Microstructure evolution during isothermal forging and subsequent heat treatment of Ti-17 alloy with a lamellar colony structure

The effect of isothermal forging and subsequent heat treatment on microstructure evolution was investigated for Ti-17 alloy with a lamellar colony structure. For this purpose, Ti-17 alloy was isothermally forged at 820 C or 860 C and subsequently heat treated for times ranging from 10 min to 8 h. The principal features of microstructure evolution were found to be strain and temperature dependent. Microstructure evolution during deformation included the rotation of alpha lamellae toward the metal flow direction. The orientation distribution of alpha lamellae changed from the two symmetrical peaks to the single one with the increasing strain. The angle (θ) in the single peak position was about 90. Additionally, length of the lamellar alpha decreased with strain and deformation temperature. Thickness of the lamellar alpha increased with deformation temperature but was independent on strain. During heat treatment, microstructure observations suggested that the process of evolution can be divided into two stages. The first included segmentation of the lamellae in the initial stage of heat treatment, whereas microstructure coarsening characterized the second one during prolonged heat treatment. Furthermore, the segmentation and coarsening processes could be accelerated by the increasing prestrain and temperature.