Designing (Ultra)Fine-Grained High-Entropy Alloys by Spark Plasma Sintering and Equal-Channel Angular Pressing

Abstract

Single-phase, face-centered cubic (FCC) high-entropy alloys (HEA) are promising materials for future applications. In order to improve the mechanical properties, especially the tensile strength of these materials, this study focuses on the combination of spark plasma sintering (SPS) and equal-channel angular pressing (ECAP). The initial fine-grained microstructure produced by SPS is further refined by ECAP in a 90°-die. Optical microscopy and electron backscatter diffraction (EBSD) confirm this considerable grain refinement, leads to a grain size below 1 µm after 1 ECAP pass. An alternating arrangement of fine-grained areas and much coarser regions, aligned under an angle of approximately 27°, is found. Moreover, a first microstructural investigation of the twin structure is conducted. The mechanical behavior was investigated by hardness measurements and tensile testing. Both the hardness and tensile strength are remarkably increased after ECAP. In contrast, the uniform elongation and elongation at fracture are significantly reduced due to the strengthening mechanisms of strain hardening and grain refinement. It is concluded that the combination of SPS and ECAP is an attractive approach for designing (ultra)fine-grained HEAs with superior properties. The investigated techniques could be applied to understand the underlying microstructural mechanisms.

An Overview of High-Entropy Alloys Prepared by Mechanical Alloying Followed by the Characterization of Their Microstructure and Various Properties

“…The microstructure of the sintered HEA confirmed the formation of ultra-fine grains, as shown in Figure 15. The mechanical properties such as the microhardness and tensile strength of SPS-consolidated CrFeCoNi HEA were excellent, and their properties were further improved by the combination of SPS and equal-channel angular pressing, as reported by the authors [46]. CoCr2-xCuFeNix HEAs using SPS; they reported a slight increase in the grain size from ten nanometers to several hundred nanometers before and after SPS, respectively.…”

“…The formation of a CrN layer improved the hardness to around 1500 HV, as shown in Figure 14c, with excellent wear and pitting corrosion-resistant properties. Rymer et al produced ultra-fine-grain-structured CrFeCoNi HEAs through the SPS technique at 1050 • C temperature under 50 MPa uniaxial pressure for 13 min [46]. The microstructure of the sintered HEA confirmed the formation of ultra-fine grains, as shown in Figure 15.…”

Strategies for Achieving Strength–Ductility Synergy in Face‐Centered Cubic High‐ and Medium‐Entropy Alloys

“…The other strengthening mechanisms in the HEAs including GB strengthening, precipitation strengthening, and dislocation strengthening are similar to those in the conventional metals and alloys, and thus classical models can be used to evaluate reasonably the strength gains in HEAs/MEAs. More importantly, these traditional strengthening strategies based on the simple introduction of dislocation barriers inevitably limit the proliferation and accumulation of dislocations, which would lead to a decline in the ductility, [36,50,86,114,145,148,149,155,159,[181][182][183][184][185][186][187][188][189][190][191][192][193][194][195] as shown in Figure 8. Therefore, in the exploration of simultaneous enhancement of strength and ductility in FCC HEAs/MEAs, the efforts of materials scientists have never stopped.…”

“…Following this idea, several strategies have been proposed with some success, some of which are discussed below. [36,50,86,114,145,148,149,156,159,[181][182][183][184][185][186][187][188][189][190][191][192][193][194][195] Here, Δε and Δσ represent the increase in uniform elongation and ultimate tensile strength caused by strengthening, respectively.…”

“…Figure 8. Strength-ductility trade-off caused by traditional strengthening strategies for FCC HEAs/MEAs, grouped by strengthening mechanisms [36,50,86,114,145,148,149,156,159,[181][182][183][184][185][186][187][188][189][190][191][192][193][194][195]. Here, Δε and Δσ represent the increase in uniform elongation and ultimate tensile strength caused by strengthening, respectively.…”