Electron microscopy study on Mo content dependence of beta to omega phase transformation due to cooling in Ti-Mo alloys

Direct observation of the formation of athermal omega phase crystals (omega(ath)) in Ti-10, 15, and 20mass%Mo alloys resulting from beta to omega phase transformations due to cooling to 131 K, has been carried out using in situ dark-field imaging technique. The formations were confirmed in all three alloys. The formation temperatures depend on the Mo content and it is noted that the higher the Mo content, the lower the formation temperature. The relationship between the formation of omega(ath) and electrical resistivity in the three alloys is discussed and it seems that only the formation of omega(ath) cannot account for the change of the resistivity of Ti-15 and 20mass%Mo alloys. Annihilation behaviours of omega(ath) in Ti-15 and 20mass%Mo alloys due to the irradiation of electron beam accelerated at 200 kV were investigated and it was found that omega(ath) in the Ti-15mass%Mo alloy disappeared faster than that in the Ti-20mass%Mo alloy. It was suggested that both results about the formation and annihilation of omega(ath) due to cooling depend on the stability of the matrix connected with the amounts of beta-phase stabilizing Mo elements.