Microstructure and texture effect on the thermal expansion of a variously aged polycrystalline superalloy IN738LC

Thermal-expansion measurements of a suitably heat-treated and aged polycrystalline superalloy, IN738LC, with various γ′ (Ni3(A1, Ti, Nb)) precipitate microstructures and annealing textures, were carried out using procedures given in ASTM E228. The preferred orientation (PO) and elasticity modulus of the alloy under the different microstructural conditions are correlated to the thermal expansivity obtained. Thermal expansion was found to decrease with decreasing γ′ precipitate size. The microstructures with coarse and medium-sized precipitates, with the 〈100〉-PO (soft directions in the fcc Ni-based alloys) and low elasticity modulus values, yield the highest thermal expansion. The microstructure with fine-sized precipitates has a lower expansion coefficient at all temperatures, while the duplex-size (fine + medium) precipitate microstructure and the single-phase solution-treated, supersaturated solid solution (SSS) condition show the lowest expansion coefficients. The low expansivity is attributed to the prevalence of the 〈111〉 and/or 〈131〉 POs in these specimens for the matrix phase and the expansion being along these relatively harder directions. Internal constraints to expansion, which determine dα/dT, are postulated to result from dislocation substructures present in the microstructural constituents and at the precipitate-matrix interface.