medium- and high- entropy alloys (M/HEAs) with systematically varied compositions (Co((80-X)/2)Cr((80-X)/2)FeXMn10Ni10 with x = 30, 40, and 50 at.%) and phase stability. Since the formation of sigma phase, usually brittle and undesirable, is a common concern, when this class of alloys is subjected to elevated temperatures (600–1000 ◦C), predicting its formation becomes essential. Thus, its formation and the phase equilibria were studied using the CALPHAD method, and two empirical methods, namely, valence electron concentration (VEC) and paired sigma-forming element (PSFE). Isothermal aging treatments at 900–1100 ◦C for 20 h were performed, since CALPHAD and VEC/PSFE predictions diverged. Both prediction methods were compared with experimental characterization by a combination of scanning electron microscopy and high-energy synchrotron X-ray diffraction. The predictions from the VEC/PSFE and CALPHAD calculations (depending on the database used) were shown to be quite accurate. 

This work proposes a methodology for designing high-strength precipitation-hardened high entropy alloys (HEAs) with an FCC matrix and L12 precipitates. High-throughput solidification calculations were conducted using the CALPHAD method, evaluating 11,235 alloys in the Cr-Co-Ni-Al-Ti system under specific boundary conditions. The acquired information was used to filter the alloys, focusing on alloys exhibiting an FCC+L12 phase field at 750 °C, a solidification interval narrower than 100 °C, and a solvus temperature under 1100 çC. The filtered alloys were analyzed to estimate their solid solution and precipitation hardening contributions to yield strength, with antiphase boundary energy (APB) assessed using two models. Three alloys were selected for testing the proposed strategy, including one with the highest yield stress and others for comparison. These alloys were produced, processed, and characterized using DSC, synchrotron XRD, SEM, and TEM. The results showed that the desired microstructure was achieved, with the alloys consisting of an FCC matrix and a high-volume fraction of L12 precipitates. Tensile tests at room temperature, 650 °C, 750 °C, and 850 °C demonstrated that the proposed model predicts well the yield strength trends, demonstrating the potential of the proposed approach for accelerating the discovery and development of novel HEAs with tailored properties. 

Abstract The individual effects of strain rate and temperature on the strain hardening rate of a quenched and partitioned steel have been examined. During quasistatic tests, resistive heating was used to simulate the deformation-induced heating that occurs during high-strain-rate deformation, while the deformation-induced martensitic transformation was tracked by a combination of x-ray and electron backscatter diffraction. Unique work hardening rates under various thermal–mechanical conditions are discussed, based on the balance between the concurrent dislocation slip and transformation-induced plasticity deformation mechanisms. The diffraction and strain hardening data suggest that the imposed strain rate and temperature exhibited dissonant influences on the martensitic phase transformation. Increasing the strain rate appeared to enhance the martensitic transformation, while increasing the temperature suppressed the martensitic transformation. 

Forming operations are known to be complex, involving many strain states, strain rates, temperatures, strain paths, and friction conditions. Material properties, such as strength and ductility, are large drivers in determining if a material can be formed into a specific part, and for selecting the equipment required for the forming operation. Predicting yielding behavior in situations such as these has been done using yield surfaces to describe material yielding in specific stress states. These models typically use initial mechanical properties, and will require correction if the material has experienced previous straining. Here, we performed interrupted uniaxial tensile testing of a 304 stainless steel to observe the effects of unloading and subsequent reloading on yielding and tensile properties. An increase in yield point developed, in which a higher yield was observed prior to returning to the bulk work hardening behavior, and the magnitude of the yield point varied with unloading conditions and strain imposed. The appearance of a yield point is attributed to strain aging or dislocation trapping at obstacles within the matrix. These results suggest that both strain aging and dislocation trapping mechanisms may be active in the matrix, which may present challenges when forming austenitic stainless and new advanced high strength steels that likely show a similar behavior. These results provide a potential area for refinement in the calculation of yielding criteria that are currently used to predict forming behavior.