Effects of microstructure on room temperature fatigue crack initiation and short crack propagation in Udimet 720Li Ni-base superalloy

Abstract

An assessment of the effects of microstructure on room temperature fatigue crack initiation and short crack propagation in a Ni-base superalloy is presented. The assessment was carried out on microstructural variants of U720Li, including as-received U720Li, U720Li-LG (large grain variant) and U720Li-LP (large intragranular coherent γ′ variant). Fatigue tests were carried out at room temperature using a 20 Hz sinusoidal cycling waveform on plain bend bars. Tests were conducted in 3-point bend under load control with an R-ratio of 0.1. A maximum load of 95% σ y was used in all tests. Room temperature fatigue crack initiation was noted to occur due to slip band cracking and from porosity on or just beneath the surface in all materials. Crack propagation was noted to be highly faceted (due to planar slip band cracking) immediately after crack initiation followed by a transition to a flatter Stage II type crack path as crack length increases. U720Li-LP was noted to show the longest fatigue lifetime, followed by U720Li-LG while U720Li shows the shortest life. The longer lifetime of U720Li-LP was linked to a higher resistance to both fatigue crack initiation and short crack propagation. U720Li and U720Li-LG show approximately similar crack initiation resistance although U720Li-LG showed slightly improved short crack growth resistance. The observations have been rationalised in terms of the microstructural characteristics of the materials, and it is believed that larger grain size, larger coherent γ′ precipitate size and higher volume fractions of both coherent and primary γ′ precipitates will improve overall fatigue lifetimes in PM Ni-base alloys which exhibit planar slip characteristics at room temperature.