Formation Of Non-equilibrium Phases Research Articles

Microstructural feature-driven machine learning for predicting mechanical tensile strength of laser powder bed fusion (L-PBF) additively manufactured Ti6Al4V alloy

The rapid solidification inherent in laser powder bed fusion (L-PBF) additive manufacturing (AM) introduces segregation phenomena and formation of non-equilibrium phases in duplex titanium alloy components, thereby impeding their suitability for high-reliability engineering applications. Consequently, heat treatment becomes indispensable for optimizing both the microstructure and mechanical properties to meet application requirements. This study aims to investigate the influence of varied annealing temperatures on the evolution of L-PBF-built Ti6Al4V alloy microstructure, subsequently elucidating their impact on tensile properties by analyzing of L-PBF process parameters, building orientations, and annealing temperatures. The findings reveal that annealing at 850 °C for 2 h facilitates the transformation of brittle martensite into a ductile lamellar (α + β) microstructure, thereby conferring excellent tensile properties upon the L-PBF-built Ti6Al4V alloy. Furthermore, to accurately predict the tensile strengths of the Ti6Al4V, we take into account the L-PBF process parameters and the as-built microstructures in a comprehensive manner, extracting the pertinent microstructural features. A machine learning (ML)-based model is built to facilitate accurate predictions. Accurate and reliable predictions are demonstrated by this model when applied to Ti6Al4V. This data-driven approach establishes a novel avenue for AM material property prediction and process parameter optimization.

Work Function, Sputtering Yield and Microhardness of an Al-Mg Metal-Matrix Nanostructured Composite Obtained with High-Pressure Torsion

Severe plastic deformation has proven to be a promising method for the in situ manufacturing of metal-matrix composites with improved properties. Recent investigations have revealed a severe mixing of elements, as well as the formation of non-equilibrium intermetallic phases, which are known to affect physical and mechanical properties. In this work, a multilayered aluminum–magnesium (Al-Mg) nanostructured composite was fabricated using constrained high-pressure torsion (HPT) in a Bridgeman-anvil-type unit. A microstructure investigation and X-ray diffraction analysis allowed us to identify the presence of intermetallic Al3Mg2 and Al12Mg17 phases in the deformed nanostructured composite. The sputtering yield of the Al3Mg2 and Al12Mg17 phases was found to be 2.2 atom/ion and 1.9 at/ion, respectively, which is lower than that of Mg (2.6 at/ion). According to density functional theory (DFT)-based calculations, this is due to the higher surface-binding energy of the intermetallic phases (3.90–4.02 eV with the Al atom removed and 1.53–1.71 eV with the Mg atom removed) compared with pure Al (3.40–3.84 eV) and Mg (1.56–1.57 eV). In addition, DFT calculations were utilized to calculate the work functions (WFs) of pure Al and Mg and the intermetallic Al3Mg2 and Al12Mg17 phases. The WF of the obtained Al-Mg nanostructured composite was found to be 4 eV, which is between the WF value of Al (4.3 eV) and Mg (3.6 eV). The WF of the Al12Mg17 phase was found to be in a range of 3.63–3.75 eV. These results are in close agreement with the experimentally measured WF of the metal matrix composite (MMC). Therefore, an intermetallic alloy based on Al12Mg17 is proposed as a promising cathode material for various gas-discharge devices, while an intermetallic alloy based on Al3Mg2 is suggested as a promising optical- and acoustic-absorbing material.

Mechanical properties of Al-Nb in situ metal-matrix composites fabricated by constrained high pressure torsion at 10 GPa and subsequent annealing

Mechanical alloying of dissimilar metals by means of severe plastic deformation techniques is known to be a perspective method of obtaining in situ metal matrix composites with advanced properties. Recent investigations found out that besides severe mixing of elements, intensive formation of non-equilibrium intermetallic phases takes place. Presence of intermetallic particles in the composite affects both its physical and mechanical properties. In this work we have realized the fabrication of Al-Nb composites in one step by means of high-pressure torsion technique. A non-trivial phenomenon of growth of tensile strength with increasing number of revolutions during HPT was revealed. It was explained by more homogeneous mixing of components and precipitation of intermetallic phase in the vicinity of Al-Nb interphases. The microstructural observations and X-ray diffraction analysis allowed to reveal Al3Nb intermetallic phase in the as deformed composites, while the equilibrium temperature of formation of this phase is above 600°C. Varying the number of revolutions and post deformation annealing allows obtaining composites with different fractions of intermetallic particles. This approach can be considered as a way for the development of composites with a tailorable complex of physical and mechanical properties.

High energy ball milling – An advanced processing route for effective development of Titanium Aluminide intermetallic alloy through mechanical alloying

Mechanical alloying is traditionally being used for developing novel alloys, which are difficult to prepare by conventional manufacturing routes, through solid state diffusion. It offers unique feature of extended solid solubility resulting in the formation of non-equilibrium immiscible phases that have huge potential in aerospace and defense applications. However, the development of intermetallic class of materials has always been a challenge. Titanium Aluminide, possessing various advantageous properties, has got limited practical usage owing to the difficulty in development. The current research focuses on developing Titanium Aluminide intermetallic material (TiAl) from elemental Aluminumand Titanium powders using high energy planetary ball milling process. The centrifugal force combined with high gravitational counterforce resulted in the formation of intermetallic phase with near stoichiometric ratio. The developed TiAl was subjected to various microstructural and morphological analysis to understand the mechanism of phase formation during the milling process. Results reveal new dimensions for developing intermetallic alloys for various advanced engineering applications.

Nonequilibrium dynamics of spontaneous symmetry breaking into a hidden state of charge-density wave

Nonequilibrium phase transitions play a pivotal role in broad physical contexts, from condensed matter to cosmology. Tracking the formation of nonequilibrium phases in condensed matter requires a resolution of the long-range cooperativity on ultra-short timescales. Here, we study the spontaneous transformation of a charge-density wave in CeTe3 from a stripe order into a bi-directional state inaccessible thermodynamically but is induced by intense laser pulses. With ≈100 fs resolution coherent electron diffraction, we capture the entire course of this transformation and show self-organization that defines a nonthermal critical point, unveiling the nonequilibrium energy landscape. We discuss the generation of instabilities by a swift interaction quench that changes the system symmetry preference, and the phase ordering dynamics orchestrated over a nonadiabatic timescale to allow new order parameter fluctuations to gain long-range correlations. Remarkably, the subsequent thermalization locks the remnants of the transient order into longer-lived topological defects for more than 2 ns.

New atomic-scale insights into the I/Ni(100) system: phase transitions and growth of an atomically thin NiI2 film.

We use a traditional surface science approach to create and study an atomically thin NiI2 film (a promising two-dimensional ferromagnetic material) formed on nickel substrate as a result of molecular iodine adsorption. The I/Ni(100) system was examined with scanning tunneling microscopy (STM), low energy electron diffraction (LEED) and density functional theory calculations. We found out that the iodine adsorption on Ni(100) at 300 K leads to the formation of non-equilibrium phases, whereas the adsorption at elevated temperature (≥390 K) gives rise to the thermodynamically stable phases. In both cases, a simple p(2 × 2) structure is formed at 0.25 ML. As more iodine is adsorbed at 300 K, the p(2 × 2) phase is replaced by the small coexisting domains of c(3 × 2) and c(6 × 2) phases both corresponding to the coverage of 0.33 ML, while adsorption at elevated temperature results in the formation of only one c(3 × 2) phase. At further iodine adsorption the c(3 × 2) phase transforms into the c(5 × 2) one, while the c(6 × 2) phase - into the one both corresponding to the coverage of 0.40 ML. In addition to simple chemisorbed phases, a new shifted-row reconstruction of Ni(100) induced by iodine adsorption was discovered. At coverages exceeding 0.40 ML, we observed complex LEED patterns and superstructures in STM and assigned them to specific surface reconstructions. We also found that prolonged iodine dosing leads to the nucleation of nickel iodide islands and the growth of a 2D atomically thin iodide film partially exfoliated from the substrate.

Non-Equilibrium Lattice Heterogeneity As the Unified Kinetic Hurdle of Ni-Rich Layered Cathode for Ultrafast Lithium Ion Batteries

The significantly reduced capacity at a fast cycling rate is one of the most serious bottlenecks that prevent the large-scale application of Ni-rich layered oxide cathode for electrical vehicle batteries. Figuring out the capacity decline mechanism is crucial to addressing such hurdle and developing ultrafast lithium ion batteries. In this work, with operando synchrotron-based spectroscopy, we captured the formation of non-equilibrium separated phases during the extremely fast cycling of various Ni-rich layered oxides, in contrast to their solid-solution structural evolution pathway at slow cycling rate. Further, such non-equilibrium phase heterogeneity was demonstrated to be an effective capacity retention descriptor for Ni-rich layered oxide cathode: the capacity retention ratio is liner inversely proportional to the lattice heterogeneity at an extremely fast cycling rate (e.g., 10C), regardless of the composition and Ni content. Those detrimental non-equilibrium phase heterogeneity could be eliminated by raising the operation temperature, contributing to much higher lithium ion transport kinetics and better capacity retention during ultrafast cycling

On the Post-Printing Heat Treatment of a Wire Arc Additively Manufactured ER70S Part.

Wire arc additive manufacturing (WAAM) is known to induce a considerable microstructural inhomogeneity and anisotropy in mechanical properties, which can potentially be minimized by adopting appropriate post-printing heat treatment. In this paper, the effects of two heat treatment cycles, including hardening and normalizing on the microstructure and mechanical properties of a WAAM-fabricated low-carbon low-alloy steel (ER70S-6) are studied. The microstructure in the melt pools of the as-printed sample was found to contain a low volume fraction of lamellar pearlite formed along the grain boundaries of polygonal ferrite as the predominant micro-constituents. The grain coarsening in the heat affected zone (HAZ) was also detected at the periphery of each melt pool boundary, leading to a noticeable microstructural inhomogeneity in the as-fabricated sample. In order to modify the nonuniformity of the microstructure, a normalizing treatment was employed to promote a homogenous microstructure with uniform grain size throughout the melt pools and HAZs. Differently, the hardening treatment contributed to the formation of two non-equilibrium micro-constituents, i.e., acicular ferrite and bainite, primarily adjacent to the lamellar pearlite phase. The results of microhardness testing revealed that the normalizing treatment slightly decreases the microhardness of the sample; however, the formation of non-equilibrium phases during hardening process significantly increased the microhardness of the component. Tensile testing of the as-printed part in the building and deposition directions revealed an anisotropic ductility. Although normalizing treatment did not contribute to the tensile strength improvement of the component, it suppressed the observed anisotropy in ductility. On the contrary, the hardening treatment raised the tensile strength, but further intensified the anisotropic behavior of the component.

Experimental validation of Scheil–Gulliver simulations for gradient path planning in additively manufactured functionally graded materials

Additive manufacturing (AM), through directed energy deposition, supports planned composition changes between locations within a single component, allowing for functionally graded materials (FGMs) to be developed and fabricated. The formation of deleterious phases along a particular composition path can cause significant cracking during the AM build process that makes the composition path unviable to produce structurally stable FGMs, but it is challenging to predict which phases will be present in as-built additively manufactured parts by analyzing only equilibrium phase relations. Solute segregation during solidification can lead to the formation of non-equilibrium phases that are stable at compositions far from the nominal composition of the melt, leading to crack formation. In this work, we developed and validated a Scheil–Gulliver simulation tool based on pycalphad. We used this tool to compare the non-equilibrium phases predicted to form during the AM build process using the Scheil–Gulliver model with experimentally measured phases at several locations with different composition in a Ti-6Al-4V to Invar-36 FGM and a commercially pure Ti to Invar-36 FGM. We showed that the phases predicted to form by the Scheil–Gulliver model agree better with the experimental results than the predictions made by assuming equilibrium solidification, proving that the Scheil–Gulliver model can be applied to FGMs. Further, we demonstrated the use of our Scheil–Gulliver simulation tool as a method of designing FGMs through screening potential FGM pathways by calculating the solidification phase fractions along the experimental gradient path in composition space.

Melting behaviour of tri-phasic Bi44In32Sn23 alloy nanoparticle embedded in icosahedral quasicrystalline matrix

Tri-phasic nanoscaled (Bi,In,Sn) alloy nanoparticles (NPs) embedded in the icosahedral quasicrystalline (IQC) matrix were synthesized by the rapid solidification route. The microstructure of the NPs was analyzed using transmission electron microscopy (TEM), which typically reveals the combination of three phases; rhombohedral (Bi), body-centered tetragonal BiIn and hexagonal (γ-Sn) in each NP. TEM study also indicates that (Bi) and BiIn bear a reasonably good lattice match with surrounding IQC, whereas (γ-Sn) does not develop any specific orientation relationship. In order to understand the melting behavior of tri-phasic NPs, extensive DSC (differential scanning calorimetry), in-situ TEM (transmission electron microscopy) and in-situ XRD (X-ray diffraction) has been carried out. The DSC result reveals substantial depression of melting temperature (∼20 °C) as compared to bulk. In-situ studies also indicate the solid-state transformation of (γ-Sn) into (β-Sn) prior to melting. As the temperature advances, melting initiates at the triple junction of (Bi)/BiIn and matrix. BiIn first melts completely, and the melt front grows into the (β-Sn) phase of the NP. Subsequently, the melt propagates into the interior of (Bi) rich region. The present study provides an insight into the formation of non-equilibrium (γ-Sn) phase instead of (β-Sn) and the mechanism of the melting behaviour of three-phase NP embedded in IQC.

Model experiment on reactive phase formation and solidification of B4C-BN composites via nanosecond pulse laser processing

In the present work we describe results of a model experiment on reactive phase formation of B4C-BN composites on a steel substrate using high-energy short laser pulses. The model experiment opens a way for high quality laser joining of boron based ceramics with steels through the reactive phase formation at the steel-ceramic interface. The study was focused on changes in microstructure, chemical and phase compositions in surface layers of the resulting B4C-BN-Fe composites. Ultrafine powders of boron carbide and boron nitride were spread on a steel surface and subjected to laser processing with nanosecond pulses. The reactions occur as a result of elevated temperature under action of laser short pulses. Subsequent rapid solidification leads to highly nonequilibrium solidification and metastable phase formation.

Impact of laser irradiation on microstructure and phase development of tungsten carbide - cobalt

Previous studies on Laser Powder Bed Fusion of tungsten carbide-cobalt (WC-Co) revealed several defects in the laser molten microstructures. To analyze the defect formation mechanisms and the fundamental impact of laser energy input on phase development and microstructure of WC- Co, single-pulse and single-track analogy experiments are conducted. Conventional WC-Co is used as substrate. Variations in cobalt content and processing conditions are investigated. It is found that a single laser exposure can be enough to induce decomposition of microstructure, WC grain growth, formation of non-equilibrium phases and thermal cracking. Based on the results, defect formation mechanisms and countermeasures are discussed.

Microstructure and oxidation behavior of NiCr-chromium carbides coating prepared by powder-fed laser cladding on titanium aluminide substrate

In the present study, a NiCr–Cr3C2 powder mixture was prepared by mechanical alloying and then coated on titanium aluminide substrates by the powder-fed laser cladding process using a set of optimum parameters. The high temperature oxidation behavior of the substrate and coating was studied by isothermal annealing at 900 °C for 5 h. It was found that the microstructure of the coating is composed of γ solid solution with different chromium carbide phases (Cr3C2, Cr7C3 and Cr23C6). The presence of different chromium carbides in the microstructure of coating can be attributed to the partial melting of primary Cr3C2 and the formation of non-equilibrium carbide phases during rapid cooling of laser cladding. The NiCr-chromium carbide laser cladded coating samples showed superior oxidation resistance compared to the substrate. The oxidation mechanism of both coating and substrate follow the parabolic law, where the parabolic rate constant of the coating was 20% of that of the substrate at 900 °C. Time-of-Flight Secondary Ion Mass Spectroscopy (ToF-SIMS) and Grazing Angle X-Ray Diffraction (GAXRD) analysis revealed that the surface of the oxide layer formed on the NiCr-chromium carbides coating and the substrate is mostly composed of Cr2O3 and TiO2, respectively.

Study of structure of copper-based composite materials during the spark plasma sintering

This paper is devoted to the study of copper-based composite materials structure formation processes in the conditions of SPS. The structures of Cu–Cr, Cu–Mo, Cu–W, Cu–Cr–Mo, Cu–Cr–W, Cu–Mo–W, Cu–Cr–SiC, and Cu–Zr–CNTs two-component and three-component systems were analyzed. The possibility of the formation of non-equilibrium phases under SPS conditions is shown, the effect of chemical transformations on the structure of bulk composite materials is considered.

Studies of thermally activated processes in gas-atomized Al alloy powders: in situ STEM heating experiments on FIB-cut cross sections

Gas atomization is the most common approach used to produce powders of metallic alloys, and the high cooling rates involved frequently lead to the formation of non-equilibrium microstructures and phases. The transformations that occur in the powders upon heating are of great interest but are challenging to study experimentally. Here we use a novel focused ion beam-based specimen preparation protocol to obtain cross sections through individual gas-atomized powder particles of three different aluminum alloys: solid solution-strengthened Al5056, precipitation-hardenable Al6061, and an Al–Cr–Mn–Co–Zr alloy which contains icosahedral quasicrystal dispersoids. In situ scanning transmission electron microscopy heating experiments were performed on these cross-sectional specimens to investigate the changes that occur in the metastable phases and non-equilibrium microstructures upon heating. The experiments reveal the details of a wide variety of thermally activated processes occurring in the particles including: solute redistribution to eliminate micro-segregation; dissolution, coarsening, transformation and decomposition of secondary phases; and precipitation within the aluminum matrix.

Zn-Sn Interdigitated Eutectic Alloy Anodes with High Volumetric Capacity for Lithium-Ion Batteries

Summary Zn-Sn interdigitated eutectic alloy (IdEA) foils are highly promising anodes for lithium-ion batteries. Synthesized via severe plastic deformation, a two-phase alloy is converted into a nanostructured foil composed of tin domains distributed throughout an electrically conductive zinc matrix. In addition to providing mechanical integrity, uncycled capacity in the zinc matrix eliminates the lithium-plating risk. These monolithic foil anodes offer an electrode-level volumetric capacity approximately double that of graphite (797 mA h cm−3), with a highly promising formation loss of

Observation of a new B2 structured phase in Ti-15Mo (wt%)

The formation of non-equilibrium and transient phases in metastable beta titanium alloys during low temperature thermal treatments is currently of great interest, as they provide a potential method of controlling the size and distribution of the equilibrium alpha phase and, hence, the resulting mechanical properties. Here, for the first time, we report on the formation of a new, B2 structured phase in the Ti-Mo system. The phase was observed in electron transparent material during in situ, and following ex situ, heat treatment at 300 °C. The B2 phase was enriched in Mo compared to the surrounding matrix material and formed in regular arrays of approximately square cross-section particles interspersed by thin beta channels. Electron diffraction indicated that the lattice parameter of this new phase was smaller than that of the parent phase, leading to significant strain in the beta channels. Critically, the B2 phase was only observed in material that had been electro-polished prior to heat treatment, and, therefore, it is hypothesised that this phase forms as a result of the preparation method and thin foil effects.

Manufacturing of intermetallic Mn-46%Al by laser powder bed fusion

Laser powder bed fusion (LPBF) provides an excellent opportunity to use custom powders for complex objects without extensive machining. This opportunity is attractive for brittle and hard intermetallics, but is challenging due to cracking, anisotropy, and the formation of non-equilibrium phases. The present investigation is focused on a development of the process parameters for pre-alloyed Mn-46 at.%Al gas atomized intermetallic powder, which is a promising magnetic material. A hierarchical approach involving optimization of the process parameters for a single track, a single layer, and then a 3D specimen was applied. The manufacturing of single tracks was performed at scanning speeds of 0.06-3.4 m/s and laser powers of 50-350 W. Test parameters guaranteeing stable single track with constant width and height, and sufficient remelting depth were selected for further manufacturing. Surface morphology, chemical composition, crack density and distribution, and the microstructures in the final materials were investigated. It was shown that the consists mostly of the ε-phase with some amounts of equilibrium γ2 and β phases and the ferromagnetic τ-phase. The presence of the ε-phase shows a potential to use heat treatment to form τ-phase magnetic phase in AM Mn-46 at.%Al. Future investigations will clarify the applicability of LPBF to manufacture Mn-46%Al for magnetic applications.

Effect of ultrasonic casting on microstructure and its genetic effects on corrosion performance of 7085 aluminum alloy

The effect of ultrasonic casting on microstructure and its genetic effects on strength, exfoliating corrosion, stress corrosion and electrochemical behavior of 7085 aluminum alloy have been investigated by optical microscope (OM), scanning electron microscope (SEM) and transmission electron microscope (TEM), together with tensile testing, exfoliation corrosion testing, electrical conductivity testing and polarization curve. The results indicate that ultrasonic casting could refine the grain, alleviate segregation and inhibit the formation of coarse nonequilibrium phase in as-cast state; in addition, the dissolution of nonequilibrium phase in the ultrasonic ingot during homogenization turns out to be more thorough. What is more, the plate processed from ultrasonic ingot holds a lower ratio of recrystallization after solid solution, and the corrosion performance of the alloy was improved under T6 temper, without sacrifice of strength, owing to the dispersive distribution of strengthening phase in the matrix and the coarse, sparse GBPs.

Scale invariance of structural transformations in plastically deformed nanostructured solids

The scale-invariant mechanical behavior of a nanostructured solid is associated with plastic distortion as a major mechanism of nano- and microscale structural transformations. Active grain boundary sliding in a deformed material (microscale) within its highly developed planar subsystem (nanograin boundaries) causes a progressive increase in lattice curvature and plastic distortion of atoms which produces nonequilibrium vacant sites in the nanostructure. The motion of nonequilibrium point defects in nanostructure curvature zones provides conditions for noncrystallographic plastic flow, dissolution or dispersion of initial phases, and formation of nonequilibrium phases in a deformed material. The possibility of reversible structural phase transformations in the presence of high lattice curvature opens the way to greatly increase the fatigue life of surface nanostructured polycrystalline materials.