Heat treatments design for superior high-temperature tensile properties of Alloy 625 produced by selective laser melting

The popular superalloy Alloy 625 was produced by Selective Laser Melting (SLM) and post-processing heat treatments were designed to optimize the inhomogeneous and constrained as-built microstructure (AB) for high temperature structural applications. A single-step solution heat treatment (RX) was designed to promote full recrystallization and approach the conventional wrought microstructure. To enhance high temperature properties, a grain boundary serration heat treatment (GBS) was successfully designed involving higher solution temperature and time to promote recrystallization and homogeneity, and a direct slow cooling step followed by a short aging to assist solute diffusion and grain boundary motion. The resulting microstructures were characterized by fully recrystallized fine equiaxed grains and fine intra and intergranular NbC precipitates. The GBS alloy also exhibited as much as 80% of serrated grain boundaries with enhanced resistance to cracking at high temperatures. Tensile properties of all three materials were evaluated at room temperature, 500 degrees C, 600 degrees C and 700 degrees C and compared with their conventional solutionized wrought Alloy 625 counterpart (Wrought). While the AB material exhibited high strength and low ductility, due for the most part to the high density of tangled dislocations resulting from SLM, both RX and GBS alloys showed tensile properties comparable to the conventional wrought material, higher strength in particular. At all temperatures, all four alloys exhibited yield strength values well over 200 MPa. Due to significantly different microstructures, deformation and fracture behaviors were different. While Wrought clearly presented irregular plastic flow at elevated temperatures typically attributed to dynamic strain aging (DSA), the materials produced by SLM and moreover those subjected to post-processing heat treatments exhibited more stable plastic deformation. The results and characterization reported in the present article highlight the predominant role of microstructure and outstanding potential of SLMed Alloy 625.