Abnormal β-phase stability in TiZrAl alloys

In titanium (Ti) alloys, metastable phases, such as β and martensite phases, have obvious effects on microstructure and properties of heat treated specimens. The β phase stability and martensitic transformation of TiZr-based alloys with different Al content were investigated. The addition of Al, the acknowledged α stabilizer of Ti-based alloys, is found to greatly modify the β phase stability and martensitic transformation of TiZr-based alloys. The β transus temperature of Ti60Zr40 binary alloy is about 640 °C. Simultaneously, as Al content increasing, β transus temperature increases significantly. When Al content reaches 15 at.%, the β transus temperature increases to nearly 850 °C. However, the phase composition of the β quenched Ti60Zr40 binary alloy is composed of whole α′ martensitic phase. Reversely, the (Ti60Zr40)85Al15 alloy, which possesses higher β transus temperatures, is mainly composed of β phase. This interesting martensitic transformation phenomenon does not conform to previous wide accepted β stabilizer theories and laws of Ti alloys. The main reasons of the abnormal β-phase stability in TiZrAl alloys are considered to be high cohesive energy, inherent low β → α phase transition temperature, and extra local stress.