Formation of β-Ti Phase in Multi-component Gamma Alloys

Abstract

ABSTRACTEffect of combined addition of elements on the formation of high-temperature β-Ti (bcc) phase in Ti-45Al-M1-M2 quaternary alloys has been examined by converting the quaternary system to quasi-ternary system using M1-equivalency of M2: [M1] eq = k [M2]. The k value is evaluated from the ratio of critical concentration, XcM, to form the β phase in the isothermal section of each ternary system at 1473 K: kM1/M2= XcM1/XcM2. In case that M1 and M2 are in the same group element, Mo-equivalency of Cr, kMo/Cr = 0.46 and Nb-equivalency of V, kNb/V = 0.57. The critical concentrations to form the β phase in the quaternary systems become lower than those estimated from these values, and the experimentally determined equivalencies become 0.8 and 1.0, respectively. These results suggest an existence of negative interaction between M1 and M2 to stabilize the β phase even if they were in the same group element. Experimentally determined quaternary phase diagram revealed that the combined addition makes the β+α two-phase region expand to lower temperatures, thereby effective in improving hot workability for development of wrought gamma alloys.