NiTi Alloy Research Articles

Laser-Induced Forward Transfer of Ni-rich NiTi Alloys for Shape Memory Applications

Laser-Induced Forward Transfer of Ni-rich NiTi Alloys for Shape Memory Applications

Molecular dynamics study on martensitic transformation behavior of SLM-NiTi alloy induced by temperature and stress

Selective laser melting (SLM) technology is currently one of the most promising additive manufacturing technologies for complex metal components. NiTi alloy has been highly regarded in advanced applications due to its excellent shape memory and good biocompatibility. However, as a new material, SLM-NiTi alloy is far from being applied in actual advanced fields. In the actual processing, such as grinding, turning, polishing, electrical discharge machining, all involve changes in temperature and stress, Therefore, it is very important to study the martensitic phase transition caused by temperature and stress changes in the precision machining process of SLM-NiTi alloy. However, it is difficult to observe the martensitic phase transition changes directly in the actual processing, so the method of molecular dynamics is adopted in this paper. Moreover, in the process of preparing NiTi alloy by selective laser melting, the ratio of Ni to Ti is very important, which determines the final forming quality. Therefore, this paper studied the martensitic transformation behavior induced by temperature and stress under different nickel proportions, different initial temperatures and different model sizes, and expounded the variation laws of stress–strain, potential energy, volume and dislocation. The microstructure and shear strain were demonstrated on the atomic scale. The results show that temperature plays an important role in the martensite transformation of SLM-NiTi alloy, low temperature will largely inhibit martensite transformation, and high temperature will promote martensite transformation. The stress induced martensite reorientation in SLM-NiTi alloy is accomplished by the migration of the interface between different martensite variants. When the nickel content is 52% and 55%, there is no inflection point between volume and potential energy with the change of temperature, when the nickel content is 50.8%, there is an obvious jump between volume and potential energy. The research in this paper is helpful to guide the processing technology of SLM-NiTi alloy, and also broadens the application of additive manufacturing materials.

Effects of Building Directions on Microstructure, Impurity Elements and Mechanical Properties of NiTi Alloys Fabricated by Laser Powder Bed Fusion.

For NiTi alloys prepared by the Laser Powder Bed Fusion (LPBF), changes in the building directions will directly change the preferred orientation and thus directly affect the smart properties, such as superelasticity, as well as change the distribution state of defects and impurity elements to affect the phase transformation behaviour, which in turn affects the smart properties at different temperatures. In this study, the relationship between impurity elements, the building directions, and functional properties; the effects of building directions on the crystallographic anisotropy; phase composition; superelastic properties; microhardness; geometrically necessary dislocation (GND) density; and impurity element content of NiTi SMAs fabricated by LPBF were systematically studied. Three building directions measured from the substrate, namely, 0°, 45° and 90°, were selected, and three sets of cylindrical samples were fabricated with the same process parameters. Along the building direction, a strong <100>//vertical direction (VD) texture was formed for all the samples. Because of the difference in transformation temperature, when tested at 15 °C, the sample with the 45° orientation possessed the highest strain recovery of 3.2%. When tested at the austenite phase transformation finish temperature (Af)+10 °C, the 90° sample had the highest strain recovery of 5.83% and a strain recovery rate of 83.3%. The sample with the 90° orientation presented the highest microhardness, which was attributed to its high dislocation density. Meanwhile, different building directions had an effect on the contents of O, C, and N impurity elements, which affected the transformation temperature by changing the Ni/Ti ratio. This study innovatively studied the impurity element content and GND densities of compressive samples with three building directions, providing theoretical guidance for LPBFed NiTi SMA structural parts.

Corrosion protection of NiTi alloy via micro-arc oxidation doped with ZnO nanoparticles and polyacrylamide sol–gel sealing

Corrosion protection of NiTi alloy via micro-arc oxidation doped with ZnO nanoparticles and polyacrylamide sol–gel sealing

Ductile Ti–Ni Alloys With an Equiaxed Microstructure Designed by Tuning the Precipitation Pathway of Ti2Ni

Ductile Ti–Ni Alloys With an Equiaxed Microstructure Designed by Tuning the Precipitation Pathway of Ti2Ni

Effects of Ga, Nb, V, Zr, and Cr on the crystal structures and thermal behavior of CuAlMn shape memory alloys

CuAlMn alloys are cheaper and more promising alloy types compared to NiTi alloys. In this study, based on CuAlMn shape memory alloy, thermodynamic and structural characteristics of some new alloys containing four elements were investigated in detail. In this study, five new quaternary shape memory alloys, each containing a CuAlMn base alloy and a different fourth additive element (Ga, Nb, V, Zr, and Cr), were produced. The properties of the obtained alloys were characterized using a series of analysis methods such as X-ray diffraction (XRD), optical microscopy, differential scanning calorimetry (DSC), and differential thermal analysis (DTA). The results showed the effects of different elements on the crystal structures of the alloys and how different elements could change the thermal behavior of the alloys.

Body temperature NiTi alloys: effect of the heat treatment on the functional thermo-mechanical properties

The design process of devices based on shape memory alloys (SMA) is particularly demanding. It is fundamental the investigation of parameters describing the thermo-mechanical behavior of these materials, such as the phase transformation temperatures and the Clausius-Clapeyron coefficient (CCC). The aim of this study is to determine how heat treatment (HT) affects the above-mentioned parameters on two commercially available body-temperature NiTi alloys. This analysis is accomplished through differential scanning calorimetry (DSC) and strain recovery (SR) tests. Finally, the two NiTi alloys have been compared to determine whether exists a significant difference between different casting batches of the same commercial alloy conformed to the ASTM F2063. It results that both alloys present a similar overall trend of Af as a function of HT temperature and time, even if the values significantly differ for treatment at temperatures higher than 773 K for all HT times. Furthermore, it is noted that the HT time presents a minor effect on the value of Af. After the SR analysis, it has been possible to highlight the effect of the HT on the values of CCC for As, Ms and Rs. The overall trend is similar between the two NiTi alloys. These results lead to the conclusion that HT highly affect not only the transformation temperatures but also the transformation path of the alloy, thus its mechanical properties.

Effect of hot isostatic pressing on the mechanical and corrosive properties of Ti–Ni alloy fabricated by selective laser melting

Hot isostatic pressing (HIP) is capable of mitigating the internal defects of materials, such as pores and cracks, resulting in improved mechanical properties and corrosion resistance, so it is particularly suitable as a post-treatment technology for the Ti–Ni alloys fabricated by selective laser melting (SLM), which display a broad application prospect in the biomedical field. Herein, the effect of HIP treatment on the mechanical and corrosion-resistant properties of SLM Ti–Ni alloys was systematically investigated. It was demonstrated that HIP treatment did not substantially affect the phase transformation behavior of the SLM Ti–Ni alloy, only showing a slight decrease in the values of As and Ms. However, HIP treatment improved the ultimate tensile strength and the elongation to fracture of the SLM Ti–Ni alloy by 67.3% and 23.2%, respectively. Meanwhile, the corrosion current density was reduced by two orders of magnitude after HIP treatment, indicating the significantly increased corrosion resistance. The enhanced mechanical properties and corrosion resistance of SLM Ti–Ni alloy were credited with the elevated temperature and pressure applied during the HIP process, which significantly eliminated the porosity defects and made the microstructure more uniform. These findings highlight the importance of performing HIP to enhance the comprehensive properties of SLM Ti–Ni alloys, which can expand the application prospects of SLM Ti–Ni alloys in the biomedical field.

In-plane measurements and computational fluid dynamics prediction of permeability for biocompatible NiTi gyroid scaffolds fabricated via laser powder bed fusion

Laser powder bed fusion (LPBF) is considered a promising technology for manufacturing porous, biomimetic, and patient-specific scaffolds for bone repair. Scaffold permeability is one of the key factors to be considered for acquiring the required mass-transport properties in bone tissue engineering. This study aims to reveal the relationship between the design parameters of gyroid-based porous structure and scaffold permeability. A set of gyroid samples was manufactured from intermetallic NiTi alloy. Nine configurations of porous structures were obtained by varying the main design parameters, namely wall thickness and unit cell size. The in-plane method was employed to measure the permeability coefficient for the gyroid structures. Computational fluid dynamics simulations of the porous structures were performed to predict the targeted properties in an implant at the design stage before LPBF manufacturing. The results of the simulations were validated with the obtained experimental results. Geometrical accuracy and surface morphology of the as-built samples were investigated with various techniques. Biocompatibility assessment of the gyroid scaffolds was performed with human cell culture experiments.&amp;nbsp;

Improvement of Cu and Zr doped Diamond-like carbon coatings on comprehensive performance of Ni-Ti alloy potentially used in braided vascular stent

Wear and corrosion between braided wires will not only release toxic nickel ions, but also cause fatigue fracture of Ni-Ti alloy-based braided stents. In this study, Cu and Zr doped Diamond-like carbon (DLC) film containing Cr intermediate layer and Cr:C gradient layer were prepared with the total thickness of about 2.17 μm and 2.05 μm, respectively. In order to study the improvement of DLC coating on the comprehensive properties of Ni-Ti based stent, the nano-mechanical, tribological, electrochemical corrosion, nickel ion release and cytotoxicity properties were all evaluated. The results showed that the nano hardness and elastic modulus for Zr-DLC film were 12.32 GPa and 138.49 GPa, respectively increased by 192% and 102% compared with bare NiTi. Cu-DLC and Zr-DLC coatings can not only significantly reduce the friction coefficient, but also greatly reduce the wear rate such as by three orders of magnitude in dry sliding condition compared with bare NiTi. Zr-DLC film reduced the icorr value with a reduction amount of 96.1%, with the values of 8.41 × 10−8 and 3.26 × 10−9 A/cm2 for NiTi and Zr-DLC, respectively. After 28 days of soak experiment in phosphate buffer solution, the total nickel ion release amount was only 5.6 ng/L for Cu-DLC and 6.9 ng/L for Zr-DLC, which was reduced by 96.1% and 95.2%, respectively. The relative cell proliferation rate results showed that NiTi and Zr-DLC were both Grade 0, which indicated good cytotoxicity result. However, the cytotoxicity of Cu-DLC reached grade 2 (the relative cell proliferation rate was 71.73%). Based on the above results, Zr-DLC showed good comprehensive performances and was expected to be used as a protective layer on the surface.

Investigation of phase transition, mechanical properties, and damping behavior of NiTip/6061Al laminar composites

The microstructure, phase composition, phase transition process, mechanical properties, and damping behavior of NiTip/6061Al laminar composites were investigated by introducing a NiTi particle layer with phase transition characteristics into the 6061 aluminum matrix using rolling composite technology. No defects, such as cracks or holes, were found in the NiTi particle layer, and no agglomeration of the NiTi particles was recorded. The interlayer interface between the alloy particle layer and the 6061Al matrix was clear and well-combined, and no new phase formation occurred in the composite. The composites exhibited phase transition characteristics similar to NiTi alloy particles, and the phase transformation temperature of the composites increased gradually with an extended aging time of the NiTi-reinforced particles. Following aging strengthening, the NiTip/6061Al laminar composites demonstrated higher yield strength than the 6061-T6 aluminum alloy, while their tensile strength remained similar. Pre-aging treatment of the NiTi particles had only a minor effect on the mechanical properties of the composites. The damping capacity of the laminar composites surpassed that of the 6061-T6 aluminum alloy due to the synergistic enhancement of phase transition damping from the NiTi particles and interparticle interface damping within the particle layer The internal friction values of the NiTip/6061Al laminar composites at 60 °C and 350 °C were up to 0.0134 and 0.1146, respectively, which were 211% and 313% higher than those of the 6061-T6 aluminum alloy at the corresponding temperature. The storage modulus and the comprehensive damping performance (EIF) of the NiTip/6061Al laminar composites were higher than those of the 6061-T6 aluminum alloy.

Environment-Induced Degradation of Shape Memory Alloys: Role of Alloying and Nature of Environment.

Shape memory effects coupled with superelasticity are the distinctive characteristics of shape memory alloys (SMAs), a type of metal. When these alloys are subject to thermomechanical processing, they have the inherent ability to react to stimuli, such as heat. As a result, these alloys have established their usefulness in a variety of fields and have in recent years been chosen for use in stents, sensors, actuators, and several other forms of life-saving medical equipment. When it comes to the shape memory materials, nickel-titanium (Ni-Ti) alloys are in the forefront and have been chosen for use in a spectrum of demanding applications. As shape memory alloys (SMAs) are chosen for use in critical environments, such as blood streams (arteries and veins), orthodontic applications, orthopedic implants, and high temperature surroundings, such as actuators in aircraft engines, the phenomenon of environment-induced degradation is of both interest and concern. Hence, the environment-induced degradation behavior of the shape memory alloys (SMAs) needs to be studied to find viable ways to improve their resistance to an aggressive environment. The degradation that occurs upon exposure to an aggressive environment is often referred to as corrosion. Environment-induced degradation, or corrosion, being an unavoidable factor, certain techniques can be used for the purpose of enhancing the degradation resistance of shape memory alloys (SMAs). In this paper, we present and discuss the specific role of microstructure and contribution of environment to the degradation behavior of shape memory alloys (SMAs) while concurrently providing methods to resist both the development and growth of the degradation caused by the environment.

Proof of concept of a self-tightening needle-less suture using a NiTi shape memory alloy

Among surgical procedures, suturing is considered simple. However, in some situations, suturing is not easy or feasible. Here, we present proof of concept of a new self-tightening needle-less suture made of a NiTi alloy. The new device is designed to overcome two demanding steps of traditional suturing: needle introduction and manual knotting. Our novel suture is composed only of a short NiTi wire that is able to change shape by exploiting a thermoelastic phase transition. This shape change is achieved by increasing the temperature from that of the operating room (e.g., 20–22 °C) to 32 °C using warm water. Suturing consists of two steps: suture introduction, in which the suture is in an open configuration, and shape recovery (i.e., closed configuration). The closed configuration is maintained at human body temperature thanks to peculiar pseudoelastic properties of the NiTi material. Finally, thermal and functional characterization verified the simplicity and effectiveness of the proposed device.Graphic abstract

A novel crystal rotation model of LDBs deformation in NiTi alloy

To investigate the deformation characteristics of Lüders-like bands (LDBs) in NiTi alloy, the uniaxial tensile testing was conducted under isothermal condition using the digital image correlation (DIC) technique. Based on the changes in the strain field near the LDBs during the initiation, propagation, and expansion of the LDBs, a novel crystal rotation model was proposed to explain the formation of the LDBs. The model focused on the microstructural evolution that occurred at the front of the phase transformation interface when the LDBs appeared, providing a fresh perspective on the mechanism of the formation of the LDBs.

Role of Zr and thermomechanical treatment on phase transformation and functional properties of NiTi-based shape memory alloys

A systematic investigation was conducted to evaluate the role of Zr addition on the martensitic phase transformation behavior and properties of NiTi-based shape memory alloys. Equiatomic binary NiTi alloy and ternary Ni50Ti45Zr5 and Ni50Ti40Zr10 alloys (at.%) were processed by a simple thermomechanical treatment including cold rolling followed by post-deformation annealing at 450 °C for 30 and 60 min. The microstructural investigation and thermal analyses reveal that martensitic phase transformation is suppressed by adding Zr. Solid solution strengthening due to Zr addition, strain hardening due to cold rolling and formation of (Ti,Zr)2Ni precipitates in a dual-phase microstructure during annealing leads to a significant increase in hardness in the thermomechanically treated alloys (e.g., up to 550 Hv for Ni50Ti45Zr5 alloy). The stress-strain curve of martensitic Ni50Ti50 alloy reveals the essential characteristics of shape memory effect, however, Ni50Ti45Zr5 alloy shows typical features of pseudoelasticity with significantly high stress plateau (∼600 MPa) and remarkable recovered strain of 3.5% during unloading.

Corrosion resistance and cytocompatibility of Ti–19Zr–11Nb–4Ta shape memory alloy for biomedical applications

The corrosion resistance and cytocompatibility of Ti–19Zr–11Nb–4Ta shape memory alloy have been investigated. Well superelasticity of 4.6% was detected in the Ti–19Zr–11Nb–4Ta alloy with a triphasic microstructure of β, α′′ and ω phases. The alloy performs a higher corrosion resistance than Ti and Ti–6Al–4V indicated by the electrochemical experiments in Hank's and 0.9% NaCl solutions. Low release rate of ions in Ti–19Zr–11Nb–4Ta alloy was also demonstrated by the 21 days immersion tests with the reference of biomedical Ti, Ti–6Al–4V and Ni–Ti alloys. Meanwhile, the Ti–19Zr–11Nb–4Ta alloy performs a well cytocompatibility by MC3T3-E1 cells' adhesion and proliferation tests, which is expected to be utilized as a new biomedical shape memory alloy.

NiTi Shape Memory Clamps with Modified Surface for Bone Fracture Treatment.

Using NiTi alloys with shape memory for long-term medical implants requires modification of their surface due to the possible occurrence of corrosion. Hence, the surface of the staples used to join fractured bone within the craniofacial region was modified by applying a titanium oxy-nitrogen layer and a hydroxyapatite coating. Surface-modified clamps were tested in vivo using New Zealand white rabbits. After determining the mechanical characteristics of the bone and considering the initial state and surface modification, the diameter of the wire (used to make the clamps with the appropriate compression force) was selected. Implantation was performed on two groups of rabbits: experimental and control. In the experimental group, an intentionally induced bone fracture was treated in one tibia. On the second tibia, two additional clamps were applied to increase the possibility of a negative impact of the NiTi alloy on a living organism. After 6 weeks of application, a proper joining of the broken bone fragments was stated. Whereas after twelve weeks, no negative impact of the clamp material on a living organism, i.e., a rabbit, was found. Hence, the clamp with the modified surface can connect bone fragments in humans as well as small and medium-sized animals, with an extended range of use up to 12 weeks.

Quasi-3D dynamic photoelastic analysis of stress distribution during preparation of simulated canals with 13 mechanical preparation systems.

The aim of this study is to compare the stress produced on the internal walls of simulated canals by nine rotary and four reciprocating systems. Sixty-five isotropic transparent blocks containing a 60° curved and tapered simulated canal were selected and distributed into 13 groups (n = 5) according to the preparation system: BioRace, HyFlex EDM, iRaCe, Mtwo, One RECI, ProTaper Next, RaCe EVO, Reciproc, Reciproc Blue, R-Motion, VDW.ROTATE, XP-Endo Rise Shaper, and XP-Endo Shaper. Each resin block was mounted in a vice and a digital camera recorded the entire sequence of each preparation system through a circular polariscope set for dark field analysis. The video frames when each instrument reached the end of the coronal, middle, and apical thirds of the canal were extracted from the recordings and analysed by two independent observers regarding the stress generated on the canal walls using a semi-quantitative evaluation on a 0-5 scale. Intra- and inter-observer agreement were subjected to the Cohen's Kappa coefficient test, whilst the experimental results were compared using Kruskal-Wallis test post hoc pairwise comparisons with Bonferroni correction (α = 5%). The inter- and intra-observer agreement were 0.98 and 1, respectively. Most instruments demonstrated acceptable performance (scores ≤ 2) in all thirds. Other instruments, such as the HyFlex EDM 25.12 (coronal and middle thirds), Reciproc Blue R25 and Reciproc R25 (coronal and apical thirds), R-Motion 30.04 (apical third), and VDW.ROTATE 20.05 (apical third) showed scores higher than 3. Statistical analysis revealed a significant difference amongst the tested systems at the coronal, middle, and apical thirds (p < .05). None of the canal instrumentation protocols were stress-free, showing varying levels of stress concentrations. Various factors seemed to influence the magnitude of stress and its distribution pattern on the canal walls. Overall, instruments characterized by a larger taper, lower speed, reciprocating motion, and made of heat-treated NiTi alloy exhibited higher patterns of stress distribution.

On Structural Sensitivity of Young’s Modulus of Ni-Rich Ti-Ni Alloy

When developing bone implants, Young’s modulus is one of the primary characteristics of the material that should be considered. This study focuses on regulating the modulus of Ti-50.8 at.% Ni alloy by varying the grain/subgrain size as well as the initial structure using subsequent aging at 430 °C for 10 h. After post-deformation annealing (PDA), the temperature dependence of Young’s modulus exhibits a pronounced V-shaped character with a minimum at the onset temperature of the forward martensitic transformation, Ms, regardless of the structure. The grain/subgrain size of B2-austenite strongly affects the modulus magnitude. This effect is ambiguous for a material with a grain size range of 0.13–3 µm and depends on the test temperature. The effect of aging on the modulus reduction depends on the initial structure; it is most pronounced in an alloy with a relatively coarse grain size of 9 µm and brings a decrease of 3.8 times at a temperature of 37 °C. Aging of the initially recrystallized Ni-rich NiTi alloy makes it possible to obtain a вone-like elastic modulus of E = 12–13 GPa at an operating temperature of 37 °C. An ultrafine-grained substructure exhibits the same Young’s modulus values in the low temperature range from −100 to 25 °C.

Deposition Temperature Induced Texture and Strengthening of Ti/Ni Multilayer Thin Films

Abstract Strong thermal effect on microstructure and mechanical properties of Ti/Ni multilayer thin films was observed from in situ heating during deposition and subsequent annealing. Films deposited at low-temperature show preferred crystallographic texture for both Ti and Ni layers, with columnar structure extending through the layers. The columnar structure becomes more distinct and complete with the increase of temperature up to 300 °C, and meanwhile, more atomic diffusion and intermixing occur along the Ti/Ni interfaces, promoting the formation of Ti-Ni intermetallic precipitates. High-temperature deposition causes disintegration of the layered structure. Columnar Ti-Ni alloys and further recrystallized alloys were detected with the preferred crystallographic texture. For material strength, an increased hardness trend was observed with increasing deposition temperature even with much larger grain size compared to room temperature case. Furthermore, for multilayer systems deposited under low temperature, post-annealing resulted in higher hardness with minimal microstructure modification, with more strengthening observed in lower deposition temperature case.