Fatigue crack growth characteristics of a 80 Ni-20 Cr alloy: the effects of mean stress and microstructural porosity

Abstract

This particular paper describes a fatigue crack propagation study aimed at examining two differing aspects, viz, the influence of porosity and mean stress on the intermediate and threshold fatigue crack growth behaviour of a 80% Ni-20% Cr alloy. This particular alloy can be utilised as a corrosion protective coating which is applied to relevant surfaces by a plasma spray technique. The microstructures of the plasma spray coating considered in this study had a austenite matrix which contained varying amounts of chromite type non-metallic inclusions and discrete porous regions; the true porosity of each plasma spray microstructures was taken as the summation of the chromite inclusions and the porous regions. It was observed that the presence of porosity markedly enhanced increased fatigue crack growth rates and reduced the threshold stress intensity range, Δ K th, levels. However, porosity did not exhibit any significant effects on Δ K th level and little effect of mean stress or R-ratio on Δ K th was also observed. This lack of R-ratio effects was attributed to a combination of (a) the fine grain of the plasma spray microstructures resulting from fast cooling during deposition and (b) the plane stress conditions under which the fatigue tests were conducted; both these factors inhibit any crack closure effects. The fatigue crack extension results from the present study showed good commonality with other data reported in the literature and the porosity effects on the fatigue crack propagation characteristics could be adequately explained by the elastic modulus values of the various plasma spray microstructures. A careful fractographic study of the fatigue fracture surfaces demonstrated extending fatigue crack preferentially encountered near porous regions and that this propensity increased with an increase in the maximum stress intensity at the crack tip, K max, of the fatigue cycle.