In Situ High‐Temperature X‐ray Diffraction Study of the Thermal Stability of the Co_0.22Cr_0.23Fe_0.29Ni_0.20Ti_0.06 High‐Entropy Alloy Produced by High‐Energy Ball Milling

Abstract

In this report, the effect of Ti addition on the phase evolution of a near‐equiatomic CoCrFeNi‐based high‐entropy alloy (HEA) during annealing in a vacuum is described. The HEA Co0.22Cr0.23Fe0.29Ni0.20Ti0.06 is synthesized through mechanical alloying of constituent elements for a milling duration of 1 h. The as‐prepared HEA is a two‐phase alloy containing body centred cubic (BCC) precipitates in an face centred cubic (FCC) matrix. The thermal stability of the HEA is experimentally studied by subjecting it to 5 h of isothermal annealing at 600, 800, and 1000 °C, using high‐temperature X‐ray diffraction (HTXRD). The HTXRD analysis indicates a noteworthy reduction in the BCC phase content after isothermal annealing at 600 °C. The formation of intermetallic σ‐phases does not occur. At temperatures of 800 and 1000 °C, the alloy matrix exhibits a single‐phase FCC solid solution. At 800 and 1000 °C, HEA undergoes partial oxidation, resulting in the formation of oxide phases on the surface and within the particles of the powder in the form of nanoscale inclusions. This oxidation depletes the metal matrix with titanium, leading to a change in the composition of the HEA. The alloy transforms into a CoCrFeNi solid solution containing 3–4 at% Ti. Thus, the alloy matrix phase remains a stable FCC solid solution.