High-temperature Phase Transformation Research Articles

High-temperature phase transformations in LiH 2PO 4 and possible solid-state polymerization

The dielectric constant and electrical conductivity were measured on heating polycrystalline LiH 2PO 4 from room temperature to 220 ∘C. Two dielectric anomalies appeared at 178 ∘C ( T p 1 ) and 196 ∘C ( T p 2 ). The electrical conductivity showed the same anomalies near the corresponding temperatures and reached the magnitude of the superprotonic phases: 3 × 1 0 − 2 Ω − 1 cm − 1 at 178 ∘C ( T p 1 ) and 10 Ω − 1 cm − 1 at 196 ∘C ( T p 2 ). Whether the superprotonic phase transformations are due to polymorphic transitions in the bulk, surface transitions, or chemical reactions (thermal decomposition) at the surface is uncertain. Considering several previous thermal studies, our experimental results seem to be related to the last case: chemical reactions (thermal decomposition) at the surface with the progressive solid-state polymerization. The anomalies at 178 ∘C ( T p 1 ) and 196 ∘C ( T p 2 ) are suggested to be the onset of dimerization and oligomerization, respectively.

Synthesis and study of the thermal stability of SrFe 1 − xM xO 3 − z (M = Mo, W) perovskites

In the present work, solid solutions of the composition SrFe 1 − x M x O 3 − z , where M = Mo, W and 0 < x < 0.5, are synthesized. The structure of the new materials depending on the composition and on oxygen stoichiometry is studied; the results of high-temperature X-ray diffraction measurements, thermal stability and phase transformations in various environments are presented.

On the theory of austenite-cementite phase equilibria in steels

A microscopic model is proposed, which describes the structure and thermodynamic properties of cementite (an ordered iron carbide with a composition of Fe3C1 − δ) and the phase equilibria between cementite and austenite (a disordered solid solution of carbon in face-centered cubic iron). Based on this model, chemical and stress-induced (deformational) interactions of carbon atoms in cementite and austenite structures are quantitatively evaluated. The “lattice” contributions in the equations of phase equilibria, which are related to changes in the crystal structure as a result of the cementite-austenite phase transition, are estimated. The proposed model well describes available data on the thermodynamics of cementite and austenite and provides a basis for the further microscopic investigation of high-temperature phase transformations in steels.

Enhanced plasticity observed in cold-worked alloys during heating

Enhanced plasticity observed in preliminary cold-worked single-phase alloys has been experimentally studied. This type of enhanced plasticity has been compared with the fine-grain superplasticity and enhanced plasticity, caused by high-temperature phase transformations. The nature of these types of enhanced plasticity is discussed.

High temperature phase transformation of tantalum nitride films deposited by plasma enhanced atomic layer deposition for gate electrode applications

Tantalum nitride thin films were deposited at 400°C by plasma enhanced atomic layer deposition using an amido-based metal organic tantalum precursor. An Ar∕N2∕H2 mixture was flowed upstream of a remote plasma system to produce the reactive species used for the nitridation process. The as-deposited film was amorphous and contained 15at.% oxygen in the bulk of the film. High resolution photoelectron spectroscopy studies of the Ta 4f feature were consistent with the presence of the semiconducting Ta3N5 phase in the as-deposited films. Electron diffraction studies were carried out by annealing the Ta3N5 film in situ in a transmission electron microscope. The high resistivity Ta3N5 phase crystallized into the cubic TaN phase at 850°C. This transformation appeared to coincide with outdiffusion of excess nitrogen from the Ta3N5 film during the anneal. The resistivity of the crystallized film was estimated to be 600μΩcm from four point probe measurements.

Densification and Grain Growth for Powder-Derived Ta2O5-TiO2 Ceramics

Thermal processing of powder-derived Ta2O5-based ceramics reveals that rapid grain growth associated with a high temperature phase transformation hinders densification, necessitating the development of a reduced-temperature processing methodology. Data are reported for the densification behavior with emphasis on microstructural changes associated with the phase transformation between a stable low-temperature phase (L-Ta2O5) and a phase which is stable at high temperatures (H-Ta2O5). The H-Ta2O5 phase is metastable at room temperature and reverts back to the L-Ta2O5 phase with thermal or mechanical treatment. TiO2 additions stabilize the H-Ta2O5 phase and result in enhanced dielectric properties. Because TiO2 additions decrease the temperature of the densification-hindering phase transformation, an alternate reduced-temperature processing route is necessary. A simple solution-coated powder method was used to produce the first-ever dense and chemically homogeneous TiO2-modified Ta2O5 ceramics in both the L- and H-Ta2O5 forms. Thus, this work represents the first comprehensive study of the effects of composition and the L⇒H-Ta2O5 phase transformation on microstructural development. The results indicate that the effect of TiO2 additions on the sintering behavior of Ta2O5 ceramics was largely limited to a reduction in the temperature for the densification-hindering phase transformation.

Phase Transformation and Structural Studies of EUROFER RAFM Alloy

High temperature phase transformations in EUROFER reduced activation ferritic martensitic (RAFM) steel were studied in-situ by means of X-ray diffraction. Results show that, during slow cooling, the austenite to ferrite transformation takes place around 755 oC. Full transformation of the austenitic phase into pure martensite is observed for cooling above 5 oC/min. This transformation was found in samples annealed at 950 oC for 3 h and quenched in liquid nitrogen. TEM analyses reveal a high concentration of carbides along the grain boundaries of the martensitic structure. The thermal expansion coefficient derived from the measurements was 12.7x10-6 K-1.

Microstructure characteristic of ceramic coatings fabricated on magnesium alloys by micro-arc oxidation in alkaline silicate solutions

Micro-arc oxidation (MAO) of AZ31B magnesium alloys was studied in alkaline silicate solutions at constant applied current densities. The microstructure, phase composition and elemental distribution of ceramic coatings were investigated using scanning electron microscopy (SEM), X-ray diffraction (XRD) and energy-dispersive spectroscopy (EDX). There are two inflections in the voltage–time response, three regions were identifiable and each of the regions was almost linear. The pores with different shapes distributed all over the coating surface, the number of the pores was decreasing, while the diameter was apparently increasing with prolonged MAO treatment time. There were also cracks on the coating surface, resulting from the rapid solidification of the molten oxide. The ceramic coating was comprised of two layers, an outer loose layer and an inner dense layer. The ceramic coating was mainly composed of forsterite phase Mg 2SiO 4 and MgO; the formation of MgO was similar to conversional anodizing technology, while formation of Mg 2SiO 4 was attributed to a high temperature phase transformation reaction. Presence of Si and O indicated that the electrolyte components had intensively incorporated into coatings.

In Situ Studies of High‐Pressure Phase Transformations in the B‐C‐N System

AbstractFor Abstract see ChemInform Abstract in Full Text.

A reappraisal of fast ion conduction in Ta, Ga, and Al-substituted Aurivillius phases

Oxygen ion conduction in the Aurivillius phases Bi 2Sr 2(Nb or Ta) 2(Ga or Al)O 11.5 has been evaluated in detail. High-temperature X-ray diffraction studies, coupled with differential scanning calorimetry, electron microscopy, and conductivity measurements were used to clearly demonstrate that Bi 2Sr 2(Nb x Ta 1− x ) 2(Ga y Al 1− y )O 11.5 with x = 0, 1 and y = 0, 0.5, and 1 form only multiphase specimens. The origin of the previously observed fast ion conduction is a result of interconnected Bi 2O 3 grains that undergo a high-temperature phase transformation to the δ-Bi 2O 3 polymorph. The Aurivillius phase component of the multiphase specimens does not demonstrate any type of order–disorder transformation, however, the phase transformations of the Bi 2O 3 are clearly observed using high-temperature diffraction and calorimetry.

Experimental and modelling studies into high temperature phase transformations

Experimental and modelling studies have been conducted to probe high temperature phase transformations in Fe–C alloys. Novel experimental techniques have been developed for the in situ study of solidification phenomena, in particular the peritectic transition, using laser-scanning confocal microscopy (LSCM). Experimental results have been used to benchmark simulations generated using Micress, a multi-component, multi-phase phase field-modelling package. The combination of in situ experiments and modelling studies has led to new insights into the role of cooling rate and solute segregation on transformation kinetics.

Crystallographic aspects related to the high pressure–high temperature phase transformation of boron nitride

Crystallographic relations between different forms of boron nitride (BN) appearing at the high pressure–high temperature structural phase transformation have been revealed by high-resolution transmission electron microscopy (HRTEM). As starting materials, crystalline hexagonal BN (hBN) with different degrees of crystallinity, or with defects intentionally introduced, were used. Cubic BN (cBN) is formed only as a minor component, the rest consisting of different forms of sp 2 bonded BN: hBN, compressed, monoclinic deformed hBN, or turbostratic BN (tBN). The small cBN crystallites (300–400 nm) contain many defects such as twins, stacking faults and nanoinclusions of other BN forms: tBN, rhombohedral BN (rBN) and wurtzite BN (wBN). The cBN phase grows epitaxially on the basal plane of hBN. The nucleation sites for cBN are revealed by HRTEM. They consist of nanoarches (sp 3 hybridized, highly curved nanostructures), frequently observed at the edges of the hBN crystallites in the starting materials. Based on HRTEM observations of specimens not fully transformed, a nucleation and growth model for cBN is proposed which is consistent with existing theoretical and experimental models.

Photoemission study of onionlike carbons produced by annealing nanodiamonds

Photoelectron spectroscopy has been used to study the products resulting from high temperature phase transformation of nanodiamonds (ND). Depending on the temperature of annealing various particles with a diamond core covered by nanometer sized fullerene-like shells, and onionlike carbon (OLC) were formed. Analysis of the $\mathrm{C}1s$ photoemission lines of the intermediates of ND transformation, prepared at temperatures of 1420 and 1600 K and then exposed to atmosphere, reveals the presence of oxygen-containing groups and both ${\mathit{sp}}^{\mathit{2}}$ and ${\mathit{sp}}^{\mathit{3}}$ carbon. The ${\mathit{sp}}^{\mathit{2}}$ component for these samples has binding energies of $284.70\ifmmode\pm\else\textpm\fi{}0.05\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$ (for the sample prepared at 1420 K) and $284.50\ifmmode\pm\else\textpm\fi{}0.05\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$ (for the sample prepared at 1600 K). A difference of $1.3\ifmmode\pm\else\textpm\fi{}0.1\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$ in the binding energy of the ${\mathit{sp}}^{\mathit{3}}$ and ${\mathit{sp}}^{\mathit{2}}$ components was observed. The ${\mathit{sp}}^{\mathit{2}}$ component for OLC prepared at 1800, 1900, and 2140 K has a binding energy of $284.45\ifmmode\pm\else\textpm\fi{}0.05\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$. The shift towards higher binding energies of the ${\mathit{sp}}^{\mathit{2}}$ component of the samples prepared at lower temperatures is explained by significant curvature of graphite layers formed in the initial stages of graphitization. The observed increase in density of states at the Fermi level for the samples prepared at 1600, 1800, and 1900 K is associated with an accumulation of different types of defects in the curved graphite layers during graphitization of diamond. The Lorentzian widths of $\mathrm{C}1s$ photoemission lines from OLC are large compared with those of HOPG. The possible reasons for this broadening are discussed.

地球内部の岩石鉱物

The recent progress of the studies on high pressure and high temperature phase transformation of earth materials are reviewed. A brief introduction on the basis on the chemistry and mineralogy of materials composing crust and mantle is also included. Several new high pressure minerals including the post-perovskite phase of MgSiO3, post-stishovite phases of SiO2 such as CaCl2 type, a-PbO2 type phases, which are likely to exist at the core-mantle boundary has been reported. Magma is highly compressible and compatible with iron relative to coexisting minerals, indicating existence of a melt-crystal density crossover at high pressure. The dense melt may play important roles at the core-mantle boundary together with in the early magma ocean stage of the earth's evolution. We can expect existence of gravitationally stable melts in the ultra low velocity layer at the core-mantle boundary. The study of the lower mantle and core is one of the most important target in the material science of the earth.

In situ studies of high-pressure phase transformations in the B–C–N system

This article briefly reviews results of our recent in situ studies of high-pressure high-temperature phase transformations of graphite-like BN–C solid solutions and turbostratic CN using powder X-ray diffraction with synchrotron radiation.

Plasticity and strength of metal oxide high-temperature superconductors (Review)

The results of research on the plasticity and strength of a wide class of metal oxide perovskite-like compounds which have the property of high-temperature superconductivity or which can be used as base compounds for making high-temperature superconductors (HTSCs) are systematized and presented from a unified point of view. The mechanical properties of materials with different morphology—single crystals, polycrystals, and composites,—measured by different methods of mechanical testing in the low-temperature, room-temperature, and high-temperature regions, are discussed. The characteristic defects of the crystal structure for these compounds are considered, the crystallography of two modes of plastic deformation—slip and twinning—is described, and the stress-induced structural rearrangement of the twin structure that appears at a high-temperature phase transformation is discussed. The features of plastic deformation and fracture of metal oxide materials due to structural microdefects (dislocations, impurities, twin and grain boundaries) and macrodefects (voids, cracks, heterophase inclusions) are noted, and the role of heavy-cation diffusion in the kinetics of high-temperature deformation is discussed. The influence of structural phase transformations and the superconducting transition on the mechanical properties of metal oxides is considered. This review is a continuation of a review of the elastic and acoustic properties of HTSCs published earlier by the authors in Fiz. Nizk. Temp. 21, 475 (1995) [Low Temp. Phys. 21, 367 (1995)].

High-temperature Raman spectroscopy and phase transformations in phosphates and vanadates Ca3 − 3x Nd2x (AO4)2 (A = P, V; 0 ≤ x ≤ 0.14)

The phases of variable composition Ca3 − 3xNd2x(AO4)2 (A = P, V; 0 ≤ x ≤ 0.14) based on calcium orthophosphate or orthovanadate are studied by Raman spectroscopy. The influence of the composition and temperature on the structural features of these compounds is considered. The changes observed in the Raman spectra at high temperatures are associated with the reversible phase transition due to the redistribution of calcium over different positions and partial reorientation of the AO4 tetrahedra.

Concentric Solidification for High Temperature Laser Scanning Confocal Microscopy

A new experimental technique defined as concentric solidification has been developed to improve in-situ observations of solidification and high temperature phase transformations using laser scanning confocal microscopy (LSCM). The technique consists of applying a radial thermal gradient across a 10 mm diameter sample such that the maximum temperature is focused in the centre of the specimen. Careful control over the sample thickness, heating rate and peak temperature results in the formation of a liquid pool in the centre of the specimen. Surface tension balance between solid, liquid, gas and crucible result in minimal meniscus formation on the liquid pool, leading to a greatly enhanced in-situ observations. Examples of the range of observations possible as well as unique observations of segregation related phenomena are presented.

Study of microstructure and solute partitioning in a cast Ti-48Al-2Cr-2Nb alloy by quenching during directional solidification technique

One major hindrance to effective implementation of cast gamma TiAl-based intermetallic alloys in aircraft engines lies in the variability of their mechanical properties resulting from chemical and microstructural heterogeneities. In the present work, the buildup of microsegregation in a cast Ti-48Al-2Cr-2Nb alloy is investigated through experiments of quenching during directional solidification (QDS). The solidification process, as well as the partitioning of alloying elements, between the solid and liquid phases, is investigated. Considering experimental conditions, the α-hcp phase is found to be the primary solidifying phase. A low dendrite tip temperature of 1475 °C was estimated from thermal recordings. These observations could be explained considering the value of the thermal gradient (around 4 °C/mm). Quantitative values of partition coefficients are proposed for Al, Cr, and Nb. In addition to Al, Cr is found to segregate in interdendritic regions, whereas Nb tends to be retained in the Ti-rich inner dendrites. Considering experimental cumulative solute distributions, the buildup of microsegregation can be satisfactorily represented on the basis of Gulliver-Scheil assumptions. Due to high-temperature quenching, the QDS experiments are also found to be appropriate to the study of high-temperature phase transformations and microstructural development of TiAl-based alloys. The results of QDS experiments are discussed with regard to the range of microstructural and chemical heterogeneities determined within Ti-48Al-2Cr-2Nb investment castings. Finally, regarding solid-state phase transformations subsequent to solidification, the study attempts to explain the formation of B2 phase particles stabilized by the ternary additions.

High pressure and high temperature phase transformations in LiNbO3

A behavior of LiNbO3 under high pressure and temperature has been studied up to 90 GPa by means of high pressure in situ x-ray observations. Recovered samples were analyzed by transmission electron microscope (TEM). When the LiNbO3 was compressed at room temperature, a transformation occurred at about 25 GPa. The powder x-ray diffraction pattern of this “room-temperature and high-pressure” (RT–HP) phase was successfully explained by the NaIO3-type structure. No further transformation was observed at room temperature up to 90 GPa and reverse transition to starting phase occurred at about 10 GPa, thus this phase was unquenchable on release of pressure. When this RT–HP phase was heated at above 30 GPa, a phase appeared which can be recovered to ambient condition. X-ray diffraction and TEM analysis of this “high-temperature and high-pressure” (HT–HP) phase clarified that this phase has hexagonal symmetry with a most likely space group of P63. The quenched sample reverts to the starting phase on heating above 650 K. This HT–HP phase is opaque, suggesting the change of electronic property. The density of these RT–HP and HT–HP phases are, respectively, 21% and 23% higher compared to the starting LiNbO3 phase at ambient condition.