High-temperature Β-phase Research Articles

Negative thermal expansion in Co2-xMnxP2O7

Negative thermal expansion (NTE) is an abnormal phenomenon that only exists in few materials but is of importance in tailoring the coefficients of thermal expansion (CTE) in some advanced devices and instruments. Herein, we find that a large volume shrinkage occurs in Co2P2O7 crossing its structural transformation from α (P21/C) to β phase (C2/m) around 584 K which can be tuned even to below room temperature (RT) by the substitution of Mn2+ for Co2+. It is also found that the sintering temperature and the content of Mn2+ are crucial factors in controlling the proportions of α- and β-phases crystallized at RT and the rates of phase transition. A zero thermal expansion within a wide temperature window (173–573 K) is realized for Co1.4Mn0.6P2O7 sintered at 1173 K. The NTE arises from the lattice volume difference between the low temperature α-phase and high temperature β-phase while a higher content of Mn2+ and lower sintering temperature favorites for the coexistence of both phases at RT and lowering the phase transition rate. This work not only adds new members for the NTE family but also presents a method for regulating the onset, width, and rate of NTE by the interplay of substitution content and sintering temperature.

Influence of Current Density upon Hydrogenation on the Shape Memory Effect of Binary TiNi Alloy Single Crystals

Some results concerning the hydrogen effect at electrolytic saturation at a current density of j = 1500 and 3500 A/m2 for 3 h at room temperature on the temperature dependence of the yield stress σ0.1(T) and the shape memory effect (SME) under tension of the [011]-oriented Ti-50.55%Ni (at.%) alloy single crystals are presented. It was shown that hydrogen is in a solid solution and forms particles of titanium hydride TiH2 after hydrogenation at j = 1500 and 3500 A/m2, respectively. Both hydrogen in the solid solution and TiH2 particles led to a decrease in the Ms temperature of the onset of the forward martensitic transformation (MT) upon cooling and the Md temperature (Md is the temperature at which the stresses for the onset of the stress-induced MT are equal to the stresses for the onset of plastic flow of the high-temperature B2 phase), and increased the yield stress σ0.1 of the B2 phase at the Md temperature compared to hydrogen-free crystals. It was found that the SME under stress depends on the tensile stress level and current density. The maximum SME εSME = 10 ± 0.2% at σex = 200 MPa and εSME = 10.5 ± 0.2% at σex = 300 MPa was observed in the hydrogen-free crystals and after hydrogenation at j = 1500 A/m2, respectively, which exceeded the theoretical value of lattice deformation ε0 = 8.95% for the B2-B19′ MT in [011] orientation under tension. At j = 1500 A/m2, the physical reason for the excess of the SME of the theoretical ε0 value was due to the increase in the plasticity of B19′ martensite upon hydrogenation. At j = 3500 A/m2, εSME = 8.0 ± 0.2%, and it was less than ε0 = 8.95% for B2-B19′ MT in [011] orientation under tension. The decrease in SME after hydrogenation at j = 3500 A/m2 was associated with the interaction of two types of B19′-martensite: oriented under stress and non-oriented, formed near TiH2 particles. It was shown that the redistribution of hydrogen in the bulk of the crystals during long-term holding for 168 h at 263 K after hydrogenation at j = 1500 A/m2 increases the SME relative to crystals without long-term holding: 3.5 times at 50 MPa and 1.8 times at 100–150 MPa. After long-term holding, εSME = 9.5 ± 0.2% at 150 MPa, which exceeds the theoretical value ε0 = 8.95% for B2-B19′ MT in [011] orientation under tension.

Analysis of the mechanism of orientations evolution during hot rolling and mechanical properties of TiBw/TA15 composites based on crystal plasticity finite element model

An in-depth understanding of the crystal orientation evolution during hot rolling of TiB whisker (TiBw) /TA15 composites and the anisotropy of the as-rolled plates can help fully utilize the material properties. In this paper, the crystal plasticity finite element models of high-temperature (HT) β-phase and room-temperature (RT) α-phase were constructed from electron backscattering diffraction data. Based on this, the orientation evolution during hot rolling in the single-phase region and the effects of the matrix texture on the mechanical properties of the as-rolled plates were analyzed. The effect of TiBw on the anisotropy was studied by the composites finite element model. Results showed that the α-fiber texture of the β-phase was formed during HT rolling. This texture was converted to the T-texture of the α-phase at RT during cooling according to the Burgers orientation relationships. The TiBw had little effect on the matrix texture composition. The TiBw and matrix texture caused the matrix to have higher strength along the rolling direction and the transverse direction, respectively. The matrix texture dominated the difference in mechanical properties because its effect exceeded that of TiBw. The effect of the matrix on the mechanical properties was caused by the Schmid factors (SFs) and the critical resolved shear stress (CRSS) of the slip system together. The slip mode was influenced by SFs determined by the angular relationship between the crystal orientation and the loading direction. The CRSS of the activated slip system determined the yield strength.

First-principles study of Ti50Ru50-xYx (Y = Ni, Pd and Pt) with a prospect to induce SME on stable CsCl-type Ti50Ru50 phase

Most CsCl-type intermetallics composed of group IV and VIII–XI transition metals have shape memory effect (SME), a phenomenon that occurs on a certain class of materials with an ability to undergo martensitic transformation (MT) during cooling. This advanced functional materials’ property is enabled by MT from high-temperature B2 phase of high symmetry to lower symmetry phases such as L10, B19 or B19’ upon cooling. Peculiarly, Ti50Ru50 with similar ordered B2 at high temperature remains ordered and stable with no phase transition down to room temperature. In this study, first-principles calculations based on density functional theory (DFT) are used to investigate the structural, thermodynamic and electronic properties of the stable Ti50Ru50 compound by systematically substituting part of the Ru atoms with ductile group 10 metal (Ni, Pd and Pt). This is an attempt to destabilizing B2 phase at 0 K through Ti50Ru50-xYx ternary alloying to promote MT that could yield SME.Graphical

Multiscale crystal plasticity finite element simulations of the deformation inhomogeneity of TiBw/TA15 composites with network-structured during hot rolling

Understanding the micromechanical responses during hot forming between TiB whiskers (TiBw) and matrix titanium alloy as well as grains is essential to effectively control the properties of the final products. In this work, a method to study the deformation inhomogeneity of network-structured TiBw/TA15 composites during rolling in high temperature single-phase regions at macro and micro scales was proposed. A crystal plastic finite element model with dislocation slip and grain boundary (GB) sliding as the deformation mechanism was established. Results showed that the discontinuous distribution of TiBw reinforcements along the GB reduced the strain difference between the network boundary region and the matrix region during high temperature deformation. Therefore, the plasticity of the composites was improved. For the high temperature β-phase, the {110}<111> with higher activity was the main deformation mode. The difference in stress distribution between the grains was caused by the different combinations of the “soft” and “hard” states of adjacent grains, which was determined by whether the value of Schmid factors was greater than 0.45. The local stress concentrations tended to spread at the interface of “soft-hard” grains. The TiBw reinforcements increased the GB strength and reduced the deformation difference between the GB and intragranular.

CHANGES IN THE STRUCTURAL-PHASE STATE AND DISLOCATION DENSITY OF TI49.8NI502 ALLOY DEPENDING ON THE ISOCHRONOUS ANNEALING TEMPERATURE AFTER SEVERE PLASTIC DEFORMATION BY ABC PRESSING AT 573 K

X-ray diffraction studies were conducted to examine changes in the structural-phase state and dislocation density of Ti498Ni50.2 alloy depending on the isochronous annealing temperature after severe plastic deformation by abc pressing at 573 K. The total true strain achieved in the alloy samples during abc pressing was e = 9.55. Isochronous annealing was carried out for 1 hour at 573, 673, 773, 873 and 973 K. Room-temperature analysis of all studied samples revealed the coexistence of R and B19' phases, whose relative fractions varied with annealing temperature. The high-temperature B2 phase was not detected. It was found that the most rapid decrease in the dislocation density, which was measured at 393 K (in the B2 state), occurs after annealing the samples at 673 and 773 K. After annealing at 773 K, the dislocation density reaches a minimum, and its value decreases by more than an order of magnitude compared to the value immediately after abc pressing. In the same temperature range there is a significant decrease in the root-mean-square B2 lattice microdistortions (e2)1/2 and a slight increase in the average size of coherent scattering regions. After abc pressing and after isochronous annealing, the main contribution to the physical peak broadening is made by the B2 lattice microdistortions, while the contribution from coherent scattering regions is insignificant. The obtained results show that intense recrystallization in Ti49 8Ni50.2 alloy after abc pressing at 573 K begins at T &gt; 773 K.

First-principles study on the effect of Ni addition on the stability of B2 Ti50Ru50 – a supercell approach

Shape memory alloys (SMA's) are a special kind of metal alloys with unique structural properties, such as shape memory effect (SME) and pseudo-elasticity (SE) that were well-observed in TiNi alloy, which commercially trades as Nitinol. This polymorphic crystal behaviour arose from the reversible phase transition between an ordered high-temperature B2 and low-temperature martensite phase (L10/B19/B19′). Due to the high ductility of Ru, this work employed a supercell approach to track the effect of nickel addition on highly stable B2-Ti50Ru50 with the intent to stimulate SME. Thermodynamic, mechanical, electronic structure and lattice dynamic characteristics were estimated using first-principles calculations. The phonons spectra of B2 Ti50Ru50 consisted of only positive frequencies, but the addition of Ni above 18.75 at. % resulted in positive and negative frequencies, indicative of a possible martensitic transformation, which is a strong character for SME.

Окисление циркония, легированного хромом, при воздействии компрессионных плазменных потоков

The results of investigation of the zirconium structure on the first stage of oxidation at temperature 700 °C in the air atmosphere after 15 minutes are presented. The commercial pure zirconium alloy as well as zirconium alloyed with chromium atoms by means of compression plasma flows impact are investigated. The results showed the possibility to form the top layer in zirconium alloyed with chromium when a thin chromium coating (1 mm) was deposited on the surface and the compression plasma flow with the absorbed energy density 25 – 43 J/cm2 influenced. Due to alloying with chromium, the high-temperature β-phase of zirconium is stabilized at room temperature as β-Zr(Cr) solid solution and martensite α′-Zr phase is formed. The oxidation at temperature 700 °C demonstrated the higher oxidation resistance of the zirconium samples treated with compression plasma flows at the highest absorbed energy density 43 J/cm2. This regime modified the top layer of the zirconium sample without β-Zr(Cr) solid solution formation which enhances the oxygen diffusion.

Atomic-scale observation of non-classical nucleation-mediated phase transformation in a titanium alloy.

Two-phase titanium-based alloys are widely used in aerospace and biomedical applications, and they are obtained through phase transformations between a low-temperature hexagonal closed-packed α-phase and a high-temperature body-centred cubic β-phase. Understanding how a new phase evolves from its parent phase is critical to controlling the transforming microstructures and thus material properties. Here, we report time-resolved experimental evidence, at sub-ångström resolution, of a non-classically nucleated metastable phase that bridges the α-phase and the β-phase, in a technologically important titanium-molybdenum alloy. We observed a nanosized and chemically ordered superstructure in the α-phase matrix; its composition, chemical order and crystal structure are all found to be different from both the parent and the product phases, but instigating a vanishingly low energy barrier for the transformation into the β-phase. This latter phase transition can proceed instantly via vibrational switching when the molybdenum concentration in the superstructure exceeds a critical value. We expect that such a non-classical phase evolution mechanism is much more common than previously believed for solid-state transformations.

Effect of marforming on superelasticity and shape memory effect of [001]-oriented Ni50.3Ti49.7 alloy single crystals under compression

The shape memory effect (SME) and superelasticity (SE) of Ni50.3Ti49.7 alloy single crystals were studied after quenching and marforming (deformation in the martensite phase, followed by annealing at 713 K for 0.5 h) along the [001] direction, under compression, in the high-temperature B2-phase. It was shown that marforming leads to the following: strengthening of the high-temperature B2-phase; an increase in the temperature range of SE; a decrease in the mechanical Δσ and thermal ΔТh hysteresis; and an increase in the transformation hardening coefficient Θ = dσ/dε, in comparison with the crystals after quenching. The reasons for improvements in the functional properties of [001]-oriented single crystals after marforming are discussed.

Effects of internal oxide contents on the oxidation and β-phase depletion behaviour in HOVF CoNiCrAlY coatings

This paper studied the effects of internal oxide contents on the high temperature oxidation and β-phase depletion behaviour of high velocity oxygen fuel (HVOF) thermally sprayed CoNiCrAlY coatings. Two oxygen-to-kerosene ratios were employed in the HVOF thermal spraying process to generate different internal oxide contents in the as-deposited CoNiCrAlY coatings. The isothermal oxidation behaviour of the as-deposited CoNiCrAlY coatings were examined in air at 1100 °C. The as-deposited HVOF CoNiCrAlY coatings before and after oxidation were characterised by X-ray diffractometry (XRD), scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS). It was shown that the high oxygen-to-kerosene ratio caused significant oxidation of in-flight particles, resulting in a large volume fraction of the internal oxides in the as-deposited CoNiCrAlY coatings. Furthermore, it was found that the CoNiCrAlY coating with a higher content of internal oxides exhibited larger mass gain and significant β-phase depletion during oxidation. This is likely due to that the Al in the coating was initially consumed during the spraying process and the discrete internal oxide stringers were not effective diffusion barriers. • Two CoNiCrAlY coatings were prepared at different oxygen-to-kerosene ratios. • The internal oxides were found to be Al-rich oxides by EDS analysis. • High mass gain was found in the coating with high internal oxide content. • Rapid β-phase depletion occurred in the coating with high internal oxide content.

Исследование выделений вторых фаз в наноструктурном технически чистом титане

The results of studies on the process of precipitation of dispersed second phases in commercially pure titanium Grade 4 and the effect of secondary precipitates on its structure and mechanical properties in two states, coarse-grained and nanostructured ones, are presented. The nanostructured state was obtained by high-pressure torsion (HPT) under a pressure of 6 GPa up to N =10 revolutions at room temperature. A particular consideration is given to the study of changes in the phase composition and microstructure of titanium subjected to deformation processing after annealing at an elevated temperature of 700°C for 30 minutes. In this work, by means of studies in a transmission electron microscope, it was shown that at a temperature of 700°C and higher, in the structure of the samples, nanoparticles of the second phases which differ in size and morphology are precipitated in both states. The nature of the observed particles was studied in SEM, by indexing the electron diffraction patterns taken from the particles, and by carrying out X-ray phase analysis by the “transmission” method. Particles of the second phases are modifications of the high-temperature β-phase of titanium. The HPT treatment of the alloy, according to the XRD data, leads to an increase in the volume fraction of precipitated particles after annealing and, as a result, to an increase in the microhardness of the states under study. The results of microhardness measurements at varying regimes of deformation and annealing are presented. Combination of HPT up to N = 5 revolutions and annealing at 700°C for 30 minutes followed by additional torsion straining also up to N = 5 revolutions provides the highest microhardness values in commercially pure titanium, which reaches 423 HV.

On the crystal structures of lithium thiocyanate monohydrate LiSCN [formula omitted] 1 H2O and the phase diagram LiSCN – H2O

Hydration of anhydrous LiSCN by stoichiometric amounts of water vapor leads to the formation of LiSCN ⋅ 1 H2O. At room temperature, LiSCN ⋅ 1 H2O crystallizes in the monoclinic space group C2/m with a = 15.0271(3) Å, b = 7.5974(1) Å, c = 6.7070(1) Å, β = 96.147(6) ° and V = 761.32(2) ų (α-phase). At 49 °C, the material transforms into a high temperature β-phase with the orthorhombic space group Pnam and a = 13.2258(2) Å, b = 7.0619(9) Å, c = 8.1663(1) Å and V = 762.72(2) ų. Structural and thermodynamic investigations were conducted using infrared (IR) spectroscopy, ex situ and in situ X-ray powder diffraction (XRPD) while varying temperature or water partial pressure pH2O, differential scanning calorimetry (DSC) and direct measurements of equilibrium pH2O. The results indicate that LiSCN ⋅ 1 H2O is a special case when compared to LiSCN ⋅n H2O (n = 0 or 2), both regarding crystal structure and hydration thermodynamics. These results are combined with literature date into an updated phase diagram of the LiSCN – H2O system, showing a detailed picture of the phase formation behavior of the LiSCN ⋅n H2O hydrates.

Behavior of Inelastic and Plastic Strains in Coarse-Grained Ti49.3Ni50.7(at%) Alloy Deformed in B2 States

The regularities of the change in inelastic strain in coarse-grained samples of the Ti49.3Ni50.7 (at%) alloy are studied when the samples are given torsional strain in the state of the high-temperature B2 phase. During cooling and heating, the investigated samples underwent the B2–B19′ martensite transformation (MT); the temperature of the end of the reverse MT was Af = 273 K. It was found that at the temperature of isothermal cycles “loading-unloading” Af + 8 K, when the specimen is assigned a strain of 4%, the effect of superelasticity is observed. With an increase in the torsional strain, the shape memory effect is clearly manifested. It is assumed that the stabilization of the B19′ phase in unloaded samples is due to the appearance of dislocations during deformation due to high internal stresses at the interphase boundaries of the B2 phase and the martensite phase during MT. The appearance of dislocations during the loading of samples near the temperatures of forward and reverse MT can also be facilitated by the “softening” of the elastic moduli of the alloy in this temperature range. At a test temperature above Af + 26 K, the superelasticity effect dominates in the studied samples.

Realizing Bi-doped α-Cu2Se as a promising near-room-temperature thermoelectric material

Copper selenide (Cu2Se) has been extensively studied as an eco-friendly thermoelectric candidate owing to the outstanding thermoelectric performance of its high-temperature β-phase. In this study, we propose that α-Cu2Se is also a promising thermoelectric material at near-room-temperature. We synthesize nanostructured Cu2-xSe via a facile solvothermal method, and densify the samples using Spark Plasma Sintering to maintain the small grain sizes. Although the as-prepared α-Cu2Se has intrinsically low lattice thermal conductivity, its high Cu deficiency leads to a high carrier concentration, therefore, a low thermoelectric performance. As an electron donor, bismuth can effectively compress the Cu vacancies of the as-prepared α-Cu2Se and in turn reduce its carrier concentration from 4.1 × 1020 to 2.0 × 1020 cm−3 at room temperature, achieving a significantly enhanced power factor and a reduced carrier thermal conductivity. Consequently, a zT of 0.43 at 373 K is obtained in Cu1.982Bi0.006Se, showing great potential in developing high-performance near-room-temperature α-Cu2Se-based thermoelectric materials.

Influence of substitution of Fe by Co on structural and magneto-mechanical properties of Fe-27Ga alloy

This paper investigates the influence of substitution of 18 at.% of Fe by Co in Fe-27Ga alloy as an effort to improve magneto-mechanical coupling. Study of phase transitions by neutron diffraction reveals that the formation of close packed phases is suppressed in the as-cast Fe-18Co-27Ga alloy. The degree of D03 ordering in the as-cast state for the Fe-18Co-27Ga alloy is lower than that for the Fe-27Ga alloy as estimated by intensities of super-lattice reflections of neutron diffraction spectra in these alloys. Investigation of magnetic properties by vibrating sample magnetometry and magnetic thermal gravimetry proved that the high-temperature B2 phase in the Fe-18Co-27Ga is ferromagnetic, whereas that of the Fe-27Ga is non-ferromagnetic. The Curie temperature (TC) here and later in the text in slowly-cooled Fe-18Co-27Ga is 726 °C, which is higher than that (650 °C) of Fe-27Ga. The mechanical and magneto-mechanical properties samples were studied through impulse excitation of vibration (IEV) method by the Resonance Frequency and Damping Analysis (RFDA) device. The results demonstrate that substitution of 18 at.%Fe by Co in Fe-27Ga alloy could enhance both the elastic, E0, and shear moduli, G0, at zero filed and the total (flexure mode) damping capacity, (ΔW/W)F0, under the measuring conditions.

Internal strain induced superconductivity in arc melted Ti0.97Fe0.03 alloy

Alloys with a small amount of iron dissolved in α-titanium are known to have superconducting transition temperatures (TC) up to ∼3.5 K which are higher than that expected from the density of states. The origin of this enhancement still remains unanswered in spite of extensive theoretical and experimental studies. We show here with the help of x-ray diffraction, metallography, resistivity and heat capacity measurements as well as the calculation of density of states that the internal strain developed during rapid cooling of the sample from the high temperature β-phase is responsible for the anomalous high value of TC = 3.3 K in the arc melted Ti0.97Fe0.03 alloy. The relaxation of the lattice by expelling a large amount of iron from the α-phase in the equilibrium state lowers the TC below 2.4 K when the Ti0.97Fe0.03 sample is annealed inside the α-phase. These results indicate that the superconductivity in Ti1−xFex alloys is mostly confined to the α-phase.

A High-Temperature β-Phase NaMnO2 Stabilized by Cu Doping and Its Na Storage Properties**Supported by the National Key Technologies R&D Program of China under Grant No 2016YFB0901500, and the National Nature Science Foundation of China under Grant Nos 51725206 and 51421002.

The high-temperature β-phase NaMnO2 is a promising material for Na-ion batteries (NIBs) due to its high capacity and abundant resources. However, the synthesis of phase-pure β-NaMnO2 is burdensome and costineffective because it needs to be sintered under oxygen atmosphere at high temperature and followed by a quenching procedure. Here we first report that the pure β phase can be stabilized by Cu-doping and easily synthesized by replacing a proportion of Mn with Cu via a simplified process including sintering in air and cooling to room temperature naturally. Based on the first-principle calculations, the band gap decreases from 0.7 eV to 0.3 eV, which indicates that the electronic conductivity can be improved by Cu-doping. The designed β-NaCu0.1 Mn0.9O2 is applied as cathode in NIBs, exhibiting an energy density of 419Wh/kg and better performance in terms of rate capability and cycling stability than those in the undoped case.

β- (Me1, Me2) and MeNx films deposited by magnetron sputtering: Novel heterostructural alloy and compound films

The article reports on the formation of high-temperature β-phase films prepared by magnetron sputtering. The principle of formation of the β-phase films is explained. It is shown that the β-phase films are composed of elements that crystallize in different crystal structures and are a novel class of heterostructural films. The properties of the β-phase films are unique and they are controlled by the crystal structures in which the elements contained in the films crystallize. It means that there are three basic parameters which decide on the resulting properties of the β-phase films: (1) the energy delivered into the film during its growth, (2) the elemental composition of the film and (3) the crystal structure of elements or phases in the film. The existence of many possible combinations of the crystal structure of elements and/or phases in the film makes it possible to develop new advanced heterostructural films with new unique properties. It is a great challenge for the further progress in the field.

Temperature-induced phase transitions for stuffed tridymites SrGa2O4 and CaGa2O4

Temperature-induced phase transitions for stuffed tridymite AGa2O4 (A = Sr, Ca) were investigated by experimental and theoretical calculations. A simple annealing of SrCO3 and Ga2O3 led to the formation of γ-SrGa2O4 (P21/n) below 1200°C, and transform to β-SrGa2O4 (P21/c) when heated at 1200°C. A similar phenomenon was found for CaGa2O4, and the temperature boundary between α-CaGa2O4 (Pna21) and the high temperature polymorph β-CaGa2O4 (P21/c) was about 1350°C. Rietveld refinements provided detailed structural information for these polymorphs and suggest that the driving force of these phase transitions is the under-bonded nature of the alkaline earth cations. In other words, the need of larger space for Sr2+/Ca2+ in the high temperature β-phase forces the 6-membered-ring channel expand through increasing the Ga-O-Ga angles. Density functional theory calculations proved the formation energies for γ-SrGa2O4 and α-CaGa2O4 were both lower than their high temperature β-polymorphs, in accordance with the experimental observations.