The effect of directional recrystallization on the low cycle fatigue response of a powder metallurgy nickel-based superalloy at elevated temperatures

A comparison has been made between the LCF response of APK-6 given only a standard heat treatment (HT material) with that which has additionally been directionally recrystallized (DR material). Temperatures between 300 and 900 °C have been employed and the modes of failure compared investigated by optical and scanning electron microscopy. HT material consists of 20 μm grains with high-angle boundaries containing massive γ′ phase, whereas DR has a 〈110〉 fibre texture of low-angle longitudinal boundaries which are not associated with coarse γ′ precipitation. For HT material a continuous fall in the LCF life is seen between 300 and 900 °C, with the greatest reduction in properties occurring in the range from 700 to 900 °C. This is attributed both to a loss of ductility and to a fall in the fracture strength of the material, arising from enhanced grain boundary oxidation damage. The LCF life of the DR alloy at 700 °C is comparable with that of the HT alloy, despite a differing mode of failure. At 900 °C the DR material shows superior LCF behaviour. This is related to a higher fracture strength of the DR material compared to HT material at this temperature.