Microstructural Design of Ni-base Superalloys by Hot Isostatic Pressing

Single-crystal Ni-base superalloys (SXs) are used as a first-stage blade material in high-pressure turbines for aero engines or in stationary gas turbines. They operate at temperatures close to their melting point where they have to withstand mechanical and chemical degradation. Casting and extensive solution heat-treatments of such blades introduce porosity that can only be reduced by hot isostatic pressing (HIP). Recent developments in HIP plant technology enable simultaneous HIP-heat-treatments due to rapid quenching at the end of such treatments. This work gives an overview of the opportunities that such a unique HIP offers for the solution heat-treatment of conventionally cast SXs or directionally solidified Ni-base superalloys fabricated by selective electron beam melting (SEBM). The influence of temperature, pressure, and cooling method on the evolution of the y/y'-morphology and on the pore shrinkage is investigated. The cooling method has a strong impact on the y'-particle size and shape whereas the combination of temperature and pressure during the HIP-treatment mainly influences porosity reduction. In a final approach a HIP treatment is satisfactorily used to fully re-establish the y/y'-microstructure after high-temperature creep degradation.