Thermal and Structural Stability of the TiZrHfNbTa Solid Solution

Abstract

A high-entropy Ti_0.2Zr_0.2Hf_0.2Nb_0.2Ta_0.2 alloy shows promise as a material for tensometric applications; however, data on its thermal stability at different temperatures are incomplete. Prepared samples were subjected to heat treatment (annealing in a vacuum) at 523 and 673 K for 0, 10, 25, 50, 100, 200, 400, and 800 h for X-ray diffraction studies and for 1, 2, 6, 10, 25, 50 100, 200, 400, and 800 h for measuring the microhardness of the solid solution. For all Ti_0.2Zr_0.2Hf_0.2Nb_0.2Ta_0.2 samples, the chemical composition, lattice parameters, and the evolution of the microstructure and microhardness in the course of complete heat treatments are studied. The cast alloys prepared by repeated electric arc melting are found to form a bcc single-phase solid solution, which is characterized by dendritic grain growth and interdendritic segregation. During annealing at 523 K, the Ti_0.2Zr_0.2Hf_0.2Nb_0.2Ta_0.2 alloy is thermally stable for 800 h and does not undergo phase transitions; however, isothermal holding leads to the formation of a nonequilibrium structure characterized by a high content of defects and concentration inhomogeneities. The decomposition of the solid solution takes place at the beginning stage of annealing at 673 K, and the long-term holding for 800 h favors the formation of multiphase structure. Whatever the annealing temperature (523, 673 K), the dendrite growth morphology changes. The behavior of time dependences of the microhardness correlates with X-ray diffraction data. In the course of annealing of experimental alloys at 523 K, no abrupt variations in the lattice parameter and hardness are observed. During annealing at 673 K, an abrupt increase in the microhardness from 365 to 560 HV and a change in the lattice parameter from 3.4128(1) to 3.3865(1) Å are observed, which indicate a phase transition. The data obtained allow us to determine the upper limit of the temperature range of operation of the alloy, which is 523 K.