Microstructural evolution and grain refinement in an intermetallic titanium aluminide alloy with a high molybdenum content

As a strong β-stabilizing alloying element, Mo has gained importance for intermetallic β/γ-TiAl alloys. In general, TiAl alloys containing a significant volume fraction of the disordered body-centered cubic β-phase exhibit improved processing characteristics during hot-working. To increase the understanding of the alloying effect of Mo, a model alloy with the chemical composition Ti-44Al-7Mo-0.1B (in at.%) was investigated. In this work, the microstructural evolution after individual heat-treatment steps was studied by means of scanning as well as conventional and in-situ transmission electron microscopy. Additionally, macro-hardness and nanoindentation measurements were conducted to study the change in hardness due to grain refinement and solid-solution hardening. The variation of the observed macro- and nano-hardness corresponds well with the microstructural evolution. The obtained grain refinement effect leads to a significant increase in the macro-hardness, whereas the increase in the average nano-hardness of the individual phases is related to solid-solution hardening.