High cycle fatigue of tantalum carbide reinforced nickel base eutectics at room temperature

Abstract

The high cycle fatigue response of two advanced tantalum carbide strengthened eutectic superalloys has been determined at room temperature. Since these alloys will be coated in service, the effects of variables associated with coating processes were given special attention. Both alloys showed a well defined fatigue limit. It was concluded that the maximum stress obtained in the cycle at the fatigue limit coincided with the tensile stress at which the matrix yielded. Above this stress level cyclic deformation of the matrix resulted in fiber failure, a necessary precursor of specimen failure. Detailed observation of the sequence of events leading to fiber failure and subsequent early crack growth in the matrix over a broad range of alturnating stresses confirmed that fiber fracture was the critical crack nucleation step in high cycle fatigue failure of the two alloys. It was shown that several changes in the surface condition did not affect the fatigue life of the alloys. However, when the samples were prestrained to crack the carbide fibers, they failed when cycled, at alternating stresses below the fatigue limit for virgin material. A similar loss of fatigue life was observed after heat treatment of one of the alloys. Fractographic examination indicated that this degradation resulted from enhanced crack nucleation at sigma platelets which were present as a result of the heat treatment.