Study On Prior Particle Boundary (PPB) Precipitation In P/M Superalloy FGH96

The second-generation powder metallurgy (P/M) superalloy FGH96 has been applied to produce turbine disc for high thrust-weight ratio engine. This alloy has shown the advantages of superior creep strength, crack propagation resistance and damage tolerance. Prior particle boundary (PPB) is one of the three major defects in P/M superalloys, which is also found in as-HIPed alloy FGH96. The main composition of PPB precipitation in this alloy was identified in the present paper. Based on thermodynamic calculation, the measures such as sub-solidus diffusion annealing and sub-solidus hot isostatic pressing (SS-HIP) were adopted in order to reduce PPB precipitation. Finally, mechanical properties of both kind of samples treated as per standard heat treatment were evaluated.The results of EDS (energy dispersive spectrometer) analysis indicate that PPB precipitation in as-HIPed alloy FGH96 is constituted of MC carbides enriched with Ti and Nb as well as some oxycarbides. According to thermodynamic calculation, partial carbides can be dissolved if the alloy is treated at very high temperature. Although PPBs cannot be completely eliminated when the alloy was treated at 1230℃for 8h/AC, large numbers of carbides at PPBs dissolved into matrix. The precipitates at PPBs became discrete particles rather than continuous. The grain size have grown larger (the mean size is 44.53μm) and become uniform when the alloy was treated as per the schedule of standard heat treatment, and better microstructure with serrated grain boundary was obtained. It seems that SS-HIP process has little effect on reducing PPBs of alloy FGH96. Although the precipites at PPBs were reduced and became smaller, their distribution were more dense compared to the sample fabricated by the former process. The microstructures of these samples exhibit finer average grain size (about 32.98μm), while the distribution is less homogeneous.Mechanical properties results show that the samples prepared by SS-HIP process and those ones treated as 1230℃×8h/AC have met the specification requirements, except the stress rupture ductility at 650℃/970MPa of the samples with high temperature treatment. The similar or superior properties have been got by the two processes compared to conventional process.