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 polycrys talline superalloy, IN738LC, with various gamma' (Ni-3(Al, Ti, Nb)) precipi tate microstructures and annealing textures, were carried out using procedu res given in ASTM E228. The preferred orientation (PO) and elasticity modul us of the alloy under the different microstructural conditions are correlat ed to the thermal expansivity obtained. Thermal expansion was found to decr ease with decreasing gamma' precipitate size. The microstructures with coar se and medium-sized precipitates, with the [100]-PO (soft directions in the fee Ni-based alloys) and low elasticity modulus values, yield the highest thermal expansion. The microstructure with fine-sized precipitates has a lo wer 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 co efficients. The low expansivity is attributed to the prevalence of the [111 ] and/or [131] POs in these specimens for the matrix phase and the expansio n being along these relatively harder directions. Internal constraints to e xpansion, which determine d alpha/dT, are postulated to result from disloca tion substructures present in the microstructural constituents and at the p recipitate-matrix interface.