The Effects of Machined Topography on Fatigue Life of a Nickel based Superalloy

The surface topography of engineering components is known to affect fatigue behaviour. However, defining topography based on the average roughness (Ra) parameter is considered to be inadequate when correlating to fatigue performance. Alternative correlations utilising areal topographic parameters would therefore be valuable. This has been addressed through areal topographic measurements and fatigue testing of an advanced nickel based superalloy, Alloy720Li. Low cycle and high cycle fatigue tests were performed at ambient temperature on specimens manufactured via different turning parameters to obtain S-N curves for different finishes. Supporting fractography is presented to emphasise the role of surface topography on fatigue crack initiation. The effects of different turned surface finishes on fatigue crack initiation are demonstrated and discussed.