The relationship between the mechanical properties and the thermal instability of β Ti-Mo alloys was studied by hardness testing, the detailed observations of the surfaces and microstructures and the investigation of the behaviour of tensile deformation on Ti-5, 14 and 20 wt%Mo alloys. The results botained are as follows.(1) It is shown from the dependence of hardness changes upon isochronal ageing temperature that the thermal instability is largest and the ω phase is readily formed in Ti-14 wt%Mo alloy.(2) By cold rolling, the formation of the ω phase in Ti-14 wt%Mo alloy is apparently promoted. However, such a phenomenon cannot be detected in Ti-20 wt%Mo alloy.(3) In tensile testing at 77 to 300 K, it appears that Ti-14 wt%Mo alloy has excellent mechanical properties having homogeneous deformation with remarkable elongation and linear work hardening rate, and Ti-20 wt%Mo alloy shows local deformation with small elongation.(4) Ti-14 wt%Mo alloy in tension is mainly deformed by {332}β twinning and in the twin bands, an athermal ω phase as stress induced product is formed. Ti-20 wt%Mo alloy in tension is mainly deformed by slipping, and dislocation bundles and stacking faults are observed though twins are not detected.(5) In such a thermally instable β phase such as in Ti-14 wt%Mo alloy, it seems that there is a definite relation between the formation of the ω phase and twinning, and that the remarkable elongation correlates to the formation of twins.
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Hida et al. (1980) studied this question.