Crystallographic Phase Transformation Research Articles

Influence of galia (Ga2O3) addition on the phase evolution and grain growth behavior of voided yttria stabilized zirconia (YSZ) powder

The effect of a galia (Ga2O3) addition on the crystallographic phase transformations and the grain growth behavior of yttria stabilized zirconia (YSZ) were investigated regarding powders containing different amounts of galia in the range of 0–25 mol %. Ternary compositions of galia- YSZ sponge-like cryogels were prepared by the sol-gel method combining freeze-drying process and calcination of dried powders at different temperatures for 2 h in air. Crystallographic phase transitions were analyzed via X-ray diffraction (XRD) and exceptional powder particle morphology of internal nano voids derived from the freeze-drying technique was investigated by using Scanning Transmission Electron Microscope (STEM). It was shown that galia supressed the grain growth process in yttria stabilized zirconia and had no effect on the internal voids obtained.

Crystallography of phase transformation during quenching from β phase field of a V-rich TiAl alloy

Well-developed $$ \alpha_{2}^{{\prime }} $$ -martensitic laths were formed in a Ti–40Al–10V (at.%) alloy after a homogenization treatment within the β phase field followed by brine quenching. The morphology and crystallography of martensite were examined by electron backscatter diffraction. It was found that the martensitic transformation was incomplete and about 35 vol% β phase was retained to room temperature. No diffusional or massive transformation has been detected in the β grain interior. All the 12 martensite variants were formed according to the Burgers orientation relationship (BOR) with the parent β grain. While global variant selection has not been detected, local variant selection occurred so that three self-accommodant variants which shared a common 〈11.0〉 axis were predominant in local regions. Based on the traces of the variants, the habit plane was determined to be close to {679}β by using a simply geometrical method. Besides, $$ \alpha_{2}^{{\prime }} $$ plates were frequently observed along the β grain boundaries, which accorded to a strict BOR with one of the adjacent β grains, but tended to grow into the other β grain and hence resulted in serrated interface. Such features were an indication of massive transformation. Moreover, an apparent variant selection criterion for the grain boundary $$ \alpha_{2}^{{\prime }} $$ plates was noted. That is, if a $$ \alpha_{2}^{{\prime }} $$ variant has, in addition to an exact BOR with one of the two adjacent β grains, a near BOR with the other β grain, this $$ \alpha_{2}^{{\prime }} $$ plate would be very large and even extends along the entire grain boundary.

High-calcium fly ash recovery from wet-stored condition and its properties

Fly ash can undergo changes in its properties during the wet-stored conditions, leading to reactivity variability consequently affecting its potential applications. Evaluation of the material properties is considerably essential. This work reports on an investigation of changes in physical and chemical properties of high calcium fly ash after recovering from the wet-stored conditions. Herein the high calcium lignite fly ash with calcium oxide constituent in a range of 20%–30% is the focus of this study. Important characteristics of fly ash have been studied, including physical properties, morphologies, chemical composition, and crystallographic phase transformation. Relation of oxide composition including silica, alumina, ferric oxide, and sulfur trioxide, to wet-storing period has been established to visualize the effect of short-term storage in water on changes in the content of mineral bearing species in fly ash. The strength activity indices of the recovered fly ash samples cured for 7 and 28 days has been presented. The significant amount of free lime and soluble calcium, aluminum, and sulfur-bearing minerals varying with storage times are believed to play their pronounced role governing compressive strength.

Texture evolution, phase transformation and mechanical properties of selective laser melted Ti-13Nb-13Zr

The crystallographic texture and phase composition associated with the performance of materials have been well documented, but there are very few studies on their controlling strategies. In this work, the crystallographic texture orientation and phase transformation of Ti-13Nb-13Zr alloy were tailored for the first time by selective laser melting with different scanning strategies. 〈011〉 crystal orientation along the building direction is preferred in sample A (unidirectional scanning along one axis), while 〈001〉 is preferred in sample B (unidirectional scanning with a 90° rotation in each layer). The evolution of the volume fraction of α′ phase was investigated using X-ray photoelectron spectroscopy. The results indicated that sample B has the highest content of α′ phasein sample B. The hardness and wear resistance have been studied through nanoindentation and the results turn out that all samples show higher hardness and better wear resistance (hin sample B. The hardness and wear resistance have been studied through nanoindentation and the results turn out that all samples show higher hardness and better wear resistance (higher Hd/Er and Hd3/Er2 ratios) than widely used CP Ti. A simple micromechanical model was used to calculate the elastic moduli of α′ and β phases, which were determined to be 151 GPa and 64 GPa, respectively.

A Spin-Crossover Molecular Material Describing Four Distinct Thermal Pathways.

Spin-crossover (SCO) molecular solids are valued switchable materials for their common abrupt and reversible thermal transitions, large thermal hysteresis, or guest-dependent effects. These properties usually involve crystallographic transitions coupled to the SCO events. These phenomena are of great value for the understanding of solid-state transformations and also for exploiting them. We present here a lattice of the complex [FeL(bbp)](ClO4)2 (1; L and bbp are tris-imine ligands) featuring an unprecedented rich succession of SCO and crystallographic phase transformations. Magnetometry measurements unveil a thermally irreversible sequence of spin conversions that delineate four different thermal pathways. All of these are single-crystal-to-single-crystal processes and can thus be monitored by single crystal X-ray diffraction using one unique specimen. Fresh crystals of 1 contain one molecule of acetone per Fe center (1·ac) that abandons the lattice upon warming at the same time that a SCO from an ordered mixed spin state (1:1 high spin/low spin; HS/LS) to a fully HS state, 1α, occurs. This crystallographic phase, accessed through a template effect by the solvent, converts into another one, 1β, upon cooling, as triggered by a HS to LS SCO. Warming of 1β induces a new SCO (LS to ordered HS/LS) coupled to another crystallographic phase transition, 1β → 1γ. The fully HS state of 1γ can not be reached before decomposition of the compound. Instead, this phase cycles between the HS/LS and the LS states through superimposable pathways, different from that of the prerequired 1β → 1γ phase change. Analysis of the thermal variation of the free energy, G, through density functional theory methods provides trends in agreement with the observation of these transformations and clarifies the possible metastable nature of the various phases identified. This unique behavior allows the access to four different magnetic responses depending on the thermal history of the sample, within a given range of temperatures near the ambient conditions.

Unexpected Crystallographic Phase Transformation in Nonstoichiometric SrUO4- x: Reversible Oxygen Defect Ordering and Symmetry Lowering with Increasing Temperature.

In situ synchrotron powder X-ray diffraction measurements have demonstrated that SrUO4 undergoes a reversible phase transformation under reducing conditions at high temperatures, associated with the ordering of oxygen defects resulting in a lowering of crystallographic symmetry. When substoichiometric rhombohedral α-SrUO4- x, in space group R3̅ m with disordered in-plane oxygen defects, is heated above 200 °C in a hydrogen atmosphere it undergoes a first order phase transformation to a (disordered) triclinic polymorph, δ-SrUO4- x, in space group P1̅. Continued heating to above 450 °C results in the appearance of superlattice reflections, due to oxygen-vacancy ordering forming an ordered structure δ-SrUO4- x. Cooling δ-SrUO4- x toward room temperature results in the reformation of the rhombohedral phase α-SrUO4- x with disordered defects, confirming the reversibility of the transformation. This suggests that the transformation, resulting from oxygen vacancy ordering, is not a consequence of sample reduction or decomposition, but rather represents a change in the energetics of the system. A strong reducing atmosphere is required to generate a critical amount of oxygen defects in α-SrUO4- x to enable the transformation to δ-SrUO4- x but once formed the transformation between these two phases can be induced by thermal cycling. The structure of δ-SrUO4- x at 1000 °C was determined using symmetry representation analysis, with the additional reflections indexed to a commensurate distortion vector k = ⟨1/4 1/4 3/4⟩. The ordered 2D layered triclinic structure of δ-SrUO4- x can be considered a structural distortion of the disordered 2D layered rhombohedral α-SrUO4- x structure through the preferential rearrangement of the in-plane oxygen vacancies. Ab initio calculations using density functional theory with self-consistently derived Hubbard U parameter support the assigned ordered defect superstructure model. Entropy changes associated with the temperature dependent short-range ordering of the reduced U species are believed to be important and these are discussed with respect to the results of the ab initio calculations.

K3V2(PO4)2F3 as a robust cathode for potassium-ion batteries

Potassium-ion batteries have emerged as promising candidates for low-cost and sustainable energy storage systems. The development of potassium-ion batteries is relatively slow due to the large size of potassium ions, rendering great difficulty in designing appropriate host materials. Herein, a K3V2(PO4)2F3 cathode is inherited from Na3V2(PO4)2F3 analog. The crystallographic structure and phase transformations are unveiled through in-situ X-ray diffraction, which shows only minor volume change of 6.2% during potassium ions insertion/extraction. Nearly two potassium ions could be provided by the electrode, delivering a capacity of over 100 mA h g−1 with a high average potential of ~3.7 V vs. K+/K. An energy density of around 400 W h kg−1 together with a respectable rate capability have been obtained. Coupling with a graphite anode, a 3.4 V-Class battery has been demonstrated, making potassium-ion batteries promising contenders to sodium ion batteries in large-scale energy storage. This discovery also sheds insights into the quest for potential electrodes from the analogs in Li/Na-ion batteries.

Hydrogen Flux through Size Selected Pd Nanoparticles into Underlying Mg Nanofilms

AbstractThe application of Mg for hydrogen storage is hindered due to the slow absorption of hydrogen in Mg films. Herein, the hydrogenation process is explored theoretically using density functional theory calculations, and energy barriers are compared for hydrogen diffusion through Pd nanoparticle/Mg film interfaces and their variations, i.e., Pd(H)/Mg(O). Decomposing the mechanism into basic steps, it is shown that Pd undergoes a strain‐induced crystallographic phase transformation near the interface, and indicated that hydrogen saturation of Pd nanoparticles enhances their efficiency as nanoportals. Using energetic arguments, it is explained why hydrogen diffusion is practically prohibited through native Mg oxide and seriously suppressed through existing hydride domains. Hydrogen flux is experimentally investigated through the nanoportals in Pd‐nanoparticle decorated Mg films by pressure‐composition isotherm measurements. An r ≈ t1/3 relationship is theoretically calculated for the radial growth of hemispherical hydride domains, and this relationship is confirmed by atomic force microscopy. The diffusion constant of hydrogen in Mg films is estimated as DHfilm ≈ 8 × 10−18 m2 s−1, based on transmission electron microscopy characterization. The unique nanoportal configuration allows direct measurement of hydride domain sizes, thus forming a model system for the experimental investigation of hydrogenation in any material.

Atomic-scale investigation of a new phase transformation process in TiO2 nanofibers.

Crystallography of phase transformation combining transmission electron microscopy (TEM) with in situ heating techniques and X-ray diffraction (XRD) can provide critical information regarding solid-state phase transitions and the transition-induced interfaces in TiO2 nanomaterials theoretically and experimentally. Two types of reduced titanium oxides (Ti3O5, Ti6O11) are found during ex situ and in situ heating of TiO2 (B) nanofibers with a specific morphology of the {100} single form (SF) in air and vacuum. The results indicate that the phase transformation process from TiO2 (B) follows the TiO2 (B) → Ti3O5 → Ti6O11 → anatase sequence for the nanofibers with the {100} SF. The occurrence of such a phase transition is selective to the morphology of TiO2 (B) nanofibers. The corresponding orientation relationships (COR) between the four phases are revealed according to the TEM characterization. Four types of coherent interfaces, following the CORs are also found. They are TiO2 (B)/Ti3O5, TiO2 (B)/Ti6O11, Ti6O11/anatase and TiO2 (B)/anatase respectively. The habit plane for the TiO2 (B) to Ti3O5 transition is calculated as the {100}TB by using the invariant line model. The detailed atomic transformation mechanism is elucidated based on the crystallographic features of the four phases.

Characterization of crystal structure and precipitation crystallography of a new MgxAl2−xGd phase in an Mg97Al1Gd2alloy

The composition, crystal structure and precipitation crystallography of a newly found precipitate are characterized by Cs-corrected scanning transmission electron microscopy. The composition of the plate-like precipitate could be expressed as MgxAl2−xGd (x= 0.38), and its crystal structure is the same as the face-centered cubic type Laves phases Mg2Gd and Al2Gd, with a lattice parameter of 7.92 Å (space group No. 227, Fd\overline 3m). The orientation relationship between the matrix and precipitate is found to be (0001)m//(111)pand [10\overline 10]m//[1\overline 10]p, and the habit plane is parallel to the (0001)m//(111)pplane. In addition, this preferred crystallography of phase transformation is well explained on the basis of the atomic matching at the interface.

Crystallography of phase transformation in the self-inclined InAs nanowires grown on GaAs{111}

During growth of InAs nanowires (NWs) on GaAs{111}, the self-inclined InAs NWs were reproducibly observed. At the self-inclined interface, the h.c.p. InAs phase was transformed into another f.c.c. InAs phase. In order to investigate the crystallography of phase transformation, a recently-developed model, edge-to-edge matching (E2EM), was applied in such self-inclined NWs for crystallographic investigation. The orientation relationships (ORs), between f.c.c. InAs and h.c.p. InAs, were predicted using the E2EM model, which agrees well with experimental results. The results indicate the importance of crystallographic study in design and construction of nanostructured semiconductors with improved physical properties.

Microstructure, crystallography of phase transformations and multiple precipitations in PH 15-7Mo stainless steel

The microstructure and crystallographic features of a semi-austenitic precipitation hardening steel PH 15-7Mo during solution treatment, roddrawing and aging were investigated by means of optical microscope, X-ray diffraction analyzer and transmission electron microscope. It was found that the microstructure of the steel was consist of dominant austenite, small amount of martensite and 10–15 vol.% δ-ferrite after solution treatment at 1050 °C followed by cooling in water at room temperature. The austenite transformed into lath martensite during tensile roddrawing about 60% deforming companied with some coherent fine β-NiAl particles precipitated within martensite. With higher aging temperature and longer holding time, tiny carbide M23C6 particles precipitated from martensite, which kept the cubic–cubic orientation relationship (OR) with austenite and G-T OR with martensite which is different with all the reported orientations. The OR between tiny carbide M23C6 particles G-T OR with martensite was discussed in terms of crystallography of phase transformations.

Effect of martensitic phase transformation on the behavior of 304 austenitic stainless steel under tension

The present work integrates in-situ neutron diffraction, electron backscatter diffraction and crystal plasticity modeling to investigate the effect of martensitic phase transformation on the behavior of 304 stainless steel under uniaxial tension. The macroscopic stress strain response, evolution of the martensitic phase fraction, texture evolution of each individual phase, and internal elastic strains were measured at room temperature and at 75°C. Because no martensitic transformation was observed at 75°C, the experimental results at 75°C were used as a reference to quantify the effect of formed martensitic phase on the behavior of 304 stainless steel at room temperature. A crystallographic phase transformation model was implemented into an elastic–viscoplastic self-consistent framework. The phase transformation model captured the macroscopic stress strain response, plus the texture and volume fraction evolution of austenite and martensite. The model also predicts the internal elastic strain evolution with loading in the austenite, but not in the martensite. The results of this work highlight the mechanisms that control phase transformation and the sensitivity of modeling results to them, and point out to critical elements that still need to be incorporated into crystallographic phase transformation models to accurately describe the internal strain evolution during phase transformation.

Shear transformation of coupled β1/β′ precipitates in Mg–RE alloys: A quantitative study by aberration corrected STEM

The coupled distribution of β1 and β′ particles is frequently observed in aged microstructures of Mg–RE (RE represents rare-earth elements) alloys. The crystallography of these coupled β1/β′ particles is characterized using atomic-resolution high-angle annular dark-field scanning transmission electron microscopy, and the shear and dilatation strains associated with the migration of the β1/β′ interface are analysed. It is found that the orientation relationship between the β1 and β′ phases, and the irrationally oriented and yet fully coherent interface associated with this orientation relationship, can both be well described by the crystallography of phase transformations. It is also found that there exists a large shear strain for the β1/β′ transformation or the migration of the fully coherent β1/β′ interface. The magnitude and sign of this shear strain are compatible, but opposite to those of the shear strain involved in the α-Mg/β1 transformation. Hence the coupled distribution of β1/β′ particles offers a unique opportunity to almost fully self-accommodate the shear strains involved in the α-Mg/β1 and β1/β′ structural transformations. This is perhaps why the β1 plates with β′ particles attached to their ends are the most commonly observed distribution in aged microstructures of Mg–RE alloys.

Structure and properties of nanostructured Cobalt processed by high pressure torsion at temperatures of 300 and 77 K

Several nanostructured states of high purity cobalt were achieved by high pressure torsion (HPT) at pressures of 4 and 8 GPa and temperatures of 300 and 77 K. Changes of crystallographic texture, grain sizes and phase transformations as a result of HPT and subsequent annealing have been measured and analyzed. Mechanical properties of nanostructured Co were studied by microhardness measurements at 300 K, and uniaxial compression at temperatures 300, 77 and 4.2 K. Comparison of the strength characteristics, plasticity and strain hardening of the nanostructured and coarse grained Co has been carried out. Micromechanisms of plastic deformation of Co during HPT deformation and uniaxial compression are discussed.

The Influence of Crystallographic Orientation on the Chemo-Elastic Response of Reconstructed LixCoO2 Cathode Particles

This work investigates the inhomogeneous and anisotropic, chemo-elastic response of a particular, but representative, reconstructed 3.6-μm LixCoO2 single-crystal cathode particle from a Li-ion battery. The results are based on a three-dimensional computational model that represents coupled electrochemical and mechanical response throughout the lithiation process (0.50 ⩽ x ⩽ 0.94) associated with a 1C galvanostatic discharge. The study focuses on the roles that phase transformation and crystallographic orientation play in the Li-concentration and stress-strain fields. Results show very high local peaks of diffusion-induced stresses, indicating the significant potential for particle fracture. Anisotropic chemo-elastic fields give rise to local Li-concentration bands, causing chemically induced misfit strains and high stresses, and such occurrences are determined to be irrespective of the particle morphology. The locally high stresses are associated primarily with a crystallographic phase transformation between two hexagonal phases that is prominent at x ≈ 0.75, being closely related to bands of chemical-misfit strains developed in the Li0.75CoO2 material. The chemo-mechanical band structure depends significantly on crystallographic orientation within the electrode particle.

Hydrogen desorption of Mg–Mg2Ni hypo-eutectic alloys in air, Ar, CO2, N2 and H2

Abstract The hypo-eutectic Mg–Mg 2 Ni system can be modified by trace elemental additions of Na which change the microstructure and the functional properties of the alloy. The modified microstructure results in improvements to the hydrogen storage properties. In alloys of optimal composition, it has been shown that the reversible storage of 6.5–7 wt% H 2 is possible at a rate of reaction that is realistic for industrial applications. This research investigates the release of H 2 in air under atmospheric pressure as well as under 0.2 MPa atmospheres of Ar, CO 2 , N 2 , and H 2 . The release kinetics were characterised using in situ crystallographic phase transformation analysis obtained by synchrotron Powder X-ray diffraction (PXRD) at the Australian Synchrotron. The mole fraction of MgH 2 , Mg, Mg 2 Ni, and Mg 2 NiH 4 was determined by Rietveld refinement using RIETAN-FP. It was found that the hydrogen release temperature largely depends on the atmosphere.

Synthesis, structural characterization and Mössbauer study of LnV0.5Fe0.5O3 perovskites (Ln = Y, La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho and Er)

Perovskites LnV0.5Fe0.5O3 (Ln=Y, La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho and Er) were synthesized by rapid solidification from arc-melted samples and characterized by the study of their crystal structure and hyperfine properties. These metastable solid solutions crystallized in the Pbnm symmetry, with the iron and vanadium cations randomly distributed in the transition metal octahedral sites. Depending on the lanthanide present at the A site of the perovskite, iron is present with two valences (i.e., Fe3+ and Fe2+). The volume of the unit cell for these perovskites increases linearly with the lanthanide ionic radius, as the perovskite approaches its ideal structure. At room temperature, the quadrupolar splitting of the trivalent paramagnetic Mössbauer component works as an indirect measurement for the Goldshmidt tolerance factor. Close to or below 100K, these perovskites undergo a crystallographic phase transformation, probably due to orbital ordering of the V3+ cations, originating two different magnetic iron sites.

Preparation of α-Fe2O3 nanotubes via electrospinning and research on their catalytic properties

In this paper, smooth α-Fe2O3 nanotubes have been successfully synthesized by electrospinning of ferric nitrate–polyvinyl alcohol solution followed by calcination in air. The morphologies and structures of the samples were characterized by transmission electron microscopy, scanning electron microscopy and X-ray diffraction. The catalytic properties were studied by differential thermal analysis and thermogravimetric analysis. The results indicated that the as-prepared α-Fe2O3 nanotubes showed a continuous morphology and an extremely high degree of crystallization. The average inner and outer diameters of the obtained α-Fe2O3 nanotubes were about 60 nm and 100 nm, respectively. The obtained α-Fe2O3 nanotubes were able to lower the temperature of the high-temperature thermal decomposition of ammonium perchlorate, while they had little effect on the crystallographic phase transformation and the low-temperature thermal decomposition.

Anisotropic behavior of energetic materials at elevated pressure and temperature

First principles and molecular dynamics methods where used to study energetic materials (EM) of varying sensitivity, particularly nitromethane (NM) and pentaerythritol tetranitrate (PETN). Molecular and electronic structure, as well as anisotropic elastic properties, variation of charge distributions, and stress-strain behavior of these three materials were investigated. Information obtained from different levels of theory is useful to reveal and assess the anisotropic sensitivity of the studied systems. For NM, we observe a crystallographic phase transformation and a drastic change in its mechanical behavior. PETN is observed to be mechanically stable within the studied pressure range, with a marked anisotropic behavior when compressed at elevated temperatures.