Diffusionless isothermal omega transformation in titanium alloys driven by quenched-in compositional fluctuations

In titanium alloys, the ω(hexagonal)-phase transformation has been categorized as either a diffusion-mediated isothermal transformation or an athermal transformation that occurs spontaneously via a diffusionless mechanism. Here we report a diffusionless isothermal ω transformation that can occur even above the ω transformation temperature. In body-centered cubic β-titanium alloyed with β-stabilizing elements, there are locally unstable regions having fewer β-stabilizing elements owing to quenched-in compositional fluctuations that are inevitably present in thermal equilibrium. In these locally unstable regions, diffusionless isothermal ω transformation occurs even when the entire β region is stable on average so that athermal ω transformation cannot occur. This anomalous, localized transformation originates from the fluctuation-driven localized softening of 2/3[111]β longitudinal phonon, which cannot be suppressed by the stabilization of β phase on average. In the diffusionless isothermal and athermal ω transformations, the transformation rate is dominated by two activation processes: a dynamical collapse of {111}β pairs, caused by the phonon softening, and a nucleation process. In the diffusionless isothermal transformation, the ω-phase nucleation, resulting from the localized phonon softening, requires relatively high activation energy owing to the coherent β/ω interface. Thus, the transformation occurs at slower rates than the athermal transformation, which occurs by the widely spread phonon softening. Consequently, the nucleation probability reflecting the β/ω interface energy is the rate-determining process in the diffusionless ω transformations.Tane M., Nishiyama H., Umeda A., et al. Diffusionless isothermal omega transformation in titanium alloys driven by quenched-in compositional fluctuations. Physical Review Materials 3, 043604 (2019); https://doi.org/10.1103/PhysRevMaterials.3.043604

Effects of solute oxygen on kinetics of diffusionless isothermal ω transformation in β-titanium alloys

We have investigated the effects of solute oxygen on the kinetics of diffusionless isothermal omega (DI-ω) transformation in β-titanium vanadium alloys. This transformation constitutes a third category of ω transformation beside the athermal and isothermal modes. Thermal analysis, hardness and internal friction measurements were conducted after quenching oxygen-containing and near-oxygen-free alloys with ~ 21 at%V from the β-stable temperature. At this level of vanadium concentration, the athermal ω transformation is not expected. It is found that the DI-ω transformation more rapidly progresses in the low oxygen alloy and the relaxation strength of the elementary process of {111}β collapse is significantly reduced

Toward Tungsten Electrodeposition at Moderate Temperatures Below 100 °C Using Chloroaluminate Ionic Liquids

The electrodeposition of tungsten at moderate temperatures (<100 °C) has been of significant interest for the fabrication of thin films and microelectromechanical system components to decrease energy consumption and increase process safety. In this study, we investigated the electrochemical reduction of WCl₄ and WCl₅ in 1-ethyl-3-methylimidazolium chloride (EMIC) and EMIC–AlCl₃ ionic liquids at 80 °C–120 °C. W-rich films with a thickness of approximately 1 μm were obtained from the Lewis acidic EMIC–AlCl₃–WCl₅ bath, whereas the other baths did not yield any deposits. The films obtained from the EMIC–AlCl₃–WCl₅ bath at 80 °C had higher W contents of 54 at.% than those obtained at 120 °C. X-ray absorption near-edge structure spectra of the W-rich films indicated that W existed in an oxidized state. The findings of this study can be used as a guide to explore optimal electrolytes and electrolytic conditions for the electrodeposition of metallic W at moderate temperatures

Evolution of microstructure and variations in mechanical properties accompanied with diffusionless isothermal ω transformation in β -titanium alloys

Diffusionless isothermal omega (DI-ω) transformation, which was recently defined in Phys. Rev. Materials 3, 043604 (2019)2475-995310.1103/PhysRevMaterials.3.043604, is classified into a third category of the ω transformation modes, other than the well-known isothermal and athermal modes. This work reveals the characteristic features of the DI-ω transformation in the β-titanium vanadium alloy system, specifically, focusing on variations in the microstructure and mechanical properties with the proceeding of the DI-ω transformation. After quenching alloys of Ti-15at%V, Ti-21at%V, and Ti-27at%V down to below room temperature from the β-stable temperature, in addition to the occurrence of the athermal ω transformation for Ti-15at%V, all of the alloys gradually undergo the DI-ω 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}β pairs to form a transient DI-ω phase. Comparison of inelastic x-ray scattering and ultrasound measurements can see a trail of the DI-ω 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 ω and DI-ω transformations is finally discussed