Development of ductile γ+α2 titanium aluminide through ingot metallurgy route, thermomechanical processing and characterization

Abstract

γ+α2 Titanium aluminide (Ti-28Al-10Nb-3Cr-0.02Bwt%) was processed through vacuum arc remelting (VAR) route using Ti sponge and pure alloying elements. Electrode for VAR melting which was designed to obtain uniform chemistry across the cross-section and throughout the height of the billet was realized through cold compaction. The alloy was hot worked in the temperature range of 1200–1100°C. Tensile properties of the alloy at ambient temperature were evaluated at different strain rates ranging from 10−4s−1 and 10−1s−1. The alloy exhibited ultimate tensile strength in the range of (386–553MPa) when tested at ambient temperature and strain rates of 10−4s−1 and 10−1s−1, whereas it varied from 630–673MPa at intermediate strain rates of 10−3s−1 and 10−2s−1. Percentage elongation is found to be 4.87–5.49% at strain rates of 10−4s−1 and at 10−1s−1 whereas it showed a relatively higher % elongation (5.42–7.19%) at intermediate strain rates of 10−3s−1 and 10−2s−1, indicating intermediate strain rates are better for optimum strength as well as for ductility.The alloy is found to have predominantly two phases γ (TiAl) and α2 (Ti3Al) with moderately random texture and presence of small volume fraction of β-phase. Microstructure showed presence of varying width of α2+γ lamellar structure. Transmission electron microscopy (TEM) confirmed presence of extensive twinning, dislocations and stacking faults with two phase structure indicating twinning has played a significant role in deformation. Presence of minor amount of β phase has been found to be contributing factor for improvement in ductility. Fracture analysis shows failure was mainly by rupture of lamellar interfaces. Occasional deformation of lamellae is also noticed in the uniaxial tensile tested samples at intermediate strain rates of 10−3s−1 and 10−2s−1 wherever small increase in ductility is observed. Marginally lower ductility at lower strain rate is due to growth of microcracks at interlamellar sites of dissimilar orientation and at higher strain rate due to faster growth of microcracks at lamellae of similar orientation.