The microstructure evolution and strengthening mechanism of a γ′-strengthening superalloy prepared by induction-assisted laser solid forming

The microstructure evolution and strengthening mechanism of a γ′-strengthening superalloy prepared by induction-assisted laser solid forming (LSF) were clarified by an investigation of the LSFed IN-738LC alloy with different solution treatments (at 1070 °C, 1120 °C and 1160 °C). The results show that both the as-deposited and heat-treated deposits were dominated by the columnar grains with a width of 100–270 μm. The equiaxed grains mainly originated from the columnar-to-equiaxed transition (CET) during LSF while a small portion of the equiaxed grains originated from recrystallization during the heat treatment, due to the relatively low temperature gradient in the induction-assisted LSF. Fine grain strengthening was still working in the columnar-grains dominated LSFed IN-738LC deposits. However, γ′ strengthening was found to play a major role in the strengthening mechanism by comparing the microstructure and properties of different heat-treated deposits. The γ′ dissolution and coarsening kinetics were calculated and the interaction mechanism between the dislocation and γ′ phase in LSFed IN-738LC was clarified. The deformation of the coarse (d > 200 nm) near cubic γ′ was found to be beneficial to the increase of ultimate tensile strength (UTS) and elongation (EL), while a brittle continuous phase formed at the grain boundary (GB) which was rich in Ni, Cr, Al and Ti was found to be detrimental to the deposit's plasticity. The γ′ phase deformation and desquamating, carbide breaking and GB phase breaking are the predominant fracture failure mechanisms. On the basis of relatively high strength, the IN-738LC deposits with a lower yield strength (YS)/UTS ratio than the cast and SLMed ones were obtained when solution-treated at 1070 °C.