Porous NiTi Shape Memory Alloy Research Articles

Stress–strain behavior of porous NiTi alloys prepared by powders sintering

Porous NiTi shape memory alloys have recently attracted extensive interests for their potential use as biomedical materials. Stress–strain behaviors of porous NiTi alloy fabricated by element powder sintering in argon atmosphere were investigated in this paper by compression and flexural tests. The results indicated that the ultimate compressive strength and the flexural strength of the porous NiTi alloy depended on the densities of the porous NiTi alloy and the sintering conditions. In compression experiments, the pre-strain could completely recover while it was less than 2%. The shape recovery of porous NiTi alloy rested with the unique SME of NiTi phase and pores structure. The pores have adverse effects on the shape recovery of NiTi alloy under the experiment conditions.

Effects of aging temperature on the martensitic transformation in porous Ni-rich NiTi shape memory alloy fabricated by combustion synthesis

Effects of aging temperature on the martensitic transformation in porous Ni-rich NiTi shape memory alloy fabricated by combustion synthesis

Control of porosity and superelasticity of porous NiTi shape memory alloys prepared byhot isostatic pressing

Porous Ni-rich NiTi SMAs have been successfully prepared from elemental Ni and Tipowders, using capsule-free hot isostatic pressing processing. The fabricated alloys haveisotropic and uniform pore distributions of spherical pores ranging from 50 to200 µm. The porosities of these NiTi alloys range from 30 to 40% by volume. The microstructure,martensitic transformation behaviors and mechanical properties of the fabricated porousNiTi SMAs aged under different conditions were studied. It has been found that the phasetransformation behaviors of porous NiTi SMAs are similar to those of conventionally castNi-rich NiTi alloys. It is encouraging that the porous NiTi SMAs exhibit excellentsuperelasticity at room temperature after the appropriate thermomechanical treatment.Experimental results reveal that the superelasticity, which is controlled by the porecharacteristics, such as pore shape, pore size and pore distribution, is important forimproving the toughness, elastic properties and compression strength of the porous NiTiSMAs.

Laser fabrication of porous surface layer on NiTi shape memory alloy

Porous NiTi shape memory alloys provide enhanced osteointegration by facilitating tissue ingrowth and transport of body fluid to the implant. To retain the excellent thermomechanical properties of bulk NiTi and to improve its biocompatibility, a porous surface layer was fabricated on bulk NiTi samples using laser surface engineering techniques. A powder mixture of NiTi and TiH 2 was laser surface alloyed on the SMA. The macrostructure and microstructure of the modified surface layer were studied. A porous layer of a few hundred micrometers thick was formed, composed of NiTi, Ti 2Ni, TiNi 3 and TiN. The pores in the layer exhibited an inter-connected morphology in 3D. The Ni content was reduced from 49.2 at.% in the substrate to 12.5 at.% near the surface of the modified layer.

Experimental characterization and micromechanical modeling of superelastic response of a porous NiTi shape-memory alloy

Porous shape-memory alloys are usually brittle due to the presence of various nickel–titanium intermetallic compounds that are produced in the course of most commonly used synthesizing techniques. We consider here a porous NiTi shape-memory alloy (SMA), synthesized by spark-plasma sintering, that is ductile and displays full shape-memory effects over the entire appropriate range of strains. The porosity however is only 12% but the basic synthesizing technique has potential for producing shape-memory alloys with greater porosity that still are expected to display superelasticity and shape-memory effects. The current material has been characterized experimentally using quasi-static and dynamic tests at various initial temperatures, mostly within the superelastic strain range, but also into the plastic deformation regime of the stress-induced martensite phase. To obtain a relatively constant strain rate in the high strain-rate tests, a novel pulse-shaping technique is introduced. The results of the quasi-static experiments are compared with the predictions by a model that can be used to calculate the stress–strain response of porous NiTi shape-memory alloys during the austenite-to-martensite and reverse phase transformations in uniaxial quasi-static loading and unloading at constant temperatures. In the austenite-to-martensite transformation, the porous shape-memory alloy is modeled as a three-phase composite with the parent phase (austenite) as the matrix and the product phase (martensite) and the voids as the embedded inclusions, reversing the roles of austenite and martensite during the reverse transformation from fully martensite to fully austenite phase. The criterion of the stress-induced martensitic transformation and its reversal is based on equilibrium thermodynamics, balancing the thermodynamic driving force for the phase transformation, associated with the reduction of Gibbs’ free energy, with the resistive force corresponding to the required energy to create new interface surfaces and to overcome the energy barriers posed by various microstructural obstacles. The change in Gibbs’ free energy that produces the driving thermodynamic force for phase transformation is assumed to be due to the reduction of mechanical potential energy corresponding to the applied stress, and the reduction of the chemical energy corresponding to the imposed temperature. The energy required to overcome the resistance imposed by various nano- and subnano-scale defects and like barriers, is modeled empirically, involving three constitutive constants that are then fixed based on the experimental data. Reasonably good correlation is obtained between the experimental and model predictions.

Fabrication and properties of porous NiTi shape memory alloys for heavy load-bearing medical applications

Porous NiTi shape memory alloys (SMA) with isotropic three-dimensionally interconnected pore structure, higher strength and less undesired Ti 2Ni phases were fabricated using the improved self-propagating high-temperature synthesis (SHS) technique by means of providing the higher heat flux density for SHS reactions using the improved ignition technique and adopting the powder compacts with a larger volume–surface area ratio to reduce heat losses. The combustion temperature agrees well with the melting point of compound NiTi (1583 K) under the two preheating temperatures (623 and 673 K). The porous NiTi SMAs fabricated by such SHS technique have the higher compressive strength than that of the compact human bone and the similar Young's modulus to that of natural bones, which is anticipated for the ideal biomaterials used as the heavy load-bearing hard tissue implants. Moreover, the transformation temperatures of the porous NiTi SMAs could be easily adjusted to the suitable values for medical applications by heat treatment.

Characterization of transformation behavior in porous Ni-rich NiTi shape memory alloy fabricated by combustion synthesis

Characterization of transformation behavior in porous Ni-rich NiTi shape memory alloy fabricated by combustion synthesis

SMS-10: Fabrication, Characterization and Modeling of Porous NiTi Shape Memory Alloy(SMS-II: SMART MATERIALS AND STRUCTURES, NDE)

SMS-10: Fabrication, Characterization and Modeling of Porous NiTi Shape Memory Alloy(SMS-II: SMART MATERIALS AND STRUCTURES, NDE)

Compression behavior of porous NiTi shape memory alloy

Porous NiTi alloy with several different porosities was processed by spark plasma sintering. The compression behavior of the porous NiTi was examined with the aim of using it possibly as a high energy absorbing material. A model for the macroscopic compression behavior of porous shape memory alloy (SMA) is presented in this work, where Eshelby’s inhomogeneous inclusion method is used to predict the effective elastic and superelastic behavior of a porous SMA based on the assumption of stress–strain curve. The analytical results are compared with experimental data for porous NiTi with 13% porosity, resulting in a reasonably good agreement.

Phase transformation behaviors in porous Ni-rich NiTi shape memory alloy fabricated by combustion synthesis

The phase transformation behaviors that change with the aging temperature in the SHS-synthesized porous Ni-rich NiTi SMA after solution treated at 1050 °C for 2 h were investigated by differential scanning calorimeter (DSC). The experimental results show that the type of phase transformation changes from one-step (after solution treatment) to two-step (after subsequent aging treatment). With increasing aging temperatures, the transformation temperatures ( R s and A s ) decrease, and the positions of the first peaks on the DSC charts during cooling and the last peaks on the DSC charts during heating shift to lower temperatures. The positions of the last peaks on DSC charts on cooling of the porous Ni-rich NiTi SMA aged at different temperatures between 400 and 550 °C for 1 h correspond to the position of the single DSC peak obtained for the solution treated materials. The interpretation for the features in the phase transformation behaviors of the porous Ni-rich NiTi SMA fabricated by combustion synthesis is proposed.

DSC study of the effect of aging temperature on the reverse martensitic transformation in porous Ni-rich NiTi shape memory alloy fabricated by combustion synthesis

The effect of aging temperature on the reverse martensitic transformation in the SHS-synthesized porous Ni-rich NiTi SMA after solution treated at 1050 °C for 4 h is investigated using differential scanning calorimeter (DSC) and discussed. The type of reverse martensitic transformation changes from one step B19′→B2 (after solution treated) through two steps B19′→R→B2 (after aged at 400, 450 and 475 °C) back to one step B19′→B2 (after aged at 500 °C). Both the austenite finish temperature ( A f) and the peak temperature corresponding to the transformation of R→B2 decrease with the rise of the aging temperature. The experimental results also indicate that shape memory properties of porous NiTi SMA fabricated by SHS can be improved by optimizing the aging technology.

Influences of solution treatment on compressive properties of porous NiTi shape memory alloy with the porosity of 53.4 vol% fabricated by combustion synthesis

Influences of solution treatment on compressive properties of porous NiTi shape memory alloy with the porosity of 53.4 vol% fabricated by combustion synthesis

Microstructure and martensitic transformation behavior of porous NiTi shape memory alloy prepared by hot isostatic pressing processing

Porous NiTi shape memory alloy (SMA) has been successfully prepared by hot isostatic pressing (HIP) of elemental Ni and Ti powder. The alloy has isotropic and uniform pore distribution of spherical pores ranged from 50 to 200 μm. The microstructure, martensitic transformation behavior and mechanical properties of this porous NiTi SMA aged at different conditions were studied. It has been found that the phase transformation behavior of porous NiTi SMA is similar to that of the dense Ni-rich NiTi alloys. The porous NiTi SMAs can exhibit almost complete superelasticity at human body temperature.

Processing of porous TiNi shape memory alloy from elemental powders by Ar-sintering

Porous TiNi shape memory alloys attracted extensive interests for their potential use as biomedical materials because of their similar microstructure to human bone and the unique mechanical properties. In this paper, an equiatomic porous TiNi alloy was prepared from elemental powders by Ar-sintering. The pore features of the porous TiNi alloy were studied by optical microscopy and scanning electronic microscopy (SEM). The mechanism of pore-formation and pore-development were proposed. X-ray diffraction (XRD) and differential scanning calometry (DSC) were used to investigate the microstructure and the temperature of phase transformation of the alloy. Compression and flexural test were carried out to determine the mechanical properties of the porous TiNi alloy. The porous TiNi alloy sintered at 980 °C in argon for 8 h was found to be composed mainly of TiNi phase with promising properties, of which the ultimate compressive and flexural strength was 677 and 246 MPa, respectively. The porosity ratio of porous TiNi alloy was up to 45%. The DSC measurements revealed the A s and A f temperatures of the alloy was 32.5 and 45.6 °C, respectively.

Porous TiNi Biomaterial Produced by Self-Propagating High-Temperature Synthesis: Pore Structure, Mechanical Property and Application

Porous TiNi shape-memory alloy (TiNi SMA) bodies were produced from the (Ti+Ni) powder mixture by self-propagating high-temperature synthesis (SHS) method. Various processing variables such as ignition temperature and heating schedule were used to control the pore structure. Relationship between pore structure and mechanical property was also investigated. An in vivo test was performed to evaluate bone tissue response and biocompatibility of porous TiNi SMA. It showed no apparent adverse reactions such as inflammation and foreign body reaction. Bone ingrowth was found in the pore space of all implanted blocks. Introduction Titanium-nickel intermetallic compound is well known as one

Properties of Al/TiNi composite using porous TiNi fabricated by a self-propagating high temperature synthesis method

In the present study, equi-atomic porous TiNi shape-memory alloys have been successfully prepared by self-propagating high-temperature synthesis (SHS) using elemental titanium and nickel powders. The porous TiNi alloys thus obtained have an open porous structure with about 64 vol.% porosity, and the pore size is about 1.8 mm. The effect of preheating temperature on the microstructure has been investigated. It was found that the pore size increased with increasing preheating temperature. Moreover, the preheating temperature was shown to have a significant effect on the microstructure of the SHS-synthesized porous TiNi shape memory alloys. Aluminum was infiltrated into porous TiNi shape-memory alloys and the damping property of Al/TiNi was measured by a vibration analyzer (FFT analyzer). The specific damping capacity of Al/TiNi composites is 70% that of a pure TiNi ingot.

Porous TiNi shape memory alloy with high strength fabricated by self-propagating high-temperature synthesis

Porous Ti–51 at.% Ni shape memory alloy (SMA) with high strength has been fabricated by the improved self-propagating high-temperature synthesis (SHS) method. The compressive strength of the porous TiNi SMA with the general porosity of 52.8 vol.% obtained under the preheating temperature of 400 °C could reach up to 500 MPa. Moreover, the transformation temperatures of the porous TiNi SMA with B2(TiNi) and B19′(TiNi) as the predominant phases could be easily adjusted to the suitable values for medical applications by heat treatment.

Modeling of transformation-induced plasticity and its effect on the behavior of porous shape memory alloys. Part II: porous SMA response

The thermomechanical constitutive model for fully dense shape memory alloys (SMAs) developed in Part I [D.C. Lagoudas, P.B. Entchev, to be published. Modeling of transformation-induced plasticity and its effect on the behavior of porous shape memory alloys. Part I: constitutive model for fully dense SMAs, Mechanics of Materials, this issue] of this two-paper series is used in a micromechanical averaging scheme to establish a model for the effective mechanical behavior of porous shape memory alloys. An incremental formulation of the Mori–Tanaka averaging scheme is employed in this work, where the porous material is treated as a two-phase composite with an SMA matrix and the pores as the second phase. The simultaneous development of transformation and plastic strain in the SMA matrix is taken into account. Expressions for the effective elastic and tangent stiffness tensors are presented as well as an expression for the evolution of the effective inelastic strain. The material parameters used by the model are estimated for the case of porous NiTi SMA processed from elemental Ni and Ti powders. Two different porous NiTi alloys are considered in the current work––material with large pore size (∼1 mm) and with small pore size (∼25 μm). The results of the model simulations for compressive mechanical loading of both types of porous SMAs are compared with the experimental data.

Fabrication of porous NiTi shape memory alloy for hard tissue implants by combustion synthesis

Porous NiTi shape memory alloy (SMA) with ideal pore characteristics and high strength for hard tissue implants has been fabricated by combustion synthesis in this paper. Microstructure and mechanical properties of the product were studied by optical microscope, scanning electron microscope (SEM), X-ray diffraction (XRD) and compression test respectively. The experimental results show the combustion temperature of Ni+Ti reactant system using an improved ignition technique can reach 1310 °C at the pre-heating temperature of 350 °C. Porous NiTi SMA with three-dimensionally interconnected pores has the isotropic pore structure, whose general porosity is about 57.3 vol.% and the open porosity ratio is about 86%. Besides two second phases (Ti 2Ni and Ni 4Ti 3) in small amount, the desired products, such as B2(NiTi) and B19′(NiTi) are the predominant phases in porous NiTi SMA. The undesired Ti 2Ni phase as discrete precipitates formed during the SHS process is often present on or near grain boundaries and difficult to be removed by solution treatment. Porous NiTi SMA has a high compressive strength (208 MPa) and a low Young’s modulus (2.26 GPa), which is similar to natural bones and can meet the mechanical property demands of hard tissue implants for heavy load-bearing applications.

Effect of pores on corrosion characteristics of porous NiTi alloy in simulated body fluid

Porous NiTi shape memory alloy (SMA) is a novel biomaterial used for human surgical implant. The corrosion characteristics of porous NiTi SMAs prepared from combustion synthesis have been investigated by potentiodynamic polarization tests in physiological and Hank’s solutions at 310K. Meanwhile, solid NiTi SMA has been chosen as a control. The preliminary data indicate that pores have strong effects on corrosion characteristics of porous NiTi SMA. Porous NiTi SMA is less corrosion resistant than solid one.