Phase transformations in Ti-Nb-Ta and Ti-Nb-Ta-Zr alloys

Abstract

Phase transformations in solution treated and quenched Ti-(13-26) Nb-(22-38) Ta (wt.%) and Ti-(13-35.5) Nb-(5-22) Ta-(4-7.2) Zr alloys have been studied. It has been observed that phase transformations in these alloys are sensitive to both composition and cooling rate. In ternary alloys, water and oil quenching resulted in the formation of orthorhombic martensite (α′′) in a retained β + ωathermal matrix, whereas slower cooling showed evidence of fine α and ωisothermal formation within the β matrix. Increase of Nb + Ta content decreases the volume percentage of martensite. Moreover, addition of Zr stabilized the β phase, lowered the martensite start temperature and suppressed ω formation. Finally, dynamic moduli of air cooled quaternary alloys showed that the modulus was sensitive to the composition, a minima at Nb/Ta ratio of 12.0 and 5 at% Zr being observed, this minimum in dynamic modulus being consistent with ω phase suppression.