Porous NiTi Research Articles

A Unified Empirical Equation for Determining the Mechanical Properties of Porous NiTi Alloy: From Nanoporosity to Microporosity

The mechanical characteristics of a monolithic (non-porous) crystalline or amorphous material are described by a well-defined set of quantities. It is possible to change the mechanical properties by introducing porosity into this material; as a rule, the strength values decrease with the introduction of porosity. Thus, porosity can be considered an additional degree of freedom that can be used to influence the hardness, strength and plasticity of the material. In the present work, using porous crystalline NiTi as an example, it is shown that the mechanical characteristics such as the Young’s modulus, the yield strength, the ultimate tensile strength, etc., demonstrate a pronounced dependence on the average linear size l¯ of the pores. For the first time, an empirical equation is proposed that correctly reproduces the dependence of the mechanical characteristics on the porosity ϕ and on the average linear size l¯ of the pores in a wide range of sizes: from nano-sized pores to pores of a few hundred microns in size. This equation correctly takes into account the limit case corresponding to the monolithic material. The obtained results can be used directly to solve applied problems associated with the design of materials with the necessary combination of physical and mechanical characteristics, in particular, porous metallic biomaterials.

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.

In vitro evaluation of cytotoxicity and genotoxicity of porous nickel titanium dental implants produced by metal injection molding technique.

Porous NiTi (pNiTi) is a promising biomaterial for functional long-term implantation that has been produced using various manufacturing techniques and tested for biocompatibility. pNiTi produced using a more recent technology of Metal Injection Molding (MIM) has shown better physical and mechanical properties than those produced by earlier manufacturing methods, but its biocompatibility has yet to be determined. Hence, extracts from pNiTi dental implants produced by MIM were tested for cytotoxicity and genotoxicity in this work. Its toxicity was evaluated at the cellular and in vitro levels using elution and 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyl-2H-tetrazolium bromide (MTT) assays. Short-term testing revealed that pNiTi extract was cytocompatible with L-929 fibroblast and V79-4 lung cells, with no cell lysis or reactivity observed, respectively (USP grade 0). Following exposure to varied extract concentrations, good cell viability was observed where the lowest concentration showed the highest optical density (OD) and cell viability (2.968 ± 0.117 and 94%, respectively), and the highest concentration had the least OD and cell viability (2.251 ± 0.054 and 71%, respectively). pNiTi extracts demonstrated genocompatibility in two independent assays: mutagenic potential using a bacterial reverse mutation test and a clastogenic effect on chromosomes using the micronucleus test. Similar to the negative control reactions, there was no significant increase in revertant colonies following exposure to 100% pNiTi extract with and without metabolic activation (p = .00). No DNA clastogenic activity was caused by pNiTi at varied extract concentrations as compared to the negative control when tested with and without metabolic activation (p = .00). As a result, both cytotoxic and genotoxic investigations have confirmed that pNiTi dental implants utilizing the MIM process are cytocompatible and genocompatible in the short term, according to the International Standard, ISO 10993 - Parts 3, 5, and 33.

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.

Mechanical and shape memory properties of NiTi triply periodic minimal surface structures fabricated by laser powder bed fusion

Porous NiTi lattice structures are widely used in the manufacture of crucial components owing to their excellent shape memory effect, superelasticity, and high damping capacities. However, the specific strength and lightweight characteristics of porous NiTi lattice structures fabricated by conventional technologies are limited by unpredictability. In this work, three types of porous NiTi structures based on triply periodic minimal surface (TPMS) – Diamond, Gyroid, and Primitive – were designed and manufactured by the laser powder bed fusion (LPBF) additive manufacturing process. This work demonstrates LPBF is a feasible and efficient approach to fabricate highly accurate porous NiTi TPMS structures. Moreover, the influence of each of these structures on the mechanical and shape memory properties was investigated. Among the three structures, Gyroid had the smallest volume fraction deviation. Furthermore, the Diamond structure had the largest compressive modulus (782.82 MPa) and ultimate yield strength (163.14 MPa). The Gyroid and Primitive structures exhibit excellent elastic recovery deriving from high values of compressive modulus (662.44 MPa, and 703.29 MPa), and can maintain reliable structural robustness. The Primitive structure exhibited the lowest mechanical properties (37.80 MPa). During the cyclic compression test, Gyroid and Primitive show a smaller unrecovered strain than Diamond. Primitive shows the largest recovered strain during the heating process (6.98%). The higher mechanical flexibility of Primitive structure endows this structure with higher recovery ratio. During the direct compression test, the residual strain exhibits a positive correlation with the loading strain. All three structures exhibit good deformation recovery capability with a strain of 4%. At a strain of 12%, recovered strain during heating became the dominant factor in the recovery of the TPMS structure. Overall, porous NiTi TPMS structures are capable of reversible compressibility composed of rapid elastic recovery and controllable shape memory recovery. The unique performance of porous NiTi TPMS structure fabricated by LPBF renders it a highly efficiency energy-absorbing structure.

Preparation of porous TiNi intermetallic compound and the corrosion behavior in NaCl solution

Porous TiNi intermetallic compounds have a promising future as biomaterials, with some challenges for their corrosion resistance. In this work, TiNi intermetallic compound was prepared by reactive synthesis of elemental powders. The evolution of the microstructure and phase composition of TiNi at different temperatures was studied. And the electrochemical corrosion behavior of TiNi intermetallic compound in NaCl solution was investigated by electrochemical tests. The results show the formation of a core-shell structure during the sintering process, where Ni is the core and Ti2Ni, TiNi and TiNi3 are the shells. With uniform diffusion, a large pore replaces the Ni core due to the Kirkendall effect. TiNi intermetallic compound has a well-developed open pore structure with an open pore ratio of 50.98%. TiNi intermetallic compound exhibits better corrosion resistance compared to Ni and Ti, which can be attributed to the generation of passivation film on the surface.

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

Compression Behavior and Failure Mechanisms of Bionic Porous NiTi Structures Built via Selective Laser Melting

Compression Behavior and Failure Mechanisms of Bionic Porous NiTi Structures Built via Selective Laser Melting

Plasticity of Porous NiTi Alloys Obtained by Self-propagating High-temperature Synthesis in Closed and Open Gas Flow Reactors

ABSTRACT Porous NiTi alloys were obtained by self-propagating high-temperature synthesis (SHS) using layer-by-layer combustion in closed and open gas flow reactors under a protective argon atmosphere. The maximum compressive strain of porous NiTi alloys synthesized in the closed reactor was 34% compared to 7% in the open gas flow reactor. X-ray diffraction, differential scanning calorimetry, scanning electron microscopy, energy dispersive spectroscopy, and optical microscopy showed that the reaction products in the two-phase gas zone of peritectic crystallization are in the form of isolated Ti2Ni crystalline clusters in the TiNi matrix. The ductility and strength of the recrystallized Ti2Ni phase hard dendrites increase the effective stiffness of porous NiTi alloys and decrease the maximum compressive strain. A highly porous NiTi alloy with improved mechanical properties was obtained by the SHS method in a closed reactor.

Preparation of Porous Tini Intermetallic Compound and the Corrosion Behavior in Nacl Solution

Preparation of Porous Tini Intermetallic Compound and the Corrosion Behavior in Nacl Solution

Preparation of Porous Tini Intermetallic Compound and the Corrosion Behavior in Nacl Solution

Preparation of Porous Tini Intermetallic Compound and the Corrosion Behavior in Nacl Solution

Numerical and experimental study of porous NiTi anisotropy under compression

Porous NiTi samples were obtained by self-propagating high-temperature synthesis (SHS) and cut to specific dimensions by electrical discharge machining. Quasi-static compression of the porous SHS-NiTi alloy demonstrated that the deformation was elastic-plastic. Three characteristic types of fracture surfaces of bridges can be distinguished. 1) Ductile fracture of the austenite phase in the form of a cup and cone relief. 2) Brittle fracture with the formation of cleavage steps. 3) Large areas of plastic shear, where cup and cone, and cleavage facets are nucleated. An algorithm for obtaining a porous framework model suitable for finite element method calculations has been developed. The effective moduli of elasticity and shear for the porous volume of the material were determined. It is shown that the studied porous volume has orthotropic elastic properties due to the geometry of the porous framework.

Structure and Mechanical Properties of Porous TiNi Alloys with Ag Nanoparticles

The work studies the influence of the silver dopant (<0.5 at.%) on the structure and mechanical properties of porous TiNi alloys obtained by self-propagating high-temperature synthesis. These alloys are of high scientific and practical interest in medicine. The presence of silver in the TiNi alloy will ensure improved cytocompatibility and antibacterial properties. The TiNi porous alloys with 0.2 and 0.5 at.% Ag nanoparticles have multiphase composition. Quantitative X-ray diffraction analysis of the obtained alloys showed that an increase in the silver content is accompanied by a quantitative decrease in the austenite phase TiNi(B2) and an increase in the martensite phase TiNi(B19’), as well as in secondary phases Ti2Ni, Ti4Ni2O. Evenly distributed silver nanoparticles up to 10 nm were found in the surface layer by transmission electron microscopy. The results of the scanning electron microscopy showed that inclusions containing silver are located mainly in the zones of Ti2Ni peritectic crystallization. The mechanical characteristics were studied by means of compression tests and it was found that with an increase in the silver dopant, the elastic modulus and elastic limit decrease, but the maximum deformation to fracture increases significantly. It was found that with an increase in the volume fraction of silver, the plastic properties of the alloy increase. No dependency of the tensile strength on the amount of silver was found.

Anisotropy of elastic properties of SHS-synthesized porous titanium nickelide

Samples of porous NiTi were obtained by the method of self-propagating high-temperature synthesis. The mechanical characteristics of the porous ones were studied by quasi-static compression. When the samples of porous titanium nickelide were subject to quasi-static compression, the deformation was of an elastic-plastic nature. Three characteristic types of the fracture surface under quasi-static compression of the porous SHS – TiNi alloy were identified: 1) ductile fracture of the austenite phase in the form of a cup relief, 2) brittle fracture accompanied by the formation of cleavage steps, 3) large areas of plastic shear deformation, on which cups and cleavage facets were nucleated. To determine the anisotropy of the porous TiNi alloy properties, the volume of the porous sample was simulated, and estimated calculations were carried out. Based on the results of reconstructing 3D neutron high resolution tomography of the porous volume of titanium nickelide and the numerical parameters of the model porous medium, an algorithm was developed for obtaining a solid-state 3D model of the porous framework for using in finite element calculations. The studied porous titanium nickelide alloy, as well as spongy bone tissues, was shown to have orthotropic elastic properties conditioned by the geometric features of the porous framework. The effective moduli of elasticity and shear for the porous volume of the material were determined. The calculation results of the elastic moduli for the studied model of porous titanium nickelide numerically agree with the results obtained by compressing the samples of porous TiNi. The porous TiNi alloy under uniaxial compression was established to be destroyed under the action of tangential shear stresses at an angle of 45 degrees to the direction of uniaxial compression. The research was carried out with financial support of the Russian Science Foundation under the Grant No. 22-72-10037, https://rscf.ru/project/22-72-10037/.