Porous NiTi Shape Memory Alloy Research Articles

Fabrication of porous Ni-Ti shape memory alloy via binder jet additive manufacturing and solid-state sintering

This paper explores the additive manufacturing of nickel-titanium (Ni-Ti) shape memory alloys (SMAs) using binder jetting and subsequent solid-state sintering under an Ar atmosphere. The consolidated 3D-printed Ni-Ti parts exhibit a correlation between pore morphology, pore fraction, and phase formation at different solid-state sintering temperatures. At a sintering temperature of 1175 °C, NiTi grains with TiC precipitates along grain boundaries are observed, accompanied by irregular, interconnected pores constituting ∼15 % of the volume. Increasing the sintering temperature to 1185 °C leads to the formation of a minor phase of Ni4Ti3 within NiTi grains, along with grain boundary TiC precipitates, and isolated pores with a volume fraction of ∼5 %. The higher sintering temperature corresponds to a higher average nanohardness value (8.1 GPa or 763 Hv compared to 6.9 GPa or 654 Hv), indicating the presence of a greater abundance of secondary phases and higher densification at the elevated sintering temperature. While the transformation temperatures (TTs) were undetectable in the bulk-printed samples, a different structure featuring designed channels and thin struts, sintered under similar conditions, exhibited detectable TTs, with a martensite start temperature of 34 °C. Biocompatibility tests demonstrated cell spreading and attachment on both sintered Ni-Ti samples. These findings offer valuable insights into the potential use of binder jetted Ni-Ti for medical implants and tissue engineering applications.

Effect of pore on the deformation behaviors of NiTi shape memory alloys: A crystal-plasticity-based phase field modeling

Porous NiTi shape memory alloys (SMA) have been developed into important biomedical and damping materials. However, the effect of pore on the martensitic microstructure evolution, functional properties, and plastic deformation has not been fully clear yet. In this work, to reveal the functional properties including superelasticity (SE), elastocaloric effect (eCE), one-way shape memory effect (OWSME), and stress-assisted two-way shape memory effect (SATWSME), as well as plastic deformation of porous NiTi SMAs, the polycrystalline SMA system is considered as a composite containing the grain-interior (GI) and grain-boundary (GB) phases, a thermo-mechanically coupled, grain-size-dependent and crystal-plasticity-based phase field model is proposed for the GI phase, and an elasto-viscoplastic constitutive model is established for the GB phase. The effects of porosity, pore diameter, pore aspect ratio, pore orientation, and pore distribution, as well as the interaction between pore and grain size, are comprehensively revealed and discussed. The simulations demonstrate that the pores cause a heterogeneous stress field, and the interaction among pores and that between pores and grain boundaries can enhance such heterogeneity, which can promote the initiation of martensitic transformation and reorientation. Moreover, the geometrical constraints caused by the pores can significantly affect the morphology and evolution of martensite microstructures. However, due to the diverse microstructure evolutions during SE, OWSME, and SATWSME, their interactions with the pores are quite different, resulting in various dependencies of each functional property and plastic deformation on the pore, and such dependencies are strongly influenced by porosity, pore diameter, pore aspect ratio, pore orientation, and pore distribution. The simulated results in this work are helpful to comprehensively and deeply understand the pore effect and its microscopic mechanism.

Non-negligible role of gradient porous structure in superelasticity deterioration and improvement of NiTi shape memory alloys

Bone-mimicking gradient porous NiTi shape memory alloys (SMAs) are promising for orthopedic implants due to their distinctive superelastic functional properties. However, premature plastic deformation in weak areas such as thinner struts, nodes, and sharp corners severely deteriorates the superelasticity of gradient porous NiTi SMAs. In this work, we prepared gradient porous NiTi SMAs with a porosity of 50% by additive manufacturing (AM) and achieved a remarkable improvement of superelasticity by a simple solution treatment regime. After solution treatment, phase transformation temperatures dropped significantly, the dislocation density decreased, and partial intergranular Ti-rich precipitates were transferred into the grain. Compared to as-built samples, the strain recovery rate of solution-treated samples was nearly doubled at a pre-strain of 6% (up to 90%), and all obtained a stable recoverable strain of more than 4%. The remarkable superelasticity improvement was attributed to lower phase transformation temperatures, fewer dislocations, and the synergistic strengthening effect of intragranular multi-scale Ti-Ni precipitates. Notably, the gradient porous structure played a non-negligible role in both superelasticity deterioration and improvement. The microstructure evolution of the solution-treated central strut after constant 10 cycles and the origin of the stable superelastic response of gradient porous NiTi SMAs were revealed. This work provides an accessible strategy for improving the superelastic performance of gradient porous NiTi SMAs and proposes a key strategy for achieving such high-performance architectured materials.

Microstructure Characteristics of Porous NiTi Shape Memory Alloy Synthesized by Powder Metallurgy during Compressive Deformation at Room Temperature

Porous NiTi shape memory alloys (SMAs) possess compatible mechanical properties with human bones and can effectively reduce the risk of stress shielding and stress concentration; therefore, they have been termed promising candidates for orthopedic implants. However, microstructure characteristics of porous NiTi SMAs during plastic deformation have rarely been investigated. The present study aims to specifically investigate microstructure characteristics and the corresponding underlying mechanisms of fabricated porous NiTi SMAs via a conventional sintering (CS) process with NaCl space holder during compressive deformation at room temperature. To realize the aforementioned target, X-ray diffraction (XRD), scanning electron microscope (SEM), electron backscattered diffraction (EBSD), transmission electron microscopy (TEM), and high-resolution transmission electron microscopy (HRTEM) are applied in the present study. The results show that the fabricated porous NiTi SMA is 51.8% for porosity, 181.65 μm for the average pore size, and 0.78 μm for the average grain size. Many Ni4Ti3 and NiTi2 phases are formed in the mixed matrix with dominant B2 (NiTi) and some B19′ (NiTi). Severe inhomogeneous deformation happens within compressed specimens, leading to the occurrence of tangled dislocation and shear bands. Microcracks occur within fabricated porous NiTi SMAs at a deformation degree of 9.2%; then, they extend quickly to form macrocracks, which finally results in the failure of regions between pores. The observed nanocrystallization and amorphization around microcrack tips within the 12.5%-deformed sample can be attributed to the relatively small grain size and the grain segmentation effect via statistically stored dislocation (SSD) and geometrically necessary dislocation (GND).

Effects of porosity and cyclic deformation on phase transformation of porous nanocrystalline NiTi shape memory alloy: An atomistic simulation

Utilizing molecular dynamics simulation, this study aims to explore the phase transformation behavior of porous nanocrystalline (NC) NiTi shape memory alloys (SMAs) when subjected to cyclic deformation. The influences of porosity and cyclic deformation on the phase transformation of NC NiTi SMAs are examined and discussed. The simulation results show that the increase in the porosity and number of cycles leads to a decrease in both the critical phase transformation stress and peak stress whereas an increase in the residual martensite, phase boundary, and interstitial atoms; the related results can be supported by previous experiments. After cyclic deformation, the reduction in the potential energy for the entire system during the tensile phase occurs at an earlier stage, indicating that the martensitic transformation occurs earlier as the number of cycles increases. Notably, the dissipated energy demonstrates a decrease with an increasing number of cycles, and the potential energy during the austenite elastic unloading stage undergoes a transition from a decreasing to an increasing trend due to the presence of residual martensite increasing with the number of cycles.

Constitutive modeling of porosity and grain size effects on superelasticity of porous nanocrystalline NiTi shape memory alloy

Constitutive modeling of porosity and grain size effects on superelasticity of porous nanocrystalline NiTi shape memory alloy

High-strength porous NiTi shape memory alloys with stable cyclic recovery properties fabricated using elemental powders

Based on previous research regarding the high-vacuum sintering of elemental powder (EP) NiTi alloys, this study investigates the preparation of high-strength, highly cyclically stable porous NiTi shape memory alloys. The microstructures, martensitic transformation behaviors, compressive and compressive recovery properties, and cyclic stabilities of the sintered and aged porous NiTi alloys were compared and analyzed. The porosity and average pore size increased with the addition of NaCl, with the oxygen content increasing slightly. The sintered alloy displayed an inhomogeneous distribution of the Ni4Ti3 precipitate, which exhibited a dispersed and homogeneous distribution after aging. The sintered porous NiTi alloy exhibited a multistage phase transformation. The martensitic transformation was suppressed after aging owing to changes in the size and distribution of the precipitates, and only R-phase transformation was observed. Porosity and aging precipitation influenced the dual yield points and recovery properties of porous NiTi. The pinning effect of aging precipitation increased the yield point and reduced the residual strain in the recovery. However, at a high porosity, the volume fraction of NiTi matrix decreased, resulting in a small strengthening contribution of precipitation. The disruption of porous structure was the dominant causes of irreversible strain. As the porosity increased, the porous structure became increasingly susceptible to damage during cyclic loading. Overall, the EP porous NiTi alloy exhibited a higher compressive strength, enhanced compression recovery properties and cyclic stability compared to those reported in previous studies. This is because of the optimization of high-temperature homogenization via sintering at 1250 °C and enhancement effects of aging precipitation. This study provides a novel approach for developing porous EP NiTi alloys.

The Influence of Ultrasonic Activation on Microstructure, Phase Transformation and Mechanical Properties of Porous Ni-Ti Shape Memory Alloys via Self-Propagating High-Temperature Synthesis.

Porous Ni-Ti shape memory alloys (SMAs) have been widely studied in biomedical and engineering applications. Porous Ni-Ti SMAs were obtained via self-propagating high-temperature synthesis (SHS), and their microstructure, phase transformation, and mechanical properties were investigated. This article presents the results of a study of changes in the microstructure, phase transformation, and mechanical properties of porous Ni-Ti SMAs when Ni and Ti metal powders were preliminarily subjected to ultrasonic activation at various periods. It was determined that the porosity of the obtained alloy samples was 62-68 vol%. The microstructure was composed of the main matrix Ni-Ti phase and the accompanying Ti and Ti2-Ni phases. The results show that the hardness 34.1-86.8 HB and elastic modulus 4.2-10.8 GPa increased with an increase in the ultrasonic activation time of the samples. The phase transformation temperature of the Ni-Ti shape memory alloy remained almost unchanged under the influence of ultrasonic treatment.

Effects of Thermal Cycling and Porosity on Phase Transformation of Porous Nanocrystalline NiTi Shape Memory Alloy: An Atomistic Simulation

Herein, the effects of thermal cycling and porosity on the martensitic phase transformation behavior of porous nanocrystalline (NC) NiTi shape memory alloys (SMAs) at the atomic scale are investigated by molecular dynamics simulation. The simulation results show that thermal cycling causes the NC NiTi SMAs to repeatedly undergo martensitic phase transformation and inverse phase transformation processes. The martensitic phase transformation capability is suppressed with increasing porosity and number of cycles, but the suppression effect of thermal cycling is weakened with increasing porosity. Moreover, the start temperature of martensitic transformation, residual martensitic phase, and the fraction of interstitial atoms increase with increasing porosity. Thermal cycling causes the disordered structures and shear plastic deformation to accumulate as cycling increases. Recoverable shear deformation is generated within the grain, but shear plastic deformation is mainly concentrated at both grain boundaries and pore surfaces. Residual martensitic phase increases after thermal cycling. These results provide important explanations and references for a deeper understanding of phase transformation behavior and pore effects caused by thermal cycling.

Effect of Nb on the Damping Property and Pseudoelasticity of a Porous Ni-Ti Shape Memory Alloy.

In order to develop novel high damping materials with excellent pseudoelasticity (PE) properties to meet the application requirements in aerospace, medical, military and other fields, porous Ni50.8Ti49.2 shape memory alloy (SMA) was prepared by the powder metallurgy method. Different contents of Nb element were added to regulate the microstructures. It was found that after adding the Nb element, the number of precipitates significantly decreased, and the Nb element was mainly distributed in the Ni-Ti matrix in the form of β-Nb blocks surrounded by Nb-rich layers. Property tests showed that with the increase in Nb content, the damping and PE increased first and then decreased. When the Nb content reached 9.0 at.%, the highest damping and the best PE could be achieved. Compared with the porous Ni-Ti SMA without Nb addition, the damping and PE increased by 60% and 35%, respectively. Correlated mechanisms were discussed.

New Insights into the Mechanical Properties, Functional Fatigue, and Structural Fatigue of Ni-Ti Alloy Porous Structures

Ni-Ti shape memory alloys (SMAs) are widely noticed and have captured great interest due to their unique shape memory effect and super elasticity. Porous Ni-Ti SMAs have the typical characteristics of both porous metals as well as shape memory alloys. Because of the uneven stress distribution, cyclic loading has a more significant effect on the phase transformation and plastic deformation of Ni-Ti porous compared with Ni-Ti bulk. This paper overviews the structural and functional fatigue experiments and numerical simulation progress of Ni-Ti porous. The factors affecting the fatigue performance of the Ni-Ti lattice structure and the methods for enhancing its fatigue performance are elaborated. More importantly, the point of the coupling analysis of structural fatigue performance and functional fatigue performance is proposed for the study of porous Ni-Ti shape memory alloys.

Molecular dynamics simulation of the porosity effect on transformation mechanism of nanocrystalline porous NiTi shape memory alloy

Molecular dynamics simulation of the porosity effect on transformation mechanism of nanocrystalline porous NiTi shape memory alloy

INVESTIGATION OF THE EFFECT OF SINTERING TEMPERATURES ON THE PRODUCTION OF POROUS NITI ALLOY BY LOOSEN SINTERING METHOD

This article aims to produce NiTi shape memory alloys, which show superelasticity and shape memory effect, as well as good biocompatibility and corrosion properties, in open and porous sizes ranging from 100-500 μm, which are required for use as implants. This structure of pores is necessary to allow tissue growth and fluid flow inside the implants. Many powder metallurgy methods have been used in producing porous NiTi shape memory alloys. However, the packaging pressure used in these methods has not successfully created the desired pore distribution, shape, and size. The methods by which it can be produced are costly in terms of cost. In the study, production was carried out by sintering the powder mixture poured into molds without pressure with the help of binder polymers. This sintering process was carried out in an argon atmosphere for 1 hour at temperatures 1050, 1125, and 1200 °C. The study shows that pressureless loosen sintering can produce porous NiTi alloys, which is the more straightforward method. The pore distribution and proportions were examined. Homogeneous distribution and pores in desired sizes are created. It has also been determined that the binder polymer has a space-retaining effect. It was determined at which temperature the alloy sintered at different temperatures contained the desired B2 austenite phase for superelasticity. Austenite start and finish temperatures were determined for the alloy produced at each sintering temperature. As a result of this research, it was determined which phase was denser at which temperature, and the phase transformation temperatures were found. The exact temperature measurements can be calculated by changing the sintering time parameter. In addition, the change in phase transformation temperatures can be measured by heat treatment after sintering. Porous NiTi alloys can be used as dental and medical implants thanks to their excellent biocompatibility and corrosion resistance. This method will decrease production costs, and more people will have access to this material. In general, the mechanism of sintering methods is joining the points in contact with the packaging pressure by necking. In this study, the combination of the grains with the polymer without packaging pressure with the thermal expansion mechanism reveals the originality of the study.

The effect of heat treatment on pseudoelastic behavior of spark plasma sintered NiTi

Porous NiTi shape memory alloys are considered as artificial bio-materials for possible use in implant applications. Their extraordinary pseudoelastic response under uniaxial loading through phase change needs further analysis. The objective of this work is to show the effect of heat treatment on the pseudoelastic behavior of NiTi SMA produced by spark plasma sintering (SPS). NiTi samples were prepared from pre-alloyed Ti-50.7 at.% Ni powders of 100-150 μm average diameter, and subjected to a wide range of homogenization and/or aging at different temperatures and duration. Transformation characteristics of heat treated samples were obtained from DSC, their phase composition was identified using X-ray diffraction (XRD), and checked with scanning electron microscopy (SEM) and energy dispersive X-ray analysis (EDX). Instrumented micro-indentation was carried out to measure the hardness altered by aging. Uniaxial compression tests were performed on heat treated samples, and their pseudoelastic behavior was shown on the stress–strain diagrams; the results showed a variation in pseudoelasticity. In general, Ni concentration of the matrix resulted in partial-pseudoelastic behavior of samples. Homogenization prior to aging improved the pseudoelasticity of the SPSed NiTi. The pseudoelastic response of the samples that have not shown the expected full pseudoelastic behavior was enhanced by applying a considerably long period of homogenization and aging.

Porous NiTi Shape Memory Alloy Fabricated via Powder Metallurgy Technique Using Pore Forming Agent

Porous NiTi shape memory alloy is of special interest for biomedical purposes especially for human bones application due to its attractive features such as lower stiffness to minimize the effect of stress shielding and good strength to prevent deformation and fracture apart from its shape memory effect and superelastic behavior. With all these great benefits, however, the challenge is to produce porous NiTi which resembles cancellous bone. Therefore, in this research, pore forming agent such as calcium hydride, CaH2, is added to the equiatomic of Ni and TiH2 powder mixture to produce porous NiTi with higher porosity level using powder metallurgy technique. Here, the effect of composition of pore forming agent on porosity level, phase formation and transformation behaviour of porous NiTi were investigated. From the observation, the pores formation exhibits small closed pores instead of interconnected pores. The result also shows that by adding 3wt% composition of pore forming agent, the porosity level of sample sintered can reach up to 32%. For phase transformation behavior, there are martensitic transformation peaks observed both upon cooling and heating for all samples, however the overall enthalpy changes are significantly lower (<2 J/g). This due to undesirable phase such NiTi2, Ni-rich phase and also Ni3Ti that co-exist with NiTi formation, thus jeopardize the transformation enthalpy for porous NiTi.

Transformation temperature and corrosion behavior of porous NiTi and NiTiCu shape memory alloy

The present research work deals with the fabrication of porous NiTi and NiTiCu shape memory alloy (SMA) by powder metallurgy process. These SMAs have number of applications in medical and space industries. The transformation temperature of both SMAs is measured by differential scanning calorimetry (DSC). Corrosion behavior is measured by potentiodynamic polarization curve in the presence of artificial saliva (pH value 7.4). Result shows that transformation temperature decreases by 8 °C to 10 °C after copper addition. Also, the corrosion current density decreases for NiTiCu, which exhibit more corrosion resistant than equi-atomic NiTi alloy. The corrosion current density of NiTiCu is less than NiTi by a factor of 31.

Superelastic behaviors of additively manufactured porous NiTi shape memory alloys designed with Menger sponge-like fractal structures

Additively manufactured porous NiTi alloys hold unprecedented promise in metallic implants due to their low elastic modulus and superelastic behavior. Such porous structures are usually topologically ordered and designed with periodically-repeating unit cells. However, the superelastic behaviors of fractal porous structures have never been studied. The Menger sponge-like fractal structures consisting of non-periodic fractal pores mimick the architectures and biomechanical properties of human bone. In this investigation, porous NiTi alloys designed with Menger sponges were fabricated by additive manufacturing using selective laser melting technology, and their superelastic behaviors were systematically characterized for the first time. Additively manufactured bulk NiTi alloys exhibit fully recoverable superelastic responses with slim stress hysteresis. The mechanical properties of bulk NiTi alloys match the properties of human cortical bones. The Menger sponges display excellent superelastic recovery strain ratios even at high porosity levels. The mechanical properties of highly fractalized Menger sponges are almost identical to the properties of human cancellous bones. The deformation mechanism undergoes a transformation from bending-dominated to stretching-dominated mode when the porous structures are highly fractalized. The unique combination of fractal topology, nanostructured microstructure, highly controllable elastic modulus and large recoverable deformation make the NiTi Menger sponge a promising candidate for metallic implants.

Tailored pore canal characteristics and compressive deformation behavior of bionic porous NiTi shape memory alloy prepared by selective laser melting

Inspired by the Bouligand structure within the crab claw, the NiTi-based bionic helicoidal porous part was fabricated based on selective laser melting (SLM). The helicoidal porous structure (HPS) was created by controlling the geometric characteristics and distribution of metallurgical pore canals within the part. The compressive deformation behavior of the metallurgical pore canal was disclosed. It was shown that the metallurgical pore canals kept the same deformation feature as the whole HPS at an early period (800 MPa) and then experienced a rapid transformation from completely recoverable state to completely unrecoverable state, with the applied stress level increasing. When the sample came to break, a zigzag fracture edge was found, indicating a twisting propagation path of a crack. Influence of laser processing parameter on the pore canal characteristic and quasi-static compressive behavior of the HPS was evaluated in details. The metallurgical pore canals with three different kinds of cross-sectional configurations were obtained at different laser processing parameters. As the larger layer thickness d together with the smaller hatch spacing h was applied, the pore canal tended to become more applanate. In this condition, the experiment and simulation results suggested that stress concentration emerging nearby the pore canal became more significant during the compression, consequently leading to a deteriorative superelasticity due to an increase of residual strain after unloading, as well as an early failure of the sample.

Constitutive Modeling of Porous Shape Memory Alloys Using Gurson–Tvergaard–Needleman Model Under Isothermal Conditions

Based on the Gurson–Tvergaard–Needleman (GTN) model, a constitutive relationship considering both the effects of strain hardening and hydrostatic stress for porous shape memory alloys (SMAs) is proposed. To capture the relationship between microscopic and mesoscopic behaviors, a representative volume element (RVE) containing an array of spherical voids is presented. In this paper, an approximate solution including strain hardening exponent [Formula: see text] is deduced by considering the porous SMA as a two phase composite with the SMA matrix and the second phase representing voids. The model parameters, [Formula: see text] and [Formula: see text], accounting for interactions between voids are investigated to take into account their influences on strain hardening, critical phase transformation stress and yield surface. In addition, the evolution equations of phase transformation are derived and then applied to the simulation of porous Ni–Ti SMAs with a porosity of 13%. Using the calibrated GTN model parameters, the critical phase transformation stress closer to experimental data is obtained. The predictions of stress–strain curve by the proposed constitutive model are found to be in excellent agreement with published experimental data and finite element results. The results prove that the model is capable of reproducing the features of porous SMAs such as superelasticity, tensile-compressive asymmetry and internal loops under uniaxial even combined loading conditions. A conclusion is drawn that the present constitutive relationship is powerful and useful for the analysis of porous SMAs.

To the relation between the orientation of the pore channels and the mechanical properties of porous NiTi shape memory alloy

To the relation between the orientation of the pore channels and the mechanical properties of porous NiTi shape memory alloy