Microstructure stability during high temperature deformation of CoCrFeNiTa eutectic high entropy alloy through nano-scale precipitation

Abstract

Dual-phase microstructures arising out of eutectic reactions offer several advantages: ease of casting, composite properties and tunable characteristic length scale of the phases. Eutectic high entropy alloys (EHEA) with a multi-component solution phase and a hard intermetallic phase are candidate materials to identify alternate high-temperature materials. Alloying elements can be chosen to improve the resistance to lamellar microstructure degradation and coarsening. In this work, we present the CALPHAD guided design of a hypo-eutectic high entropy alloy CoCrFeNiTa0.395 with primary dendritic FCC phase and fine eutectic (FCC solution + Laves phase) microstructure possessing good high-temperature mechanical properties. The primary FCC phase fraction is 0.42 ± 0.02. The interlamellar spacing of the eutectic is 0.69 ± 0.12 μm. The alloy exhibited a balanced yield strength of 1303 ± 18 MPa, a fracture strength of 2237 ± 23 MPa and a fracture strain of 0.3 under room temperature compression testing. The mechanism for change in the lamellar morphology during deformation at temperatures beyond 0.8 TE is explained schematically. Strain field distribution obtained by FEM simulation correlates well with the observed microstructure gradients in the deformed samples. The formation of nano-scale precipitates in the low strain rate deformation is attributed to thermal effects. High-temperature precipitation of two types of precipitates (L12 ordered and Ni3Ta type) in the FCC-solution phase extended the useable temperature range of this alloy.