Temperature Dependence of the Deformation Behavior of High-Entropy Alloys Co20Cr20Fe20Mn20Ni20, Co19Cr20Fe20Mn20Ni20С1, and Co17Cr20Fe20Mn20Ni20С3. Mechanical Properties and Temperature Dependence of Yield Stress

Abstract

This paper discusses the temperature dependence of the mechanical properties of multicomponent alloys Co₂₀Cr₂₀Fe₂₀Mn₂₀Ni₂₀ (Cantor alloy), Co₁₉Cr₂₀Fe₂₀Mn₂₀Ni₂₀С₁, and Co₁₇Cr₂₀Fe₂₀Mn₂₀Ni₂₀С₃ under uniaxial static tension in the temperature range from 77 to 473 K. It is shown that all the alloys acquire an fcc crystal structure after thermomechanical treatment, but the alloy with 3 at % carbon exhibits large incoherent chromium carbides unlike single-phase Co₂₀Cr₂₀Fe₂₀Mn₂₀Ni₂₀ and Co₁₉Cr₂₀Fe₂₀Mn₂₀Ni₂₀С₁ alloys. Doping with carbon causes solid solution strengthening of the austenitic phase as well as dispersion hardening and moreover promotes grain refinement in the Cantor alloy. Solid solution strengthening contributes to an increase in the athermal and thermal stress components of σ_0.2, leading to higher yield stress values and stronger temperature dependences σ_0.2(T) in Co₁₉Cr₂₀Fe₂₀Mn₂₀Ni₂₀С₁ and Co₁₇Cr₂₀Fe₂₀Mn₂₀Ni₂₀С₃ alloys than in the Cantor alloy. The results of X-ray diffraction and microscopic analysis indicate that, despite the difference in the total concentrations of interstitial atoms in Co₁₉Cr₂₀Fe₂₀Mn₂₀Ni₂₀С₁ and Co₁₇Cr₂₀Fe₂₀Mn₂₀Ni₂₀С₃ alloys, the concentrations of carbon dissolved in the crystal lattice of the austenitic phase are close. However, the higher strength properties of Co₁₇Cr₂₀Fe₂₀Mn₂₀Ni₂₀С₃ compared to Co₁₉Cr₂₀Fe₂₀Mn₂₀Ni₂₀С₁ are determined primarily by grain boundary strengthening and, to a lesser extent, by dispersion hardening. Both factors such as lowering the deformation temperature and doping with carbon contribute to an increase in the deforming stresses of the Cantor alloy. It is shown that carbon doping affects the staged plastic flow of the Cantor alloy: the tensile curves of Co₁₉Cr₂₀Fe₂₀Mn₂₀Ni₂₀С₁ carbon alloy exhibit a well-defined stage of microplastic deformation, and the flow curves of Co₁₇Cr₂₀Fe₂₀Mn₂₀Ni₂₀С₃ alloy have a parabolic shape at the initial stages of plastic flow, which is typical of the deformation of alloys with large incoherent particles. The elongation to failure of Co₂₀Cr₂₀Fe₂₀Mn₂₀Ni₂₀ and Co₁₉Cr₂₀Fe₂₀Mn₂₀Ni₂₀С₁ alloys increases linearly with decreasing deformation temperature, i.e. the mechanical properties of single-phase alloys are improved in the region of low test temperatures. For Co₁₇Cr₂₀Fe₂₀Mn₂₀Ni₂₀С₃ alloy, an increase in strength properties during low-temperature deformation is accompanied by a decrease in elongation, and the alloy becomes brittle from the macromechanical viewpoint.