Implications of dislocation micromechanisms for changes in orientation and shape of single crystal superalloys

Abstract

The development and exploitation of gas-turbine blades cast from nickel-base superalloys in single crystal form has been one of the most successful industrial and commercial ventures relating to advanced structural materials over the last twenty years or so. The benefits of producing gas-turbine blades from single crystal superalloys has resulted from at least two factors. Firstly, the elimination of grain boundaries from the component has increased the creep ductility by removing sites where cavities develop and has allowed the chemistry of superalloys, designed specifically for use in the single crystal form, to be free of grain boundary strengthening elements thus increasing the melting temperature and potential operating temperature. Secondly, by controlling the orientation of the single crystal casting, normally having the low modulus direction parallel to the centrifugally stressed blade axis, thermal stresses are minimized and the thermal fatigue resistance is greatly enhanced. However, these advantages have been obtained at the expense of dealing with a highly anisotropic material. In order to exploit the full potential of single crystal superalloys, an effective anisotropic design strategy is required.