Growth behaviour of small surface cracks in inconel 718 superalloy

Abstract

Abstract In order to study the fatigue damage of the Ni-base superalloy Inconel 718 at room temperature and 500°C, plain specimens were fatigued under constant stress amplitude and in two-step loading. Experimental results showed that the resistance to slip and crack initiation decreased due to the softening of the matrix at elevated temperature. With regard to the effect of the elevated temperature on the crack growth, the growth rate dl/dN of large cracks was accelerated, whereas the growth of microcracks less than 50μm was suppressed relative to R.T.. There was no effect of the stress change on the crack growth behaviour for low-to-high and high-to-low block loading at room temperature, nor for low-to-high block loading at 500°C. For high-to-low block loading at 500°C, however, the growth behavior of microcracks less than 50μm at the second stress level was affected by the first-stress revel. The results are discussed from the viewpoint of the softening of the matrix and of the oxide films formed at the elevated temperature.