Deformation and strength behavior of two nickel-base turbine disk alloys at 650 °C

Two powder metallurgy nickel-base turbine disk alloys, RENE'95* and KM4, were studied for strength and deformation behavior at 650 degreesC. Two classes of microstructures were investigated: unimodal size distributions of gamma ' precipitates with particle sizes ranging from 0.1 to 0.7 mum and commercially heat-treated structures with bimodal or trimodal size distributions of gamma ' precipitates. The strength and deformation mechanisms were heavily influenced by the microstructure. In both alloys, deformation during compression tests consisted of a combination of a/2(110) antiphase boundary (APB)-connected dislocation pairs and a/3(112) partials creating superlattice intrinsic stacking faults (SISFs). In unimodal alloys, the fault density increased with decreasing particle size and decreasing strain rate. These trends, observed in compression testing, are consistent with earlier studies of similar alloys, which were tested in creep. As the gamma ' size was reduced, the nature of the faults changed from being isolated within single precipitates to being extended across entire grains. Commercially heat-treated alloys, containing a bimodal distribution of gamma ' particles, exhibited significantly more faulting than unimodal alloys at the same cooling gamma ' size. This augmentation of the faulting in commercial alloys was apparently due to the presence of the fine, aging gamma ' particles. The two typical commercial heat treatments (supersolvus and subsolvus) resulted in different deformation structures: the subsolvus behavior was similar to that of unimodal alloys with gamma ' sizes between 0.2 and 0.35 Am, while the supersolvus deformation was similar to that of unimodal alloys with the 0.1 am gamma ' size. These differences were attributed to differences in the size of the fine, aging gamma ' particles. Creep deformation in a commercially heat-treated material at 650 degreesC occurred solely by SISF-related mechanisms, resulting in a macroscopic slip vector of 112. The effects of alloy chemistry, APB energy, and microstructure on the deformation and mechanical behavior are discussed in detail, and possible effects of the faulting mechanisms on the mechanical behavior are explored. Finally, models for yield strength as a function of microstructure, for bimodal alloys with large volume fractions of precipitates are found to be in need of development.