Effects of annealing treatment on microstructure and mechanical properties of laser melting-deposited VCoNi medium-entropy alloy

Abstract

Additive manufacturing (AM) is a promising approach to producing high- or medium-entropy alloys (HEAs or MEAs) with excellent properties. However, some undesirable microstructural defects and unstable precipitated phases were formed owing to the repeated heating and cooling steps and the rapid solidification nature of the AM process. Annealing treatment after AM fabrication is often beneficial for adjusting the comprehensive mechanical properties and stabilizing the microstructures. In this study, a VCoNi MEA single wall sample was fabricated successfully via laser melting deposition (LMD), and the effects of post-annealing treatments on the microstructures and mechanical properties were explored. The grain size and σ-phase particle size increase and the σ-phase particles undergo fragmentation, spheroidization, and coarsening during annealing. As the annealing time increases, the tensile strain increases, but the tensile strength decreases. An excellent combination of strength and ductility can be obtained after annealing for 1.5 h (The yield strength, ultimate tensile strength, and tensile strain are 741.2 MPa, 1287.8 MPa, and 30.5 %, respectively). HDI hardening is the most important strengthening mechanism in this work. The decrease in tensile stress after the annealing treatment is mainly attributed to the decrease of heterogeneity of microstructure, and the increase in tensile strain after the annealing treatment is mainly attributed to the decrease of the pores and the spheroidization of the rod-like σ-phase particles. This work provides practical insight into adjusting the microstructure and comprehensive mechanical properties of HEAs and MEAs for AM.