Porous Shape Memory Alloy Research Articles

Characterization of NiTi and NiTiCu Porous Shape Memory Alloys Prepared by Powder Metallurgy (Part I)

In this paper, the effect of compacting pressure (150–450 MPa) and copper (Cu) additions (2.5, 5, 7.5 and 10 wt%) on the microstructure and physical properties of NiTi-based shape memory alloys prepared by powder metallurgy is studied. Many characterization techniques were employed in this study such as X-ray diffraction method and scanning electron microscope. The chemical composition of the prepared alloys and microstructure was achieved by using scanning electron microscope equipped with EDS. Differential scanning calorimetric is utilized to measure the transformation temperature of the prepared alloys. Several physical tests such as particle size distribution measurements, density and porosity of green compacted samples and density and porosity after sintering are achieved. XRD test shows that the master sintered samples consist of three phases at room temperature: NiTi monoclinic phase, NiTi cubic phase and Ni3Ti hexagonal phase. Furthermore, (CuNi2Ti) intermetallic compound is appeared in the samples with 2.5, 5, 7.5 and 10 wt% of Cu. According to the results, it was found that compacting pressure has an essential effect on the decreases in porosity percentage and the results show that (Cu) additions increase porosity percentage in all weight percentages of additives.

A 3-D constitutive model for pressure-dependent phase transformation of porous shape memory alloys

Porous shape memory alloys (SMAs) exhibit the interesting characteristics of porous metals together with shape memory effect and pseudo-elasticity of SMAs that make them appropriate for biomedical applications. In this paper, a 3-D phenomenological constitutive model for the pseudo-elastic behavior and shape memory effect of porous SMAs is developed within the framework of irreversible thermodynamics. Comparing to micromechanical and computational models, the proposed model is computationally cost effective and predicts the behavior of porous SMAs under proportional and non-proportional multiaxial loadings. Considering the pressure dependency of phase transformation in porous SMAs, proper internal variables, free energy and limit functions are introduced. With the aim of numerical implementation, time discretization and solution algorithm for the proposed model are also presented. Due to lack of enough experimental data on multiaxial loadings of porous SMAs, we employ a computational simulation method (CSM) together with available experimental data to validate the proposed constitutive model. The method is based on a 3-D finite element model of a representative volume element (RVE) with random pores pattern. Good agreement between the numerical predictions of the model and CSM results is observed for elastic and phase transformation behaviors in various thermomechanical loadings.

Effect of Pore Structure Regulation on the Properties of Porous TiNbZr Shape Memory Alloys for Biomedical Application

Recently, porous Ti-Nb-based shape memory alloys have been considered as promising implants for biomedical application, because of their non-toxic elements, low elastic modulus, and stable superelasticity. However, the inverse relationship between pore characteristics and superelasticity of porous SMAs will strongly affect their clinical application. Until now, there have been few works specifically focusing on the effect of pore structure on the mechanical properties and superelasticity of porous Ti-Nb-based SMAs. In this study, the pore structure, including porosity and pore size, of porous Ti-22Nb-6Zr alloys was successfully regulated by adjusting the amount and size of space-holder particles. XRD and SEM investigation showed that all these porous alloys had homogeneous composition. Compression tests indicated that porosity played an important role in the mechanical properties and superelasticity of these porous alloys. Those alloys with porosity in the range of 38.5%-49.7% exhibited mechanical properties approaching to cortical bones, with elastic modulus, compressive strength, and recoverable strain in the range of 7.2-11.4 GPa, 188-422 MPa, and 2.4%-2.6%, respectively. Under the same porosity, the alloys with larger pores exhibited lower elastic modulus, while the alloys with smaller pores presented higher compressive strength.

Plastic and transformation interactions of pores in shape memory alloy plates

A three-dimensional constitutive model for shape memory alloys (SMAs) is developed along the lines of the Stebner–Brinson (SB) implementation of the Panico–Brinson model. Plastic kinematic hardening behavior is simulated in addition to elastic deformation and phase transformation. A series of finite element simulations is carried out using this model to investigate the localization effects of the stress and strain field on NiTi plates with structured arrays of pores. The application of this model on porous architectures provides insight into how geometric features influence the mechanics of the structure. The incorporation of plastic deformation shows a marked decrease in the maximum stress levels; these results are more consistent with experimental data as compared to the original SB model. Furthermore, the new results demonstrate that clustered pores lead to more distributed stresses and transformation compared to a dispersed configuration of pores, indicating the importance of pore geometry in determining the stress and strain distribution. The improved model provides a practical tool toward design and optimization of porous SMA structures.

Effects of aging on the structure and damping behaviors of a novel porous CuAlMn shape memory alloy fabricated by sintering–dissolution method

Effects of aging on the microstructures and damping behaviors of a porous CuAlMn shape memory alloy were studied. It is found that after aging at 350°C both the damping capacity of the alloy in the martensite state and the height of the internal friction peak arising from the reverse martensitic transformation increase due to the refining of martensitic structure and the annihilation of quenched-in vacancies. It is also found that the atomic ordering and the annihilation of quenched-in vacancies can promote the reverse martensitic transformation to take place, resulting in a significantly decreased temperature of the internal friction peak.

Response of porous SMA: a micromechanical study

Lately porous shape memory alloys (SMA) have attracted great interest as low weight materials characterized by high energy dissipation capability. In the present contribution a micromechanical study of porous SMA is proposed, introducing the simplifying hypothesis of periodic distribution of voids. The mechanical response of the heterogeneous porous medium is derived by performing nonlinear finite element micromechanical analyses considering a typical repetitive unit cell made of a circular hole in a dense SMA matrix and prescribing suitable periodicity and continuity conditions. The constitutive behavior and the dissipation energy capability of the porous Nitinol are examined for several porosity levels. Numerical applications are performed in order to test the ability of the proposed procedure to well capture the overall behavior and the key features of the special heterogeneous material.

A phenomenological constitutive model for Functionally Graded Porous Shape Memory Alloy

Functionally Graded Porous Shape Memory Alloy (FGP-SMA), whose porosity varies continuously along gradient direction, has attracted wide attention in the field of smart materials. Considering the Gibbs free energy, a phenomenological constitutive model which can be used to predict the mechanical behaviors of FGP-SMA is presented here by using the theory of thermodynamics. A new transformation function considering the effect of hydrostatic stress is proposed. A finite element model is also provided to investigate the mechanical properties of a FGP-SMA cylinder. The uniaxial compression test of the FGP-SMA cylinder is simulated by using those two models mentioned above. Numerical results show a reasonable agreement with experimental results, which implies that the models established in this work are valid. What’s more, the average stress–strain relation, the stress distribution on the cross section and the energy absorption properties of the FGP-SMA cylinder are investigated in detail. The results obtained demonstrate several interesting features of this new material, which may have potential applications in practice.

A constitutive model of porous SMAs considering tensile–compressive asymmetry behaviors

A constitutive model of the macroscopic behaviors of porous shape memory alloys (SMA) is developed in this work. A yield function for porous SMAs considering both the effect of hydrostatic stress and the tensile–compressive asymmetry is proposed. Combining the constitutive model of dense SMAs and the macroscale and microscale analysis, the evolution equation for the overall transformation strain is then derived. Examples for the response of both dense SMA and porous Ni–Ti SMA subjected to uniaxial tension and compression loads are supplied. Good agreement between the numerical prediction results and the published experimental data is observed. Numerical result shows that the yielding stresses, loop width and length, strain-hardening behaviors of porous SMAs under pure tensile and pure compressive are different. Importantly, the transformation initiation stress is much closer to the experiment result than simulated by Zhao et al. (2005).

Density Dependence of the Macroscale Superelastic Behavior of Porous Shape Memory Alloys: A Two-Dimensional Approach

Porous Shape Memory Alloys (SMAs) are of particular interest for many industrial applications, as they combine intrinsic SMA (shape memory effect and superelasticity) and foam characteristics. The computational cost of direct porous material modeling is however extremely high, and so designing porous SMA structure poses a considerable challenge. In this study, an attempt is made to simulate the superelastic behavior of porous materials via the modeling of fully dense structures with material properties modified using a porous/bulk density ratio scaling relation. Using this approach, direct modeling of the porous microstructure is avoided, and only the macroscale response of the model is considered which contributes to a drastic reduction of the computational cost. Foam structures with a gradient of porosity are also studied, and the prediction made using the fully dense material model is shown to be in agreement with the mesoscale porous material model.

Phase transformation and damping behavior of lightweight porous TiNiCu alloys fabricated by powder metallurgy process

Porous TiNiCu ternary shape memory alloys (SMAs) were successfully fabricated by powder metallurgy method. The microstructure, martensitic transformation behavior, damping performance and mechanical properties of the fabricated alloys were intensively studied. It is found that the apparent density of alloys decreases with increasing the Cu content, the porous Ti50Ni40Cu10 alloy exhibits wide endothermic and exothermic peaks arisen from the hysteresis of martensitic transformations, while the porous Ti50Ni30Cu20 alloy shows much stronger and narrower endothermic and exothermic peaks owing to the B2-B19 transformation taking place easily. Moreover, the porous Ti50Ni40Cu10 alloy shows a lower shape recovery rate than the porous Ti50Ni50 alloy, while the porous Ti50Ni30Cu20 alloy behaves reversely. In addition, the damping capacity (or internal friction, IF) of the porous TiNiCu alloys increases with increasing the Cu content. The porous Ti50Ni30Cu20 alloy has very high equivalent internal friction, with the maximum equivalent internal friction value five times higher than that of the porous Ti50Ni50 alloy.

Density dependence of the superelastic behavior of porous shape memory alloys: Representative Volume Element and scaling relation approaches

As the use of Shape Memory Alloys (SMAs) grows increasingly common in many industrial applications, the porous form of SMA is of particular interest as it associates both the shape memory effect and superelasticity with the characteristics of foam. However, numerical prediction of the mechanical response of SMA foam is very challenging due to the micro–macro nature exhibited by the material, as the porous microstructure is several orders of magnitude smaller than the overall dimensions of the macroscopic porous sample. To circumvent, or at least alleviate this computational weight, an attempt is made to describe the superelastic behavior of SMA foams using two approaches: Representative Volume Element (RVE) and scaling relation; the latter is based on modeling the fully-dense material with mechanical properties equivalent to those of its porous counterpart. This approach avoids direct modeling of the porous microstructure and thus contributes to a drastic reduction of the computational cost. A validation is made by comparing the numerical results obtained in this study with experimental results taken from the literature.

Fabrication and internal friction behaviors of novel porous CuAlMn shape memory alloy filled with polystyrene

Polystyrene layer with adjustable thickness was introduced into a porous CuAlMn shape memory alloy, and the internal friction behaviors of the resultant materials were investigated using a multifunctional internal friction apparatus through the method of forced vibration. Two relaxational internal friction peaks accompanied by a sharp decrease of the relative dynamic modulus were found at around 95°C and 119°C, respectively. The two internal friction peaks have been ascribed to the relaxation process in a thermal equilibrium state of the polystyrene over its glass transition temperature. The damping capacity of the resultant materials was greatly elevated due to the superposition of several individual damping sources in them.

Influence of chemical composition and pre-heating temperature on the structure and martensitic transformation in porous TiNi-based shape memory alloys, produced by self-propagating high-temperature synthesis

A study of the influence of the chemical composition and the pre-heating temperature of the Ti–Ni powder mixture on kinetics of martensitic transformation and structure in the porous TiNi alloys produced by SHS was carried out. It was found that a variation in the nickel concentration in the powder mixture influenced the volume fraction of the TiNi phase undergoing martensitic transformation. An increase in the nickel concentration from 45 at.% to 52 at.% led to an increase in the volume fraction of the TiNi phase from 60% to 90% in the alloy and a decrease in the alloy volume undergoing martensitic transformation from 60% to 10%. It was shown that in the porous TiNi alloys with the Ni concentration varied from 48 at.% to 50 at.% the martensitic transformations were found in two temperature intervals. The first interval was from 347 K to 304 K and it was due to the B2 → B19′ transformation occurring in the volume of the TiNi phase with a nickel concentration close to 50.0 at.%. The second temperature interval was from 252 K to 198 K and it was caused by the same transformation occurring in the volumes of the TiNi phase with a nickel concentration close to 51%. The value of the pre-heating temperature did not influence the characteristics of the martensitic transformations, except in the Ti – 48 at.% Ni alloy where an increase in the pre-heating temperature resulted in an increase in the volume fraction of the TiNi phase undergoing martensitic transformation from 30% to 60%.

Wear Properties of Porous NiTi Orthopedic Shape Memory Alloy

Porous NiTi shape memory alloy (SMA) scaffolds have great potential to be used as orthopedic implants because of their porous structure and superior physical properties. Its metallic nature provides it with better mechanical properties and Young’s modulus close to that of natural bones. Besides allowing tissue ingrowth and transfer of nutrients, porous SMA possesses unique pseudoelastic properties compatible to natural hard tissues like bones and tendons, thus expediting in vivo osseointegration. However, the nickel release from debris and the metal surface may cause osteocytic osteolysis at the interface between the artificial implants and bone tissues. Subsequent mobilization may finally lead to implant failure. In this study, the wear properties of porous NiTi with different porosities processed at different treatment temperatures are determined. The results of the study show that the porosity, phase transformation temperature, and annealing temperature are major factors influencing the wear characteristics of porous NiTi SMA.

A Brief Review of Biomedical Shape Memory Alloys by Powder Metallurgy

In the realm of bioimplantation, titanium-based Shape Memory Alloys (SMAs) exhibit phenomenal versatility, with successful application in diverse fields. One area of particular interest is that of orthopaedics, where the unique properties of SMAs offer a range of benefits. That said, existing alloys still have unresolved issues concerning biocompatibility and osseointegration. Primary concerns include carcinogenicity, allergenicity and a significant mismatch between the Young’s moduli of bone and osteoimplants; issues that could be addressed via a novel porous titanium alloy. With that in mind, this paper seeks to provide a review identifying promising candidates for new, perfectly biocompatible alloys for production via powder metallurgy. Furthermore, an attempt will also be made to summarise existing research into appropriate methods for the production of a porous Ti-based SMA implant.

Effect of spherical micro-voids in shape memory alloys subjected to uniaxial loading

In this study the effect of micro-voids on the superelastic–plastic behavior of shape memory alloys is investigated. A new constitutive model for porous shape memory alloys, based on the Gurson–Tvergaard–Needleman formulation, is proposed. The model is able to reproduce both forward and reverse stress induced phase transformation, as well as plastic deformation. In addition, the model accounts for the presence of micro-voids and void-growth through a void volume fraction. A one dimensional implementation has been conducted, and results from the new constitutive model is compared with finite element unit-cell analyses. The model does well in reproducing results from unit-cells with void volume fractions f0<0.05. However, some discrepancy is found for void volume fractions f0⩾0.05 that can be attributed to the highly inhomogeneous stress field in the unit-cells which the new model does not account for. The main results show that even for relatively small micro-voids the stress at which transformation and plastic yielding is initiated is lowered. Also, the presence of micro-voids leads to a narrowing of the stress–strain hysteresis which affects the amount of energy dissipated during a superelastic cycle.

A phenomenological two-phase constitutive model for porous shape memory alloys

We present a two-phase constitutive model for pseudoelastoplastic behavior of porous shape memory alloys (SMAs). The model consists of a dense SMA phase and a porous plasticity phase. The overall response of the porous SMA is obtained by a weighted average of responses of individual phases. Based on the chosen constitutive model parameters, the model incorporates the pseudoelastic and pseudoplastic behavior simultaneously (commonly reported for porous SMAs) as well as sequentially (i.e. dense SMAs; pseudoelastic deformation followed by the pseudoplastic deformation until failure). The presented model also incorporates failure due to the deviatoric (shear band formation) and volumetric (void growth and coalescence) plastic deformation. The model is calibrated by representative volume elements (RVEs) with different sizes of spherical voids that are solved by unit cell finite element calculations. The overall response of the model is tested against experimental results from literature. Finally, application of the presented constitutive model has been presented by performing finite element simulations of the deformation and failure in unaixial dog-bone shaped specimen and compact tension (CT) test specimen. Results show a good agreement with the experimental data reported in the literature.

Magnetic response of porous NiCoMnSn metamagnetic shape memory alloys fabricated using solid-state replication

NiCoMnSn porous metamagnetic shape memory alloys were successfully fabricated using solid-state replication of ammonium bicarbonate powders via sintering at 1273 K. Around 50% porosity of 300–500 μm interconnected pores was achieved. Oxygen contamination from the space holder and binder, which negatively influences the magneto-thermal coupling, was prevented using a titanium oxygen getter during sintering. The resulting porous samples demonstrate a metamagnetic response similar to the homogenized powder, offering possible utilization in magnetic refrigeration and actuation.

Internal Friction Peaks in a Porous CuAlMn Shape Memory Alloy

Internal friction behavior of a porous CuAlMn shape memory alloy was examined. Two internal friction peaks appear at around 350 oC and 380 oC during the first heating process in the as-quenched specimen, but they disappear during the second or more heating process. Two other internal friction peaks are found at around 150 oC and 325 oC from the second heating process. Correlated mechanisms of the peaks are discussed.

Shape memory characteristics of Ti–Ni–Mo alloys sintered by sparks plasma sintering

Abstract Porous shape memory alloys have attracted great interest as low-weight materials with high energy dissipation properties. In the biomedical field, owing to their biocompatibility and their promise to exhibit high strength and low modulus, porous Ti–Ni alloys have been tested as bone implant materials, successfully exhibiting a significant amount of bone ingrowth. In order to use the excellent superelastic property of shape memory alloys in many medical applications, the austenite transformation finish temperature ( A f ) must be about 37 °C which is human body temperature. In this study, the transformation temperature could be controlled through the substitution of Mo for Ni in a Ti–Ni alloy. For the fabrication of bulk near-net-shape shape memory alloys and porous metallic biomaterials, Ti 50 Ni 49.9 Mo 0.1 and Ti 50 Ni 49.7 Mo 0.3 shape memory alloy powders were prepared by gas atomization and the transformation temperatures and microstructures of those powders were investigated as a function of powder size. The dependence of powder size on the martensitic transformation temperature is also very small in the powders ranging from 25 to 150 μm. The phase transformation temperatures of the porous specimens are almost the same as those of as-atomized powders and the A f of Ti 50 Ni 49.9 Mo 0.1 and Ti 50 Ni 49.7 Mo 0.3 porous specimens is 40.4 and 34.4 °C, respectively. According to compressive tests, the porous samples exhibit shape memory effect and it is found that the recovered stains of the Ti 50 Ni 49.9 Mo 0.1 and Ti 50 Ni 49.7 Mo 0.3 specimens are 1.5 and 2.0%, respectively.