Formation Of Ω-phase Research Articles

The α→ω and β→ω phase transformations in Ti–Fe alloys under high-pressure torsion

The formation of ω-phase under high-pressure torsion (HPT) has been studied in Ti–Fe alloys. Seven alloys with Fe concentration from 0 to 10 wt % have been annealed between 600 and 950 °C, quenched and HPT-treated at 7 GPa, 1 rpm, 5 and 0.1 anvil rotations (equivalent strain eeq = 156 and = 3.1, respectively). The strain after 0.1 rot. corresponds to the transient state of HPT, and that after 5 rot. corresponds to the HPT steady-state and to the dynamic equilibrium between formation and annihilation of microstructure defects. A defect-rich high-pressure ω-phase forms after HPT and persists in the samples also after the pressure release. The amount of retained ω-phase after HPT depends on the iron concentration. It increases from 40% in pure titanium, reaches maximum of 95% at 4 wt % Fe and then decreases again to 10% at 10 wt % Fe. It is because the addition of iron influences the lattice parameters in β and ω-phases in a different manner. The minimal lattice mismatch between β- and ω-phases is reached at 4 wt % Fe. A good conformity between the lattices of the β- and ω-phases enhances the probability of the martensitic (diffusionless) β→ω transformation. Based on the XRD and TEM observations, the crystallography and mechanisms of α→ω and β→ω phase transformations (which can be diffusionless as well as controlled by mass transfer) under the influence of pure shear by HPT are discussed.

Unusual dynamic precipitation softening induced by dislocation glide in biomedical beta-titanium alloys

Softening of metallic materials containing precipitates during cyclic deformation occurs through dissolution of the precipitates, because the to-and-fro motion of the dislocation causes dissolution of the precipitate particles by cutting them. Here, however, we found the completely opposite phenomenon for the first time; a “dynamic precipitation softening” phenomenon. In a Ti-35Nb-10Ta-5Zr body-centered cubic structured β-Ti alloy single crystal developed for biomedical implant, the to-and-fro motion of the dislocation “induced” the selective precipitation of the ω-phase whose c-axis is parallel to the Burgers vector of the moving dislocation, which led to the significant cyclic softening of the crystal. The formation of the ω-phase is generally believed to induce significant hardening of β-Ti alloys. However, the present results suggest that this is not always true, and control of the anisotropic features of the ω-phase via control of crystal orientation can induce unusual mechanical properties in β-Ti alloys. The unique anisotropic mechanical properties obtained by the cyclic-deformation-induced oriented ω-phase formation could be useful for the development of “single-crystalline β-Ti implant materials” with advanced mechanical performance.

Microstructural evolution and functional fatigue of a Ti–25Ta high-temperature shape memory alloy

Abstract

Ternary Ti-Zr-Nb and quaternary Ti-Zr-Nb-Ta shape memory alloys for biomedical applications: Structural features and cyclic mechanical properties

Ti-18Zr-14Nb, Ti-18Zr-13Nb-1Ta, Ti-18Zr-12Nb-2Ta and Ti-18Zr-11Nb-3Ta (at%) shape memory alloys were obtained by vacuum arc remelting and subjected to different thermomechanical treatment (TMT) schedules. The transmission electron microscopy analysis revealed that a TMT, which consisted of cold rolling with a true logarithmic strain of e = 0.3 and post-deformation annealing at 600°C (30min), led to a subgrained structure formation with a size of structural elements of 200–300nm and to a room temperature superelastic behavior. The X-Ray diffraction (XRD) analysis showed that a gradual replacement of Nb content by its Ta equivalent decreases the impact of alloying elements on the β-phase stabilization phenomenon and suppresses the ω-phase formation: at room temperature, Ti-18Zr-14Nb and Ti-18Zr-13Nb-1Ta alloys contain mainly parent β-phase, while Ti-18Zr-12Nb-2Ta and Ti-18Zr-11Nb-3Ta alloys contain considerable amount of α”-martensite. The crystallographic resource of recovery strain for all the alloys exceeds 5% and reaches 6%, for the Ti-18Zr-11Nb-3Ta alloy. The fatigue life of the Ti-18Zr-13Nb-1Ta alloy is almost twice as high as that of the Ti-18Zr-14Nb alloy. During multi-cycle mechanical testing, the following regularity appeared for all the alloys tested: during 10–15 first loading-unloading cycles, the mechanisms of plastic deformation and stress-induced martensite formation and stabilization prevail, while after that, the β⇄α” martensitic transformation mechanisms become dominant, and the alloys’ behavior turns from elasto-plastic into superelastic.

Strengthening in high-pressure quenched Zr

ABSTRACTThe present study examined the effects of pressure (range: 1–6 GPa) on microstructure and mechanical properties of pure Zr. Pressure significantly affected refining of Zr microstructure. When 5 GPa pressure was applied, ω-phase was observed in processed specimen, and volume fraction sharply increased to 57.4% for specimen pressurized-quenched at 6 GPa. Benefitting from refinement of acicular-shaped α (α′) plates and the formation of equiaxial ω-phase, the yield strength of the sample quenched from 6 GPa reached ∼616 MPa, which is almost twice as that of coarse-grained Zr.

Comparative study of structure formation and mechanical behavior of age-hardened Ti–Nb–Zr and Ti–Nb–Ta shape memory alloys

This work sets out to study the peculiar effects of aging treatment on the structure and mechanical behavior of cold-rolled and annealed biomedical Ti–21.8Nb–6.0Zr (TNZ) and Ti–19.7Nb–5.8Ta (TNT) (at.%) shape memory alloys by means of transmission electron microscopy, X-ray diffractometry, functional fatigue and thermomechanical testing techniques. Dissimilar effects of aging treatment on the mechanical behavior of Zr- and Ta-doped alloys are explained by the differences in the ω-phase formation rate, precipitate size, fraction and distribution, and by their effect on the alloys' critical stresses and transformation temperatures. Even short-time aging of the TNZ alloy leads to its drastic embrittlement caused by “overaging”. On the contrary, during aging of the TNT alloy, formation of finely dispersed ω-phase precipitates is gradual and controllable, which makes it possible to finely adjust the TNT alloy functional properties using precipitation hardening mechanisms. To create in this alloy nanosubgrained dislocation substructure containing highly-dispersed coherent nanosized ω-phase precipitates, the following optimum thermomechanical treatment is recommended: cold rolling (true strain 0.37), followed by post-deformation annealing (600°C, 15–30min) and age-hardening (300°C, 30min) thermal treatments. It is shown that in TNT alloy, pre-transition diffraction effects (diffuse reflections) can “mask” the β-phase substructure and morphology of secondary phases.

ω Transformation in cold-worked Ti–Nb–Ta–Zr–O alloys with low body-centered cubic phase stability and its correlation with their elastic properties

The ω transformation and its correlation with elastic properties were investigated in cold-worked Ti–36Nb–2Ta–3Zr–xO mass% alloys with low body-centered cubic (β) phase stability, known as gum metal. Analysis of the temperature dependence of the ω (hexagonal) phase formation using transmission electron microscopy and of the elastic properties of solution-treated and cold-worked alloys using resonant ultrasound spectroscopy revealed that in the solution-treated 0.36% and 0.51% O alloys, the high concentration of oxygen suppressed ω-phase formation from room temperature to a fairly low temperature of ∼13K. However, the ω phase was formed by cold working at room temperature in the 0.30% and 0.47% O alloys. Importantly, the fraction of the ω phase clearly increased upon cooling, which indicates that the formation of the ω phase is thermodynamically favorable near and below room temperature in the cold-worked 0.30% and 0.47% O alloys. This formation of the ω phase and the low stability of the β phase related to the low electron/atom (e/a) ratio were the dominant factors determining the elastic properties near and below room temperature in the cold-worked Ti–Nb–Ta–Zr–O alloys.

Special features of formation of omega-phase in titanium alloys due to hardening

Special features of formation of athermal ω-phase in titanium alloys of the Ti – Nb, Ti – V, and Ti2AlNb-base systems are considered from the standpoint of changes in the electron concentration of the β-solid solution. The conditions of formation of ω-phase with anomalous (“raft”) morphology are determined. A dislocation model explaining the morphological features is suggested. The effect of hydrogen alloying of titanium alloys on the process of formation of ω-phase in hardening is described. Amethod for computing electron concentration for titanium alloys with hydrogen is suggested.

Age-hardening behaviour of two metastable beta-titanium alloys

Two novel β-titanium alloys, of compositions Ti–10V–2Ni and Ti–10V–6Cu (wt.%) and with distinct potential for athermal ω-phase formation, have been selected and the sensitivity of the age-hardening responses to the cooling rate post solution treatment investigated. Upon rapid water quenching, the solution-treated Ti–10V–2Ni alloy contained a significantly higher volume fraction of ω-phase than the Ti–10V–6Cu alloy. It was observed that the Ti–10V–6Cu alloy is remarkably insensitive to cooling rates in the range of water quenching to air cooling, in terms of both microstructural evolution and age-hardening behaviour, when aged isothermally at 350 °C and 500 °C. The microstructure is thermally stable and the hardness remains approximately constant for extended aging (48 h) at both temperatures, implying that it may have potential as a deep-hardenable alloy. In comparison, the aging response of the Ti–10V–2Ni alloy was marginally influenced by cooling rate when aged at 500 °C, the effect being ascribed to autocatalytic formation of α-phase in the course of slow air cooling. The results are discussed with respect to the β-phase stability and the time-temperature-transformation (TTT) characteristics of metastable beta-titanium alloys.

Effect of Oxygen Addition on Isothermal Omega Phase Stability in Ti-29Nb-13Ta-4.6Zr

A peculiar effect of oxygen on ω-phase stability, i.e., the enhancement of the isothermal ω-phase formation during aging due to oxygen addition was observed in the Ti-29Nb-13Ta-4.6Zr; this effect is contradictory to that reported conventionally. The effect was analyzed from the viewpoint of distribution of alloying elements. Oxygen and/or zirconium may dissolve in the ω phase during aging, resulting in the stabilization of the ω phase in this alloy.

Features of the ω-phase formation in zirconium and its alloys under quasi-hydrostatic pressure and dynamic loading

X-ray diffraction, optical and transmission electron microscopy and measurement of microhardness were used to study zirconium, Zr–Ti and Zr–Nb alloys loaded by quasi-hydrostatic pressure and spherical converging shock waves of different intensity. It was revealed that the phase and structural states of the specimens depend on the loading type, the loading intensity, the niobium content, and the depth of the layer in the sphere. It has been established that there are two types of defects in the ω-phase structure: linear defects of displacements of [0001] atomic rows and stacking faults in the planes {2¯ 1¯ 10} irregularly distributed on a crystal.

The ω-phase formation in titanium upon deformation under pressure

The ω-phase formation in titanium of the VT1-00 grade upon deformation under pressure has been investigated by X-ray diffraction analysis and diffraction electron microscopy. The deformation was effected by two methods: shear under a pressure of 6 GPa in Bridgman anvils and high-strain-rate equal-channel angular pressing at a pressure of about 2 GPa. Upon deformation under a pressure of 6 GPa, the ω phase is formed as grains that are isolated or clustered into groups. Upon deformation at a pressure of 2 GPa, this phase arises in the form of nanosized particles that are orientationally related to the α phase. After deformation by shear under pressure using one revolution of the anvils, new grains of the α phase up to 2–3 μm in size have been detected. The grains are nearly free of dislocations and have wavy boundaries. The origin of these grains is tracealle the reverse ω → α phase transformation that takes place upon pressure release and occurs via the “normal” rather than martensitic mechanism, at the expense of migration of the inter-phase boundaries. Upon heating, the reverse ω → α transformation at 100°C does not yet begin, whereas at 220°C the transformation proceeds almost completely. A temperature distribution in the titanium sample upon shear under pressure at a rate of 0.3 rpm has been calculated; according to this distribution, the maximum temperature rise is 12 K.

Microstructural modulation of Ti–Fe–V ultrafine eutectic alloys with enhanced mechanical properties

Systematic microstructural investigation on a series of (Ti 70.5 Fe 29.5 ) 100− x V x alloys with x = 0, 1, 3, 5 and 7 reveals that addition of V content causes microstructural refinement and formation of ω-phase. By adding small amount of V (1–3 at.%) into the Ti 70.5 Fe 29.5 binary eutectic alloy, both the strength and the plasticity of the alloys are spontaneously improved up to 2000–2200 MPa and ∼3.5%, respectively. However, large amount of V (5–7 at.%) leading to the high volume fraction of the ω-phase results in degradation in both strength and plasticity. These results suggest that the mechanical properties of the Ti–Fe–V ultrafine eutectic alloys are strongly governed by volume fraction of the eutectic matrix and ω-phases.

Formation of nano-scale ω-phase in arc-melted micron-scale dendrite reinforced Zr 73.5Nb 9Cu 7Ni 1Al 9.5 ultrafine composite during heat treatment

An arc-melted Zr 73.5Nb 9Cu 7Ni 1Al 9.5 alloy consists of dendritic body-centered cubic solid solution and a mixture of ultrafine body-centered tetragonal Zr 2Cu-type and hexagonal close-packed MgZn 2-type phases in the ultrafine matrix. This composite exhibits two exothermic reactions upon heating to 973 K with a heating rate of 40 K/min. After the first exothermic reaction nano-scale ω-phase particles have formed in the β-phase dendrites indicating a decomposition of the supersaturated solid solution upon heating. At the early stage during the decomposition of the β-phase the ω-phase is coherent with the parent phase without clear structural differences. However, semicoherency between the β- and the nano-scale ω-phase regions was observed during the growth of the ω-phase. This structural semicoherency causes nano-scale twinning in the β-phase dendrites around the ordered ω-phase in order to release the mismatch energy at the interfaces between the ordered ω- and β-phases.

Evidence of α → ω phase transition in titanium after high pressure torsion

Abstract It is well known that a high pressure ω-phase is formed in Ti at high pressures in the range between 2 and 8 GPa. This martensitic-type transformation demonstrates very large hysteresis, and hence the ω-phase can be retained in the material after release of pressure. Additionally, applied shear stresses are known to facilitate the α → ω transformation. This paper describes an investigation on the ω-phase formation after high pressure torsion under a wide range of pressures and shear strains by means of X-ray diffraction and transmission electron microscopy. We show that the ω-phase forms in Ti upon high pressure torsion deformation after 300 s under a pressure of 3 GPa. This suggests that the transformation kinetics are notably increased as compared with the kinetics of pressure-induced transformation.

First-principles study of ω-phase formation in the Ti3Al2V system

Using first-principles methods, the phase stability of the underlying body-centered-cubic (bcc) structureof Ti3Al2V and slightly rearranged atomic structures are investigated. Thecalculated ground-state energies show an instability in the ternaryTi3Al2V alloy withrespect to the ω structure-type atomic displacement. A Mulliken population analysis shows strong bondingbetween the transition metals and Al. It is shown that Ti–Al is the strongest bond and thatω-type displacements increase the population overlap for this bond and reduce the energy ofthe system. The first-principles calculations are extended to finite temperature andvarious contributions to the free energy are calculated within the quasiharmonicapproximation. It is shown that, at high temperatures, the bcc structure is stabilized bythe contribution of the low-energy modes to lattice entropy. In agreement withexperiment and in contrast to the Ti–Al–Nb system, we find that the metastableB82 structure cannot form in this alloy.

Substrate effect on the formation of ω-phase in sputtered zirconium films

Zirconium films were prepared by magnetron sputtering on steel, glass and silicon substrates. The effects of the substrate, inter-layer and compressive stress in the films on the formation of ω-phase are presented. Steel substrate promotes the growth of biphased films whereas a chromium inter-layer promotes the growth of single ω-phase.

63Cu NMR analysis of microstructure evolution in Al–Cu–Mg alloys

63Cu NMR spectroscopy has been used to detect metastable Guinier–Preston–Bagaryatsky (GPB) zones and nanoscale precipitates of equilibrium S-phase (Al2CuMg) in dilute alloys of aluminium containing copper and magnesium with compositions which lie in the α + S phase field. The GPB zones are observed to form rapidly at room temperature with a time development closely related to the Vickers hardness. The final development of S-phase in the alloy has been confirmed by the observation of a line shape in the alloy identical to that observed in a specimen prepared from stoichiometric Al2CuMg. Analysis of the hyperfine structure of the 63Cu line shape observed for S-phase shows clearly that two Cu sites are present with approximately equal population. This result suggests that possibly two crystallographically distinct Al2CuMg phases are present. The addition of small amounts of silver to Al–Cu–Mg alloys in the α + θ phase field is known to induce the formation of Ω-phase: a slight distortion of tetragonal θ-phase Al2Cu. A hyperfine-structured 63Cu line shape assigned to Ω-phase, indicating one distinct Cu site, has been observed in two separate Al–1.7 at.% Cu–0.33 at.% Mg alloys containing 0.1 and 0.18 at.% Ag, but not in the same Al–Cu–Mg alloy without Ag.

ω-phase formation in NiAl and Ni2Al alloys

The phase stability of bcc-like NiAl and Ni2Al with respect to phase transformations into the ω phase is examined by first-principles, full-potential, linear muffin-tin orbital method (FPLMTO), band-structure calculations. In agreement with experiment we find that bcc-like NiAl is stable, whereas Ni2Al is unstable. We have also performed some calculations in slightly disordered phases, and find that the disordering tends to inhibit the transformation. This supports the suggestion that a combined displacive/ordering transformation mechanism is involved in the phase transformations of the Ni2Al system. Calculated lattice parameters and the degree of internal shuffling of atoms for the ordered Ni2Al phase were found to be in excellent agreement with the experiment

ω-phase formation in a rapidly solidified Cr-40 at.% Al alloy

The diffuse ω structure has been identified by electron diffraction for the first time in a C11b matrix of the melt-spun Cr-40 at.% Al alloy ribbon. The C11b matrix consists of nanometre-scale ordered domains produced by a long-period chemical ordering in the precursor A2-B2 structure. Each C11 b unit cell in the Cr-40 at.% Al alloy is based on three cubic B2 cells along the c axis and contains two antiphase boundaries. The structure of the diffuse ω is consistent with that observed by De Fontaine. The relative stability of the crystalline ω structure has been predicted with respect to bcc-type precursor phases as a function of the displacement parameter z from calculation of the cohesive energy of the ω phase using the full-potential linear muffin-tin orbital method. The results show that the three-dimensional crystalline ω structure is stable with respect to bcc-type precursor phases in the Cr-Al system and reveals the physical background as to why the ω structure in Cr-40 at% Al is diffuse in nature.