B2 NiTi Research Articles

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.

Interdiffusion in BCC_B2 Ni–Ti–V alloys at 1223K–1323K

Establishing a Ni–Ti–V system diffusion kinetics database contributes to a better understanding of the precipitation processes in shape memory alloys. In this paper, eleven sets of BCC_B2 Ni–Ti–V single-phase diffusion couples were prepared and annealed at temperatures of 1223 K, 1273 K, and 1323 K. The composition-distance curves were determined using the electron probe microstructure analysis technique. The interdiffusion coefficients were extracted using the HitDIC software, which is based on the numerical inverse method. The software demonstrated high accuracy in predicting the composition curves of the prepared diffusion couples. The interdiffusion coefficients inferred from the HitDIC software and the ones calculated by the Matano-Kirkaldy method achieved good agreement. This demonstrates the reliability and rationality of the evaluated interdiffusion coefficients. This study also investigated the composition and temperature dependencies of interdiffusion coefficients.

Effects of WC/TiC addition on the thermodynamic behavior of TiB2-based cermet during sintering process

The heat absorption, weight change and exhaust behavior of TiB2-based cermet powder mixtures with additions of WC, TiC and WC-TiC were analyzed by a simultaneous thermal analyzer coupled with a mass spectrometer (TG/DSC-QMS). Thermodilatometry was used to investigate the densification behavior due to sintering process of cermets. During the sintering process, the order of degassing is: water vapor, CO2 released by the reduction of adsorbed oxygen in the raw material powder, CO and CO2 released by the reduction of surface oxides of Co and Ni powder, WC powder, TiB2 and TiC powder. The addition of WC, TiC and WC-TiC accelerated the reduction of oxides on the surface of Co and Ni powders, and decreased the temperature point of liquid phase formation in TiB2-based cermet. WC and TiC dissolved in liquid CoNi binder and then precipitated on the undissolved TiB2 particles to form a typical core-rim structure during the sintering. This study is expected to lay a solid foundation for the development of high-performance TiB2-based cermets based on TiB2-CoNi, TiB2-WC-CoNi, TiB2-TiC-CoNi and TiB2-WC-TiC-CoNi cermets in the future.

Effect of Sintering Temperature on Microstructure Characteristics of Porous NiTi Alloy Fabricated via Elemental Powder Sintering.

To investigate the effect of the sintering temperature on the microstructure characteristics of porous NiTi alloys, two types of porous NiTi alloys with equal atomic ratios were fabricated via elemental powder sintering at 950 °C and 1000 °C. Afterwards, optical microscopy (OM), scanning electron microscopy (SEM) and transmission electron microscopy (TEM) were collectively applied to investigate the pore characteristics and microstructure of the fabricated porous NiTi alloy. The results show that when the sintering temperature increases from 950 °C to 1000 °C, the average pore size increases from 36.00 μm to 181.65 μm, owing to the integration of these newly formed small pores into these pre-existing large-sized pores. The measured density increases from 2.556 g/cm3 to 3.030 g/cm3, while the porosity decreases from 60.4% to 51.8%. This is due to the occurrence of shrinkage after the sufficient diffusion of atoms. Furthermore, the characterization results confirm that a change in the sintering temperature would not change the phase types within a porous NiTi alloy; namely, the matrix consists primarily of B2 NiTi, with a significant amount of Ni4Ti3 precipitates and a small amount of Ni3Ti precipitates and Ti2Ni precipitates. However, as the sintering temperature increases, the number of Ni4Ti3 precipitates decreases significantly. The formation of a Ni4Ti3 phase in the present study is closely related to the enrichment of Ni content in the matrix owing to the diffusion rate difference between Ni atoms and Ti atoms and the absence of a transient liquid phase (TLP) during the sintering process owing to the relatively low sintering temperature (lower than the eutectic temperature). Moreover, the increasing sintering temperature speeds up the atom diffusion, which contributes to a reduction in the enrichment of Ni as well as the number of formed Ni4Ti3 precipitates.

Corrosion and wear interplay: Tribo-electrochemical evaluation of NiTiNOL60 alloy in sulfuric acid

The combined corrosion and wear behaviour of NiTiNOL60 alloy is important in various engineering applications. While previous studies have explored its performance in artificial seawater environments, limited information exists regarding its tribocorrosion characteristics in acidic conditions. This paper investigates the synergistic interaction between sliding wear and electrochemical processes for NiTiNOL60 alloy in a sulfuric acid environment, employing experimental procedures compliant with ASTM standards. Microscopic analysis confirms the alloy's Ni-rich composition, featuring a dense network of B2 NiTi + Ni4Ti3 cubic and rhombohedral crystal matrix structures. Our research and results reveal that material degradation during tribocorrosion arises from the combined effects of sliding wear and electrochemical reactions. Consequently, multiple wear mechanisms influenced by tribochemical wear are observed, with delamination and micro-cracks being notably prominent under higher applied loads due to surface tensile stress and contact pressure, leading to crack propagation perpendicular to the sliding direction. These findings have practical implications for optimising the performance and durability of NiTiNOL60 alloy in acidic environments, offering valuable insights for load-bearing engineering applications.

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).

Microstructure Evolution and Shear Strength of TiAl‐Based Intermetallics Joint Vacuum Brazed with TiH2–Ni–TiB2 Powder Filler

Vacuum brazing of Ti–47Al–2Nb–2Cr–0.15B (at%) alloy with TiH2–Ni–TiB2 powder filler in situ assisted 20 vol% TiB whiskers is performed at 1190–1250 °C for 10 min. The microstructure evolution of the joint at various brazing temperatures and the relationship between microstructure and joint properties are systematically investigated. In the results, it is indicated that the interfacial microstructure of the γ‐TiAl joint brazed at 1230 °C is γ‐TiAl/Ti3Al + Al3NiTi2 (layer I)/Al3NiTi2 + Ti3Al (layer II)/Al3NiTi2 + TiB + Ti3Al (layer III)/Al3NiTi2 + Ti3Al (layer II)/Ti3Al + Al3NiTi2 (layer I)/γ‐TiAl. The layer II containing high‐hardness Al3NiTi2 is always the weakest part in the brazed joint. The microstructure refinement and load‐bearing behavior of layer III are increased with brazing temperature due to the increasing length of the TiB whisker. Variation of the content of low‐hardness Ti3Al in layer II as well as its microstructure refinement and internal bonding strength, the shear strength of γ‐TiAl joints is first increased and then decreased with brazing temperature, but that of Al3NiTi2 in layer II is reversed. The joint brazed at 1230 °C possesses the maximum shear strength of 332.87 MPa and a quasi‐cleavage fracture occurs in the layer II mainly consisted of Al3NiTi2.

Third element diffusion induced amorphization of NiTi in a NiTi-Nb nanocomposite

The paper reports a unique and rare phenomenon observed in a NiTi-Nb nanowire in-situ composite system. The phenomenon is the amorphization of the B2 NiTi matrix caused by the diffusion of Nb through the B2 lattice. The diffusion of Nb through the NiTi matrix is driven by the effect of Oswald ripening associated with the fragmentation and granulation of Nb nanowires at elevated temperatures and the amorphization of the NiTi matrix is attributed to the effect of Ni-Ti atomic swapping, i.e., the formation of anti-site defects in the B2 structure, caused by the substitutional diffusion of a third element, Nb in this case. The phenomenon is observed by transmission electron microscopy examination and x-ray diffraction measurement and the hypothesis of explanation is supported by molecular dynamic simulation studies.

Effects of pore diameter and B2–NiTi crystal on plasticity of amorphous Ni–Ti alloy based on molecular dynamics simulation

The poor plasticity of metallic glasses (MGs) limits its applicability as engineering materials. Therefore, it is particularly important to find ways to improve the plasticity of metallic glasses. In this study, the effects of pore diameter and B2–NiTi crystal on the compressive deformation of Ni50Ti50 MGs are studied by molecular dynamics simulation. The results demonstrate that pores effectively inhibit rapidly stress reduction. Pore experiences pressure in the direction of the load under compression process, which induce the formation of high shear strain atoms around the pores. The presence of pore induces the formation of shear transition zones (STZs) and shear bands. Specifically, larger pore diameters result in lower yield strengths, flatter stress-strain curves, and increase plasticity of MGs. As the B2–NiTi crystal is highly resilient in MGs, it could not be easily destroyed. In the compression process, the B2–NiTi crystal reduce the deformation of the system and impede the propagation of shear bands. As a result, the shear bands only propagate in the cracks of crystals. The larger number of crystals lead to a larger inelastic deformation interval in the nanocomposites and improve the ability to block the propagation of shear bands. These findings elucidate the relationship between yield strength, shear band, plasticity, pore diameter, and B2-phase crystals, and provide theoretical guidance for the development of MGs with improved plasticity.

Synthesis and characterization of novel NiTi–Ni3Ti/SiC nanocomposites prepared by mechanical alloying and microwave-assisted sintering process

Equiatomic Ni–Ti alloy reinforced with 0, 2.5, 5, and 10 vol % SiC nanoparticles was synthesized through mechanical alloying and microwave sintering process. In this method, a mixture of Ti and Ni powders and SiC nanoparticles (0–10 vol%) were mechanically milled for 30 h, compressed under 1 GPa pressure, and then sintered at 900–1100 °C by microwave heating. Microstructural analysis of the synthesized samples was performed using X-ray diffraction and scanning/transmission electron microscopy, and their hardness was determined using a Vickers microhardness tester. Structural studies revealed the formation of a complex phase structure consisting of the austenite (B2–NiTi), the martensite (B19′-NiTi), and the nickel-rich (Ni3Ti) and titanium-rich (NiTi2) phases. HR-TEM investigations confirmed an increased Ni3Ti phase in the austenitic NiTi matrix nearby the martensitic NiTi phase. It was found that adding SiC nanoparticles to Ni–Ti increased the amount of hard Ni3Ti phase, promoted the stability of the martensite phase at room temperature, and retarded the matrix grain growth during the sintering process. Microhardness tests showed that when the content of SiC nanoparticles increased up to 5 vol %, the microhardness increased considerably to 9 GPa and then declined. The high microhardness of synthesized nanocomposites is due to the formation of Ni3Ti and NiTi martensitic phases and their nanocrystalline structure.

Surface Structure and Properties of Hydroxyapatite Coatings on NiTi Substrates

Hydroxyapatite coatings were deposited for 1, 2, and 3 h on NiTi substrates using plasma-assisted radio frequency sputtering. The matrix consisted of NiTi B2 and NiTi B19’ phases and Ti2Ni, Ni3Ti, and Ni4Ti3 intermetallic compounds. The surface coating was monoclinic hydroxyapatite. Increasing the deposition time to 3 h made it possible to form a dense hydroxyapatite layer without visible defects. The phase contrast maps showed that the coating consisted of round grains of different fractions, with the smallest grains in the sample deposited for 3 h. The wettability tests showed that the coating deposited for 3 h had the highest surface energy, reflected in the proliferation density of the MCF-7 cell line.

Microstructure and phase evolution of functionally graded multi-materials of Ni–Ti alloy fabricated by laser powder bed fusion process

The layer-by-layer laser powder bed fusion (L-PBF) process was applied to fabricate functionally graded multi-materials (FGMs) of nickel-titanium alloy. The FGMs Ni–Ti alloy was built with composition transitioning from pure Ni incrementally graded to pure Ti by different composition gradients. The microstructure, chemical composition, and phase evolution were characterized using SEM, EDS, XRD, and EBSD along the build direction. By varying the proportions of Ni and Ti, the microstructure and phases were changed gradually across the build direction. Several phase transformations, γ-Ni → NiTi B2 + intermetallic phases → α-Ti, appeared through the compositional gradient. Cracks were found in the gradient zone, and the results were explained in terms of the various phases present. The potential to accomplish such a completely deposited FGMs Ni–Ti alloy and considerable changes in composition are made possible by a new strategy to make innovative FGMs Ni–Ti alloys together with 3D components using L-PBF additive manufacturing.

Ductility reduction upon cobalt substitution in B2 NiTi

This paper reports the effect of ternary addition of cobalt to B2 NiTi by varying concentrations × = 0.125, 0.25 and 0.375 in Ni0.5−xCoxTi0.5 and the impact on their crystal structure, electronic and mechanical properties using first-principles calculations. The third element Co was substituted on both Ni and Ti sub-lattices using the supercell approach, site preference energy and formation energy suggest the preference of Co substitution on Ni sub-lattice agreeing with literature. The evolution of the electronic and mechanical properties from undoped NiTi to TiCo by substitution of Ni by Co is hereby discussed. Young’s (E) and shear (G) modulus is found to gradually increase as concentration (x) is increased. Importantly, ductility decreases with increasing cobalt concentration. From DOS, it is observed that the delocalized d states are reduced upon increasing Co addition due to strong d-d hybridization which reduces the metallicity and in turn ductility. This increase in Young’s modulus and decrease in ductility upon cobalt addition is indicative of the overall stiffness and resistance to deformation.

Microstructure and Hydrogen Permeability of Nb-Ni-Ti-Zr-Co High Entropy Alloys.

Hydrogen separation membranes are one of the most promising technologies for hydrogen purification. The development of high-entropy alloys (HEAs) for hydrogen separation membranes is driven by a "cocktail effect" of elements with different hydrogen affinities to prevent hydride formation and retain high permeability due to the single-phase BCC structure. In this paper, equimolar and non-equimolar Nb-Ni-Ti-Zr-Co high entropy alloys were fabricated by arc melting. The microstructure and phase composition of the alloys were analyzed by scanning electron microscopy and X-ray diffraction, respectively. The hydrogen permeation experiments were performed at 300-500 °C and a hydrogen pressure of 4 bar. In order to estimate the effect of composition and lattice structure on hydrogen location and diffusivity in Nb-Ni-Ti-Zr-Co alloy, ab initio calculations of hydrogen binding energy were performed using virtual crystal approximation. It was found that Nb-enriched and near equimolar BCC phases were formed in Nb20Ni20Ti20Zr20Co20 HEA while Nb-enriched BCC and B2-Ni(Ti, Zr) were formed in Nb40Ni25Ti18Zr12Co5 alloy. Hydrogen permeability tests showed that Nb20Ni20Ti20Zr20Co20 HEA shows lower activation energy and higher permeability at lower temperatures as well as higher resistance to hydrogen embrittlement compared to Nb40Ni25Ti18Zr12Co5 alloy. The effect of composition, microstructure and hydrogen binding energies on permeability of the fabricated alloys was discussed.

Density functional theory study of dissociative adsorption of H2 molecules on NiTi (001) surfaces

Ti-based alloys have the potential to be used as hydrogen storage units, including NiTi. In contrast, NiTi alloy is sensitive to H atoms. It has been found that hydrogen can cause embrittlement in NiTi alloys. Thus, it is become indispensable to elucidate the reaction of H2 molecules on the NiTi surface. Using density functional theory, we investigated the dissociation mechanism of H2 molecules on the B2 NiTi (001) surface. We found that H atoms tend to come closer to Ni atoms on the Ti- and Ni-terminated (NiTi) (001) substrate. The calculation results showed that the adsorption energy of H atoms at the hollow site was higher than that at the top site. We identified two dissociation mechanisms of H2 molecules on Ti and Ni terminated on NiTi (001) substrates via the hollow sites of the adsorption route. The adsorption energy values obtained were extremely low, that is, 0.23 and 0.38 eV for the Ni and Ti terminated of NiTi (001) substrates, respectively. The dissociation reaction of H2 molecules, which is an exothermic reaction, can quickly occur on the NiTi (001) surface because of the low activation energy.

Heat treatment of hot-isostatic-pressed 60NiTi shape memory alloy: Microstructure, phase transformation and mechanical properties

60NiTi alloy is considered to be a promising material for specialized bearing and gear applications due to its high hardness, strength, and low modulus. However, fabricating 60NiTi through conventional processing methods is challenging due to the brittleness and poor workability. In this study, 60NiTi with high relative density was successfully fabricated directly from pre-alloyed powder through hot isostatic pressing. The effects of solution and aging treatments on microstructure and mechanical properties were systematically studied by advanced characterization techniques. The hot-isostatic-pressed 60NiTi showed low average hardness and elastic strain due to the formation of a soft Ni3Ti phase and B2 NiTi matrix. Solution treatment above 1000 °C dissolved the Ni3Ti phase and promoted the formation of nanoscale Ni4Ti3 precipitates, which significantly improved the hardness, strength, and elastic strain of 60NiTi. The formation of the Ni4Ti3 phase can be mainly attributed to the driving forces induced by the chemical supersaturation and mechanical stress concentration. Finally, the phase transformation mechanisms during heat treatment and compression test were discussed.

Tribological Behaviour of Al–Ni–TiB2 Composite Coating on AA1100 Al-alloy Prepared Using TIG Torch Welding Route

Tribological Behaviour of Al–Ni–TiB2 Composite Coating on AA1100 Al-alloy Prepared Using TIG Torch Welding Route

Structure and Properties of Metal-Matrix Composites Based on an Inconel 625–TiB2 System Fabricated by Additive Manufacturing

This research work studies the structural phase parameters and physicomechanical properties of metal-matrix composite materials based on a Ni–TiB2 system obtained by additive manufacturing (specifically, direct laser deposition). The properties of the composites obtained were investigated at high temperatures (up to 1000 °C). The feasibility of the fabrication of a composite nanostructure of alloy with advanced physicomechanical properties was shown. The introduction of reinforcing TiB2 particles into an Inconel 625 matrix was confirmed to increase the microhardness and tensile strength of the material obtained. Apparently, the composite structure of the samples facilitates the realisation of several strengthening mechanisms: (1) a grain boundary mechanism that causes strengthening and dislocation movement; (2) a mechanism based on the grain structure breakdown and Hall–Petch relationship realisation.

Superelasticity preservation in dissimilar joint of NiTi shape memory alloy to biomedical PtIr

Laser microwelding was used to join, for the first time, superelastic NiTi to biomedical PtIr which can be used in multicomponent biomedical devices. By process optimization, it was possible to control the formation of the B2 NiTiPt phase, with no intermetallic compounds being formed. The NiTiPt phase inside the fusion zone had a strong metallurgical bonding with the NiTi base material due to the smooth transition of its grain orientation towards 〈111〉 B2 NiTi. The major finding of the present work is the preservation of the NiTi superelastic response in the welded joint as evidenced by the load/unloading cycling up to 6% strain, significantly higher than typically required for biomedical applications.

Thermal, microstructural and elastic modulus behavior of Ti50Ni50−xNbx (x = 0–25%at) shape memory alloys obtained by plasma arc melting

Ternary alloys based on the NiTi system with the addition of Nb have attracted attention for several areas of engineering, due to improvements in the shape memory properties. In this work, six TiNiNb compositions were obtained by plasma arc melting under controlled atmosphere, with Nb contents of 5%, 10%, 15%, 20% and 25% Nb in the Ti50Ni50−xNbx system (% at). This work aims to evaluate the influence of Nb content on the microstructure, phase transformation and elastic modulus behavior of the ternary alloy compared to the equiatomic binary NiTi alloy. Differential scanning calorimetry (DSC) show that after a heat treatment of 900 °C for 1 h, all studied compositions present a single-step phase transformation during cooling and heating. Nb addition has a limited effect over phase transformation temperatures, with the onset of martensite transformation remaining practically constant. The microstructure of the alloys is constituted by B2 NiTi austenite matrix in the samples with 0% and 5% Nb, and by B19′ NiTi martensite in the samples with 10–25% Nb, all surrounded by an eutectic phase rich in Nb (β-Nb). Scanning electron microscopy (SEM) revealed microstructures with the presence of dendrites with a large amount of β-Nb that evolved in size with increasing Nb content. The Nb addition and consequent microstructure changes contributed to the decrease in the average hardness, going from 527 HV in NiTi to values between 259 and 396 HV in TiNiNb alloys. The Nb content, however, had a weaker effect over the highest modulus of elasticity, which was highest in the Ti50Ni30Nb20 alloy (66 GPa) and lowest in the Ti50Ni45Nb5 alloy (52 GPa), representing a less than 10% variation.