Omega Phase Research Articles

Improvement of pseudoelasticity and ductility of Beta III titanium alloy--application to orthodontic wires

The pseudoelasticity of metastable Beta III titanium alloy (TMAtrade mark) used for orthodontic applications is obtained by cold wiredrawing. This wire has higher rigidity than cold-drawn NiTi (Nitinoltrade mark, superelastic NiTi SE) and lower recoverable deformation. The low ductility value of Beta III is due to the deformation imposed by wiredrawing. The aim of this research was to improve the behaviour of this alloy by modifying the microstructural parameters to decrease the rigidity and increase the recoverable deformation and ductility of the alloy. The effects of second phase precipitate, grain size, and deformation on the wire mechanical properties were also examined. The isothermal precipitation of alpha (alpha) or omega (omega(isoth)) phases precludes the expression of the pseudoelastic effect. The presence of an omega(isoth) phase considerably increases fracture strength, whereas the alpha phase strongly decreases the ductility and adversely affects the strain recovery (epsilon(r)). To control the grain size, the growth of the recrystallized grains was studied by considering several parameters, which are known to have an influence on grain size, including the cold rolled strain, the temperature, the time of annealing, and the initial grain size. A structure with coarse grains, quenched from a temperature higher than the beta transus (T(beta)), associated with a plastic pre-deformation, contributed to an improved pseudoelastic behaviour, due to the presence of a reversible martensite phase (alpha'') induced by the pre-deformation.

Novelties of cyanide displacement reaction in ibuprofen amide process by phase transfer catalysis: Solid–liquid versus solid–liquid (omega)–liquid systems

This work deals with the phase transfer catalysed cyanide displacement reaction on 1-(4-isobutyl phenyl) ethyl chloride to synthesize 2-(4-isobutyl phenyl) propionitrile, which is an intermediate for the synthesis of ibuprofen analogs, belonging to a class of NSAID (nonsteroidal anti-inflammatory drugs). The reaction was studied using solid–liquid phase transfer catalysis (S-L PTC) with trace quantities of water, forming the so-called omega phase at 90 °C. The rates of reaction and selectivity to the product are enhanced in the S-L(org.)-L (ω) PTC in comparison with S-L PTC, which in turn is superior to L-L PTC; the latter suffers from the disadvantage of side reactions in the aqueous phase. In the current work, the effects of various parameters such as catalyst structure, catalyst loading, substrate loading and temperature were studied on the conversion and rates of reaction of 1-(4-isobutyl phenyl) ethyl chloride with solid sodium cyanide under S-L and S-L(ω)-L PTC at 90 °C with toluene as the organic solvent. Tetrabutylammonium bromide (TBAB) was found to be the best catalyst. The role of omega liquid phase in intensification of the S-L PTC was theoretically and experimentally investigated. The kinetic constants have been determined and the apparent activation energy is found as 4.2 kcal/mol, which suggests that the reaction is quite fast, which is likely to bring in mass transfer effects.

Nucleation of athermal omega phase in Cu–Zn system

Nucleation of the athermal omega (ω) phase transformation in the body-centered cubic parent phase is studied by evaluating the reciprocal free energy which includes the properties of coherent strain energy and the bulk free energy change. The droplet model of the omega formation is combined with the Landau-type of the free energy expression of phase transformation. The metastable ω cluster having a two dimensionally expanded dual phase structure is suggested to form in the heterophase fluctuation. Quantitative calculation of free energy has identified the critical transformation temperature T 0 = 300 K in β-brass that is defined as the temperature at which the free energy of the metastable phase is the same as the free energy of the parent phase. The continuous transformation from the metastable ω cluster to a stable ω phase is shown to occur at the transformation temperature T ω that is well below T 0.

The influence of oxygen content on the α to ω phase transformation and shock hardening of titanium

In this study the influence of alloy chemistry on the propensity of omega phase formation in two titanium alloys during shock loading is examined. The effect of peak shock stress on the phase stability and substructural evolution of high purity and A-70 (3700ppm oxygen) titanium was probed utilizing velocity interferometer system for any reflector and post shock substructural analysis. While in the high purity titanium the alpha to omega phase transformation was found to occur at 10.4GPa, no transformation was observed in the A-70 material for stresses up to 35GPa. Transmission electron microscopy analysis and neutron diffraction of shock-recovered samples confirmed these results and probed the details of twin and dislocation structures. Debye temperature data are also presented and the Debye-Waller temperatures for the alpha and omega phases in the high purity titanium are calculated. Finally, the compressive, quasistatic reload behaviors of both high purity and A-70 titanium are compared to the as-annealed counterparts at comparable strains and discussed in terms of the phase transformation and the shock-induced defect generation.

Laser‐deposited Ti‐Nb‐Zr‐Ta orthopedic alloys

The complex quaternary Ti-35Nb-7Zr-5Ta orthopedic alloy has been successfully deposited from a powder feedstock consisting of a blend of elemental titanium, niobium, zirconium, and tantalum powders, using the laser engineered net-shaping (LENStrade mark) process. In the as laser-deposited form, these alloys exhibit a substantially higher tensile strength as compared with more conventionally processed counterparts of similar composition, while maintaining excellent ductility and a low modulus. Furthermore, the as-deposited alloys appear to exhibit a <001> texture, with a substantially large number of grains of the beta phase aligning one of their <001> axes nearly normal to the substrate or parallel to the growth direction. The microstructure of the as-deposited as well as tensile-tested alloys have been characterized in detail using scanning electron microscopy (SEM), orientation microscopy (OM), and transmission electron microscopy (TEM). Formation of a high density of shear bands, possibly arising from slip localization due to precipitates of the omega phase in the beta matrix, is clearly evident in the tensile-tested sample. The enhanced tensile strength and low modulus in these laser-deposited alloys coupled with the ability to form near-net shape components makes LENS an attractive processing technology for orthopedic implants.

Three Phase Catalytic Reactions: Role of Omega Aqueous Phase in Solid-Liquid Phase Transfer Catalyzed Etherification of 2'- Hydroxy Acetophenone

Phase Transfer Catalyst (PTC) is used in more than 600 commercial processes, with wide ranging applications in the pharmaceuticals, agrochemicals, polymer industries. Despite thousands of publications on the PTC, several intricacies of PTC in reactions are not understood or properly modeled. Liquid-liquid (L-L) PTC is inferior to solid-liquid (S-L) phase transfer catalyzed reactions, which in turn can be converted into Solid (reactant)-Liquid-(aqueous omega)- Liquid (organic) using trace quantities of water to realize 100% selectivity to the product. There is great enhancement in rates of reactions in comparison with corresponding S-L or L-L PTC. Thus, the O-alkylation of sodium salt of 2'-hydroxy acetophenone was achieved using 1-bromopentane using tetrabutylammonium bromide as the catalyst at 90 0C.2’-Pentyloxy acetophenone is used as a starting material for synthesis of fungicides. The current study deals with the efficacy of so-called omega phase with special emphasis on kinetic and modeling aspects.

Rate intensive and selective etherification of vanillin with benzyl chloride under solid–liquid phase transfer catalysis by aqueous omega phase

The alkylation of vanillin, containing three reactive groups, with benzyl chloride is challenging and can lead to both C- and O-alkylated products, particularly when liquid–liquid (L–L) phase transfer catalysis (PTC) is used. In the current studies, solid–liquid (S–L) PTC has been employed in the reaction of solid sodium salt of vanillin and benzyl chloride in toluene at 90 °C to make 4-benzyloxy vanillin, an ether, which is used as a perfume and also as a starting material for the synthesis of thalifoline, ephedradine as alkaloids and in synthesis of flavonoid compounds. The selectivity towards the desired product under S–L PTC is 100%. The rates of reaction are enhanced greatly by using trace quantities of water (the so-called omega phase (ω)) with 100% selectivity to the ether. Efficacy of various phase transfer agents such as TBAB (tetra- n-butyl ammonium bromide), TBAHS (tetra- n-butyl ammonium hydrogen sulfate), TPAB (tetra- n-propyl ammonium bromide), TEAB (tetraethyl ammonium bromide) and ETPB (ethyl triphenyl phosphonium bromide) was evaluated under otherwise similar conditions at 90 °C and explained. The order of activity was as follows: TBAB (maximum) > TBHS > TPAB > TEAB > ETPB (minimum). A theoretical model is developed to account for the enhancements in rates of reaction and selectivity to the ether. The kinetic constants have been evaluated and the apparent activation energy is determined as 9.64 kcal/mol. The results are novel.

Structural Properties and Stability of Metastable Phases in the Zr-Nb System

The lattice parameters of the bcc (b) and omega (W) phases occurring metastably 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 b and W phases in alloys with 10 and 18 at.% Nb was monitored using neutron thermodiffraction. A method of analysis is applied to the data which involves a confrontation between the observed structural properties and an idealised -or ``reference'- behaviour (RB) which admits a simple mathematical description. A generalised form of Vegard's law is adopted as RB for the b phase, whereas a specific RB is proposed for the W 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 W to the b phase. Finally, the neutron diffraction data on the W phase are 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.

Heterophase fluctuations of incommensurate omega phase in Cu–Zn system

A heterophase fluctuation of omega (ω) phase in the body-centered cubic matrix is studied by evaluating the reciprocal free energy modulus which includes the properties of coherent strain energy and the bulk free energy change. As a consequence, analytical expressions are obtained for the concentration, dimensions and distribution of heterophase ω clusters. The incommensurate ω cluster having a dual phase (ω and β) structure is suggested to form in the heterophase fluctuation. On the basis of the theoretical analysis of the distribution of ω clusters, quantitative calculation of diffuse intensity of X-ray diffraction was carried out for the Cu–Zn system. The estimated diffuse intensities I( h)/ I 0 are in good agreement with the experimental results: the offset of the diffuse intensity peak and ellipsoidal shape of the intensity having streaks towards the nearest fundamental spot. The diffuse peak of X-ray diffraction is manifested to behave like a central phonon peak of heterophase fluctuation as T/( T − T 0) where T 0 is the coherent transformation temperature.

Growth Kinetic Model for Isothermal Omega Phase Particles in Low-Cost Beta Titanium Ti&amp;ndash;6.8Mo&amp;ndash;4.5Fe&amp;ndash;1.5Al Alloy

The growth kinetics of the isothermal omega phase in Ti–6.8Mo–4.5Fe–1.5Al can be monitored by the continuous measurement of the electrical resistivity as shown in our previous publication in this Journal. The rate law for the growth of these isothermal omega particles radius riso is obtained assuming the resistivity to be proportional to the surface of the interface between the beta matrix and the quasi spherical isothermal omega particles. The rate law r3iso∝t is observed at the beginning of the growth process, that is before the interaction of the outward diffusion fields from the particles. A mathematical model based on a diffusional transfer of the beta stabilizers from the ωiso particles through the interface between the ωiso particles into the beta matrix is developed. The growth rate slows down when the outward diffusional fields from the particles interact. Ostwald ripening is not observed.

Heterophase fluctuation of omega phase and X-ray diffuse scattering from dual phase structure

Abstract Heterophase fluctuation of athermal omega embryos has been analyzed by assuming a dual phase structure of omega embryos composed of omega and bcc matrix phase. The two-dimensional modulation of dual phase was suggested from the quantitative estimation of coherent free energy of omega embryos using microscopic theory of elasticity and the Landau anharmonic theory for phase transformation. The X-ray diffraction theory was developed in connection to the formation of omega embryos having the dual phase structure. The offset of the diffuse peak position from the ideal omega point in the X-ray diffraction pattern is attributed to the dual phase (incommensurate phase) of omega embryos. It was also shown that the ellipsoidal shape of the diffuse intensity tailing toward the fundamental spot of the matrix phase is originated from the equilibrium shape of the omega embryo. The quantitative estimation of elastic energy modulus (EEM) in the disordered bcc matrix and in the ordered bcc matrix indicates a difference in the deviation amount of the minimum point k ( q m ) from the ideal omega point k ( q ω ) and a difference in the elliptical shape of embryos.

Fatigue Performance of Low Rigidity Titanium Alloy for Biomedical Applications

Microstructures of Ti-29Nb-13Ta-4.6Zr (TNTZ) aged at temperatures between 573 and 723 K after solution treatment at 1063 K have super fine omega phase, or both super fine alpha and omega phases, respectively in beta phase with an average grain diameter of 20 µm. Plain fatigue strength of TNTZ aged after solution treatment is much greater than that of as-solutionized TNTZ in both low cycle fatigue and high cycle fatigue life regions. This is due to the improvement of the balance of strength and ductility by the precipitation of alpha phase. Fretting fatigue strength of TNTZ conducted with various heat treatments decreases dramatically as compared with their plain fatigue strength in both low cycle fatigue and high cycle fatigue life regions. In this case, the decreasing ratio of fretting fatigue life increases with increasing the small crack propagation area where both the tangential force and frictional force at the contact plane of pad exist. In fretting fatigue in air, the ratio of fretting damage (Pf/Ff), where Pf and Ff stand for plain fatigue limit and fretting fatigue limit, respectively, increases with increasing elastic modulus. In fretting fatigue in Ringer’s solution, the passive film on specimen surface is broken by fretting action in TNTZ, which have excellent corrosion resistance, and, as a result, corrosion pits that lead to decreasing fretting fatigue strength especially in high cycle fatigue life region, are formed on its surface.

Design of new titanium alloys for orthopaedic applications.

Parallel to the biofunctionalisation of existing materials, innovation in biomaterials engineering has led to the specific design of titanium alloys for medical applications. Studies of the biological behaviour of metallic elements have shown that the composition and structure of the material should be carefully tailored to minimise adverse body reactions and to enhance implant longevity, respectively. Consequently, interest has focused on a new family of titanium alloys: Ti-6Mo-3Fe-5Ta, Ti-4Mo-2Fe-5Ta and Ti-6Mo-3Fe-5Zr-5Hf alloys. The non-toxicity of the specially designed titanium alloys compared with osteoblastic cells has been ascertained using MTT and RN tests. In addition, phase transformations upon thermal processing have been investigated, with comparison with a well-defined beta titanium alloy. Optimum thermal processing windows (above 550 degrees C) have been designed to generate a stable and nanostructured alpha phase from the isothermal omega phase that precipitates in a low temperature range (150-350 degrees C). The generation of such nanostructured microstructures should provide a promising opportunity to investigate tissue-biomaterial interactions at the scale of biomolecules such as proteins.

Characterization of microstructure in laser-surface-alloyed layers of aluminum on nickel

In order to obtain basic understanding of microstructure evolution in laser-surface-alloyed layers, aluminum was surface alloyed on a pure nickel substrate using a CO2 laser. By varying the laser scanning speed, the composition of the surface layers can be systematically varied. The Ni content in the layer increases with increase in scanning speed. Detailed cross-sectional transmission electron microscopic study reveals complexities in solidification behavior with increased nickel content. It is shown that ordered B2 phase forms over a wide range of composition with subsequent precipitation of Ni2Al, an ordered omega phase in the B2 matrix, during solid-state cooling. For nickel-rich alloys associated with higher laser scan speed, the fcc gamma phase is invariably the first phase to grow from the liquid with solute trapping. The phase reorders in the solid state to yield gamma' Ni3Al. The phase competes with beta AlNi, which forms massively from the liquid. The beta AlNi transforms martensitically to a 3R structure during cooling in solid state. The results can be rationalized in terms of a metastable phase diagram proposed earlier. However, the results are at variance with earlier studies of laser processing of nickel-rich alloys.

Novelties of Solid−Liquid Phase Transfer Catalysed Synthesis of α-Isopropyl-p-chlorophenyl Acetonitrile from p-Chlorophenyl Acetonitrile

C-Alkylation is an important class of synthetic reaction employed in the fine chemical industry. Although a number of C-alkylation reactions are reported under liquid−liquid (L−L) phase transfer catalysis (PTC) leading to intensification of reaction rates, there is a dearth of information on similar reactions under the solid−liquid (S−L) PTC including kinetics and mechanism. S−L PTC offers higher rates of reaction and better selectivities to the desired product vis-à-vis L−L PTC. This contribution reports the novelties of S−L PTC for C-alkylation of p-chlorophenyl acetonitrile to α-isopropyl-p-chlorophenyl acetonitrile which is an important intermediate in the synthesis of a pyrethroid insecticide named fenvalerate. p-Chlorophenyl acetonitrile can be mono- or dialkylated, depending on the reaction conditions, and it is a versatile synthetic intermediate in the sense that the nitrile function can be hydrolysed, reduced, or added to by organometallics after an alkylation has been carried out. It is found that the S−L PTC process is more intensified and selective. The mechanistic details of C-alkylation under basic conditions and the unexpected side reaction of oxidative decyanation are studied. The production of water as a byproduct in the reaction to form the omega (ω) phase and the mechanism in reference to the S−L−ω-phase system is described. A numerical model is composed to calculate the values of various pseudo rate constants and validate the simulated profiles against experimental data. The finer aspects of selectivity in C-alkylation reaction are analysed.

Mechanical properties and grindability of dental cast Ti-Nb alloys.

Aiming at developing a dental titanium alloy with better mechanical properties and machinability than unalloyed titanium, a series of Ti-Nb alloys with Nb concentrations up to 30% was made. They were cast into magnesia-based molds using a dental casting machine and the mechanical properties and grindability of the castings were examined. The hardness of the alloys with Nb concentrations of 5% and above was significantly higher than that of titanium. The yield strength and tensile strength of the alloys with Nb concentrations of 10% and above were significantly higher than those of titanium, while the elongation was significantly lower. A small addition of niobium to titanium did not contribute to improving the grindability of titanium. The Ti-30% Nb alloy exhibited significantly better grindability at low grinding speed with higher hardness, strength, and Young's modulus than titanium, presumably due to precipitation of the omega phase in the beta matrix.

Effect of chromium content on structure and mechanical properties of Ti-7.5Mo-xCr alloys.

The present work is a study of a series of Ti-7.5Mo-xCr alloys with the focus on the effect of chromium content on the structure and mechanical properties of the alloys. Experimental results show that low hardness, strength and modulus binary Ti-7.5Mo alloy is comprised primarily of fine, acicular martensitic alpha' phase. When 1 wt % Cr is added, a small amount of beta phase is retained. With 2 wt % or more chromium added, the entire alloy becomes equi-axed beta phase with bcc crystal structure. The average beta grain size decreases with Cr content. When the alloy contains about 2-4 wt % Cr, a metastable omega phase is present. In Ti-7.5Mo-2Cr alloy appears the highest omega intensity accompanied with high microhardness, bending strength and modulus. The omega-induced embrittling effect is most profound in Ti-7.5Mo-2Cr alloy that exhibits a terrace type fracture surface covered with numerous micron-sized dimples. The alloys with higher Cr contents show normal ductile type fractography with much larger deformation dimples. The present results indicate that Ti-7.5Mo-(4-6)Cr alloys seem to be potential candidates for implant application.

An Analytical Electron Microscopy Study of the Initial Stage of Formation Processes of Aged Omega Phase Crystals in a Ti-15Mo Alloy due to Aging at 323 K

An Analytical Electron Microscopy Study of the Initial Stage of Formation Processes of Aged Omega Phase Crystals in a Ti-15Mo Alloy due to Aging at 323 K

Beta to Omega Phase Transformation in a Ti-Mo Alloy Deformed in an Impact Compression Mode Due to Aging

Beta to Omega Phase Transformation in a Ti-Mo Alloy Deformed in an Impact Compression Mode Due to Aging

Microstructural Features of Fusion Welded 2024-T351

Microstructural features have been investigated in fusion welded samples of the established Al-Cu-Mg-Mn airframe alloy, 2024-T351. Specifically, the local strengthening and precipitate conditions have been assessed for variable polarity plasma arc (VPPA) welding of 13mm thick plate. Microstructures were studied using a combination of differential scanning calorimetry (DSC) and transmission electron microscopy (TEM). Micro-hardness measurements (Vickers hardness) were carried out, identifying a two peak structure in the heat affected zone (HAZ) and a minimum plateau in the fusion zone. The influences of aging, re-solutionising and re-precipitation are noted across the HAZ, along with the incidence of the omega(Al2Cu) phase.