Effects of post annealing on the microstructure, precipitation behavior, and mechanical property of a (CoCrNi)94Al3Ti3 medium-entropy alloy fabricated by laser powder bed fusion

Abstract

• The (CoCrNi) 94 Al 3 Ti 3 medium-entropy alloys with different strength-ductility combinations were fabricated by laser powder bed fusion and subsequent annealing processes. • Cr-rich σ-phase precipitates formed after 1073 K and then gradually dissolved at 1373 K, while fine L1 2 coherent precipitates were formed at 1373 K. • Precipitation and grain boundary strengthening both markedly increased owing to the formation of precipitation particles and recrystallization-induced grain refinement after annealing at 1073 K. The additive manufacturing of multi-principal element alloys has remarkable potential for industrial applications. In this study, a (CoCrNi) 94 Al 3 Ti 3 medium-entropy alloy (MEA) with adequate strength-ductility synergy was prepared via laser powder bed fusion. The microstructural evolution, mechanical property, and deformation mechanisms of the MEA were investigated after post annealing for a short period (0.5 h) at a temperature range of 773-1373 K using various microstructural characterization techniques and quantitative analysis. The static recrystallization temperature of the (CoCrNi) 94 Al 3 Ti 3 MEA ranged from 973 to 1073 K. The average grain size first decreased and then increased, while the dislocation density persistently decreased and texture gradually weakened with increasing annealing temperature. Cr-rich σ-phase precipitates formed after 1073 K and then gradually dissolved at 1373 K, while Ni, Al, and Ti elements were aggregated to form a small amount of fine L1 2 coherent precipitates with an average diameter of approximately 70 nm at 1373 K. The evolution of the dislocation density, grain size, and precipitates significantly influenced the propensity of deformation twins and stacking faults, which consequently affected the strain hardening behavior and mechanical properties. The quantitative calculation of strengthening mechanisms showed that dislocation strengthening played a dominant role at annealing temperatures below 1073 K, and it significantly weakened at 1373 K. Precipitation and grain boundary strengthening both markedly increased owing to the formation of precipitation particles and recrystallization-induced grain refinement after annealing at 1073 K.