MICROSCOPIC APPROACHES TO ISOTHERMAL TRANSFORMATION OF INCOMMENSURATE OMEGA PHASE ZONES IN TI-20WT-PERCENT MO ALLOY STUDIED BY XDS, HREM AND EXAFS

The as-quenched omega zones in Ti-20%Mo alloy showed incommensurate X-ray diffuse scattering (XDS). Dark field images showed that the omega zone was a chain-like array of 3-5 white dots, and each dot was a 3 atom molecule. The omega zones were observed as weak modulated fringes on lattice images micrographs. The variants ( omega //1 and omega //3, 5-10 nm in size) of isothermal commensurate omega phase formed after an incubation period showed lattice fringe spacings of 0. 28 and 0. 40 nm corresponding to (222)/3 and (1 bar1 bar 2)/3 points, respectively. During the incubation period, there were some morphologically unidentified products which were the precursors of omega variants ( omega //2 and omega //4).