Recent material design aims at fabricating heterogeneous structures to overcome the strength-ductility trade-off. A severe plastic deformation process is adopted to a multi-interstitial high-entropy alloy to form precipitate-rich and precipitate-lean domains. By this approach, the alloy achieved a yield strength of similar to 1020 MPa with a tensile elongation of similar to 34 %.

This study investigated the effects of deformation-induced martensitic transformation (DIMT) on the stretchability and fracture behavior of thin sheets made from ferrous medium-entropy alloys (MEAs). Analyses of punch load-stroke curves from Erichsen tests revealed that DIMT occurred in the 60 at% Fe MEA, 64 at% Fe MEA, and 304 stainless steel (60Fe, 64Fe, and 304SS alloys, respectively). This DIMT affects necking onset, uniform elongation, and the Erichsen index (EI) under diverse stress conditions. DIMT plays a significant role in strain hardening and mechanical instability during uniaxial tests, enhancing strain hardening through martensite formation and also initiating necking due to non-uniform deformation. Particularly in 64Fe, DIMT-induced early necking resulted in reduced uniform elongation. On the other hand, biaxial Erichsen test results demonstrated further strain hardening due to martensite-induced strain accommodation, leading to enhanced resistance to deformation during biaxial loading. The distinctive shapes of punch load-stroke curves in biaxial tests, compared to stress-strain curves from uniaxial tensile tests, are a result of the interaction between DIMT and stretching directions, as well as its alloy-dependent phase stability. In 60Fe, transformed martensite contributed to strain hardening and delayed necking initiation, while extensive martensite transformation in 64Fe enhanced strain hardening further. This understanding of the complex correlation between DIMT, strain hardening, and mechanical instability holds potential for alloy optimization and developing processing strategies, applicable to automotive manufacturing and structural engineering industries.

The fabrication of high-entropy alloys (HEAs) using metal injection molding (MIM) represents a significant leap forward in the field of materials engineering, enabling the production of components with complex shapes while maintaining the unique mechanical properties of HEAs. This study investigates the effect of heating rate variations on the densification behavior in the equiatomic CoCrFeMnNi HEA, focused on microstructural analysis and densification behavior for each thermal schedule. By optimizing the sintering behavior with single and double-step heating rates, significant findings reveal that sintering under double-step heating rates leads to marked improvements in densification and microstructural uniformity. Microstructural analyses indicate that the modulated heating rates during sintering crucially affect the segregation of Cr and secondary phase formation. This research highlights the importance of sintering profiles in optimizing the microstructure and properties of HEAs processed through MIM, contributing valuable insights to a refined strategy for the improved microstructure and mechanical strength of HEAs in powder metallurgy.

One of the key goals of processing 316L stainless steel by powder metallurgy (PM) techniques is to achieve industrial-viable tensile properties without structural defects like poor densification, undesired phase transitions, and oxidation during high-temperature sintering. To address this, this study adopts high-pressure torsion to fabricate a fully dense structure at ambient temperature through cold consolidation. The samples fabricated by the present PM-based technique exhibits considerably enhanced tensile properties compared to counterparts processed by conventional PM techniques, with a remarkable yield strength of 1 GPa and total elongation of 46%. Additionally, the segregation of certain elements during subsequent annealing results in a unique microstructure with nano-scale sigma precipitates which induces dislocation pile-up, leading to improved yield strength and retarded dislocation motion. The results indicate that the present PM-based route is an applicable technique to achieve the strength-ductility synergy in 316L stainless steel.

High-entropy alloys (HEAs) have been reported to have better properties than conventional materials; however, they are more expensive due to the high cost of their main components. Therefore, research is needed to reduce manufacturing costs. In this study, CoCrFeMnNi HEAs were prepared using metal injection molding (MIM), which is a powder metallurgy process that involves less material waste than machining process. Although the MIM-processed samples were in the face-centered cubic (FCC) phase, porosity remained after sintering at 1200°C, 1250°C, and 1275°C. In this study, the hot isostatic pressing (HIP) process, which considers both temperature (1150°C) and pressure (150 MPa), was adopted to improve the quality of the MIM samples. Although the hardness of the HIP-treated samples decreased slightly and the Mn composition was significantly reduced, the process effectively eliminated many pores that remained after the 1275°C MIM process. The HIP process can improve the quality of the alloy.

The current research work introduces a novel processing technique involving a combination of hot rolling and a direct quench and partitioning treatments to produce an ultra-high strength, low-carbon and lean-composition steel with superior mechanical properties and enhanced stretch flangeability. The methodology involves the introduction of a secondary partitioning step after a one-step direct quenching and partitioning (DQP) process. A detailed investigations on microstructures, tensile properties and stretch flangeability (using hole expansion testing) were carried out. The martensite-austenite two phase microstructure resulted in a remarkably improved product of strength and elongation (PSE, 24 GPa.

Metal additive manufacturing (MAM) offers an excellent capability for designing complex geometries with topology-optimized and near-net-shaped structures. The optimization-designed MAM parts with constrained volume require superior mechanical properties to broaden their utilization in the industry. However, the intrinsic defects generated during the building process deteriorate the mechanical functionality and limit utilization in various industrial applications. In this study, we propose a new strategy to reduce generated defects using ultrasonic nanocrystal surface modification (UNSM) on laser powder bed fusion (LPBF) processed 316L stainless steel. Subsurface pores and high surface roughness in MAM parts were significantly improved through the impact of UNSM treatment, and a gradient structure with mechanical incompatibility was developed. Consequently, the LPBF-processed samples after UNSM treatment show the excellent combination of strength and ductility, which is attributed to the high synergistic hardening from the gradient microstructure and the suppression of damage evolution by controlling built defects.

Developing metal additive manufactured (AM) parts with excellent mechanical properties broaden the utilization of AM parts in various industrial applications. Recently, gradient structures have received significant attention owing to their excellent mechanical properties. In this study, we propose a new strategy to obtain a superior gradient structure using annealing and ultrasonic nanocrystal surface modification (UNSM) on laser powder bed fusion (LPBF) fabricated 316 L stainless steel. The post-LPBF annealing treatment disturbed the cellular dislocation structure, which led to the optimized materials for gradient structure. The resulting gradient structure after the UNSM treatment has a thicker gradient layer with a significant strain partitioning between the domains than the LPBF followed by UNSM without annealing. As a result, the present LPBF-annealing-UNSM alloys show superior strength-ductility synergy compared to the LPBF-UNSM samples.

In the current study, a metastable Fe-15Ni-8Mn-8Co-3Ti-1Si ferrous medium entropy alloy is subjected to powder high-pressure torsion (HPT) followed by annealing. The body-centered cubic microstructure ob-tained from the HPT method largely changes into a metastable face-centered cubic phase decorated by Ti -Si-rich precipitates during the isothermal post-annealing process. The optimum heat treatment results in materials with high ultimate tensile strength and total elongation of 748 MPa and 68.81%, respectively. The material is strengthened by the numerous grain boundaries created during recrystallization, which effec-tively impede dislocation movement and postpone the deformation-induced martensitic transformation (DIMT) to the applied strain of over 20%. High ductility and high strength are combined by employing the simultaneous effects of controlled DIMT, grain boundary engineering, and precipitate strengthening, re-sulting in multistep strain hardening of the corresponding specimen.& COPY; 2023 Elsevier B.V. All rights reserved.