Achieving superior superplastic properties in fine grained intermetallic alloys based on γ-TiAl + α2‑Ti3Al

The present paper is a brief review of superplastic behavior of intermetallic alloys based on γ-TiAl+α2-Ti3Al with varying niobium content and refined microstructure obtained by hot working. Three γ+α2 alloys having the nominal compositions Ti-44.2Al-1.9Cr-0.7Nb-0.4B, Ti-45Al-8Nb-0.2C and Ti-43.7Al-4.2Nb-0.5Mo-0.2B-0.2C (at.%) are considered in the paper. The hot working included quasi-isothermal hot forging or extrusion in the α+γ or α+β(B2)+γ phase field and hot forging at T=950°C. The hot working led to formation of refined partially rectystallized/globularized microstructures in the Ti-44.2Al-1.9Cr-0.7Nb-0.4B and Ti-43.7Al-4.2Nb-0.5Mo-0.2B-0.2C alloys and fully ultrafine grained microstructure in the Ti-45Al-8Nb-0.2C alloy. The fine grained materials were used to prepare specimens for tensile testing. The tensile tests were performed at T=800-1000°C and έ~10-4-10-3 s-1 in air without any protection against oxidation. Superplastic elongations and higher values of the strain rate sensitivity coefficient (m>0.3) were reached at T=850-1000°C for all alloys under study. The comparison of the superplastic properties of the alloys suggests that superplastic elongations were significantly higher owing to more intensive hot working and in the alloys with higher niobium content and volume fraction of the α2 phase. Microstructure examination showed that the last two factors provided slow dynamic grain growth during superplastic flow leading to sustainable superplastic flow and superior superplastic elongations (δ>1000%) in the Ti-43.7Al-4.2Nb-0.5Mo-0.2B-0.2C and Ti-45Al-8Nb-0.2C alloys. Particularly, incredible high superplastic elongations (δ=1270-2860%) were attained in the Ti-43.7Al-4.2Nb-0.5Mo-0.2B-0.2C alloy at T=900-1000°C. The obtained results show that slow dynamic grain growth during superplastic flow is the key precondition for achieving superior superplastic properties in fine grained γ+α2 alloys.