NiTi Shape Memory Alloy Research Articles

High-Strength Dissimilar Welds Between a NiTi Shape Memory Alloy and Titanium Obtained by Intermixing Niobium Using Pulsed Laser Beam Welding

Joining of NiTi shape memory alloys with commercially pure titanium (Ti) is of great interest for manifold industrial applications. However, dissimilar fusion welding of these materials is associated with the formation of extremely brittle intermetallic compounds, e.g., Ti2Ni and TiNi3, which drastically limit the mechanical properties of NiTi/Ti joints. The present investigation seeks to improve both chemical compatibility and mechanical performance through intermixing of a niobium foil as filler material into a NiTi/Nb/Ti butt-joint configuration by means of pulsed laser beam welding. Different pulse durations are applied to tailor intermixing and evaluate the deviating chemical compositions of the weld metal. It is demonstrated that intermixing of niobium significantly increases the weldability of the material combination NiTi/Ti compared to autogenous welding. However, the proportion of intermixed filler material exerts a substantial impact on the microstructural evolution and mechanical properties. Ultimate tensile strengths of up to 679 MPa with fracture occurring in the titanium base metal at an elongation at break of 18.7 pct is obtained by a reduction of the pulse duration, which represents a major improvement over previous studies focusing on fusion welding of NiTi to titanium and its alloys.

Influence of high temperature ternary and quaternary additions on the phase transformation and actuation fatigue characteristics of NiTi shape memory alloys

Influence of high temperature ternary and quaternary additions on the phase transformation and actuation fatigue characteristics of NiTi shape memory alloys

Characterization of corrosion properties, structure and chemistry of titanium-implanted TiNi shape memory alloy

The near-equiatomic TiNi shape memory alloy passivated through electropolishing and ion implantation with titanium has been studied toward corrosion performance in simulated body fluids (0.9 wt% NaCl, artificial blood plasma). Corrosion rate, nickel oxidation, repassivation and charge transfer has been examined by conventional methods of potentiodynamic polarization, electrochemical impedance spectroscopy and cyclic voltammetry. Experimental validation of the electrochemical results was performed using electron-microscopic (SEM/TEM) techniques and Auger electron spectroscopy. It was revealed that the thickness of the passive layer (TiO + TiO2) could be increased by ∼5 times after ion implantation. Regardless of the corrosion environment, the TiNi alloys exhibiting different surface finishes still suffer from pitting corrosion associated with leaching of nickel ions via oxidation reaction. The Ti-implanted alloy shows satisfactory corrosion resistance in comparison with the reference electropolished TiNi alloy. After ion implantation, the dissolution of the surface layer during anodic polarization was restricted due to the formation of the Ni-depleted amorphous sublayer. Auger Ti- and Ni-LMM peaks are found to be shifted to lower energies due to the contribution of NiO and TiO bonding. It has been shown that not the thickness, but rather the structure and phase composition of the oxide layer are main factors responsible for corrosion performance.

Study on removal of recast layer of NiTi shape memory alloy machined with magnetic field-assisted WEDM-ECM complex process

Study on removal of recast layer of NiTi shape memory alloy machined with magnetic field-assisted WEDM-ECM complex process

3D Chiral Energy-Absorbing Structures with a High Deformation Recovery Ratio Fabricated via Selective Laser Melting of the NiTi Alloy.

Excellent energy-absorbing structures have been highly sought after in engineering applications to improve devices and personal safety. The ideal energy absorption mechanism should exhibit characteristics such as lightweight, high energy absorption capacity, and efficient reusability. To address this demand, a novel three-dimensional (3D) chiral lattice structure with compression-twist coupling deformation is fabricated by combining the left and right chiral units. The proposed structure was fabricated in NiTi shape memory alloys (SMAs) by using laser powder bed fusion technology. The compression experiment result indicates that the shape recovery ratio is as high as 94% even when the compression strain is over 80%. Additionally, the platform strain reaches as high as 66%, offering high-level specific energy absorption, i.e., 213.02 J/g. The obtained results are of great significance for basic research and engineering applications of energy-absorbing structures with high deformation recovery ratios.

Experimental and numerical study on the mechanical properties of NiTi-SMA bars

This paper presents an experimental investigation that aims to study the mechanical properties of NiTi shape memory alloy (SMA) bars, with a particular emphasis on comprehensively evaluating their superelastic properties and energy dissipation capabilities. The study examines the influence of thermal treatment processes, loading rates, pre-training, and loading amplitudes on crucial parameters such as residual strain, energy dissipation capacity, initial stiffness, and equivalent viscous damping ratio of SMA bars. Additionally, the microstructure of the SMA bars' cross-sections, both prior to and following thermal treatment, is examined using scanning electron microscopy. The existing Graesser constitutive model is improved, and its validity is verified through numerical fitting. The findings demonstrate that SMA bars with a direct working section of 10 mm exhibit favorable superelastic properties after undergoing heat treatment at 350 °C for 35 min. It is recommended that the loading rate of SMA should not exceed 0.15 % per second, and appropriate pre-training measures should be implemented in practical engineering applications. Microstructural analysis reveals distinct dimples and intergranular fracture marks in the cross-sections of heat-treated SMA bars, indicating a high level of material toughness. Moreover, the improved Graesser constitutive model accurately calculates the mechanical characteristics of SMA bars and effectively describes their residual strain behavior. The research outcomes of this study provide valuable theoretical and technical insights for the practical application of shape memory alloy bars in engineering contexts.

Insights into the influence of Ni4Ti3 precipitates and martensite transformation on the glide of a[100] dislocation in austenitic NiTi alloys: an atomistic simulation study

Dislocation-dominated plastic deformation in austenitic NiTi alloys is highly detrimental and it could be suppressed by precipitation of Ni4Ti3. Additionally, at a certain temperature range, not only Ni4Ti3 precipitate but also the B19’ martensite produced from stress-induced martensitic transformation may prevent dislocation glide in austenitic NiTi alloys. In order to shed further light on this issue, four atomic configurations were constructed using an edge dislocation (a [100]) and two Ni4Ti3 precipitate variants (precipitate-V1 and precipitate-V2) to investigate dislocation glide in austenite containing Ni4Ti3 precipitate under shear at temperatures of 500 K or 700 K. As a result, all configurations at 500 K undergo the shear-induced martensitic transformation, producing B19’ martensite that impedes dislocation glide. And at 700 K, dislocation was pinned by precipitate or distortion zones caused by the superposition of strain field of precipitate and dislocation, without martensitic transformation. Furthermore, it was proven that precipitate-V1 impedes dislocation glide more strongly than precipitate-V2 at both temperatures of 500K and 700 K. It was attributed to the different in spatial relative location between the precipitate and dislocation and explained by the Orowan dislocation-precipitate bypass stress. Finally, three strategies for achieving high shear fatigue resistance in NiTi shape memory alloys were proposed based on these findings.

Preparation of a Zn/hydroxyapatite/chitosan composite coating with an antibacterial ability and cytocompatibility on a NiTi alloy

Zn has been extensively used to prevent microbial infections in orthopedic implants; however, high amounts of Zn inhibit cell proliferation. This study investigated the ability to decrease the cytotoxicity of Zn-doped coatings on NiTi shape memory alloys (NiTi SMAs) while maintaining their antibacterial activity. The Zn3(PO4)2 distributed in the chitosan coating, annotated as ZHAP, was obtained by electrophoretic deposition in a chitosan solution with Zn acetate acid and hydroxyapatite particles. The ZHAP inhibited bacterial growth and exhibited a good antibacterial activity (86 %) against Staphylococcus aureus. In addition, the MG63-cell adhesion and cytocompatibility with ZHAP were better than those with the Zn-doped coating due to the precipitation of Zn3(PO4)2 which decreased the release of Zn2+ ions from the chitosan matrix. The coatings did not affect neither the superelasticity (< 0.5 % residual strain) nor the shape memory ability (100 % recovery rate) of the NiTi SMAs.

Structure and Properties of TiNi Shape Memory Alloy after Quasi-Continuous Equal-Channel Angular Pressing in Various Aged States

The effect of quasi-continuous (QC) equal-channel angular pressing (ECAP) in various pre-aged states on the structure formation and mechanical and functional properties of a hyper-equiatomic titanium nickelide (TiNi) shape memory alloy is studied. QC ECAP with a channel intersection angle of 110° is carried out at a temperature of 450 °C after aging for 1 and 5 h for three passes. To investigate the obtained structure and properties, the following research methods are applied: transmission electron microscopy, XRD analysis, calorimetric study, tension and hardness tests, and a special technique for the determination of functional properties. QC ECAP allows for the considerable refinement of structural elements and results in obtaining a mixed fine-grade structure, with structural elements of average sizes of 92 nm after pre-aging for 1 h and 115 nm after pre-aging for 5 h. Pre-aging for 5 h before QC ECAP, in combination with QC ECAP and post-deformation aging at 430 °C for 1 h, provides the best combination of mechanical and functional properties: a dislocation yield stress of 1410 MPa, ultimate tensile strength of 1562 MPa, and total recoverable strain of 11.6%. These values are comparable with the best results obtained for titanium nickelide and expand opportunities for the application of smart shape memory devices.

Effects of Point Defects on the Monoclinic Angle of the B19″ Phase in NiTi-Based Shape Memory Alloys

This study investigated the effects of a variety of point defects on the monoclinic angle of the B19″ martensite in NiTi, including Ni and Ti vacancies, Ni–Ti anti-sites in the lattice, third element substitution, and Ni and Ti enrichments in the B2 stoichiometry. Their effects were studied by density functional theory calculations on the monoclinic B19″ phase as a proxy for the experimentally observed B19′ phase. It was found that vacancies and Ni–Ti anti-sites reduce the monoclinic angle of B19″, whereas third element substitution of Ni and Ti may affect the monoclinic angle in both directions depending on the third element and the host element it replaces. Enriching Ni or Ti content to deviate from the B2 stoichiometry decreases the monoclinic angle. A change in the monoclinic angle of the B19″ phase implies variation in the characteristic strains associated with the transformation. Thus, these findings may provide a new angle on possible interpretations of the variations of recoverable transformation strains experimentally observed of NiTi shape memory alloys.

Characterization of NiTiNb10.5 shape memory alloy: microstructural aspects and high-temperature oxidation behavior

The influence of Niobium addition on the high-temperature oxidation behavior of NiTi shape-memory alloys was investigated. Mass-gain kinetics were determined for up to 100 h oxidation at 800, 900 and 1000 °C. Niobium addition drastically reduced the oxidation rates. The surface and cross-section of the resulting oxide layers were characterized by XRD and SEM/EDS. The presence of Nb changed the morphology, phase assemblage, and composition of the scale product, which directly influenced the observed results. The postulated reasons for the improved oxidation resistance were the Nb-doping of TiO2 and the formation of an Nb-rich layer at the scale/alloy interface.

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).

A novel strategy for fabricating phase transforming NiTi shape memory alloy via multiple processes of severe plastic deformation

NiTi shape memory alloy was successfully fabricated from Ni/Ti multilayers via an innovatively combinative route consisting of accumulative roll bonding (ARB), high-pressure torsion (HPT) and annealing. Due to the severe plastic deformation induced by ARB and HPT, the subsequent post annealing time was significantly reduced. Differential scanning calorimetry (DSC) analysis confirms the occurrence of austenitic and martensitic transformations during heating and cooling process, implying that the shape memory effect was obtained.

Effect of robotic weaving motion on mechanical and microstructural characteristics of wire arc additively manufactured NiTi shape memory alloy

Abstract The effect of robotic weaving motion on the crystallographic orientation, texture and mechanical properties of thin-walled nickel titanium shape memory alloy fabricated via wire arc additive manufacturing is studied. Crystallographic orientation analysis reveals that the weaving motion alters the epitaxial growth of grains, which reduces the fraction of (001) oriented grains but increased the fraction of (110) and (111) oriented grains. Moreover, the weaving motion increases the fraction of equiaxed grains and homogeneously distributes Ni4Ti3 phases. These changes improve the hardness and strength of the build and also introduce isotropy in mechanical properties. Results of the study reveal that arc weaving strategy has a great potential in achieving near isotropic tensile characteristics and is beneficial in terms of tailoring texture in building nickel titanium thin-walled smart material structures.

Nanoindentation behaviour simulation of a polycrystalline NiTi shape memory alloy using the crystal plastic finite element method

Polycrystalline NiTi shape memory alloys (SMAs) are inevitably involved in plastic deformation under the action of an external force, thus significantly affecting the mechanical behaviour of materials. To deeply understand the plastic deformation mechanism of the NiTi SMA at 400 ℃, the representative volume element (RVE) model combined with the crystal plastic finite element method and the improved Voronoi algorithm were used to construct a three-dimensional polycrystalline spherical nanoindentation model for studying the contribution of different slip systems to the plastic deformation. The calculation results show that the constructed polycrystalline model can well match the experimental curve of the macroscopic stressstrain response curve of the polycrystalline NiTi SMA; the nanoindentation curve also confirms this result. Based on the constructed 3D polycrystalline nanoindentation model, three slip systems ({110}<100>, {010}<100>and {110}<111>) were introduced at the same time. According to the average cumulative shear strain calculated during the indentation process, the results show that the {110}<111> slip system provides the largest contribution to plastic deformation, and the {010}<100>slip mode provides the smallest contribution. The elastic modulus of the model was calculated using the Oliver-Pharr method, and the calculated results are consistent with the experimental results, which proves the validity of the model.

Direct-Current-Assisted Healing for Functional Degradation of Nanocrystalline Superelastic NiTi Shape Memory Alloys

Direct-Current-Assisted Healing for Functional Degradation of Nanocrystalline Superelastic NiTi Shape Memory Alloys

Microstructure and properties of NiTi shape memory alloy fabricated by double-wire plasma arc additive manufacturing with a nearly equal atomic ratio

In this study, a Ni49Ti51 shape memory alloy was innovatively prepared using the double-wire arc additive manufacturing process with plasma arc as the heat source and pure Ni and Ti welding wires as the raw materials. The microstructure and properties were systematically studied. The results indicate that the microstructure of the prepared Ni49Ti51 shape memory alloy mainly consists of NiTi (B19′), NiTi (B2), and NiTi2 phases. The ultimate compressive strength and fracture strain are 2241.3 MPa and 31.7 %, respectively. These values have increased by 39.7 % and 252.2 %, respectively, compared to the plasma arc powder melting process. Moreover, the shape memory effect is superior, exhibiting a shape memory recovery rate of up to 88 %.

Analysis of Shape-Memory-Alloy Fiber Interface Strength for Optimization of Self-Healing Composites

Self-healing materials, possessing an innate ability to mend damage and restore structural strength, have tremendous potential to improve safety and reliability, especially in space applications where recovery or manual performance of repairs may be prohibitive, dangerous, or impossible. Self-healing metallic materials developed with Nickel Titanium (NiTi) Shape Memory Alloy (SMA) reinforcement showed a promising prospect for recovering from larger scale damage. However, NiTi is known to form an inert oxide (TiO2) surface layer that is extremely hard to attach to. In this research, to promote better strength between the fiber and matrix, TiO2 surface layer has been removed by etching the NiTi wires in an inert environment and a pull test has been performed to evaluate the resulting change in adhesion strength between the fiber and metallic matrix. The experimental investigation of these interface fracture scenarios will enable the efficient design of composite materials.

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.

Unleashing multi-scale mechanical enhancement in NiTi shape memory alloy via annular intra-laser deposition with homogenized Ti2Ni nanoprecipitates

Additive manufacturing (AM) of composition-sensitive NiTi shape memory alloys (SMAs) is hindered by uncontrollable non-equilibrium microstructures, leading to functional performance degradation and limitations in high-throughput net-shaping applications. To address these challenges, we propose a novel approach combining annular intra-laser directed energy deposition (AIL-DED) with a tailored gradient post-treatment process. This enables the fabrication of nanocomposite microstructures in NiTi SMAs with precise control over the homogenization of Ti2Ni nanoprecipitates. In-situ monitoring and characterization of thermal history, microstrain, and constitutional phases, were utilized to uncover the relationship between multi-scale structural features and mechanical responses. The interplay between laser reflectivity, energy input, and lattice thermal vibrations affects forming quality in AIL-DED. Dynamic temperature variations were studied before reaching thermal equilibrium, revealing insights into thermal and mechanical interactions during the process. These findings contribute to understanding challenges in fabricating high-quality NiTi SMAs. Examining phase transformation behavior and its correlation with processing parameters deepens understanding of microstructural evolution and provides insights for tailoring material properties. The stress relaxation and dislocation evacuation at elevated temperatures caused the expansion of lattice and interplanar spacing in the age-treated SMA's B2 matrix phase, exhibiting a stable two-step phase transformation behavior (B19′↔R↔B2). AIL-DED combined with two-stage heat treatment strategy demonstrated enhanced homogeneity of non-equilibrium microstructures, resulting in the formation of high-density, well-dispersed Ti2Ni nanoprecipitates, with a semi-coherent interface to B2 matrix. The combination of load-bearing strengthening through well-dispersed Ti2Ni nanoprecipitates and Orowan strengthening at the semi-coherent interface yielded simultaneous enhancements in nanohardness, ultimate compressive strength, compressive fracture strain, and deformation recovery, following the AIL-DED combined with two-stage heat treatment process. The fracture mechanism transitioned from a mixture of brittle-ductile behavior to predominantly ductile characteristics. This study provides valuable insights that advance additive manufacturing and enhance the functional performance of SMAs in critical applications.