Influence of coatings and hot corrosion on the fatigue behaviour of nickel-based superalloys

Abstract

The fatigue behaviour of materials and components is strongly affected by the surface conditions, i.e. the surface finish and the materials properties in the near- surface region. Therefore any change in these conditions during fabrication or service may change the fatigue properties. It is common practice to coat turbine blades with materials that are resistant to hot corrosion. However, this treatment involves chemical and mechanical changes in the materials surface properties which could also change the fatigue properties of these components. The recommended heat treatment for the coating normally differs from that which produces the best mechanical or creep properties of the base metal and hence may cause additional changes in the fatigue properties. Chemical and mechanical changes in the surface conditions of an uncoated component may also take place during operation. Additionally, a large amount of notching may occur since oxidation and hot corrosion are not homogeneously distributed but take place preferentially at sites such as grain boundaries. In this work we investigate the effect of Pt-Al coatings on the high cycle fatigue (HCF) behaviour of the cast nickel-based alloys IN 738LC and IN 939 which are commonly used in large industrial gas turbines. A reduction in the fatigue life due to the coating was observed. However, the simultaneous occurence of hot corrosion attack and cyclic loading was much more detrimental. Fractographic and metallographic investigations showed that, in the as-coated condition, the crack initiation sites in the Pt-Al-coated alloys were internal pores situated just below the surface of the substrate. After aging or hot corrosion cracks initiate at the surface probably as a result of notch development by the attack. When HCF and hot corrosion are acting concurrently the coatings are expected to give a beneficial effect by protecting the surface from accelerated crack initiation.