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

Two powder metallurgy nickel-base turbine disk alloys, RENE'95* and KM4, we re studied for strength and deformation behavior at 650 degreesC. Two class es of microstructures were investigated: unimodal size distributions of gam ma ' precipitates with particle sizes ranging from 0.1 to 0.7 mum and comme rcially heat-treated structures with bimodal or trimodal size distributions of gamma ' precipitates. The strength and deformation mechanisms were heav ily influenced by the microstructure. In both alloys, deformation during co mpression tests consisted of a combination of a/2(110) antiphase boundary ( APB)-connected dislocation pairs and a/3(112) partials creating superlattic e intrinsic stacking faults (SISFs). In unimodal alloys, the fault density increased with decreasing particle size and decreasing strain rate. These t rends, observed in compression testing, are consistent with earlier studies of similar alloys, which were tested in creep. As the gamma ' size was red uced, the nature of the faults changed from being isolated within single pr ecipitates to being extended across entire grains. Commercially heat-treate d 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 apparen tly due to the presence of the fine, aging gamma ' particles. The two typic al commercial heat treatments (supersolvus and subsolvus) resulted in diffe rent deformation structures: the subsolvus behavior was similar to that of unimodal alloys with gamma ' sizes between 0.2 and 0.35 Am, while the super solvus deformation was similar to that of unimodal alloys with the 0.1 am g amma ' size. These differences were attributed to differences in the size o f the fine, aging gamma ' particles. Creep deformation in a commercially he at-treated material at 650 degreesC occurred solely by SISF-related mechani sms, resulting in a macroscopic slip vector of 112. The effects of alloy ch emistry, APB energy, and microstructure on the deformation and mechanical b ehavior are discussed in detail, and possible effects of the faulting mecha nisms on the mechanical behavior are explored. Finally, models for yield st rength as a function of microstructure, for bimodal alloys with large volum e fractions of precipitates are found to be in need of development.