Plastic deformation of beta-Ti-Mo alloys with isothermal omega phase

National Institute for Materials Science, Tsukuba, Japan

Abstract

Aging of metastable beta-Ti alloys leads to the precipitation of omega phase, resulting in a significant loss in ductility (omega brittleness). This presentation reports the results of 3D observations of isothermal omega phase by an orthogonal FIB/SEM system. It was revealed that the shape of omega phase particles varies with the particle size. By serial sectioning, it was possible to obtain a 3D image for the volume of several micrometer cube containing numerous omega particles. The volume fraction of omega phase was qualitatively determined. The value was much lower than the previous investigation by X-ray diffraction by Hickman in 1969. The SEM and TEM revealed the formation of shear bands in the cold rolled samples, where the shearing of omega phase and transformation into beta phase while maintaining the depletion of Mo was found. After the HPT deformation, white etching layers are formed near the median plane of the disc, where the omega phase particles were absent and exhibit significantly lower hardness values. These findings may help to clarify the cause of brittleness in beta Ti alloys with isothermal omega phase.