NiTi Alloy Research Articles

Microstructure-dependent deformation mechanisms and fracture modes of gradient porous NiTi alloys

Gradient porous structures based on triply periodic minimal surfaces offer exceptional specific strength and multi-functionality. However, strain heterogeneity complicates their deformation mechanisms and fracture modes. In this study, we fabricated a series of gradient porous NiTi alloys based on gyroid (G), diamond (D), and I-WP (I) unit cells, with porosities ranging from 50% to 70%. Surprisingly, the I structure exhibited an abnormally high compressive strength of nearly 600 MPa at around 50% porosity, more than twice that of the G and D structures. This performance gap was attributed to the activation of mixed deformation mechanisms and distinct fracture modes. While the G structure showed a uniform radial strain distribution, the D and I structures displayed pronounced radial strain gradients with concentrated central strains. The D structure deformed through relative sliding of oblique struts, while the I structure maintained macrostructural stability, with thick edge struts providing continuous strain hardening until monolithic fracture. This unique deformation behavior results from the synergistic effects of its intrinsic macrostructural stability, significant radial strain gradients, and sufficiently thicker edge struts. The integration of metallurgical and architectural design principles represents a novel approach to tailoring the mechanical properties of gradient porous metals for advanced applications.

Influence of Jewelry Surface Vibromechanical Processing on the Structure and Mechanical Properties of TiNi Alloy

Influence of Jewelry Surface Vibromechanical Processing on the Structure and Mechanical Properties of TiNi Alloy

Accelerated improvement in tensile superelasticity of electron beam directed energy deposition manufactured NiTi alloys by artificial thermal cycling combined with low temperature aging treatment

ABSTRACT The low-temperature aging treatment at 250°C can significantly improve the tensile superelasticity of NiTi alloys fabricated by electron beam directed energy deposition (EB-DED). However, it requires a very long aging duration (up to 200 h) to achieve excellent tensile superelasticity due to inherent coarse grain size. To accelerate the aging process, the high-density dislocations are introduced by artificial thermal cycling treatment prior to the aging treatment (the original dislocation content in EB-DED processed NiTi alloys is very low), which will promote the subsequent uniform precipitation of nanoscale Ni4Ti3 particles during low-temperature aging treatment. The phase transformation behaviour always maintains a stable two-stage martensitic phase transformation. Under a cyclic tensile test at 6% strain, 24 h aged sample with thermal cycling maintains a recovery rate exceeding 90% even after 10 cycles, comparable to the performance of the sample aged for 200 h without thermal cycling, indicating a substantial improvement in aging efficiency.

Hot deformation behavior of NiTiHf alloy under compression: Effect of deformation heating on flow softening

High-temperature shape memory alloys are suitable materials for substituting hydraulic elements in the aerospace industry due to light weight and high reactive stresses developing during thermoelastic martensitic transformation and acceptable recoverable strain. These properties may be improved by plastic deformation; however, the plastic deformation of NiTiHf alloys along with structure investigation has yet to be studied systematically. Here, the hot deformation behavior of a NiTiHf alloy with hafnium content >20 at.% has been investigated, and the processing map has been elaborated according to the dynamic material model (DMM) based on the Prasad instability criterion. The deformation heating effect has been shown to affect the flow curves and accounts for profound softening during deformation due to quasi-adiabatic deformation mode at high strain rates. NiTiHf alloys have a much higher activation energy of plastic flow, and the instability region of plastic flow is much larger compared to binary NiTi alloys. Microstructural observations have revealed that in the instability regions, oxide bands and crack nucleation occur, whereas outside this region, such defects are not observed. Electron backscatter diffraction has demonstrated that at high temperatures and slow strain rates, discontinuous dynamic recrystallization occurs resulting in the necklace-type structure, while at high strain rates, this phenomenon has not been observed. Based on the obtained results, the preferable deformation conditions are established to be 850–1000 °C and 0.003–0.05 s−1.

The Effect of Ageing on Phase Transformations and Mechanical Behaviour in Ni-Rich NiTi Alloys.

In this article, the results of research on a NiTi alloy with a high nickel content (51.7 at.%), produced using the additive technology SLM method and subjected to isothermal ageing after solution annealing, are presented. The study involved the determination of the sequence of phase transformations occurring using differential scanning calorimetry (DSC) and the determination of the temperature range of these transformations. In parallel, the phase composition was determined using the XRD method; the hardness and the Young's modulus were also determined. The analysis of the DSC results obtained indicates the following characteristic features of the NiTi alloy, which change with ageing time: (1) During cooling (from +150 °C to -50 °C), the type of transformation changes from a one-step transformation after solution annealing to a two-step transformation after the ageing process over 1, 20, and 100 h at 500 °C; (2) during heating (from -50 °C to +150 °C) for all the samples, regardless of the ageing time, only a one-step transformation from martensite M(B19') to austenite A(B2) is observed; (3) the temperature at which the transformation starts increases with the ageing time; (4) the width of the total temperature range of the transformation M(B19') → A(B2) during heating changes from large (ΔT = 49.7 °C), after solution annealing, to narrow (ΔT = 19.3 °C and ΔT = 17.9 °C after 20 h and 100 h of ageing); and, most importantly, (5) a comparison with the literature data shows that, irrespective of the composition of the NiTi alloy and the manufacturing technology of the alloy samples (regardless of whether this was traditional or additive technology), a sufficiently long ageing process period leads to the occurrence of the martensite → austenite transformation in the same temperature range.

Cyclic Fatigue of Different Ni-Ti Endodontic Rotary File Alloys: A Comprehensive Review.

Modern endodontics aims to decrease the bacterial load from the complex endodontic space. Over the years, improvements in the operative phases have led to a considerable increase in the success rate of endodontic treatments. The shaping phase has seen the development of new techniques supported by technological innovations that have led to higher treatment predictability. Endodontic instruments have experienced a series of changes that have led to modifications in their design, surface treatments, and heat treatments. The clinical use of rotating nickel-titanium instruments has become widespread and consolidated, a success due primarily to the alloy's mechanical characteristics, which are superior to steel ones, but also to innovations in instrument design. The advent of the Ni-Ti alloy has kept the concepts and requirements of shaping the same but has modified its implementation in endodontics. The following review followed the Preferred Reporting Items for Systematic Reviews and Meta-analyses (PRISMA) protocol. The research question focused on Ni-Ti endodontic instruments whose cyclic fatigue was evaluated by analyzing cyclic fatigue strength and the incidence of fracture. At the end of the research, 10 systematic reviews and 1 randomized controlled trial were included in this comprehensive review. The most frequently analysed alloys were M-wire, conventional Ni-Ti, and CM-wire. In seven articles, instruments made of M-wire alloy were used; in eight articles, instruments made of conventional Ni-Ti; and in seven articles, instruments made of CM-wire alloy. The technological evolution of Ni-Ti alloys has led to the development of increasingly high-performance endodontic files that are resistant to cyclic fatigue during clinical practice and have greater resistance to sterilisation practices, making treatment easier and more predictable over time. In particular, heat-treated nickel-titanium root canal instruments present greater resistance to cyclic fatigue than untreated ones and those used with reciprocating kinematics concerning continuous rotation.

Damage accumulation and its effect during thermal- and stress-induced cycling martensite transformation of laser powder bed fused (LPBF) NiTi alloy

Nitinol (NiTi) alloys produced via laser powder bed fusion (LPBF) exhibit promising potential across diverse applications, yet their suitability for devices necessitating repetitive actuation confronts uncertainties regarding microstructural evolution and functional fatigue behavior during cyclic operation. Our investigation unveils that damage accumulation during thermal cycling stems from geometrically necessary dislocations formed through finite-scale incoherent interface motion, contrasting earlier hypotheses attributing LPBF-induced dislocation activity as the primary contributor. Damage accumulation during mechanical cycling is characterized by notable multiplication of cyclic dislocations via dislocation twinning interactions, rather than severe martensitic plasticity. These newborn cycling dislocations inherit martensite's crystallographic characteristics, exerting a distinct influence mechanism on martensite transformation and deformation behavior, diverging from conventional studies on function fatigue in traditional NiTi alloys. This study sheds light on the distinctive damage accumulation traits observed during thermal/mechanical cycling of LPBF-fabricated NiTi alloys, offering valuable insights for probing their functional fatigue mechanisms.

The influence of hydroxyapatite coatings with different structure and crystallinity on osteogenesis stimulation

NiTi alloys are actively used in medicine as implants because of their mechanical properties. At the same time, however, the NiTi surface is not biologically active, which has a negative effect on osteogenesis. In addition, Ni is capable of causing various toxic reactions in the human body. It is therefore necessary to create a composite coating that is both bioactive and prevents the release of Ni ions. It has been shown that by changing the modes of plasma assisted RF sputtering of a calcium phosphate target, it is possible to obtain coatings with different contents of the amorphous phase. The coatings formed consist of hydroxyapatite and amorphous calcium phosphate. On the surface of a sample not treated with argon plasma, there are no phases containing calcium or phosphorus, although these elements are present on the surface. The different ways in which the CaP coatings were applied also affected their structure. It was found that the CaP coatings of samples M2 and M3 have mediocre corrosion resistance, dissolving quickly. This has a positive effect on the ability to osteogenesis, which was discovered as a result of in vitro tests. These tests have shown that the sample obtained at Idischarge = 30 A and Ubias = 30 V has the highest bioactivity and produces the lowest percentage of dead cells.

Exploring the Friction and Wear Mechanisms of Nickel Titanium Alloy Fabricated by Laser Powder and Wire-Based Directed Energy Deposition

The NiTi alloy has attracted significant interest owing to its notable wear and corrosion resistance, suggesting the significant promise of utilizing additively manufactured NiTi alloys in tribological applications. In this study, dry sliding wear tests with sliding velocity of 0.1 m/s for total distance of 250 m at room temperature, 50 °C, 100 °C, and 200 °C were carried out on samples of NiTi alloy samples fabricated using laser powder and laser wire directed energy deposition (LP-DED and LW-DED) processes. Enhanced friction and wear behavior of LP-DED samples were captured at lower temperatures due to higher hardness and the stable dominating NiTi B2 austenite phase in the sample microstructure as confirmed from x-ray diffraction (XRD) analysis. Accumulated transferred material at lower temperatures played a crucial role in modifying the wear mechanisms. However, at higher temperatures both LP-DED and LW-DED samples presented similar wear mechanisms.

Exploring the structural mechanics of titanium nickel solid alloy using COMSOL Multiphysics: A Poisson equation and continuity equation perspective

This study investigates the structural mechanics of titanium-nickel (Ti-Ni) alloy thin film using computational modelling through COMSOL Multiphysics based on Poisson’s equation and continuity equation for stress check by considering its linear elastic, conservation of charge and providing insight into nanomaterial deformation. In the COMSOL environment the parameters for titanium nickel (Ti-Ni) are embedded in the COMSOL Simulink interface. The Thin film layer was designed by defining the layer geometry of the size and shape of the layer with a width of 500 μm, depth of 200 μm and height of 3 μm subjected to boundary conditions such as von – mises stress, surface temperature, iso-surface temperature, multi-slice electric potential, displacement component, surface elastic strain energy density and total enthalpy. The results displayed a trend that is, as the surface temperature increases there will be an increase in the current densities associated with high electrical conduction. On the same note, the designed thin film layer will pass the percolation threshold. The results of surface elastic strain energy density and total enthalpy imply that the designed thin film layer is effective and efficient as a structural pseudopotential device (photodiode).

Characterization of the temperature-dependent superelastic and elastocaloric effects of a NiTi tube under compression at 293–330 K

Comprehensive characterizations of the superelastic and elastocaloric effects of NiTi and NiTi-based shape memory alloys (SMA) in the operation temperature region are highly desirable for using them in elastocaloric coolers with a large temperature lift. In this article, we report the superelastic and elastocaloric effects of a commercially available superelastic polycrystalline NiTi SMA tube with an outer diameter of 5 mm and a wall thickness of 1 mm between 293 and 330 K. The NiTi tube sample was subjected to a training of 250 cycles to stabilize its superelastic and elastocaloric effects. We observed that temperature dependencies existed for both superelastic and elastocaloric effects of the NiTi tube, and stress–strain curves differed much between isothermal and adiabatic loading conditions. The largest temperature rise and temperature drop measured at 293 K under an applied strain of 3.66% and a strain rate of 0.1 s−1 during loading and unloading were 21 and 11 K, respectively. The loading conditions (loading function and holding time) also impacted the superelastic effect of the NiTi tube. We identified two major reasons for the irreversibility of the adiabatic temperature change: the hysteresis heat dissipation and the temporary residual strain after unloading, and they affected the cooling performance of the elastocaloric cooler in different ways. We investigated the dependencies of the superelastic and elastocaloric effects on the maximum applied strain and the temperature distribution on the NiTi tube during loading and unloading. The results are beneficial to the modeling of elastocaloric coolers with large temperature lifts.

Experimental study on the corrosion of AlN refractories used as crucibles for induction melting of TiNi alloys

Vacuum induction melting has proven to be a cost-efficient way to fabricate TiNi alloys with excellent compositional homogeneity and uniformity. However, the high reactivities of TiNi alloys generate aggressive melt attacks with a particular ceramic crucible. Here, the performance of AlN crucible for melting TiNi alloys was investigated, and the underlying mechanisms for the interfacial interactions were revealed. The results demonstrated the feasibility of using AlN crucibles for induction melting of TiNi alloys. Specifically, melt infiltration was suppressed and thus physical erosion was diminished by reducing the apparent porosity of the crucible. Further investigations combined with a theoretical analysis revealed that the TiN barrier layer distributed on the crucible surface was formed in situ due to crucible/melt interactions, which protected the ceramic matrix from further melt attack and limited the contaminant levels. Our work provides a viable pathway for the preparation of high-quality TiNi alloys via induction melting.

Characteristic variations in a pulsed‐anodized NiTi alloy surface by the lower voltage setting

Pulsed anodization of a nearly equiatomic NiTi alloy in HNO3 leads to the formation of a nearly Ni‐free oxide layer, resulting in the suppression of Ni‐ion release from the alloy surface. The core technology involves introduction of a lower‐voltage period, which promostes chemical reactions between the alloy and electrolyte, to obtain a surface layer with better corrosion protection. In this study, a higher voltage of 3.5 V was applied, and the lower voltage was varied within 0–3.5 V. As the lower voltage was increased, nanometer‐sized pores present on the anodized surface gradually expanded, while the layer thickness decreased. Although the corrosion protectivity of the layer did not change significantly during the electrochemical experiments, the amount of Ni‐ion released into the physiological solution significantly decreased when the voltage was below 1.8 V. X‐ray photoelectron spectroscopy analyses revealed the presence of Ni (OH)2 on the topmost surface, and its concentration decreased at a voltage <1.8 V. Lower voltages affected the concentration of Ni (OH)2 on the topmost surface and thus considerably influenced the Ni‐ion release behavior. These findings will contribute to the production of NiTi alloys with improved biocompatibility.

Self-Heating and Fatigue Assessment of Laser Powder Bed Fusion NiTi Alloy with High Cycle Fatigue Mechanisms Identification

Rapid methods for assessing the fatigue properties of materials have been developed, among which the self-heating method stands out as particularly promising. This approach analyzes the thermal signal of the specimen when subjected to cyclic loading. In this research, the self-heating method was utilized for the first time with laser powder bed fusion (LPBF) of NiTi alloys, examining two specific loading conditions: loading ratios of 0.1 and 10. A thorough examination of the material self-heating behavior was conducted. For comparative purposes, conventional fatigue tests were also conducted, alongside interrupted fatigue tests designed to highlight the underlying mechanisms involved in high cycle fatigue and potentially self-heating behavior. The investigation revealed several key mechanisms at play, including intra-grain misorientation, the emergence and growth of persistent slip bands, and the formation of stress-induced martensite. These findings not only deepen our understanding of the fatigue behavior of LPBF NiTi alloys but also highlight the self-heating method potential as a tool for studying material fatigue.

Nickel nitride-mediated nitric oxide generation for combating implant-associated infections

NiTi alloys are widely used in the fabrication of biomedical implants, and it is essential to endow them with selective antibacterial properties to avoid implant-associated infection. In this study, nickel nitride (Ni3N), as a promising new inorganic nitric oxide (NO) donor, was in situ constructed on the surface of NiTi alloys through a straightforward two-step plasma immersion ion implantation (PIII) strategy. Nickel sites were first exposed by physical bombardment with argon or metal plasma, followed by reaction with nitrogen plasma to form Ni3N. Ni3N can disrupt the transmembrane proton electrochemical gradient by capturing protons to produce ammonia species (NHx), resulting in the accumulation of reactive oxygen species (ROS) as a result of bacterial metabolic disruption. On the other hand, the generated NHx can further react with ROS to produce NO and reactive nitrogen species (RNS). As a result, the Ni3N films exhibited more than 99 % antibacterial efficiency against both gram-negative and gram-positive bacteria. They also demonstrated effective antibacterial properties in a rat subcutaneous implant model. In addition, the modified NiTi alloys exhibit good corrosion resistance and cytocompatibility. This study provides a green, safe, and inexpensive method for constructing metal nitride films on implant surfaces and offers new ideas for the clinical design and fabrication of novel antibacterial implants.

Tribo-electrochemical investigation of 60NiTi alloy in saline solution

This research explores the tribocorrosion behaviour of 60NiTi alloy, also known as NiTiNOL60, when exposed to a saline environment. Our investigation focuses on understanding the relationship between corrosion and wear rates and assessing surface damage and material degradation. To conduct our experiments, we employed a linear reciprocating ball-on-plate tribometer coupled with electrochemical polarisation using a three-electrode cell configuration to assess the combined effects of corrosion and sliding wear. Surface characterisation was carried out through scanning electron microscopy and energy dispersion spectroscopy, revealing the material to be a Ni-rich 60NiTi alloy, with surface oxidation evident in the electrolyte medium. Our electrochemical findings indicate the occurrence of localised corrosion in both cathodic and anodic regimes, with corrosion pit nucleation, cavities, and cracks being accelerated by reciprocating sliding and corrosion potential. These interactions exposed the material surface to various wear mechanisms, including abrasive, adhesive, oxidative, corrosive, and fatigue processes. This study underscores the significant influence of mechanical properties on the rate of material degradation due to corrosion, while also highlighting the substantial impact of prevailing electrochemical conditions on the rate of mechanical material removal. This paper offers valuable insights for designers working on load-bearing structures in saline environments.

Preparation of a (Ca,Sr,Ba)ZrO3 Crucible by Slip Casting for the Vacuum Induction Melting of NiTi Alloy.

Vacuum induction melting is a more energy-efficient process for the preparation of a titanium alloy with good homogeneity and low cost. But the crucial problem for this technology is in developing a crucible refractory with high stability. In the present work, a novel (Ca,Sr,Ba)ZrO3 crucible was prepared by slip casting and its performance in melting NiTi alloy was studied. The results showed that a single solid solution was formed with a homogeneous distribution of metal elements after sintering at 1500 °C. It was found that the total content of oxygen and nitrogen remaining in the TiNi alloy after melting in the (Ca,Sr,Ba)ZrO3 crucible was 0.0173 wt.%, which fulfills the ASTM standard on biomedical TiNi alloys. The good resistance of the (Ca,Sr,Ba)ZrO3 crucible to molten NiTi has a relationship with the sluggish diffusion effect of high-entropy ceramics. This study provides insights into the process of designing highly suitable crucible material for melting a NiTi alloy.

Evaluation of Interface and Residual Strain of NiTi Layer Deposited on NiTiX Substrate by Laser Powder Bed Fusion

This study investigates the microstructure and properties of functionally graded NiTi alloy bilayers. The NiTi layer is printed by laser powder bed fusion on a NiTiX (where X is Hf or Cu) substrate prepared by vacuum arc remelting. Specimens produced with different thicknesses of layers, but constant thickness ratio, are examined by optical and scanning electron microscopy prior to and postannealing process at 1000 °C for 16 h. Scanning electron microscopy– energy‐dispersive X‐ray spectroscopy and transmission electron microscopy studies reveal the presence of Ti2Ni and Ni4Ti3 precipitates in the as‐printed NiTi/NiTiCu samples and Ti2Ni type precipitates in as‐printed NiTi/NiTiHf. Digital image correlation quantifies residual strain in the as‐printed bilayer and enables strain relief to be monitored during heating. It has been shown that microcracks occurring along interfacial zones during the laser powder bed fusion are diminished after annealing heat treatment. The microcrack closure occurs by diffusion of third elements to the open microcracks, leading to precipitation and accumulation of third elements in the interfaces. Eventually, the as‐printed NiTi/NiTiCu sample displays two‐way shape memory effects with about 24.5% shape recovery. This work enhances understanding of controlling fabrication to yield tailored properties in additively manufactured functionally graded NiTi‐based materials.

Resistance Feedback of a Ni-Ti Alloy Actuator at Room Temperature in Still Air.

This paper illustrates an experimental activity for the closed-loop position control of an actuator made using shape memory alloy (SMA) wire. A solution with the self-sensing effect was implemented to miniaturize the systems, i.e., without external sensors. A proportional control algorithm was initially used, demonstrating the idea's feasibility; the wire can behave simultaneously as an actuator and sensor. An experimental investigation was subsequently conducted for the optimization of the developed actuator. As for the material, a Flexinol wire, Ni-Ti alloy, with a diameter of 0.150 mm and a length of 200 mm, was used. Preliminarily, characterization of the SMA wire at constant and variable loads was carried out; the characteristics detected were elongation vs. electric current and elongation vs. electrical resistance. The control system is PC based with a data acquisition card (DAQ). A drive board was designed and built to read the wire's electrical resistance and power it by pulse width modulation (PWM). A notable result is that the actuator works with good precision and in dynamic conditions, even when it is called to support a load up to 65% different from that for which the electrical resistance-length correlation has previously been experimentally obtained, on which the control is based. This opens up the possibility of using the actuator in a counteracting configuration with a spring, which makes hardware implementation and control management simple.

Nickel–Titanium Alloy Laser Micromachining: A Review of Nitinol Laser Processes and Optimization for High‐Speed Laser Cutting

The binary nickel–titanium (NiTi) alloy, “Nitinol”, has become a prominent name in the medical device industry among medical device manufacturers. The unique properties of the material, such as the shape‐memory effect and pseudoelasticity, have earned the material increasing popularity for new product innovations. Among the various manufacturing processes available for metal alloys, laser micromachining has taken the lead for processing Nitinol‐based products. This literature review provides an analysis of the history and applications of Nitinol, an overview of the microstructure of the material, and a review of the current manufacturing methods for laser cutting medical‐grade Nitinol. A more in‐depth focus is placed on the realm of laser processing and the challenges associated with the manufacturing process. Ultrafast femtosecond pulse processing delivers promising results and quality for manufacturing Nitinol medical devices. However, there exists a need to investigate potential approaches to increase the cutting speed of the process to enhance throughput and stay competitive in a growing market due to the cutting speed being over 4‐times slower than long pulse cutting. Many tactics to address the problem are discussed, ranging from laser selection, processing parameters, and nontraditional laser machining approaches.