Alloying effects on superplastic behaviour of Ti-Fe-Al-Ni alloys

The aim of the present investigation was to develop a new titanium alloy with improved superplastic performance (higher elongation and lower forming temperature) through alloying with iron, nickel, and aluminium. A total of nine alloy compositions were used in which the iron content was fixed while nickel and aluminium contents were varied. Superplastic tension tests were conducted at various temperatures in the range 625-750°C with a strain rate of 10^-3 s^-1. Step strain rate tests were performed in a strain rate range 10^-4-2·5 × 10^-2 s^-1 at 700, 725, and 750°C to study some important superplastic deformation properties. The alloy Ti-5Fe-4Al-2Ni proved to be promising in that it gave a large elongation of more than 1500% at a low deformation temperature of 650°C, which was possible mainly because of the existence of iron and nickel. Alloying with nickel within a suitable amount was found to increase the strain rate sensitivity and to decrease the flow stress, while the effect of aluminium addition was found to be insignificant regarding superplastic behaviour, but its content should be limited. Possible mechanisms for super plasticity in the present alloy system are also discussed.