Order Phase Transformation Research Articles

Phase-field modeling of interdiffusion between dissimilar Fe-Cr-Ni alloys during non-isothermal hot isostatic pressing

Powder metallurgy hot isostatic pressing (PM-HIP) has emerged as a promising alternative to welding for joining dissimilar metals. During HIP, interfacial bonding is mediated by solid state diffusion. The interdiffusion zone across the interface depends on processing conditions, calling for the need for accurate numerical tools capable of simulating interdiffusion and possible phase transformation in order to optimize processing parameters. Here, a phase-field (PF) model based on CALPHAD-based free energy functionals is developed to simulate the interdiffusion and phase evolution between dissimilar Fe–Cr–Ni based steels undergoing HIP and is demonstrated using the interface between 316L and SA508 steels. To overcome the numerical challenges caused by the singular magnetic and entropy terms in the CALPHAD free energy models in the Fe–Cr-Ni system, polynomial functions are fitted with temperature dependent coefficients represented by Fourier series to accurately describe the phase stability of both fcc and bcc phases in the composition and temperature space. This enables simulations of non-isothermal HIP cycles. Diffusivity data from commercial software and literature are taken to parameterize the kinetic parameters. A discrete nucleation model is incorporated for possible phase transformation. The modified thermodynamic models are validated against previous experiments at 923 K and 1273 K. The interdiffusion kinetics are benchmarked against new HIP experiments joining powder and bulk 316L to bulk SA508 with three different HIP cycles. The good agreement between simulations and experiments on both phase stability and interdiffusion indicate that the model is suitable for simulating interdiffusion between Fe–Cr–Ni alloys during HIP cycles. It is also found that using powder and bulk 316L gives similar interdiffusion profiles at elevated temperature when a dense interface forms during HIP.

Friction Force Mapping of Molecular Ordering and Mesoscopic Phase Transformations in Layered-Crystalline Organic Semiconductor Films.

It is critical to understand molecular ordering processes in small-molecule organic semiconductor (OSC) films in optimizing electronic device applications, although it is difficult to observe and investigate the ordering characteristics at a mesoscopic or device scale. Here, we report that friction force microscopy (FFM) allows visualizing the ordering transformation process from a thermodynamically metastable phase to a stable phase at a mesoscopic scale. We utilized 2-octyl-benzothieno[3,2-b]naphtho[2,3-b]thiophene (2-C8-BTNT) as a typical highly layered-crystalline OSC. We found that the friction force between an AFM tip and spin-coated OSC films significantly depends on whether local film states are in metastable monolayer phase or stable bilayer-type herringbone (b-LHB) phase that exhibits high carrier mobility. The formation of the stable b-LHB phase leads to lower friction than the metastable monolayer phase, clearly visualizing the molecular order. Force map (Fmap) analysis indicates that the lower friction in the b-LHB phase should be associated with the reduction of interfacial adhesion force. Notably, the observed results demonstrate that the spin-coated thin film changes from continuous film with the monolayer phase to rugged microcrystal grains with the b-LHB phase when left at ambient conditions. By contrast, an appropriate post-thermal annealing process facilitates the phase transformation without inducing such morphological changes. The technique provides a unique and effective tool for revealing the relationship between processing conditions and device performance in polycrystalline OSC films.

Accelerated precipitation and enhanced mechanical properties of Cu-3.5Ti alloy via electropulsing treatment

Aging is an important step in improving the comprehensive properties of Cu-3.5Ti alloy. However, both prolonged holding times and high heat treatment temperatures lead to significant oxidation and substantial energy consumption. The effects of heat treatment (HT) and electropulsing treatment (EPT) on the microstructure and mechanical properties of the solid solution Cu-3.5Ti alloy were comparatively examined in this study. The findings demonstrated that EPT quickly completed the processes of spinodal decomposition and short-range ordered to long-range ordered phase transformation, thereby fully precipitating fine and diffusive β′-Cu4Ti phase, which greatly enhanced the precipitation strengthening effects. In addition to enhancing the strength of the alloy, EPT significantly reduced the peak aging time (to just 1/12 of that of HT) and greatly increased the plasticity (elongation to failure rose by 47 % compared to HT). The main reason for the rapid completion of the spinodal decomposition and the full precipitation process by the EPT was the additional free energy provided by the electropulsing, which significantly accelerated the Ti atomic diffusion and lowered the nucleation barrier of the β′-Cu4Ti phase. EPT is a toughening heat treatment process for metallic materials, which offers the advantages of high efficiency, low energy consumption, and low cost.

Highly Coercive L10-Phase Dots Obtained through Low Temperature Annealing for Nano-Logic Magnetic Structures

Nano-logic magnetic structures are of great interest for spintronic applications. While the methods used for developing arrays of magnetic L10-phase dots are, in most cases, based on deposition followed by annealing at high temperatures, usually around 700 °C, we demonstrate here a technique where a much lower annealing temperature (i.e., 400 °C) is needed in order to promote fully the disorder–order phase transformation and achievement of highly coercive L10-phase dots. In order to develop building blocks based on arrays of L10-phase FePt dots for further spintronic applications, an engraving technique using electron beam lithography is employed. This paper describes the fabrication, as well as the morphological and magnetic characterization, of regularly placed FePt dots of various shapes, as pre-requisites for integration into nano-logic devices. As a proof of concept, regular arrays of FePt circular dots were devised and their structural characterization, using X-ray diffraction (XRD) and transmission electron microscopy (TEM), was performed. It has been shown that annealing at only 400 °C for 30 min proved the occurrence of the tetragonal L10 phase. Moreover, structural characterization showed that the disorder–order phase transformation was complete with only the L10 phase detected in high resolution TEM. The magnetic characterization provided more insight into the potential of such arrays of magnetic devices with convenient values of magnetic coercivity, remanent and saturation magnetization. These findings show good potential for developing regular arrays of uniformly shaped magnetic entities with encouraging magnetic performances in view of various applications.

Nanophase transformation and strengthening - Toughening mechanism of the Cu–15Ni–8Sn alloy during aging

The Cu–15Ni–8Sn alloy ingot was prepared in the medium-frequency induction melting furnace. The hot extruded alloy was treated by solid solution at 840 °C for 60 min and aging at 400 °C for different times (0 miñ480 min). The effect of aging time on the evolution of the nanophase of Cu–15Ni–8Sn alloy and the strengthening-toughening mechanism were studied. The results show that at the beginning of aging, the alloy undergoes spinodal decomposition, the wavelength of the modulated structure is approximately 3∼7 nm, the ordered phase transformation of D022 and L12 occurs simultaneously, and the ordered phase size is approximately 5 nm. With the extension of aging time, the modulated structure became coarser, and the wavelength of the modulated structure increased to 5–9 nm. At the same time, the D022 ordered phase changed to the L12 ordered phase, and a continuous distribution of elliptical discontinuous precipitation appeared at the grain boundary. The ratio of the long and short axes of the discontinuous precipitation was approximately 4.7. In the late aging period, the D022 ordered phase was basically transformed into the L12 ordered phase, and discontinuous precipitation, which is incompatible with the matrix, extended into the crystal. After 120 min of aging at 400 °C, the hardness, strength, and elongation of the alloy are 105.4 HRB, 839.8 MPa and 9.6 %, respectively. The peak-aging states D022 and L12 coexist in an orderly phase at the nanoscale, and this nanoscale strengthening phase is completely integrated with the matrix, which improves the strength and toughness of the alloy. The research results can provide support for the development of high-performance copper alloys in the ocean.

Theoretical Investigation of Pressure Dependent Structural and Elastic Properties of MnSi Compound

MnSi is a metallic compound that exhibits a structural phase transition as a function of pressure. At ambient conditions, MnSi has a cubic structure such as NaCl type crystal structure. However, at high pressures, it undergoes a structural phase transition to a CsCl type crystal structure. The pressure-dependent structural phase transition in MnSi has been studied using various experimental and theoretical techniques. In the present work, we have used an efficient inter-ionic potential approach to predict pressure dependent structural phase change and associated volume collapse in MnSi. Therein, the potential includes the long-range Coulomb, van der Waals (vdW) interaction, and the short-range repulsive interaction up to second neighbour ions. It has been demonstrated that the Hafemeister and Flygare approach successfully evaluates the equation of state for change in volume with respect to applied pressure. We have identified a structural phase transition from B1(NaCl) type structure to B2 (CsCl) type structure in this compound. The estimated value of phase transition pressure (Pt) is 42 GPa, which is consistent with the available reported data. The identified first order phase transformation showed a volume collapse of about 10% in the vicinity of transition. In addition, we have also investigated second order of elastic constants for MnSi compound.

Novel Rare Earth (RE)-Free Nanocomposite Magnets Derived from L10-Phase Systems.

In the quest for novel rare earth (RE)-free magnetic materials, which also exhibit other additional properties such as good corrosion resistance and potential to operate at higher temperatures, an alloy deriving from the binary FePt system, with Mo and B addition, has been synthesized for the first time, using the out-of-equilibrium method of rapid solidification form the melt. The alloy with the composition Fe49Pt26Mo2B23 has been subjected to thermal analysis through differential scanning calorimetry in order to detect the structural disorder - order phase transformation as well as to study the crystallization processes. For the stabilization of the formed hard magnetic phase, the sample has been annealed at 600 °C and further structurally and magnetically characterized by means of X-ray diffraction, transmission electron microscopy, 57Fe Mössbauer spectrometry as well as magnetometry experiments. It has been proven that after annealing at 600 °C the tetragonal hard magnetic L10 phase emerges via crystallization from a disordered cubic precursor and becomes the predominant phase in terms of relative abundance. Moreover, it has been revealed by quantitative analysis via Mössbauer spectroscopy that the annealed sample exhibits a complex phase structure, where the L10 hard magnetic phase is accompanied by few other soft magnetic phases, in minority abundance: the cubic A1, orthorhombic Fe2B and residual intergranular region. The magnetic parameters have been derived from 300 K hysteresis loops. It was shown that, contrary to the as-cast sample which behaves as a typical soft magnet, the annealed sample presents strong coercivity and high remanent magnetization, accompanied by a large saturation magnetization. These findings offers good insight into the potential developing of novel class of RE-free permanent magnets, based on Fe-Pt-Mo-B, where the magnetic performance emerges from the co-existence of hard and soft magnetic phases in controlled and tunable proportions, capable of finding good applicability in fields requiring good catalytic properties and strong corrosion resistance.

Кинетика формирования сверхструктуры L10 в сплаве Cu–56Au (ат. %): резистометрическое исследование

Due to the improved strength properties compared to the equiatomic Cu–50 at. % Au alloy, non-stoichiometric Cu–56 at. % Au alloy can be used both in dentistry and as a corrosion-resistant conductor of weak electrical signals in tool engineering. The work studies the kinetics of the disorder→order phase transformation in the Cu–56Au alloy, during which the disordered fcc lattice (A1-phase) is rearranged into an atomically ordered one with the L10 superstructure. The initial disordered state of the alloy was obtained in two ways: applying plastic deformation by 90 % or quenching at a temperature of above 600 °C (i. e., from the region of the A1-phase existence). To form the L10 superstructure, annealing was carried out at temperatures of 200, 225, and 250 °C. The annealing duration ranged from 1 h to 2 months. Resistometry was chosen as the main technique to study the kinetics of the disorder→order transformation. The temperature dependences of the electrical resistivity of the alloy in various structural states are obtained. The authors constructed the graphs of the electrical resistance dependence on the annealing time logarithm, based on which, the rate of the new phase formation was estimated. To evaluate the structural state of the alloy at various transformation stages, the authors used X-ray diffraction analysis (XRD). The crystal structure rearrangement during the transformation is shown by the example of splitting the initial cubic A1-phase peak (200) into two tetragonal ordered L10 phase peaks – (200) and (002). Based on the resistometry and X-ray diffraction analysis data, the authors carried out a quantitative assessment of the rate of the disorder→order phase transformation in the alloy under the study. It is established that the values of the converted volume fraction (resistometry) and the long-range order degree (X-ray diffraction analysis) are close. The study shows that in the temperature range of 200–250 °C, the rate of atomic ordering according to the L10 type in the nonstoichiometric alloy Cu–56 at. % Au is maximum at 250 °C. It is identified that the disorder→order transformation in the initially quenched specimens of the investigated alloy proceeds approximately an order of magnitude faster than in preliminarily deformed specimens.

Tuning the guest-induced spatiotemporal response of isostructural dynamic frameworks towards efficient gas separation and storage

One-atom exchange in a flexible MOF changes the 2nd order phase transformation to 1st order, enabling efficient gas storage and separation.

The Improvement in Ordered Phase Transformation Temperature of Cu75Fe05Pd20 Alloy for Advanced Engineering Applications

The Improvement in Ordered Phase Transformation Temperature of Cu75Fe05Pd20 Alloy for Advanced Engineering Applications

Ordered phase transformation and Cu doping effects in room-temperature ferromagnetic Sr3YCo4O10.5+δ.

Sr3YCo4O10.5+δ (314-SYCO), with an unusual ordered structure and a high Curie temperature (Tc ≈ 335K), is attracting increasing attention. Herein, to improve the electrical performance of 314-SYCO, Cu-doped Sr3YCo4-x Cu x O10.5+δ (x = 0-0.8) ceramics were prepared using a solid-state reaction method. Systematic research was conducted on both the ordered phase transformation and the effects of Cu doping on the microstructure, electrical transport characteristics, and magnetic properties. For x = 0-0.4, the (103) and (215) planes were observed and combined with Rietveld refinement results for the X-ray diffraction data, confirming the formation of ordered tetragonal Sr3YCo4-x Cu x O10.5+δ . This phase was formed with a mass gain of ∼0.8% and heat released at ∼1,042°C. With increasing Cu content, the concentration of hole carriers also increased, leading to a substantial reduction in electrical resistivity. The electrical resistivity decreased by 92-99% at 300K. The polycrystalline materials have semiconducting behaviour with a three-dimensional Mott variable-range hopping mechanism. For the magnetic properties, a Hopkinson peak was observed at 319K, and the Tc was approximately 321K for x = 0. The magnetisation and Tc decreased with increasing Cu content, and a G-type antiferromagnetic-to-ferromagnetic phase transition occurred due to the spin state change for some Co3+ ions from high/intermediate spin to low/intermediate spin. These results lay the groundwork for refinement of the sintering procedure and doping parameters to enhance the performance of 314-SYCO in the context of current applications such as microwave absorbers and solid oxide fuel cell cathodes.

On the interplay between a novel iron and iron-carbide atomic layer deposition process, the carbon nanotube growth, and the metal–carbon nanotube coating properties on silica substrates

The fabrication of iron and iron carbide nanoparticles (NPs) for catalytic reactions such as the growth of carbon nanotubes (CNTs) compete with the challenge of covering a wide range of substrates with perfect control of the NP reactivity. We present in this work a novel atomic layer deposition (ALD) process to grow Fe/Fe3C thin films over silica flat substrates. The depositions were carried out exposing the surface through various number of ALD cycles, resulting in Fe-based films with thicknesses ranging from 4 nm to almost 40 nm. After a thermal treatment, the film dewetts into nanoparticles, where the efficiency to grow CNTs will depend on the average size distribution of the nanocatalyst. X-ray diffraction and x-ray photoelectron spectroscopy were used to track the elemental, phase, and shape (film to particles) transformation in order to identify the key features of the nanocatalyst, thereby controlling the CNT nucleation and growth. Thin film thickness of around 5 nm promotes the growth of a dense CNT forest. Furthermore, the metal–CNT films reveal optical properties that are totally tailored by the initial number of ALD cycles.

Integrating Superlattice to Regulate P2‐O2 Phase Transition and Improve Cycling Stability in Sodium‐Ion Batteries

AbstractP2‐Na2/3Ni1/3Mn2/3O2 is a promising cathode material for sodium‐ion batteries (SIBs), but it faces the rapid capacity decay due to P2‐O2 phase transition. The superlattice is a stable structure, compositing it can effectively eliminate the P2‐O2 phase transition. Based on superlattice of NaLi1/3Mn2/3O2, a series of xNaLi1/3Mn2/3O2 ⋅ (1‐x)Na2/3Ni1/3Mn2/3O2 (x=0, 0.3, 0.4, 0.5) were first synthesized by compounding NaLi1/3Mn2/3O2 into Na2/3Ni1/3Mn2/3O2 in this article. The XRD and SAED show that special superlattice guarantees the structure stable of materials, resulting in a first order phase transformation to a solid solution reaction in high voltage. The composite cathode materials realize excellent cycle performance and discharge specific capacity. When x=0.4, the capacity retention rate is 82.64 % after 100 cycles, which is greatly improved compared with 30.60 % of Na2/3Ni1/3Mn2/3O2. This paper provides a new idea for using superlattice to stabilize the structure and improve the electrochemical performance for SIBs.

Disordered to ordered phase transformation: Correlation between microstructure and magnetic properties in Fe–Pd thin films

The kinetics of the transformation from the disordered to the ordered phase has been studied in Fe56Pd44 thin films deposited on silicon substrates by Joule evaporation. Subsequent thermal treatments at 550 °C in vacuum were performed at different annealing times to promote the formation and the optimization of the ordered L10 FePd phase. X-ray diffraction allowed us to investigate the microstructure of the samples (grain size, volume fraction of the transformed phase, order parameter of the ordered phase, and average order of the alloy), whereas magnetic measurements allowed us to evaluate coercivity and to independently estimate the volume fraction of the ordered L10 phase. The kinetics of the transformation turned out to reach completion after 20 min of annealing time with the grain size growing with a power law with an exponent equal to 8.24. The resulting monophasic film has a high density of defects, which contribute to the magnetic coercivity. A linear relationship between the coercive field and the average order parameter of the alloy has been established. Finally, the evolution of the transformed phase obeys Avrami's law with an exponent of ∼0.4, indicating that the transformation is controlled by the nucleation mechanism without grain growth.

The spall and anomalous inelastic response of Galfenol to shock loading

Plate impact experiments were performed to probe the dynamic response of Galfenol, specifically the Hugoniot elastic limit (HEL), spall strength, and failure characteristics. Galfenol exhibits low HEL and high spall strength as compared to α-Fe, possibly owing to the large grain size and strong texture. A cusp is observed on the velocity–time profiles that exhibits pronounced stress and strain rate dependence. The nature of this cusp is not well understood, but it may possibly be related to the ferromagnetic-to-paramagnetic second order phase transformation induced by shock waves. Shock-recovered specimens exhibit brittle intergranular fracture under compressive loading with cracks initiating and growing along the shock direction. It is expected, though not confirmed, that these cracks grow along grain boundaries owing to embrittlement by Ga segregation at the boundaries.

Theoretical studies of pressure induced structural phase transformation and elastic properties in nickel silicide

In this study, we have explored pressure dependent structural phase transformation and related volume collapse in the vicinity of transition in Nickel Silicide (NiSi) with the help of an effective inter-ionic potential approach. It was observed that by employing an effective interionic interaction approach, which is basically based on phenomenological models of lattice dynamics, includes the long-range Coulomb, van der Waals (vdW) interaction, and the short-range repulsive interaction in the framework of the Hafemeister and Flygare approach. Our results indicate that the material parameters evaluated by such an approach provide a successful evaluation of the equation of state for change in volume with respect to the applied pressure. The estimated value of phase transition pressure (Pt) is 25 GPa for NiSi, which is in line with the information that we have in the literature. This transition corresponds to the transformation of a B1 (NaCl) type structure to a B2 (CsCl) type structure and is a first-order phase transition. This first order phase transformation showed a volume collapse of about 11.44% at 25 GPa for NiSi. Later on, the relationship between pressure and the variation of second-order elastic constants is examined in greater detail.

Mechanical properties of sunflower oil under pressure

This article presents the results of research on the influence of high pressure on the mechanical properties of sunflower oil. The pressure value was changed by the stepwise method to obtain thermodynamic equilibrium. Dependencies of changes in the volume and compressibility of sunflower oil on pressure were investigated. A discontinuous (step) change was observed indicating the appearance of the first order phase transformation. The phase transition for sunflower oil was observed in the pressure range from 450 to 500 MPa after about 170 h. Most likely, then, there was a change to the double crystal phase. The time after which this transformation took place was the longest of all times that the authors have observed so far for the study.

Role of Disordered Precursor in L10 Phase Formation in FePt-Based Nanocomposite Magnet

In order to prove the usefulness of having a structurally disordered precursor to the formation of FePt L10 phase and to facilitate the co-existence of exchange coupled hard and soft magnetic phases with optimized magnetic properties in various conditions of annealing, a Fe-Pt-Zr-B melt spun alloy has been synthesized and detailed structural and magnetic investigations have been undertaken to probe its phase evolution during annealing. The dynamics of formation of the hard magnetic L10 phase during the gradual disorder–order phase transformation has been monitored by using a complex combination of X-ray diffraction methods and 57Fe Mössbauer spectroscopy methods, over a wide range of annealing temperatures. Multiple phases co-existing in the annealed sample microstructures, observed in XRD, have been reconfirmed by the Mössbauer spectra analysis and, moreover, accurate quantitative data have been acquired in what concerns the relative abundance of each of the observed crystalline phases in every stage of annealing. It is shown that the formation of the hard magnetic phase, emerging from the chemically disordered precursor, is gradual and occurs via complex mechanisms, involving the presence of a disordered Fe-Zr-B-rich intergranular region which contributes to an increase in the abundance of the L10 phase for higher annealing temperatures. Magnetic measurements have confirmed the good performances of these alloys in terms of coercivity and remanence. These results contribute to the development of these alloys as the next generation of rare earth, free permanent magnets.

A nanostructural approach to the interfacial phenomena in spark plasma sintered TiB2 ceramics with vanadium and graphite additives

A novel TiB2-based composite, with metallic vanadium and nano-graphite additives, was fabricated by spark plasma sintering. A comprehensive investigation was carried out to clarify the sintering behavior and interfacial phenomena. The in-situ synthesis of crystalline V3B4, V6C5, and TiC phases was proved. The formation of V6C5 was attributed to the ordering phase transformation promoted by carbon vacancies in the initially-synthesized VC. TiC was formed due to the reduction of TiO2 impurity with graphite additive. Simultaneous addition of vanadium and graphite not only resulted in the reduction of B2O3 impurity, but also the synthesis of V3B4 nano-crystals, which shows the critical role of oxide impurities on the final phase arrangement. The liquid state of B2O3 at high temperatures promoted the possibility of such reactions. The in-situ formation of such phases led to significant improvements in the mechanical properties. Compared to the monolithic TiB2 consolidated through similar conditions, 50% increase in fracture toughness and 27% enhancement in elastic modulus were observed.

Magnetic Phase Coexistence and Hard-Soft Exchange Coupling in FePt Nanocomposite Magnets.

With the aim of demonstrating phase coexistence of two magnetic phases in an intermediate annealing regime and obtaining highly coercive FePt nanocomposite magnets, two alloys of slightly off-equiatomic composition of a binary Fe-Pt system were prepared by dynamic rotation switching and ball milling. The alloys, with a composition Fe53Pt47 and Fe55Pt45, were subsequently annealed at 400 °C and 550 °C and structurally and magnetically characterized by means of X-ray diffraction, 57Fe Mössbauer spectrometry and Superconducting Quantum Interference Device (SQUID) magnetometry measurements. Gradual disorder–order phase transformation and temperature-dependent evolution of the phase structure were monitored using X-ray diffraction of synchrotron radiation. It was shown that for annealing temperatures as low as 400 °C, a predominant, highly ordered L10 phase is formed in both alloys, coexisting with a cubic L12 soft magnetic FePt phase. The coexistence of the two phases is evidenced through all the investigating techniques that we employed. SQUID magnetometry hysteresis loops of samples annealed at 400 °C exhibit inflection points that witness the coexistence of the soft and hard magnetic phases and high values of coercivity and remanence are obtained. For the samples annealed at 500 °C, the hysteresis loops are continuous, without inflection points, witnessing complete exchange coupling of the hard and soft magnetic phases and further enhancement of the coercive field. Maximum energy products comparable with values of current permanent magnets are found for both samples for annealing temperatures as low as 500 °C. These findings demonstrate an interesting method to obtain rare earth-free permanent nanocomposite magnets with hard–soft exchange-coupled magnetic phases.