Crystallographic Transformation Research Articles

Effect of rare earth ion substitution on phase decomposition of apatite structure.

The paper describes an investigation of phase decomposition of apatite lattice doped with rare earth ions (cerium, samarium, and holmium) at temperatures ranging from 25 to 1200 ºC. The rare-earth ion-doped apatite minerals were synthesized using sol-gel method. In situ high-temperature powder X-ray diffraction (XRD) was used to observe phase changes and the lattice parameters were analyzed to ascertain the crystallographic transformations. The expansion coefficient of the compounds was determined, and it was found that the c-axis was the most expandable due to relatively weak chemical bonds along the c-crystallographic axis. Differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA) were used to examine the decomposition properties of the materials. Due to rare earth ion doping, the produced materials had slightly variable decomposition behaviour. The cerium and samarium ions were present in multiple oxidation states (Ce3+, Ce4+, Sm3+, Sm2+), whereas only Ho3+ ions were observed. Rare earth ion substitution affects tri-calcium phosphate proportion during decomposition by regulating concentrations of vacancies. X-ray photoelectron spectroscopy (XPS) analysis indicated that cerium and samarium ion-doped apatite yielded only 25% tricalcium phosphate during decomposition. This finding advances our understanding of apatite structures, with implications for various high-temperature processes like calcination, sintering, hydrothermal processing, and plasma spraying.

Singular Interfacial Structures at Two Levels: Their Roles in the Development of Phase Transformation Crystallography

Singular Interfacial Structures at Two Levels: Their Roles in the Development of Phase Transformation Crystallography

Development of ferroelectricity with crystallographic phase transformation in Hf0.5Zr0.5O2 thin films upon initial stimulation of an electric field exceeding the coercive field

This study demonstrates a drastic transformation of ferroelectricity and crystallographic phase in Hf0.5Zr0.5O2 (HZO) thin films through the first stimulation of an electric field larger than the coercive field in metal-ferroelectric-metal (MFM) capacitors. Initially, capacitance–voltage (C–V) measurements in fresh MFM capacitors with voltage sweep smaller than the coercive field indicated constant capacitance value without hysteresis. Applying the electric field exceeding the coercive field subsequently, the C–V behavior changed to a typical dual-peak ferroelectric feature in positive and negative sweeps. X-ray diffraction analysis of the HZO layers before and after a single electric-field application revealed small-angle shifts in diffraction peaks, confirming a crystallographic phase transformation induced by the single electric-field stimulation exceeding the coercive field. These results suggest a possibility that the first electric field apply initiates development of ferroelectric phase transformation from a non-ferroelectric phase.

Effects of acicular ferrites on crystallography and incomplete transformation of low-carbon low-alloy steel

In this study, we investigated the effects of acicular ferrite (AF) on the crystallographic and incomplete transformation characteristics of low-carbon low-alloy steel. Intermediate annealing after austenitization induced a transformation yielding AFs, accelerating the isothermal transformation kinetics and regulating the microstructure. This process formed highly dense packet grain boundaries and improved the stability of untransformed austenite. AF formation increased the number of packet interfaces, increasing the crack propagation energy. The reduction and refinement of martensite/austenite (M/A) islands increased the crack nucleation energy. A displacive model was used to fit the transformation kinetics curve, indicating the significant effect of the nucleation rate on the isothermal transformation of the experimental steel at 450, 475, and 500 °C. The findings of this study indicate that nucleation regulation can accelerate transformation kinetics to strengthen and toughen bainitic structures.

Intragrain impurity annihilation for highly efficient and stable perovskite solar cells

Intragrain impurities can impart detrimental effects on the efficiency and stability of perovskite solar cells, but they are indiscernible to conventional characterizations and thus remain unexplored. Using in situ scanning transmission electron microscopy, we reveal that intragrain impurity nano-clusters inherited from either the solution synthesis or post-synthesis storage can revert to perovskites upon irradiation stimuli, leading to the counterintuitive amendment of crystalline grains. In conjunction with computational modelling, we atomically resolve crystallographic transformation modes for the annihilation of intragrain impurity nano-clusters and probe their impacts on optoelectronic properties. Such critical fundamental findings are translated for the device advancement. Adopting a scanning laser stimulus proven to heal intragrain impurity nano-clusters, we simultaneously boost the efficiency and stability of formamidinium-cesium perovskite solar cells, by virtual of improved optoelectronic properties and relaxed intra-crystal strain, respectively. This device engineering, inspired and guided by atomic-scale in situ microscopic imaging, presents a new prototype for solar cell advancement.

On a mathematical theory for the crystallography of shear transformations in metals: Unification of mechanisms

A theory for shear transformations in crystalline materials linking their Crystallography, atomic-scale mechanisms and Thermodynamics is introduced. The lattice and transformation function are treated as vector space and functions respectively to overcome shortcomings of classical matrix-based theories for martensite crystallography. The extremum principle is used to obtain the atomic paths minimising and maximising the energy from all possible solutions, therefore explaining which sequences are more Thermodynamically feasible. The face-centred cubic (FCC)→body-centred cubic (BCC) transformation in Fe is used as case study. The theory predicts that the average values of the extrema correspond to the crystallography, shear magnitude, interface defects and habit planes of main transformations in Fe, i.e. Widmanstätten Ferrite, Bainite, lath martensite, plate and lenticular martensite. Bainite represents the macroscopic average minimum energy configuration, whereas Widmanstätten Ferrite corresponds to a local and discrete minimum energy configuration. Lath martensite corresponds to the average shear configurations minimising atomic displacements when there is no other driving force, e.g. diffusion, whereas the maximum energy configurations correspond to plate and lenticular martensite forming twin pairs. Targeted results for the FCC→hexagonal close-packed lattice transformation are presented to demonstrate the robustness of the theory. The predictions are combined with elasticity theory to compute the Driving Force and predict the start temperatures – therefore connecting for the first time – local and macroscopic crystallographic features with the Thermodynamics of shear-controlled structures in Fe. This work presents new results towards addressing longstanding challenges in Materials Science such as theoretically demonstrating that Bainite is a shear transformation.

Twisting two-dimensional iron sulfide layers into coincident site superlattices via intercalation chemistry.

Layered van der Waals (vdW) materials are susceptible not only to various stacking polymorphs through translations but also twisted structures due to rotations between layers. Here, we study the influence of such layer-to-layer twisting through the intercalation of ethylenediamine (EDA) molecules into tetragonal iron sulfide (Mackinawite FeS). Selected area electron diffraction patterns of intercalated FeS display reflections corresponding to multiple square lattices with a fixed angle between them, contrary to a single square lattice seen in the unintercalated phase. The observed twist angles of 49.13° and 22.98° result from a superstructure formation well described by the coincident site lattice (CSL) theory. According to the CSL theory, these measured twist angles lead to the formation of larger coincident site supercells. We build these CSL models for FeS using crystallographic group-subgroup transformations and find simulated electron diffraction patterns from the model to agree with the experimentally measured data.

Operando Neutron Diffraction Analysis of Commercial Positive Electrodes of Lead Batteries

We have analyzed by operando neutron diffraction the charge/discharge crystallographic transformation processes inside the positive active mass (PAM) of industrial lead batteries, getting information onthe spatial distribution of the phases. Our study was focused on the positive electrode because it is there where the main limitations to the charge/discharge efficiency are originated. The experiments were carried out in the VULCAN instrument at the Spallation Neutron Source of the Oak Ridge National Laboratory (Tennessee, USA) using the volume gauge technique. VULCAN is a high brightness time-of-flight diffractometer which provides fast volumetric mapping and, therefore, the possibility to study kinetic behaviors. Accordingly, both static and dynamic mapping experiments were performed during charge-discharge cycles, comparing fresh and cycled cells. The diffractograms obtained were analyzed using VDRIVE and GSAS software, together with dedicated Python scripts. In addition to compositional maps (Figure 1), information on lattice parameters, crystallinity, stoichiometry and hydration was also obtained as a function of the state of charge. Local inhomogeneities and differences in the behavior between fresh and cycled cells were observed. Moreover, we have used the Pb signal from the positive grid as internal calibrating element to obtain the absolute molar concentration of the component phases, comparing the PbSO4 PbO2 transformation with the effective charge driven by the cell during charge and discharge processes. The b-PbO2 mass utilization during the C5 100% to 20% discharge was about 0.3. In the same way, the charge factor was estimated to about 70%, from the start of charging until the PbO2 concentration reaches a near flat regime, and about 5% after reaching the flat regime. Giving for the full charging cycle a value about 50%.As far as we know, this is the first time that diffraction information has been obtained from inside commercial thick electrodes (3.4 mm in thickness) during real operation conditions, mapping the PbO2 and PbSO4 evolution in a 2.24 cm x 7.2 cm region of the positive electrode. Figure 1

Crystallography of stress-induced martensitic transformation and giant elastocaloric effect in a A textured Ni27Cu21Mn46Sn6 shape memory alloy

Shape memory alloys exploit the latent heat yielded by stress-induced martensitic transformation to achieve elastocaloric effect, being suited for solid-state cooling technology as an alternative to conventional vapor-compression refrigeration. In this work, the crystallography of stress-induced martensitic transformation and elastocaloric effect in a polycrystalline Ni27Cu21Mn46Sn6 shape memory alloy with <001>A preferential orientation produced by means of directional solidification were studied. By performing ex-situ electron backscatter diffraction (EBSD) measurements, the transition from austenite to a single variant of 6 M martensite induced by compressive loading was evidenced, following a specific transformation orientation relationship of {1 0 1}A//{1 –2 –6}M and <1 0 − 1>A//<–6 –6 1>M between two phases. Moreover, owing to the synergistic effect of large transformation entropy change tailored through manipulating the lattice volume change across the phase transformation and microstructure texturing promoted by temperature gradient effect during solidification process, the present alloy can demonstrate very remarkable elastocaloric effect, with an extremely high value of adiabatic temperature variation up to –31.8 K upon releasing a moderate compressive loading of 460 MPa. This work is expected to give insight into stress-induced martensitic transformation crystallography and offer some inspirations for designing high-performance elastocaloric materials.

Dislocation-mediated migration of the α/β interfaces in titanium

Interphase boundaries are essential in the deformation and phase transformations in titanium (Ti) alloys. While static structures of semicoherent α/β interfaces in various Ti alloys have been carefully examined, their migration behavior at atomic scales is far less clear. In this study, we employed molecular dynamics simulations to investigate the migration of the semicoherent α/β interface in pure Ti. The interface migration behavior shows a shear-coupled feature with the interface dislocation glide and a macroscopic shear. The simulation reveals that both the glide direction of the dislocations with respect to the interface and the dislocation spacing strongly influence the migration rate, and the low-index glide plane of the interface dislocation plays a minor role. The dependence of interface mobility on temperatures confirms the critical role of thermal activation during the interface migration, especially for activating the interface dislocation glide. Furthermore, the shear-coupled interface migration driven by element partition is simulated using a newly developed Ti-Mo potential, consistent with the displacive-diffusional features previously observed in the surface precipitates. The simulated interface migration mode is validated by comparing it with the crystallography features of surface precipitates in a Ti-Cr alloy. The interface energy and mobility obtained from simulations further explain why the distinctive crystallographic features of the surface precipitates observed experimentally are favored over other candidate interfaces. The present study has explored an approach for systematically examining thermodynamic and kinetic factors governing the development of phase transformation crystallography at different temperatures and chemical driving forces.

Synthesis and characterization of Celsian by solid‐state reaction process from mullite

AbstractMonolithic barium aluminosilicate (Celsian), with good thermal shock resistance, high thermal stability, and strong versatility, is attractive material as a ceramic matrix for high‐temperature composites. However, Celsian cannot be precipitated directly during the synthesis process but can only be generated by crystallographic transformation of the metastable Hexacelsian at high temperatures, and this process is sluggish. In this study, mullite was chosen as the main raw material for the synthesis of Celsian with almost fully monoclinic through a simple one‐step solid‐state reaction process at a lower temperature and shorter time, which was investigated using differential thermal analysis, phase identification, microstructure observation, and FactSage thermodynamic calculations. The results showed that mullite is a favorable raw material for the synthesis of Celsian, effectively avoiding the appearance of BaAl2O4 in Hexacelsia, making it possible to synthesize Celsian with almost fully monoclinic with only 4.5 h of holding at 1300°C. This is a significant improvement compared to the higher temperatures and longer holding times required for the synthesis of Celsian from other raw materials, according to previous literature.

Pretreatment with ChOH/Urea under mild conditions for the efficient deconstruction of poplar cell wall

In this study, DES consisting of Choline hydroxide (ChOH) and Urea (Ur) was used to pretreat poplar under mild conditions (60 ℃). The 72 h saccharification efficiency of the pretreated samples for 10 min was significantly improved (69.97 %), which amounted to a 2.4-fold increase in sugar yield over the raw material (29.89 %). The remarkable enhancement could be correlated to the topochemical and morphological changes occurring in poplar cell walls, including the removal of lignin and hemicelluloses, decreased crystallinity and microstructural disruption. Noteworthy, X-ray diffraction analysis showed that cellulose I underwent a crystallographic transformation from cellulose I to cellulose II during DES pretreatment. Our study demonstrated that ChOH/Ur pretreatment was quite effective in deconstructing poplar cell walls and increasing the enzymatic hydrolysis efficiency of cellulose under mild conditions.

Phase Transformation Crystallography in Pipeline HSLA Steel after TMCP

Thermo-mechanical controlled processing (TMCP) is employed to obtain the required level of mechanical properties of contemporary high-strength low-alloy (HSLA) steel plates utilized for gas and oil pipeline production. The strength, deformation behavior and resistance to the formation and propagation of running fractures of the pipeline steel are mainly determined by its microstructure and crystallographic texture. These are formed as a result of austenite deformation and consequent γ→α-transformation. This present study analyses the crystallographic regularities of the structural and textural state formation in a steel plate that has been industrially produced by means of TMCP. The values of the mechanical properties that have been measured in different directions demonstrate the significance of the crystallographic texture in the deformation and failure of steel products. An electron backscatter diffraction (EBSD) method and crystallographic analysis were utilized to establish the connection between the main texture components of the deformed austenite and α-phase orientations. This paper demonstrates that the crystallographic texture that is formed due to a multipath γ→α-transformation results from the α-phase nucleation on the special boundaries between grains with γ-phase orientations. The analysis of the spectra of the α-γ-interface boundary angle deviations from the Kurdjumov–Sachs (K–S), Nishiyama–Wassermann (N–W), and Greninger–Troiano (G–T) orientation relationships (ORs) allows to suggest that the observed austenite particles represent a secondary austenite (not retained) that precipitates at intercrystalline α-phase boundaries and correspond to the ORs with regard to only one adjacent crystallite.

Structure, dielectric, and ferroelectric properties of (1-x)Bi0.5Na0.5TiO3 – X K0.5Na0.5NbO3 (0 ≤ x ≤ 0.1) solid solution

The correlation of the phase structure, dielectric, and ferroelectric properties of lead-free (1-x)(Na0.5Bi0.5)TiO3–xK0.5Na0.5NbO3 (NBTKNx) (0 = x ≤ 0.1) polycrystalline ceramics, fabricated via a solid state reaction technique, were investigated. The Rietveld refinement allowed identifying the crystallographic transformation from a rhombohedral to a coexisting rhombohedral-tetragonal or tetragonal long range-ordered ferroelectric (FE) phase. The dielectric investigations showed an increase of the dielectric diffuseness (1.53 = γ ≤ 1.73) and a clear shift of the depolarization temperature (Td) to a lower temperature while increasing substitution. More importantly, the lattice disorder also generated a plateau-like dielectric anomaly, leading to a thermally stable ϵr ∼2859 ± 20% (120–500 °C) and ∼3112 ± 10% (120–420 °C) for x = 0.075 and 0.1 samples, respectively. At room temperature (RT), Raman spectroscopy investigations revealed a downshift of the frequencies as a function of the composition with an inhomogeneous broadening of the Raman lines. On heating, Raman spectra showed changes in the region where the dielectric transitions are observed. Moreover, the composition dependence of the current peaks in the I-E loops confirmed the occurrence of a phase transition from a non-ergodic polar phase to an ergodic weakly polar after the applying of an electric field of 60 kV/cm−1.

Effects of UV Radiation on the Carbonation of Cement-Based Materials with Supplementary Cementitious Materials

Solar light with high-energy ultraviolet (UV) radiation acting on the surface of cement-based materials easily changes the properties of cement-based materials by affecting their carbonation reaction. In order to elucidate the difference in the carbonation process under UV radiation in cement-based materials with different supplementary cementitious materials (SCMs), the carbonation depth (apparent pH values), chemical composition (XRD, FTIR, and TG analysis), and mechanical properties (compressive strength and microhardness) of cement-based materials were evaluated. The results revealed that UV radiation acting on the surface of cement-based materials accelerated the carbonation reaction, which enhanced the decrease rate of pH and formation of stable calcite, thereby improving the macromechanical and micromechanical properties of cement-based materials. In addition, the carbonation process under UV radiation differs according to the added SCM. In particular, silica fume substantially increased the carbonation of cement-based materials under UV radiation, resulting in a 53.3% increase in calcium carbonate coverage, a 10.0% increase in compressive strength, and a 20.9% increase in mean microhardness, whereas the incorporation of blast furnace slag resulted in a smaller effect on UV irradiation-induced carbonation. In addition, UV radiation facilitates the crystallographic transformation process of cement-based materials containing metakaolin, resulting in more stable crystals of carbonation products. This study provides a theoretical framework and serves as an important reference for the design of cement-based materials under strong UV radiation for practical engineering applications.

Evidence for austenite to non-modulated martensite transformation crystallography and variant organization in Ni-Mn-Ga-Co ferromagnetic shape memory alloys

Knowledge of transformation crystallography and variant organization of product phase was a prerequisite for tuning microstructure and enhancing functionalities in materials with displacive structure-transformation. However, the transformation orientation relationship (OR) between parent austenite and non-modulated (NM) martensite in traditional NiMnGa ferromagnetic shape memory alloy was not yet well-determined via direct experimental results, thus leading to ambiguity in understanding the self-accommodated configuration. In this work, by generating a microstructure with coexisting austenite and NM martensite through minor Co substitution and proper heat treatment, the transformation OR was unambiguously determined to be the N-W relation with {111}A//{101}NM and <2¯11>A//<101¯>NM on the basis of accurate EBSD orientation measurements. Analysis on deformation gradient matrix constructed from the N-W OR revealed that, the internal nano-twined structure allowed a maximum profit for eliminating the overall latticed deformation caused by transformation, overweighing the “sandwich” micro-variant pair structure. Such strain accommodation mechanisms consequently contributed to the resultant self-accommodated hierarchically twinned structure of martensite. Moreover, it was found that the prior austenite grain boundaries (PAGBs) provided dominated nucleation sites for NM martensite variants, where the preferential variant selection at PAGBs mainly depended on the inclination angle between the PAGB and matching close-packed direction (<2¯11>A//<101¯>NM) of variants. The present study provided comprehensive information on transformation crystallography and displacive characters of austenite to NM martensite transformation, which was useful for the efforts in property optimization and theoretical simulation.

Analysis and design of bistable and thermally reversible metamaterials inspired by shape-memory alloys

In this work, we study lattice structures that exhibit a bistable behavior, i. e., they can snap from one stable state to another, and are also completely reversible, capable of reverting back to its original state through a heat treatment. We design this behavior by constructing lattice structures using networks of nonlinear springs that display tension–compression asymmetry and have different thermal expansion coefficients. The mismatch in the thermal expansion coefficients induces residual stresses in the springs which results in the lattice structure exhibiting bistability at low temperatures and monostability at high temperatures. This behavior mimics the crystallographic phase transformations of shape memory alloys, but here artificially introduced in a structural lattice. By analyzing a representative unit cell, we quantify the effect that the stiffness and the thermal expansion coefficient of the springs have on the stability of the structural lattice. In addition, for simple 2D lattices, using the concept of universal unfoldings of singularity theory, we perform a perturbation analysis to identify the key variables of the structure where controlling defects is important, as they lead to drastic changes in the bifurcation behavior of the lattice. Finally, we verify numerically our analytical predictions in both 2D and 3D simulations using continuation techniques. The examples proposed confirm that the bistable and reversible features of the unit cell carry on to the macroscale, opening the route for the design of lattice structures for energy absorption applications that can heal with a heat treatment.

Combustion synthesis, characterization, and photodegradation performance of La2-xBixCuO4

Quaternary metal oxide system of the type, La2-xBixCuO4, where x = 0, 0.01, 0.03, 0.06, 0.08 have been synthesized by the combustion method. The crystalline phases and purity of nanostructures were confirmed by X-ray diffraction, which showed that they crystallized in the orthorhombic system of Bamb space group. Furthermore, they tend to adopt the quadratic system upon the incorporation of Bi atom into the lattices of the compound. A vibrational study shows the disappearance of absorption band around 680 cm−1 due to crystallographic system transformation. Morphological characterization by scanning electron microscopy (SEM) indicated that the micrographs are composed of nanometric crystallite particles with porous microstructure. The photocatalytic property of these materials was evaluated by monitoring the degradation of Malachite green under sunlight and the disappearance of the band characteristic of malachite green around 620 nm occurred after 240 min. The efficiency of La2-xBixCuO4 is 64, 68, 75, 89, and 83% for the nanostructures for x = 0, 0.01, 0.03, 0.06, 0.08, respectively.

Effect of thermally induced microstructural changes on the mechanical properties and ballistic performance of poly (p-phenylene terephthalamide) fibers

Microstructural variations have a strong influence on the load transfer capacity of the high-performance polymeric fibers, which is also reflected in their ballistic property changes. The focus of the present study is to investigate thermally induced microstructural changes and their reflection on the mechanical properties and theoretical ballistic limit of poly ( p-phenylene terephthalamide) fibers by a correlation. From the quantitative analysis of XRD, thermally induced changes in unit cell a-dimension show profound sensitivity in affecting the tenacity and modulus of the fibers. Based on the physicochemical changes in FTIR and FESEM analysis, significant surface deterioration and changes in the chemical network are observed. However, dimensional variations of the crystal structure along a-direction show a stronger influence than the chemical and morphological changes, reflecting sigmoidal responses with tenacity, modulus and theoretical V50 by correlations. As an effect of unit cell dimensional variation, changes in crystallinity are resulted and lead to the loss in theoretical ballistic limit of the fibers by following first-order kinetics. Lastly, angular separation and (200) orientation angle are determined to build a global correlation with modulus and theoretical ballistic limit for quickly decoding macro-changes in terms of micro-properties. The given correlations can help to identify crystallographic transformations upon other induction techniques and view their effect on mechanical and ballistic parameters. In addition, the given approach can be extended for different ballistic materials under any environmental conditions.

Molybdenum disulfide of modified in situ as a nanofiller for enhanced mechanical properties and thermal properties of polypropylene composites

AbstractFew‐layer molybdenum disulfide (MoS2) was prepared by liquid phase exfoliation with in‐situ modification as a nanofiller for polypropylene (PP). Scanning and transmission electron microscopies (SEM, TEM) showed uniform dispersion and excellent adhesion of MoS2 within the PP matrix. When MoS2 was centrifuged at 1000 rpm, the mechanical properties of the composite were greatly improved by adding only 0.5 wt.% of MoS2. The yield strength, bending modulus, and impact strength increased by 2.3%, 8.9%, and 44.03%, respectively. X‐ray diffraction (XRD) and differential scanning calorimetry (DSC) showed that the enhanced mechanical properties of the composites were related to PP crystallographic transformation. Thermogravimetric analysis (TGA) showed that the incorporation of 2 wt.% MoS2 increased the maximum weight‐loss temperature of the composites by approximately 30°C. The mechanisms for increasing thermal stability have been thoroughly demonstrated by band energy analysis. MoS2 nanosheets have high aspect ratios and strong insulation with a change in band gap width compared to bulk MoS2, which is resulting in the stronger interactions at the MoS2/PP substrate interface. Furthermore, due to the outstanding carrier for rigid and thermally stable materials, MoS2 composites displayed excellent mechanical and thermal properties. In addition, unlike PP, the melt flow rate (MFR) of composite materials is more conducive to downstream product processing. This will further promote the application scope of MoS2/PP composites.