Effect of Fe content on structure and mechanical properties of a medium: Entropy Fex(CoNi)100-xCr9.5C0.5 (x = 60 and 65) alloys after cold rolling and annealing

Abstract

In this study two medium-entropy Fe60Co15Ni15Cr9.5C0.5 (hereinafter referred to as Fe60) and Fe65Co12.5Ni12.5Cr9.5C0.5 (Fe65) alloys (both compositions in at%) were studied after thermomechanical processing involving cold rolling and subsequent annealing at 800 and 1000 ℃. The Fe60 alloy was expected to have a higher stability of a fcc phase and to be less prone to the martensite formation than those of the Fe65 alloy as suggested the ∆Gfcc→bcc values calculated using a Thermo-Calc software. Both the Fe60 and Fe65 alloys underwent deformation-induced microstructure refinement upon cold rolling and recrystallization in the fcc phase and precipitation of Cr-rich carbides after annealing. However, the Fe60 alloy had the fcc-based structure, while the Fe65 alloy contained a significant amount (35–60 %) of the bcc martensite. After cold working, the Fe65 alloy was stronger than the Fe60 alloy (yield strength of 1050 MPa and 1390 MPa, respectively). Annealing resulted in considerable softening of the alloys - yield strength was in a range of 205–360 MPa, and no significant difference in strength between Fe60 and Fe65 was found. The latter finding was associated with the plastic flow initiation with the softer fcc phase. Both alloys demonstrated the transformation-induced plasticity (TRIP) effect associated with the bcc martensite formation during tensile deformation. At room temperature, the TRIP effect was more developed in the Fe65 alloy due to which a comprehensive combination of properties after cold rolling and annealing at 800 °C can be attained; i.e. yield strength of 360/1100 MPa, ultimate tensile strength of 1060/1900 MPa, uniform elongation of 20/26 %, and impact toughness (KCV) of 1300/885 kJ/m2 at 293/77 K, respectively. Quantitative analysis of the structure-properties relationships in the program alloys was performed to reveal similarities and differences in the behavior of the alloys.