The effect of ta content on phase constitution and aging behavior of Ti-Ta binary alloys

Using Ti-Ta alloy to which Ta was added systematically, i.e. Ti-5 mass%Ta, 10 mass%Ta, 20 mass%Ta, 30 mass%Ta, 40 mass%Ta and 50 mass%Ta alloys, the effect of Ta concentration on phase constitution in the solution treated and quenched state and aging behavior is studied by electrical resistivity and Vickers hardness measurements and X-ray diffactometry. All alloys were solution treated at 1173 K for 3.6 ks and then quenched into ice water (STQ). STQed specimens were isochronally heat treated front 323 to 1173 K. STQcd specimens of Ti-50 mass%Ta alloy were isothermally aging at five different temperatures, 573 K, 623 K, 673 K, 723 K and 773 K. In the solution treated and quenched state, only reflections of hexagonal martensite, alpha' were obserevd by XRD in Ti-5Ta to 30Ta alloys, whereas only orthorhombic martensite, alpha", was identified by XRD in Ti-40Ta and 50Ta alloys. alpha" in Ti-50Ta alloy is reversely transformed into beta upon isochronal heat treatment: the starting temperature is situated at a temperature between 623 K and 673 K. Isothermal aging of this alloy shows that alpha" directly decomposes to a and phases without reverse transformation of alpha" at 573 K and 623 K aging. On isothermal aging above 673 K, alpha" reversely transforms into beta and then isothermal or phase precipitates in the reversed beta.