Multiple Phase Transformations Research Articles

Phase Transformation, Microstructural Evolution and Tensile Properties of a TiH2-Based Powder Metallurgy Pure Titanium

Multiple phase transformations were carried out during dehydrogenation process of TiH2-based powder metallurgy. The influence of phase transformation on the microstructure is highly worthy of attention. In situ synchrotron radiation was employed to investigate the phase transformation sequence of TiH2 powder compact during the vacuum sintering process. It was found that a transformation route TiH1.971 → TiH2 + TiH + TiH0.71 + α(H) → TiH + TiH0.71 + α(H) → TiH + TiH0.71 + α(H) + β(H) → α(H) → α took place, resulting in an equiaxed microstructure. Increasing heating rate and avoiding the intense dehydrogenation to retain hydrogen-rich β (β(H)) and TiHx aciculae at the interfaces is found to be a feasible method to fabricate hierarchical α-Ti structures. A fully dense fine martensitic microstructure was produced after fast heating the TiH2 powder compact to 1100 °C and an immediate hot extrusion. Subsequently, by vacuum annealing treatment at 700 °C, composite α/βt lamellar structures were generated and a simultaneously enhanced tensile strength of 746 MPa and excellent elongation to fracture of 33.8% were achieved. It is suggested that adjusting the dehydrogenation reactions of TiH2-based powder metallurgy is conductive to generating hierarchical lamellar structures with a highly promising combination of strength and ductility for pure Ti.

Multiple Structural and Phase Transformations of MOF and Selective Hydrocarbon Gas Separation in its Amorphous, Glass Phase States.

The first wide-view image of multiple structural and phase transformations for MOFs from crystal state transformations further to the extreme limit approaching liquid/glass phase, was presented based on a square-layer framework of [Co2(pybz)2(CH3COO)2]·DMF (Co2). The process involves i) an initial crystalline transformation brings to a 3-fold interpenetrated and ordered vacancies contained framework [Co(pybz)2(CH3OH)2]·2CH3OH (CoM) due to in-situ disassemble-reassemble, ii) thermal induced departure of a pair of cis-form coordinated methanol in CoM leads to amorphous framework (a-dCoM), iii) glass transition (Tg = 566 K) to super-cooled liquid (scl-dCoM, spanning 38 K), iv) obtaining MOF glass g-dCoM upon quenching the super-cooled liquid, and v) re-crystallization of super-cooled liquid to six-fold interpenetrated dia-net framework [Co(pybz)2]6n (rec-dCoM) under heating above 604 K. The access to glass from CoM, provides a new self-perturbation strategy to create more MOF glasses without melting. The wider pore size distribution in amorphous/glassy MOFs than crystalline precursor realized the first time selective hydrocarbon gas separation by breakthrough experiments, which bring efficient separation of 1:99 C2H2/C2H4 by either a-dCoM or g-dCoM and produce polymer grade C2H4 with purity ≥ 99.5% after a single adsorption process. Furthermore, the mixture of 50:50 C3H6/C3H8 can be separated by a-dCoM.

Multiple phase transformation of ZIF-L to LDH and Ag nanoparticles decorated on CoNi layered double hydroxides for high-performance supercapacitor and highly efficient hydrogen evolution reaction

ZIF-L-derived binary layered double hydroxide (LDH) established active catalysts have been shown to have potential as electrode materials in the development of high-performance supercapacitors (SCs) and hydrogen evaluation reactions (HER). This study uses conductive nano-silver (Ag) particles in hierarchically integrated CoNi-LDH nanosheets (NSs), which are described. The developed Ag@CoNi-LDH NSs electrode has a very high specific capacitance of 2878 F g−1 at 0.5 A g−1 and an impressive rate capability. This is due to consistent morphology, plentiful electroactive sites, homogeneous particle size, and decreased electronic/ionic impedance. The fabrication of solid-state hybrid supercapacitors (HSCs) using Ag@CoNi-LDH NSs as the positive electrode and activated carbon (AC) as the negative electrode in PVA/KOH polymer gel electrolytes has been described. The prepared supercapacitors achieve a maximum energy density of 105 Wh kg−1 at power densities of 1183 Wh kg−1, coupled with excellent initial capacitance retentions of 97 % after 20,000 cycles. Ag@CoNi-LDH NSs demonstrate effective electrocatalytic activity towards HER with an ultra-low overpotential of 26 mV at 10 mA cm−2 and the associated Tafel value of 35 mV dec−1 using 0.5 M acidic electrolyte. It is possible to attribute the extraordinary electrochemical performance to the thoughtful mixture of two electroactive materials in the future.

Multiple structural phase transitions in single crystal silicon subjected to dynamic loading

Single crystal Silicon (SC Si), as one of the most extensively used elements, has a series of complex phase changes, however, it is not fully understood under dynamic loading conditions. In this letter, both the shock induced multiple solid-solid phase transformations and solid-liquid transition are systematically investigated using large-scale molecular dynamics (MD) simulations. With the increase of particle velocities up to 2.8 km/s, the SC Si is revealed to undergo a continuous structural phase transitions from cubic diamond (cd) to β-tin, bct5, near-simple cubic (sc), and body centered cubic (bcc) structure. Furthermore, the liquid Si is distinguished and shows solid-liquid coexistence under intensive shocks. Our work dissects the continues multiple phase transitions in shocked Si for the first time in MD simulations, which provides new understandings of phase transitions of Si under dynamic loadings, and strongly supports and complements the relevant reported experimental researches.

Study of the effect of multiple phase transformations and relaxation annealing on the microstructure of a martensitic TiNi alloy in different structural states

The work was devoted to studying the effect of multiple phase transformations in the temperature range of phase transformations of a TiNi alloy with a martensitic structure in various initial states - coarse-grained, ultrafine-grained, nanostructured. The conducted studies have shown that in the process of thermal cycling, the accumulation of defects in the crystalline structure occurs - dislocations, which contribute to a decrease in the size of structural elements. Subsequent relaxation annealing, slightly reducing the dislocation density, made it possible to obtain a stable and more equilibrium microstructure, including the nanostructural state.

Structural Anisotropy-Driven Atomic Mechanisms of Phase Transformations in the Pt-Sn System.

Using in situ atomic-resolution scanning transmission electron microscopy, atomic movements and rearrangements associated with diffusive solid to solid phase transformations in the Pt-Sn system are captured to reveal details of the underlying atomistic mechanisms that drive these transformations. In the PtSn4 to PtSn2 phase transformation, a periodic superlattice substructure and a unique intermediate structure precede the nucleation and growth of the PtSn2 phase. At the atomic level, all stages of the transformation are templated by the anisotropic crystal structure of the parent PtSn4 phase. In the case of the PtSn2 to Pt2Sn3 transformation, the anisotropy in the structure of product Pt2Sn3 dictates the path of transformation. Analysis of atomic configurations at the transformation front elucidates the diffusion pathways and lattice distortions required for these phase transformations. Comparison of multiple Pt-Sn phase transformations reveals the structural parameters governing solid to solid phase transformations in this technologically interesting intermetallic system.

Preparation of low cobalt-doped LiNiO2 materials by the spray pyrolysis method

LiNiO2 (LNO) is recognized as the most promising cathode material for next-generation lithium-ion batteries owing to its highest energy density and economic benefit in the layered oxide cathode materials systems. However, severe cation disordering, multiple phase transformations and poor lithium-ion diffusion kinetics, which result in rapid degradation of cycling stabilities and rate properties, hinder its further practical applications. Cobalt doping is an effective strategy to address such issues, but the influences of cobalt on LNO materials have not been figured out completely yet. Hence, a systematical investigation of cobalt doping for LiNiO2 cathode materials prepared by spray pyrolysis is conducted. The addition of cobalt enhances the stabilities of layered structures and suppresses the cation disordering effectively. The multiple-phase transformations during cycling are also alleviated significantly. Furthermore, cobalt doping can decrease charge transfer resistances and improve lithium-ion diffusion kinetics, especially at high de-lithiation states. As a result, the electrochemical performances are improved comprehensively after cobalt doping. The LNO with optimal doping amount exhibits an improved capacity retention of 78.0% after 100 cycles as compared with 64.9% for pristine LNO, and an outstanding rate performance of 167.2 ​mA ​h ​g−1 with the current density of 5 ​C (1 ​C ​= ​200 ​mA ​g−1).

Re alloying-driven stacking fault energy decrease enabled strength-ductility synergy in NiCoCrFe high-entropy alloys

Metallic materials have long been enslaved in the dilemma of strength-ductility trade-off, which limits their potential applications. Here, we report on a new class of non-equiatomic Re-added Ni-Co-Cr-Fe face-centered cubic (FCC) high-entropy alloys (HEAs) with superior yield strength to those of Mo-added and other FCC HEAs without compromising ductility. The strengthening mechanisms of these HEAs were discussed in detail by experimental observation of the recrystallized microstructure, dislocation configuration and deformation substructure combined with theoretical evaluation of yield stress and stacking fault energy (SFE). It has been clarified that the unique work-hardening capacity and deformability of these HEAs is attributed to solid solution strengthening and the grain boundary strengthening caused by refined crystalline on the one hand. And what's more important, the appropriate addition of Re greatly reduces SFE so as to modulate the dislocation slip and dissociation behavior and promote phase transformation. It has been confirmed that the plastic deformation process was controlled by abundant microbands, multiple nanoscale deformation twins and phase transformation in the Re-added HEAs.

In Situ Atomic-Scale Deciphering of Multiple Dynamic Phase Transformations and Reversible Sodium Storage in Ternary Metal Sulfide Anode.

Ternary metal sulfides (TMSs), endowed with the synergistic effect of their respective binary counterparts, hold great promise as anode candidates for boosting sodium storage performance. Their fundamental sodium storage mechanisms associated with dynamic structural evolution and reaction kinetics, however, have not been fully comprehended. To enhance the electrochemical performance of TMS anodes in sodium-ion batteries (SIBs), it is of critical importance to gain a better mechanistic understanding of their dynamic electrochemical processes during live (de)sodiation cycling. Herein, taking BiSbS3 anode as a representative paradigm, its real-time sodium storage mechanisms down to the atomic scale during the (de)sodiation cycling are systematically elucidated through in situ transmission electron microscopy. Previously unexplored multiple phase transformations involving intercalation, two-step conversion, and two-step alloying reactions are explicitly revealed during sodiation, in which newly formed Na2BiSbS4 and Na2BiSb are respectively identified as intermediate phases of the conversion and alloying reactions. Impressively, the final sodiation products of Na6BiSb and Na2S can recover to the original BiSbS3 phase upon desodiation, and afterward, a reversible phase transformation can be established between BiSbS3 and Na6BiSb, where the BiSb as an individual phase (rather than respective Bi and Sb phases) participates in reactions. These findings are further verified by operando X-ray diffraction, density functional theory calculations, and electrochemical tests. Our work provides valuable insights into the mechanistic understanding of sodium storage mechanisms in TMS anodes and important implications for their performance optimization toward high-performance SIBs.

Dislocation Density of Electron Beam Powder Bed Fusion Ti–6Al–4V Alloys Determined via Time-Of-Flight Neutron Diffraction Line-Profile Analysis

Ti–6Al–4V alloys undergo a multiple phase transformation sequence during electron beam powder bed fusion (EB-PBF) additive manufacturing, forming unique dislocation substructures. Thus, determining the dislocation density is crucial for comprehensively understanding the strengthening mechanisms and deformation behavior. This study performed time-of-flight neutron diffraction (TOF-ND) measurements of Ti–6Al–4V alloys prepared via EB-PBF and examined the dislocation density in the as-built and post-processed states using convolutional multiple whole profile (CMWP) fitting. The present TOF-ND/CMWP approach successfully determined the bulk-averaged dislocation density (6.8 × 1013 m−2) in the as-built state for the α-matrix, suggesting a non-negligible contribution of dislocation hardening. The obtained dislocation density values were comparable to those obtained by conventional and synchrotron X-ray diffraction (XRD) measurements, confirming the reliability of the analysis, and indicating that the dislocations in the α-matrix were homogeneously distributed throughout the as-built specimen. However, the negative and positive neutron scattering lengths of Ti and Al, respectively, lowered the diffraction intensity for the Ti–6Al–4V alloys, thereby decreasing the lower limit of the measurable dislocation density and making the analysis difficult.

Adsorption-catalysis design with cerium oxide nanorods supported nickel-cobalt-oxide with multifunctional reaction interfaces for anchoring polysulfides and accelerating redox reactions in lithium sulfur battery

The charge and discharge working mechanisms in lithium sulfur batteries contain multi-step complex reactions involving two-electron transfer and multiple phase transformations. The dissolution and diffusion of lithium polysulfides cause a huge loss of active material and fast capacity decay, preventing the practical use of lithium sulfur batteries. Herein, CeO2 nanorods supported bimetallic nickel cobalt oxide (NiCo2Ox) was investigated as a cathode host material for lithium sulfur batteries, which can provide adsorption-catalysis dual synergy to restrain the shuttle of polysulfides and stimulate rapid redox reaction for the conversion of polysulfides. The polar CeO2 nanorods with abundant surface defects exhibit chemisorption towards lithium polysulfides and the excellent electrocatalytic activity of NiCo2Ox nanoclusters can rev up the chain transformation of lithium polysulfides. The electrochemical results show that the battery with NiCo2Ox/CeO2 nanorods can demonstrate high discharge capacity, stable cycling, low voltage polarization and high sulfur utilization. The battery with NiCo2Ox/CeO2 nanorods unveils a high specific capacity of 1236 mAh g−1 with a very low capacity fading of 0.09% per cycle after 100 cycles at a 0.2C current rate. Moreover, the excellent performance with high sulfur loading (>5 mg cm−2) verifies a huge promise for future commercial applications.

Coupling Quantification of Pulverization with Galvanostatic Cycling of Bulk Film Alloy-Type Anodes

Alloy-type anodes, such as metallic antimony, demonstrate promise as alternative electrode materials for lithium-ion battery systems due to their high theoretical capacity of 660 mAh/g. However, antimony undergoes anisotropic volume expansion and multiple crystallographic phase transformations upon lithiation and delithiation, which often leads to fracture. This fracture can result in loss of electrical contact and poor cycling stability. These pulverization phenomena are often observed, primarily during delithiation, but are not quantified in terms of physical bulk material lost. To quantify the mass, size, volume, and density of pulverized regions in a bulk antimony film in a lithium system, we have developed a cell that enables the use of optical microscopy to capture videos of lithiation and delithiation. Using ImageJ and MATLAB® scripts developed for analysis, we have begun to quantify the amount of material lost as a function of deposition parameters.

‘Aqueous Processed’ O3-Type Transition Metal Oxide Cathodes Enabling Long-Term Cyclic Stability for Na-Ion Batteries

Among the potential cathode material classes for Na-ion batteries, O3-type layered NaxTMO2s (TM => transition metal ion) are of importance due to their high starting Na-content (of ~1 per formula unit; x). However, the O3-type NaxTMO2s suffer from multiple structural phase transformations during electrochemical charge/discharge cycles, TM-dissolution into electrolyte [1-2] and, more importantly, inherent sensitivity to moisture [3]. The moisture sensitivity of these ‘layered’ NaxTMO2s necessitates the usage of toxic/hazardous non-aqueous solvents like N-Methyl-2-pyrrolidone (NMP) during electrode preparation. Against this backdrop, a carefully designed composition has been developed in this work, which addresses the aforementioned problems, in particular, the air/water-instability. Partial/complete substitution of Ti-ion for Mn-ion in Na(Li0.05Mn0.5-xTixNi0.30Cu0.10Mg0.05)O2 eliminated the presence of Mn3+ (which dissolves in electrolyte) at the particle surface, supressed increment in impedance and voltage hysteresis during electrochemical cycling and, thus, significantly improved cyclic stability of Ti-substituted O3-type layered NaxTMO2s. The Mn-containing Na-TM-oxides were found to be extremely unstable in terms of phase/structure retention upon exposure to air and water; progressively evolving O’3 and P3 phases due to spontaneous Na-loss and thereby forming undesired NaOH and Na2CO3 phases on the particle surface (see Fig. 1a), causing increase in electrochemical impedance. By contrast, no phase/structural change occurred upon partial/complete Ti-substitution (for Mn-ion), even after 40 days of air-exposure and 12 h of soaking, as well as stirring, in water (viz., very stringent hydration condition) (see Fig. 1b). Such excellent stability against hydration, which was partly due to reduced Na-ion ‘inter-slab spacing’ in the presence of Ti-ion, was not reported earlier for O3-type Na-TM-oxides. The excellent stability of the optimized O3-type NaTMO2 enables the usage of environment/health-friendly and economical ‘aqueous-binder’ (viz., Na-alginate) and water (as solvent) for electrode preparation. Overall, the ‘aqueous-processed’ cathode exhibits first cycle capacity of ~125 mAh/g (between 2-4 V; vs. Na/Na+), with smooth electrochemical cycling profiles (see Fig. 2a) and excellent long-term cyclic stability, with a capacity retention of ~56% after 750 cycles at C/5 (see Fig. 2b). Overall, the present work, as published in ref. [4], has established important correlations between the composition, structure (viz., reduction in ‘inter-slab spacing’), stability against hydration (viz., in air and water), feasibility for health/environmental-friendly ‘aqueous processing’ of electrodes, electrochemical impedance, stability of average voltages and cyclic stability of O3-type Na-TM-oxide based cathode materials for Na-ion batteries. Keywords: Na-ion battery; layered transition metal oxide cathode; air/water-stability; aqueous processing; electrochemical behaviour Reference s : [1] S. Komaba, N. Yabuuchi, T. Nakayama, A.Ogata and T. Ishikawa, Inorg.chem, 51, 6211–6220 (2012).[2] P. F. Wang, Y. You, Y. X. Yin and Y. G. Guo, J. Mater. Chem. A, 4, 17660–17664 (2016).[3] H. R. Yao, P. F. Wang, Y. Gong, J. Zhang, X. Yu, L. Gu, C. Ouyang, Y. X. Yin, E. Hu, X. Q. Yang, E. Stavitski, Y. G. Guo and L. J. Wan, J. Am. Chem. Soc., 139, 8440–8443 (2017).[4] B. S. Kumar, A. Pradeep, A. Dutta, A. Mukhopadhyay., J. Mater. Chem. A, 8, 18064-18078 (2020). Figure 1

Preparation of calcium carbonate nanoparticles from waste carbide slag based on CO2 mineralization

This study proposes a novel method for simultaneously dealing with the waste carbide slag (WCS) and CO2 mitigation. In addition, the ultrafine vaterite CaCO3 with high economic value was produced under mild conditions of 25 °C and 0.1 MPa. Ammonium sulfate ((NH4)2SO4) was employed to efficiently extract calcium from WCS into the solid phase calcium sulfate dehydrate (CaSO4·2H2O). Subsequently, it was subjected to “dropwise carbonation” leading to the formation of a calcium carbonate with controllable particle size between 100 nm–1 μm through multiple crystalline phase transformations. The results showed that nano-sized CaCO3 with nearly 90% purity and whiteness was prepared at 25 °C, CO2-500 mL/min, and NH4+/Ca2+ = 2.4 conditions. Results of TG and FTIR analyses confirmed that CaCO3 polymorphs with vaterite and metastable were formed. Based on the calcium conversion ratio of the entire process, treatment with 1 ton of WSC captured ∼0.5 tons of CO2 and yields about 1.15 tons of nano-sized CaCO3 per each cycle operation. This study provides a valuable method for identifying potential application for scale preparation of vaterite nanoparticles from low cost but abundant calcium resource such as WCS and gypsum.

Crystal structures and thermal expansion of Yb2Si2O7–Gd2Si2O7 solid solutions

Solid solutions within the Yb2Si2O7–Gd2Si2O7 system were synthesized by a solid state reaction route and characterized for their structural and thermal properties. Incorporation of Gd2Si2O7 into Yb2Si2O7 resulted in multiple phase transformations, with the solid solutions exhibiting the monoclinic β-RE2Si2O7, monoclinic γ-RE2Si2O7, and orthorhombic δ-RE2Si2O7 crystal structures. The stability of crystal structures with additions of Gd2Si2O7 are attributed to space filling and minimization of molar volume. The thermal expansion coefficients (CTE) of the β and γ structures were largely unchanged by the addition of Gd2Si2O7 to the lattice, although the orthorhombic Yb2Si2O7–Gd2Si2O7 solid solution exhibited slightly less anisotropy in CTE of the lattice directions (25°C–1500°C) than undoped orthorhombic δ-Gd2Si2O7.

Accurate numerical prediction of thermo-mechanical behaviour and phase fractions in SLM components of advanced high strength steels for automotive applications

Conventional crash absorber in automotive applications, so called crash boxes are fabricated via deep drawn sheet metal resulting in significant lead times and costs. Laser Powder Bed Fusion processes, like Selective Laser Melting (SLM) offer an attractive alternative for the fabrication of crash parts while eliminating any need for costly forming dies and reducing the lead times, provided required material properties are achieved. Reliable numerical simulation model to predict the SLM build process with greater spatial resolution and accuracy is indispensable to understand the process further in order to ensure its applicability to crash structures. In this paper, an improved simulation methodology for SLM process is presented to predict the material behaviour via temperature, deformation, hardening, flow stress and phase fractions throughout the component with increased accuracy and greater resolution. To achieve desired spatial resolution, the equivalent layers are subdivided into individual tracks, which are then deposited sequentially to simulate the printing process. The material is a medium manganese (7­-8 %) transformation induced plasticity (TRIP) steel with austenite and martensite primary phases. The multiple solid-state phase transformation cycles undergone by the material are modelled in the simulation and the final phases are predicted. The results indicate improved accuracy and higher resolution in predictions for temperature, phase fractions and deformation.

Термическое разложение химически осажденного нанопорошка гидроксида алюминия

In this study, the phase formation of aluminum oxide nanopowder during thermal heating of chemically precipitated aluminum hydroxide was discussed. Nanopowders of aluminum oxide and hydroxide were characterized by X-ray diffraction and thermogravimetric analysis, electron microscopy, energy dispersive X-ray spectroscopy analysis. XRD analysis detected a complex phase composition consisting of different modifications of aluminum hydroxide. It was shown that the precipitated aluminum hydroxide has a flake morphology with an average particle diameter of 5.5 nm. The differential thermal analysis showed that as-deposited aluminum hydroxide undergoes a multiple stage phase transformation. The deconvolution of the experimental differential curve by the Gaussian function and comparison of the temperature intervals of mass change with the literature data revealed that the precipitated aluminum hydroxide powder consists of bayerite α-Al(OH)3 and aluminum oxyhydroxide AlOH. It was found that bayerite is transformed to boehmite in the temperature range of 100 – 400 °C. After that γ-Аl2O3 is formed during the thermal decomposition of boehmite in the temperature range of 400 – 530 °C. The second hydroxide phase formed during the chemically precipitated method has a single stage phase transformation from AlO(OH) to γ-Аl2O3 in the temperature range of 260 – 500 °C. The nanopowder of γ-Аl2O3 formed at the temperature of higher than 600 °C has an equiaxed particle shape and an average size of 7 nm.

Microstructure and mechanical properties of low-carbon steel fabricated by electron-beam additive manufacturing

The microstructure and mechanical properties of a billet obtained by electron-beam additive manufacturing (EBAM) using an industrial welding wire of low-carbon steel were studied. After EBAM, the steel billet possesses the phase composition constant in volume (ferrite with carbides). Deformation behavior of samples of steel obtained by the additive methods depends on their position in the billet. In its lower part, which is characterized by a high cooling rate in the EBAM-process, predominantly equiaxed ferrite grains with an average size of 15 μm are formed. The mechanical properties and deformation behavior (the presence of a yield plateau, the stages of plastic flow) of this part of the billet are close to those of normalized low-carbon steel obtained by conventional methods of metallurgy and thermomechanical processing. In the middle and top parts of the billet, the coarse non-equiaxed ferritic grains (hundreds of micrometers) form. The mechanical properties in this part weakly depend on the position of the samples and their orientation relative to the building direction of the billet — the yield plateau disappears; the yield stress, the ultimate tensile strength, the elongation become lower than those of the normalized low-carbon steel obtained by the conventional method. Despite the predominance of ferritic grains (with carbides), a small portion of grains with a lamellar ferrite morphology resembling martensite or bainite is observed in all parts of the billet. The latter is the result of a complex thermal history of the billet, which can undergo multiple phase transformations during successive heating and cooling cycles.

Tuning Transition Metal Oxides Towards Achieving Water-Stability and Electrochemical Stability As Electrode Materials for Sodium-Ion Batteries

Transition metal (TM) oxides are a fascinating class of materials, whose properties can be suitably tuned in a variety of ways; such as by selecting TM-ions/dopants having preferred electronic configurations, engineering the crystallographic site occupancy by dopants, controlling/modifying the degree of covalence of TM-O bonds, modifying lattice spacing(s), tuning phase assemblage etc. Such modifications done from the fundamental perspectives influence the performances of TM-oxides for a variety of applications, including their widespread usage as electrode-active materials in alkali metal-ion batteries.In the context of the upcoming Na-ion battery system, O3-type ‘layered’ Na-TM-oxides are promising as cathode-active materials due to their inherently high initial Na-content (as compared to the P2 counterparts). However, they suffer from instabilities caused due to multiple phase transformations during Na-removal/insertion and sensitivity to air/moisture. Against this backdrop, with the help of a dopant, having d 0 electronic configuration (viz., no octahedral site preference energy), we have been able to tune the composition and structural features to suppress the phase transitions upon Na-removal/insertion and also improve the air/water-stability in significant terms; so much so that long-term cyclic stability has been achieved with health/environment-friendly ‘aqueous processed’ electrodes (sans, usage of toxic/hazardous/expensive chemicals like NMP and PVDF) [1]. As will be explained in more elaborate terms during the talk, the changes in structural features, which have led to such outstanding water-stability, include differential contraction/dilation of the Na-‘inter-slab’/TM-‘slab’ spacing and partial occupancy of the dopant at tetrahedral sites of the structure.On the anode front, successful development of Na-ion battery system necessitates looking beyond hard carbon based anodes, which possess safety hazards due to the Na-insertion potential being a bit too close to the Na-plating potential. In this context, Na-titanates promise to be ‘safe’ anode materials; but suffer from cyclic instability [2]. Against this backdrop, we have developed carefully tuned bi-phase Na-titanate based electrodes, having Na2Ti3O7 and Na2Ti6O13 as the primary and secondary phases, respectively. The as-developed phase assemblage has been able to address the cyclic instability of single-phase Na-titanate, leading to long-term cyclic stability even at high current densities (up to 50C!) [3]. These are important steps towards the development of health/environment-friendly, cost-effective, safe and high-performance Na-ion batteries.Acknowledgement to RRCAT, Indore, India, for enabling the usage of Synchrotron facility.Parts of the concerned works have been published as (i.e., the associated publications); B. S. Kumar, A. Pradeep, A. Dutta, and A. Mukhopadhyay; J. Mater. Chem. A 8 (2020) 18064H. S. Bhardwaj, T. Ramireddy, A. Pradeep, M. K. Jangid, V. Srihari, H. K. Poswal, and A. Mukhopadhyay; ChemElectroChem 5[8] (2018) 1219A. Pradeep, B. S. Kumar, A. Kumar, V. Srihari, H. K. Poswal, and A. Mukhopadhyay; Electrochim. Acta 362 (2020) 137122

Modelling of Microstructure Formation in Metal Additive Manufacturing: Recent Progress, Research Gaps and Perspectives

Microstructures encountered in the various metal additive manufacturing (AM) processes are unique because these form under rapid solidification conditions not frequently experienced elsewhere. Some of these highly nonequilibrium microstructures are subject to self-tempering or even forced to undergo recrystallisation when extra energy is supplied in the form of heat as adjacent layers are deposited. Further complexity arises from the fact that the same microstructure may be attained via more than one route—since many permutations and combinations available in terms of AM process parameters give rise to multiple phase transformation pathways. There are additional difficulties in obtaining insights into the underlying phenomena. For instance, the unstable, rapid and dynamic nature of the powder-based AM processes and the microscopic scale of the melt pool behaviour make it difficult to gather crucial information through in-situ observations of the process. Therefore, it is unsurprising that many of the mechanisms responsible for the final microstructures—including defects—found in AM parts are yet to be fully understood. Fortunately, however, computational modelling provides a means for recreating these processes in the virtual domain for testing theories—thereby discovering and rationalising the potential influences of various process parameters on microstructure formation mechanisms. In what is expected to be fertile ground for research and development for some time to come, modelling and experimental efforts that go hand in glove are likely to provide the fastest route to uncovering the unique and complex physical phenomena that determine metal AM microstructures. In this short Editorial, we summarise the status quo and identify research opportunities for modelling microstructures in AM. The vital role that will be played by machine learning (ML) models is also discussed.