Ni Shape Memory Alloy Research Articles

Microstructure and corrosion behaviour of Cu-Al-Ni shape memory alloys with Ag nanoparticles

In this study, the effect of silver nanoparticles on the transformation temperatures, microstructural and corrosion characteristics of the ternary Cu–Al–Ni shape memory alloys (SMA) was investigated. It was subsequently observed that the addition of Ag nanoparticles controlled the phase morphology and orientations of the parent phases, along with the formation of the Ag-rich precipitates. Furthermore, it was shown that the addition of Ag nanoparticles can affect the martensitic transformation temperature, due to the microstructure changes that are associated with the formation of intermetallic compounds and/or precipitates. The corrosion behaviour of the Cu–Al–Ni shape memory alloy with and without nano-Ag addition were investigated using electrochemical tests in a NaCl solution, and their results showed that the corrosion potential of Cu–Al–Ni–Ag (nanoparticles) SMA (−277.1 mVSCE) was nobler than the Cu–Al–Ni SMA. It was also shown that there is a decline in corrosion current density from 6.93 to 5.61 µA/cm2 after the addition of nano-Ag to Cu–Al–Ni SMA. The combination of SEM, together with EDX and XRD, also showed that the formation of the corrosion products silver chloride, cuprous oxide, aluminium oxide/hydroxide and silver oxide act as a protective layer and improve the corrosion resistance of Cu–Al–Ni–Ag SMA.

Effect of annealing on stress‐induced transformation behaviors at low temperatures in a Ti–51.8 at.% Ni shape memory alloy

AbstractFor polycrystalline Ti‐51.8Ni (at.%) specimens annealed at 473 and 943 K, the microstructures at room temperature were observed by transmission electron microscopy and the stress‐induced transformation behaviors at low temperatures were investigated by compressive test. The precipitation of Ti3Ni4 was confirmed in the both annealed specimens, where the precipitates at 943 K had a lenticular shape in micrometer size, and those at 473 K seemed to be clusters in nanometer size. Superelasticity with almost complete shape recovery was detected at temperatures below 200 K for all the annealed specimens. In comparison to the as‐homogenized specimen, although the temperature dependence on the stress hysteresis only slightly changed in each annealing condition, the equilibrium stresses in the specimen annealed at 473 and 943 K apparently increased and decreased, respectively.

Thermal and structural characterization of Cu–Al–Mn–X (Ti, Ni) shape memory alloys

In this study, the Cu–Al–Mn–X (X = Ni, Ti) shape memory alloys at the range of 10–12 at.% of aluminum and 4–5 at.% manganese were produced by arc melting. We have investigated the effects of the alloying elements on the transformation temperatures, and the structural and the magnetic properties of the quaternary Cu–Al–Mn–X (X = Ni, Ti) shape memory alloys. The evolution of the transformation temperatures was studied by differential scanning calorimetry with different heating and cooling rates. The characteristic transformation temperatures and the thermodynamic parameters were highly sensitive to variations in the aluminum and manganese content, and it was observed that the nickel addition into the Cu–Al–Mn system decreased the transformation temperature although Ti addition caused an increase in the transformation temperatures. The effect of the nickel and the titanium on the thermodynamic parameters such as enthalpy and entropy values was investigated. The structural changes of the samples were studied by X-ray diffraction measurements and by optical microscope observations at room temperature. It is evaluated that the element Ni has been completely soluble in the matrix, and the main phase of the Cu–Al–Mn–Ni sample is martensite, and due to the low solubility of the Ti, the Cu–Al–Mn–Ti sample has precipitates, and a martensite phase at room temperature. The magnetic properties of the Cu–Al–Mn, Cu–Al–Mn–Ni and Cu–Al–Mn–Ti samples were investigated, and the effect of the nickel and the titanium on the magnetic properties was studied.

Influence of addition of carbon nanotubes on structure–properties of Cu–Al–Ni shape memory alloys

The influence of the addition of 0·2 wt-% carbon nanotubes on the microstructure and mechanical properties of Cu based shape memory alloy with a composition of Cu–11·5 wt-%Al–4·5 wt-%Ni were investigated using optical microscopy, X-ray diffraction, scanning electron microscopy and a universal testing machine. The experimental results show that the carbon nanotubes’ (CNTs’) addition has an effective influence on the martensite–austenite transformation temperatures, type, behaviour and morphology. Furthermore, the addition of CNTs can control the grain size and thus the tensile strength, elongation, fracture stress–strain, yield strength and shape memory effect can be enhanced. Remarkably, the shape recovery ratio reached ∼68% of the original shape; this can be attributed to the precipitation and formation of the CNTs into the microstructure, effectively on the grain boundaries, which can help to reduce/relief the concentration of internal stress.

OS0909 Ti-50.4at%Ni形状記憶合金の疲労寿命特性に及ぼす表面処理の影響

In this study, the effects of surface smoothness on fatigue characteristics of Ti-50.4at%Ni shape memory alloy (which is used as self-expanding stent) wire are investigated. The Ti-Ni specimen is a wire 1mm in diameter. The surface smoothness is varied by the physically polishing and electro-polishing. The surface and fractured surface are observed by scanning electron microscope (SEM). The surface of electro-polished specimen is smoother than the other specimen. The fatigue characteristics of specimen is investigated by the rotary bending tests in the water kept 310K. The applied vending strain (ε_<ap>) is varied from 1 to 6% in this study. When the ε_<ap>, is above 3%, fatigue characteristic of electro-polished specimen is better than the other specimen, while ε_<ap> is below 3%, physically polished specimen also become good as well as fatigue characteristic of electro-polished specimen. Furthermore, the fracture origins of all specimen are found on the surface of each specimen. In consequence, fatigue characteristics are improved with increasing of the surface smoothness.

Structure of the Passive Film formed on Fe&amp;ndash;Mn&amp;ndash;Si&amp;ndash;Cr&amp;ndash;Ni Shape Memory Alloy after Wet and Dry Corrosion Test

The corrosion resistance of an Fe­Mn­Si­Cr­Ni shape memory (16 Mn) alloy was estimated by the wet and dry corrosion tests, and the structure of the passive film formed on the alloy was examined by primary EELS (Electron Energy Loss Spectroscopy) using TEM (Transmission Electron Microscopy) analysis. The electrochemical behavior of 16 Mn alloy was investigated by EIS (Electrochemical Impedance Spectroscopy). 16 Mn alloy showed much higher corrosion resistance than the carbon steel (SM) in wet and dry corrosion tests. In EIS measurement following corrosion test, 16 Mn alloy had much larger value of corrosion resistance (Z1mHz) by forming the passive film as compared to SM. In AES (Auger Electron Spectroscopy) and XPS (X-ray Photoelectron Spectroscopy) analysis, the passive film of 16 Mn alloy was shown to contain Fe, Mn, Cr, Si and Ni. From TEM-EELS, the passive film was found to consist of 2 layers of a Cr­Si-rich in inner and an Mn-rich outer layer of Fe oxide. It was found that 16 Mn alloy could maintain the passive film which composed of effective elements in the wet and dry environment. [doi:10.2320/matertrans.M2014029]

Low Temperature Ageing Effects in NiTi Shape Memory Alloys

This paper reports the comprehensive study of the characteristics of the R-phase transformations observed in low temperature aged (@ 1500C) Ti-49.85 atomic percent Ni shape memory alloys (SMAs). The stabilizations of the R-phase transformations occurred with two separate transformation peaks were observed in DSC study in prolonged aged conditions. In conventional ageing temperature ageing at 3000C, R-phase transformations exhibit an increase in Af temperature; but in low temperature aged NiTi shows that measured Af temperature is lower than the solutionized sample and R-phase appeared well below the original B2→B19’ transformation temperature initially; this result is very peculiar for this ageing treatment. The energy required for the nucleation of this phase ∆HB19’→R on heating cycle was 2.24 J/g, which was much lower than the reverse transformation (on cooling) ∆HB19’→R = –12.6 J/g. This loss of about –8.55 J/g, is believed to be covered by the transformations between the two peaks of B19’→R and R→B2 in heating. The occurrence of the R-phase transformation is attributed to precipitation-induced inhomogeneity of the matrix, both in terms of composition and internal stress fields.

Effect of Ageing Temperatures on the Transformation Behaviour of Ti-50.7at.%Ni Shape Memory Alloy

Ti-50.7at.%Ni as one of Ni-rich TiNi shape memory alloy is very responsive to precipitation ageing treatment due to the formation of Ti3Ni4 precipitates. This study indicates that this alloy exhibits different transformation sequences under different ageing temperatures. Precipitation of Ti3Ni4 has introduced the formation of intermediate R-phase prior to martensitic transformation. The lack of 2-step R-phase transformation, B2→R1 and B2→R2 transformation in this alloy, suggested that Ti3Ni4 precipitation occurs homogeneously.

Characterisation and modelling of behaviour of a shape memory alloys

Shape memory alloys (SMAs) provide an attractive solid-state actuation alternative to engineers in various fields due to their ability to exhibit recoverable deformations while under substantial loads. This feature is of particular importance when utilising the smart composite materials reinforced by SMA. Many constitutive models describing this repeatable phenomenon have been proposed, where some models also capture the effects of rate-independent irrecoverable deformations in SMAs. This paper presents experimental investigations and numerical simulations on shape memory alloys. First, by consisting in determining the transformations of equiatomic Ti–Ni shape memory alloys by differential scanning calorimeter. Then, in order to validate a 3D numerical model of the pseudoelastic behaviour of SMA allowing a finite strain analysis, a set of experimental tests at various initial temperatures is proposed. Finally, the numerical simulations of uniaxial tests performed on shape memory alloys are presented and compared with experimental data, permitting the validation of the proposed modelling. Reasonably good correlation is obtained between the experimental and model predictions.

A Review on Influence of Alloying Elements on the Microstructure and Mechanical Properties of Cu-Al-Ni Shape Memory Alloys

Cu–Al–Ni shape memory alloys (SMAs) have been developed for high temperatures engineering components such as sensor and actuators, due to their ability to work at temperatures near 200°C, rather than NiTi and Cu–Zn–Al alloys whose maximum working temperatures around 100°C. These alloys are widely used because they are much cheaper than NiTi/Cu-Zn-Al and do not require any complicated processing during their manufacturing as do for other shape memory alloys. In addition, these alloys have a small hysteresis and high transformation temperatures compared with other alloys. Despite all these advantages, these alloys have their limitations such as brittleness and low phase recovery strains and stress. The present review describes the role of alloying elements on the properties of Cu-Al-Ni shape memory alloys. It has been found that the additions of alloying elements have a significant effect on the formation, morphology, and structure of the obtained martensite, therefore, the properties of these alloys varied in accordance of these effects.

Impact of the volume change on the ageing effects in Cu–Al–Ni martensite: experiment and theory

The time evolution of the physical properties of martensite during martensite ageing is traditionally explained by the symmetry-conforming short-range order (SC-SRO) principle, which requires the spatial configuration of crystal defects to follow the symmetry change of the host lattice. In the present study, we show that the volume change of the host lattice also contributes to the ageing effects in Cu–Al–Ni shape memory alloy besides the symmetry change. To substantiate this statement the gradual increase of the storage modulus with time at constant temperature was measured by dynamic mechanical analysis (DMA) and the experimental results were quantitatively described in the framework of the symmetry-conforming Landau theory of martensitic transformations in a crystal with defects. The comparison of experimental and theoretical results confirmed that the time dependence of the storage modulus is caused by two different physical mechanisms. Evaluations showing that the first mechanism is driven by the spontaneous symmetry change and the second mechanism is caused by the volume change after the martensitic transformation was carried out.

Martensitic phase transition in Cu–14%Al–4%Ni shape memory alloys studied by Brillouin light scattering

The paper presents the influence of the martensitic phase transition on hypersonic thermally excited surface acoustic waves propagating in Cu–14%Al–4%Ni (wt%) shape memory alloy. Non-destructive and non-contact testing using Brillouin light scattering spectroscopy permitted determination of the elastic constants of austenite versus temperature. Experimental results obtained for martensite were interpreted using the proposed model of the cubic to orthorhombic martensitic phase transition based on the Landau model of a first-order phase transition. Additionally we adopted the approximation of the domain structure of martensite by a polycrystalline sample using the Voigt–Reuss–Hill procedure of averaging the elastic constants.

Continuously cast Cu–Al–Ni shape memory alloy – Properties in as-cast condition

Abstract In respect to the cost of constituents, Cu–Al–Ni alloys can be regarded as low-cost shape memory alloys. However, the processing is quite costly due to the brittleness of the ingots. Therefore it would be advantageous to produce semi-finished products directly from the melt via highly productive technique. A laboratory device for vertical continuous casting directly from the vacuum induction melting furnace was experimentally employed for continuous casting of the Cu-13 wt.% Al-4 wt.% Ni shape memory alloy. A water-cooled copper crystalliser with a graphite insert was used to cast ϕ 16 mm strands. Diverse sets of casting parameters were tested, the average casting speeds being within the range from 250 mm min −1 to 625 mm min −1 . The castability of the alloy using this technique was good. Strands were examined in the as-cast condition, and no cracks or other defects were discovered. Medium casting speeds resulted in good surface quality, and a fully martensitic microstructure was obtained even at the lowest average casting speed. The paper presents and discusses the surface quality, microstructures and mechanical properties.

Effects of Annealing Temperature on Thermomechanical Properties of Cu–Al–Ni Shape Memory Alloys

The effects of the annealing temperature on structural properties and the phase transformation of a Cu–14.1Al–3.9Ni (mass %) shape memory alloy (SMA) have been investigated. The annealing process was carried out at temperatures in the range of $$700\,^{\circ }{\mathrm{C}}$$ to $$850\,^{\circ }{\mathrm{C}}$$ . The structural changes of the as-quenched and annealed samples were studied by optical microscope and X-ray diffraction measurements. The evolution of the transformation temperatures was studied by differential scanning calorimetry with different heating and cooling rates. The activation energy and thermodynamic parameters of the samples were determined. It was found that the heat treatment has an effect on the characteristic transformation temperatures and on thermodynamic parameters such as enthalpy, entropy, and activation energy. The crystallite size of the as-quenched and annealed samples were determined. Vickers hardness measurements of the as-quenched and annealed samples were also carried out. It is evaluated that the transformation parameters of a CuAlNi SMA can be controlled by heat treatment.

Dynamic impact behavior of Ni-rich and Ti-rich shape memory alloys

Stress vs. strain curves of two kinds of Ti–Ni shape memory alloys under dynamic compressive loading condition were obtained by the split Hopkinson pressure bar experiment. One is Ni-rich, 50.75Ni–49.25Ti (at%), alloy and the other is Ti-rich, 49.5Ni–50.5Ti (at%). The former exists as austenite phase to show superelastic behavior at the room temperature, and the latter is martensitic phase at the room temperature. The dynamic stress–strain curves were compared with quasi-static compressive test results. The Ni-rich specimen displayed a much higher stress level at the high strain rate than at the quasi-static test, but the Ti-rich specimen exhibited a stress level alike in the two types of tests. Temperature measurement on the surface of the impact specimens was carried out during the impact test. The temperature rising was about 3–5°C in the considered test conditions.

Relationship between transformation temperatures and alloying elements in Cu–Al–Ni shape memory alloys

Cu–Al–Ni shape memory alloys are good candidates for high temperature applications. We have investigated the effects of alloying elements on transformation temperatures, heat-capacity values, and structural properties of Cu–13.73Al–4.3Ni and Cu–13Al–4.3Ni (wt%) shape memory alloys. The evolution of the transformation temperatures was studied by differential scanning calorimetry with different heating/cooling rates. The heat-capacity measurements of the samples were made. It was found that the mass percentage of the alloying element has an important effect on the characteristic transformation temperatures and thermodynamic parameters. The structural changes of the samples were studied by X-ray diffraction measurements and optical microscope observations at room temperature. It is evaluated that the transformation parameters of CuAlNi shape memory alloy can be controlled by the change of the mass percentages of the alloying elements.

Effect of Ageing Treatment on the Microstructures of Ti-50.7at.%Ni Shape Memory Alloy

In this study, a commercial Ti-50.7at.%Ni was subjected to solution treatment at 800oC and 900oC for one hour followed by ageing at 300oC, 400oC, 500oC and 600oC for 1 hour, 2 hours and 4 hours respectively. Characterization of the material using optical microscope, XRD, FESEM and EDX has shown that the presence of Ti2Ni and Ti-rich precipitates in the as-received material remained unchanged even after heat treatment. The presence of oxygen in the air condition heat treatment has stabilized Ti2Ni since this condition causing the formation of Ti4Ni2O which is also determined as Ti2Ni due to their same crystal structure. The formation of Ti3Ni4 precipitates during ageing encouraged the establishment of R phase instead of martensite due to the depletion of Ni content in the matrix. Ageing treatment at 300oC generates two steps of R phase transformation due to the differences in Ni composition in the matrix at the grain boundary and grain interior region. However, for samples subjected to ageing at higher temperature, 400-500oC, only one step of R phase transformation occurred because Ni atomic diffusion was not forbidden in the high temperature. Hardness test performed on the samples revealed that increasing the ageing time will increase the hardness of material; however for the sample aged at 600oC, increasing the ageing time would reduce the hardness due to dissolution of Ti3Ni4 precipitaes into the matrix.

Manufacturing of monolithic superelastic rods with variable properties for spinal correction: Feasibility study

A new concept of monolithic spinal rod with variable flexural stiffness is proposed to reduce the risk of adjacent segment degeneration and fracture associated with rigid spinal fixation techniques while providing adequate stability to the spine. The concept is based on the use of Ti–Ni shape memory alloy rods subjected to different processing schedules implying local annealing, cold work, or a combination of both. A feasibility study of the concurrent technological routes is performed by comparing their potential to locally control material microstructure and properties.

Thermodynamics of Ti–Ni shape memory alloys

The thermodynamics of the Ti–Ni system are reviewed, and CALPHAD descriptions of metastable intermetallic phases are presented. These phases play an important role as precipitates in shape memory alloys. Metastable Ti3Ni4 and Ti2Ni3 are described as line compounds. Their thermodynamic model parameters are optimized with experimental solvus data and molar enthalpies at 0K from new first-principles analysis. Best results are obtained, when the thermodynamic description of the D024-ordered TiNi3 phase is re-optimized with new thermodynamic data. This also requires adjustment of the other phase descriptions, including B2 austenite and B19′ martensite. The modifications have important consequences on the computed start temperature of the martensitic transformation, which is a crucial property for the shape memory effect. R-phase, a metastable intermediate martensite, is considered in the thermodynamic modeling. The following thermodynamic standard data for the metastable intermetallic phases are obtained at 298.15K: ΔH°m(Ti3Ni4)=−34,714.5J/mol, ΔH°m(Ti2Ni3)=−36,742J/mol, ΔH°m(R-phase)=−35,649J/mol, S°m(Ti3Ni4)=31.91J/molK, S°m(Ti2Ni3)=29.76J/molK, S°m(R-phase)=27.87J/molK.

Effect of copper addition on the microstructure and shape recovery of Fe–Mn–Si–Cr–Ni shape memory alloys

The effect of Cu addition on the microstructure and shape recovery behaviour of Fe–Mn–Si–Cr–Ni shape memory alloys has been studied. Microstructural characterization revealed that Cu is soluble in these alloys at least up to 3wt% in the austenite (γ) phase and acts as a strong γ stabilizer. The γ–ε martensite transformation start temperature (MS) and the reverse ε–γ transformation start temperature (AS) were observed to decrease with Cu addition, although at different rates. Copper addition promotes the formation of incoherent Fe5Ni3Si2 type intermetallic π-phase precipitate during ageing and its amount increases with the amount of Cu in the alloy. Most importantly, Cu results in the deterioration of shape recovery of Fe–Mn–Si–Cr–Ni shape memory alloys, by increasing stacking fault energy (SFE), by decreasing the resistance to plastic deformation and by lowering the MS temperature of the γ phase.