Intermetallic NiTi Research Articles

Effect of post-deposition heat treatment on microstructure and mechanical properties of NASA HR-1 cold spray coatings

The primary goal of this work was to examine the impact of heat treatment on the evolution of microstructure and mechanical properties of NASA HR-1 cold spray coatings. Coatings were produced employing a high-pressure cold spray system using N2 and He process gases and the effect of process gas and deposition temperature on the microstructure and mechanical properties of the coatings was examined. Microstructural characterization was performed using optical microscopy, scanning electron microscopy, micro X-ray computed tomography, and electron backscatter diffraction. NASA HR-1 coatings produced with He process gas exhibited improved plastic deformation and resulted in the lowest porosity compared to those deposited with N2 process gas. All coatings showed brittle failure with limited ductility in the as-deposited condition. Heat treatment of the NASA HR-1 cold spray coatings was performed at 550 °C and 950 °C for 1 h to improve the mechanical strength of the coatings. Heat treatment improved the particle bonding and enhanced the mechanical properties of the cold spray coatings. Heat treatment performed at 550 °C resulted in higher mechanical strength of coatings, whereas the heat treatment performed at 950 °C resulted in recrystallized microstructure and improved ductility of the coatings. However, heat treatment at 950 °C resulted in forming the Eta (η) phase and Ni-Ti intermetallics in all the NASA HR-1 cold spray coatings. The overall results suggest that using He as process gas contributed to reduced porosity and enhanced mechanical properties of the cold spray coatings.

Mechanical and electrochemical properties of porous NiTi intermetallics synthesized via rapid thermal explosion

In this paper, porous NiTi intermetallic was synthesized via a modified thermal explosion (TE) method, utilizing Ni (74 μm, 38 μm, 5 μm, 100–200 nm) and Al element powders as raw material. The exothermic behavior and phase transition of porous NiTi intermetallic compounds were studied. The results showed that with the refinement of Ni particle size, the heat release decreased, and the Ti-74 μm sample was the product of two eutectic reactions. Their mechanical properties were verified by the Vickers hardness tester and universal testing machine, and the corrosion behavior of porous NiTi intermetallic compounds in phosphate-buffered saline (PBS) solution was studied by electrochemical tests. Among them, Ti-5 μm exhibits the most homogeneous microstructure and favorable mechanical properties, which has a porosity of 31.7 % and a compressive strength of 321.71 MPa. However, corrosion resistance is sacrificed due to the high open porosity. This study holds great promise for precise control microstructural characteristics and optimization of mechanical and electrochemical properties of porous NiTi intermetallics.

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. 

In-Situ Alloying of CoCrFeNiX High Entropy Alloys by Selective Laser Melting

High Entropy Alloys are a class of alloys which have been shown to largely exhibit stable microstructures, as well as frequently good mechanical properties, particularly when manufactured by additive manufacturing. Due to the large number of potential compositions that their multi-component nature introduces, high throughput alloy development methods are desirable to speed up the investigation of novel alloys. Here, we explore once such method, in-situ alloying during Additive Manufacture, where a powder of a certain pre-alloyed composition is mixed with the required composition of powder of an additional element, such that alloying takes place when powders are melted during the process. To test the effectiveness and capability of the approach, selective laser melting has been used to manufacture pre-alloyed CoCrFeNi, and also CoCrFeNiCu and CoCrFeNiTi alloys by combining pre-alloyed CoCrFeNi powder with elemental powders of Cu and Ti. Processing parameter variations are used to find the highest relative density for each alloy, and samples were then characterised for microstructure and phase composition. The CoCrFeNi alloy shows a single phase face centred cubic (FCC) microstructure, as found with other processing methods. The CoCrFeNiCu alloy has a two phase FCC microstructure with clear partitioning of the Cu, while the CoCrFeNiTi alloy has an FCC matrix phase with NiTi intermetallics and a hexagonal close packed (HCP) phase, as well as unmelted Ti particles. The microstructures therefore differ from those observed in the same alloys manufactured by other methods, mainly due to the presence of areas with higher concentrations than usually encountered of Cu and Ti respectively. Successful in-situ alloying in this process seems to be improved by the added elemental powder having a lower melting point than the base alloy, as well as a low inherent tendency to segregate. While not producing directly comparable microstructures however, the approach does seem to offer advantages for the rapid screening of alloys for AM processability, identifying, for example, extensive solid-state cracking in the CoCrFeNiTi alloy.

Additive manufacturing of Ti-Ni bimetallic structures

Bimetallic structures of nickel (Ni) and commercially pure titanium (CP Ti) were manufactured in three different configurations via directed energy deposition (DED)-based metal additive manufacturing (AM). To understand whether the bulk properties of these three composites are dominated by phase formation at the interface, their directional dependence on mechanical properties was tested. X-ray diffraction (XRD) pattern confirmed the intermetallic NiTi phase formation at the interface. Microstructural gradient observed at the heat-affected zone (HAZ) areas. The longitudinal samples showed about 12% elongation, while the same was 36% for the transverse samples. During compressive deformation, strain hardening from dislocation accumulation was observed in the CP Ti and transverse samples, but longitudinal samples demonstrated failures similar to a brittle fracture at the interface. Transverse samples also showed shear band formation indicative of ductile failures. Our results demonstrate that AM can design innovative bimetallic structures with unique directional mechanical properties.

Enhancing the phase stability of TiNi intermetallic compound via nanocrystallization in an irradiated multicomponent vanadium alloy

Intermetallic compounds often have a low radiation tolerance due to the low recombination rate of radiation-induced defects. In the present work, we designed a novel multicomponent vanadium-based alloy (MVA), V34Ti25Cr10Ni30Pd1, containing a micron-scale TiNi matrix phase with VCr nanoprecipitates (NPs) and a micron-scale VCr matrix phase with TiNi NPs. The MVA was irradiated with 6 MeV Ti3+ ions with a radiation dose of 5×1015ions/cm2 at room temperature. Results indicated that the micron-scale intermetallic TiNi matrix along with VCr NPs inside both became amorphous, while the micron-scale VCr matrix including numerous intermetallic TiNi NPs both exhibited a high structural stability after ion irradiation. The intermetallic TiNi matrix becomes amorphous due to the accumulation of radiation-induced defects, and the intermetallic TiNi NPs in VCr matrix have a high stability of crystallographic structure due to high-density interfaces between NPs and matrix. These results indicate that the phase stability of TiNi intermetallic compound is increased after nanocrystallization. Besides, the discrepancy of two matrix phases including precipitates after ion irradiation, and the underlying mechanisms are discussed in detail in this work. This work gives a guidance for designing new vanadium alloys and intermetallic compounds with enhanced structural stability under ion irradiation.

Microstructure and mechanical property improvement of dissimilar metal joints for TC4 Ti alloy to Nitinol NiTi alloy by laser welding

Abstract To avoid the formation of Ti-Ni intermetallics in a joint, three laser welding processes for Ti alloy–NiTi alloy joints were introduced. Sample A was formed while a laser acted at the Ti alloy–NiTi alloy interface, and the joint fractured along the weld centre line immediately after welding without filler metal. Sample B was formed while the laser acted on a Cu interlayer. The average tensile strength of sample B was 216 MPa. Sample C was formed while the laser acted 1.2 mm on the Ti alloy side. The one-pass welding process involved the creation of a joint with one fusion weld and one diffusion weld separated by the remaining unmelted Ti alloy. The mechanical performance of sample C was determined by the diffusion weld formed at the Ti alloy–NiTi alloy interface with a tensile strength of 256 MPa.

Phase constitution, microstructure and properties of pulsed laser-clad ternary CrNiTi medium-entropy alloy coating on pure titanium

A ternary CrNiTi medium-entropy alloy (MEA) coating with excellent surface performance (hardness and wear resistance) was successfully prepared on pure Ti sheet by pulsed laser cladding. Microstructural characteristics of the MEA coating were probed by combined use of multiple characterization techniques and reasons for the formation mechanisms of various phases in the coating were well explored. Results show that fine cellular grains are formed in the MEA coating during the ultrafast non-equilibrium solidification process induced by pulsed laser cladding. These grains have an average size less than 1 μm and correspond to a BCC solid-solution phase. There appears irregular-shaped Cr2Ti Laves phase (C14-type) inside most of the cellular grains, while intergranular structures are demonstrated to be NiTi intermetallics. Hardness tests reveal that the CrNiTi MEA coating has a hardness of 940 ± 35 HV which is ~8 times that of the pure Ti substrate (119 ± 9 HV). Also compared to the pure Ti substrate, a much lower wear rate is noted for the coating demonstrating greatly improved wear resistance. Comprehensive analyses show that the excellent surface performance of the CrNiTi MEA coating can be ascribed to combined contributions from the solid-solution hardening and grain refinement hardening of the BCC phase, as well as second phase hardening produced by Cr2Ti Laves phase and NiTi intermetallics.

Microstructure development and nanoindentation behaviour of annealed Ni-P-Ti coatings

ABSTRACT Electroless Ni-P-NiTi composite coatings were prepared on API X100 steel substrates by co-depositing Ti particles and Ni-P then followed by annealing Ni-P-Ti coatings. The formation of austenite NiTi phase was validated by XRD, line scan and point EDS. The Gibbs free energies for the formation of various intermetallic NiTi phases during annealing were calculated. The nanoindentation behaviour of annealed Ni-P-Ti coatings was investigated. Mechanical properties such as hardness and Young’s modulus of Ni-P matrix, Ni3Ti, and NiTi are acquired by nanoindentation tests. Elastic recovery ratio was calculated from the load-depth curves of nanoindentation tests. Nanoindentation results revealed that the NiTi phase has much higher recovery ratio compared to Ni-P matrix and Ni3Ti. The mechanism for the high recovery ratio was discussed.

Shell structure of the porous TiNi-framework obtained by the SHS method

Porous SHS-TiNi alloys, presented as a porous framework with a complex structure of the Core-Shell type, were studied by microscopic and structural methods. The core is the porous framework of an intermetallic TiNi phase. The shell was found to be a complex three-layer complex. An oxygen-hardened peritectic TiNi–Ti2Ni layer 1-6 μm thick forms an inner shell layer. Thermo and corrosion-resistant amorphous-nanocrystalline intermetallic oxycarbonitrides, saturated with O, N, C interstitial impurities, form middle and outer layers. The middle dense double layer 60-80 nm thick protects the TiNi matrix. The outer layer 1-15 μm thick has a loose layered structure, it is enriched in the MAX-phases Ti3SiC2, Ti3AlC2. The dense nano double layer provides a chemical passivity to the porous nickelide titanium alloy with shape memory, does not limit mobility and improves the ability to integrate into biological tissues.

Study on the microstructure optimization and mechanical properties of dissimilar TC4-304L arc-brazing joints

In this paper, the microstructure optimization and mechanical property of joints are studied when the Cu, CuSi3, and CuNi10 wires are used to Cold Metal Transfer arc-braze TC4 to 304L. Based on the microstructure analysis and thermodynamic calculation, the existing state of Ti and Fe atoms are transferred from high brittleness Ti-Fe to less brittle Ti-Fe-Si intermetallics when the welding wire is changed from Cu to CuSi3 due to the lower Gibbs free energy of Ti-Fe-Si intermetallics. Furthermore, the (Fe, Ni) solution and Ti-Ni intermetallics with the lowest hardness are formed when CuNi10 wire is used. Thus, three Ti-Fe suppression mechanisms are concluded: suppressed by forming intermetallics and diffusion inhibition, suppressed by forming intermetallics, and suppressed by forming intermetallics and solution. Based on the microstructure optimization, the max hardness of intermetallics layer decreases from 769 to 654 Hv and the max tensile strength increases from 186.4 to 319.4 MPa. However, all cracks of the joints with different wires will initiate from the bottom of the seam and propagate in the TC4/seam transition zone due to the high Schmid factor. Besides, the crack propagation rate of more than 200 m/s can be in-situ measured by high-speed camera with 200000 fps.

Synthesis of co-sputter deposited Ni–Ti thin alloy films and their compositional characterization using depth sensitive techniques

Ni–Ti (Nickel–Titanium) thin alloy films of different concentration and thickness were synthesized using magnetron co-sputtering process utilizing separate Ni and Ti targets. Concentrations of Ni and Ti in the deposited films were optimized by controlling the power ratio of individual Ni and Ti targets. Thickness and density of the layered structures present in the films were evaluated by X-ray Reflectivity (XRR). Atomic concentrations of Ni and Ti were determined by Energy Dispersive X-ray Spectroscopy and matched with the concentrations obtained through XRR measurements. Secondary Ion Mass Spectrometry and Rutherford Backscattering Spectrometry were utilized to investigate the elemental depth distributions of Ni, Ti and O content in the films. Depth distribution studies confirmed that oxygen as a surface contaminant was present in all the deposited films, although films having thickness in the range of 21.5 nm −22.7 nm, were found to contain oxygen within the films too. Thicker Ni–Ti alloy films, in the range of 51.3 nm −52.3 nm, did not reveal any oxygen contamination in the bulk of the films. X-ray Photoelectron Spectroscopy (XPS) was used to determine elemental and molecular content of the deposited films. XPS analysis confirmed the presence of NiO at the surface and intermetallic Ni–Ti within the deposited films.

High Speed In Situ Synchrotron Observation of Cyclic Deformation and Phase Transformation of Superelastic Nitinol at Ultrasonic Frequency

The near equi-atomic intermetallic Ni Ti alloy Nitinol is used for medical implants, notably in self-expanding stent grafts and heart valve frames, which are subjected to several hundred million load cycles in service. Increasing the testing frequency to the ultrasonic range would drastically shorten the testing times and make the very-high cycle regime experimentally accessible. Such tests are, however, only meaningful if the material response at ultrasonic frequency is identical to that observed in conventional fatigue tests. A novel fatigue testing setup where superelastic Nitinol dog bone specimens are loaded at ultrasonic cycling frequency is presented. Loading conditions resemble in vivo loading (i.e., repeated cyclic loading with relatively small strain amplitudes, specimens in a pre-strained multi-phase state). Strains and phase transformations during ultrasonic frequency cycling are quantitatively measured in an X-ray diffraction (XRD) synchrotron experiment and compared to the material response at low frequency. The XRD experiment confirms that forward and reverse stress-induced phase transformation from austenite to martensite via the intermediate R-phase occurs during low frequency (0.1 Hz, strain rate dot{varepsilon}approx 10−3 s−1) and ultrasonic frequency (20 kHz, dot{varepsilon}approx 102 s−1) cycling. Since the same deformation mechanisms are active at low and ultrasonic frequency, these findings imply a general applicability of the ultrasonic fatigue testing technique to Nitinol.

Diffusivity of Ti-Ni Diffusion Couple Enhanced by Pulse Current During Spark Plasma Sintering

The effect of pulse current (PC) on the interdiffusion of Ti-Ni system was investigated under spark plasma sintering. The growth rate of Ni-Ti intermetallics under PC was 3.22 to 6.44 times higher than that with direct current (DC). The reaction activation energy of intermetallics at PC was 18.4 to 79.0 kJ/mol, which is lower than the activation energy of 275.1 to 313.9 kJ/mol at DC; thus, the increased growth rate at PC may have resulted from the decrease of activation energy.

Obtaining Functional Intermetallic Ni–Ti Coatings by Heterophase Transfer Combined with Laser Treatment

The results of the investigation of the intermetallic Ni–Ti system coating obtained by the complex method of cold gas dynamic spraying and laser treatment are presented. The dependence of the coating charge composition on the initial powder composition is shown. Its structure, porosity, and microhardness are studied, and also the results of X-ray phase analysis confirming the presence of the intermetallic phase are presented. The microhardness is increased by more than a factor of 5 to 632 HV, and the porosity is decreased by a factor of 25 to 0.2%. Investigation of the substrate–coating boundary shows the presence of mutual diffusion.

On the versatility and distinctiveness in the use of microwave energy for the ignition of low exothermic Ni–Ti intermetallics combustion synthesis

Microwave energy has been employed in the present work in order to reach ignition of the combustion synthesis in the binary Ni–Ti system without the need of any separate additional pre-heating step. Indeed, the latter is known to be necessary with more conventional ignition strategies, due to the low exothermic character of the reaction between elemental Ni and Ti powders. On the opposite, the unique microwaves heating mechanism allows directly transferring electromagnetic energy in the reactive specimen that absorbs and transforms it into the heat necessary to reach ignition of the exothermic reaction. According to the electromagnetic field distribution inside the microwave single mode applicator employed, it was possible to realize both Thermal Explosion (TE) and Self-propagating High-temperature Synthesis (SHS) ways of combustion.Moreover, the use of a single mode applicator also allowed separately investigating the effect of electric and magnetic fields strengths, at the 2450 MHz microwave frequency, on the combustion characteristics. This significantly affected the microstructure as well as the phase evolution in the synthesized products.

Zn-Sn Interdigitated Eutectic Alloy Anodes with High Volumetric Capacity for Lithium-Ion Batteries

Summary Zn-Sn interdigitated eutectic alloy (IdEA) foils are highly promising anodes for lithium-ion batteries. Synthesized via severe plastic deformation, a two-phase alloy is converted into a nanostructured foil composed of tin domains distributed throughout an electrically conductive zinc matrix. In addition to providing mechanical integrity, uncycled capacity in the zinc matrix eliminates the lithium-plating risk. These monolithic foil anodes offer an electrode-level volumetric capacity approximately double that of graphite (797 mA h cm−3), with a highly promising formation loss of

Obtaining the functional intermetallic Ni–Ti coatings by heterophase transfer combined with laser treatment

The results of the investigation of the intermetallic Ni–Ti system coating obtained by complex method of cold gasodynamic spraying and laser treatment are presented. The dependence of the coating charge composition on the initial powder composition is shown. Its structure, porosity, and microhardness are studied, and also the results of X-ray phase analysis confirming the presence of intermetallic phase are presented. The microhardness increased more than 5 times to 632 HV, and porosity decreased 25 times up to 0.2%. Investigation of the substrate-coating boundary showed the presence of mutual diffusion.

Properties of Ni-Ti-(Fe,Co,Al) Shape Memory Alloys Prepared by Self-Propagating High-Temperature Synthesis

Shape memory, superelasticity and pseudoplasticity are the exceptional properties of Ni-Ti alloys, which are listed among SMAs (Shape Memory Alloys). The carrier of these properties is the intermetallic NiTi phase. In addition to this phase, the Ti2Ni phase is always present, which is hard and brittle, and therefore it is undesirable in the Ni-Ti alloys. The aim of this work was to determine the influence of selected alloying elements (iron, cobalt and aluminium) in two different amounts (1 wt. % and 3 wt. %) on the microstructure, phase composition and amount of the Ti2Ni phase, hardness and transformation temperature of Ni-Ti-X alloy. The samples examined were prepared by self-propagating high-temperature synthesis at sintering temperature of 1100 °C. By altering the selected elements, the Ti2Ni phase was not reduced, on the contrary aluminium increased its amount. The Ni-Ti alloys alloyed with these three alloying elements proved to be harder. The phase transformations were observed only in the sample with 1 wt. % of aluminium at heating and cooling curves from differential scanning calorimetry.

Post-spray modification of cold-sprayed Ni-Ti coatings by high-temperature vacuum annealing and friction stir processing

In this study, two post-spray treatments, i.e. high-temperature vacuum annealing (HTVA) and friction stir processing (FSP), were employed to modify the cold-sprayed (CS) Ni-Ti coatings, obtained from a mechanically blended powder. The phase transformation occurred at different HTVA temperatures, but with the formation of pores due to the Kirkendall effect. FSP proved to provide a new opportunity for the formation of intermetallic phases in the CSed Ni-Ti coating. A comparison of the microstructures, microhardness and tribological behaviors between the CSed and FSPed coatings, reveals that the microstructure of the modified Ni-Ti coating exhibits a mechanically alloyed layer with the in-situ synthesized Ni-Ti intermetallics, and no obvious defects are found. The microhardness of the improved layer is increased to 1003.5 ± 65.9 HV0.1, which is about 4.5 times compared to the CSed coating (222.5 ± 6.6 HV0.1). In addition, the mechanically alloyed layer by FSP contains multiplicate Ni-Ti intermetallic compounds, exhibiting better wear resistance than that of the CSed coating, thus proving the effectiveness of FSP on the microstructural modification and properties improvement for the CSed metal-metal composite coatings.