Phase Transformation Mechanism Research Articles

Atomic level mechanism of disorder-order transformation kinetics at nanoscale in FePt based systems

L10 ordered FePt is one of the most promising materials for spintronic and recording media applications. In the present work, the mechanism of L10 phase transformation in FePt based films with varying initial structures is examined at the nanoscale to understand the ordering process using synchrotron based GIXRD, MOKE, VSM, and techniques with sub nanometer depth selectivity like XRR and SIMS. Precisely controlled compositions of the films are deposited using magnetron sputtering. Rapid thermal annealing is used for post-deposition processing. It is evaluated experimentally that for a shorter annealing time of 70 s at 400 °C, besides volume diffusion, short circuit diffusion paths along the intercrystallite region owing to the presence of nanostructured grains play a dominant role in alloying behavior. A study of the L10 ordering process reveals the crucial role of film structure in controlling the transformation kinetics, texturing of nanograins, and magnetic coercivity. Diffusion studies disclose that type B diffusion kinetics is activated for the annealing time during which L10 transformation occurs in the films.

Low-temperature creep performance of additive manufactured Ti–6Al–4V

Abstract Additive manufacturing enables the fabrication of versatile and cost-effective metallic-alloy components from a digital data model. This study explores the prospects of selective laser melting (SLM), an additive manufacturing technique, for fabricating Ti6Al4V alloy components from Ti6Al4V alloy powders. Selective laser melting parameters, such as laser power, scanning speed, powder thickness, hatching space, and scanning strategy, are carefully selected through a series of experiments. The metallurgical characteristics (microstructure, grain orientation, and phase composition), microhardness, and creep performance of the as-fabricated specimens are tested and analyzed. The kinetics of phase transformation and rupture mechanism are determined using advanced instrumental characterization tools, such as field emission scanning electron microscope, energy dispersive X-ray spectroscope, X-ray diffractometer, and transmission electron microscope.

Phase Transformation Behavior and Mechanical Properties of NiTi Shape Memory Alloys Fabricated by Directed Laser Deposition

Directed laser deposition shows promise as a method for fabricating complex structure NiTi shape memory alloy parts in engineering, aerospace, automobile, and biomedical fields. The dislocation density has an impact on phase transformation behavior and mechanical properties of NiTi manufactured by directed laser deposition. In this work, NiTi samples were produced by directed laser deposition, and the mechanism of martensitic phase transformation and its interactions with dislocation density were revealed through molecular dynamics. Laser power and scanning speed are the main factors affecting dislocation density, the phase compositions of the samples are B2, B19′, and Ti2Ni phase, and the samples with dislocation density below 2 × 1015 m-2 show single-step B2-B19′ martensitic transformation, stress plateau and improved elongation, the samples with dislocation density above the value exhibit no significant transformation peak and no stress plateau. The movement of (100) twin planes and the growth of advantageous variants are observed in the simulation which can explain stress-induced martensitic reorientation and the stress plateau phenomenon. The bending test suggested that the sample under the optimal parameters has an 85% recovery ratio within 8% pre-strain.

Phase transformation mechanism of nominal Si(001) surface driven by hydrogen thermal annealing

Herein, we report the phase transformation mechanism of the nominal Si(001) surface driven by hydrogen thermal annealing. The surface energies of H-terminated Si(001) surface with different phase structures were calculated by density functional theory. The results show that the surface phase with monoatomic steps can transform into the surface phase with diatomic steps under proper ranges of hydrogen chemical potential. Combining thermodynamic and kinetic factors, the phase transformation can’t occur when annealing temperature lower or higher than 800 °C. In addition, surface phases with different types of diatomic steps are alternately transformed through the intermediate phase with monoatomic steps and the imperfection of the transformation process gradually increases with the extension of annealing time. Finally, different experiments have been carried and the experimental results are in good agreement with the phase transformation mechanism. This study provides complete theoretical mechanism and process parameters for controlling the phase structures of the nominal Si(001) surface through hydrogen thermal annealing.

The anisotropic sublimation induces rhombohedral to rock-salt phase transformation of chalcogenide SnSb2Te4 two-dimensional nanomaterial

We report the sublimation and phase transformation mechanisms of a two-dimensional (2D) chalcogenide SnSb2Te4 using a lab-developed in-situ electro-thermal testing system on a Cs-corrected transmission electron microscope. It reveals that the sublimation process is primarily governed by the surface energy mechanism, following an anisotropic route. The van der Waals (vdW) layers ending with Te atoms lead to the preferential sublimation on (003) plane, with the (1 1‾ 4) and (117) planes accommodating the sublimation. It is uncovered for the first time, that the preferential and accommodated sublimation of Te and Sb atoms along the vdW planes results in short-range diffusion inducing a phase transition from rhombohedral SnSb2Te4 to rock-salt (Sn1-xSbx)Te. These results highlight complex physics processes of 2D materials under service conditions and the route of atomic-resolved electro-thermal research platforms for investigations of complex atomistic mechanisms of 2D materials by thermal-electric external fields.

Achieving high ductility at medium temperature in a nonequiatomic CoCrNi alloy

The tensile strength and ductility of face-centered cubic (FCC) multi-principal element alloys (MPEAs) typically decline monotonically with increasing temperature. In this work, we conducted tensile tests at different temperatures and characterized deformation microstructures for a nonequiatomic CoCrNi alloy with low stacking fault energy (SFE). Exceptional ductility is obtained at medium temperature, breaking the general sense of monotonic change with temperature. The effect of deformation mechanisms of dislocations, stacking faults and phase transformation on the ductility was discussed. This work reveals the unique temperature effect on the mechanical properties of metals with very low SFE.

Interfacial phase transformation and element diffusion mechanism of high-speed laser cladding Inconel 625 in high-temperature environment

It is essential to reveal the phase transformation and element diffusion mechanism at the interface between the Inconel 625 coating prepared by high-speed laser cladding and the Q245R substrate to meet the requirement of high-temperature service environment. A banded area formed at the bottom of the laser cladding layer after being heated at 700 °C. The banded area thickened as the duration of high-temperature treatment increased. The cellular and columnar crystals in the banded area were not visible. Laves phase at the banded area dissolved, and M23C6 precipitated and accumulated significantly. The main reasons for phase transformation in the banded area could be attributed to C diffusion from the substrate to the laser cladding layer. Additionally, the lamellar structure contained α-Fe and Fe3C, which formed in the substrate beneath the interface due to the C enrichment at the interface. Simultaneously, element diffusion and phase transformation led to the increase of lattice misfit, dislocations, and vacancies at the interface. Ultimately, element diffusion and phase transformation led to an increase in the microhardness at the bottom of the laser cladding layer and a decrease in the microhardness at the heat affect zone.

Stabilization versus Penetration Dynamics Induced by Localized Concentration Polarization in Solid Polymer Electrolytes

While the onset of dendrites found inside solid polymer electrolytes was typically analyzed by the dilute solution theory, nonideal behaviors such as dendrites at underlimiting current densities were often reported. Here, we consider two critical factors that were often neglected in existing studies, the severe heterogeneous current distribution and the dynamic change of modulus during the polarization process. Polymers with different dynamic mechanical properties were assessed, exploiting the recently discovered mechanism of phase transformation inside low-salt-concentration polymers. Analyses of the operando images revealed two characteristic points on the potential curve, the local and total concentration depletion which each corresponded to the starting and stopping point of dendrites. We further assess these dynamics at different degrees of heterogeneity controlled by different electrode sizes. The penetration dynamics and Sand’s time scaling exponent were heavily affected by both the initial concentration and the electrode size, which stress the significance of interfacial dynamic heterogeneity in working batteries.

Structural transformation behavior of oxide scale during coiling of 0.9 wt% Cr-containing high-strength steel

The phase transformation mechanism of oxide scale on the surfaces of chromium (Cr)-free carbon steel and 0.9 wt% Cr-containing steel at different coiling temperatures and cooling rates was studied using thermal simulation and electron backscattering diffraction experiments. The results showed that the eutectoid transformation of both the steel followed the “C” curve relationship. The Cr-containing steel had a larger eutectoid transformation area fraction than the Cr-free carbon steel. The nose tip temperature of the eutectoid transformation of the oxide scale on the surfaces of Cr-free carbon steel and 0.9 wt% Cr-containing steel were 420∼510 °C and 420–480 °C, respectively. For the 0.9 wt% Cr-containing steel, the proportion of Σ3 and Σ13b grain boundaries in cubic Fe3O4 increased, followed by a decrease, when the coiling temperature decreased from 610 to 410 °C. The Σ3 and Σ13b grain boundaries of the Fe3O4 low-dimensional coincident site lattice had the highest proportion for 510 °C coiling temperature. The low-dimensional coincident site lattice grain boundaries could be used to enhance crack resistance and improve the oxide scale on the steel plate surface.

Probing the roles of surface characteristic of suspended nanoparticle in shear thickening suspensions

The surface characteristic of suspended nanoparticle is an extremely important property for the non-Newtonian behavior of shear thickening suspensions. Easy access to distinct surface characteristics and a deep understanding of the role of friction and adhesion forces remains challenging yet critical to study the suspension performance. In this work, by synthesizing mesoporous silica with different surface roughness and groups, the influence of surface characteristics on the non-Newtonian behavior of suspension is comprehensively explored. The results show that the suspensions containing particles with rough surface and hydroxyl groups on the surface have higher shear thickening power, lower critical shear rate, lower discontinuous shear thickening transition fraction and lower jamming fraction. Furthermore, the test of the first normal stress difference N1 shows that the interparticle friction and adhesion both reduce the critical volume fraction of the transition from fluid lubrication dominance to frictional contact dominance in the suspension. This finding deepens the understanding of phase transformation mechanism, and open an approach to accelerate the advanced shear thickening suspensions design for next-generation intelligent materials.

Synthesis and high-temperature phase transformation behavior of Dy2O3–Sc2O3 co-stabilized ZrO2 powders prepared by cocurrent chemical coprecipitation

In this study, Dy2O3–Sc2O3 co-stabilized ZrO2 (DyScSZ) powders with various stabilizer ratios were synthesized by cocurrent chemical coprecipitation method. The difference analysis of phase composition and microstructure was conducted on the synthesized powders in detail. The phase evolution and phase transformation behaviors of the synthesized powders were also investigated. Results show that all of the powders were synthesized with nanoscale and exhibit subglobose morphology. The excessive elevation of Sc2O3 fraction in total stabilizers was adverse to the formation of the synthesized powders with pure tetragonal phase, which also exhibit a significant effect both on the phase transformation behaviors and the final volume fractions of transform products. The 2.5Dy1.0ScSZ powders exhibit favorable phase stability resulting from the nano-size effect and the synergies of Dy2O3 and Sc2O3 stabilizers during the phase transformation process. Moreover, a phase transformation mechanism of DyScSZ was proposed based on the new phase nucleation and growth theory.

Barium Titanate Synthesis in Water Vapor: From Mechanism to Ceramics Properties

A facile and environmentally benign method for single-phase barium titanate synthesis in a water vapor medium was studied to reveal the mechanism of phase transformation of the initial simple oxide mixture and estimate the capability of the product to be used as a raw material for low-frequency dielectric ceramics. The composition and structure of the reactants’ mixture, treated in vapor at 130–150 °C as well as at 230 °C for various time periods, were investigated by means of XRD, SEM, TEM, EDX, and FTIR methods. The kinetics of the occurring phase transformation can be described using the Johnson–Mehl–Avrami–Erofeev equation. The reaction between the initial oxides was considered as a topochemical process with an apparent activation energy of 75–80 kJ mol−1. A crucial role in this process belonged to the water vapor medium, which facilitated the generation of the reaction zone and the spreading inward of the solid particles. The synthesized tetragonal barium titanate powder (mean particle size of 135 nm) was sintered using a conventional technique at 1250 °C to obtain ceramics with grains of about 2 μm. Capacitance measurements identified a permittivity and dielectric loss factor of the ceramics that reached 3879 and 6.7 × 10−3, respectively, at 1 kHz and room temperature.

A Comprehensive Understanding of Bainite Phase Transformation Mechanism in TRIP Bainitic-supported Ferrite Steel

A Comprehensive Understanding of Bainite Phase Transformation Mechanism in TRIP Bainitic-supported Ferrite Steel

Mesophase induced by alternating-current poling in relaxor ferroelectric single crystals

Alternating-current poling (AC-Poling) has attracted wide interests in enhancing piezoelectric performance of ferroelectric single crystals. However, the mechanism of phase transformations and domain morphology underlying AC-Poling has not been thoroughly investigated. In this work, we have systematically studied the AC-poling effect on domain structures and piezoelectric properties in the relaxor ferroelectric single crystals of 0.24Pb(In1/2Nb1/2)O3–0.44Pb(Mg1/3Nb2/3)O3–0.32PbTiO3. It is shown that the piezoelectric constant (d33) and permittivity (ε33) are greatly enhanced via [001]-AC-poling compared to DC-poling, related to the presence of modified domain configurations as indicated previously. For [110]-oriented specimens, the piezoelectricity through AC-poling is only half as that for the DC-poled single crystals. We attribute this to the transition from the monoclinic-B phase to an orthorhombic phase under AC-poling, confirmed by synchrotron radiation X-ray diffraction and piezo-response force microscopy. The transition to the orthorhombic phase is likely a consequence of lower Gibbs energy. This study on orientation-dependent characteristics of AC-poling for PIN-PMN-PT single crystals will guide the development of AC-poling technology in applications.

Exploration of Electrode‐Electrolyte‐in‐One System Based on Fluorine/Chloride Ion Battery

AbstractWith an increasingly profound understanding of battery materials, strategies designed for interface protection have become more sophisticated. However, it is inherent that different materials used in electrodes and electrolytes tend to react with each other. To address this issue, this study proposes an “Electrode‐Electrolyte‐In‐One” (EEIO) concept, where a single energy storage material can serve as both electrode and solid‐state electrolyte (SSE) during its different ion storage stages. The EEIO materials of chloride ion batteries and fluorine ion batteries are preliminarily studied, establishing reasonable screening procedures and predicting their operational mechanisms. The analysis of Cl− and F− affinity indicates that the EEIO system in the anion storage batteries comprises the anode and SSE. Through comprehensive evaluation of ion binding capacity, phase transformation mechanism, electrical conductivity, and ion diffusion kinetic properties, several potential EEIO materials are identified. In the EEIO system, the material interface and the conductive‐insulation interface are separated, which solves the material compatibility problem between the anode and SSE. Furthermore, the EEIO system has an inherent correlation between the anode electrochemical reaction and the SSE decomposition reaction, ensuring that the electrochemical voltage of EEIO batteries never exceeds the decomposition voltage of SSE. Therefore, SSE in the EEIO battery has absolute electrochemical stability.

Lattice Strain Enhanced Phase Transformation of NaYbF4: 2% Er3+ Upconverting Nanoparticles by Tuning the Molar Ratio of Na+/Yb3+

AbstractNaYbF4 upconverting nanoparticles (UCNPs) have enhanced optical properties compared to the NaYF4 UCNPs. However, synthesis of monodisperse NaYbF4 with controllable size and optical properties poses challenges, and the mechanism of phase transformation remains to be understood. Here, they report on the effect of Na+/Yb3+ molar ratio on the morphological and optical properties of upconverting NaYbF4: 2% Er3+ nanoparticles. Controllable transformation of cubic phase nanoparticles produced with [Na+]/[Yb3+] = 1 to hexagonal phase is achieved by increasing Na+ content. The hexagonal UCNPs produced with [Na+]/[Yb3+] = 4 have significantly enhanced intensity of optical emission of ≈600 times compared with the pure cubic phase crystal. The work reveals that the increasing dislocation of sodium and ytterbium distribution cause the accumulation of the lattice strain with increasing Na+ content, and triggers the lattice strain‐mediated phase transformation in cubic cell, as confirmed by the Density Function Theory simulations. These results provide new insights into the growth of UCNPs and pave the way for developing controlled synthesis of UCNPs for applications as bio‐probes and for energy harvesting.

Analysis of the interfacial microstructure, phase transformation, and diffusion mechanism of Ti2AlNb/TA2 joint diffusion-bonded at low temperature

Ti2AlNb-based alloys are mostly diffusion-bonded with conventional titanium alloy at a high temperature of above 920 °C. However, the high temperature could reduce the mechanical properties of the matrix alloy, especially the significant decrease in ductility. Therefore, trying to lower the temperature of the hot process of Ti2AlNb-based alloy is a work worth exploring. The microstructure evolution of the Ti2AlNb-based alloy and the conventional titanium during the bonding process at low temperatures has not been analyzed in detail. In this work, Ti2AlNb-based alloy is diffusion-bonded with TA2 titanium at a low temperature of 800 °C under the pressure of 10 MPa for 1 h. The detailed microstructure and phase transformation of the dissimilar joint are analyzed to reveal the interface reaction during the diffusion process. The ternary interdiffusion coefficients of the TA2/Ti2AlNb alloys diffusion couple are calculated to detect the diffusion behavior and mechanism of different elements at the low temperature of 800 °C. The result shows that the interfacial reaction phases are Al-rich α phase / Nb-rich β + ω phase / tiny α phase / striped α2 phase / β/B2 + O + α2 phase respectively from the TA2 titanium (α phase) matrix to the Ti2AlNb-based alloy (β/B2 + O phase). The interdiffusion coefficients obtained suggest that the diffusion rate of Al is higher than that of Ti, while Ti diffuses faster than Nb. Additionally, it was observed that Ti facilitates the diffusion of Al and hinders the diffusion of Nb. On the other hand, both Al and Nb impede the diffusion of Ti in the TA2/Ti2AlNb alloy diffusion couple during the 800 °C diffusion process.

Synergistically Coupling Atomic-Level Defect-Manipulation and Nanoscopic-Level Interfacial Engineering Enables Fast and Durable Sodium Storage.

Through inducing interlayer anionic ligands and functionally modifying conductive carbon-skeleton on the transition metal chalcogenides (TMCs) parent to achieve atomic-level defect-manipulation and nanoscopic-level architecture design is of great significance, which can broaden interlayer distance, optimize electronic structure, and mitigate structural deformation to endow high-efficiency battery performance of TMCs. Herein, an intriguing 3D biconcave hollow-tyre-like anode constituted by carbon-packaged defective-rich SnSSe nanosheet grafting onto Aspergillus niger spores-derived hollow-carbon (ANDC@SnSSe@C) is reported. Systematically experimental investigations and theoretical analyses forcefully demonstrate the existence of anion Se ligand and outer-carbon all-around encapsulation on the ANDC@SnSSe@C can effectively yield abundant structural defects and Na+ -reactivity sites, accelerate rapid ion migration, widen interlayer spacing, as well as relieve volume expansion, thus further resolving the critical issues throughout the charge-discharge processes. As anticipated, as-fabricated ANDC@SnSSe@C anode contributes extraordinary reversible capacity, wonderful cyclic lifespan with 83.4% capacity retention over 2000 cycles at 20.0 A g-1 , and exceptional rate capability. A series of correlated kinetic investigations and ex situ characterizations deeply reveal the underlying springheads for the ion-transport kinetics, as well as synthetically elucidate phase-transformation mechanism of the ANDC@SnSSe@C. Furthermore, the ANDC@SnSSe@C-based sodium ion full cell and hybrid capacitor offer high-capacity contribution and remarkable energy-density output, indicative of its great practicability.

Phase transformation mechanisms of NiTi shape memory alloy during electromagnetic pulse welding of Al/NiTi dissimilar joints

The phase transformation mechanisms of NiTi shape memory alloy (SMA) during electromagnetic pulse welding (EMPW) of Al/NiTi dissimilar alloy joints were studied. Two types of amorphization behaviors were observed in the NiTi SMA of the Al/NiTi dissimilar joint. These are: i) fast cooling-induced amorphization of the joint interfacial reaction layer; and ii) severe deformation-induced amorphization. Additionally, the martensitic transformation was observed in the NiTi SMA during the high strain rate plastic deformation imposed by the EMPW process. The study shows that the formation of B19' martensite during high strain rate plastic deformation of NiTi SMA is influenced by both the strain rate and the peak pressure of the deformation during welding.

Comparison of hydrostatic and non-hydrostatic compression of glassy carbon to 80 GPa

Understanding new mechanisms for phase transformation in carbon is of considerable interest. This study investigates on the compression conditions required to create recoverable diamond during room-temperature high-pressure compression of glassy carbon. Under non-hydrostatic compression conditions when shear is present, glassy carbon transforms into an oriented graphitic structure at ∼45 GPa, and then forms mixed diamond and lonsdaleite nanocrystals when the pressure is higher than ∼80 GPa. In contrast, during hydrostatic compression no significant changes in the microstructure was observed, highlighting glassy carbon’s resilience under compression. Molecular dynamics modelling supports the proposed model that shear drives the phase transition mechanism and causes a temperature spike that drives crystallisation. Our work demonstrates that shear is key to high-pressure diamond formation in the absence of heating.