Solidification microstructure and tensile deformation mechanisms of selective electron beam melted Ni₃Al-based alloy at room and elevated temperatures

Selective electron beam melting (SEBM) was used to process crack-free Ni₃Al-based IC21 alloy (low density superalloy) containing ~85% γ′-volume fraction. There are distinct differences between dendrites and inter-dendritic regions with the presence of coarse γ+γ′ eutectic and secondary solidification microconstituents (Cr and Mo-rich) in the latter. The pronounced inter-dendritic eutectic regions suggest that a significant elemental partitioning between the liquid and solid occurred during the SEBM. The terminal liquid is trapped at boundaries between dendrites and grains, as evidenced by the liquid films on cracked surfaces. In contrast to extensive studies indicating the segregation of Zr and B, we show unambiguously the segregation of Si to low melting point liquid films and thereby enhancing the susceptibility to solidification cracking in IC21 produced by SEBM. The tensile specimens extracted from the crack-free IC21 samples exhibit superior properties at room temperature (RT) and 1000 °C. The RT deformation mechanism is characterised by cutting γ′-phase with two paired dislocations and antiphase boundaries in between. At 1000 °C tensile deformation, the well-developed γ/γ′ interfacial dislocation networks are in good agreement with their promising high-temperature performance (σ_y = 518 ± 10 MPa, σ_UTS = 560 ± 16 MPa, 20.5% for ductility).