Temperature-induced Phase Transformation Research Articles

Structural Phase Transitions of Mg2NiH4 Induced by NiH4 Molecular Ion Distortion and Metal Lattice Deformation under High Pressures.

Experimental observations indicate that pressure treatments transform the monoclinic and orthorhombic low-temperature phases of Mg2NiH4 into an unresolved pseudocubic phase even at room temperatures, resembling the temperature-induced phase transformation of the material. This pressure-induced phase transformation is anticipated to involve deformations of the tetrahedrally bonded NiH4 molecular ion and the metal lattice. However, the precise mechanism underlying this transformation remains unclear. To elucidate this behavior, this study adopted first-principles density functional theory calculations to investigate the effect of applied pressure on the observed phase transition. The results revealed that the phase transition from the low-temperature phases to the pseudocubic phase occurs at approximately 8-9 GPa. Furthermore, at this pressure, the metal lattice deforms into an antifluorite-like structure, and the NiH4 molecular ion undergoes substantial distortion, resulting in the alignment of hydrogen atoms along the crystal axes of the metal lattice. Furthermore, through a comprehensive structural exploration, we successfully identified several tetragonal and orthorhombic models that are energetically more stable than the low-temperature phases at low pressures. These models are potential candidates for elucidating the pseudocubic phase observed in experiments. Overall, our findings are anticipated to enhance the current understanding of the structural changes occurring during pressure and temperature-induced phase transitions.

Effect of particle stiffness on microgel self-assembly and suspension phase behavior over a broad temperature range

We synthesized thermoresponsive poly(N-isopropylacrylamide) (PNIPAM) colloidal microgel particles of different stiffnesses by controlling the concentration of a polar crosslinker in a precipitation polymerization synthesis method. When suspended in an aqueous medium, the particles collapsed by expelling water as the temperature was raised toward the volume phase transition temperature (VPTT) of ≈ 34 °C. We noted that the sizes of the stiffer particles, synthesized with higher crosslinker concentration, collapsed less abruptly. Using Fourier transform infrared spectroscopy, we observed enhanced particle dehydration with increasing temperature and decreasing particle stiffness. Oscillatory rheology experiments on dense aqueous PNIPAM suspensions, prepared at a fixed particle effective volume fraction ϕeff = 1.5 at 25°C, revealed that suspensions constituted by the stiffest particles are the most elastic over a broad temperature range. Above the VPTT, suspensions of particles of intermediate stiffnesses exhibited two-step yielding, a typical signature of fragile gel formation. Zeta potential measurements showed that PNIPAM particles of lower stiffnesses are rendered electrostatically unstable in aqueous suspension. Combining cryogenic scanning electron microscopy and rheology, we noted a glass–glass transition when the temperature of a dense suspension of stiff PNIPAM particles was raised across the VPTT. In contrast, suspensions of particles of the lowest stiffnesses showed a gel-viscoelastic liquid–gel transition during an identical temperature ramp experiment. Our study reveals that temperature-induced phase transformations in dense PNIPAM suspensions depend sensitively on the stiffness of the constituent particles and can be explained by considering amphiphilicity-driven morphological changes in the suspension microstructures.

Unveiling Temperature-Induced Structural Phase Transformations and CO2 Binding Sites in CALF-20.

The increase in atmospheric carbon dioxide concentration linked to climate change has created a need for new sorbents capable of separating CO2 from exhaust gases. Recently, an easily produced metal-organic framework, CALF-20, was shown to withstand over 450,000 adsorption/desorption cycles in steam and wet acid gases. Further development and industrial application of such materials require an understanding of the observed processes. Herein, we demonstrate that conditioning as-synthesized CALF-20 single crystal transforms it into a different phase, γ-CALF-20. The transformation resulted in significant structural changes, including the binding of water molecules to Zn(II), accompanied by a reduction of 9% in the unit cell volume. Our experimental findings were supported by the energy-volume dependence of CALF-20 in the presence and absence of water molecules calculated from density functional theory. We have also monitored the sorption process of the dominant greenhouse gas, CO2, on the initial phase of CALF-20 at atomic resolution using in situ single-crystal X-ray diffraction under specific pressure. The new understanding of CALF-20 chemistry from these studies should facilitate development of novel sorbents for gas adsorption technologies.

Physical mechanism for nanocrystalline NiTi alloy with ultrasmall-sized grains exhibiting one-way shape memory effect without temperature-induced phase transformation

In the present work, a novel finding is that nanocrystalline NiTi alloy with ultrasmall-sized grains (where the diameter of grains is less than or equal to 10 nm) presents one-way shape memory effect (OWSME) without temperature-induced phase transformation, where the involved physical mechanism is revealed by combining experiments with molecular dynamics simulation (MD). The results show that fraction of grain boundaries (GBs) is so much that temperature-induced martensitic transformation is suppressed, but it leads to a certain unrecoverable strain. The following loading results in increase of strain and decrease of Gibbs free energy, which leads to stress-induced martensitic transition. After unloading, Gibbs free energy is increased by the release of elastic strain, which leads to partial reverse martensitic transition. Consequently, OWSME in nanocrystalline NiTi alloy with ultrasmall-sized grains is aroused by retained stress-induced martensitic transition followed by its reverse phase transition rather than temperature-induced one followed by its reverse phase transition.

Fabrication of Ln2Zr2O7 Fluorite and LnAlO3 Perovskite (Ln = La, Nd, Sm) Compounds to Catalyze the OCM Reaction: On the Temperature-Induced Phase Transformation and Oxygen Vacancy.

Through synthesizing Ln2Zr2O7 and LnAlO3 (Ln = La, Nd, Sm) catalysts, the origin of active sites for oxidative coupling of methane (OCM) on A2B2O7 fluorite and ABO3 perovskite compounds has been compared and elucidated. Ln2Zr2O7 catalysts show much better reaction performance than the respective LnAlO3 catalysts at low temperatures (500-600 °C), but the difference will be mitigated significantly above 600 °C. The reaction performance ranks in the order of La2Zr2O7 > Nd2Zr2O7 > Sm2Zr2O7 > LaAlO3 > NdAlO3 > SmAlO3. It is revealed that the unit cell free volume (Vf) plays an important role in affecting the catalytic activity, and the Ln2Zr2O7 catalysts with a disordered defect fluorite phase have inherent oxygen vacancies, which can directly activate gas-phase O2 molecules to generate OCM reactive O2- anions. However, the oxygen vacancies of LnAlO3 with a perovskite structure can only be generated by lattice distortion/transformation above 600 °C. Moreover, Ln2Zr2O7 fluorites have weaker B-O bonds than LnAlO3 perovskites, thus making it easier to generate surface vacancies as well as active O2- sites. The surface alkalinity is intimately relevant to the active oxygen species, which act together to decide the OCM performance on both types of catalysts. Indeed, this explains that LnAlO3 catalysts show much worse performance than Ln2Zr2O7 catalysts below 600 °C, which will be evidently improved at elevated temperatures due to phase transformation.

A Thermal, Mechanical, and Materials Framework for a Shape Memory Alloy Heat Engine for Thermal Management.

Shape memory alloy (SMA) heat engines possess an inherent property of sensing a change in temperature, performing work, and rejecting heat through the shape memory effect resulting from a temperature-induced phase transformation. This work presents a framework for the design and implementation of an SMA-based Stirling heat engine for maximum torque or speed incorporating and combining mechanical, thermal, and material aspects. There is a growing need for such engines for reliable thermal management and energy recovery in both ground and space applications. Mechanical aspects were addressed from force balances in the SMA element and focused on the resulting stress distribution. Thermal aspects considered heat transfer between the SMA element and both the heat source and the heat sink. Materials aspects considered the chemical, elastic, and frictional contributions to the enthalpy of the transformation. The roles of nano- and microstructure through composition, precipitates, variant interfaces, training, cycling, texture, defects, nucleation sites (bulk vs. surface), and multi-step transformations (e.g., a trigonal R-phase transformation) in NiTi based-alloys are also emphasized. The aforementioned aspects were combined to present a figure of merit to aid in the design and implementation of a Nitinol Stirling heat engine operating to maximize torque or maximize speed.

An interatomic potential for ternary NiTiHf shape memory alloys based on modified embedded atom method

NiTi-based high temperature shape memory alloys (HTSMAs) can operate in high temperature applications, where binary NiTi cannot function. NiTiHf is among the most popular HTSMAs due to its lower preparation cost and good thermal stability. Various aspects of material properties of NiTiHf and the effect of different factors, such as composition, heat treatment and precipitates, on its behavior is yet to be understood. Molecular dynamics (MD) simulations can provide insights to acquire such an understanding. However, MD simulations of NiTiHf have not been possible due to the absence of a reliable interatomic potential, which is a necessary tool for such simulations. In this study, density functional theory (DFT) simulations were employed and a ternary Second Nearest Neighbor Modified Embedded Atom Method (2NN MEAM) potential was calibrated for NiTiHf. To perform this study, the related unary and binary potentials were calibrated by fitting the values of their reproduced physical properties to the DFT results. The final ternary MEAM potential was checked for reliability and transferability by performing MD simulations. The results showed that the developed potential can appropriately capture the temperature-induced and stress-induced martensitic phase transformations in NiTiHf.

Effects of thermomechanical treatments on Cu–Al–Mn Shape Memory Alloys

In this paper, the authors, building on the established state-of-the-art knowledge, explore the effects of thermomechanical treatments on the Shape Memory Effect (SME) and damping performance of the Cu–Al–Mn Shape Memory Alloys (SMA). Specifically, to explore the potential of application of this SMA in the form of sheets, for the purpose of designing lightweight structures with high damping performance. The casting and the thermomechanical treatment process were fully laid out in the paper. The characterization of the material in terms of chemical composition and microstructure was performed through optical microscopy and SEM coupled with EDX analysis. The temperature-induced phase transformation behavior of the alloy was explored using DSC. The damping performance was quantified through the loss factor which was measured at different levels of static tensile preload using a DMA machine. The alloy has shown significant workability which permits the fabrication of sheets. However, the material displays high sensitivity to oxidation due to a combined effect of impurities and the hot-rolling process necessary for obtaining thin SMA sheets. Nonetheless, the alloy shows competitive damping performance with respect to commercially available NiTi SMA.

A large compressive recoverable strain induced by heterogeneous microstructure in a Ni50.6Ti49.4 shape memory alloy via laser powder bed fusion and subsequent aging treatment

Laser induced highly metastable microstructure provides the possibility for the tunable precipitation behavior and mechanical/functional properties via direct aging treatment. In this paper, Ni-rich NiTi-based samples were fabricated by laser powder bed fusion (LPBF) and then were directly aged for different aging times that spanning three time-scales (10 min-100 h). The results showed that through the aging treatment, a heterogeneous microstructure consisting of Ti 4 Ni 2 O x nanoparticles, nanoscale Ni-rich precipitates, dislocation structures, and martensitic twin variants formed in the matrix. Due to the change in the elastic strain field, the short time aging treatment tended to induce (001) compound twins, while the long time aging assisted in the formation of< 011 > type II twins. The temperature-induced and deformation-induced phase transformation behavior were further studied. With the progress of precipitates, it was found that a rapid evolution from a single-stage phase transformation to a multi-stage one occurred. During the cyclic compression, the sample aged for 1 h showed the most excellent superelasticity with initial recoverable strain of 0.089 and steady recoverable strain of 0.087, as well as good cyclic stability. • Microstructure evolution of LPBF-ed Ni-rich NiTi SMA with the aging time was disclosed. • Effects of aged microstructure on transformation behavior and pseudoelasticity were elaborated. • A large compressive recoverable strain and its good cyclic stability were achieved via the proper aging treatment.

Atomistic simulations of AuTi high-temperature shape memory alloys

Herein, the atomic-scale phase transformation and deformation processes of AuTi shape memory alloys for high-temperature applications were investigated using molecular dynamics simulations based on a newly developed interatomic potential for the Au-Ti binary system. The developed second nearest-neighbor modified embedded-atom method potential exhibited good accuracy and transferability to reproduce various physical properties of the target alloy system, particularly in properties related to the reversible phase transformation of AuTi high-temperature shape memory alloys. As a conceivable application of the interatomic potential, the temperature-induced phase transformation of AuTi single-crystal shape memory alloys and the effect of off-stoichiometry on the phase transformation behavior were examined. Molecular dynamics simulations revealed that the transformation temperature and thermal hysteresis decreased with increasing Ti content, consistent with the reported experimental trend, suggesting that the reduced volume difference between the austenite and martensite phases was closely related to the reduced thermal hysteresis. We further examined the mechanical response and resultant superelasticity of nanocrystalline AuTi alloys to demonstrate that the cyclic deformation of nanocrystalline alloys exhibited a transformation ratcheting behavior caused by the plastic deformation of amorphous-like grain boundary region.

Temperature induced phase transformation in Co

Temperature dependent phase transformation behavior in cobalt from hexagonal close-packed (hcp) to face centered cubic (fcc) has been found to be contradictory to that reported earlier. It is found that hcp phase stabilizes at both low and high temperature (sim 873 K) while fcc phase is stabilized at sim 500 K. At 298 K, hcp Co has been found to be predominant (sim 70%) where hcp magnetic phase is sim 60%. At 973 K, hcp phase is again predominant (sim 73%), but it is mainly the non-magnetic phase (sim 67%). Contrary to present results, it was found earlier that fcc phase was stabilized at high temperature and hcp to fcc transformation occured at sim 700 K. Present results from perturbed angular correlation measurements, therefore, requires a new theoretical interpretation for Co phase transformation. From present measurements, hyperfine magnetic fields in Co at room temperature for the hcp and fcc phases have been found to be 18.7(6) and 12.8(3) T, much lower than earlier reported results. The hyperfine magnetic fields at ^{181}Ta impurity atom have been calculated by density functional theory (DFT) employing the full potential (linearized) augmented plane wave method (FP-LAPW). Present calculated results for both hcp and fcc phases corroborate our experimental results.

A flexible and smart shape memory alloy composite sheet based on efficient and bidirectional thermal management

ABSTRACT Leveraging advances in shape memory alloys (SMAs) and flexible thermoelectric devices (f-TEDs), this paper presents a structural and functional integrity composite sheet to address the inefficient and bulky activation of 2-D SMAs. A series of experimental tests were performed to reveal the generation, change, and transfer mechanisms of different types of electrically-induced heat in the f-TED, as well as the temperature-induced SMA phase transformation behaviors. The results show that the composite sheet exhibited good bidirectional thermal management capacity. The austenite deformation can be completed within 10s at an operating current of 2 A. The cooling recovery, in particular, performs much better than in conventional modes (the temperature declines exponentially with time). Finally, through two functional prototypes, a light switch and a flexible gripper, the application potential of the proposed composite was further experimentally demonstrated.

Manipulation of wave motion in smart nonlinear phononic crystals made of shape memory alloys

Thanks to the functional role of shape memory alloys (SMAs) in controlling the mechanical behavior of structures, researchers have started investigating the possibility of manipulating wave motion in phononic crystals using SMAs. While SMAs were used before to tune the wave propagation in linear phononic crystals, in this work, we aim to extend their utilization to nonlinear lattices. For this purpose, SMA helical springs are used to manipulate the dispersion curves and the location of stop-bands in weakly nonlinear monoatomic and diatomic lattice chains. Using Brinson’s formulation to describe the thermo-mechanical behavior of SMA wires and Lindstedt-Poincaré method to solve the derived governing equations, closed-form nonlinear dispersion relations in monoatomic and diatomic lattice chains are obtained and the effects of temperature-induced phase transformation and stiffness nonlinearity on the wave propagation are investigated. The results reveal that the dispersion curves of a weakly nonlinear monoatomic chain are formed at lower frequencies through the austenite-to-martensite phase transformation. Similarly, both the acoustic and optical branches of a diatomic lattice are moved to lower frequencies during the phase transformation in the cooling process. Therefore, the generated stop-bands in nonlinear diatomic lattices are also moved to lower frequencies. In addition, using auxiliary SMA ground springs, new classes of nonlinear monoatomic and diatomic chains exhibiting additional low-frequency attenuation zones are introduced. These low-frequency stop-bands are tunable and their frequency range can be modulated by exploiting the temperature-induced phase transformation in the SMA springs. The results obtained from analytic formulations are verified by numerical calculations and an excellent agreement is observed. Such tunability and the potential for adding stop-bands in low frequencies reveal that SMAs can be very helpful in designing nonlinear phononic and acoustic devices, such as vibration mitigators and wave filters with pre-defined attenuation zones.

Effects of precipitate on the phase transformation of single-crystal NiTi alloy under thermal and mechanical loads: A molecular dynamics study

Molecular dynamics simulations have been carried out in this study to investigate the stress-induced and temperature-induced phase transformations in NiTi shape memory alloy in the presence of Ni4Ti3 precipitate. NiTi alloys form the most popular class of shape memory alloys that have many applications in various industries. Therefore, computational simulation of the material behavior can extend our understanding of the material properties and help the development of alloys with tailored properties. In the simulations, an Ni4Ti3 precipitate (with a rhombohedral crystal structure) was generated and embedded into the NiTi matrix (having a B2 crystal structure). The transformation temperatures were computed by applying a cooling-heating cycle starting from a high temperature. At the austenitic state (at a temperature above Af), a monotonically increasing uniaxial compressive force was applied to the models with and without precipitate and, as expected, both models exhibited superelastic behavior. Furthermore, similar models were made for different directions of the compressive force relative to the crystallographic directions to study the direction dependence of the mechanical response. For all the loading directions that phase transformation happened, a larger hysteresis occurred for the precipitated model compared to the pristine NiTi due to the hindrance of reverse phase transformation in presence of the precipitate. The results show that the precipitate has a pronounced effect on the superelastic behavior in 〈100〉 loading directions.

Thermodynamic and kinetic mechanism of phase transformation of levofloxacin hydrochloride

In this work, thermodynamic and kinetic factors that affect formation and phase transformation process of different solid forms of levofloxacin hydrochloride were investigated in detail. Dynamic vapor sorption experiments (DVS) and varying temperature-powder X-ray diffraction (VT-PXRD) experiments were carried out to study moisture-dependent stability, thermal stability as well as the transformation process between Form I and Form II. Critical water activities of levofloxacin hydrochloride were determined in temperature range of 5.0–45.0 °C. Raman spectroscopy was applied to in situ monitor the temperature-induced phase transformation and the hydration process of levofloxacin hydrochloride, and one possible mechanism consisting of multiple chemical equilibria was proposed to analyze the effect of thermodynamic and kinetic factors on the formation and transformation of different solid forms. Results show that the anhydrate, Form II would transform to the monohydrate, Form I more easily with the increasing water content. And the transition point moves toward higher water contents as the temperature increases. The results suggested that, except for thermodynamic factors, kinetic factors also play an essential role in controlling the solid forms of polymorphic compounds.

Temperature-induced structural phase transformation in samples of Co3O4 and Co3-xNixO4 for CoO

Undoped and Ni-doped cobalt oxide (Ni=4%) powders were prepared by hydrothermal synthesis and subsequently examined. The powders obtained were subjected to thermal annealing at temperatures ranging between 600 °C and 1000 °C. The experimental results indicate that the annealing affects the structural properties with the phase change Co3O4 to CoO. X-Ray diffraction (XRD) analysis reveals the presence of pure spinel phase up to 800 °C in powders obtained with and without the addition of nickel. From 900 °C, the CoO phase was observed. Rietveld refinement was conducted with main GOF values ​​between 1.09 and 1.26 and also x2 values ​​between 1.28 and 1.57, indicating a good correspondence between the simulated and the experimental pattern. In addition, SEM measurements evidenced that particle size increased with temperature; however, samples doped with Ni exhibited a slightly smaller size compared to pure samples. The synthesized powders showed shifted and broad bands Raman, with increasing temperature, mainly for doped samples. These changes in the position of Raman vibrational modes are related to defects and distortions in the crystal lattice of the material as a result of the increase in temperature and the addition of Ni. Furthermore, such changes may also indicate loss of crystallinity in the material, or the beginning of the phase change.

Architected material analogs for shape memory alloys

Architected material analogs for shape memory alloys

Phase constitution of the noble metal thin-film complex solid solution system Ag-Ir-Pd-Pt-Ru in dependence of elemental compositions and annealing temperatures

Multiple-principal element alloys hold great promise for multifunctional material discovery (e.g., for novel electrocatalysts based on complex solid solutions) in a virtually unlimited compositional space. Here, the phase constitution of the noble metal system Ag-Ir-Pd-Pt-Ru was investigated over a large compositional range in the quinary composition space and for different annealing temperatures from 600 to 900 °C using thin-film materials libraries. Composition-dependent X-ray diffraction mapping of the as-deposited thin-film materials library indicates different phases being present across the composition space (face-centered cubic (fcc), hexagonal close packed (hcp) and mixed fcc + hcp), which are strongly dependent on the Ru content. In general, low Ru contents promote the fcc phase, whereas high Ru contents favor the formation of an hcp solid-solution phase. Furthermore, a temperature-induced phase transformation study was carried out for a selected measurement area of fcc-Ag5Ir8Pd56Pt8Ru23. With increasing temperature, the initial fcc phase transforms to an intermediate C14-type Laves phase at 360 °C, and then to hcp when the temperature reaches 510 °C. The formation and disappearance of the hexagonal Laves phase, which covers a wide temperature range, plays a crucial role of bridging the fcc to hcp phase transition. The obtained composition, phase and temperature data are transformed into phase maps which could be used to guide theoretical studies and lay a basis for tuning the functional properties of these materials.

Deformation induced twinning in hcp/bcc Al10Hf25Nb5Sc10Ti25Zr25 high entropy alloy – microstructure and mechanical properties

High-entropy alloys of hexagonal structure commonly revealing high strength but very limited ductility, still remain a challenging task. Advanced strategy is proposed in present work in order to develop high-entropy alloy of chemical composition Al10Hf25Nb5Sc10Ti25Zr25 at.%. Crystallographic features and microstructures of as-cast and annealed Al10Hf25Nb5Sc10Ti25Zr25 at.% alloy are characterized by X-ray diffraction, scanning and high-resolution transmission electron microscopies, whereas differential scanning calorimetry is used to follow temperature-induced phase transformation in the as-cast system. The cast alloy reveals microstructure containing fine orthorhombic needle-like plates within solid-solution of body-centered cubic structure. Such a dual-phase microstructure leads to enhanced combination of high strength and good ductility. The alloy annealed at 1000 °C for 5h acquires tensile strength increased by 38%, yield strength more than ten times higher, and almost 3-fold increased plasticity as compared with the as-cast alloy. Its Vickers hardness becomes higher by 25%. These meaningful changes are associated with temperature-induced transformation of its microstructure, which include diffusion growth of hexagonal plates enriched in Zr, Sc, and Hf within the bcc-type matrix enriched in Nb and Ti. Microstructure reveals twins within the hcp plates which plays a crucial role in plastic deformation and strengthening of the materials. Ab initio calculations based on the density functional theory are employed to investigate stability and mechanical properties of bcc, hcp, and orthorhombic phases of 5-component Al15Hf25Sc10Ti25Zr25 at.% and 6-component Al10Hf25Nb5Sc10Ti25Zr25 at.% high entropy alloys. The hcp phases of both systems are energetically preferred at T = 0 K, brittle and have rather high Vickers hardness. The bcc phases of both high-entropy alloys stabilize and become energetically favored above 790 K for Al15Hf25Sc10Ti25Zr25 at.% and 1150 K for Al10Hf25Nb5Sc10Ti25Zr25 at.% alloys. Depending on the type of crystallographic structure, the phases enriched in 5 at% of Nb show either reduced or enhanced values of their elastic moduli and Vickers hardness as compared to the Nb-free phases. No transformation from brittle to ductile state and vice versa is observed upon doping the considered phases of Al-Hf-Sc-Ti-Zr system with Nb.

Pressure-dependent Raman scattering of polycrystalline KNb1−xTaxO3 solid solutions

We report experimental and calculated Raman scattering investigations of KNb1−xTaxO3 (x = 0.4, 0.5, 0.6) solid solutions as a function of hydrostatic pressure. The observed phase transitions sequence in the range from room pressure to 12 GPa is similar to the temperature-induced structural phase transformations: orthorhombic (ferroelectric) to tetragonal (ferroelectric) to cubic (paraelectric). Furthermore, it was observed that the domain of stability of ferroelectricity at high pressures increases with Nb-content. For the first time, some DFT calculations of theoretical Raman spectra are reported in order to support the experimental observations of pressure-induced phase transitions.