Phase Transformation Propagation Research Articles

Stress Wave and Phase Transformation Propagation at the Atomistic Scale in NiTi Shape Memory Alloys Subjected to Shock Loadings

A unique property of Nickel–Titanium (NiTi) shape memory alloys is their ability to dissipate the shock loading energy by two complementary mechanisms: (a) through deformation-induced phase transformations caused by the structural vibrations, and (b) through the phase transformations caused by the stress wave propagation in the material. Despite extensive research work on the former mechanism, the latter one is still highly unknown, particularly at the atomistic scale. In this paper, the phase transformation, and consequently the energy dissipation, caused by the propagation of stress waves in single-crystal and polycrystalline NiTi alloys under shock wave loadings are investigated using molecular dynamics (MD) method. The nanostructure and dynamic response of the material, when subjected to a shock loading, are studied at the atomistic level. The effects of various nanoscale properties, including the orientation of lattice with respect to the shock loading direction, average grain size, and the effect of grain boundaries on the stress wave propagation, phase transformation propagation, and the energy dissipation in polycrystalline NiTi alloys are studied.

Modeling of unstable behaviors of shape memory alloys during localization and propagation of phase transformation using a gradient-enhanced model

In this article, a multi-dimensional gradient-enhanced constitutive model of shape memory alloys is developed. The model is developed to capture unstable behaviors of shape memory alloys during both the forward and reverse phase transformations. Also, influence of loading history on the start of the phase transformation during both forward and reverse transformations is considered in the model by introducing new transformation limits and phase fraction formulations. The model is implemented in a finite element code, and using a numerical framework, effects of loading, boundary condition, inhomogeneous deformations, imperfection, and geometry on the unstable behaviors of different shape memory alloy samples during nucleation and phase transformation are investigated. The obtained results are compared with the available experimental and numerical results in the literature. The obtained results show that the gradient enhancement removes pathological localization effects which would typically result from the softening influence. In addition, the numerical study proves that the mode can capture the basic features of phase transformation front patterns and their evolution during transformations.

A 2D finite element based on a nonlocal constitutive model describing localization and propagation of phase transformation in shape memory alloy thin structures

Here, the effects of localization and propagation of martensitic phase transformation on the response of SMA thin structures subjected to thermo-mechanical loadings are investigated using nonlocal constitutive model in conjunction with finite element method. The governing equations are derived based on variational principle considering thermo-mechanical equilibrium and the spatial distribution of the nonlocal volume fraction of martensite during transformation. The nonlocal volume fraction of martensite is defined as a weighted average of the local volume fraction of martensite over a domain characterized by an internal length parameter. The local version of the thermo-mechanical behavior model derived from micromechanics considers the local volume fraction of martensite and the mean transformation strain. A 4-noded quadrilateral plane stress element with three degrees of freedom per node accounting for in-plane displacements and the nonlocal volume fraction of martensite is developed. Numerical simulations are conducted to bring out the influence of material and geometrical heterogeneities (perturbations/defects) on the localization and propagation of phase transformation in SMA thin structures. Also, a sensitivity analysis of the material response due to the localization and the other related model parameters is carried out. The detailed investigation done here clearly shows that the localization of phase transformation has significant effect on the response of shape memory alloys.

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.

Modelling of localization and propagation of phase transformation in superelastic SMA by a gradient nonlocal approach

In this work, a nonlocal phenomenological behavior model is proposed in order to describe the localization and propagation of stress-induced martensite transformation in shape memory alloy (SMA) wires and thin films. It is a nonlocal extension of an existing local model that was derived from a micromechanical-inspired Gibbs free energy expression. The proposed model uses, besides the local field of the internal variable, namely the martensite volume fraction, a nonlocal counterpart. This latter acts as an additional degree of freedom, which is determined by solving an additional partial differential equation (PDE), derived so as to be equivalent to the integral definition of a nonlocal quantity. This PDE involves an internal length parameter, dictating the global scale at which the nonlocal interactions of the underlying micromechanisms are manifested during phase transformation. Moreover, to account for the unstable softening behavior, the transformation yield force parameter is considered as a gradually decreasing function of the martensite fraction. Possible material and geometric imperfections that are responsible for localization initiation are also considered in this analysis. The obtained constitutive equations are implemented in the Abaqus® finite element code in one and two dimensions. This requires the development of specific finite elements having the nonlocal volume fraction variable as an additional degree of freedom. This implementation is achieved through the UEL user’s subroutine. The effect of martensitic localization on the superelastic global behavior of SMA wire and holed thin plate, subjected to tension loading, is analyzed. Numerical results show that the developed tool correctly captures the commonly observed unstable superelastic behavior characterized by nucleation and propagation of martensitic phase. In particular, they show the influence of the internal length parameter, appearing in the nonlocal model, on the size of the localization area and the stress nucleation peak.

Effect of TiN Addition on the Low Temperature Degradation of Ceramic Tool Materials 3Y-TZP

Ceramic tool materials, 3Y-TZP added by TiN particles, were fabricated through hot-pressing techniques. The effects of TiN on their low-temperature degradation at 220# in air were investigated. It is shown that TiN can improve the stability of t-ZrO2 and inhibit the transformation from tetragonal to monoclinic phase, and that the content of TiN affects the stability of tetragonal phase and the propagation of tetragonal-to-monoclinic transformation into the specimen interiors. It is suggested that the grain-boundary phase prevents the nucleation of transformation, and that the high elastic modulus of TiN can prevent the propagation of phase transformation by resisting the volume expansion of transformation. When the content of TiN is 20wt%, the ceramic material shows better low temperature degradation resistance.

形状記憶合金に生じる不均一変形挙動の計測

Shape memory alloy (SMA) has been paid attention as smart material revealing shape recovery by heating. Many researchers have investigated its basic properties and applications. However, many problems have existed in usage of SMA such as change of properties (transformation temperature, Young's modulus) by process of fabrication, shape memory treatment, condition of phase and so on. Therefore, it has been pointing in recent years that expressing these phenomena using conventional constitutive relation of SMA has been difficult. In the present study, phenomena needed to construct novel constitutive relation of SMA, such as nucleation and propagation of phase transformation and so on, were measured using digital image correlation method. Here, tensile loading and unloading tests were performed for NiTi alloy plate in two temperature regions where the SMA shows the shape memory effect (26°C) and pseudoelasticity (60°C). The obtained results were as follows; (1) The propagation of the phase transformation during the stress-induced martensitic transformation and austenite transformation progressed from the both ends to the center of the specimen. (2) Staying phenomena of deformation was confirmed at nucleation point of phase transformation. Deformation behavior of SMA can be classified broadly into three categories, such as “Nucleation of phase transformation”, “Process between nucleation and propagation” and “Propagation of phase transformation”. (3) The transformation had a fixed angle along the longitudinal direction of the specimen, propagating from the both ends to the center of the specimen.

Propagation of Phase Transformation under Strain Variations of TiNi based Shape Memory Alloy Wire

Propagation in a localized region of stress-induced phase transformation under strain variations in TiNi SMA is analyzed experimentally, by tension tests for wire and theoretically, by numerical simulation, FEM that is based on J 2 F theory. From the infrared thermograph during tension tests, it is found that a localized region of transformation appears near the end of the wire. The localized region of transformation propagates according to extension of the specimen, which gives rise to typical characteristics in the nominal stress-strain curve under strain variables of SMA wire (For example, the overshoots, the flat stress regions for reverse transformation and the nonlinear phenomena near the finishing points of transformation). Based on the constitutive assumptions of the softening type of the true stress-logarithmic strain curve, the simulated result exhibits the former typical behaviors associated with propagation of transformation. The validity of the constitutive assumptions and the mechanisms of the former behaviors are discussed by comparison of the experimental and the simulated results.

Surface crack initiation in 2Y-TZP ceramics by low temperature aging

Abstract Surface crack initiation in 2Y-TZP specimens during aging at 250°C was investigated. Surface upheaval due to the isothermal phase transformation appeared on aging, in order to reduce the change of strain free energy for the transformation. The nucleation of a microcrack was induced by surface upheaval at the beginning of aging, and the microcrack was grown to a macrocrack with aging. The crack surface created by isothermal phase transformation shows a completely intergranular fracture mode. The propagation of phase transformation from surface to interior during aging is continous and uniform and the transformed region shows a porous microstructure.