Interfacial structure of widmanstatten plates in a TiCr alloy

Abstract

A TEM study was conducted of the interphase boundary structure of the two types of h.c.p. proeutectoid α plate formed in the b.c.c. β matrix of a hypoeutectoid Ti-7.15% Cr alloy. Ill-formed normal α plates were found to be partially coherent at their edges as well as along their broad faces. Misfit dislocations observed on the broad faces are of a-type, with Burgers vector a 3 〈11 2 ̄ 0〉 α spaced 20 nm apart, and on the edges are of c + a-type with b = a 3 〈11 2 ̄ 3〉 α and 89 nm apart. Very well formed black plates, developed at lower reaction temperatures, exhibit an interdislocation spacing of ca 35 nm on their broad faces. 0-lattice analysis indicates that the broad faces of neither morphology correspond to the b.c.c.:h.c.p. boundary with the smallest structural component of interfacial energy. This analysis also suggests that second arrays of misfit dislocations were absent because they are either too closely spaced or lie at too small an angle to the first arrays. The average spacing between the growth ledges on the broad faces of black plates is markedly greater than that on the broad faces of normal α plates, consistently with their smaller thickness/length ratio and higher lengthening rate.