Β-phase Stability Research Articles

Influence of Aging After Hydrogen Absorption on Fatigue Properties of Ti-20 V-4 Al-1 Sn Alloy

The influence of aging after hydrogen absorption on the fatigue properties of a β-type titanium alloy, Ti-20 V-4 Al-1 Sn, was examined. Hydrogen was absorbed to the alloy through acid-cleaning in order to make three model materials possessing different hydrogen quantities. Since hydrogen is a strong β-phase stabilizer, the precipitation of a fine α phase during aging was strongly suppressed and age-hardening was delayed with increasing absorbed hydrogen quantity. In the materials aged after hydrogen absorption, even if their hardness and tensile strength were leveled out by adjusting the aging time, fatigue strength was slightly decreased with the increase in hydrogen quantity. It was thought that such reduction in the fatigue strength resulted from the existence of weak parts where the density of the α phase precipitated with aging was lower.

First-Principles Study on Elastic Properties and ElectronicStructures of Ti-Based Binary and Ternary Shape Memory Alloys

The elastic properties and electronic structure of B2 phase binaryTiM (M = Fe, Co, Ni, Pd, Pt and Au) and ternaryTi50Ni43.75Pd6.25, Ti50Ni43.75Cu6.25 shapememory alloys are studied by the plane-wave psedudopotential methodwithin the local density approximation. The elastic constants anddensity of states are calculated. Our calculations show that themartensitic transformation behaviour of these alloys is closely relatedto their elastic properties. The Ti d DOS at the Fermi level is mainlyresponsible for the B2 phase stability of these alloys.

First-principles study on alloying effect on martensitic transformation behavior of TiNi alloy

The alloying effect on the martensitic transformation behavior of TiNi alloy is investigated for the first time from the elastic properties and electronic structure by the plane-wave pseudopotential method within the local density approximation. The elastic constants and density of states are calculated. The results show that the alloying effect on the martensitic transformation behavior of TiNi alloy is close related to its elastic properties. The Ti d density of states at Fermi level is mainly responsible for the B2 phase stability of TiNi alloy with the low addition of the third element. As for TiNiPd ternary alloy, the high martensitic transformation temperature stems from the stabilization of B19 phase TiNiPd by the high addition of Pd content.

Ti-base bulk nanostructure-dendrite composites: Microstructure and deformation

Bulk nanostructured/glassy Ti-base multicomponent alloys are attractive advanced high-strength materials for structural applications or as functional materials, e.g. for medical applications. Typically, the strength and elastic strain limits of such alloys are significantly higher than for conventional microcrystalline alloys. It is of strong interest to develop a one-step process that can directly produce such alloys in bulk form, e.g. through casting, with an in situ formed composite microstructure that combines phases with different desirable properties. Along this line, a series of (Ti–Zr–Ta)–Cu–Ni alloys designed from glass-forming alloy compositions with different volume fractions of β-Ti phase have been investigated. The introduction of the β-Ti dendritic solid solution phase enhances the ductility of the material. A further improvement can be achieved by adding isomorphous β-phase stabilizers in (Ti–Nb–Ta)–Si/In alloys. These alloys exhibit low elastic modulus (50 GPa) and very large deformability (>100% plastic strain) at room temperature without appearance of any microcracks. The microstructure of these alloys consists of a high volume fraction of bcc β-Ti phase coexisting with a nanostructured interdendritic matrix. The superplastic-like deformability of the near beta alloys is correlated with copious twinning in the micrometer-size β-Ti-dendrites leading to a nano/ultrafine filamentary structure.

First‐principles study on the elastic properties and electronic structure of TiNi‐based ternary shape memory alloys

AbstractThe elastic properties and electronic structure of B2 phase TiNi and ternary Ti50Ni43.75Fe6.25, Ti50Ni43.75Pd6.25, Ti50Ni43.75Cu6.25 shape memory alloys are studied by the plane‐wave psedudopotential method within the local density approximation. The elastic constants and density of states are obtained for the first time. Our calculations show that the martensitic transformation behavior of these alloys is closely related to their elastic properties. The Ti d DOS at Fermi level is mainly responsible for the B2 phase stability of these alloys. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Phase-Stability Dependence of Plastic Deformation Behavior in Ti-Nb-Ta-Zr-O Alloys

The authors investigated the effects of alloy content on mechanical properties to make clear a correlation between plastic deformation behavior and β-phase stability in Ti-Nb-Ta-Zr-O alloys. It was realized that there was specific compositional area in which the alloy exhibited little work hardening and minimum Young’s modulus value. The specific area was expressed by the bond order (Bo), based on theDV-Xα method, of 2.87 and the averaged electron/atom ratio (e/a) of 4.24, which corresponded to those of a multifunctional β titanium alloy, “Gum Metal.” These electronic conditions also minimized ideal strength required for plastic deformation without any dislocation activity. The deformation behavior of alloys in the specific compositional area revealed that the unique behavior could be characterized by a “giant fault.” It was also confirmed that such a compositional area corresponded to the phase boundary between the α″ martensite and β phases at room temperature.

Phase transition studies in BIMEVOX solid electrolytes using AC impedance spectroscopy

The use of Arrhenius plots of total conductivity derived from ac impedance spectroscopy measurements in characterisation of phase transitions is discussed with reference to Bi 4V 2O 11− δ and the BIMEVOX family of oxide ion-conducting solid electrolytes. The technique is shown to be superior in some cases to standard laboratory methods such as thermal analysis and variable temperature X-ray diffraction. Examples include the identification of the metastable intermediate ε-Bi 4V 2O 11− δ , β-phase stability in BIMGVOX and a kinetic study of the γ → γ’ transition in BIZNVOX.

The effect of grain size and stability on ambient temperature tensile and creep deformation in metastable beta titanium alloys

The effect of grain size in the range of 100–500 μm on the ambient temperature tensile and creep deformation behavior was investigated in a metastable Ti–9.4 wt% Mn alloy at 95% yield stress. It was observed that, as grain size increases the creep strain increases, the slip lines become coarser and the density of slip increases. An interrupt creep test showed that new slip lines were generated with time and most of the deformation occurred during the first few hours of testing. The effect of β-phase stability was studied by comparing the creep and tensile behavior of Ti–9.4 wt% Mn in this study, with previously reported results obtained for Ti–14.8 wt% V and Ti–13.0 wt% Mn at ambient (room) temperature. The Molybdenum Equivalency, which is a measure of the stability of these alloys, are 9.9, 14.3, and 19.9, respectively. The main mode of deformation in Ti–9.4 wt% Mn alloy was observed to be slip. The modes of deformation changed with the increase in stability from slip accompanied by Stress Induced Plate formation to only slip. It was also found that metastable ω-phase was present in lower stable alloys, irrespective of the observed mode of deformation. At 95% yield stress, the amount of creep strain in 200 h was found to increase with a decrease in stability of the β-phase.

Correlation Between Diagrams of Isothermal and Anisothermal Transformations and Phase Composition Diagram of Hardened Titanium Alloys

A brief review of works devoted to phase transformations in titanium alloys and published by the authors during a long period and of recent publications on the topic is presented. When plotting phase diagrams of hardened titanium alloys (metastable diagrams) the authors used as a basis their relationship to the equilibrium diagrams and the transformations of the β-phase of various compositions during hardening from the β-range temperatures. After studying the structure of hardened alloys they investigated their transformations during aging, isothermal treatment, and continuous cooling from the β-range temperatures. Using the interrelation between the phase diagrams of hardened alloys and the phase transformations of β-phase under various kinds of heat treatment the authors developed theoretical diagrams of isothermal and anisothermal transformations, which were later confirmed by experimental diagrams for pilot and commercial alloys. The suggested classifications of diagrams of isothermal and anisothermal transformations are applicable to titanium alloys of various structural classes with allowance for the coefficient Kβ of stabilization of the β-phase.

Non-empirical study of phase competition in A2–B2 mixture in Fe–Al alloy

In the framework of the coherent potential approximation we show that the interaction parameter V(0) for disordered A2 FeAl alloy strongly depends on the concentration. Data of non-empirical calculations for different compositions of B2 phase were used to extract V(0) and were compared with the parameter treated from the X-ray scattering data. Another energy parameter V(k→s) which defines the stability of B2 phase with respect to its temperature disordering is negative for all concentrations under consideration.

Effect of Co addition on oxidation behavior of IrAl

Although iridium is promising to be as an effective oxygen diffusion barrier (ODB) for ultrahigh-temperature structural materials, iridium itself is not applied to be a practical oxidation resistant coating due to its poor oxidation resistance caused by gaseous IrO 3 formation. Recently the authors proposed B2 IrAl-base smart coatings which can form an iridium layer as ODB and an aluminum layer as a protective coating on the iridium layer by oxidation. However, the oxidation resistance of binary IrAl alloys is still insufficient. In this study, the effect of Co addition to IrAl on oxidation behavior was studied where Co addition was expected to enhance B2 phase stability. Binary IrAl and IrAl containing 10 mol% Co were fabricated by reactive hot pressing method and oxidation behavior was evaluated using simultaneous differential scanning calorimetry (DSC) and thermogravimetry (TG) in O 2 up to 1873 K. X-ray difrractometry (XRD) and Scanning electron microscopy (SEM) were also carried out for alloy characterization. It was revealed for heating oxidation up to 1863 K in O 2 that the oxidized products confirmed by XRD were Ir, IrO 2 and Al 2O 3 for both the alloys and that Co 2AlO 4 was recognized only for Co-added IrAl. It was also found for isothermal oxidation up to 1673 K that thin and continuous Al 2O 3 layer was formed at surface only for Co-added IrAl. It is concluded that oxidation resistance of IrAl alloys was dramatically improved by Co addition.

Defect structures in FeAl B2 alloys

Two alloys of the Fe–Al system, having compositions in the B2 phase stability range, have been investigated concerning the kinds and concentrations of the lattice defects that can be frozen into the material by water quenching from temperatures as high as 1000°C. The specimens were characterised using positron annihilation methods, Mössbauer spectroscopy and X-ray diffraction. The values of the positron lifetimes indicate that lattice defects in the samples quenched from lower temperatures are different from those found in specimens treated at higher temperatures. The overall picture on the actual type and concentration of such defects, as also obtained from the other analytical techniques, is consistent with the literature data on similar alloys.

Sol–Gel‐Prepared Pure and Lithium‐Doped Hexacelsian Polymorphs: An Infrared, Raman, and Thermal Expansion Study of the β‐Phase Stabilization by Frozen Short‐Range Disorder

Monoclinic and hexagonal forms of celsians (BaAl2Si2O8) were synthesized by hydrolysis–polycondensation of Si/Al alkoxide mixtures. Dilatometry, Raman scattering, IR absorption, and X‐ray diffraction techniques were used to identify various hexacelsian phases and their conversion into the monoclinic phase as a function of thermal treatments and doping. Emphasis is given to the relationship between the shrinkage behavior and the static and dynamic short‐range disorders in the XO4 tetrahedra (X = Si/Al) arrangement. The α hexagonal phase exhibits a well‐defined α–βHT transition at about 300°C but a “statically disordered”βSD form, which does not vary very much with temperature and is very similar to the βHT form obtained below 300°C for some materials. This statically disordered phase is preferentially observed for fast‐sintered lithium‐free compositions and transforms gradually in the ordered form by annealing above 1100°C. Such stabilization of the high‐temperature phase by the static disorder arising from the peculiar synthesis through alkoxide hydrolysis and the gel route, which freeze the dynamic disorder of gel‐forming entities in a static disorder at the gelation step, is discussed in the light of previously observed cases. The thermal expansion behavior is very sensitive to the synthesis conditions, doping, and thermal treatments. The ordered α phase is more easily achieved with lithium‐doped compositions, but lithium addition shifts the hexagonal‐to‐monoclinic phase transformation onset toward lower temperatures and promotes complete transformation with thermal annealing.

Influence of thermal annealing on the martensitic transitions in Ni–Ti shape memory alloys

We report mainly on resistance R(T) measurements between 4.2 and 1300 K on NixTi100−x alloys, with 51<x<54.5at%, quenched from different annealing temperatures TA to room temperature. When quenched from the B2-phase stability range (TA=1273 K), alloys with 51<x<54 show an increase in R(T) with decreasing T below 300 K. Subsequent annealing at TA=653 K (1 h) and quenching leads to a reduction of the R(T) anomaly below 320 K and the occurence of a martensitic transition (MT) from the B2- to the R-phase, with TR=310 K, rather independent of x. After annealing at 723 K (1 h) and 823 K (1 h), respectively, two-step MTs occur from B2 to R and subsequently to B19′, with MS(B19′) depending on x and TA. After annealing at 923 K or higher TA, MTs cannot be found anymore, and the R(T) behaviour is similar to that after quenching from 1273 K. Studies of R(T) at high T on samples quenched from 1273 K reveal the occurence of mainly two annealing stages. The first one at around 500 K marks structural changes inducing the martensitic phases at lower temperatures. The second one at about 900 K marks the formation of the B2-phase and the disappearance of other phases triggering the MT. The R(T) results are compared with the thermal expansion a(T) and X-ray investigations. The structural phase diagram of Ni–Ti around NiTi is discussed.

Electronic structure and stability of Ti-based B2 shape-memory alloys: By LMTO-ASA

The electronic and structural properties for TiM alloys (where M stands for Fe, Co, Ni, Pd, Pt, Cu, Ag and Au) with B2 (CsCl) structure are systematically studied using an LMTO-ASA method. Our discussion gives a detailed explanation of correlations between B2 phase stability and electronic structure for these alloys. Using two parameters, (1) overlapped area of site and momentum decomposed DOS, (2) shift of the center of partial band, the mechanism of interbonding which strongly determines phase stability can be explored further. The difference electron densities contour maps also help to give a clear picture of the interbonding situation in some of these alloys. Our results may give a direction to the work of designing new B-structured functional materials.

The Effects of Coke Deposition on NiMoO4Used in the Oxidative Dehydrogenation of Butane

The effects of coke deposition on the NiMoO4catalyst used for oxidative dehydrogenation ofn-butane have been investigated. The coke deposition rate and the catalyst changes during this process were studied by TG, FTIR, and XRD. Coke deposition induces segregation of the NiMoO4into MoO3and NiO. It was observed that the coke elimination in He/air flow for 30 min is not complete and the catalyst recovers only partially its structure. On the other hand, it was observed that coke deposition pretreatments lead to stabilization of the NiMoO4β-phase at room temperature. If the coke is totally eliminated, the β-phase is again unstable. The expected catalyst deactivation with coke deposition was not observed. To the contrary, the conversion increases about 40% after such treatments, and the selectivity to dehydrogenation products also increases. Especially, the butadiene selectivity is more than twice higher. The increase of conversion and selectivity is ascribed to coke catalytic activity and β-phase stabilization.

Size effects in phase transitions of semiconductor nanoparticles embedded in glass

Phase transitions of copper halide particles have been investigated by optical means. A decrease of particle melting temperature with decrease of size has been found. It is shown that the observed size dependencies of crystallization and melting temperatures can be described with practically the same value of surface tension in the framework of homogeneous nucleation theory and particle melting model in which the idea about thermally activated formation of a liquid layer on the particle surface is used. It has been found that the lowest temperature at which high temperature modification of CuBr (β-phase) can be stabilized due to extra annealing of samples depends weakly on particle size. Decrease of melting temperature prevents stabilization of β-phase for particles having radii less than 5 nm.

Interfacial Reactions in Metal-Matrix Composites

ABSTRACTThe interfacial reaction between Al2O3 (alumina) and a β-Ti alloy has been characterized by transmission electron microscopy, scanning electron microscopy, and X-ray energy-dispersive spectroscopy. Diffusion bonding single-crystal alumina and a β-Ti alloy was found to produce three interfacial regions: a region of intermetallics (Tl3Al and TiAl) located near the alumina interface, an α-Ti region, and a β-Ti region (rich in Mo, the β-phase stabilizer). Of the intermetallics to form, Ti3Al was found to form first and have an aligned, planar interface with the alumina. TiAl formed second and was found to separate grains of Ti3Al and the alumina. Reaction products observed in the diffusion-bonded alumina/β-Ti couples are compared with those observed in metal-matrix composites (MMCs), where a β-Ti alloy matrix is reinforced with alumina fibers. Different coatings used in MMCs are investigated for their ability to prevent the reaction between the matrix and fibers.

Electron-positron annihilation in Ti based compounds with B2 (CsCl) structure

The angle correlations of annihilation radiation ( ACAR ) in TiMe shape memory intermetallic compounds ( where Me stands for Fe, Co, Ni, Pd, Pt, and Au ) were investigated. It was established that in the sequence of compounds studied, the ACAR gradually changes from the average of the correlations corresponding to the two pure components toward those close to the pure Ti. Consideration of the conduction electrons as free electron gas allowed us to determine the valence band widths and the numbers of sp electrons. Using positron annihilation we also obtained information concerning the B2 phase stability in the studied compounds.

Electronic structure and stability of Ti-based B2 shape-memory compounds: X-ray and ultraviolet photoelectron spectra.

The evolution of x-ray photoelectron and ultraviolet spectra and their modification at the phase transformation B2-B19(B19') of equiatomic TiM and quasibinary Ti(Ni,M) compounds, where M stands for Fe, Co, Ni, Pd, Pt, Au, and Cu, are presented. Investigation of x-ray photoemission spectroscopy includes valence bands, satellites, and core-level energy studies. As the atomic number of M increases (both within the same period and along the group), its d states become more localized, the maximum and the center of gravity of the d bands shift towards the bottom of the valence band, and the M contribution to the density of states (DOS) at the Fermi level, N(${\mathit{E}}_{\mathit{F}}$), degrades. The DOS localization is accompanied by a spatial localization of the M d electrons resulting in weakening of the d-d covalent bonds between the alloy components and, thus, destabilizing the B2 phase. While the contribution of M d electrons to the DOS at ${\mathit{E}}_{\mathit{F}}$ decreases, the Ti d electron portion increases to such an extent that, e.g., in TiPd and TiAu, the M contribution to N(${\mathit{E}}_{\mathit{F}}$) is almost negligible. The increase in core-level energy of all partners in compounds with respect to the elemental state is discussed in terms of covalency. The M d band localization leads to an increase of many-body interactions and, as a consequence, we managed to observe the Pd satellites without resonance enhancement. The B2-B19' phase transition is accompanied by modification of the DOS which affects the Ti d states in general. Having analyzed the electronic structure we introduce parameters controlling the B2 phase stability and suggest a phenomenological formula for the martensitic-transformation temperature which satisfactorily describes the experimental behavior.