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

Abstract

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, a_i, and fatigue life, N_f, was independent of defect types and was formulated as
N_f·Δσ^m_eq=h·a^1-m/2_i
Δσ_eq=Δσ√1+2Eg(n)/n+1·Δε_p/Δσ
where m: the exponent in crack propagation law, da/dN=CΔK^m, h: a material constant, E: Young's modulus, g(n): a function of cyclic strain hardening exponent, n, and Δε_p: plastic strain range.
The relation between threshold stress, Δσ_th, and defect size, a_i, was also expressed as
ΔK_th=2/π·Δσ_th√π(a_i+a₀)
where ΔK_th: 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.