Microstructure and cracking behavior of Ni₃Al-based IC21 alloy fabricated by selective laser melting

Understanding the formation mechanism of microstructure and cracking behavior of Ni₃Al-based IC21 alloy fabricated by selective laser melting (SLM) is of great importance to promote its development and application in advanced aeroengine field. In this study, SLM was applied to Ni₃Al-based IC21 for the first time and cubic IC21 alloy samples were prepared by SLM and their microstructure was characterized. The average primary dendrite arm spacing (PDAS) is only 700 nm due to the extremely high cooling rate in the molten pool, which is three orders of magnitude smaller than that in traditional directional solidified parts. The microstructure consists of the long striped γ and γ’ phase in the dendritic trunk and the granular NiMo phase particles at the interdendritic regions and grain boundaries. This unique microstructure of aligned γ and γ’ phase is distinct from the cubic γ’ phase precipitated in γ matrix in conventionally manufacturing since PDAS was significantly refined to a scale of precipitated phase, thus the solid-state transformation of γ to γ’ occurs epitaxially from the long striped peritectic γ’ phase at interdendritic regions. The solidification cracking appears at high angle grain boundaries (HAGB) with dendritic morphology, where Mo, Ta and Re elements are enriched and many granular NiMo phase forms. Decreasing content of Mo element is a method to reduce NiMo phase at grain boundary which can eliminate crack.