Phase Transformations in Ti-15Mo Investigated by in situ Electrical Resistance

In this study phase transformations in metastable beta Ti-15Mo alloy were investigated by an in situ electrical resistance measurement in a wide range of temperatures from -196°C to 850°C. Different temperature ranges of the evolution of electrical resistance were correlated with underlying phase transformations. In the low temperature range, stage I (from -196°C to 220°C) the decrease of electrical resistance with increasing temperature is caused by the dissolution of ω_{ath} (formed during quenching by athermal shuffle transformation) which is accompanied by the relaxation of lattice strain, while the diffusional assisted growth of ω_{iso} in the range from 220°C to 380°C (stage II) is the main mechanism causing the increase of resistance. Another decrease of the resistance in the range from 380°C to 550°C (stage III) is explained by the dissolution or transformation of ω_{iso}. The increase of resistance above 550°C (stage IV) is related to the growth of α-phase particles. The fully reversible character of ω_{ath} growth and dissolution during heating and cooling in the stage I up to 100°C was confirmed by temperature cycling during repeated in situ resistance runs from RT. Pre-ageing of samples at 300°C promotes the formation of ω_{iso} particles. Subsequently, ω_{ath} particles are not created, which is fully consistent with electrical resistance measurements. The presence of ω_{ath} and the orientation relationship between ω and β were identified by the electron diffraction.