Reasons for Formation of Triangular [alpha] Precipitates in a Ti-15V-3Cr-3Sn-3Al [beta] Titanium Alloy

The triangular phase in Ti-15V-3Cr-3Sn-3Al is observed after two-step aging. The three-dimensional shape of the phase is a triangular pyramid that is composed of three precipitates. The reason for the formation of the triangular shape is investigated by means of crystallographic analysis and high-resolution TEM observations. According to the HR-TEM observations, it is possible that the triangular phase has Potters orientation relationship with the matrix despite the existence of Burgers orientation relationship in an intragranular precipitate, as reported in previous studies. This suggests that in the alloy under consideration, the orientation relationship between the precipitate and the matrix is selectable in terms of lattice correspondence. As for the growth direction of the precipitate, an invariant line is investigated. In our previous study, the growth direction determined in trace analysis by TEM observation was mostly in agreement with the direction of the invariant line. Although the three precipitates have a common direction, which is [2110]α/[111]β, their invariant lines are not parallel with each other, i.e., [447]α1, [744]α2, and [474]α3. Therefore, the precipitates expand along each direction while growing and form a triangular shape.