Effect of heat treatment on the microstructures and hardness of selective laser melted Rene88DT superalloy

Abstract

Selective laser melted Rene88DT specimens with columnar grains (specimen C) and near-equiaxed grains (specimen E) were fabricated, on which solution-aging and direct aging treatments were applied, respectively. The evolution of microstructures and hardness of both specimens was investigated. Results show that specimen C showed columnar grains with aspect ratio of 3.6 and 5% difference in hardness between vertical and horizontal directions, while specimen E with near-equiaxed grains (aspect ratio of 1-2) was isotropic. The as-deposited alloys consisted of γ-matrix, carbides and Laves phases. After solution-aging treatment, equiaxed grains (56 μm) with annealing twins, blocky μ phases (1 μm) and dispersively distributed γ′ (40 nm) were observed in specimen C. After direct aging treatment, rod-like γ′ phases with 40 nm in length and 15 nm in diameter were precipitated in specimen E. Specimen E showed higher hardness than specimen C because of stronger grain boundary strengthening, substructure strengthening and second-phase strengthening.