Creep fracture of complex turbine alloy under fluctuating stress

Abstract

In this paper the influence of superimposing a fluctuating stress on the tensile creep behaviour of Inconel alloy X-750 at 750°C is discussed. Cyclic axial stresses from 0 to ±82·2 MN/m² were superimposed upon specimens of varying grain size (9–200 μm average dia.) which were subjected to a mean axial stress of 411 MN/m². Fracture life was markedly improved by the superimposition of small cyclic stresses (e.g. for a grain size of 200 μm and a cyclic stress of ±6·9 MN/m², life was increased by a factor of 17). With increasing cyclic stress, the improvement in life was less significant, and life was virtually unaffected by the superimposition of a cyclic stress of 82·2MN/m². Further increases in the superimposed cyclic stress reduced life: the tensile strength of the alloy was about 620 MN/m² at 750°C. This behaviour has been explained in terms of the reduced rate of interlinkage and propagation of intergranular cracks owing to crack blunting at small superimposed cyclic loads. The microstructural variable of grain size has been studied, and it was observed that fatigue life increased as the grain size increased, according to the equation N_f=aD^b, where N_f is the number of cycles to failure, D is the grain size, the exponent b is about 0·6, and a is a function of the cyclic stress level. The results emphasize the significance of strain and the final crack-linkage process in controlling high-temperature intergranular embrittlement.