Influences of PPB, PPB affect zone, grain boundary and phase boundary on crack propagation path for a P/M superalloy FGH4096

Abstract

The fatigue crack growth (FCG) tests for hot isostatic pressed (HIPed), isothermal forged (IFed) and heat treated (HTed) FGH4096 superalloys were conducted in air at 650–750 °C with and without dwell time to investigate the effect of microstructural characteristics on FCG paths. Optical microscope (OM) and scanning electron microscope (SEM) with energy dispersive spectrometer (EDS) were used to observe and analyze the microstructures. After FCG tests, fracture surfaces and central sectioned surfaces were observed to investigate the crack propagation paths. FCG rate curves were also plotted to assist the analysis of FCG behaviors. The results indicated that prior particle boundaries (PPBs) were prevalent in HIPed specimens, but they transformed to PPB affect zones (PAZs) in IFed specimens by isothermal forging. After the standard heat treatment, PAZs were eliminated, and grain boundaries which were decorated with continuously distributed carbides became prevalent in HTed specimens. Grain boundaries in HTed specimens made the greatest contribution to FCG, and the highest FCG rate was attributed to the predominant intergranular failures. PPBs also had a pronounced influence on FCG, and the dominant failures of interparticle resulted in the high FCG rate in HIPed specimens. However, PAZs made the least contribution to FCG mainly owing to their particular microstructures. Thus, the prevalent failures of PAZs resulted in the lowest FCG rate in IFed specimens. Phase boundaries of large γ′ precipitates in PPBs or PAZs also had accelerating effect on FCG, while their contributions were relatively small.