Damage Tolerance of P/M Turbine Disc Materials

The influence of heat treatments on the microstructure and mechanical properties of a powder metallurgy (P/M) turbine disc alloy MERL76, was studied. A damage tolerant microstructure (DTM) was designed for the alloy using a modified heat treatment (MHT) sequence. Comparisons of mechanical properties between the DTM and the conventional microstructure (CM) were made on the basis of tensile strength, low cycle fatigue (LCF) life, creep rupture life, and fatigue crack growth rates (FCGRs). The DTM significantly improved the creep rupture life (310 times) at 760°C but resulted in a marginal loss. in tensile strength. The FCGR tests showed that FCGRs of the DTM at 700°C were much lower than the CM material by a factor of 10 at a frequency of 0.15 Hz whereas the FCGRs at a frequency of 1 Hz were comparable over a wide range of AK values. The LCF lives of the two microstructures at 700°C were also comparable.