Abstract

The object of the present work has been to compare the elevated temperature fatigue characteristics of APK-6 manufactured by the direct extrusion of loose powder (DEJLP) with that which has been directionally recrystallized (DR). DE/LP material consists of 20 pm grains with high-angle boundaries containing massive y' phase, whereas DR has a x110> fibre texture of low-angle longitudinal boundaries, which are not associated with coarse y' precipitation. LCF tests were conducted with R = -1 at 300, 500 and 700% on the DE/LP material, and DR material has been tested at 700°C. It is shown that the elastic strain amplitude dominates the fatigue life over most of the data, arising from the combination of high yield strength and low ductility exhibited by these materials. The DE/LP material showed prolific subsidiary grain boundary cracks. The main fracture was predominantly transgranular in character, however, lying parallel to active slip bands. The DR material fails wholly transgranularly, with the fracture appearing to nucleate on intense slip bands. It is suggested that, due to the coarse grain size, crack nucleation occurs after fewer cycles in DR than in DE/LP material, but propagation to final failure is slower due to the longer, highly serrated crack profile imposed by the grain dimensions. The result is that the total LCF lives of the two microstructures are comparable at 700°C. Superalloys 1992 Edited by S.D. Antolovich, R.W. Stusrud, R.A. MacKay, D.L. Anton, T. Khan, RD. Kissinger, D.L. Klarstmm The Minerals, Metals & Materials Society, 1992

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