Tuning δ → γ transformation types to relieve mechanical property degradation in a Co-free face-centered cubic metastable high-entropy alloy

Abstract

In face-centered cubic Fe-Mn-based high manganese alloys, stacking fault energy could be lowered remarkably by silicon alloying and thus the martensitic transformation of γ-austenite to ε-martensite could be facilitated. Inspired by this, a Co-free face-centered cubic Fe59Mn15Cr11Si9Ni6 metastable high-entropy alloy was developed by replacing cobalt by silicon and adjusting chemical compositions on the basis of Fe-Mn-Cr-Co-Ni high-entropy alloys. The Fe59Mn15Cr11Si9Ni6 alloy solution treated at 1273 K for 30 min (295 MPa yield strength, 776 MPa ultimate strength and 63.6% elongation) exhibited better mechanical properties than a typical Fe50Mn30Co10Cr10 dual-phase metastable high-entropy alloy (~250 MPa yield strength, ~730 MPa ultimate strength and ~50.0% elongation) since their austenitic grains were around 45 µm. When the Fe59Mn15Cr11Si9Ni6 alloy was heat-treated at 1548 K for 30 min, δ → γ massive transformation and δ → γ long-range diffusional transformation would occur after brine quenching and air cooling, respectively. The degradation of ultimate strength and elongation was much weaker after δ → γ massive transformation than after δ → γ long-range diffusional transformation with reference to the solution-treated Fe59Mn15Cr11Si9Ni6 high-entropy alloy. The degradation of mechanical properties derived from δ → γ phase transformation should be relieved through promoting the occurrence of δ → γ massive transformation. It should be a helpful strategy to relieve the degradation of mechanical properties for Fe-Mn-Cr-Si-Ni metastable high-entropy alloys prepared by casting, welding and additive manufacturing.