High-Temperature Twinning in Single Crystals of the Cr20Fe20Mn20Co35Ni5 High Entropy Alloy

Abstract

The paper reports on the deformation mechanism (slip and twinning), strain hardening rate Θ(ε), and plasticity of \([\bar 111]\)- and \([\bar 144]\)-oriented single crystals of the Cr₂₀Fe₂₀Mn₂₀Co₃₅Ni₅ (at %) high entropy alloy (HEA) under tension in the temperature range 373–573 K. It is shown that the onset of plastic flow in this temperature range is associated with the slip of a/2‹110› dislocations highly split into a/6‹112› partial Shockley dislocations, and critical shear stresses for slip \(\tau _{{\rm{cr}}}^{{\rm{sl}}}\) are independent of the crystal orientation. The development of high-temperature twinning at the temperature 373–573 K and strain 5 and 20% is first discovered in the \([\bar 111]\)- and \([\bar 144]\)-oriented single crystals of the Cr₂₀Fe₂₀Mn₂₀Co₃₅Ni₅ HEA, respectively. High-temperature twinning is facilitated by the combination of a low stacking fault energy γ₀ = 14 mJ/m² and heavy lattice distortion as a result of mixing of substitutional atoms in equal or nearly equal atomic concentrations. It is found that, at high temperatures, \(\tau _{{\rm{cr}}}^{{\rm{tw}}}\) does not depend on the crystal orientation and the test temperature: \(\tau _{{\rm{cr}}}^{{\rm{tw}}} = (80...110) \pm 5\) MPa at the \([\bar 111]\) orientation and \(\tau _{{\rm{cr}}}^{{\rm{tw}}} = (100...110) \pm 5\) MPa at the \([\bar 144]\) orientation. The maximum plasticity of 70–90% is realized for the \([\bar 144]\)-oriented crystals when twinning develops mainly in one system. The dependence of the strain hardening rate Θ(ε) is characterized by stages, which are observed during twinning in face-centered cubic polycrystals.