Dynamic fracture response of Cantor-derived medium entropy alloys

Abstract

The dynamic behavior for five Cantor-derived alloys with four elements in equiatomic composition was investigated. The composition was systematically modified to: CrMnFeNi, MnFeCoNi, CrMnCoNi, CrMnFeCo, and CrFeCoNi. Under identical homogenization, deformation, and recrystallization processing conditions, these materials exhibited significant differences in the microstructure and the corresponding mechanical response, which was measured through uniaxial stress compression loading at strain rates ranging from 10⁻⁴ to 10³ s⁻¹. Additionally, spall recovery experiments were performed at strain rates of ∼10⁴ s⁻¹ to further understand the effect of strain rate on damage and failure in tension. Four of the five alloys were face-centered cubic and exhibited ductile failure by void nucleation, growth, and coalescence, primarily at the grain boundaries. Surprisingly, the significant differences in compressive strength (ranging from ∼200 to ∼600 MPa at 10⁻⁴ s⁻¹) did not manifest themselves in the spall strength, which varied from 2.08 to 2.61 GPa (not including the CrMnFeCo alloy). This is attributed to similar processes of void growth, which occur at the scale of a few micrometers and do not incorporate the mesoscopic response (such as Hall-Petch), which determines the yield strength. One of the alloys, CrMnFeCo, exhibited especially low ductility in compression and fragmented into pieces during both compression and spall testing. This behavior was attributed to the formation of a brittle Sigma-like phase. The effect of both chemistry and processing was observed to affect the specific spall strength and damage evolution. Specifically, Cr segregation at the grain boundaries had a deleterious effect. This study provides a template for the choice of superior dynamic response among high and medium entropy alloys.