Latent Heat Of Transformation Research Articles

Study on the rate-dependent cyclic deformation of super-elastic NiTi shape memory alloy based on a new crystal plasticity constitutive model

In this paper, a crystal plasticity based constitutive model (Yu et al., 2013) is extended to describe the rate-dependent cyclic deformation of super-elastic NiTi shape memory alloy by considering the internal heat production. Two sources of internal heat productions are included in the proposed model, i.e., the mechanical dissipations of inelastic deformation and the transformation latent heat in the NiTi shape memory alloy. With an assumption of uniform temperature field in the alloy specimen, a simplified evolution law of temperature field is obtained by the first law of thermodynamics and the heat boundary conditions. An explicit scale-transition rule is adopted to extend the proposed single crystal model to the polycrystalline version. The capability of the extended polycrystalline model to describe the rate-dependent cyclic deformation of super-elastic NiTi shape memory alloy is verified by comparing the predictions with the corresponding experimental ones. The comparison demonstrates that the proposed constitutive model considering the internal heat production predicts the rate-dependent cyclic deformation of super-elastic NiTi shape memory alloy fairly well.

Effect of thermal cycling on the martensitic transformation in Ni-Mn-In alloys

The influence of thermal cycling on the characteristics of the martensitic transformation in Ni45.4Mn40.9In13.7 alloy was investigated. It was shown that after 1000 repeated thermal cycles trough temperature interval of the martensitic transformation, application of magnetic field of 1.1 T resulted in higher transformation temperatures shift of 10 K/T compared to 7 K/T before cycling. On the other hand, the measurements display a steady increase of electric resistivity with the number of cycles that along with decrease of the transformation latent heat indicate the phase hardening. The means to prevent functional degradation of magnetocaloric materials due to phase hardening are suggested.

Determination of Local Thermophysical Properties and Heat of Transition from Thermal Fields Measurement During Drop Calorimetric Experiment

This paper proposes a non-contact original method to estimate local thermophysical properties (heat capacity and thermal conductivity) and heats of transition from plane thin specimens. This method is based on measurement of temperature fields with an infrared camera during a drop calorimetric experiment. A studied specimen and a reference specimen, with similar geometries, are simultaneously tested. Firstly, the method is validated by estimating heat capacity and thermal conductivity of Vanadium specimens and by comparing the determined values with those obtained by Differential Scanning Calorimetry and by a laser flash method, respectively. Secondly, the method is used to determine latent heats of martensitic transformations. These heats of transition are determined during homogeneous and heterogeneous drop calorimetric experiments of NiTi shape memory alloys specimens. Measured transformation temperatures and latent heats are in good accordance with results obtained by Differential Scanning Calorimetry.

Enhanced reversibility and unusual microstructure of a phase-transforming material

Materials undergoing reversible solid-to-solid martensitic phase transformations are desirable for applications in medical sensors and actuators, eco-friendly refrigerators and energy conversion devices. The ability to pass back and forth through the phase transformation many times without degradation of properties (termed 'reversibility') is critical for these applications. Materials tuned to satisfy a certain geometric compatibility condition have been shown to exhibit high reversibility, measured by low hysteresis and small migration of transformation temperature under cycling. Recently, stronger compatibility conditions called the 'cofactor conditions' have been proposed theoretically to achieve even better reversibility. Here we report the enhanced reversibility and unusual microstructure of the first martensitic material, Zn45Au30Cu25, that closely satisfies the cofactor conditions. We observe four striking properties of this material. (1) Despite a transformation strain of 8%, the transformation temperature shifts less than 0.5 °C after more than 16,000 thermal cycles. For comparison, the transformation temperature of the ubiquitous NiTi alloy shifts up to 20 °C in the first 20 cycles. (2) The hysteresis remains approximately 2 °C during this cycling. For comparison, the hysteresis of the NiTi alloy is up to 70 °C (refs 9, 12). (3) The alloy exhibits an unusual riverine microstructure of martensite not seen in other martensites. (4) Unlike that of typical polycrystal martensites, its microstructure changes drastically in consecutive transformation cycles, whereas macroscopic properties such as transformation temperature and latent heat are nearly reproducible. These results promise a concrete strategy for seeking ultra-reliable martensitic materials.

A micromechanical analysis of the coupled thermomechanical superelastic response of textured and untextured polycrystalline NiTi shape memory alloys

In this paper a micromechanical model that incorporates single crystal constitutive relationships is used for studying the pseudoelastic response of polycrystalline shape memory alloys (SMAs). In the micromechanical framework, the stress-free transformation strains of the possible martensite twinned structures, correspondence variant pairs (CVPs), obtained from the crystallographic data of NiTi are used, and the overall transformation strain is obtained by defining a set of martensitic volume fractions corresponding to active CVPs during phase transformation. The local form of the first law of thermodynamics is used and the energy balance relation for the polycrystalline SMAs is obtained. Generalized coupled thermomechanical governing equations considering the phase transformation latent heat are derived for polycrystalline SMAs. A three-dimensional finite element framework is used and different polycrystalline samples are modeled based on Voronoi tessellations. By considering appropriate distributions of crystallographic orientations in the grains obtained from experimental texture measurements of NiTi samples, the effects of texture and the tension–compression asymmetry in polycrystalline SMAs are studied. The interaction between the stress state (tensile or compressive), the number of grains and the texture on the mechanical response of polycrystalline SMAs is studied. It is found that the number of grains (or size) affects both the stress–strain response and the phase transformation propagation in the material. In addition to tensile and compressive loadings, textured and untextured NiTi micropillars with different sizes are also studied in bending. The coupled thermomechanical framework is used for analyzing the effect of loading rate and the phase transformation latent heat on the response of both textured and untextured samples. It is shown that the temperature changes due to the heat generation during phase transformation can affect the propagation of martensite in samples subjected to high strain rates.

Surface Effects on Structural Phase Transformations in Nanosized Shape Memory Alloys

We systematically investigated the role of surface on the B2–B19 transformation of free-standing nanoparticles by classical molecular dynamics simulations. It was found that the surface of particles has a shell structure with a thickness of 2 unit cells, suppressing the B2–B19 transformation locally. Below a critical size, this surface shell leads to the reduction of the transformation latent heat and the vanishing of B2–B19 transformation. We propose a Landau-type model based on the core–shell structure of particles, which reproduces well the relation between transformation temperature and particle size. Our results provide a microscopic insight into the origin of surface-induced size effect on the structural phase transformations in nanosized shape memory alloys.

Simulation of the thermomechanical and metallurgical behavior of steels by using ABAQUS software

Simulation is a very helpful and valuable work tool in the field of heat treatment of steels. It allows behavior laws and algorithms to be learned and tested. Simulation provides a low cost, secure and fast analysis tool. For a fine assimilation of the microstructure and distortion evolution concepts which characterize the surface hardening process, a simulation of the thermo-metallurgical and mechanical coupling is elaborated by using ABAQUS software linked to phase transformation and elastic–plastic modules which we developed taking into account of interactions between these fields. The application of this method makes it possible to highlight the metallurgical and mechanical behavior laws and procedures used to calculate phase fractions, austenite grain size, hardness, microstructure effect on physical properties, transformation latent heat, deformation progress, and stress genesis during a surface hardening; which cannot be feasible by using standard software based simulation. Predicted thermo-metallurgical results and residual stress have been compared with measured ones; the coincidence between simulated and experimental values confirms the validity of the chosen simulation tool.

EFEITOS DA ADIÇÃO DO INOCULANTE Al-3%Ti-1%B ÀS LIGAS DO SISTEMA Al-Si

Grain refinement of the as-cast macrostructure by the addition of the inoculant Al-3%Ti-1%B is examined for Al-3%Si, Al-7%Si, and Al-11%Si alloys using thermal and Fourier analyses as well as quantitative metalography. The effects of inoculant addition on the cooling curves, on the grain size, on the solid fraction evolution with time, and on the latent heat of transformation are investigated. The results show that the cooling curves are changed significantly with the inoculant addition only at the beginning of primary solidification. The evolution of solid fraction with time, however, occurs more slowly with the addition of inoculant. For the Al-3%Si alloy, the time rate of solid formation decreases from the beginning of solidification to a solid fraction of 50%. The average grain size decreases substantially with the addition of the inoculant, suggesting the presence of a larger number of grains during solidification

Computer-Aided Development and Simulation Tools for Shape-Memory Actuators

This article discusses the dynamic properties of shape-memory alloy (SMA) actuators, which are characterized by their rate of heating and cooling procedures and can be described only insufficiently for different boundary conditions. Based on an analysis of energy fluxes into and out of the actuator, a numerical model implemented in MATLAB/SIMULINK (MathWorks, Natick, MA) is presented. Besides the fluxes, the time-variable parameters like the latent heat of transformation or the influence of stress on the transformation temperatures are also included in the model. These parameters, depending on actuator geometry and temperature, the fraction of martensite, and the environmental conditions, are considered in the simulation in real time. In addition, this publication sums up the needed empirical data (e.g., fatigue behavior) to create a general-purpose engineering tool. The SMA wire-based actuation system can be configured by drag-and-drop tools and finally simulated and graphically displayed for different actuator systems. The development and verification of such a tool (called CASMADA) from theoretical equations to the verification on real elements is the main topic of this publication.

Effect of phase transformation latent heat on prediction accuracy of strip laminar cooling

The cooling of hot-rolled strips is of great importance due to its effects on the mechanical and metallurgical properties of the final product. In this paper, the effect of phase transformation on accurate prediction of strip temperature at the run-out table is investigated. For this purpose, a model is developed for the heat transfer and phase transformation in hot-rolled strips during the cooling process. The model is capable of calculating strip temperature and the volume fraction of steel phases during cooling at the run-out table under the two different situations of “regarding phase transformation latent heat” (RPTLH) and “disregarding phase transformation latent heat” (DPTLH). Actual data from a hot rolling line were used to evaluate the accuracy of the model. The model was used to predict strip temperature under the above two scenarios. Comparison of actual and predicted values in the two DPTLH and RPTLH cases revealed that disregarding the released heat would lead to considerable errors in temperature predictions. The results for different steel grades predicted by the model also showed that the higher the carbon content of the steel, the higher the error due to disregarding phase transformation latent heat.

DEVELOPMENT OF HOT FORMING TECHNOLOGY FOR ULTRA HIGH STRENGTH STEEL AND ITS MECHANICAL PROBLEM

Hot forming is the advanced technology that integrates traditional heat treatment and cold stamping. The ultra high strength component with the tensile strength 1600 MPa can be manufactured by hot forming. The principle of hot forming,one-step method and multi-step method technology,the key manufacturing technology in hot forming process are introduced.The evolutions of microstructure and mechanical property of the material are analyzed before hot forming,in hot forming and after hot forming.The material model of austenitic phase at high temperature with dependence on temperature and strain rate and its application method in hot forming are presented.The formability of the material in hot forming is studied.The influence of hardness ability value on formability is investigated and the optimized temperature range is presented.The anisotropy of the rolling direction is analyzed and the experimental results indicate the anisotropy of crystal lattice disappear in hot forming.The constitutive relations of the material are descripted in hot forming,especially the thermal-mechanical-transformation coupled relationship.The multi-phase mixed relationship is introduced to analyze the effective thermo-mechanical parameters and mechanical properties of multi phases during the hot forming.By defining the concepts,the phase-transformation volume stress and phase-transformation plastic stress are expressed to explain the mechanism of thermal-mechanical-transformation coupled relations.Then the numerical simulation of hot forming is introduced,especially the dynamic explicit algorithm,the static explicit algorithm and the numerical simulation of the temperature field based on the independently developed commercial CAE software for sheet metal forming,named KMAS(King-Mesh Analysis System).The phase transformation latent heat is introduced into the analysis of temperature field during the hot forming process. Based on the analysis of the crash problem for car body,the functional design methods of hot-forming parts used in the vehicle body design are presented.Finally prospective developments of hot forming are suggested.

Thermodynamic assessment of the stabilization effect in deformed shape memory alloy martensite

When a martensitic shape memory alloy is deformed, the reverse transformation occurs at higher temperature than that of undeformed martensite. This is a typical case of the stabilization effect of martensite that is commonly observed in shape memory alloys. Regarding previous results measured by electric resistance and/or dilatometoric methods in NiTi and CuAlNi shape memory alloys, this study has performed calorimetric measurement in these alloys in order to re-examine the stabilization effect in terms of thermodynamics. Experimental evidence for appreciable changes in the reverse transformation temperature due to variant change of the martensite is presented. The elastic energy stored in the deformed martensite and the irreversible energy dissipated during the reverse transformation are estimated from the transformation temperatures, the stress–strain curves of the martensite and the latent heat of transformation. The temperatures of the reverse martensitic transformation have been related to these energies in explicit form.

Lasers in Periodontics- A dilemma

With the development of the first laser by Mainam in 1960, over decades, the subject has grown by leaps and bounds in dentistry over the conventional therapies. However, despite a large number of studies, the application of lasers for the treatment of periodontitis remains controversial. Lasers used today are either a continuous, pulsed (gated) or running pulse waveform. E.g. CO2, Nd:YAG, Ho:YAG, Er:YAG, Er,Cr:YSGG, Nd:YAP, GaAs(diode) and Argon. Two main factors that determine the extent of energy absorption is the wavelength of the light used and the optical properties of the tissue. Others include pigmentation, water content, mineral content, heat capacity and latent heat of transformation. Lasers tissue interactions include the physiologic and mechanical processes of heat conduction and dissipation, the degree of tissue inflammation and vascularity and the availability of progenitor cells to participate in the healing process. The present article is a literature review designed to assess the rationality of using lasers in periodontal therapy.

Influence of Welding Parameters on Residual Stress in 9% Ni Steel for Low Temperature Service

Based on welding heat transfer theory, heat elastoplasticity theory and phase transformation theory, welding residual stress in multi-layer welded joint of 9% Ni steel for liquefied natural gas (LNG) storage tanks was simulated using SYSWELD finite element analysis software. Taking into account nonlinear relationships between temperature and mechanical properties, together with influence of transformation latent heat on temperature field, double ellipsoid heat source model was applied in the simulation of the three fields (temperature field, microstructure field and stress-strain field). Distribution pattern of welding residual stress in 9%Ni steel welded joint was obtained through numerical simulation. Results showed that heat affected zone (HAZ) showing high-level residual stress was the main stress concentrated part of the welded joint. Longitudinal residual stress appeared to be tensile in the weld and HAZ, and compressive in regions remote from the weld. A match of low heat input and high interpass temperature should be employed to lower residual stress level in HAZ. Simulation results were well consistent with theoretical analysis.

INFLUENCES OF HEAT TREATMENT ON TRANSFORMATION BEHAVIORS OF NEAR-EQUIATOMIC POROUS NITI SHAPE MEMORY ALLOY

The pore characteristics and pore size distribution of porous near-equiatomic NiTi shape memory alloy fabricated by self-propagating high-temperature synthesis (SHS) are described in detail. The effects of different heat treatments on the transformation of porous NiTi alloy were investigated by differential scanning calorimetry (DSC), x-ray diffraction (XRD), and scanning electron microscopy (SEM). The results indicate that heat treatment had strong influences on the transformation temperatures and latent heats of transformation. When the porous alloy was annealed at 648K and 748K for 3.6ks, two steps transformation including R transformation occurred during cooling and heating and the R transformation temperatures are lower than B 2↔ B 19' transformation temperatures. However, no transformation was detected within the experimental temperature range if the porous alloy was solution treated at 1133K for 2.4ks. This novel phenomenon was the results of extensive Ti2Ni intermetallic compound precipitation. The transformation temperatures of porous NiTi alloy after annealing at 1323K for 3.6ks were much lower than those of the untreated alloy.

Thermal and stress-induced martensitic transformations in quaternary Ni 50Mn 37(In, Sb) 13 ferromagnetic shape memory alloys

This study investigated the martensitic transformation and the mechanical behaviour of quaternary Ni 50Mn 37In 13− x Sb x ( x = 0, 2, 6, 8, 10) Heusler alloys. Substitution of Sb for In was found to reduce the orthorhombic distortion of the martensite form the L2 1 austenite, but increased the transformation volume change. The critical temperatures of the transformation are found to decrease nearly linearly with increasing valence electron concentration caused by Sb substitution for In, with a linearity coefficient of ∼15 K per at% of Sb. The transformation latent heat and the transformation entropy change decreased with increasing Sb content, with two different composition dependencies in concentration ranges of 0 < x < 6 and x > 6. This is attributed to different magnetic states of the martensite. A recoverable transformation strain of 0.46% was obtained in the polycrystalline Ni 50Mn 37In 3Sb 10 alloy. An effect of stress-induced martensite stabilization was observed in the Ni 50Mn 37In 7Sb 6 sample.

Preparation and Microstructure Analysis of Ti-Al Sheet by Electron Beam Physical Vapor Deposition

By using electron beam physical vapor deposition (EB-PVD) technology, Ti-Al thin sheet with dimension of 450mm×450mm×0.2mm was prepared and the microstructure of Ti-Al deposit was investigated by means of scanning electron microscopy (SEM), atom force microscopy (AFM) and X-ray diffraction (XRD), and then the effect on deposit by re-evaporation of Al was explored by calculating the ratio of re-evaporating capacity with depositing capacity of Al on the substrate. The results indicate that there existed equiaxed crystal and columnar crystal along the cross-sectional may resulted from the transformation latent heats released during the transition course of atoms from gaseous state to solid state, and the variation of target-substrate distance would take effect on the phase composition due to the changing of atoms collision probability and radiant heat quantity absorbed by substrate. The effect on deposit by re-evaporation of Al could be neglected because the re-evaporating capacity of Al was far below that of the depositing capacity.

Structural, Thermal and Magnetic Properties of Ga Excess Ni-Mn-Ga

The martensitic transition and the ferro- to paramagnetic transition have been studied in a series of Ga excess Ni-Mn-Ga specimens [Ni2-xMnGa1+x (0.4≤ x≤ 0.9)] by differential scanning calorimetry and magnetization measurements. The martensitic transition exhibits a hysteresis whose width is similar to Ni2MnGa, indicating that the transition is thermoelastic. The latent heat of transformation is comparable with other Ni-Mn-Ga alloys. A substantial increase in the martensitic transition temperature is observed due to Ga doping. Interestingly, the x-ray diffraction pattern of all the compositions studied show a modulated martensitic structure in the martensitic phase.

TIPS AND TRICKS FOR CHARACTERIZING SHAPE MEMORY ALLOY WIRE: PART 3-LOCALIZATION AND PROPAGATION PHENOMENA

This is the third paper in our series to identify unusual phenomena and to provide recommendations for the thermo-mechanical characterization of shape memory alloy (SMA) wire. Part 1 provided some basic background of the martensitic transformations responsible for the shape memory effect and superelasticity 1 . The characterization of two typical NiTi SMA alloys began with differential scanning calorimetry (DSC) thermograms to measure their respective transformation temperatures, specific heats, and latent heats of transformation. It included an experiment for each alloy showing both shape memory and superelasticity, but in different temperature regimes. Part 2 reviewed the various methods for obtaining a fundamental set of isothermal mechanical responses and provided data on the same two SMA wire alloys over their relevant temperature windows 2 . In the process, it showed stress-induced transformations, which lead to an introduction of strain localization and propagation of phase transformation fronts. In this paper(Part 3), we narrow ourfocus on certain unusual phenomena occurring during superelastic tension responses, namely localization of strain and temperature fields. These are often overlooked by the novice, yet they play an important role in the kinetics of stress-induced phase transformation, and in turn, exacerbate the material response’s sensitivities to loading rate and ambient media (which will be the subject of the next paper in this series). Here, we introduce special experimental techniques for (otherwise) difficult to measure features in the underlying material response. We focus on the superelastic response of one of the two NiTi alloys that was used in our previous articles, that is, the alloy with an Austenite finish temperature (Af) below room temperature (designated as superelastic wire). Figure 1 shows four isothermal superelastic experiments, magnified from the set of fundamental mechanical responses of Fig. 8 in 2 . Each was done on a virgin specimen under elongation control at a slow rate, ˙L =± 1 × 10 −4 s −1 , in a temperature-controlled air chamber at different Editor’s Note: This ET feature series is intended as an introduction to this exciting area of experimental mechanics. It aims to increase awareness of active materials and to promote their consistent characterization by disseminating best practices from leading researchers in the field. Each article in the series will address the characterization of one commercially significant active material. Series editors: Nilesh D. Mankame and Paul W. Alexander.

Comparison between the thermal properties of fully dense and porous NiTi SMAs

In this paper the results and methodology for evaluating the dependence of the thermal properties of porous and fully dense NiTi SMAs on temperature and porosity using an experimental–numerical approach is reported. In the experimental work a cylindrical NiTi sample was uniformly heated at one side and temperature histories at different positions recorded. A variety of heating rates ranging from 10 to 100 K/s and maximum temperature values of 400 K or 1400 K at the top of the NiTi sample were used to carry out experiments. The numerical code based on the 1D unsteady heat diffusion equation considers the sample as a solid featured by the average density depending on its porosity and permits the evaluation of the effective thermal properties. The total heat capacity of each sample is defined on the basis of the transformation latent heat and data reported in literature, while the thermal conductivity is obtained by comparison of the numerical results to the experimental curves. Samples featuring porosity levels of 0%, 30%, 48%, 68% have been tested and the total heat capacity and thermal conductivity dependence on temperature and porosity are defined in the temperature range from 300 K up to 1400 K.