Novel experimentation techniques to study damage accumulation and microstructural instability during creep of super alloy single crystals at high temperatures (>1000°C)

The objective of the present work is to demonstrate that it is well worth the effort to develop novel experimentation techniques to study creep in Ni base super alloy single crystals above 1000°C. A miniature tensile creep specimen and a double shear creep specimen have been developed. These are useful in high temperature technology because they can be directly taken out from cast turbine blades (miniature tensile creep specimens) and provide multiaxial creep data (double shear creep specimens). Most importantly, these new techniques can be used to increase the level of microstructural understanding of creep. Thus miniature tensile creep experiments can be used to demonstrate that <100> directions parallel to, and perpendicular to, the solidification direction differ in creep rupture behaviour. This is related to the presence of cast micropores which align along primary dendrites. The double shear creep specimens, on the other hand, help in understanding the influence of stress multiaxiality on rafting, the directional coarsening of the γ′ phase. In a macroscopic crystallographic shear system of type <110> {110}, rafting occurs at 45° to the shear direction and is governed by the maximum principal stress.