Temperature-induced Phase Transformation Research Articles

Organic Molecule-Modulated Phase Evolution of Inorganic Mesostructures

The involvement of alkane in the P123-TEOS-NH4F-H3O+ synthesis system alters the phase behavior of the complex emulsion system dramatically. Changing one of the reaction parameters (such as the initial reaction temperature, IRT) will result in diverse solution mesostructures. With subsequent condensation of silicate species, interesting inorganic materials with various mesostructures are obtained. The present work is aimed at understanding the phase evolution behavior of this complex alkane (C6-C12)-P123-TEOS-NH4F-H3O+ emulsion system, with emphasis on the influence of alkane chain number (ACN) and IRT. HREM (high-resolution electron microscopy), XRD (X-ray diffraction), nitrogen sorption, FFEM (freeze-fracture electron microscope), and interfacial tension techniques have been used to investigate the phase behavior of the emulsion system and the structure of the inorganic products. A linear relationship between the phase-transformation temperature (PTT) and ACN has been established, which could be attributed to the modification of alkane with respect to the hydrophobic-hydrophilic properties of the complex emulsion system. Moreover, the right combination of reaction temperature, ACNs, and thus-induced swelling of hydrophobic PPO blocks as well as the modification of hydrophilicity of PEO brushes by silicate oligmers is the driving force in altering the packing parameter/geometry of the copolymers surfactant (P123) aggregates. This leads to the diverse structures of the obtained mesoporous silicas. A temperature-induced phase-transformation mechanism has also been proposed and discussed.

OS1001 多軸応力状態における形状記憶合金の応力および温度誘起相変態挙動の構成式モデル(OS10-01 形状記憶材料の特性と応用1,OS10 形状記憶材料の特性と応用)

Ti-Ni形状記憶合金を対象に,相変態挙動を伴う非弾性変形挙動を記述する構成式モデルを開発した。モデルにおいては,材料の物質点が種々の方位を向いた微視結晶粒からなり,それぞれの微視結晶粒のひずみは物質点のひずみに等しいという等ひずみモデルで記述されるものとしている。そして,それぞれの結晶粒の変態システムにおける分解せん断応力が限界値に達すると,変態・逆変態および変態の再分布が生じるとする。このようにして扱うことにより,変態に際して働くアコモデーション機構を再現することができる。構成式は,応力および温度誘起変態・逆変態を扱うことができ,多軸応力状態での形状記憶挙動および超弾性挙動を記述できる。

Multiscale bifurcation and stability of multilattices

A lattice-level model is developed for active materials, such as shape memory alloys, that undergo martensitic phase transformations. The model is investigated using equilibrium path following and bifurcation techniques. It is shown that a multiscale stability criterion is essential for correctly interpreting the stability of crystal equilibrium configurations under both thermal- and stress-loading conditions. A two-stage temperature-induced phase transformation is predicted from a cubic B2 phase to an orthorhombic Cmmm phase to a final orthorhombic B19 phase. Under stress-loading conditions, martensitic transformations from the B2 austenite phase to a number of possible martensite phases are identified. These include reconstructive transformations to B11, B33, and C2/m structures and proper transformation to a C2/m monoclinic phase which displays characteristic tension-compression asymmetry. The prediction of both temperature-induced and stress-induced proper martensitic transformations indicates the likelihood that the current model will exhibit shape memory behavior.

Effect of temperature on sintered austeno-ferritic stainless steel microstructure

The influence of temperature on microstructural changes of sintered austeno-ferritic steels has been investigated. PM stainless steels have been obtained by sintering mixtures of austenitic and ferritic stainless steel powders. Only temperature-induced phase transformation was observed in austenite, as a result of elements interdiffusion between both phases. Microstructural characterization was completed with atomic force microscopy (AFM) and micro- and nano-indentation test, it is revealed an increase in the hardness with respect to the solutionized materials.

A model for temperature-induced phase transformations in finite-strain elasticity

We propose a model for phase transformations that are driven by changes in the temperature. We consider the temperature as a prescribed quantity like an applied load. The model is based on the energetic formulation for rate-independent systems and thus allows for finite-strain elasticity. Time-dependent Dirichlet boundary conditions can be treated by decomposing the deformation as a composition of a given deformation satisfying the time-dependent boundary conditions and a part coinciding with the identity on the Dirichlet boundary.

On the anelastic contribution to the temperature dependence of lower mantle seismic velocities

There is little agreement regarding the importance of anelastic contributions to the temperature dependence of seismic wave velocities. Some studies consider the anelastic contribution to be negligible, whereas others argue that it may be as important as the purely elastic contribution. In order to evaluate these claims, we review physically plausible models for mantle relaxation processes and construct a model of activation enthalpy based on solid state physics as well as on recent experimental results. We then deduce self-consistent 1-D Q profiles and evaluate the anelastic contribution to the temperature dependence of V s and V p . Physically plausible values of activation enthalpies yield anelastic contributions that increase with depth and account for at least 20% of the velocity temperature derivatives at 2600 km. We show that anelasticity may significantly moderate temperature variations in hot (slow velocity) zones. This suggests that inferred lateral changes in temperature, as well as arguments regarding the presence of chemical heterogeneity, and/or temperature induced phase transformations, are likely to be significantly affected by anelastic effects in the mantle. We also compute values of the dimensionless parameters R s/ p , R ϕ/ s , and R ρ/ s . We find that anelasticity can either increase or decrease R s/ p , but the magnitude of the correction, of the order of 15%, is smaller than the uncertainty in the elastic value. Anelasticity cannot change the sign of R ϕ/ s and R ρ/ s , but it can decrease their values by as much as 50%. However, given current uncertainties in seismic and mineral data, the effects of anelasticity on the dimensionless parameters do not bring any quantitative constraints on lateral variations of temperature, chemical composition and/or mineral phase.

Structural and magnetic properties of hcp and fcc Ni nanoparticles

The face-centered-cubic (fcc) and hexagonal-close-packed (hcp) Ni nanoparticles were synthesized with citrate by sol–gel method and heat-treating technique. The structure, morphology and magnetic properties of the samples were characterized using X-ray diffraction (XRD), transmission electron microscopy (TEM), and vibrating sample magnetometer (VSM). Our XRD and TEM results suggested that hcp Ni nanoparticles were successfully synthesized when the heating temperature reaches 300 °C. With a further increase in temperature to 400 °C, a temperature-induced phase transformation of hcp to fcc was observed. Moreover, the VSM results demonstrated the existence of ferromagnetic behavior in the synthesized fcc and hcp Ni nanoparticles. Nevertheless, the magnetic measurement suggested that the magnetic properties in hcp nanoparticles is probably the sum of two contributions: superparamagnetic and ferromagnetic one. The unsaturated magnetization is much smaller than 47.6 emu/g for the fcc nanoparticles obtained at 400 °C and 55 emu/g for the bulk material. It was also found that changes of the stress, grain size and crystal structure during heat-treating have significant influences on the magnetic properties of the Ni nanoparticles.

Temperature Induced Phase Transformation on the 4H-SiC(11-20) Surface

The atomic structure of the 4H-SiC(11 2 0) surface including possible phase transformations via Si deposition and annealing has been investigated using low energy electron diffraction (LEED), Auger electron spectroscopy (AES), X-ray photoelectron spectroscopy (XPS) and atomic force microscopy (AFM). The sample is initially prepared by hydrogen etching before loading into the ultra-high vacuum system. The sample is then out-gassed to remove oxygen from the surface. To explore the existence of ordered surface phases, Si is deposited on the sample at 850°C for 15 minutes followed by a series of sequential annealing steps. Throughout this process, the surface is monitored by LEED, AES and XPS. LEED shows that the surface continuously maintains a (1×1) periodicity. Yet, two unique and distinguishable (1×1) phases can be identified. The changes between these phases are clearly demonstrated by the LEED spot intensities. Simultaneously, the Auger and XPS data show a decrease in Si intensity.

Phase evolution in the alkane–P123–water–TEOS quadru-component system: a feasible route to different complex mesostructured materials

Directed by the temperature-induced phase transformation in the alkane (from heptane to dodecane)–P123–water–TEOS quadru-component system, i.e., from a swollen inverse hexagonal phase (H2) to a swollen lamellar phase (Lα) and then to a swollen normal micelles phase (L1), complex silica materials, such as mesoporous nanofibres, multilamellar vesicles (MLVs) and mesocellular foams (MCFs), are constructed.

Stability of crystalline solids—II: Application to temperature-induced martensitic phase transformations in a bi-atomic crystal

This paper applies the stability theory of crystalline solids presented in the companion paper (Part I) to the study of martensitic transformations found in shape memory alloys (SMA's). The focus here is on temperature-induced martensitic transformations of bi-atomic crystals under stress-free loading conditions. A set of temperature-dependent atomic potentials and a multilattice description are employed to derive the energy density of a prototypical SMA ( B 2 cubic austenite crystal). The bifurcation and stability behavior are then investigated with respect to two stability criteria (Cauchy–Born (CB) and phonon). Using a 4-lattice description five different equilibrium crystal structures are predicted: B 2 cubic, L 1 0 tetragonal, B 19 orthorhombic, Cmmm orthorhombic, and B 19 ′ monoclinic. For our chosen model only the B 2 and B 19 equilibrium paths have stable segments which satisfy both the CB- and phonon-stability criteria. These stable segments overlap in temperature indicating the possibility of a hysteretic temperature-induced proper martensitic transformation. The B 2 and B 19 crystal structures are common in SMA's and therefore the simulated jump in the deformation gradient at a temperature for which both crystals are stable is compared to experimental values for NiTi, AuCd, and CuAlNi. Good agreement is found for the two SMA's which have cubic to orthorhombic transformations (AuCd and CuAlNi).

Stress and Strain Evolution in Cast Refractory Blocks during Cooling

A computational model has been developed to study the stress and strain histories experienced by zirconia-containing cast refractory blocks when they are cooled from the casting temperature. Incorporated into the model are strong temperature dependencies of the following material properties: elastic modulus, flow strength, thermal conductivity, and thermal expansion. The pressure-sensitive Drucker-Prager plasticity model has been used to account for the substantial difference between the tensile and compressive stress-strain behaviors found in these refractories. The temperature-induced phase transformation in the zirconia, as well as the overall thermal contraction, are united by introducing a coefficient of total dilatation. A nonuniform radial distribution of the transforming phase is also incorporated. The parameters that control the time-dependent stress and strain responses are identified by performing calculations that span the range of expected variables. Process strategies for manufacturing crack-free blocks are suggested.

Low temperature-induced intermartensitic phase transformations in Ni–Mn–Ga single crystal

Abstract Single crystalline Ni 52.6 Mn 23.6 Ga 23.8 alloy has been found to undergo a sequence of temperature-induced, reversible, intermartensitic transformations (IMTs). Such transformations are accompanied by anomalies in the calorimetric, mechanical, magnetic and electrical resistivity measurements, both on cooling and heating the samples. TEM observations have confirmed the transformation sequence austenite → 10M → 14M → 2M. The low-temperature forward IMTs are controlled by the internal stresses and elastic energy stored in the martensitic state after the performed transformation sequence; while this causes a notable variability of the forward transformation temperatures for different specimens, the corresponding reverse transformations are much more reproducible. Application of an external magnetic field of 50 kOe is observed to produce shifts of ∼5 K in all the transformation temperatures. Theoretical considerations suggest that in the intermartensitic transformations, this shift is predominantly produced by the anisotropic magnetoelastic interactions, while for the martensitic transformation, the jump in the absolute value of magnetization and magnetoelastic contribution are equally important.

Three-dimensional Landau theory for multivariant stress-induced martensitic phase transformations. III. Alternative potentials, critical nuclei, kink solutions, and dislocation theory

In part III of this paper, alternative Landau potentials for the description of stress-and temperature-induced martensitic phase transformations under arbitrary three-dimensional loading are obtained. These alternative potentials include a sixth-degree (2-4-6) polynomial in Cartesian order parameters and a potential in hyperspherical order parameters. Each satisfies all conditions for the correct description of experiments. The unique features of the potentials are pointed out and a detailed comparison of the potentials is made for NiAl alloy. Analytic solutions of the one-dimensional time-independent Ginzburg-Landau equations for the 2-3-4 and 2-4-6 potentials for a constant-stress tensor and invariant-plane strain are obtained and compared. Solutions include martensitic and austenitic critical nuclei and diffuse martensite-austenite and martensite-martensite interfaces. The widths and energies of the nuclei and interfaces are functions of the thermodynamic driving force, the gradient energy coefficient, and a parameter that characterizes the stability of austenite. The splitting of a martensite-martensite interface into two austenite-martensite interfaces is interpreted as a potentially new mechanism---namely, barrierless austenite nucleation---which might be observed experimentally at the interface between two invariant-plane-strain variants. The widths, energies, and gradient energy coefficients of the martensite-martensite and austenite-martensite interfaces are estimated for NiAl. Finally, we outline a version of phase field theory for dislocations based on our theoretical framework for phase transformations.

Cost-Effective Grinding of Zirconia Using the Dense Vitreous Bond Silicon Carbide Wheel

Results of grinding zirconia using wheels with fine grain size SiC and dense vitreous bond are presented. Wheel wear results demonstrated that this type of SiC wheel could grind fully and partially stabilized zirconia (PSZ) very effectively. X-ray diffraction was used to analyze the percentage of monoclinic phase in the PSZ base material, ground surface, and debris. As expected, due to the stress- and temperature-induced phase transformation during grinding, the percentage of monoclinic phase on the ground surface was increased relative to the base material. However, X-ray diffraction showed no monoclinic phase in the PSZ debris. This suggests that, during grinding, the low thermal conductivity of zirconia and SiC, compared to that of diamond, facilitates heat retention in the chip and softens the work-material. This makes the efficient grinding of PSZ possible. Grinding temperature measurement results supported this hypothesis.

Temperature induced phase transformations of molecular nanoclusters

We report results from canonical molecular dynamics (MD) and jump-walking Monte Carlo (MC) simulations, which show two sequential solid–solid transformations of nanosized (∼2 nm) free TeF 6 clusters on cooling below their solidification point. MC and MD simulations define the same temperature interval (76±4) K for the discontinuous (first order) transformation of 59-molecule TeF 6 clusters and (29±5) K for the continuous transformation. We conclude that the ergodic requirements can be satisfied in solid–solid transformations of these specific clusters. The discontinuous transition is characterized with dynamically coexisting phases in a single cluster observed in MD simulations. The coexisting phases are stabilized by kinetics of small systems, although they might be thermodynamically unstable in the bulk of the same material.

Analysis of a thermally induced phase transformation strain state for the TiNi shape memory alloy under a complex stress state

In the paper an analysis of isostatic testing results for the Ti-51.0 at. % Ni shape memory alloy are presented. Tests were performed in a complex stress state, the tubular specimen being loaded by a combination of axial load and torque. After achieving a specified level of the load under complex and various stress states, a thermal cycle was started. During the test, the temperature-induced phase transformations were observed as variations in the transformation strain tensor components. These strain components were measured with the use of resistive strain gauges, digitally recorded and processed to extract pure phase transformation strains. For selected complex stress states, the transformation strain tensor components, principal transformation strains and characteristic temperatures of the phase transformations were found. On the basis of the experimental data, basic relations between characteristic parameters of an imposed stress state and the temperature-induced phase transformation strain state were determined.

Coupled thermomechanical dynamics of phase transitions in shape memory alloys and related hysteresis phenomena

In this paper the nonlinear dynamics of shape memory alloy phase transformations is studied with thermomechanical models based on coupled systems of partial differential equations by using computer algebra tools. The reduction procedures of the original model to a system of differential-algebraic equations and its solution are based on the general methodology developed by the authors for the analysis of phase transformations in shape memory materials with low dimensional approximations derived from center manifold theory. Results of computational experiments revealing the martensitic-austenitic phase transition mechanism in a shape-memory-alloy rod are presented. Several groups of computational experiments are reported. They include results on stress- and temperature-induced phase transformations as well as the analysis of the hysteresis phenomenon. All computational experiments are presented for Cu-based structures.

Effect of solid-solid phase transformations on elastic constitutive relations

A two-dimensional (2D) Ginzburg-Landau (GL) theory for temperature induced phase transformations in single crystals is extended to incorporate the effects of mechanical and thermal (expansion or dilatational) strain effects. This extension of the GL theory allows the single crystal GL model to be implemented into polycrystalline homogenization schemes. The proposed model is capable of examining the interactions of applied mechanical loading states and phase transformation processes within single crystals.

The cracking of zirconia refractory tubes under hot shock

The evolution of stresses and strains in a zirconia-containing refractory tube subjected to a hot shock on the outer surface and convective cooling at the inner surface is analysed with the method of finite elements. To account for the temperature-induced phase transformation in the zirconia as well as the overall thermal expansion, a coefficient of total dilatation is introduced. The parameters that control the time-dependent stress and strain responses are identified by performing finite element calculations that span the range of variables relevant to steel making. The effects of tube thickness, hot shock duration, initial temperature, temperature dependence of elastic modulus, and transformation amplitude on stress and strain distributions are discussed, and heating and process strategies to eliminate surface cracking are suggested.

Heating Effects on The Young's Modulus of Films Sputtered onto Micromachined Resonators

AbstractSurface-micromachined polysilicon lateral resonant structures were fabricated and used to determine the temperature dependence of the Young's modulus of the polysilicon. This is done by passing a dc current through the beams during resonance testing, resulting in Joule-heating. The temperatures are calibrated by increasing the dc current until the melting point of silicon is attained. The calculated Young's moduli agree well with reported values for single crystal silicon.In addition, metal films were sputter-deposited onto the polysilicon resonators, and similar experiments performed on the composite devices to determine the temperature dependence of the modulus of the sputtered films. Ni films demonstrate a linear decrease in Young's modulus with temperature. TiNi films demonstrate two distinct modulus values with an intermediate transition region, due to the temperature-induced reversible phase transformation exhibited by TiNi.