Microstructural design and mechanical properties of a cast and heat-treated intermetallic multi-phase γ-TiAl based alloy

Advanced intermetallic multi-phase γ-TiAl based alloys, such as TNM alloys with a nominal composition of Ti-43.5Al-4Nb-1Mo-0.1B (in at.%), are potential candidates to replace heavy Ni-base superalloys in the next generation of aircraft and automotive combustion engines. Aimed components are turbine blades and turbocharger turbine wheels. Concerning the cost factor arising during processing, which - additionally to material costs - significantly influences the final price of the desired components, new processing solutions regarding low-cost and highly reliable production processes are needed. This fundamental study targets the replacement of hot-working, i.e. forging, for the production of turbine blades. But without forging no grain refinement takes place by means of a recrystallization process because of the lack of stored lattice defects. Therefore, new heat treatment concepts have to be considered for obtaining final microstructures with balanced mechanical properties in respect to sufficient tensile ductility at room temperature as well as high creep strength at elevated temperatures. This work deals with the adjustment of microstructures in a cast and heat-treated TNM alloy solely by exploiting effects of phase transformations and chemical driving forces due to phase imbalances between different heat treatment steps and compares the mechanical properties to those obtained for forged and heat-treated material.