Intermediate-Temperature Creep Deformation and Microstructural Evolution of an Equiatomic FCC-Structured CoCrFeNiMn High-Entropy Alloy

Abstract

The tensile creep behavior of an equiatomic CoCrFeNiMn high-entropy alloy was systematically investigated over an intermediate temperature range (500–600 °C) and applied stress (140–400 MPa). The alloy exhibited a stress-dependent transition from a low-stress region (LSR-region I) to a high-stress region (HSR-region II). The LSR was characterized by a stress exponent of 5 to 6 and an average activation energy of 268 kJ mol−1, whereas the HSR showed much higher corresponding values of 8.9–14 and 380 kJ mol−1. Microstructural examinations on the deformed samples revealed remarkable dynamic recrystallization at higher stress levels. Dislocation jogging and tangling configurations were frequently observed in LSR and HSR at 550 and 600 °C, respectively. Moreover, dynamic precipitates identified as M23C6 or a Cr-rich σ phase were formed along grain boundaries in HSR. The diffusion-compensated strain rate versus modulus-compensated stress data analysis implied that the creep deformation in both stress regions was dominated by stress-assisted dislocation climb controlled by lattice diffusion. Nevertheless, the abnormally high stress exponents in HSR were ascribed to the coordinative contributions of dynamic recrystallization and dynamic precipitation. Simultaneously, the barriers imposed by these precipitates and severe initial deformation were referred to so as to increase the activation energy for creep deformation.

High-Temperature Nano-Indentation Creep of Reduced Activity High Entropy Alloys Based on 4-5-6 Elemental Palette

“…The calculated higher activation energy of 900-1000 kJ/mol for HEAs compared to pure W may be associated with severe lattice distortion and sluggish diffusion in HEAs resulting in a greater degree of dislocation interaction and supporting their higher creep resistance [48]. Activation energy for CoCrFeNiMn [12] and precipitation-hardened (FeCoNiCr)94Ti2Al4 [49] HEAs were reported to be ~300-400 kJ/mol and ~300-800 kJ/mol, respectively, from tensile tests. However, to the best of the authors' knowledge, there are no reports on activation energy of HEAs by the nano-indentation creep test.…”

“…In service, one of the essential design criteria for engineering components is the time-dependent deformation behavior (i.e., creep resistance). Macroscopic bulk compression or tension [10][11][12], micro-pillar compression [13,14], and nano-indentation technique [15][16][17][18][19][20][21] have been used to investigate the deformation behavior of materials in a wide range of length scales. Major discrepancies between the data obtained by nano-indentation and via conventional creep testing (i.e., compression/tension) have been reported [22,23].…”

New Advances in High-Entropy Alloys

“…Especially the CoCrFeMnNi alloy is famous as the Cantor alloy for its high plasticity and comparable strength. The tensile creep behavior of the CoCrFeNiMn HEA was systematically investigated by Cao et al [ 14 ] over an intermediate temperature range (500–600 °C) and applied stress (140–400 MPa). The alloy exhibited a stress-dependent transition from a low-stress region to a high-stress region, which was characterized by the dynamic recrystallization and the grain-boundary precipitation.…”

Long-Term Creep Behavior of a CoCrFeNiMn High-Entropy Alloy

“…The literature of creep in high-entropy alloys (HEAs) is varied as to sources (alloy design and processing execution) and methods (indentation versus compression versus tension). Some authors use nano-indentation or small punch creep tests ( Ref 30,[33][34][35][36][37][38][39]. Others use stress relaxation techniques or compression to measure creep behavior (Ref 31).…”

“…Others use stress relaxation techniques or compression to measure creep behavior (Ref 31). Many authors use small buttons, remelted, and then heat treated at some prescribed temperature for some arbitrary duration as the manufacturing approach (Ref 30, [33][34][35][36][37][38][39] with inherent unreproducibility in the approaches. However, it is questionable as to the effectiveness of using these small melts in representing creep performance that might arise from larger ingots produced by typical manufacturing methods used for commercial components (Ref 40,41).…”