Influence of upset forging and heat treatment on the microstructure and mechanical properties of a new β-solidifying γ-TiAl alloy

Abstract

Our previous works revealed that alloying of a β-solidifying intermetallic γ-TiAl alloy with Zr and Hf instead of Nb led to improvement in the strength and creep resistance due to effective solid solution hardening, a strong partitioning preference γ>α2 for Zr and a lower lattice misfit of the γ-TiAl and α2-Ti3Al phases. On this basis, a new β-solidifying γ-TiAl alloy with a composition Ti-44Al-X(Nb,Zr,Hf)-0.15B (at.%) (designated as TNZ γ-alloy) has been designed. The as-cast ingot had a fine lamellar structure with a small amount of the metastable β(βo) phase. Compression tests performed at T = 950–1200 °C showed that the new alloy possessed good forgeability even at 950 °C without signs of strain localization. The alloy ingot was subjected to upset forging at T = 950 °C followed by heat treatments. The applied processing led to formation of refined duplex and lamellar type structures almost free of the β(βo) phase. The obtained microstructural conditions were examined and then the tensile and creep tests were performed. The mechanical tests showed that with increasing the lamellar constituent and decreasing the lamellar spacing the creep resistance expectedly increased and ductility decreased. The obtained results are discussed from the viewpoint of processability, mechanical properties and oxidation resistance of the new alloy.