Influence of Al addition on microstructures and mechanical properties of Ti-Fe alloys

The influence of Al content on the evolution of microstructures and mechanical properties of Ti-xFe-yAl alloys quenched from a β field was investigated via hardness measurements, tensile tests, scanning electron microscopy (SEM) equipped with electron back scattering pattern (EBSP) analyzer, and conventional transmission electron microscopy (TEM). The hardness of Ti-(3-6)Fe-yAl decreased remarkably on addition of 1% Al. In comparison to binary alloys, the maximum hardness decrease was achieved in Ti-4Fe-lAl. The hardness of Ti-4Fe-yAl decreased with increasing Al content up to around 5%, reaching a minimum, then gradually increased. Though all of the Ti-3Fe-yAl alloys showed the martensite structure, Ti-4Fe-yAl showed a structural change of β+ω → β+ω+ martensite → β+ω with increasing Al content. Consequently, Ti-4Fe-7Al had the lowest density in the β-type alloys equivalent to that of Ti-6A1-4V. The ratio of reciprocal distance d*_0002ω/d*_222β on selected-area diffraction (SAD) pattern, which is often used as an indicator of \{332\}<113> twin formation, was nearly constant up to 5% Al of Ti-4Fe-yAl, but decreased over 6% Al. The tensile tests of both Ti-4Fe-5Al and Ti-4Fe-7Al showed ductility. The deformation behavior of Ti-4Fe-5Al producing strain-induced martensite resembled that of Ti-8Mo. On the other hand, Ti-4Fe-7Al was deformed by a coarse slip of \{112\}<111> in the same manner as Ti-20Mo, and the SAD pattern analysis revealed that the diffuse reflection of the ω phase in the non-deformed area turned into the spots at the deformation band. It was suggested that Al behaves as a β stabilizer element when added to the lowest β alloy and is available for weight saving, while maintaining the β structure.