Combined effects of thin-section size, grain size and cavities on the high temperature creep fracture properties of a nickel-base superalloy

Abstract

The creep fracture characteristics of a conventionally cast (CC) MAR-M 002 superalloy, controlled by the grain-boundary diffusion mechanism, have been investigated at various specimen section-sizes D, and grain sizes, d. It is observed that the creep rupture strain (or ductility, eR, is controlled by the D2/(nGl) ratio, where nG is the number of grains per cross-section of specimen and l is the half-cavity spacing, at the creep conditions (900° C/ 300MPa). A rapid improvement in creep rupture life can be made by reducing the (dC/d)/D ratio [ or, equivalently, the (dCnG)/D2 ratio] below a critical value (∼ 100×10-8 10μm-1), where dC is the cavity size. The thin-section size dependent creep rupture life, tR/D, and creep rupture strain, eR/D, are explained on the basis of grain boundary sliding (GBS) and creep crack growth (CCG) behaviour of the alloy. eR/D and tR/D can be improved by reducing the GBS rate. A large improvement in tR/D can be achieved by reducing the GBS and CCG rates below the critical values of these rates by reducing the crack size through increasing the grain size above a critical value. (Above a critical grain size value the crack size becomes so small that, as a result, a large increment of tR is achieved.)