NiTi Matrix Research Articles

Effect of Cooling Rates on the Transformation Behavior and Mechanical Properties of a Ni-Rich NiTi Alloy

Slightly nickel-rich Ni–Ti alloys (typically 50.5–51% atomic percent nickel) are commonly used to produce devices that are superelastic at body temperature. This excess nickel can be tolerated in the NiTi matrix when its temperature is above the solvus of about 600 °C, but will precipitate out during lower temperatures. Recent work has been done on exploring the effect lower temperatures have on the material properties of NiTi. Findings showed that properties begin to change at temperatures as low as 100 °C. It is because of these results that it was deemed important to better understand what may be happening during the quenching process itself. Through running a combination of DSC and tensile tests on samples cooled at varying rates, it was found that the cooling rate has an effect on properties when heat treated above a specific temperature. Understanding how quickly the alloy must be cooled to fully retain the supersaturated NiTi matrix is important to optimizing processes and anticipating material properties after a heat treatment.

Oxidation Behavior of NiTi-Al Based Alloy with Nb and Mo Additions

Oxidation behavior of a Ni-45Ti-5Al-2Nb-1Mo (composition in at %) alloy was studied in comparison with the Ni-45Ti-5Al alloy by X-ray diffraction (XRD), scanning electron microscopy (SEM) and cyclic oxidation tests. The results show that the microstructure of Ni-45Ti-5Al-2Nb-1Mo alloy is composed of NiTi matrix, Ti2Ni and (Nb, Ti) solid solution phases. By 2 at% Nb and 1 at% Mo addition, the Ni-45Ti-5Al-2Nb-1Mo alloy shows excellent oxidation resistance. The mass gain of Ni-45Ti-5Al-2Nb-1Mo alloy after 100 h exposure at 800 °C is 2.29 mg/cm2, which is much lower than that of Ni-45Ti-5Al alloy. This is mainly resulted from the doping effect of Nb and Mo in TiO2 and the formation of a continuous Al2O3 layer by Nb and Mo addition. The oxide scale of the present alloy is with multi-layer structure in order of TiO2 layer/ Al2O3 + NiAl2O4 mixture layer/ TiO2 layer/ Nb-rich TiO2 layer/ Ni3Ti layer from outside.

Effect of SiC Particle Size on the Physical and Mechanical Properties of NiTi Alloys

NiTi shape memory alloys have attracted significant interest due to their unique superelasticity and high damping performance. In this work, the effect of SiC particle size on both physical and mechanical properties of NiTi matrix composite was investigated. Ni and Ti powders with particle sizes of 40 µm were used with the SiC addition with varying particle sizes of 20 µm and 40 µm, respectively. Composites of NiTi with 1wt. % SiC were fabricated by powder metallurgy technique. The effects of SiCp addition on hardness, relative density and wear behavior of NiTi composites have been investigated. The samples were examined by scanning electron microscope, for microstructural studies and phase development. The results showed that the distribution of the reinforced particle was uniform. Moreover, as the SiC particle size decreases, hardness and wear resistance increase. It was demonstrated that SiC particle size significantly enhanced the wear resistance of NiTi composite.

High Damping of Lightweight TiNi-Ti2Ni Shape Memory Composites for Wide Temperature Range Usage

A bimodal porous TiNi-Ti2Ni shape memory alloy composite (SMAC) with 59% porosity was fabricated by sintering Ti-46at.%Ni elemental powders with pore-forming agent. The porous TiNi-Ti2Ni SMAC contains two irregular pores of about 400 and 120 μm. We investigated the microstructure and pore morphology correlated with the mechanical properties and damping capacities of the SMAC. Ti2Ni intermetallic phases with size of 1-3 μm were homogeneously distributed in the TiNi matrix. The porous TiNi-Ti2Ni SMAC exhibits exceptionally high inverse mechanical quality factor (Q −1) of ~0.25 at < 40 °C, which is among the highest value reported for porous/dense shape memory alloys or composites to best of our knowledge, and it shows very high compressive fracture strain of about 25%. Moreover, the fabricated porous SMAC at relatively low strain amplitude can exhibit considerable high Q −1 of 0.06 ~ 0.11 for a wide range of temperature between − 90 and 200 °C, which is attributed to the stress concentration distribution provided by the bimodal structure of pores and the massive interfaces between pore/matrix and TiNi/Ti2Ni. These porous SMACs can be an ideal candidate for using as a lightweight damping material in the energy-saving applications.

Single walled carbon nanotubes reinforced intermetallic TiNi matrix nanocomposites by spark plasma sintering

We report the processing of single walled carbon nanotubes (SWCNTs) reinforced TiNi intermetallic matrix nanocomposites from Ti/Ni and SWCNTs powders using spark plasma sintering (SPS) at temperatures from 1000 °C to 1200 °C. The SWCNTs are doped into the TiNi matrix from 0.0 to 1.0 wt%. The effect of SWCNTs reinforcement contents on the relative density, phases, microstructure and microhardness of TiNi intermetallics matrix andCNTs/TiC/TiNi nanocomposites are studied. The experimental results show that the TiNi sintered at T= 1200 °C reinforced with 0.8 wt% SWCNTs has the highest Vicker’s microhardness and relative density, which were HV 5.29 GPa and 96%, respectively. That can be explained by the precipitation of TiC and Ti2Ni in the matrix.This study explores the possibility of developing novel TiNi matrix nanocomposites with shape memory effect and biocompatibility.

Elastocaloric cooling of additive manufactured shape memory alloys with large latent heat

The stress-induced martensitic phase transformation of shape memory alloys (SMAs) is the basis for elastocaloric cooling. Here we employ additive manufacturing to fabricate TiNi SMAs, and demonstrate compressive elastocaloric cooling in the TiNi rods with transformation latent heat as large as 20 J g−1. Adiabatic compression on as-fabricated TiNi displays cooling ΔT as high as −7.5 °C with recoverable superelastic strain up to 5%. Unlike conventional SMAs, additive manufactured TiNi SMAs exhibit linear superelasticity with narrow hysteresis in stress–strain curves under both adiabatic and isothermal conditions. Microstructurally, we find that there are Ti2Ni precipitates typically one micron in size with a large aspect ratio enclosing the TiNi matrix. A stress transfer mechanism between reversible phase transformation in the TiNi matrix and mechanical deformation in Ti2Ni precipitates is believed to be the origin of the unique superelasticity behavior.

Microstructure and martensitic transformation of Ni–Ti–Pr alloys

The effect of Pr addition on the microstructure and martensitic transformation behavior of Ni50Ti50−x Pr x (x = 0, 0.1, 0.3, 0.5, 0.7, 0.9) alloys were investigated experimentally. Results show that the microstructures of Ni–Ti–Pr alloys consist of the NiTi matrix and the NiPr precipitate with the Ti solute. The martensitic transformation start temperature decreases gradually with the increase in Pr fraction. The stress around NiPr precipitates is responsible for the decrease in martensitic transformation temperature with the increase in Pr fraction in Ni–Ti–Pr alloys.

Bioinspired metal\u2013polymer thin films with varying hydrophobic properties

Nanocomposites involve the inclusion of one material into the layers of another material at a nanoscale level. Inspired by nature, nanocomposites material systems offer functionalities over their bulk forms which in some cases have evolved over millions of years. Here, thin film coatings have been fabricated by PVD sputtering, comprising a soft PTFE phase which is combined with a hard metallic NiTi phase. A series of coatings with PTFE ranging from 10 to 75 vol% have been prepared, and their surface energies and microstructures investigated. The surface energy of the nanocomposite films changes with the PTFE content, falling in the range between PTFE and NiTi with water contact angles between 80° and 102° for a thin film with 25 and 75 vol% of PTFE, respectively. Here, both TEM and EDX reveal PTFE forming along NiTi column boundaries. Coatings with PTFE content greater than 50 vol% failed due to a build-up of intrinsic stress. The degree of hybridization between NiTi and PTFE was found to be dependent on the PTFE layer thickness. SEM analysis of this coating reveals PTFE at the surface embedded within the NiTi matrix.

Thermomechanical cyclic stability of porous NiTi shape memory alloy

It is known that shape memory and phase transition response of NiTi shape memory alloy (SMA) is dependent on mechanical or thermal background, and the presence of phases such as B2(NiTi), B19'(NiTi), Ni4Ti3, Ni3Ti2 and NiTi2. The volume fraction of metastable Ni4Ti3 phase in NiTi matrix can be easily changed by heat treatments. Both Ni4Ti3 phases and Ni ingredient affect the phase transition kinetics and the shape memory response. This work deals with the effect of thermomechanical cycling on the shape memory response of a porous Ni50.8Ti49.2 SMA, which is homogenized at 1050°C and aged at 800°C, afterwards slow cooled. Full recoverable shape memory response and cyclic stability were achieved during thermomechanical cycling under a compressive stress of 5MPa.

Structure of multi-functional calcium phosphates/TiO2layers deposited on NiTi shape-memory alloy

The structure of the NiTi matrix covered by multi-layer was studied applying X-ray diffraction techniques supported by electron microscopy. Multi-layer was composed from titanium oxide (passivation) followed by mixture of the hydroxyapatite (HAp) andβ-tricalcium phosphate (β-TCP) (electrophoresis). Conditions of deposition as well as sintering did not change the nominal ratio of HAp/TCP and saved their original structure. Also, the passivated NiTi matrix and with HAp/TCP-deposited layer did not change structure. However, sintering, done for HAp/TCP consolidation, introduced local differences in the lattice parameter as well as phase composition of the NiTi matrix. In consequence of that, two-steps martensitic transformation occurred in sintered NiTi/TiO2/Hap–TCP composite.

In situ synchrotron high-energy X-ray diffraction study of microscopic deformation behavior of a hard-soft dual phase composite containing phase transforming matrix

This study explored a novel intermetallic composite design concept based on the principle of lattice strain matching enabled by the collective atomic load transfer. It investigated the hard-soft microscopic deformation behavior of a Ti3Sn/TiNi eutectic hard-soft dual phase composite by means of in situ synchrotron high-energy X-ray diffraction (HE-XRD) during compression. The composite provides a unique micromechanical system with distinctive deformation behaviors and mechanisms from the two components, with the soft TiNi matrix deforming in full compliance via martensite variant reorientation and the hard Ti3Sn lamellae deforming predominantly by rigid body rotation, producing a crystallographic texture for the TiNi matrix and a preferred alignment for the Ti3Sn lamellae. HE-XRD reveals continued martensite variant reorientation during plastic deformation well beyond the stress plateau of TiNi. The hard and brittle Ti3Sn is also found to produce an exceptionally large elastic strain of 1.95% in the composite. This is attributed to the effect of lattice strain matching between the transformation lattice distortion of the TiNi matrix and the elastic strain of Ti3Sn lamellae. With such unique micromechanic characteristics, the composite exhibits high strength and large ductility.

NiTi-Enabled Composite Design for Exceptional Performances

In an effort to further develop shape memory alloys (SMAs) for functional applications, much focus has been given in recent years to design and create innovative forms of SMAs, such as functionally graded SMAs, architecture SMAs, and SMA-based metallic composites. This paper reports on the progress in creating NiTi-based composites of exceptional properties stimulated by the recent discovery of the principle of lattice strain matching between the SMA matrix and superelastic nanoinclusions embedded in the matrix. Based on this principle, different SMA–metal composites have been designed to achieve extraordinary shape memory performances, such as complete pseudoelastic behavior at as low as 77 K and stress plateau as high as 1600 MPa, and exceptional mechanical properties, such as tensile strength as high as 2000 MPa and Young’s modulus as low as 28 GPa. Details are given for a NiTi–W micro-fiber composite prepared by melt infiltration, hot pressing, forging, and cold rolling. The composite contained 63% in volume of W micro-fibers of ~0.6 μm thickness. In situ synchrotron X-ray diffraction revealed that the NiTi matrix underwent martensite transformation during tensile deformation while the W micro-fiber deformed elastically with a maximum strain of 0.83% in the loading direction, implying a W fiber stress of 3280 MPa. The composite showed a maximum high tensile strength of 2300 MPa.

Laser ignition in Self-propagating High temperature Synthesis of porous NiTinol Shape Memory Alloy

Self-propagating High temperature Synthesis (SHS) is a challenging method for producing porous near net-shape materials. Starting from pure metallic powders several compounds can be fast synthesized. Among promising porous structured materials for biomedical applications NiTinol Shape Memory Alloy can be suitable for their peculiar properties, like shape memory effect or pseudo-elasticity, high biocompatibility and lightness.In the present work SHS process on Ni and Ti elemental powders, ignited by a laser beam, was studied. The effect of main laser parameters (laser power and exposition time) was studied on porosity, microstructure and martensitic transformation temperatures. Samples with porosity in the range 42–48% were produced; NiTi matrix and other secondary phases, like NiTi2, Ni3Ti and Ni4Ti3 were detected. No dependence of microstructure and transformation temperatures with the process parameters was found in the investigated range.

Effect of La addition on the microstructure and martensitic transformation of Ni-Ti-La alloys

The effect of the addition of the rare earth element La on the microstructure and martensitic transformation behavior of Ni50Ti50-xLax (x = 0, 0.1, 0.3, 0.5, 0.7, 0.9) alloys was investigated experimentally. The microstructure of Ni-Ti-La alloys consists of the near-equiatomic TiNi matrix and LaNi precipitates. One-step martensitic transformation was observed in all Ni-Ti-La alloys. The martensitic transformation start temperatures decrease gradually with the increase of La fraction in Ni-Ti-La alloys.

Fabrication, microstructure and mechanical properties of W[sbnd]NiTi composites

New two-phase tungsten-based composites containing 88 wt% tungsten powders and 12 wt% nearly equiatomic NiTi alloy deforming by martensite variant detwinning were fabricated by infiltration and hot pressing in this study. The change of Ti/Ni ratio in NiTi mater alloy and the effect of addition of Nb element on the microstructure, martensitic transformation and mechanical properties of WNiTi composites were investigated by comparison of WNi50Ti50, WNi44Ti56, WNi42Ti58 and WNi42Ti53Nb5 composites. The results showed that brittle Ni3Ti formed in the WNi50Ti50 and WNi44Ti56 composites and brittle Ti2Ni formed in the WNi42Ti58 composites while no brittle intermetallics formed in the WNi42Ti53Nb5 composite. The WNi42Ti53Nb5 composite exhibited the sharpest martensitic transformation with the largest transformation enthalpy among the four different composites. The WNi42Ti53Nb5 composite exhibited a double-yielding phenomenon under compression with an ultimate compressive strength of 3820 MPa and a deformation of 50.4%. In-situ synchrotron high-energy X-ray diffraction measurements revealed the first yielding was caused by the martensite reorientation of the NiTi matrix and the second was due to the commencement of massive plastic deformation of the reoriented martensite and is also attributed to the microscopic internal fracturing of the W particles.

TEM phase analysis of NiTi shape memory alloy prepared by self-propagating high-temperature synthesis

The effect of annealing a Ni–48at.%Ti alloy prepared by self-propagation high-temperature synthesis (SHS) by means of transmission electron microscopy (TEM) was studied. The alloy in the as-sintered condition shows the presence of B2 NiTi matrix, relatively large particles (2–100 μm) of cubic NiTi2 phase and very fine densely distributed particles of rhombohedral Ni4Ti3 phase. The phase fractions were determined by the Rietveld refinement of neutron diffraction. Twelve-hour annealing at 1000 °C leads to coarsening of Ni4Ti3 particles. Slow furnace cooling from the annealing temperature results in a transformation of the rhombohedral Ni4Ti3 phase to a metastable Ni3Ti2 phase with cubic symmetry. Its particles, coherent with the B2 matrix, have the form of thin oval platelets with lenticular cross-section. The orientation relation is either cube-to-cube or [115]B2 || [111]P and (552)B2 || (121)P. An additional isothermal annealing of the water cooled samples at 720 °C for 3 and 10 h was carried out. According to the scanning and transmission electron microscopy observation of the sample annealed for 3 h, the cubic Ni3Ti2 phase transforms to the high-temperature tetragonal Ni3Ti2–H variant (space group I4/mmm).

Investigation on microstructure and martensitic transformation of neodymium-added NiTi shape memory alloys

The effect of rare earth element neodymium (Nd) addition on the microstructure and martensitic transformation behavior of Ni[Formula: see text]Ti[Formula: see text]Nd[Formula: see text] ([Formula: see text] = 0, 0.1, 0.3, 0.5 and 0.7 at.%) shape memory alloy was investigated by scanning electronic microscope, X-ray diffraction and differential scanning calorimetry. The results show that the microstructure of Ni–Ti–Nd ternary alloy consists of NiNd phase, NiTi2 and the NiTi matrix. A one-step martensitic transformation is observed in the alloys. The martensitic transformation temperature Ms increases sharply increasing 0.1–0.7 at.% Nd content is added.

Anomalous expansion of Nb nanowires in a NiTi matrix under high pressure

Under high pressure, materials usually shrink during compression as described by an equation of state. Here, we present the anomalous volume expansion behavior of a one-dimensional Nb nano-wire embedded in a NiTi transforming matrix, while the matrix undergoes a pressure-induced martensitic transformation. The Nb volume expansion depends on the NiTi transition pressure range from the matrix, which is controlled by the shear strain induced by different pressure transmitting media. The transformation-induced interfacial stresses between Nb and NiTi may play a major role in this anomaly. Our discovery sheds new light on the nano-interfacial effect on mechanical anomalies in heterogeneous systems during a pressure-induced phase transition.

Thermal Analysis of Ni-Ti-X Alloys Prepared by Self-propagating High-temperature Synthesis

In this work, the influence of alloying elements on the transformation temperatures and temperatures of formation of NiTi intermetallic phases were investigated. NiTi alloys are characterized by shape memory effect, pseudoplasticity and superelasticity. These properties strongly depend on the alloy composition in binary Ni-Ti and ternary Ni-Ti-X. Because these alloys are used in various branches of industry, such as aerospace, engineering or medicine, the addition of ternary element can influence their application significantly. Especially, the presence of Ti2Ni and Ni3Ti in the NiTi matrix may cause a degradation of the shape memory behaviour and mechanical properties. For this reason, we observed the formation of intermetallics by differential thermal analysis (DTA). Differential scanning calorimetry (DSC) experiments were performed to monitor the evolution of the transformation characteristics. We report new results, which show the strong dependence of the transformation temperatures between austenite and martensite on the alloy composition, and how the alloying elements (Mg, C, Zr) influence the formation of Ni-Ti phases.

Metal Injection Moulding of Superelastic TiNi Parts

TiNi shape-memory properties are successfully used today for the fabrication of various technical devices. The limited machinability and high cost of TiNi encourage the use of near-net shape production techniques such as metal injection moulding. In this work TiNi alloys tensile test specimens are produced by metal injection moulding from pre-alloyed powders. A binder based on a mixture of polyethylene, paraffin wax and stearic acid is used. Parts with a density of about 96.6% of theoretical density are obtained. Scanning electron microscopy coupled with EDX measurements reveals a microstructure consisting of a TiNi matrix with small Ti4Ni2Ox and TiC inclusions. DSC and X-ray diffraction observations indicate the presence of additional Ni4Ti3 precipitates. The parts exhibit full superelasticity at room temperature even for strains of up to 4%, without the need for additional thermal post-treatments. Ultimate tensile strengths up to 980 MPa are obtained.