Influence of the γ/γ′ Misfit on the Strain-Age Cracking Resistance of High-γ′ Ni and CoNi Superalloys for Additive Manufacturing

A series of new printable Ni and Co–Ni high γ′ superalloysSuperalloys designed for additive manufacturingAdditive manufacturing have been evaluated for strain-age crackingStrain-age cracking (SAC) resistance. Printability studies and heat treatmentHeat treatment experiments were conducted to identify processingProcessing windows and characterize the overall resistance to SAC. High-resolution synchrotron X-ray diffractionX-ray diffraction experiments were performed to measure the γ and γ′ lattice parametersLattice parameter as a function of the temperature. All the superalloysSuperalloys tested were found to have a positive γ/γ′ misfitMisfit at room temperature which decreases as the temperature increases. The misfitMisfit of a SAC-prone alloy, 247LC, decreases rapidly and turns negative at about 600 °C, whereas the misfitMisfit of superalloysSuperalloys with intermediate resistance to strain-age crackingStrain-age cracking remains slightly positive. In the three most SAC-resistant superalloysSuperalloys, the misfitMisfit remains larger than 0.05% until at least 883 °C. The results show that a critical aspect for designing SAC-resistant alloys is ensuring that the misfitMisfit between γ and γ′ remains positive at all temperatures to generate compressive stresses on grain boundariesGrain boundary. Furthermore, the work also highlights a critical positive misfitMisfit value of 0.05% to prevent crackingCracking.