FATIGUE-CRACK GROWTH-CHARACTERISTICS OF A 80-NI 20-CR ALLOY - THE EFFECTS OF MEAN STRESS AND MICROSTRUCTURAL POROSITY

This particular paper describes a fatigue crack propagation study aime d at examining two differing aspects, viz. the influence of porosity a nd mean stress on the intermediate and threshold fatigue crack growth behaviour of a 80% Ni-20% Cr alloy. This particular alloy can be utili sed as a corrosion protective coating which is applied to relevant sur faces by a plasma spray technique. The microstructures of the plasma s pray coating considered in this study had a austenite matrix which con tained varying amounts of chromite type non-metallic inclusions and di screte porous regions; the true porosity of each plasma spray microstr uctures 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 thresho ld stress intensity range, DELTAK(th), levels. However, porosity did n ot exhibit any significant effects on DELTAK(th) level and little effe ct of mean stress or R-ratio on DELTAK(th) was also observed. This lac k of R-ratio effects was attributed to a combination of (a) the fine g rain of the plasma spray microstructures resulting from fast cooling d uring deposition and (b) the plane stress conditions under which the f atigue tests were conducted; both these factors inhibit any crack clos ure effects. The fatigue crack extension results from the present stud y showed good commonality with other data reported in the literature a nd the porosity effects on the fatigue crack propagation characteristi cs 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 pr eferentially encountered near porous regions and that this propensity increased with an increase in the maximum stress intensity at the crac k tip, K(max), of the fatigue cycle.