Abstract
Microstructure evolution of a eutectic VNbTiSi medium entropy alloy (MEA) was systematically investigated during annealing heat treatment. Polymorphic transformation from beta-(Nb, X)(5)Si-3, a high-temperature metastable phase, to '-or gamma-(Nb, X)(5)Si(3 )is revealed and is demonstrated to be closely related to element diffusion. There was mutual diffusion between V and Si, forming a diffusion transition, and the Ti diffusion promoted the formation of gamma-(Nb, X)(5)Si-3 mainly distributed at the grain boundary because of the Ti su-persaturation in the BCC phase during solidification. Strong elemental diffusion promotes the formation of a thermal groove at the three junctions of sub-grain boundaries, contributing to the fast spheroidization of (Nb, X)(5)Si-3, while the termination migration dominates the subsequent coarsening stage. Polymorphic transformation of silicides brings in sub-grain boundaries or low angle grain boundaries (LAGBs), con-tributing to silicide refinement. As a result, the compressive strength at elevated temperatures is greatly improved due to the increasing volume fraction of (Nb, X)(5)Si-3 after heat treatment. (C) 2022 Elsevier B.V. All rights reserved.
NSTL
Elsevier