Deformed microstructure of the single-crystal superalloy NASAIR 100 at 1050 °C

The microstructure of the [001] oriented single-crystal superalloy NASAIR 100, solidified respectively with the planar, cellular, coarse-dendritic, and fine-dendritic solid/liquid (S/L) interfaces, deformed at 1050 °C and 160 MPa, was investigated with the attempt to reveal the variation of γ′ structures in the necked zones of creep specimens and, furthermore, to explain that the dendritic single crystals constantly possessed superior stress-rupture lives. During the stress-rupture tests, it was found that directional coarsening of γ′ precipitates occurred and that γ′ rafts perpendicular to the tensile load axis, [001] orientation, formed. The γ′ rafts in the necked zones of tensile creep specimens were thickened significantly and were no longer perpendicular to, but gradually inclined to, the tensile load axis. Additionally, the γ′ rafts were much thicker and showed less perfect morphology in both the planar and cellular single crystals than in the dendritic single crystals. From the observation of dislocation configurations, it was evident that the shearing of γ′ precipitates was the main movement of dislocations when passing the γ′ particles. At the primary creep stages of both the planar and cellular single crystals, the slip of dislocations was inhomogeneous. However, the shearing of γ′ precipitates in the dendritic single crystals was homogeneous, resulting in higher creep resistance.