Fundamental Studies of Beta Phase Decomposition Modes in Titanium Alloys.

Abstract

Abstract : A TEM study of the interphase boundary structure of proeutectoid alpha plates in a Ti-7.15% Cr alloy has shown that both the broad faces and edges of the plates are partially coherent. At the broad faces, misfit dislocations are 1/3 1120 whereas at the edges they are 1/3 1123 . For normal alpha plates, the dislocations are ca. 20nm apart at the broad faces and 8 nm. apart at the edges. At black plates, the low temperature morphological variant of proeutectoid alpha, the dislocations are ca. 35nm. apart. Growth ledges are typically 100 - 350 nm. apart at the broad faces of normal plates and ca. 700 nm. apart on black plates. A detailed O-lattice analysis has demonstrated that a simple correlation between the misfit dislocation and the growth ledge structures does not exist. Growth kinetics studies of grain boundary allotriomorphs in Ti-3.9 w/o Co and Ti-7.15 w/o Cr have shown for the first time that the rejector plate mechanism is operative when the matrix phase has a bcc crystal structure. As previously predicted, however, this mechanism accelerates growth substantially less at a given homologous temperature than previous work has shown it to do in fcc matrices. The feasibility of detailed fundamental studies of the influence of beta grain size upon the development of Widmanstatten alpha sideplates has been demonstrated. Preliminary observations have been made on sideplate evolution from grain boundary allotriomorphs by both local morphological instability and sympathetic nucleation.