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

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 (alpha ) in a retained beta + omega(athermal) matrix, whereas slower cooling showed evidence of fine alpha and omega(i)sothermal formation within the beta matrix. Increase of Nb + Ta content decreases the volume percentage of martensite. Moreover, addition of Zr stabilized the beta phase, lowered the martensite start temperature and suppressed omega 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 omega phase suppression. (C) 2000 Kluwer Academic Publishers.