Temperature-induced Phase Transformation Research Articles

Evolution of Martensitic Nanostructure in NiAl Alloys: Tip Splitting and Bending

A phase-field (PF) model for the phase transformation (PT) between austenite and martensite and twinning between two martensite is presented where PT is described by a single order parameter. Such a description helps us to obtain the analytical solution of interface energetics and kinetics. PF-elasticity problems are solved for cubic-to-tetragonal PT in NiAl. The stress and temperature-induced PT and corresponding twinning and growth of the martensitic phase inside a nanocrystal are simulated. It reproduces nontrivial experimentally observed nanostructure such as splitting and bending of martensitic nanostructure as well as twins crossing. The evolution and morphology of such interesting nanostructures are discussed.

Multiresponsive Spin Crossover Driven by Rotation of Tetraphenylborate Anion in an Iron(III) Complex

Dynamic molecular materials, which involve mechanical motions in crystals, usually exhibit tunable properties related to conformational polymorphs. Herein, we describe a solvent-free molecular crys...

Eutectic crystallized FePd nanoparticles for liquid metal magnet.

Magnetically hard nanoparticles have been widely explored in colloidal solution synthesis, while a high temperature-induced phase transformation is indispensable to achieve its high magnetocrystalline anisotropy. However, a long-standing challenge of magnetic nanoparticles is the inaccessibility of size-controlled growth without sintering-induced agglomeration. Here, we report a universal one-pot eutectic reaction scheme of magnetically hard FePd nanoparticles, in which the crystallization conditions are critical for its magnetic performance. We demonstrate that the stoichiometry between transition metal and eutectic salt and sintering temperature can play an important role in the magnetic coercivity of FePd nanoparticles. In addition, gallium liquid metal is employed as the conductivity filler for the formation of a magnetorheological fluid after mixing with metallic FePd nanoparticles. The liquid composite shows a high metallic and thermal conductivity as an unconventional cooling metallic ferrofluid conductor, and we further demonstrate its potential application in sensors, conductors and thermal interfaces.

Evolution of hard Fe–C electrodeposits with temperature

ABSTRACT The high microhardness of electrodeposited Fe–C coatings with around 800 HV after electrodeposition increased to about 1300 HV by dedicated annealing, reflecting a huge potential for surface engineering. The coatings remain nanocrystalline during annealing, but co-deposited carbon and oxygen result in temperature-induced phase transformations. Energy-dispersive synchrotron diffraction revealed that carbides and oxides form during annealing of the as-deposited coating, which consisted mainly of carbon-free ferrite. The understanding of occurring precipitations was supplemented and verified by in-situ thermal analysis, which revealed the temperatures of thermal events related to decomposition reactions, the evolution of gasses and associated mass changes during annealing.

Size, structure, and luminescence of Nd2Zr2O7 nanoparticles by molten salt synthesis

Pyrochlore materials with novel properties are in demand with multifunctional applications such as optoelectronics, scintillator materials, and theranostics. Many reports have already indicated the importance of the synthesis technique for Nd2Zr2O7 (NZO) nanoparticles (NPs); however, no explanation has been provided for the reason behind the nature of its phase selectivity. Here, we have explored the structural and optical properties of the NZO NPs synthesized by a molten salt synthesis method. We have synthesized size-tunable NZO NPs and correlated the particle size with their structural behavior and optical performance. All NZO NPs are stabilized in defect fluorite phase. Neutron diffraction provided insight on the behavior of oxygen in the presence of heavy atoms. We have collected bright amalgam of blue and green emission on UV irradiation due to the presence of oxygen vacancies from these NPs. We have carried out in situ XRD and Raman investigations to observe the temperature-induced phase transformation in a controlled argon environment. Interestingly, we have not observed phase change for the molten salt synthesized fluorite NZO NPs; however, we observed phase transformation from a precursor stage to pyrochlore phase by in situ XRD directly. These observations provide a new strategy to synthesize nanomaterials phase-selectively for a variety of applications in materials science.

Multi-Objective Design Optimization of a Shape Memory Alloy Flexural Actuator

This paper presents a computational model and design optimization strategy for shape memory alloy (SMA) flexural actuators. These actuators consist of curved SMA wires embedded within elastic structures; one potential application is positioning microcatheters inside blood vessels during clinical treatments. Each SMA wire is shape-set to an initial curvature and inserted along the neutral axis of a straight elastic member (cast polydimethylsiloxane, PDMS). The elastic structure preloads the SMA, reducing the equilibrium curvature of the composite actuator. Temperature-induced phase transformations in the SMA are achieved via Joule heating, enabling strain recovery and increased bending (increased curvature) in the actuator. Actuator behavior is modeled using the homogenized energy framework, and the effects of two critical design parameters (initial SMA curvature and flexural rigidity of the elastic sleeve) on activation curvature are investigated. Finally, a multi-objective genetic algorithm is utilized to optimize actuator performance and generate a Pareto frontier, which is subsequently experimentally validated.

The Analysis of Thermal-Induced Phase Transformation and Microstructural Evolution in Ni-Ti Based Shape Memory Alloys By Molecular Dynamics

Shape memory alloys has been widely applied on actuators and medical devices. The transformation temperature and microstructural evolution play two crucial factors and dominate the behavior of shape memory alloys. In order to understand the influence of the composition of the Ni-Ti alloys on the two factors, molecular dynamics was adopted to simulate the temperature-induced phase transformation. The results were post-processed by the martensite variant identification method. The method allows to reveal the detailed microstructural evolution of variants/phases in each case of the composition of Ni-Ti. Many features were found and having good agreement with those reported in the literature, such as the well-known Rank-2 herringbone structures; the X-interface; Ni-rich alloys have lower transformation temperature than Ti-rich alloys. In addition, some new features were also discovered. For example, the Ti-rich alloys enabled an easier martensitic transformation; the nucleated martensite pattern determined the microstructural evolution path, which also changed the atomic volume and temperature curves. The results generated in the current study are expected to provide the design guidelines for the applications of shape memory alloys.

High temperature-mediated rocksalt to wurtzite phase transformation in cadmium oxide nanosheets and its theoretical evidence.

Herein, a high temperature-induced phase transformation (PT) in chemically grown CdO thin films is demonstrated, and its corresponding electronic origin further investigated by density functional theory. In particular, the cubic rocksalt to hexagonal wurtzite PT in the CdO thin film annealed at 900 °C was confirmed by X-ray diffraction (XRD), which was consistent with the high-resolution transmission electron microscopy (TEM) results. Moreover, atomic force microscopy and scanning electron microscopy clearly evidenced the morphological evolution via the formation of a nanosheet network in the wurtzite-phase CdO film. The high temperature treatment also led to a significant enhancement in the optical band gap from 2.2 to 3.2 eV, as manifested by UV-visible spectroscopy. The enhanced surface roughness of the nanosheet caused a deviation in the net dipole moment, which may break the polarizable bonds and help in reducing the average dielectric constant, resulting in a band gap opening for the transformed phase. Furthermore, X-ray absorption spectroscopy at the oxygen k-edge revealed a notable shift in the inflection point of the absorption edge, while the X-ray photoelectron spectroscopy (XPS) Cd 3d and O 1s spectra suggested a gradual reduction in the CdO2 phase with an increase in annealing temperature. In addition, different complementary techniques including Rutherford backscattering and Raman spectroscopy were exploited to understand the aforementioned PT and its structural correlation. Finally, molecular dynamics simulation together with density functional theory calculation suggested that the symmetry modification at the Brillouin zone boundary provides a succinct signature for the PT in the CdO thin film.

Mechanism of high temperature induced phase transformation and magnetic properties of Mn3O4 crystallites

Abstract This paper is an experimental study of the different phases of oxides of manganese, prepared by solution combustion method using mixture of urea and glucose as fuels. To validate the thermal stability of Mn2O3, the as-synthesized sample was subjected to heat treatment from 350 °C to 1000 °C with 100 °C interval. The change in Mn2O3 to Mn3O4 was identified by powder X-ray diffraction (PXRD) to show the temperature dependent phase transformation that occur during the heat treatment process. The PXRD patterns show that Mn2O3 is stable between temperatures of 350 °C to 850 °C, and a new phase i.e Mn3O4 appears at 1000 °C. The Mn2O3 and Mn3O4 have their distinct surface morphologies viz., spherical and rod shapes. The detailed morphology, elemental compositions and structure investigation were carried out by using Scanning Electron Microscopy (SEM-EDAX) and Raman spectroscopy. Sintered sample of Mn3O4 were subjected to magnetic measurements. The Mn3O4 crystallite with diameters of 50 nm shows ferromagnetic behavior with a phase transition from ferromagnetic to paramagnetic phase at 48 K. A large coercivity up to 3000 Oe and a small remanence of 0.1092 emu/g at 10 K, was observed. The temperature dependent magnetization measured in a magnetic field of 100 Oe, increases significantly with decreasing temperature around 45 K, both for the FC and ZFC measurements. The susceptibility measurement of the Mn3O4 crystallites were investigated between 0 and 300 K by applied magnetic fields up to 5 T. A magnetic transition is observed near 48 K which is attributed to the formation of the ferrimagnetic Mn3O4.

Temperature induced phase transformation and in vitro release kinetic study of dihydromyricetin-encapsulated lyotropic liquid crystal

In this paper, several lyotropic liquid crystals (LLCs) formed by AEO20-Tween80 (1:1, wt:wt)/isoamyl acetate/H2O with different mass ratios of 45/5/50 (S1), 45/10/45 (S2), 45/15/40 (S3), 35/20/45 (S4), 35/15/50 (S5),35/10/55 (S6) were prepared to encapsulate dihydromyricetin (DMY). The effect of temperature on the rheological properties of LLCs loaded DMY were further studied. Interestingly, S1 with a high AEO20-Tween80/isoamyl acetate ratio of 45/5 showed the same viscoelastic liquid-like properties at temperature range from 25 °C to 45 °C, corresponded to the stable hexagonal phase (H1) structure confirmed by small angle X-ray scattering (SAXS). While in vitro release results of S1 showed that the drug cumulative release percentage and release rate gradually increased as temperature rose, indicating that the release behaviors were dominated by temperature. As a contrast, by decreasing AEO20-Tween80/isoamyl acetate ratio to 35/15 (S5), an obvious transition from elastic gel-like properties to viscoelastic liquid-like properties appeared with the increase of temperature, corresponded to the mixed cubic phase changing to the lamellar phase (Pn3m + Fd3m at 25 °C → H1 + Fd3m at 31 °C → H1 + Lα at 37 °C → Lα at 45 °C). In vitro release results of S5 showed that the drug cumulative release rate dramatically increased with temperature increased, implying that the release behaviors were simultaneously dominated by structure and temperature. This provides a possibility for exploring controlled release liquid crystal carriers of dihydromyricetin.

Real time in situ x-ray diffraction study of the crystalline structure modification of Ba0.5Sr0.5TiO3 during the post-annealing

We report about an in situ study of crystalline structural changes during thermal treatment of a Ba0.5Sr0.5TiO3 (BSTO) film grown on MgO. The study covers the complete cycle of heating, annealing and cooling and reveals simultaneous phenomena of phase transitions and strain evolution, which have been characterized by in situ 2D reciprocal space mapping (2D-RSM) using high-resolution synchrotron x-ray diffraction in coplanar and grazing incidence geometries. In this way, temperature induced phase transformation from the BSTO2 to the BSTO1 phase has been monitored and the appearance of a further crystalline phase was detected. Moreover, for both BSTO phases, transitions between in-plane compressive and tensile states have been determined during thermal treatment. Furthermore, a contraction of the out-of-plane lattice components has been observed during the annealing phase while the in-plane lattice components remain leading to the change of the residual in-plane strain towards tensile state. The in situ 2D-RSM findings provide valuable and versatile insights into strain engineering and structure modification upon thermal treatment.

Adaptive locally resonant metamaterials leveraging shape memory alloys

Locally resonant metamaterials leveraging shape memory alloy (SMA) springs are explored in this work in an effort to develop adaptive metamaterial configurations that can exhibit tunable bandgap properties as well as enhanced damping capabilities. An analytical model for a locally resonant metamaterial beam in transverse vibrations is combined with an SMA model for the resonator springs to investigate and leverage the potential of temperature-induced phase transformations and stress-induced hysteretic behavior of the springs. Two case studies are presented for this new class of smart metamaterials and the resulting finite metastructures. In one case, SMA resonators operate in the linear elastic regime, first at low temperature (martensitic behavior) and then at high temperature (austenitic behavior), demonstrating how the bandgap can be tuned to a different frequency range by altering the SMA elastic modulus with temperature. In the second case, the SMA springs are kept at high temperature at all times to operate in the nonlinear regime, so that the hysteresis associated with the SMA pseudoelastic effect is manifested, yielding additional dissipation over a range of frequencies, especially for the modes right outside the bandgap.

Coexistence and compatibility of martensite reorientation and phase transformation in high-frequency magnetic-field-induced deformation of Ni-Mn-Ga single crystal

High-frequency magnetic-field-induced Martensite Reorientation (MR) is one of the most important advantages of Ferromagnetic Shape Memory Alloys (FSMAs), but its stability is threatened by dissipation heat accumulation (“self-heating”) of cyclic frictional twin boundary motion, which can cause temperature-induced Phase Transformation (PT) and reduce the output-strain amplitude significantly. In this paper, the interaction of the temperature-induced PT and the magnetic-field-induced MR during high-frequency magnetic actuation on FSMA is studied with in-situ observations of local-strain evolution in conjunction with microstructure compatibility analysis. Based on the nominal strain and temperature responses and the corresponding local-strain maps, it is revealed that, when the temperature-induced PT takes place during the high-frequency field-induced MR, the specimen is divided into three zones: non-active austenite zone (with a constant deformation), active martensite zone (with cyclic deformations of MR) and buffering needle zone (interfacial zone) with a fine-needle-twin structure which plays an important role in maintaining the compatibility between austenite and martensite zones with different cyclic deformations during the dynamic loading. A novel mechanism is revealed that, under the magnetic actuation with changing ambient airflow, the “self-heating” temperature-driven phase boundary motion and the magnetic-field-driven twin boundary motion can coexist, because the specimen needs to self-organize the different phases/variants to satisfy all the thermo-magneto-mechanical boundary conditions. Taking advantage of this mechanism, the volume fractions of austenite and martensite zones can be adjusted with changing ambient airflow velocity, which provides an effective way to tune the nominal output strain amplitude (from 1% to 6% in the current study) while the working temperature is kept almost constant (around Ms and Mf).

Coupled behavior of shape memory alloy-based morphing spacecraft radiators: experimental assessment and analysis

Thermal control is an important aspect of spacecraft design, particularly in the case of crewed vehicles, which must maintain a precise internal temperature at all times in spite of significant variations in the external thermal environment and internal heat loads. Future missions beyond low Earth orbit will require radiator systems with high turndown ratios, defined as the ratio between the maximum and minimum heat rejection rates achievable by the radiator system. Current radiators are only able to achieve turndown ratios of 3:1, far less than the 12:1 turndown ratio requirement expected for future missions. An innovative morphing radiator concept uses the temperature-induced phase transformation of shape memory alloy (SMA) materials to achieve turndown ratios that are predicted to exceed 12:1 via substantial geometric reconfiguration. Developing mathematical and computational models of these morphing radiators is challenging due to the strong two-way thermomechanical coupling not present in traditional fixed-geometry radiators and not widely considered in the literature. Although existing simulation tools are capable of analyzing the behavior of some thermomechanically coupled structures, general problems involving radiation and deformation cannot be modeled using publicly available codes due to the complexity of modeling spatially evolving boundary fields. This paper provides important insight into the operational response of SMA-based morphing radiators by employing computational tools developed to overcome previous shortcomings. Several example problems are used to demonstrate the novel radiator concept. Additionally, a prototype morphing radiator was designed, fabricated, and tested in a thermal environment compatible with mission operations. An associated finite element model of the prototype was developed and executed. Model predictions of radiator performance generally agree with the experimental data, giving confidence that the tools developed are able to accurately represent the thermomechanical coupling present in morphing radiators and that such tools will be useful in future designs.

Phase-field approach for stress- and temperature-induced phase transformations that satisfies lattice instability conditions. Part 2. simulations of phase transformations Si I[formula omitted] Si II

A complete system of equations of the advanced phase-field theory for martensitic phase transformations (PTs) under a general stress tensor is presented. Theory includes a fully geometrically nonlinear formulation for the general case of finite elastic and transformational strains as well as anisotropic and different elastic properties of phases. Material parameters are calibrated, in particular, based on the crystal lattice instability conditions from atomistic simulations for martensitic PTs between cubic Si I and tetragonal Si II phases under complex triaxial compression-tension loading. A finite element algorithm and numerical procedure is developed and implemented in the code deal.II. Various 3D problems on lattice instabilities and following nanostructure evolution in single-crystal silicon are solved for compression in one direction under lateral stresses and analyzed. Strong effects of the stress states and local stress hysteresis on the interface width and nanostructure evolution are presented. In particular, the interface width diverges when lateral stress tends to the region in which instability stresses for direct and reverse PTs coincide. Direct and reverse transformations both occur in the unique homogeneous way without hysteresis, energy dissipation, and damage due to internal elastic stresses. Stress fields within a sample and especially within interfaces are determined and their effect on the nanostructure evolution is analyzed. Problems with definition of the elastic interfacial tension (stress) are analyzed. It is demonstrated that the instability stresses for initiation of the PTs are independent of the prescribed stress measure; however, this does not mean that PT will be completed at such stresses.

Thermal effects on high-frequency magnetic-field-induced martensite reorientation in ferromagnetic shape memory alloys: An experimental and theoretical investigation

Ferromagnetic Shape Memory Alloys (FSMAs) exhibit large strains by the magnetic-field-induced martensite reorientation. But, due to the high-frequency field-induced cyclic frictional martensite twin boundary motion in FSMAs, the dissipation heat can cause a large temperature rise. Thus, the output strain amplitude of FSMAs would decrease significantly if the temperature increases to be high enough to trigger the Martensite-Austenite phase transformation. Such thermal effects on the dynamic responses of FSMAs are unclear in literature because most existing dynamic experiments were performed only for a short-time period (a few seconds) to avoid the temperature rise. In this paper, systematic long-time experiments (>100 s) on a Ni-Mn-Ga single crystal are conducted at various levels of magnetic field frequency, initial compressive stress and ambient airflow velocity. It is found that, during the long-time actuation, the specimen temperature increases and then saturates at a certain level (stable temperature) while the strain oscillation evolves to a stable cycle; both the stable temperature and the stable strain amplitude depend on the frequency, the stress level and the heat exchange condition (i.e., ambient airflow velocity). Particularly, when the specimen temperature reaches a critical level to partially transform the martensite to the austenite, the output strain amplitude reduces suddenly because of less martensite reorientation. Changing the ambient heat-exchange condition (by the airflow) can modify the specimen temperature evolution to avoid the phase transformation, but it also changes the behaviors of the martensite reorientation that is sensitive to temperature. Eventually, the output strain amplitude depends on the airflow velocity non-monotonically, i.e., there exists a critical heat exchange condition to achieve the maximum stable strain amplitude. Based on the systematic experiments and a simplified one-dimensional heat-transfer model, the critical condition can be determined. The new experimental phenomena of the thermal effects can be well understood and described by the heat-transfer model. Further, instead of avoiding the temperature rise and the phase transformation, we propose to take advantage of the interaction between the temperature-induced phase transformation and the magnetic-field-induced martensite reorientation to develop a special “isothermal” FSMA actuator with a tunable output strain amplitude and a constant working temperature. This paper provides systematic experimental data and theoretical analysis for understanding the thermo-magneto-mechanical coupling in FSMAs and developing reliable high-frequency long-time running FSMA-actuators.

Phase field approach for stress- and temperature-induced phase transformations that satisfies lattice instability conditions. Part I. General theory

Recently, results of molecular dynamics (MD) simulations were obtained for the crystal lattice instability conditions for the phase transformations (PTs) between semiconducting Si I and metallic Si II under action of all six components of the stress tensor (Levitas et al. (2017a, b)). These conditions are linear in terms of stresses normal to the cubic faces of Si I and are independent of the shear stresses. In the current paper, we (a) formulated the requirements for the thermodynamic potential and transformation deformation gradient tensors and (b) developed a phase field approach (PFA) for the stress-induced martensitic PTs for large strains while allowing for interfacial stresses, which are consistent with the obtained instability conditions. The general system of equations for coupled PFA and nonlinear elasticity is presented. Crystal lattice instability criteria are derived within a PFA, and it is proven that they are independent of the prescribed stress measure. In order to reproduce the lattice instability conditions obtained with MD: (a) one has to use the fifth degree polynomial interpolation functions of the order parameter for all material parameters; (b) each component of the transformation strain tensor should have a different interpolation functions; and (c) the interpolation functions for tensors of the elastic moduli of all ranks should have zero second derivatives for the parent and product phases, so that terms with elastic moduli, which are nonlinear in stresses, do not contribute to the lattice instability conditions. Specific interpolation and double-well functions have been derived for all parts of the Helmholtz free energy and for two models for the transformation deformation gradient. For these models, explicit expressions for the Ginzburg-Landau equations and lattice instability conditions are derived. Material parameters have been calibrated using results of MD simulations. In Part II of this paper, the developed model is further refined and studied, and applied for the finite element simulations of the nanostructure evolution in Si under triaxial loading.

Nanoscale multiphase phase field approach for stress- and temperature-induced martensitic phase transformations with interfacial stresses at finite strains

A thermodynamically consistent, novel multiphase phase field approach for stress- and temperature-induced martensitic phase transformations at finite strains and with interfacial stresses has been developed. The model considers a single order parameter to describe the austenite↔martensitic transformations, and another N order parameters describing N variants and constrained to a plane in an N-dimensional order parameter space. In the free energy model coexistence of three or more phases at a single material point (multiphase junction), and deviation of each variant-variant transformation path from a straight line have been penalized. Some shortcomings of the existing models are resolved. Three different kinematic models (KMs) for the transformation deformation gradient tensors are assumed: (i) In KM-I the transformation deformation gradient tensor is a linear function of the Bain tensors for the variants. (ii) In KM-II the natural logarithms of the transformation deformation gradient is taken as a linear combination of the natural logarithm of the Bain tensors multiplied with the interpolation functions. (iii) In KM-III it is derived using the twinning equation from the crystallographic theory. The instability criteria for all the phase transformations have been derived for all the kinematic models, and their comparative study is presented. A large strain finite element procedure has been developed and used for studying the evolution of some complex microstructures in nanoscale samples under various loading conditions. Also, the stresses within variant-variant boundaries, the sample size effect, effect of penalizing the triple junctions, and twinned microstructures have been studied. The present approach can be extended for studying grain growth, solidifications, para↔ferro electric transformations, and diffusive phase transformations.

Influence of Temperature on the Mechanical Properties and Reactive Behavior of Al-PTFE under Quasi-Static Compression.

Al-PTFE (aluminum-polytetrafluoroethylene) is a typical kind of Reactive Material (RM), which has a variety of potential applications in weapon systems. In this paper, quasi-static compression experiments were carried out for a pressed and sintered mixture of Al and PTFE powders using a microcomputer-controlled electronic universal testing machine. The results show that both the mechanical property and reactive behavior of Al-PTFE are strongly temperature-dependent. The material undergoes a brittle-ductile transition associated with a temperature-induced crystalline phase transformation of the PTFE matrix. At low temperatures (−18, 0, and 16 °C), samples of Al-PTFE failed with shear crack and no reaction was observed. As the temperature increased (22, 35, and 80 °C), Al-PTFE exhibited a high toughness and violent reaction occurred in all of the tested samples. Scanning electron microscope observations showed different fracture mechanisms of the PTFE matrix and the increase in toughness was due to the formation of PTFE fibrils which could dissipate energy and bridge crack plane during plastic deformation.

Temperature induced phase transformations and negative electrocaloric effect in (Pb,La)(Zr,Sn,Ti)O3 antiferroelectric single crystal

Temperature induced phase transitions and electrocaloric effect (ECE) of (Pb,La)(Zr,Sn,Ti)O3 (PLZST) single crystals have been comprehensively studied. Based on the in situ evolution of domain structures and dielectric properties of the PLZST crystals, the phase transitions during heating are in the sequence of orthorhombic antiferroelectric → rhombohedral ferroelectric → cubic paraelectric. Coexistence of the negative and positive ECEs has been achieved in the PLZST single crystals. A negative ECE value of −1.26 °C and enhanced electrocaloric strength of −0.21 K mm/kV near the Curie temperature have been obtained. A modified Landau model gives a satisfactory description of the experimentally observed unusual ECE. Moreover, a temperature–electric field phase diagram is also established based on theoretical analysis. Our results will help people understand better the electrocaloric family, particularly on the negative and/or positive effect in antiferroelectrics and ferroelectrics.