The effect of heat treatment on microstructure and mechanical properties of Ti3Zr1.5NbVAl0.25 refractory high entropy alloys fabricated by directed energy deposition

Abstract

Directed energy deposition (DED) is recognized as an advanced technology for fabricating refractory high entropy alloys (RHEAs), and appropriate heat treatment of DED materials may further enhance their properties. In this work, Ti₃Zr_1.5NbVAl_0.25 RHEAs were prepared using DED, and the effects of annealing (550 °C/5 h) and solid solution (800 °C, 1000 °C, and 1200 °C/30 min) treatment on the microstructure and mechanical properties of the RHEAs were systematically investigated. The results indicate that needle-like C14 Laves phases precipitated in the matrix after annealing, which significantly degraded the plasticity. Following solid solution treatment at temperatures above 800 °C, the C14 Laves phase dissolved, resulting in a single phase BCC structure in the matrix. As the solid solution temperature increased from 800 °C to 1200 °C, abnormal grain growth occurred, with the average grain size expanding from 30.2 μm to 292.0 μm. The solid solution treatment had a negligible impact on strength and microhardness, while it reduced plasticity but enhanced impact toughness. The 1000 °C solid solution samples demonstrated the most balanced mechanical properties, with a yield strength of 871 MPa and a fracture elongation of 24.7 %, showing minimal deviation from the DEDed samples. Importantly, the impact toughness reached 55.0 J/cm², representing a remarkable increase of 201 % compared to the DEDed samples. This study provides valuable insights for the development of RHEAs that achieve an optimal combination of strength and toughness.