Porous Shape Memory Alloy Research Articles

High Damping of Lightweight TiNi-Ti2Ni Shape Memory Composites for Wide Temperature Range Usage

A bimodal porous TiNi-Ti2Ni shape memory alloy composite (SMAC) with 59% porosity was fabricated by sintering Ti-46at.%Ni elemental powders with pore-forming agent. The porous TiNi-Ti2Ni SMAC contains two irregular pores of about 400 and 120 μm. We investigated the microstructure and pore morphology correlated with the mechanical properties and damping capacities of the SMAC. Ti2Ni intermetallic phases with size of 1-3 μm were homogeneously distributed in the TiNi matrix. The porous TiNi-Ti2Ni SMAC exhibits exceptionally high inverse mechanical quality factor (Q −1) of ~0.25 at < 40 °C, which is among the highest value reported for porous/dense shape memory alloys or composites to best of our knowledge, and it shows very high compressive fracture strain of about 25%. Moreover, the fabricated porous SMAC at relatively low strain amplitude can exhibit considerable high Q −1 of 0.06 ~ 0.11 for a wide range of temperature between − 90 and 200 °C, which is attributed to the stress concentration distribution provided by the bimodal structure of pores and the massive interfaces between pore/matrix and TiNi/Ti2Ni. These porous SMACs can be an ideal candidate for using as a lightweight damping material in the energy-saving applications.

Distinctive Features of the Phase Composition of Porous TiNi-based Alloys Obtained by Reaction and Diffusion Sintering

The present article is concerned with questions of reaction and diffusion sintering of porous shape-memory TiNi-based alloys. The comparative analysis of structural features of the porous alloys obtained by diffusion sintering of TiNi powder and reaction sintering of Ti and Ni powders was conducted. It is observed that the main feature of structure of the porous alloys is related to fraction of the TiNi phase which occupies about 90 vol.% at diffusion sintering, and 20÷50 % of the total volume of multiphase alloy for reaction sintering. The mechanisms of the structure formation on the solid phase and liquid phase sintering stages of these methods were considered. The role of Ti2Ni phase during sintering was disclosed. The Ti2Ni phase not only provides the necessary quality of sintering, activates recrystallization processes for diffusion sintering, modifies the phase composition of the sintered specimen for reaction sintering, but also participates in the formation of the TiNi phase, increasing its fraction.

Dynamic and Static Compressive Behaviors of Highly Porous Ti–Ni–Mo Shape Memory Alloy

Dynamic and Static Compressive Behaviors of Highly Porous Ti–Ni–Mo Shape Memory Alloy

3D hierarchical multiscale analysis of heterogeneous SMA based materials

A 3D hierarchical multiscale computational homogenization framework with periodic Representative Volume Element (RVE) is proposed in the present study for analysis of shape memory alloy (SMA) fiber reinforced composite and porous SMA specimen. The shape memory alloy, which exhibits forward and reverse transformations, is wired through a matrix, while a dense SMA matrix encircles voids as inclusions in the porous model. The asymptotic mathematical homogenization approach is used to express the microscopic and macroscopic governing equations. An iterative solution is used at each point in order to derive the tangential constitutive response of the corresponding periodic RVE. Several numerical problems are solved and the results are compared with those obtained from a fully meshed direct numerical model. Detailed discussions are presented regarding the distribution of field variables, in particular the martensitic volume fraction and the stress state inside RVEs located at critical points of the macro models, subjected to cyclic stress loads in the form of tension, flexure and torsion. Moreover, a micromechanical analysis is conducted to investigate the effect of the shape of ellipsoid porosities on the overall response of porous SMA.

A beam model of porous shape memory alloy deformation

A model for simulation of functional-mechanical properties of porous TiNi shape memory alloy samples fabricated by self-propagating high temperature synthesis has been proposed. A sample is considered to consist of pores and ligaments allowing an account of the main sample structural features associated with synthesis conditions. The longitudinal ligaments are approximated by curved beams; the groups of transversal ligaments are regarded as flat slotted springs. An analysis of shapes and sizes of pores and ligaments allows identifying characteristic sizes of the beams. A microstructural model is used for calculation of the phase deformation of the shape memory material. Simulation of stress-strain curves at different temperatures and phase deformation of a porous TiNi sample on cooling and heating under a constant stress has shown a good agreement with experimental data.

Laser additive manufacturing of bulk and porous shape-memory NiTi alloys: From processes to potential biomedical applications

Abstract

Porosity exploration of SMA by Taguchi, regression analysis and genetic programming

Porosity plays a vital role in the field of bio-medical engineering of implantations i.e. orthopedic and orthodontics. Shape memory alloys exhibit a greater strength with a higher porosity. The strength of porous shape memory alloys were found similar to the strength of bones. In the present research, NiTi SMA is fabricated by powder metallurgy process. The processing parameters of sintering and compaction (i.e. compaction pressure, sintering temperature and sintering time) play an important role in the porosity investigation of SMA. Taguchi’s method based \(\hbox {L}_{9}\) orthogonal array was selected for the planning of experiments. Sintering temperature and sintering time were the significant process parameters as compared to compaction pressure. Regression coefficients and equation was derived by use of regression analysis. Further this equation was solved with the help of genetic programming and results of both (i.e. Taguchi’ method and genetic programming) were compared to find the maximum porosity. The maximum porosity that can be achieved is 56 % and the confirmation experiments were performed at 95 % confidence level.

Effects of high O contents on the microstructure, phase-transformation behaviour, and shape-recovery properties of porous NiTi-based shape-memory alloys

Porous NiTi-based shape-memory alloys (SMAs) were aged at different temperatures and holding times to study the effects of high O contents on the composition, microstructure, phase-transformation behaviour, and shape-recovery properties of SMAs. NiTi2 and Ni2Ti4Ox phases were precipitated during the aging process; the concentration was not significant until an aging temperature of 500°C was applied for 2h (O content=1.87wt.%). Aging the SMAs at 600°C for 2h produced a coarse NiTi2 phase and an O content of 4.3wt.%. Ti atoms were consumed during the oxidation process, which caused the excess Ni atoms to dissolve into the matrix or generate Ni-rich phases, such as Ni4Ti3 or Ni3Ti. The presence of excess Ni atoms resulted in different phase-transformation behaviours. A high O content increased the elastic modulus of the porous NiTi-based SMAs, but decreased the shape-recovery rate. The samples aged at 600°C exhibited inferior mechanical properties, and hence, it was concluded that high-temperature heat treatments should be avoided.

Shape memory characteristics of porous Ti-Ni-Mo alloys prepared by solid state sintering

Ti50Ni49.7Mo0.3, Ti50Ni49.6Mo0.4 and Ti50Ni49.5Mo0.5 alloy fibers were prepared by a melt overflow process. Two-step B2-R-B19′ transformation was observed in the rapidly solidified Ti-Ni-Mo fibers. Upon increasing the Mo-content from 0.3 to 0.5at.%, the austenite transformation finish temperature (Af) of R→B2 decreased from 43 to −42°C. Porous shape memory alloy pellets with 75% porosity were fabricated by a vacuum sintering technology, using the alloy fibers. Mechanical properties and shape memory effect of the highly porous Ti50Ni49.6Mo0.4 alloy, of which Af is lower than room temperature, were investigated using a compressive test. The plateau of a stress-strain curve was observed at about 2MPa and resulted in 5% elongation associated with stress-induced martensitic transformation. It was also found that a recovered strain was 1.8% on heating after the compressive deformation. Because of the high porosity of this specimen, an elastic modulus of 0.91GPa could be obtained.

Theoretical and numerical modeling of dense and porous shape memory alloys accounting for coupling effects of plasticity and transformation

The present work develops three-dimensional phenomenological constitutive models for dense and porous shape memory alloys (SMAs). The models are extensions of a recent work and considers pressure dependent behavior for porous SMAs as well as the coupling effects of transformation and plasticity for both dense and porous SMAs. In contrast to dense SMAs, a considerable plastic strain accumulates in porous SMAs even during phase transformation. Therefore, an effective solution algorithm for simultaneous evolution of transformation and plastic strain is presented via replacement of the classical Kuhn–Tucker inequality conditions by the so-called Fischer–Burmeister complementarity function. Numerical predictions are compared with experimental results and a comprehensive study is performed on the material parameters regarding coupling effects and pressure dependency. Moreover, we implement the model using corotational formulation and perform finite element analysis of a porous SMA spring actuator, and a tube under non-proportional loading to assess the reliability of the proposed model for large rotations and general multiaxial loadings.

Homogenization techniques for the analysis of porous SMA

In this paper the mechanical response of porous Shape Memory Alloy (SMA) is modeled. The porous SMA is considered as a composite medium made of a dense SMA matrix with voids treated as inclusions. The overall response of this very special composite is deduced performing a micromechanical and homogenization analysis. In particular, the incremental Mori---Tanaka averaging scheme is provided; then, the Transformation Field Analysis procedure in its uniform and nonuniform approaches, UTFA and NUTFA respectively, are presented. In particular, the extension of the NUTFA technique proposed by Sepe et al. (Int J Solids Struct 50:725---742, 2013) is presented to investigate the response of porous SMA characterized by closed and open porosity. A detailed comparison between the outcomes provided by the Mori---Tanaka, the UTFA and the proposed NUTFA procedures for porous SMA is presented, through numerical examples for two- and three-dimensional problems. In particular, several values of porosity and different loading conditions, inducing pseudoelastic effect in the SMA matrix, are investigated. The predictions assessed by the Mori---Tanaka, the UTFA and the NUTFA techniques are compared with the results obtained by nonlinear finite element analyses. A comparison with experimental data available in literature is also presented.

Effects of asymmetric material response on the mechanical behavior of porous shape memory alloys

The demand for lightweight devices attracts attention toward porous materials. Among them, porous shape memory alloys are of interest due to superior mechanical and biological properties. High cost related to fabrication and characterization of such materials makes it necessary to model their mechanical properties before fabrication. Experimental observations of dense shape memory alloys show tension–compression asymmetry which in turn can affect the mechanical response of porous ones. In this article, the effects of this asymmetric response on the mechanical response of porous shape memory alloys are investigated by comparing three models: asymmetric, symmetric with tensile, and symmetric with compressive material parameters. To this end, a constitutive model considering asymmetric material response is proposed based on microplane theory. Then, this model is used to simulate the stress–strain response of porous shape memory alloys. The results are compared with available experimental and numerical data, and a good agreement is observed. It is concluded that in comparison with the asymmetric model, the symmetric model with tensile material parameters under-predicts the stress level while the model with compressive one over-predicts the stress level. In addition, the effects of porosity on the asymmetric response as well as hysteresis of stress–strain curve in tension and compression are assessed.

A Unit Cell Model for Simulating The Stress-Strain Response of Porous Shape Memory Alloys

Porous shape memory alloys are a new class of advanced materials with combined advantages of both shape memory alloys and porous materials. In order to manufacture a porous shape memory alloy with the desired mechanical properties, it is important to predict its mechanical properties before fabrication. In this paper, a new unit cell model is proposed to simulate the mechanical stress-strain response of porous shape memory alloys. Microplane theory is used to attribute mechanical constitutive relations of shape memory alloys to the bulk material, and the finite element method is employed for numerical simulations. The results show a good agreement with the experimental stress-strain behavior reported in the literature. The effect of pore volume fraction on the stress-strain response is also studied using the proposed approach. Random microstructures are generated in the FE model, and the effects of randomness on the mechanical behavior of porous shape memory alloys are also investigated for different values of pore volume fraction.

Remarkable superelasticity of sintered Ti–Nb alloys by Ms adjustment via oxygen regulation

Abstract Nickel-free Ti–Nb alloys were fabricated by conventional powder metallurgy sintering method. The oxygen content of sintered Ti–Nb alloys were successfully decreased to around 0.8 wt.% by putting TiH2 powders aside the samples during the sintering process. For the first time, the phase transformation behaviors of these sintered Ti–Nb alloys were observed by differential scanning calorimetry (DSC) measurement. The relationship between martensitic transformation start temperature (Ms) and mechanical properties, including elastic modulus and recoverable strain, was established by compression tests carried out at room temperature. While Ms was close to test temperature, an obvious drop of elastic modulus was observed, which was related to the “β-α” transformation. In the meantime, a remarkable recoverable strain as high as 5% was obtained at room temperature, which is the highest value reported in sintered Ti–Nb alloys until now. The results of this study reveal that the recoverable strain can be further improved by adjusting Ms close to the service temperature, which was neglected in the reported studies before and will provide some guidance for the future design and fabrication of porous Ni-free Ti-based shape memory alloys for biomedical application.

Micromechanical analysis of porous SMA

The present paper deals with computational micromechanical analyses of porous shape memory alloy (SMA). Porous SMAs are considered composite materials made of a dense SMA matrix including voids. A three-dimensional constitutive law is presented for the dense SMA able to reproduce the pseudo-elastic as well as the shape memory effects and, moreover, to account for the different elastic properties of the austenite and martensite phases. Furthermore, a numerical procedure is developed and the overall behavior of the porous SMA is recovered studying a representative volume element. Comparisons between the numerical results, recovered using the proposed modeling, and experimental data available in the literature are presented. The case of closed and open porosity is investigated. Parametric studies have been conducted in order to investigate the influence of the porosity, the shape and orientation of the pores on the overall mechanical response and, mainly, on the energy absorption dissipation capability.

Submicron-porous NiTi and NiTiNb shape memory alloys with high damping capacity fabricated by a new top-down process

In this paper, submicron-porous NiTi and NiTiNb shape memory alloys were firstly fabricated by a new top-down process, including solidified phase controlling and etching process. These porous alloys possessed fine pore size (∼0.4 μm) and homogeneous pore distribution. The mechanical properties of these porous alloys were similar to those prepared by conventional powder metallurgy techniques or bottom-up process. However, the damping capacity of the submicron-porous NiTi was ∼0.07 and 40% higher than the counterparts produced by the conventional powder metallurgy techniques. The damping capacity of the submicron-porous NiTiNb was ∼0.18 and 50% higher than its dense counterpart. The internal friction value at austenite state for submicron-porous NiTiNb was ∼0.04 and 200% higher than its dense counterpart, 300% higher than submicron-porous NiTi. The high damping capacity of these submicron-porous shape memory alloys was considered to be caused partly by their fine pore structures, which enhanced the stress concentration and the microplastic deformation or collapse of thin nodes. These porous shape memory alloys exhibited moderate stiffness as well as high damping capacity and high specific strength, making them good candidate as practical engineering damping materials.

A three-dimensional phenomenological constitutive model for porous shape memory alloys including plasticity effects

Porous shape memory alloys are a class of very interesting materials exhibiting features typical of porous metals and of shape memory alloys. In contrast to dense shape memory alloys, considerable plastic strain accumulates in porous shape memory alloys even during phase transformation. Moreover, due to the microstructure of porous materials, phase transformation and plasticity phenomena are significantly pressure-dependent. In this article, we propose a three-dimensional phenomenological constitutive model for the thermomechanical behavior of porous shape memory alloys able to predict shape memory effect, pseudo-elastic behavior and plastic behavior under proportional as well as non-proportional multiaxial loadings. To this end, proper internal variables together with free energy and limit functions are introduced. The material parameters are determined for several available porous shape memory alloys with a wide range of porosity (from 16% to 51%), and a good agreement between numerical predictions of the proposed model and experimental results is observed. Moreover, plastic and transformation strain evolution during increasing and decreasing cyclic loadings are demonstrated through several examples. Finally, to show model capabilities, shape memory effect and pressure-dependent behavior as well as model response under non-proportional loadings are also presented.

In vitro and in vivo evaluation of porous TiNi-based alloy as a scaffold for cell tissue engineering

This study aims to look into the applicability of a porous TiNi-based shape memory alloy (SMA) scaffold as an incubator for bone marrow mesenchymal cells, hepatocytes, and pancreatic islet cells. The porous TiNi-based SMA used was fabricated using a self-propagating high-temperature synthesis (SHS) technique, in which scaffold blocks measuring 4 × 4 × 10 mm were prepared. In vitro tests were done using mesenchymal stem cells (MSC) isolated from mature bone marrow of CBA/j inbred mice, and cultured in 3 different culture media – Control medium, Osteogenic medium, and Chondrogenic medium. Hepatocytes and islet cells were isolated from the livers and pancreatic glands of Wistar rats respectively, seeded on porous TiNi-based SMA scaffolds, and cultured. The scaffolds were then implanted into the abdominal cavity of Wistar rats and later harvested, at days 7, 14, 21, and 28, post-implantation. SEM imaging was performed with pre-implanted scaffolds at day 0 and harvested scaffolds at days 7, 14, 21, and 28, post-implantation. Based on weight increase percentages, the in vitro study revealed that the osteogenic group showed a 2-fold increase, and the chondrogenic group showed a 1.33-fold increase, compared to the control group. The in vivo study, on the other hand, showed that from day 7 post-implantation, the cellular in-growth gradually invaded the inner porous structure from the periphery towards the center, and at day-28 post-implantation, all pores were closed and completely filled with cells and the extracellular matrix. The results show that porous TiNi-based SMA is a unique biocompatible incubator for cell cultures and can be successfully used for tissue bioengineering and artificial organs.

Modeling of Functional Properties of Porous Shape Memory Alloy

A model accounting for the microstructure of porous TiNi shape memory alloy samples fabricated by self-propagating high temperature synthesis has been proposed for simulation of their functional-mechanical properties. Structural elements of a porous sample have been approximated by curved beams. An analysis of shapes and sizes of pores and ligaments permitted to identify characteristic sizes of the beams. A mathematical object consisting of rigidly connected small curve beams has been considered. The stress-strain state of a beam was estimated by the classical methods of strength of materials. The microstructural model was used for calculation of the phase deformation of the shape memory material. Simulation of stress-strain curves and phase deformation of a porous TiNi sample on cooling and heating under a constant stress has shown a good correspondence between the experimental data and the results of modeling.

Influence of TiHX Addition on SHS Porous Shape Memory Alloy

Porous NiTi alloys are receiving considerable attention as they can be used as scaffold for bone replacement. Most production routes presented in the literature use metal powders as raw material (pure Ni and Ti or prealloyed NiTi powders): among these processes, Self propagating High temperature Synthesis (SHS) is investigated as a possible energy saving, quick and easy method of production. To obtain porous NiTi, compacted Ti and Ni powders are preheated and then ignited, avoiding high reaction temperatures at which the compound melts and consequently pores collapse. A drawback of low reaction temperatures is the formation of secondary phases. In this paper the addition of hydrided titanium (TiHX, x=1.5-1.9) powder is considered. During the reaction, hydrided titanium endothermically decomposes and can act as process controlling media. Reference Ni-Ti and Ni-TiHx mixed powders were reacted and the temperature evolution monitored. Differential Scanning Calorimetry was used to verify the presence of transforming phases (austenite, martensite). Microstructure characterization was performed with X-ray diffraction analysis and scanning electron microscope, equipped with EDX and EBSD detectors. The results confirmed that decomposition of hydrided titanium is the controlling process of the reaction, limiting the availability of Ti and absorbing reaction heat. The presence of TiHx can suppress SHS reaction, leaving un-reacted Ni and Ti powders and high amount of other intermetallic phases. If partial or complete decomposition of TiHx is allowed during preheating of reactants, NiTi production can occur: secondary phases content decreases for increased decomposition of TiHx before SHS reaction.