Single crystal nickel base superalloys are used to make turbine blades in gas turbines for power plants and aero engines. During service they are exposed to high stresses and temperatures and suffer creep, which limits their service life. The initial material contains a large volume fraction of cuboidal particles which consist of the ordered L12 phase known as γʹ (70-80%). The particles are separated by thin channels which consist of a solid solution phase, γ, that has a fcc crystal structure. The high temperature deformation is mainly caused by dislocation plasticity. In the present study diffraction contrast and analytical transmission electron microscopy have been used to study elementary processes which govern the evolution of the microstructure and which so far have only received limited attention.
In the present work, a number of questions were addressed such as how alloy elements partition between dendrites and interdendritic regions (large scale) and between γʹ particles and γ channels (small scale), how small γ particles form inside large γʹ particles and how small γʹ particles nucleate and grow in γ channels. The microstructure of a single crystal superalloy evolves during creep. The evolution of microstructures during high temperature and low stress creep tests and after annealing was investigated. Ledges and grooves form at the γ/γʹ interfaces. The presence of ledges and grooves at the γ/γʹ interfaces is an important observation which has been addressed by only a few researchers. Ledges and grooves are associated with dislocation segments. In the current work these grooves/dislocation-assemblies receive special attention. In addition, microstructures which form during selective electron beam melting (SEBM) were characterized.