Reverse Transformation of α″ and Initial β Decomposition in Quenched Ti-Nb Binary Alloys

Abstract

On Ti-20∼40%Nb alloys quenched from 1073 K, the changes in resistivity and its temperature dependence during the early stage of aging at low temperatures were related to phase transformations confirmed by X-ray diffraction. The following results were obtained:(1) 26-40%Nb alloys tended to approach a negative temperature dependence (NTD) of resistivity, which was attributed to reverse transformation of α″ orthorhombic martensite confirmed in 26-35%Nb alloys.(2) An isothermal reverse transformation of α″ continued for a long period by 473 K aging of the 35%Nb alloys.(3) As and/or Af temperatures of α″ were distinguished from the starting temperatures of athermal ω formation by reversibility of the resistivity change with cooling or heating of specimens and by the direction change of resistivity-temperature coefficients.(4) As and Af of α″ formed by quenching were lowered with increasing Nb content and were estimated to be 620 and 720 K in 26%Nb, 550 K and 610 K in 30%Nb, 450 and 500 K in 35%Nb, 400 and 450 K in 40%Nb, respectively.(5) Above the Af, the β phase formed by a perfect reverse transformation of α″ decomposed to α+β phases with diffusion. Long-period heating at a temperature slightly below the As resulted in a direct decomposition of α″ to β+α.