[en] The ω-phase (a three atom hexagonal) is formed as a metastable phase in alloys of Ti and Zr i.e group IV B elements, when alloyed with elements placed to their right in the periodic table, on rapid cooling from the high temperature β-phase (bcc) field over a limited range of solute content. This is called athermal omega meaning a diffusionless transformation. At high solute content, also, omega phase has been observed to occur in the alloys of these elements on thermal aging, called isothermal omega. The ω-phase morphology, here, in alloys, is of fine particles. The orientation relationship (O.R.) between the bcc and the co-phase has been very well explained as to be due a small displacement of a pair of (111) planes of bcc lattice to the intermediate position, leaving the next plane unaltered, collapsing the next pair again and so on. Four equivalent <111> directions in bcc phase coupled with the fact of a small volume change across the transition, successfully explained the small particle size of the ω phase, so observed, also

[en] The lattice parameters of the bcc (β) and (Ω) phases occurring metastability in a series of Zr-rich Zr-Nb alloys have been determined at and above room temperature (TR) using neutron diffraction techniques. In the first place, the effect of temperature changes upon the lattice parameters of the β and Ω phases in alloys with 10 and 18 at. % Nb was monitored using neutron thermo-diffraction. A method of analysis is applied to the data, which involve a confrontation between the observed structural properties and an idealized -or 'reference'- behavior (RB) which admits a simple mathematical description. A generalized form of the law of Vegard is adopted as RB for the β phase, whereas a specific RB is proposed for the Ω structure. The experimental data are well accounted for by this interpretation scheme, leading to a picture of the isothermal reactions occurring at high temperature, which involves the transfer of Nb from the Ω to the β phase. Finally, the neutron diffraction data on the Ω phase is combined with an electron microscopy study for the alloy with 10 at. % Nb aged at 773 K, which provides information on the composition of this phase and its evolution towards thermodynamic equilibrium. (author)

[en] ω-Zr, the high pressure phase of Zr, is believed to be caused by an electronic phase transition. To check this, positron angular correlation measurements of the electron momentum density of ω-Zr have been carried out and compared with that of the α-phase. Comparison of the normalised angular correlation curves shows significantly larger number of annihilations towards the lower momentum region in ω-Zr. The widths at half maximum are 10.3 mrad and 9.5 mrad for ω- and α-phase respectively. This can be physically interpreted in terms of increased delocalization of d-states in the B atom layers and preferential annihilation of positrons from the same. The details of the experimental set up, which utilizes ⁶⁴Cu source of about one curie strength produced by irradiation at CIRUS reactor is also presented. (author)

[en] An atomic-crystallographic mechanism of rearrangement of the lattice during the α→ω transformation in zirconium, titanium and their alloys is proposed, based on propagation of a localized displacement wave of rows of atoms <1120> of the α-phase. The main features of the α→ω transformation that have been established experimentally in zirconium and its alloys are explained in terms of this mechanism. (author)

[en] A shock recovery experiment and post-shock mechanical measurements were conducted on high-purity Zr. Mechanical properties of the Zr samples before and after the shock loading are compared and discussed in terms of the substructure evolution during shock loading. Metastable ω-phase was found in the Zr sample following shock-loading to 7 GPa. A new orientation relationship between the α and ω phases was derived which does not agree with those previously reported in hydrostatic pressure experiments. A mechanism is proposed for the observed α → ω transformation in Zr. copyright 1994 American Institute of Physics

No abstract available

[en] Studies for understanding the formation of omega phase in zirconium and zirconium based alloys is important, as these materials, because of low neutron absorption, are widely used in nuclear industry. The presence of omega phase is undesirable as it embrittles these materials. The α → ω → β transformation under application of pressure in Zr is well established; however, the mechanism of α → ω under pressure is not completely understood. Also, it is interesting to explore whether pressure-induced β → ω transformation is possible. In Zr-based alloys, the formation of ω phase occurs in the metastable β phase at ambient pressure by rapid cooling from the high temperature β phase field. The fine particle morphology of the ω phase in these cases has been explained in terms of a shuffle dominated displacive transformation involving a lattice collapsing mechanism. Our experimental study on the β-stabilized Zr-20Nb alloy reveals that it transforms to ω phase on shock compression, whereas this β → ω transition is not seen in a hydrostatic pressure condition. The plate like morphology of ω formed under shock compression is in contrast to the fine particle morphology seen in this system under thermal treatment, and the habit plane prediction from the phenomenological crystallographic theory of martensitic transformation is consistent with experimental observations. These findings are indicative of the fact that the mechanism of β → ω formation under shock treatment is different and it involves large shear component. This study also explains why under shock compression only one of the orientation relation is chosen out of the possible two orientation relations if the α → ω transition proceeds via the intermediate β phase. The results of this study will be presented. (author)

No abstract available

[en] Strain-induced $\alpha \to \omega$ phase transformation (PT) in the zirconium (Zr) sample under compression and plastic shear in a rotational diamond anvil cell (RDAC) is investigated using the finite element method (FEM). The fields of the volume fraction of the $\omega$ phase, all components of the stress tensor, and plastic strain are presented. Before torsion, PT barely occurs. During torsion under a fixed applied force, PT initiates at the center of the sample, where the pressure first reaches the minimum pressure for strain-induced $\alpha \to \omega$ PT,$p_{\epsilon }^d$, and propagates from the center to the periphery and from the symmetry plane to the contact surface. Salient increase of the shear friction stress and pressure at the center of a sample, so-called pressure self-multiplication effect observed experimentally for some other materials, is predicted here for Zr. It is caused by much higher yield strength of the $\omega$ phase in comparison with the $\alpha$ phase. Except at the very center of a sample, the total contact friction stress is equal to the yield strength in shear of the mixture of phases and the plastic sliding occurs there. Due to the reduction in sample thickness and radial material flow during torsion, the $\omega$ phase can be observed in the region where pressure is lower than $p_{\epsilon }^d$, which may lead to misinterpretation of the experimental data for $p_{\epsilon }^d$. For the same applied force, torsion drastically promotes PT in comparison with the compression without torsion. However, the PT process in RDAC is far from optimal: (a) due to the pressure self-multiplication effect, the pressure in the transformed region is much higher than that required for PT; (b) the region in which PT occurs is limited by the pressure $p_{\epsilon }^d$ and cannot be expanded by increasing a shear under a fixed force; and (c) the significant reduction in thickness during torsion reduces the total mass of the high-pressure phase. These drawbacks can be overcome by placing a sample within a strong gasket with an optimized geometry. It is shown that, due to strong pressure heterogeneity, characterization of $\alpha \to \omega$ and $\alpha \to \beta$ PTs based on the averaged pressure contains large errors. The obtained results, in addition to providing an improved understanding of the strain-induced PTs, may be beneficial for the optimum design of experiments and the extraction of material parameters, as well as optimization and control of PTs by varying the geometry and loading conditions.

[en] The formation of ω phase under electron and ion irradiation and after heat treatment has been examined in a Zr-20wt%Nb alloy. Thermal treatment as well as irradiation has been found to lead to the formation of diffuse omega maxima in selected area diffraction patterns. The origin of these maxima has been probed by carrying out high resolution electron microscopy (HREM). A comparison of omega formation by irradiation and by thermal treatment has been made with a view to understand the role of chemical short range order (CSRO) in diffuse ω scattering and to establish the mechanism of ω formation. Electron irradiation has been found to lead to dissolution of the ω phase in periods of time considerably shorter than those required for thermal treatment. The precipitation of the α phase under electron irradiation has also been investigated. (author)