Low cycle fatigue characteristics of 〈001〉 and randomly aligned superalloy single crystals

Abstract

The influence of microstructure on the low cycle fatigue behavior in a model single crystal superalloy has been measured for 〈001〉 aligned as well as randomly oriented crystals. Microstructures produced by three heat treatments in 〈001〉 aligned as well as randomly oriented crystals. Microstructures produced by three heat treatments in 〈001〉 crystals; namely fine γ′, coarse γ′ and a strain aged γ′ structure were tested under constant total strain amplitude conditions at temperatures ranging from 650 to 1000°C. Randomly oriented specimens having the coarse γ′ precipitates were tested at 870°C. SEM evaluation of the fracture surfaces showed fatigue crack initiation related to microporosity below 980°C and oxidation cracking at greater temperatures. No significant effect of the γ′ rafting on LCF life was observed. All 〈001〉 aligned crystals had cycle lives similar to other directionally solidified superalloys under similar conditions, a possible indication of similar failure mechanisms. Anisotropic fatigue crack growth patterns were identified in 〈001〉 crystals with cracks growing fastest in the 〈011〉 direction and a model was developed to predict this phenomenon. The cycle lives of randomly oriented crystals were observed to increase linearly with decreasing modulus under bulk elastic conditions and yield strain conditions were developed to explain these results.