CRYSTALLOGRAPHY OF ALPHA PHASE PRECIPITATED ON DISLOCATIONS AND DEFORMATION TWIN BOUNDARIES IN A BETA TITANIUM-ALLOY

Crystallographic orientation relationships of alpha-phases precipitated on dislocations and deformation twin boundaries in the beta-matrix have been investigated in a Ti-15V-3Cr-3Sn-3Al alloy, by means of transmission electron microscopy (TEM). Planar and widely spaced, {112}[111] beta-slip bands are formed by weak rolling at room temperature after being solutionized. By the subsequent aging at 933 K, alpha-plates precipitate preferentially on the dislocations in the slip bands. In the same slip bands, alpha-plates select particular variants of the Burgers orientation relationship. The selectivity of variants was discussed in terms of the effective relaxation of the transformation strain by the stress field associated with the dislocation. By subzero rolling at 77 K, {332}[113] beta-deformation twins are produced. By the subsequent aging at 933 K, alpha-phases precipitate on the twin boundaries, holding the Burgers orientation relationships with respect to both of the beta-matrix and the twin. It is considered that the alpha-nucleus forms coherently with both of the beta-phases to reduce the energy of the alpha/beta interface newly formed.