Study of Fatigue Crack Propagation in Powder Metallurgy Hot Isotatically Pressed Nickel-Base Alloy

摘要

L.C. Astroloy contains a complex distribution of rounded cuboidal gamma prime precipitates (0.1 micrometers edge dimension), cooling gamma prime(0.01 micrometers dia) and prior particle boundary particles of MC, ZrO2, alpha-Al2O3 and M3B2; the majority of MC carbides were not found in conjunction with ZrO2 particles as was previously assumed. Delta K sub th value were found to vary with specimen geometry with lower values being associated with more symmetrical specimen geometries. With four-point bend samples, Delta K sub th increased with increasing grain size, in agreement with previous findings. The present work has identified several anomalies which exist with respect to crack closure measurements and their correlation of FCP. rates. Closure values differed markedly in repeated K-decreasing threshold determinations even though fatigue crack growth rates remained unchanged. The introduction of an artificial asperity in the wake of the crack tip in a 2024 aluminum alloy gave rise to large measured values of closure which had little influence on the crack growth rates; apparently, crack closure had little influence on associated crack growth rates.