The mechanisms and temperature dependence of superlattice stacking fault formation in the single-crystal superalloy PWA 1480

Abstract

Deformation microstructures in PWA 1480 nickel-base superalloy single crystals were studied in the range of 20 °C to 1100 °C. Similar to previous investigations, superlattice stacking faults were observed after slow strain rate deformation at temperatures between 700 °C and 950 °C. Unlike previous studies, a high density of superlattice stacking faults was observed after deformation at 200 °C and below. The mechanisms of fault formation in the two temperature regimes were different. In the range of 700 °C to 950 °C, single isolated superlattice-intrinsic stacking faults (SISFs) were produced by the decomposition of an a/2(110) matrix dislocation in the γ/γ′ interface. The a/3(112) partial shears the particle, while the a/6(112) Shockley remains in the interface. At 200 °C and below, a high density of faults was produced on closely spaced parallel planes. The most common feature after deformation in this range is the faulted loop, which is most often observed to be a superlattice-extrinsic stacking fault (SESF). These low-temperature faults, along with their temperature dependence, were quite similar to those observed in single-phase Ll22 materials. The available evidence suggests that the low-temperature faults were produced by the dissociation of an a unit superdislocation into a pair of a/3 partials. The temperature dependence of the faulting (at low temperatures) was modeled by linear isotropic elasticity, and the results suggest that the SISF energy increases significantly from 20 °C to 400 °C. Multiplanar, overlapping superlattice faults were analyzed with respect to bond violations. This analysis suggested that an antiphase boundary (APB) on top of an SISF has a very high fault energy, similar to that of the complex stacking fault. Therefore, the presence of SISF loops on glide planes promotes further dissociation by the SISF scheme instead of the APB scheme and explains the high density of SESFs and microtwins observed in the deformation structures.