Grain boundary crystallography and segregation in Ni-based superalloy INC738 manufactured by electron-beam powder bed fusion in as-built and annealed conditions

Abstract

The excellent high-temperature properties of Ni-based superalloy INC738 are due to its hierarchical microstructure, making it an ideal engineering material for high-temperature applications. Engineering parts are now increasingly made via electron beam powder bed fusion (EPBF), an additive manufacturing technique suitable for such hard-to-weld Ni-based superalloys, due to lower thermal gradients and unmatched scan path control. The thermal cycles induced by EPBF impact characteristics of the γ-matrix, γ’ precipitates, secondary phases such as carbides, grain boundary (GB) solute segregation and, in turn, properties including GB cohesion and strength. However, a more thorough understanding of the GB microstructure evolution with focus on GB chemistry and character is required to optimise properties. We systematically investigate texture, grain structure, GB habit planes, and GB segregation in INC738 fabricated with linear versus random EPBF scanning strategies. We show that random scanning is a suitable strategy to inhibit cracking, refine grains, and decrease segregation of Cr, Mo, C, and B at GBs. For both scanning strategies, γ/γ GBs predominantly terminate on {100} planes and are decorated with C, B, Mo, and W. Upon 2 h annealing at 1180 °C and 1250 °C, the GB character and texture are shown to remain stable despite a reduction in GB interfacial excess. After 24 h annealing at 1250 °C, GB segregation and depletion are nearly eradicated, while static recrystallisation is observed with a predominant formation of annealing twins and GBs terminating on {111} planes. These findings are critical for defect-free additive manufacturing of INC738 and similar grades for superior high-temperature performance.