LCF behaviour of a single crystal nickel-base syperalloy

Abstract

Low cycle fatigue behaviour of a single crystal nickel-base superalloy designated as DD8 has been investigated using both scanning electron microscope (SEMI and transmission electron microscope (TEM). The results are presented and discussed in this paper. Variation of stress amplitude with number of cycles for samples fatigue tested with a strain of 0.66% in fully reversed total strain control mode shows slight difference in the behaviour at ambient temperature and at 850°C. SEM examination of fractured surfaces shows initiation of fatigue crack at the external surface. Planar slip and cleavage facets are observed at both the temperatures. TEM examination reveals dislocation bands containing tangles, loops and dipoles. Substructure at higher temperature indicates beginning of the cell formation. An interesting feature observed is phase transformation induced by strain both at room temperature and at 850°C.