Β-phase Transition Research Articles

Achieving superior mechanical and corrosion properties in medium-thickness Ti-6Al-4 V alloy joints by back heating assisted friction stir welding below β-phase transformation temperature

The equiaxed microstructure formed below the β-phase transition point of Ti alloys generally exhibits an excellent balance of mechanical properties and corrosion resistance. However, this desirable microstructure is impossible to be obtained in fusion welded joints but is prospective to be achieved by the solid-state friction stir welding (FSW) technique. Unfortunately, it generally generates bottom defects and severe tool wear in medium-thickness Ti alloy joints when conventional FSW was conducted below the β-phase transition point. In this study, 6 mm thick Ti-6Al-4 V plates were joined by both the conventional FSW and back heating assisted FSW (BHAFSW). Defects caused by significant tool wear and bottom phase transition differences occurred in the conventional FSW. It was found that the hardness difference between the base material (BM) and the tool increased to 35.6 % from 500 °C to 900 °C. The back heating (150 °C) was used to control welding temperatures remaining the 900 °C, thus largely reducing the tool wear by increasing the hardness difference. In addition, the back temperature compensation increased the bottom temperature and controlled the phase transition position from the bottom to the middle of the joint. The shoulder pressure contributed to the compression of the defects, and the defects were eliminated by increasing the pressure at the phase transition position. A significantly refined equiaxed microstructure with an average grain size of ∼0.9 μm was achieved below the β-phase transition temperature in the stir zone via back heating assisted FSW, while a bimodal structure with an average grain size of 3.5 μm was formed near the β-phase transition temperature. Inconspicuous reduction of the strength was detected for the joints (98 % of the BM) which possess equiaxed microstructures, and the corrosion resistance of the joints was enhanced compared to the BM. This superior synergy of mechanical and corrosion properties exceeded the majority of Ti alloy joints previously reported. This study provided an effective method for obtaining medium-thickness Ti alloy joints with ultrafine equiaxial structures with superior mechanical properties and corrosion resistance.

In Situ Annealing Effect on Thermally Co-Evaporated CsPbI2Br Thin Films Studied via Spectroscopic Ellipsometry.

All-inorganic cesium lead halide perovskites possess excellent thermal stability, a feature that renders them highly favorable for optoelectronic applications with an elevated thermal budget. Employing a coevaporation approach for their deposition holds promise for manufacturing at an industrial level, owing to improvements in device scalability and reproducibility. For unlocking the full potential of vacuum-evaporated perovskite thin films, it is crucial to delve deeper into their crystallization process, which, as a solid-state reaction, has been less investigated compared to the crystallization process of, most commonly used, solution-based methods. In this work, we employ spectroscopic ellipsometry, a nondestructive, high speed, and high accuracy characterization method, to study the real time annealing effect on thermally coevaporated CsPbI2Br thin films in a temperature range between 25 and 300 °C. We achieve this by developing a singular dynamic model that can be fitted in real time as a function of temperature, providing insights into how thermal annealing influences the perovskite film's morphology and optical constants. Based on the latter, we derive the temperature dependence of the thermo-optic coefficient and Urbach energy as well as analyze the interband transition energies via critical point analysis. We demonstrate that the γ- to β-phase transition can be identified through a pronounced shift in the bandgap energy, whereas the β- to α-phase transition can be discerned by a sharp increase in the film's roughness. We corroborate the obtained fit results with additional in- and ex situ measurements, such as in situ grazing incidence wide-angle X-ray scattering, atomic force microscopy, reflectance/transmittance, and profilometry.

In situ activation of flexible magnetoelectric membrane enhances bone defect repair

For bone defect repair under co-morbidity conditions, the use of biomaterials that can be non-invasively regulated is highly desirable to avoid further complications and to promote osteogenesis. However, it remains a formidable challenge in clinical applications to achieve efficient osteogenesis with stimuli-responsive materials. Here, we develop polarized CoFe2O4@BaTiO3/poly(vinylidene fluoridetrifluoroethylene) [P(VDF-TrFE)] core-shell particle-incorporated composite membranes with high magnetoelectric conversion efficiency for activating bone regeneration. An external magnetic field force conduct on the CoFe2O4 core can increase charge density on the BaTiO3 shell and strengthens the β-phase transition in the P(VDF-TrFE) matrix. This energy conversion increases the membrane surface potential, which hence activates osteogenesis. Skull defect experiments on male rats showed that repeated magnetic field applications on the membranes enhanced bone defect repair, even when osteogenesis repression is elicited by dexamethasone or lipopolysaccharide-induced inflammation. This study provides a strategy of utilizing stimuli-responsive magnetoelectric membranes to efficiently activate osteogenesis in situ.

Effects of HIP Process Parameters on Microstructure and Mechanical Properties of Ti-6Al-4V Fabricated by SLM

Ti-6Al-4V titanium alloy products formed by selective laser melting (SLM) are characterized by high strength and low plasticity. In addition, there may be pores inside the material, which may become a fracture sprouting point and accelerate the failure of the parts. Using an optical microscope (OM), scanning electron microscope (SEM), and electronic universal testing machine, the effects of hot isostatic pressing (HIP) parameters on the microstructure and tensile property of SLM-formed Ti-6Al-4V titanium alloy were investigated. The results show that HIP performed below the β-phase transition temperature, and the structure of the Ti-6Al-4V titanium alloy is composed of an α phase and β phase. With the increase in the HIP temperature, the α lath coarsens into a short rod, the content of the β phase increases and coarsens, and the tensile strength and yield strength of Ti-6Al-4V show a decreasing trend. With an HIP process performed at a temperature of 910 °C and pressure of 130 MPa for 2 h, the Ti-6Al-4V titanium alloy obtains the best matching of strength and plasticity.

Nanoparticle size and surface chemistry effects on mechanical and physical properties of nano-reinforced polymers: The case of PVDF-Fe3O4 nano-composites

In the present work, PVDF - Fe3O4 nanoparticle (NP) nanocomposite films were produced using the electrospinning method. We investigated the effect of NP size on the film's morphology (fiber size), mechanical properties, and physical properties (β-phase percentage). Surprisingly, while nanoparticle size acts as an enhancer for mechanical properties, it appeared to act as an inhibitor in terms of its effects on the crystallization of the β-polymorph. This result seemed in discordance with many previous results. A focus on local interactions between the NP surface chemistry and PVDF chains revealed the influence of grafted ligands at the nanoparticle surface on the crystallization of the piezoelectric phase of PVDF. The results from the molecular dynamics (MD) simulations for systems of PVDF chains with slabs of –OH and oleic acid-grafted magnetite, showed that the probability of beta phase configuration decreases when the nanoparticles are functionalized with oleic acid and becomes more probable for –OH terminated magnetite. These computational results are in accordance with our experimental results. To verify this hypothesis, we prepared films with washed nanoparticles to eliminate the excess oleic acid that acts as a β-polymorph inhibitor. As a result, the amount of β-phase obtained for washed nanoparticles increased and the difference in the amount of β-phase between the different samples decreased. Moreover, when heated, the films of nanocomposite with washed NP developed more β-phase for smaller sizes of nanoparticles. At 140 °C, isomerization occurred, and oleic acid was converted into elaidic acid, reducing the steric hindrance, and promoting the interaction between PVDF chains and the surface of the nanoparticles. This isomerization reaction seems to be an enhancer of the α- to β-phase transition. Our results prove that optimizing multiple properties in nano-reinforced polymers requires consideration of different aspects, such as NP size, surface chemistry, and processing methods.Our results based on mixed experimental and modeling approach proved the usefulness of simulation in understanding and guiding our experimental results. Our results suggest that for enhancing piezoelectric properties in PVDF magnetite nano-composites, the chemistry and the molecular morphology of the grafted ligands when combined with NP size could lead to multi-properties enhancement simultaneously.

Ultrarobust, hierarchically anisotropic structured piezoelectric nanogenerators for self-powered sensing

Despite significant efforts to develop piezoelectric nanogenerators (PENGs), scalable manufacture of polymeric PENGs with highly sensitive and stable piezoelectric output remains a great challenge. Here, we propose a noncovalent assembly-mediated solid-state drawing strategy for the electrical treatment-free fabrication of high-performance PENGs with hierarchically anisotropic structure. Along with the stress-induced anisotropic alignment of glycine modified-molybdenum disulfide nanosheets, the strong intermolecular interactions and confinement between 2D nanosheets and polyvinylidene fluoride (PVDF) dipoles can aggressively induce the self-polarized β-phase transition of PVDF in a favorable direction. The resulted nanocomposite without extra electrical treatment exhibits remarkable piezoelectric performance (d33 =24.9 pC N-1) and extraordinary mechanical performance (tensile strength of 213.3 MPa, toughness of 85.8 MJ m-3), far surpassing most reported polymeric piezoelectric materials. Taking these advantages, the PENGs-based self-powered physiological sensors show highly sensitive and stable output signals, satisfying the requirements of tiny and large human motions monitoring. This work opens an avenue toward large-area compliant and low-energy manufacturing of diverse energy harvesting devices and self-powered sensory systems.

Anisotropic thermal transport properties of quartz: from −120 °C through the <i>α</i>–<i>β</i> phase transition

Abstract. Thermal diffusivities of synthetic quartz single crystals have been measured between −120 and 800 ∘C using a laser flash method. At −120 ∘C, the lattice thermal diffusivities are D[001]=15.7(8) mm2 s−1 and D[100]=8.0(4) mm2 s−1 in the [001] and [100] directions, respectively. Between −80 and 560 ∘C, the temperature dependence is well approximated by a D(T)=1/Tn dependency (with n=1.824(29) and n=1.590(21) for the [001] and [100] directions), whereas for lower temperatures measured thermal diffusivities show smaller values. The anisotropy of the thermal diffusivity D[001]∕D[100] decreases linearly over T in α- and β-quartz, with a discontinuity at the α–β phase transition at Tα,β=573 ∘C. In the measured signal–time curves of α-quartz, an unusual radiative heat transfer is observed, which can be linked to the phase transition. However, the effect is already observed far below the actual transition temperature. The standard evaluation procedure insufficiently describes the behaviour and leads to an underestimation of the thermal diffusivity of ≥20 %. Applying a new semi-empirical model of radiation absorption and re-emission reproduces well the observed radiative heat transfer originating in the phase transition. In the β-quartz region, the radiative heat transfer is not influenced by the phase transition effect observed in α-quartz and for the thermal diffusivity evaluation common models for (semi)transparent samples can be used.

Dynamic Recrystallization Behavior and Model Study of Equiaxed Fine Grain Structure TC4

Thermal compression test of equiaxed fine grain structure TC4 was conducted by using Glebble-3800 testing machine. The deformation temperature ranged between 800-950 °C, the strain rate ranged between 0.01-10 s−1, and the deformation was 60%. The dynamic recrystallization behavior under this condition was analyzed. The results show that the dynamic recrystallization behavior of equiaxed fine grain structure TC4 is sensitive to temperature and strain rate. The grain size and volume ratio of dynamic recrystallization increases with increasing temperature and decreases with increasing strain rate. In this paper, with the β-phase transition temperature as the boundary, a dynamic recrystallization critical strain model, a dynamic model and a critical size model of the equiaxed fine grain TC4 are established. The predicted value of the established constitutive model is in good agreement with the experimental value, which provides a theoretical basis for the microstructure control of the alloy during plastic processing.

Morphological and optical properties of α- and β-phase zinc (∥) phthalocyanine thin films for application to organic photovoltaic cells.

We have investigated the morphological and optical properties of α- and β-phase Zinc Phthalocyanine (ZnPc) thin films for application to organic photovoltaic cells (OPVs). It was found that the α-phase is completely converted to the β-phase by thermal annealing at 220 °C under ultrahigh vacuum conditions. When the α- to β-phase transition takes place, the surface roughness of the ZnPc film became flat uniformly with a nanometer order of unevenness by anisotropic growth of crystalline grains along a lateral direction to substrates. Correspondingly, the optical absorbance of the β-phase film became greater by 1.5-2 times than that of the α-phase one in an ultraviolet-visible-near infrared (UV-vis-NIR) wavelength range, which plays a role in increasing the number of photogenerated excitons. On the contrary, time-resolved photoluminescence measurements showed that the average lifetime of excitons for the β-phase film became shorter by 1/6-1/7 than that for the α-phase one, which plays a role in decreasing the number of excitons achieving the donor/acceptor interface where excitons are separated to carriers (holes and electrons). Both the increase in the number and the shortening in the average lifetime have a trade-off relationship with each other for contribution to the photoelectric conversion efficiency of OPVs. Then, we examined an external quantum efficiency (EQE) of OPVs using the α- and β-phase films as a donor and obtained that the former OPV (α-phase) exhibits a higher EQE by ∼2 times than the latter one (β-phase) in the wavelength range of 400 nm-800 nm.

Friction stir welding of Ti-6Al-4V alloy: Friction tool, microstructure, and mechanical properties

Abstract In this work, 2 mm-thick Ti-6Al-4V plates were successfully friction stir welded using a newly designed friction tool, and the microstructure evolution and mechanical behaviors of weld were investigated. The process parameters of friction stir welding (FSW) were explored and defect-free welds were produced. The cross-sections of the weld showed typical bowl profiles, which can be divided into four regions: base metal (BM), shoulder-affected zone (SAZ), stir zone (SZ), and heat-affected zone (HAZ). The peak temperature in the SZ exceeded the β-phase transition temperature and the materials in the SZ underwent phase transformation during FSW. The final microstructure of the SZ was a lamellar structure with (α + β) phase and the HAZ has a bimodal microstructure consisting of equiaxed prior α and lamellar (α + β) grains. The maximum tensile strength and yield strength of the FSWed samples reached up to 92 % of the base metal and the tensile failures occurred at the SZ. The micro-hardness varied significantly along the thickness of the plate. While the micro-hardness value of SZ was lower than that of BM due to the grain coarsening and dynamic recrystallization during the welding. The tensile fracture morphologies exhibited a typical quasi-cleavage fracture.

Neutron diffraction study of the α- to β-phase transition in BaD2 under high pressure

We report high-pressure neutron diffraction, and Raman spectroscopy data from the α- (Pnma) to β- (P63/mmc) phase transition in BaD2. The transition was observed at 2.53(5) GPa on pressure increase, and 1.65(5) GPa on pressure decrease, with a relatively narrow 0.20(5) GPa region of phase coexistence. We report the isothermal equations of state for the α-phase at 300 K and the β-phase at both 300 K and 480 K, and have calculated the entropy change through the transition. The Raman data show that a low-energy Ba vibration is seen to soften with applied pressure; DFT calculations suggest that this is predominantly due to an instability in the Ba atoms prior to the first-order transition. The shift in D positions in the high-pressure phase suggest that the Wyckoff 4f-model (where one of the D atoms is split between two sites) is a better fit at lower pressures, but that the model tends towards the Wyckoff 2d-model (where D1 is localised on a single site) with increased pressure. These results are used to discuss implications on the reported increases in ionic-conductivity over the transition.

Electronic Origin of α″ to β Phase Transformation in Ti-Nb-Based Thin Films upon Hf Microalloying.

We present results on thin Ti-Nb-based films containing Hf at various concentrations grown by magnetron sputtering. The films exhibit α” patterns at Hf concentrations up to 11 at.%, while at 16 at.% Hf, the β-phase emerges as a stable structure. These findings were consolidated by ab initio calculations, according to which the α”–β transformation is manifested in the calculation of the electronic band energies for Hf contents between 11 and 18 at.%. It turns out that the β-phase transition originates from the Hf 5d contributions at the Fermi level and the Hf 6s hybridizations at low energies in the electronic density of states. Bonding–anti-bonding first neighbor features existing in the shifted plane destabilize the α″-phase, especially at high Hf concentrations, while the covalent-like features in the first neighborhood stabilize the corresponding plane of the β-phase. Thin films measurements and bulk total energy calculations agree that the lattice constants of both α″ and β phases increase upon Hf substitution. These results are important for the understanding of β-Ti-based alloys formation mechanisms and can be used for the design of suitable biocompatible materials.

Design Principles to Govern Electrode Fabrication for the Lithium Trivanadate Cathode

A full depth of discharge mathematical model for the lithium trivanadate cathode, considering lithiation of the layered α-phase, phase change, and lithiation of the rock-salt like β-phase at lower potentials, is developed. The coupled electrode-scale and crystal-scale model is fit to electrochemical data, and additionally validated with operando EDXRD. There is good agreement between the simulated and measured spatial variation of the volume fraction of the β-phase. This mathematical model is used to guide electrode fabrication, accounting for both ionic and electronic transport effects. Values of three design parameters—electrode thickness, porosity, and volume fraction of conductor—are identified, and the sensitivity of the energy density to these design parameters is quantified. The model is also used to investigate electrode design to create electrodes that deliver the maximum achievable energy density under the constraint that the α to β-phase transition is avoided, since phase change has been demonstrated to reduce cycle life. The energy density sacrificed to avoid phase change decreases at higher discharge rates, but the target values for electrode fabrication remain the same as those when optimizing the electrode for the full depth of discharge.

Phase transition behavior in Fe2O3 nanofibers

Hematite (α-Fe2O3) nanofibers were fabricated using the electro-spinning technique and followed by an annealing process in air. The crystalline structure and morphology of the nano-fibers were studied using XRD, SEM, Raman spectra and high-resolution TEM. Zero-field-cooled (ZFC), field-cooling (FC) and field heated (FH) type temperature dependence of magnetization curves of α-Fe2O3 nanofibers are measured in the temperature range 15–390 K. Two kinds of magnetization steps were observed in the temperature dependence of magnetization curves, one is at 300 k which is very weak and the other is in the temperature range of 245–257 K with large thermal hysteresis (12 K). The tiny step at 300 K can be attributed to normal Morin transition in α-Fe2O3 and this was suppressed by the consequent α- to β- phase transition, which caused the pronounced step in the temperature dependence of magnetization curves. This α- to β- phase transition in Fe2O3 nanofibers was further confirmed by in situ TEM observations. This is the first study to describe novel α- to β-phase transition upon cooling.

Recrystallization Behavior and Super-Elasticity of a Metastable β-Type Ti-21Nb-7Mo-4Sn Alloy During Cold Rolling and Annealing

Based on the d-electron alloy design theory, a new metastable β-type titanium alloy for biomedical applications, Ti-21Nb-7Mo-4Sn (wt.%) was designed in this article. This theory predicted this alloy was a metastable single β-type titanium alloy with low elastic modulus at room temperature, and the β-phase transition (β to α″) would be easy to occur during the cold rolling. The evolution of microstructure and mechanical properties of this alloy during cold rolling plus annealing were investigated by means of x-ray diffraction, optical microscope, transmission electron microscope and mechanical properties test. The results indicate that only β phase can be identified in this alloy before cold rolling, while a large amount of lath martensite (α″ phase) appears after cold rolling due to stress-induced martensitic transformation, which is in accordance with the prediction of the d-electron alloy design theory. The recrystallization nucleation occurs preferentially in the martensite lath region during the subsequent annealing process, and this region forms a large number of nano-crystals and microcrystals, showing that cold rolling plus annealing can become a new process for refining grains. Compared with casting and cold rolling samples, the annealing materials at 923 K for 10 min have excellent comprehensive mechanical properties with lower elastic modulus (58 GPa) and higher elastic recovery rate (35.39%).

The thermal behaviour of silica varieties used for tool making in the Stone Age

Before 100,000 years ago, during the Middle Stone Age (MSA) of South Africa, silica varieties of minerals and rocks were sometimes heated during tool making in order to improve their knapping properties. If the heating and cooling process is not controlled, failure results and the nodules fracture. Recently, we postulated that the reversible α- to β-phase transition may play a role in causing silcrete, a type of rock often used to make stone tools in the Western Cape, to fracture. In this new study, we analyse the thermal behaviour (520–620 °C) of silcrete and compare it to that of two chalcedony samples from different origins, together with samples of chert, agate and flint. These minerals and rocks were commonly used to make stone tools. Differential scanning calorimetry (DSC) measurements show that the α- to β-phase transformation is prominent in silcrete, agate and one of the chalcedony samples, weaker in chert and the second chalcedony sample, but non-existent in flint. X-ray fluorescence (XRF), thermogravimetric analysis (TG) and carbon and sulphur analyses show differences in elemental composition between the rocks and minerals. X-ray powder diffraction (XRD), Fourier transform infrared (FTIR) and Raman spectroscopy highlight differences in microstructure. These small differences in chemical composition and structure contribute to a variety of chemical reactions and phase transformations that can take place in rocks and minerals, which in combination determine their stability upon heating and show that care should be taken when generalising thermal behaviour.

Surface Morphology and Affected Layer in Disc-milling Grooving of Titanium Alloy

Abstract Disc-milling grooving experiment was carried out to measure milling force and temperature for titanium alloy samples. After machining, surface roughness, surface topography, residual stress, microstructure and microhardness under different milling conditions were analyzed. The results show that the surface roughness of the center on milling surface is lower than that of the edge; moreover, the surface roughness decreases with the increase of the spindle speed, but increases with the increase of depth of cut and feed speed. The residual compressive stress is produced on the machined surface and subsurface, and gradually declines to zero with increase of the depth. The microstructure of lattice tensile deformation is found along feed direction under the effect of milling force, progressing from the initial equiaxed structure to long flake lattice. The metallographic structure of plastic deformation zone changes with the temperature, transforming from the initial equiaxed microstructure to a lamellar microstructure when the temperature is up to β-phase transition temperature. The combination of mechanical and thermal loads increases the microhardness on the machined surface and subsurface

Ab-initio study of structural phase transitions and optoelectronic properties in BeH2 at increasing pressure

The structural stability, electronic and optical properties of BeH2 under high pressure have been studied using the density functional theory (DFT) employing full potential-linearized augmented plane wave (FP-LAPW) method. The exchange correlation functional has been solved using the generalized gradient approximation. The calculations show that BeH2 becomes unstable upon application of pressure. At a pressure of 29.40GPa the ground state α-BeH2 transforms to hypothetical phase β-BeH2 and further at a pressure of 53.77GPa (with respect to the ground state α-BeH2) β-BeH2 transforms to γ-BeH2. In α-BeH2 phase it remains as an insulator while in β-BeH2 phase its behavior becomes metallic. But upon further increase in pressure it becomes a semiconductor in γ-BeH2 phase. Hence the possibility of obtaining high-pressure phases with superconducting properties cannot be ruled out. There occurs a huge equilibrium volume collapse at α- to β-phase transition and relatively smaller volume changes at β- to γ-phase transition. Our obtained value of dielectric constant (3.0) for α-BeH2 is in excellent agreement with earlier reported value (3.1). Also BeH2 shows anisotropic behavior in all three studied phases.

Single-shot time resolved study of the photo-reversible phase transition induced in flakes of Ti3O5 nanoparticles at room temperature

We have performed single-shot time-resolved reflectivity measurements of the photo-reversible phase transition between λ- and β-phases of Ti3O5 nanoparticles at room temperature. These phase transitions are induced using nanosecond and millisecond laser pulses and are revealed by Raman spectroscopy. Upon nanosecond pulsed excitation, the λ- to β-phase transition is detected in about 900ns, whereas the β- to λ-phase transition is noticed in less than 20ns. The λ- to β-phase transition induced by quasi continuous wave laser happens on the millisecond time scale. We propose a scenario that accounts very well for the observed phenomena.

LiB Electrode Ageing Observed from PVdF Binder

Ageing behaviors of the positive electrode of lithium ion battery are characterized by measuring mechanical properties of the electrode reeds, such as resonance frequency and internal friction, as a function of temperature. In the measurements of the electrode reeds with a sandwich structure of active material film and current collector of Al foil, two thermally-activated relaxation processes can be observed on the polyvinylidene difluoride binder in the active material film. Namely, a surface-related relaxation at ~150 K and a relaxation corresponding to the β-phase transition at ~240 K in the polymer binder can be observed at high signal/noise ratio. The resonance frequency decreases and the internal friction increases after charge/discharge cycling. The changes in activation energies of the relaxation processes also indicate that the measurement of mechanical properties of the positive electrode is an effective method for characterizing ageing behaviors of the positive electrode as a whole.