Effects of Defects and Microstructure on the Elevated Temperature Fatigue Property of P/M Superalloy MERL76

Elevated temperature fatigue tests were conducted on P/M HIP'ed superalloy, MERL76. The defects, such as preciptitates on the prior particle boundaries (PPBs), gas-porosities and nonmentallic inclusions, were found at fatigue origins. The relation between defect size, ai, and fatigue life, Nf, was independent of defect types and was formulated as Nf·Δσmeq=h·a1-m/2i Δσeq=Δσ√1+2Eg(n)/n+1·Δεp/Δσ where m: the exponent in crack propagation law, da/dN=CΔKm, h: a material constant, E: Young's modulus, g(n): a function of cyclic strain hardening exponent, n, and Δεp: plastic strain range. The relation between threshold stress, Δσth, and defect size, ai, was also expressed as ΔKth=2/π·Δσth√π(ai+a0) where ΔKth: threshold stress intensity factor for long cracks. The value, a0, was about 0.1mm, which coincided with the size estimated by an experimental relationship between grain size and a0 on steels. The usage of Ar-treated powders and the hot-work after HIP'ing improved fatigue life since they prevented PPB failure. Furthermore, the usage of fine powders is preferable because it can reduce the size of pores and inclusions.