Omega phase in materials

A complex study of the microstructure of titanium and zirconium pseudo-single crystals subjected to severe plastic deformation in Bridgman anvils under a pressure of 8 GPa at a temperature of 80 K has been carried out. Raman spectra of the samples deformed to different degrees show a two-phase α + ω state, which indicates the realization of the pressure-induced transition during deformation process with the retention of the ω-phase in the structure of both metals after the load removal. The quantitative ratio of α- and ω- phases in the structure of deformed metals and its change with increasing degree of deformation has been analyzed.

Correlation of Raman spectroscopy results with experimental data of X-ray diffraction analysis, electron microscopic analysis, as well as literature data of calorimetry and calculated thermodynamic data by DFT method on kinetics of the reverse ω → α transformation demonstrates the efficiency of Raman spectroscopy method for estimation of the phase composition of pure metals, which experienced phase transition under loading.

Solid particle erosion (SPE) resistant coatings extend the life of gas turbine compressor blades and improve the safety of flights. Among different types of these coatings, nanolayered multi-layered and nanocomposite coatings proved to be more efficient. Further increase in the efficiency of these coatings can be achieved by introducing energy-absorbing metal porous layers. Here, we report the development of three different types of SPE and corrosion resistant Ti/TiN multilayer coatings (thickness ∼9 μm) on Ti6Al4V substrates using a sputtering technique. The coatings contain three stacks of Ti/TiN (7.3 nm) bi-layers separated by two energy-absorbing Ti layers of different porosity in each coating. The Ti film deposited at 4 mTorr chamber pressure showed a mix of ω and α phases with high porosity. The SPE studies were conducted according to ASTM-G76 standards at a temperature of 400 °C. The corrosion properties of the coatings were studied by potentiodynamic polarization in a 3.5 wt% NaCl solution. The coatings with energy-absorbing porous Ti layers showed better SPE-resistant performance compared to the coating with dense Ti layers. Whereas, vice versa behaviour was in the case of corrosion resistance performance. SPE mechanism is also analysed using field emission scanning electron microscopy images of erosion scars.

Zirconium transforms from the α→ω phase under high hydrostatic pressures. The critical transformation pressure is expected to be affected by internal stresses associated with defects. This study focuses on the effects of dislocations and twins and their associated stress fields on the transformation. Samples are pre-loaded to seed dislocations or twins. In-situ high-hydrostatic-pressure X-ray synchrotron experiments are performed revealing that microstructures with pre-existing prismatic 〈a〉 dislocations and $\left\{10\overline{1}2\right\}$ twins promote the transformation more effectively than pyramidal 〈c + a〉 dislocations or $\left\{11\overline{2}2\right\}$ twins. Post-mortem electron backscatter diffraction further shows that pre-seeded defects stabilize the ω-phase at ambient conditions. In-situ stress-hold neutron diffraction experiments are also performed combining both hydrostatic and deviatoric stresses capturing the role of deviatoric stresses on phase transformation kinetics. These results support the findings of recent atomistic simulations indicating flow of prismatic 〈a〉 dislocations at an α-ω interface promotes the growth of the ω domain.

A novel low-activation Ti_1.5ZrV_0.5Ta_0.5 refractory high-entropy alloy (RHEA) was designed as a potential candidate for nuclear reactor application. At room temperature, it had an elongation of 8.4% and a yield strength of 1096 MPa. The phase evolution of this alloy and its effect on properties was investigated. At 400 °C, the solid solution bcc₁ transformed into the fcc phase and bcc₂ phase, and the ω phase and α phase also appeared. At 600 °C, the ω phase and α phase disappeared, and the microstructure of the alloy was composed of the fcc phase and bcc₂ phase. When the temperature was up to 1200 °C, the fcc phase and bcc₂ phase re-transformed into solid solution bcc₁ phase. The precipitation of ω phase and α phase caused a sharp increase in strength and a decrease in plasticity. Meanwhile, the appearance of the fcc phase led to a simultaneous decrease in strength and ductility, due to larger stress concentrations at the fcc/bcc interface. Besides, the formation mechanism of each phase in the alloy was discussed in detail.

We present a new interatomic potential for atomistic modeling of $\alpha$- and $\beta$-phases of zirconium. The potential was developed within the framework of the N-body approach, which takes into account pair, three-body and many-body interactions. The new potential predicts a wide range of zirconium properties in agreement with known experimental and DFT data including structural energies, elastic constants, energies of point and planar defects, thermodynamic properties, such as melting temperature and $\alpha -\beta$ transition temperature. The potential predicts the mechanical instability of the $\beta$-phase of zirconium at 0 K, as well as relaxation effects of internal degrees of freedom in elastic constants. The predicted interstitial stacking fault energy is in a good agreement with the DFT data. The formation energies of self-interstitial atoms in the $\alpha$-phase are in agreement with the known DFT values, predicting the basal octahedral (BO) configuration as the most stable. The potential predicts a spontaneous $\beta -\alpha$ transition with decreasing temperature. The calculated $\alpha -\beta$ phase transition temperature is 20% higher than the experimental value. The potential accurately predicts the melting temperature of zirconium, due to which a good agreement with the experimental data was obtained on the temperature dependencies of the diffusion coefficients for the $\alpha$- and $\beta$-phases of zirconium. For both phases, the main contribution to the diffusivity is due to vacancies, while the corresponding values for interstitials are by one or two orders of magnitude lower. The potential predicts the deviation from the Arrhenius law of the temperature dependence of the diffusion coefficient for the $\beta$-phase of zirconium, due to the nonlinear dependence of the Gibbs energy of a vacancy formation on temperature. The constructed potential can be used for molecular dynamics simulations of $\alpha$- and $\beta$-phases of zirconium and further development of interatomic potentials within the framework of the N-body approach for alloys containing zirconium.

Structural transformations in the solid state dictate operating regimes of materials for engineering applications. Advanced structural characterisation facilitated by electron microscopy has resulted in significant progress in our understanding of structural transformations across resolvable length scales. We shall confine this communication to one of the metallic systems. This refers to titanium (Ti) alloys. They exhibit formation of a variety of solid solution phases, intermetallic phases, quasicrystals, incommensurate structures, and metallic glasses under different processing conditions. Additionally, newer phase formation at nanometer length scales has also been observed in Ti alloys. The exploration of properties in presence of structures at nanoscale in these alloys have not been discussed in literature extensively. Such an approach will open an avenue for nano-engineered alloys. An attempt will be made to indicate the direction of investigation in this connection succinctly. Understanding the nature and pathways of solid state structural transformations in Ti alloys seem to be important in view of the wide variety of engineering applications. Nanostructured materials have shown formation of newer phases not included in equilibrium phase diagrams. This review shall dwell on this aspect by drawing parallelism from many other alloy systems at nanoscale. In particular, $\mathrm{Au}-\text{Cu}$ nanostructures will be discussed as an example. It will be argued that size of the system will have influence on the formation of structures that are normally not observed at microscopic length scales in Ti alloys. In view of the complexities involved in phase transformations in Ti alloys, it is important to evolve or look for a model that will help us understand structural transformations by minimum geometrical distortion from a parent phase. Such an approach will offer one of the ways of comprehending formation of phases at nanoscale. In addition to this, it will also help us to consider group-subgroup relationship. It will be shown that unified structural description towards this will be helpful. A brief summary of higher dimensional structural modelling will be presented here with particular reference to phases formed in Ti alloys.