On the influence of stress state on rafting in the single crystal superalloy CMSX-6 under conditions of high temperature and low stress creep

Abstract

Single crystal superalloys have excellent high temperature strength and oxidation resistance and are therefore used as blades in aircraft engines and land based gas turbines. The microstructure of these alloys consists of two phases: (i) a high volume fraction of coherently precipitated {gamma}{prime}-cubes (L1{sub 2}) which strengthen the material separated by (ii) thin channels of face centered cubic (f.c.c.) {gamma}-matrix, as shown. However, microstructural instability (i.e. {gamma}{prime}-coarsening) is observed in these alloys during high temperature creep deformation (T > 900 C). In creep, coarsening of particles is generally considered as a softening process which increases the creep rate of an alloy. However, this process does not always dominate the overall materials response to creep loading. Thus, Mughrabi et al have clearly shown that {gamma}{prime}-coarsening can occur while the overall creep rate decreases. A number of studies on the high temperature creep deformation behavior of various single crystal superalloys have been reported in recent years. To explain rafting the authors here consider the case of a {gamma}/{gamma}{prime}-microstructure with negative misfit, i.e. where the lattice constant of the ordered {gamma}{prime}-phase is smaller than the lattice constant of the {gamma}-phase.