Abstract
Diffusionless isothermal omega ($\mathrm{DI}\text{\ensuremath{-}}\ensuremath{\omega}$) transformation, which was recently defined in Phys. Rev. Materials 3, 043604 (2019), is classified into a third category of the $\ensuremath{\omega}$ transformation modes, other than the well-known isothermal and athermal modes. This work reveals the characteristic features of the $\mathrm{DI}\text{\ensuremath{-}}\ensuremath{\omega}$ transformation in the $\ensuremath{\beta}$-titanium vanadium alloy system, specifically, focusing on variations in the microstructure and mechanical properties with the proceeding of the $\mathrm{DI}\text{\ensuremath{-}}\ensuremath{\omega}$ transformation. After quenching alloys of Ti-15at%V, Ti-21at%V, and Ti-27at%V down to below room temperature from the $\ensuremath{\beta}$-stable temperature, in addition to the occurrence of the athermal $\ensuremath{\omega}$ transformation for Ti-15at%V, all of the alloys gradually undergo the $\mathrm{DI}\text{\ensuremath{-}}\ensuremath{\omega}$ transformation upon aging at a temperature as low as 373 K, leading to a moderate increase in the hardness. The degree of the hardness increase in these alloys can be successfully explained by a local instability concept based on quenched-in thermal concentration fluctuations. It is also shown that internal friction (Ti-21at%V) diminishes after the low-temperature aging, which indicates the annihilation of such unstable regions showing a dynamic collapse of ${{111}}_{\ensuremath{\beta}}$ pairs to form a transient $\mathrm{DI}\text{\ensuremath{-}}\ensuremath{\omega}$ phase. Comparison of inelastic x-ray scattering and ultrasound measurements can see a trail of the $\mathrm{DI}\text{\ensuremath{-}}\ensuremath{\omega}$ transformation remaining even in the Ti-27at%V alloy, which has no obvious athermal omega transformation temperature. Based on the results, the difference between athermal $\ensuremath{\omega}$ and $\mathrm{DI}\text{\ensuremath{-}}\ensuremath{\omega}$ transformations is finally discussed.
Highlights
Titanium alloys have been indispensable materials for modern industrial society in that they exhibit superior properties such as mechanical strength, corrosion resistance, and biocompatibility
To clarify these unsolved problems and to construct a universal ω transformation theory, we recently explored the detailed mechanism of the diffusionless isothermal ω (DI-ω) transformation in the Ti–V alloy system [31]
For β-Ti–V alloys, we more deeply investigate the characteristic features of the DI-ω transformation in terms of the microstructure evolution and mechanicalproperty change accompanying its progress. We show that these changes can be thoroughly interpreted by using the concept illustrated in Fig. 1, and characterized by means of differential scanning calorimetry, hardness measurements, transmission electron microscopy, submegahertz internalfriction measurements at low temperatures, and inelastic x-ray scattering measurements
Summary
Titanium alloys have been indispensable materials for modern industrial society in that they exhibit superior properties such as mechanical strength, corrosion resistance, and biocompatibility. The transformation only gradually proceeds upon further cooling below Tω [10], there is usually no large temperature hysteresis found for the β ↔ ω athermal transformation in systems where this transformation clearly occurs. To clarify these unsolved problems and to construct a universal ω transformation theory, we recently explored the detailed mechanism of the diffusionless isothermal ω (DI-ω) transformation in the Ti–V alloy system [31]. Since the quenched-in compositional fluctuation at the nanometer scale is large, some local regions are inevitably lean in the β-stabilizing element, and become unstable for the β structure. What we emphasize here is that the various phenomena including room temperature aging can be successfully explained and understood based on the present mechanism
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