Growth behaviour of athermal ω-phase crystals in a Ti-15mass%Mo alloy studied by In-situ dark field and high resolution electron microscopy

摘要

The formation of athermal w-phase crystals in β-Ti alloys has been postulated based on the negative temperature dependence of electrical resistivity. However, no direct evidence has been reported. In order to clarify the formation and morphology of athermal ω-phase crystals in Ti-15mass%Mo alloys cooled to and held at 131 K, in-situ dark field image observations and high resolution electron microscope (HREM) observations were carried out using a TV-VTR system. It was found that athermal ω-phase crystals with spherical shape and a size of approximately 20 nm were formed by cooling to 131 K in areas that were not irradiated by electrons, but, no formation was observed in areas irradiated by electrons. HREM images of the athermal ω-phase crystals revealed the atomic structure of ω-phase crystals. Athermal ω-phase crystals formed by cooling were annihilated by electron irradiation, and the stability depended on the holding period at 131 K, that is, the longer the period, the more stable the crystals became. The annihilation caused by the temperature rise from 131 to 200 K was a function of time. The annihilation was noticed at 1680 s, but not at 780 s. This fact suggests that an incubation time exists in the case of the ω→←β transformations. It seems that the β→←ω phase transformations indicate hysteresis.