Effect of grain boundary serration on the fatigue, creep and cyclic creep strengths of a nickel-base superalloy in air and in hot corrosive environment.

Abstract

Fatigue, creep and cyclic creep tests have been carried out at 800°C for the variously heat-treated nickel-base superalloy Inconel 751. The heattreatments given are simple aging (SA), two-stages solution treatment (TS) and direct aging (DA), with and without coating of synthetic ash mixture composed of 90% Na2SO4 and 10% NaCl. Effect of grain boundary topography, particularly of the serrated grain boundary produced by the direct aging, on the strength properties is, then, investigated.It is clarified that hot corrosion can alter the fracture mechanism completely. In hot corrosive environment, a stress-enhanced aggressive intergranular penetration of sulfides, oxides and/or chlorides can cause a grain boundary weakening to induce a premature intergranular fracture in a brittle manner regardless of the grain boundary topography. In particular, the cyclic creep is shown to result in minimization of failure life in hot corrosive environment because the combined creep and fatigue loadings promote the aggressive intergranular attack, in contrast to the case in air in which the failure life is mainly determined by the creep damage.In general, the effect of grain boundary topography on the strength properties is significant in hot corrosive environment rather than in air. In hot corrosive environment, both the chromium-depleted zone and the straight grain boundary appear to enhance the aggressive intergranular attack, because the former can induce the low melting nickel sulfides formation and the latter is easily subjected to the locally increased strain concentration so as to provide a stress-activated path. The development of the chromiumdepleted zone tends to give a detrimental effect on the strength properties under all the loading conditions, while the detrimental effect of the straight boundary is more pronounced under the creep-dominant loading conditions. Consequently, in order to improve the strength properties in hot corrosive environment, it is important to eliminate such undesirable microstructures associated with the grain boundary, depending on the loading conditions.It is also confirmed that even in hot corrosive environment, the serrated grain boundary contributes notably to improve the strength under creepdominant loading conditions by virtue of the suppression of such an aggressive intergranular attack.