Transformation Front Research Articles

An analytical model for the phase transformation front propagation in superelastic SMA under impact tensile loading

An analytical model for the phase transformation front propagation in superelastic SMA under impact tensile loading

Modeling of localized phase transformation in pseudoelastic shape memory alloys accounting for martensite reorientation

A reliable prediction of the pseudoelastic behavior necessitates the involvement of martensite reorientation in the model. This is important not only under non-proportional loading but in general when the phase transformation proceeds in a localized manner, which results in complex local deformation paths. In this work, an advanced model of pseudoelasticity is developed within the incremental energy minimization framework. A novel enhancement of the model over its original version lies in the formulation of a suitable rate-independent dissipation potential that incorporates the dissipation due to martensitic phase transformation and also due to martensite reorientation, thus yielding an accurate description of the inelastic transformation strain. The finite-element implementation of the model relies on the augmented Lagrangian treatment of the non-smooth incremental energy problem. Thanks to the micromorphic regularization, the related complexities are efficiently handled at the local level, leading to a robust finite-element model. Numerical studies highlight the predictive capabilities of the model. The characteristic mechanical behavior of NiTi tube under non-proportional tension–torsion and the intricate transformation evolution under pure bending are effectively captured by the model. Additionally, a detailed analysis is carried out to elucidate the important role of martensite reorientation in promoting the striations of the phase transformation front.

Mechanisms of phase decomposition in a non-equimolar CrFeMnNi alloy during thermal aging

While high entropy alloys and multi principal element alloys have been largely studied and developed as single solid solutions, fewer investigations have focused on phase transformations in these multicomponent alloys. This study reports on the aging behavior at 500 °C of a non-equimolar CrFeMnNi alloy. The results unveil a complex evolution, leading to complete decomposition into three main phases, namely an L10 NiMn phase, an FCC Fe-rich phase, and a BCC Cr-rich phase. The intricate microstructure is achieved in stages through continuous precipitation of L10 Ni-Mn nanoscale precipitates, which are progressively overtaken by a sweeping discontinuous front involving the co-precipitation of the L10 NiMn and BCC Cr-rich phases in addition to the parent FCC Fe-rich phase. Furthermore, the Cr-rich phase initially forms with metastable compositions leading to further precipitation of Fe-rich precipitates within the Cr-rich region behind the transformation front. Understanding these transformation pathways through metastable states emphasizes the potential for expanding the range of achievable properties through deliberate microstructure tailoring of multi principal element alloys.

Thermomechanics of phase transformation induced localization in NiTi tubes. Part I experiments

This two-Part study aims to illustrate and elucidate the effects of thermomechanical interactions in pseudoelastic NiTi structures that arise in the course of the reversible transformations between the austenitic and martensitic phases. Transformation leads to inhomogeneous deformation with the two phases coexisting and to latent heat-induced local heating/cooling. Part I first presents the results of displacement-controlled isothermal tension and compression experiments on NiTi tubes covering the pseudoelastic temperature regime. The experiments are conducted in a constant temperature circulating bath which suppresses thermomechanical interactions so the transformation front velocity is governed strictly by the applied displacement rate. The load/unload hystereses traced, quantify the transformation stresses and strains and will be used to calibrate the constitutive model of Part II. A set of isobaric experiments in which NiTi tubes are taken through a cool/heat cycle under constant stresses of different levels follow. The experiments are conducted in a custom small-scale environmental chamber with a circulating air stream run by a feedback temperature controller, coupled to load-controlled mechanical loading. The evolution of helical bands of the alternate phase is captured using digital image correlation. The results demonstrate that under such loading histories a strong interaction develops between the evolving inhomogeneous deformation, the latent heat and the surrounding environment. The speed of propagation of the helical fronts slows down or accelerates so as to match the rate at which heat is removed/added to the specimen by the airflow. Thus, here the evolution of localized deformation is strongly coupled to the interaction between the latent heats and heat transfer from the environment. The effect of the stress level on these events is examined through eight isobaric experiments, which provide a rich data set for evaluating constitutive models and structural analyses of these experiments such as those in Part II.

ЦИФРОВІЗАЦІЯ ЯК ІНСТРУМЕНТ ТА СФЕРА ДЕРЖАВНОЇ ДОПОМОГИ СУБ’ЄКТАМ ГОСПОДАРЮВАННЯ В УКРАЇНІ

The article considers that the issue of state assistance in the digital sector is quite new, and especially relevant for Ukraine during the martial law and during the post-war reconstruction of the country. Understanding the possibilities of digital technology and the risks of being left behind in the race for digital transformation, governments around the world are increasingly putting digital transformation front and centre of their policy agenda to stimulate social development and economic prosperity. This involves significant investment, the need to address legal issues, and requires constant adaptation to the specific conditions of each country. The provision of state assistance in the digital sphere is regulated by the regulations of the EU Commission, in particular the articles regulating the provision of state assistance for innovations. It is noted that the digital portal Diya deserves special attention in the field of development of digital tools of state support for business entities. State assistance in the digital sector is focused on achieving long-term goals for the security of digital infrastructure, digital business transformation and digitalization of public services based on the widespread introduction of digital technologies. The success of digital transformation depends on the coordination of efforts of the state and entrepreneurs to expand the implementation of e-services and the transition to inclusive electronic interaction between state supervision and control bodies and business. The impact of the war proved to be an extremely powerful stimulus to the digitalization of Ukraine. The IT industry has become the only one that in 2022 showed positive growth rates, although its development indicators were much worse than expected (the increase was only 6% against the expected 40%). According to the NBU, in 2022 The IT industry provided foreign exchange earnings to the Ukrainian economy in the amount of $7.34 billion. The volume of exports increased by $400 million compared to pre-war 2021, an increase of 5.8%; the number of IT specialists in Ukraine has reached a record high of 300 thousand people.

Lüders and Portevin–Le Chatelier processes in austenitic-martensitic TRIP steel

The authors studied the nature of mobile fronts of localized deformation that generate and propagate during deformation of metastable austenitic-martensitic TRIP steel VNS9-Sh along the entire length of the loading curve from the yield point to fracture. A joint research of the nature of the deformation fronts movement and kinetics of the magnetic phase accumulation made it possible to establish that the fronts under consideration are the fronts of the thermoelastic phase transformation of metastable austenite into martensite. This transformation is realized firstly by formation of the Chernov–Lüders bands and then the Portevin–Le Chatelier bands. Both processes are consistent with staging of the deformation curve, which contains a pseudo-plateau, a section with an increasing hardening coefficient, and a section with a decreasing hardening coefficient. It is shown that the deformation-induced phase transformation corresponds to the fronts propagating on the pseudo-plateau and on the section of loading curve with an increasing hardening coefficient. The Portevin–Le Chatelier bands, which are formed in the section of the loading diagram with a decreasing hardening coefficient, are not associated with “austenite-martensite” transformation and have a twin nature. The kinetics of thermoelastic transformation fronts, as well as deformation fronts in materials with a shear mechanism of shaping, can be described in terms of the autowave concept. On the yield plateaus, the phase transformation occurs through generation and propagation of localized plasticity switching autowaves. In the section with an increasing hardening coefficient, it continues through generation and movement of excitation autowaves. The propagation regions of excitation autowaves are limited in the sample space. They are set by the zones of origin and annihilation of primary switching autowaves which were formed on the yield plateau.

(Invited) Ionic Transport and Structure of Battery Electrolytes, Electrodes and Interfaces: New NMR, Diffraction and Optical Microscopy Approaches

This talk will discuss the development and use of new characterisation methods to understand function over different time and lengthscales, to improve materials function and battery performance. We show how electrode material function can be studied with NMR to measure transport, X-ray diffraction to measure phase segregation within the bulk, optical measurements to explore processes at the particle level, and continuum modelling to tie transport with particle level observations. In the examples discussed, NMR measurements provide hopping frequencies and diffusion coefficients and operando diffraction measurements provide evidence for solid solution vs two phase behaviour as a function of cycling. New optical measurements then allow phase transformation and phase fronts to be visualized directly. The processes that underpin the formation and movement of various phase fronts can then be explored via simulations. Applications to both anode cathode materials will be described. Finally new approaches to investigate the structures of pristine and degraded electrolyte will be discussed.

Structural Anisotropy-Driven Atomic Mechanisms of Phase Transformations in the Pt-Sn System.

Using in situ atomic-resolution scanning transmission electron microscopy, atomic movements and rearrangements associated with diffusive solid to solid phase transformations in the Pt-Sn system are captured to reveal details of the underlying atomistic mechanisms that drive these transformations. In the PtSn4 to PtSn2 phase transformation, a periodic superlattice substructure and a unique intermediate structure precede the nucleation and growth of the PtSn2 phase. At the atomic level, all stages of the transformation are templated by the anisotropic crystal structure of the parent PtSn4 phase. In the case of the PtSn2 to Pt2Sn3 transformation, the anisotropy in the structure of product Pt2Sn3 dictates the path of transformation. Analysis of atomic configurations at the transformation front elucidates the diffusion pathways and lattice distortions required for these phase transformations. Comparison of multiple Pt-Sn phase transformations reveals the structural parameters governing solid to solid phase transformations in this technologically interesting intermetallic system.

An elastoplastic phase-field study of the precipitation behaviors of Mg17Al12 phase in Mg-Al-Based alloys: Part I. Modeling and the role of plasticity

An elastoplastic phase-field model for typical diffusional transformations was developed, where plastic deformation was considered through a modified time-dependent Ginzburg-Landau dynamic equation. The model was applied to investigate the precipitation in Mg-Al-based alloys, with a focus on studying the effects of elastoplastic deformation associated with phase transformation on the morphology and growth kinetics of β-Mg17Al12 precipitates (with Burgers orientation). The elastoplastic modeling results show that the precipitates adopt a lath-shaped morphology with lozenge ends, consistent with experimental observations. Although the plastic strain accumulated in the transformation front has a moderate impact on the morphology of the precipitate, it exhibits a clear dual effect on the growth of the precipitate along [112̅0]α. Specifically, plasticity promotes the growth of the precipitate during the initial stage, owing to its stress relaxation effect, and induces a solute poor zone of atomic Al with a greater concentration gradient at the transformation front. However, the plastic strain tends to retard the growth of the precipitate afterward due to its limited ability to release the increasing compressive strain between the two phases. These results shed light on understanding the role of plasticity in general diffusional transformations and provide guidelines for tailoring the microstructure of alloys.

Spatial and temporal evolution of localized deformation in NiTi tubes in a constant stress thermal cycle: Experiments and analysis

A novel experimental setup is presented that allows for precise control of thermal and mechanical loads, and simultaneous monitoring of the temperature and the full-field deformation of small SMA structures that undergo phase transformations. The facility is used to conduct two experiments in which NiTi tubes are taken through a temperature cycle under constant load that leads to phase transformations in the form of helical localization bands that propagate along the specimen. The latent heat of transformation causes a complex interaction with the prescribed load and thermal environment. By changing the rate of the airflow through the environmental chamber it is revealed that the velocities of the transformation fronts depend on the rate at which heat is removed/added by the controlled environment. The experiments are simulated using a new fully coupled thermomechanical extension of the constitutive framework developed by this research group. Key features of the framework include the modeling of the reversible A⇄M transformation through a single surface in the deviatoric stress-temperature space that obeys kinematic hardening; with the transformation strain and entropy as the internal variables governed by an associative flow rule; and the inhomogeneous deformation exhibited in tension being modeled as softening. The tube is analyzed in a finite element coupled static displacement transient temperature analysis, and taken through the cool/heat cycle of the experiment. The temperature-strain response is accurately reproduced with the two transformations initiating at essentially the same temperatures as in the experiment and propagating in similar localized banded manners at similar speeds. Reproduction of the complex behavior observed in the experiments requires the calibration of the constitutive model, its discretization, and the modeling of the structure and its boundary conditions to work together to near perfection. The simulation also demonstrated that the heat exchange between the structure and the environment, in the present analysis governed by only by convection, requires further enhancement.

Non-equilibrium time-temperature-transformation diagram for enhancing magnetostriction of Fe-Ga alloys

Coherent nanoprecipitates formed at early-stage decomposition in a number of body-centered-cubic (BCC) Fe-based alloys with nonmagnetic solute have been found to strengthen their magnetostriction significantly. Despite that the differences in structure and properties between equilibrium and metastable states have been well recognized in an extensively-studied system Fe-Ga, however, a non-equilibrium time-temperature-transformation (TTT) diagram for manipulating the intermediate nanoprecipitates towards further enhancing magnetostriction is still lacking. By systemically investigating the time- and temperature-dependent early-stage phase transformations of the magnetostriction-peak composition alloy Fe73Ga27, a non-equilibrium TTT diagram and the corresponding time-temperature-property (TTP) relation were successfully determined in this work. A nose temperature (the optimum temperature with maximal nucleation rate) of ∼400 °C to produce face-centered-tetragonal (FCT) L60 nanoprecipitates was determined in the diagram. Above the nose temperature, the L60 nanoprecipitates grow much faster and become incoherent rapidly, characterized by their enlarged tetragonality c/a towards the equilibrium face-centered-cubic (FCC) L12 phase. Below the nose temperature, the L60 nanoprecipitates grow much slower and keep coherent with the BCC matrix over a wide aging time range, but the low atomic diffusion rate and the coherent elastic energy produce extra hexagonal omega nanoprecipitates at the phase transformation front. Based on the non-equilibrium TTT diagram, the optimally-aged random polycrystalline alloy with coherent, dense and fine L60 nanoprecipitates can exhibit magnetostriction as large as 180 ppm, nearly 3 times of that of the solution-treated counterpart. Consequently, this work not only provides a processing base for enhancing magnetostriction of Fe-Ga alloys, but also may offer important guidance to tailor the microstructure of other nanoprecipitates-bearing alloys with similar diffusion-controlled phase transformation.

Ageing-resistant zirconia/graphene-based nanostructures composites for use as biomaterials

This work explores the incorporation of graphene-based two-dimensional nanostructures as moisture barriers to delay hydrothermal ageing of yttria-stabilized zirconia and strengthen its use in biomedical applications. Two sets of highly dense zirconia composites incorporating multilayered graphene with very different lateral dimensions, few layer graphene and exfoliated graphene nanoplatelets, were prepared. The effect of the addition of graphene nanostructures on zirconia ageing was investigated by conducting accelerated hydrothermal degradation experiments in an autoclave. An improved resistance to low-temperature degradation and a high tolerance to damage were achieved in the composites compared to those of monolithic zirconia. The incorporation of 1 vol% multilayered graphene was very effective in restricting the hydrothermal degradation. In particular, the composite incorporating exfoliated graphene nanosheets exhibited outstanding resistance to ageing because of their fine dispersion throughout the matrix, which effectively seemed to restrict grain growth and slow the propagation of the transformation front to the ceramic bulk.

3D spatial reconstruction of macroscopic austenite–martensite transition zones in NiTi wires induced by tension and twisting using diffraction/scattering computed tomography

Macroscopic austenite–martensite transition zones in thin superelastic NiTi wires subjected to stretching, twisting and their combination were reconstructed via diffraction/scattering computed tomography. The method allows to visualize spatial distributions of both phases from a series of X-ray diffraction scans. A cone-shaped, radially symmetric transformation front was observed in stretched wire both with and without applied pre-twist. Twisting of a pre-stretched wire (with a cone-shaped transformation band front already developed) led to the formation of a multi-prong localization morphology recognizable on the surface. Transition zones in these cases are highly heterogeneous complex 3D objects exhibiting distinct phase gradients; the longitudinal size of the zones is comparable with the wire diameter. In contrast, localized martensitic strips parallel with wire axis and with a wedge-like cross-section appeared on the surface of the wire which was twisted only. We discuss the observed localization patterns with respect to current computational simulations and micromechanical models.

Quantitative nondiagonal phase field modeling of eutectic and eutectoid transformations

We develop a three-phase field model for the simulation of eutectic and eutectoid transformations on the basis of a nondiagonal model obeying Onsager relations for a kinetic cross coupling between diffusion and the phase fields. This model overcomes the limitations of existing phase field models concerning the fulfillment of local equilibrium boundary conditions at the transformation fronts in the case of a finite diffusional contrast between the phases. We benchmark our model in the well understood one-sided case with diffusion only in the parent phase against results from the literature. In addition to this solidification scenario, the case of solid-state transformations with diffusion in the growing phases is investigated. Our simulations validate the relevance of the theory developed by Ankit et al. [Acta Mater. 61, 4245 (2013)], that describes in a single frame the two limiting regimes where diffusion mainly takes place whether in the mother phase or in the growing phases. In both the one-sided and two-sided cases, we verify the necessity of the kinetic cross coupling for quantitative phase field simulations.

Asymmetrical superelastic behavior of thermomechanically processed semi-equiatomic NiTi alloy in tensile and compressive modes of deformation

In the present work two different cold working and annealing schemes were utilized, and the asymmetric superelastic response of thermomechanically processed materials were then assessed through cyclic tensile and compressive modes of deformation. The values of transformation stress, transformation strain, and pseudoelastic strain were measured for each treated and solutionized specimens and the asymmetric response was compared. In the solution annealed state, the difference of these parameters at different deformation modes was negligible due to the weak texture of the material, while for thermomechanically treated ones, development of specific deformation and recrystallization texture components was identified to be one of the underlying reasons of intensified asymmetry. The evolved substructure during the thermomechanical processing also played a substantial role in determining the asymmetric response. The presence of fine grains and dense dislocation substructure could hinder the movement of the transformation front, thus limiting the range of transformation. In tensile mode, the transformation stress was lower, but higher transformation strain was achieved, which was discussed relying on the slip activity in specified oriented grains. The lower transformation strain in compression mode led to lower pseudoelastic strain due to the narrow transformation range which finally degraded superelastic response of the material.

HR-STEM investigation of atomic lattice defects in different types of η precipitates in creep-age forming Al–Zn–Mg–Cu aluminium alloy

High-resolution (HR) high-angle annular dark-field (HAADF) scanning transmission electron microscopy (STEM) has revealed the atomic lattice defects in different types of η precipitates in the Al–Zn–Mg–Cu aluminium alloy subjected to creep-age forming treatment (with a constant stress lower than its room-temperature yield strength during ageing). Along the zone axes of [110] Al //[2 1 ‾ 1 ‾ 0]η of η 1 and η 12 , [112] Al //[2 1 ‾ 1 ‾ 0]η of η 2 and [100] Al //[2 1 ‾ 1 ‾ 0]η of η 13 , atomic projections of (2 1 ‾ 1 ‾ 0)η have been investigated. In those types of η, elongated hexagonal lattice defects (labelled as Type I defects) can be found; they are apparently related to local disorder in atomic stackings. Furthermore, in η 12 , elongated hexagonal lattice defects with a much higher aspect ratio (labelled as Type II defects) are uniquely observed. These atomic lattice defects are presumably pertinent to the lattice accommodation in the course of creep-age forming. Additionally, in η 1 and η 12 , the features of a Penrose tiling defect connecting with Type I defects are observed, and these complex defects obviously affect the growth direction of the precipitate, resulting in a nearly spherical morphology. Alternatively, several entirely-passed faulted layers in a new type of precipitate, η 14 , consequently bring about a new orientation relationship: (51 3 ‾ ) Al //(0001)η 14 and [112] Al //[2 1 ‾ 1 ‾ 0]η 14 . Moreover, in an atomic STEM image of η 14 , the significant Z-contrast gradient adjacent to the transformation front of η 14 elucidates the Zn/Cu diffusion from the matrix to the precipitate along {1 1 ‾ 1 ‾ } Al planes at the interface. • Along the [2110]η zone axis, the edge-on atomic configurations of η 1 , η 2 , η 12 , η 13 and η 14 precipitates have been investigated to explore their atomic lattice defects. • Within the spherical-like morphologies of η 1 and η 12 , Penrose tiling features jointed with elongated hexagonal lattice defects were identified. • In η 12 precipitates, two types of elongated hexagonal lattice defects of different sizes were found. • Differently-oriented elongated hexagonal lattice defects presumably affect the local stackings, bringing about differently-oriented types of η precipitates. • In the newly-typed η 14 precipitate, some entirely-passed stacking fault layers presumably cause a new specific orientation relationship with respect to the Al matrix.

Establishing regularities in the propagation of phase transformation front during timber thermal modification

The creation of environmentally friendly protective materials for building structures made of wood could make it possible to influence the processes of stability and the physical-chemical properties at the thermal modification of hornbeam wood over a certain time. That necessitates studying the conditions for investigating phase transformations when the timber is exposed to high temperature, as well as establishing the mechanism of hornbeam wood thermal modification. Given this, a mathematical model of the phase transformation process during the transfer of heat flux to a sample was built. Based on the derived dependences, it was established that when hornbeam wood is exposed to temperature treatment, it undergoes endothermic phase transformations characterized by the heat absorption and change in the color of hornbeam wood. In particular, at a temperature of 200 °C, the temperature in the wood decreases by 5 % due to the chemical changes in the structure of cell wall components (lignin, cellulose, and hemicellulose). It was found that the process of thermal modification is accompanied by the decomposition of hemicellulose and the amorphous part of cellulose, a decrease in moisture absorption, as well as a decrease in the volume of substances that are a medium for the development of fungi. In addition, lignin and the resulting pseudo lignin undergo a process of polymerization and redistribution throughout the cell volume. At the same time, they give the cell walls higher density, hardness, increase hydrophobicity (water repellency), thereby reducing the ability to absorb moisture and swell. It was established that the most effective parameter of phase transformations is the temperature and aging duration. The results of moisture absorption have been given; it has been found that over 6 hours of modified timber exposure, its moisture absorption decreases by more than 10 times, which allows its application at facilities with high humidity

Formation mechanism and stability of austenitic islands in carbides in a Ni-Cr-Fe based high-temperature austenitic alloy undergoing carburization

We investigate a phase transformation in a Ni-Cr-Fe-based high-temperature alloy during the technologically important carburization process, with multi-length scale experimental techniques. The study focuses on the formation and stability of novel austenitic islands formed within carbide structures during the M23C6 to M7C3 phase-transformation. We demonstrate that the austenitic islands nucleate near the M23C6/M7C3 transformation front, from a supersaturation of metal atoms in the M23C6-carbide as M7C3 grows. After formation, the austenitic islands equilibrate their composition with the matrix temporally but remain relatively stable inside the carbides due to their large sizes (a few hundred nanometers in diameter) and a weak Gibbs-Thomson effect.

A numerical investigation of the nucleation and the propagation of NiTi martensitic transformation front under impact loading

A numerical investigation of NiTi SMA alloy under impact loading is presented in this paper. It is aimed at better understanding the observed behavior of NiTi SMA specimen loaded at impact velocities (up to 50m/s). A phenomenological thermodynamic model with an explicit integration frame is chosen and implemented as a user's law in Abaqus explicit commercial code. The parameters of the model are determined from the usual thermal-mechanical testing data without any tuning. The comparison between numerical and experimental results shows a satisfactory agreement, which proves the pertinence of the proposed numerical model.Using this numerical model, the reason of the experimentally observed inconstant strain level ahead of the phase transformation front is found. Instead of a material feature of NiTi SMA under impact loading, such a phenomenon is only due to the loading condition. Virtual tests at very high impact velocities are performed. Steep impulses without a rising time is chosen in order to have only one transformation front propagating inside the specimen, knowing that it is not possible to obtain such a simple condition in a real tensile test. A steady transformation front is observed and there is an asymptotical speed limit of this propagating front when the impact velocity increases. This speed limit is higher than the shear elastic wave speed but lower than the longitudinal elastic wave speed.

SIMULATION OF BEHAVIOR OF FUNCTIONALLY - HETEROGENEOUS MATERIALS UNDER TEMPERATURE LOADS

The paper considers the issue of nonlinear mathematical modeling of functionally inhomogeneous materials at temperature loads. The proposed model makes it possible to describe the thermo-pseudo-plastic behavior of the material at the point. The diagram of pseudo-elastic material consisting of three curvilinear sections is used. This approach leads to an unstable stress-strain diagram, and to describe the thermo-mechanical behavior of samples of different shapes, it is necessary to have a solution of the boundary value problem taking into account the development of the deformation front of the phase transformation. This takes into account not only the ambient temperature, but also the heat released at the point during the phase transition. A numerical procedure for calculating a material diagram has been developed, which is a curve enveloping a family of material diagrams constructed for certain laws of change in the velocity of the deformation rupture front. An integrated diagram of the material under the influence of a complex load is constructed.Keywords: phenomenological model, nonlinear material model, materials with shape memory, thermo-pseudo-plasticity, numerical procedure for calculating the diagram.