Β-phase Stability Research Articles

Influence of oxygen and plastic deformation on the microstructure and the hardness of a Ti–Nb–Ta–Zr–O Gum Metal

In this paper, the Gum Metal Ti–29Nb–13Ta-4.6Zr-xO with two different oxygen contents (2200 and 4600 ppm) was prepared by high-pressure torsion (HPT) and their microstructural evolution, thermal stability, physical and mechanical properties were investigated for specimens prepared under different processing conditions. An increase in the β-phase stability at the expense of αʺ, as well as in the hardness was observed with the combined effect of oxygen addition and HPT processing. The increase in hardness was attributed to the microstructural refinement, to the creation of a high density of defects, and to the stress-induced ω-phase formation, which were all obtained due to the severe plastic deformation. Besides that, oxygen solid solution hardening provides a great increase in hardness. Vickers microhardness of approximately 400 HV was obtained for samples with higher oxygen content and deformed by HPT. The elastic modulus did not change significantly for the different oxygen levels, with values of 53–66 GPa. The typical Gum Metal properties were attained in the present paper, and they are directly related to its deformation mechanisms such as slip, stress-induced αʺ-martensite as well as reverse martensitic transformation (αʺ → β). The unique Gum Metal properties combined with the obtained increase in hardness make these two alloys very well suited for biomedical applications such as implants.

The effect of Zr addition on the microstructure evolution and mechanical properties of hot-rolled TiAlNbZr alloy

The microstructure and mechanical properties of hot-rolled Ti–6Al–7Nb-xZr (x = 0, 5, 10, 15, 20 wt%) (TANZ) alloys were systematically studied. Herein, the effects of phase and microstructure evolution on the mechanical properties of hot-rolled TANZ alloy were investigated by differential scanning calorimetry (DSC), X-ray diffraction (XRD), electron backscatter diffraction (EBSD), transmission electron microscopy (TEM), and statistical analysis. The addition of zirconium can change the β-phase stabilization ability of TANZ alloy and thus change its phase transformation point. Metastable phases (α′, α″, and metastable β phases) are retained during hot rolling at 850 °C and subsequent water quenching. Furthermore, the recovery of the initial α phase is promoted during hot rolling and holding. During the deformation process, the presence of both the initial α phase and the metastable phase (β transformation structure) is conducive for coordinated simultaneous deformation. The uniform deformation ability and the work-hardening behavior of TANZ alloys are significantly improved by increasing Zr content. The yield strength, true stress in tensile strength tests, and uniform elongation of the TAN15Z alloy are 848 MPa, 1160 MPa, and 9.5%, respectively. The corresponding values for the TAN20Z alloy are 810 MPa, 1331 MPa, and 11.6%, respectively. A significant improvement in the trade-off between strength and plasticity was achieved. Furthermore, the fracture mechanisms of the studied alloys were also discussed in depth.

Graded hierarchical microstructure and mechanical property of electron beam melted Ti–5Al–5Mo–5V–3Cr–1Zr

The layer-wise additive manufacturing process offers a unique approach to achieve a novel graded microstructure which can fulfill the conflicting requirement of contradictory properties. The present study investigated microstructure evolution and mechanical properties of a near β alloy Ti–5Al–5Mo–5V–3Cr–1Zr fabricated by additive manufacturing using electron beam melting. In combination, the high preheating temperature in electron beam melting, the low β transus (850 °C) in Ti–5Al–5Mo–5V–3Cr–1Zr, and the increasingly accumulated heat with build height resulted in an in-situ formed graded hierarchical microstructure from an α+β lamellar microstructure at the specimen bottom to an αp+αs bi-lamellar microstructure at the specimen top. Such a gradually changing microstructure was obtained by the same electron beam melting parameter set, and its formation was related to the controlled secondary α precipitation through manipulating the stability of metastable β-phase at different specimen heights. The resultant hardening effect in the bi-lamellar microstructure was attributed to the precipitation of fine secondary α.

B2 ordering in body-centered-cubic AlNbTiV refractory high-entropy alloys

The phase stability of a bcc AlNbTiV high-entropy alloy at elevated temperatures is studied using a combination of machine-learning interatomic potentials, first-principles calculations, and Monte Carlo simulations. The simulations reveal a B2 ordering below about 1700 K, mainly caused by a strong site preference of Al and Ti. A much weaker site preference for V and Nb is observed, strongly affecting the alloys total configurational entropy. The underlying mechanisms of the B2 phase stability as opposed to the random solid solution are discussed in terms of a high persisting configurational entropy of the B2 phase due to strong sublattice site disorder.

Systematic exploration of the L-PBF processing behavior and resulting properties of β-stabilized Ti-alloys prepared by in-situ alloy formation

Aim of this work is to gain a comprehensive understanding of the effects of an increasing β-phase stability of Ti-alloys on the L-PBF processing behavior. For this purpose, seven different Ti-alloys with an increasing concentration of the β-phase stabilizing elements Fe and V were prepared by L-PBF and in-situ alloy formation. The Molybdenum equivalent (Moeq) of the examined alloys, as a measure of β-phase stability, was varied systematically between −3.3 and 25. It is shown that a homogeneous distribution of elements is achievable by in-situ alloying. The experiments prove that the investigated alloys can be processed by a single L-PBF parameter set with high relative density above 99.8%. This finding is substantiated by calculated thermo-physical material properties and an analytical model. To understand the underlying metallurgical effects governing the L-PBF results, the samples were investigated extensively by EDS, EBSD, XRD, light microscopy and compression tests. The β-phase fraction varies in dependence of the Moeq between 0% and 99%. Because of rapid solidification inherent in L-PBF a Moeq of 10 is sufficient to receive more than 90% β-phase. The same amount of β-phase after furnace cooling was only observed in alloys with Moeq of 20 or more. While all alloy compositions can be processed with high relative density of over 99.8%, alloys with a Moeq between 15 and 20 show a brittle material behavior in as-built state, resulting in cracking during L-PBF. This behavior is attributed to the formation of ω-phase during L-PBF. In contrast, the highest β-stabilized alloy with a nominal Moeq of 25 exhibits a very high ductility with a fracture strain exceeding 50%.

Structural and mechanical properties of nanocrystalline Ga2O3 films made by pulsed laser deposition onto transparent quartz substrates

This work reports on the correlation between structure, surface/interface morphology and mechanical properties of pulsed laser deposited (PLD) β-Ga2O3 films on transparent quartz substrates. By varying the deposition temperature in the range of 25 °C–700 °C, ∼200 nm thick Ga2O3 films with variable microstructure and amorphous-to-nanocrystalline nature were produced onto quartz substrates by PLD. The Ga2O3 films deposited at room temperature were amorphous; nanocrystalline Ga2O3 films were realized at 700 °C. The interface microstructure is characterized with a typical nano-columnar morphology while the surface exhibits the uniform granular morphology. Corroborating with structure and surface/interface morphology, and with increasing deposition temperature, tunable mechanical properties were seen in PLD Ga2O3 films. At 700 °C, for nanocrystalline Ga2O3 films, the dense grain packing reduces the elastic modulus Er while improving the hardness. The improved crystallinity at elevated temperatures coupled with nanocrystallinity, the β-phase stabilization is accounted for the observed enhancement in the mechanical properties of PLD Ga2O3 films. The structure-morphology-mechanical property correlation in nanocrystalline PLD β-Ga2O3 films deposited on quartz substrates is discussed in detail.

Microstructural evolution and properties of a Ti-Nb-Ta-Zr-O prepared by high-pressure torsion

In this work, the Ti-29Nb-13Ta-4.6Zr-0.07O (TNTZO) alloy was processed by high-pressure torsion (HPT), and its microstructure, phase stability, elastic behavior, mechanical properties and deformation mechanisms were determined for samples prepared under different conditions. An increase in β-phase stability relative to martensite and a significant hardening was observed after severe plastic deformation. The increase in β-phase stability is attributed to the sharp increase in the system’s Gibbs free energy that occurs when the α"-laths reach a critical nanometer-size during the HPT processing. This system’s Gibbs free energy increase triggers the occurrence of the α"→ β reverse martensitic transformation. The greater hardness can be related to three factors: microstructural refinement; high dislocation density and stress-induced formation of the ω-phase. Plastic deformation is based on conventional slip, detwinning, stress-induced martensite formation, as well as the reverse martensitic transformation. The reverse martensitic transformation observed during HPT corresponds to a recently discovered mechanism that has not been well addressed in the literature until now. In this paper, this new mechanism is clearly detected and detailed discussed based on a systematic Rietveld refinement, in-depth microscopy, and calorimetry.

Microstructures analysis and quantitative strengthening evaluation of powder metallurgy Ti–Fe binary extruded alloys with (α+β)-dual-phase

Abstract In this study, Fe, one of the inexpensive β-phase stabilizers, was employed to fabricate Ti alloys by the powder metallurgy route. The formation mechanism of unique microstructures and texture of the extruded Ti–Fe alloy was elucidated through scanning electron microscopy-electron backscatter diffraction (SEM-EBSD) and transmission electron microscopy-energy-dispersive X-ray spectroscopy (TEM-EDS). For the Ti–Fe alloys consolidated from the pre-mixed pure Ti powder and Fe particles by spark plasma sintering and following hot extrusion, the additive Fe atoms existed as solid solution atoms in β-Ti phase. The increment in the Fe content was effective in increasing the β-Ti phase volume fraction and refining the α-Ti grains. The mean α-Ti grain size of Ti-4 wt.% Fe alloy was 1.27 μm, which was about ten times less than that of the pure Ti material (12.42 μm). The α and β phases of the extruded Ti-1–4 wt.% Fe material were aligned in parallel to the extrusion direction, and they suppressed the grain growth of each other. Although yield stress (YS) and tensile strength (TS) remarkably increased to 1093 MPa and 1183 MPa, respectively, with an increase in the Fe content, a large elongation of 28–38% was obtained in the extruded Ti–Fe alloys. These tensile properties were favorable compared to the commercial Ti-6 wt.% Al-4 wt.% V alloy. The dominant strengthening factors for the Ti–Fe alloys were α-Ti grain refinement and β-Ti hard phase dispersion. In the case of 4 wt% Fe addition, 50% of the YS increment was due to the latter strengthening mechanism.

Influence of β-phase stability in elemental blended Ti-Mo and Ti-Mo-Zr alloys

This paper investigated the improvement of mechanical properties for one of the most used biomaterials, titanium-based alloy. To improve its mechanical properties, molybdenum was chosen to be added to Ti and Ti-Zr alloys through a mechanical blending process. After homogenization of Ti (12, 15) Mo and Ti (12, 15) Mo-6 Zr, the compaction pressure and sintering temperature were varied to create pellets. Characterization has been done using scanning electron microscopy (SEM), X-ray diffraction (XRD), Vickers’s hardness, Archimedes test and ultrasonic method, and 3-point bending test. Micrograph of each pellet revealed the influence of Mo content that plays a prominent role in the variation of microstructure in the alloys Ti-Mo and Ti-Zr-Mo. The porosity and density were also influenced by changing the β-phase. EBSD analysis shows the increase in β-phase with the addition of Zr. The overall results indicated that the percentage of β-phase greatly affects the mechanical properties for the specimens.

Крихкопластичний перехід в алюмінідах титану, легованих β-стабілізаторами

The temperature, structural, and rate sensitivity of the plasticity characteristics in γ-tita¬nium aluminides with different Al contents, doped with β-phase stabilization elements, are studied. Particular attention is paid to dislocation mechanisms that control the brittle-plastic transition. The main role of grain boundaries in the formation of plasticity characteristics is demonstrated. At low temperatures, the grain boundaries stop propagation of brittle transgranular cracks and confine the development of the plastic zone beyond the boundaries of an individual grain, creating the prerequisites for fracture in the microdeformation level. At elevated temperatures, the boundaries contribute to the formation of dislocations pile-up in the plastic zone with a stress concentration required to set off the Frank-Reed sources and the displacement of the plastic zone beyond the boundaries of an individual grain, changing its configuration and stress distribution and inhibiting the propagation of cracks. Acceleration of rela¬xation processes in the vicinity of the crack’s tip creates the prerequisites for the development of macrodeformation. Local relaxation processes at the crack’s tip contribute to high speed sensitivity of the plasticity characteristics. This effect has important practical consequences, since there is a temperature region near the upper working temperature of γ-TiAl alloys, where the stress value remains high (yield strength σ02 ~700 MPa and ultimate stress σul ~ 1200 MPa at bending tests) regardless of the strain rate, while deformation sharply increases at low speeds. As a result, it is possible to achieve a combination of high strength and ductility during creep tests. In samples tested by tension with low speed (10-5 s-1) the neck formation take a place. Deformation occurs by the dislocation-twinning mechanisms. At small deformations (7%) a twinning mechanism is preferable. Concentration of dislocations sharply increases at large deformations (32%) with formation of dislocation clusters. Stress relaxation on the boundary between γ-phase twins and α2-lamella, occurs by macroscopic shift on α2-lamella. Keywords: γ-titanium-aluminides, structure, strength, plasticity, brittle-ductile transition, temperature and rate sensitivity.

Single crystal shear moduli of β-phase stabilized by thermomechanical treatment in TiNbSn alloys with ultralow elastic modulus

The single crystal shear moduli is crucial to explore the origin of low Young’s modulus for β-type titanium alloys. In this study, the single crystal elastic constants of two TiNbSn alloys with Nb concentration below a critical quantity (βc) were extracted from their polycrystalline specimens using an in-situ synchrotron X-ray diffraction technique, although it was impossible to fabricate their single crystals due to the ultralow β-phase stability. The results show that the shear modulus C’ can be further reduced by decreasing the Nb concentration to below βc. Meanwhile, strong correlation between the shear modulus C44 and the Nb concentration was observed in TiNbSn alloys, although the C44 is independent on β-stabilizing elements in binary alloys. It is proposed that ultralow Young’s modulus can be realized in the alloy composition below βc by concurrently reducing C’ and C44.

Phase stability of mechanically alloyed Ti-Fe-Al alloys

Iron alloying element addition to pure titanium can be used to enhance strengthening as well as β-phase stabilizer. However, the outcome depends on the processing route to some extent. Mechanical alloy technique is adapted in this work to produce titanium alloys with 7wt% iron content and 1wt% Aluminum. The obtained nano-size particles have two phases a and β structures and the formation of the second β-phase is mainly enhanced by the milling force. Heating the produced grain compacted powder to a temperature above the beta phase transus temperature followed by quenching to ambient temperature resulted in a-martensite partial transformation. This result can indicate that the obtained β-phase for the present alloys is not fully stabilized by the iron additions.

Anharmonicity and Phase Diagram of Magnesium Oxide in the Megabar Regime.

With density functional molecular dynamics simulations, we computed the phase diagram of MgO from 50 to 2000GPa up to 20 000K. Via thermodynamic integration (TDI), we derive the Gibbs free energies of the B1, B2, and liquid phases and determine their phase boundaries. With TDI and a pseudo-quasi-harmonic approach, we show that anharmonic effects are important and stabilize the B1 phase in particular. As a result, the B1-B2 transition boundary in the pressure-temperature plane exhibits a steep slope. We predict the B1-B2-liquid triple point to occur at approximately T=10000 K and P=370 GPa, which is higher in pressure than was inferred with quasiharmonic methods alone. We predict the principal shock Hugoniot curve to enter the B2 phase stability domain but only over a very small range of parameters. This may render it difficult to observe this phase with shock experiments because of kinetic effects.

Evaluating the phase stability of binary titanium alloy Ti-X (X = Mo, Nb, Al, and Zr) using first-principles calculations and a Debye model

To realize bottom-up design of alloys based on theoretical calculations, the thermodynamic stabilities of phases in Ti binary alloys were estimated by a combination of density functional theory calculations for the internal enthalpy energy, the Bragg-Williams approximation for the mixing entropy contribution, the Debye model for the vibrational free energy, and the Sommerfeld model for the electronic excitation entropy. The special quasirandom structure (SQS) model was used to describe the disordered distribution of the alloying element in the solid solution state. We focused on Ti–Mo, Ti–Nb, Ti–Al, and Ti–Zr binary alloys, which have different phases, such as the α phase in the hexagonal close-packed (hcp) structure and the β phase in the body-centered cubic (bcc) structure, depending on the temperature and alloying element fraction. The elastic constants, bulk modulus, and Poisson's ratios were calculated using a strain energy method. Excitations from the vibrational contribution to the quasi-harmonic Debye approximation were added to the 0 K free energy originally derived from ab initio calculations. The effect of temperature up to 1000 K on phase stability was analyzed. Furthermore, to compare phase stabilities, the free energies of formation were calculated using the ground states of the constituent phases as references. The calculated elastic property indicated the mechanical instability of most bcc Ti–Al and bcc Ti–Zr alloys, hcp Ti–Mo and hcp Ti–Nb at high fraction range. The SQS supercell models showed good agreement in elastic constant, bulk modulus, and Poisson's ratio compared to the previous experimental and theoretical results. Free energy results showed that Mo and Nb are β-phase stabilizers, Al is an α-phase stabilizer, and Zr is a neutral element. As the fraction of the alloying element changed, stabilizing or destabilizing effects were observed under different temperatures. Moreover, the linear relationship between the filling of the d band and phase stability was identified in low temperature range. For the β phase, Mo had a stronger stabilizing effect than Nb; both Mo and Nb destabilized the α phase at low temperatures, whereas high temperatures increased the stability of the α phase and the temperature effect became more significant than the element effect. In the examined temperature range, the α phase Ti–Al alloys were stable at all Al fractions, where the thermal effect was negligible. All the α Ti–Zr alloys in this study had similar stabilities to their constituent phases (hcp Ti and hcp Zr) over a wide temperature range.

Comparative experimental study of the dynamic properties and adiabatic shear susceptibility of titanium alloys

Titanium and titanium alloys are prone to adiabatic shear failure when deformed at high strain rates due to their low coefficients of specific heat and thermal conductivity. Comparative study of the adiabatic shear susceptibility of titanium alloys are seldom reported in the literature. To further understand this topic, the mechanical response of five representative titanium alloys, i.e. commercially pure titanium (TA2, α), Ti-6.5Al–2Zr–1Mo–1V (TA15, near α), commercially Ti–6Al–4V (TC4, α+β), Ti–5Al–5Mo–5V–1Cr–1Fe (TC18, near β) and Ti–3Al–8V–6Cr–4Mo–4Zr (TB9, β), is characterized over a wide range of strain rates (ranging from 0.001 to 4000s−1) and temperatures (ranging from 293 to 673K). The findings of this experimental demonstrate that the yield stress of TA2 is the lowest, while it is the highest for TC18. The strain hardening effect of titanium alloys is prominent at low strain rate, while it may be decreased at high strain rates due to the presence of β-phase stabilizers. In addition, the mechanical behavior of titanium alloys is sensitive to strain rate and test temperature. The results also suggest that the formation of ASB is related to the applied strain rate and initial test temperature, and higher test temperature and strain rate promote the occurrence of adiabatic shear localization in dynamic deformation. Moreover, for titanium alloys, a lower strain-hardening coefficient and higher strength does not correspond to high sensitivity to adiabatic shear failure, and the dynamic failure energy, with its constant value, seems to be the best indicator of ASB failure. Furthermore, our results and analysis are important in enhancing the understanding of the dynamic properties and adiabatic shear susceptibility of titanium alloys. Based on this work, engineers can choose the optimal titanium alloy to fabricate components according to the service requirements to fully utilize the potential of the material.

β-Phase Stability of Two Biomedical β-Titanium Alloys During Severe Plastic Deformation

The β-phase stability and deformation behavior patterns of two β-type titanium bioalloys, viz. Ti-Nb-Ta-Zr (TNTZ) type and Ti-Nb-Ta (TNT) type, processed by a series of intense plastic deformations have been investigated theoretically and experimentally. Firstly, theoretical analysis was carried out, including an estimation of possible deformation mechanisms based on the electronic parameters of the studied alloys identified with the aid of the Bo–Md diagram. Secondly, phase composition and structural parameters determined by x-ray diffraction (XRD) analysis revealed that the application of severe plastic deformation (SPD) induces grain refinement (in particular for one of the two alloys), accompanied by residual stress generation and some partial phase transformation. Scanning electron microscopy (SEM)/transmission electron microscopy (TEM) imaging and some measurements of the texture completed the deformation behavior analysis. TNT alloy, with higher β stability (Moeq ~ 12.5 wt.%), presented an almost unmodified β-grain dimension from 29.4 nm to 24.4 nm (and thus poor β-grain refinement), coupled with a very fine dispersion of nanometric (~ 8.4 nm) crystallites of orthorhombic α″-stress-induced martensite. TNTZ alloy, also with high β stability (Moeq ~ 10.1 wt.%), showed accentuated β-grain refinement (from 27.8 nm to 9.9 nm), with a very small amount of orthorhombic α″-stress-induced martensite, but grain dimensions almost three times larger than that of the TNT alloy (~ 20.8 nm). The theoretical estimations concerning the possible deformation mechanisms are supported by the analysis of the experimental results.

Phase transformation and mechanical properties of Ti-(10–30)Zr–3Mo–1Sn alloys

Studies on three Ti–xZr–3Mo–1Sn (x=10–30 at.%) alloys found that the influences of Zr on phase transformation and mechanical properties are closely tied to its effect on β-phase stability. It decreases the β-transus by ~8.8 °C/at.%, suppresses the formation of α-phase or martensite during cooling and enhances the room temperature super-elasticity. The influence of Zr on lattice transformation strain is weak with the calculated maximum transformation strains of ~8% in all three alloys regardless of their large differences in Zr content. The largest recoverable strain of ~3.8% was obtained in alloy 30Zr after heat treatment at 600 °C. Heat treating at 600 °C produces a α+β dual phase structure with unique texture patterns in alloys 10Zr and 20Zr. Although texture patterns of α and β phases can be understood based on their crystal orientation relationship, mechanisms that caused these textures remain unknown. Aging at 200 °C for a short time increases the super-elastic strain without sacrificing material ductility and total recoverable strain.

Effect of heat-treatment on microstructure and mechanical properties of Ti alloys: An overview

The present review paper complies the information of Ti and its alloys from basic processing to mechanical properties, along with various microstructure and heat-treatment processes. The basic processing routes and reaction to develop pure titanium from ore is explained. The role of stabilizers on pure titanium and their classifications as α-phase or β-phase stabilizers are explained with the details of respective elements. The various kinds of microstructure available in Ti or its alloys are shown and explained. The microstructure of alloy can be altered with help of processing conditions, composition and/ or cooling rate and medium. The resulted microstructure obtained after heat treatment of various Titanium alloy are lamellar, bimodal, equiaxed, basket-weave, acicular and widmanstätten, which exhibits different mechanical properties. Generally, Titanium alloy with fully lamellar structure shows higher fracture toughness and crack resistance while equiaxed structure shows high cycle fatigue resistance. Cooling rate and cooling medium also results in different microstructure. It was observed that water cooled titanium alloys shows highest hardness due to martensite formation because of rapid cooling followed by air and furnace cooling.

Basic characterization of new Ti-Mn-Zr alloys

Beta titanium alloys (β-Ti) have been a subject of great interest since the 1980s. In this study, the microstructure, mechanical and corrosion behavior of new low-cost β-type Ti-14Mn-xZr (x = 0, 3 and 6 wt%) alloys are studied. The novel alloys were fabricated by arc melting followed by annealing treatments at 900 °C for 6 hrs then quenched in ice water to achieve good homogenization. The microstructure, mechanical properties, and corrosion properties of the alloys were investigated. The XRD patterns show a single β phase in all alloys and confirmed by the microstructural analyses. Both the compression strength and the hardness of the alloys were increased by increasing the Zr-content in the alloy due to the solid solution strengthening as well as the higher β-phase stability. Also, Zr addition decreased the corrosion rate of TiMnZr alloys.

Microstructure investigation of Ti-26Nb alloy manufactured from elemental powders by electron beam melting

Alloys which are planned to be used for implants fabrication must possess excellent biocompatibility, high strength, and low Young’s modulus. A low elastic modulus, close to that of the cortical bone could significantly reduce the stress-shielding phenomenon usually occurring after surgery. Beta-titanium alloys such as Ti-Nb are good candidates for this purpose. Nb is known as a biocompatible metal used for titanium β-phase stabilization. Previous investigations indicate that the increase of Nb content results in the increase of β phase amount but the decrease of β grain size. In this study, we were aiming at the investigation of the microstructural properties of a titanium alloy manufactured by electron beam melting from the elemental powders mixture of Ti and Nb with 26 at.% of Nb. The influence of operating parameters on the efficacy of dissolving Nb particles in Ti was studied. The results obtained by SEM analysis demonstrated that electron beam energy has a significant effect on the homogeneity of the manufactured specimens. To obtain homogeneous and porosity-free specimens high energy level is required. The microstructure of these specimens was characterized.