Effects of ω-phase precipitation on β → α, α" transformations in a metastable β titanium alloy

Effects of omega phase precipitation on beta --> alpha, alpha" transformations in a metastable beta titanium alloy have been studied by means of transmission electron microscopy. Isothermal holding at 300 degreesC for 100 min after the solution-treatment at 1000 degreesC yielded homogeneously distributed spherical omega phase particles of similar to 10 nm in diameter. With raising the holding temperature, omega phase particles of the same orientation variant precipitated as groups. At temperatures above 350 degreesC, alpha phase laths nucleated at the omega/beta interfaces and grew into both the matrix and omega particles with consuming omega phase particles. Thus, the alpha/omega Takemoto and the omega/beta Burgers relationships were held simultaneously, and the orientation of or laths nucleating at an omega/beta interface was determined uniquely. In the specimens quenched directly from beta single-phase region, alpha" martensite plates nucleated preferentially at beta grain boundaries. The alpha" formation, however, was accelerated by a short time holding at temperatures between 600 and 300 degreesC, but was largely retarded by the progress of omega phase precipitation. (C) 2001 Elsevier Science B.V. All rights reserved.