Ni Shape Memory Alloy Research Articles

Effect of Heat-treatment Time on Performance Recovery of Ti-50.4at%Ni Shape Memory Alloy

Effect of Heat-treatment Time on Performance Recovery of Ti-50.4at%Ni Shape Memory Alloy

Effect of Repeated Heat-treatment for Expansion on Shape Memory and Mechanical Properties of Ti-50.4at%Ni Shape Memory Alloy for Stent

Effect of Repeated Heat-treatment for Expansion on Shape Memory and Mechanical Properties of Ti-50.4at%Ni Shape Memory Alloy for Stent

Effect of proportional control treatment on transformation behavior of Ti–50.9at.% Ni shape memory alloys

► We designed TGA furnace for gradient annealing. ► R-phase and martensitic transformation temperature were changed in the specimen. ► The dɛ / dT of TGA specimens was lower than that of isochronously annealed specimens. ► TGA treatment was most effective at 773 K in case of R-phase and martensitic transformation. ► TGA effect increased with increasing applied stress in case of R-phase and martensitic transformation. In this study, the shape memory behavior of Ti–50.9 at.% Ni alloys with time gradient annealing (TGA) was investigated via differential scanning calorimetry (DSC) and thermal cycling tests under constant load. The ingot was prepared using a high-frequency induction vacuum furnace. The as-cast ingot was hot forged and extruded, followed by cold-drawing and intermediate annealing to produce wires of 1.0 mm in diameter, with a final cold-drawing of 30% reduction in cross-section. For the TGA treatment, a new type of radiant furnace was designed to maintain a constant temperature and create a time gradient along the length of the specimen. According to DSC measurements, a 34 K variation in the R-phase transformation interval (i.e., R s − R f ) was obtained along the length of the specimen (80 mm) that was time-gradient annealed from 3 min to 20 min at 773 K. The results of thermal cycling tests under constant load revealed that the temperature dependence of transformation elongation ( dɛ / dT ) of the TGA specimen is smaller than that of the isochronously annealed specimen at every heat-treatment temperature (673–773 K). The lowest dɛ / dT of R-phase transformation (0.0031%/K) was obtained for the specimen that was TGA-treated at 673 K. The difference in dɛ / dT of R-phase and the martensitic transformation of the TGA-treated and isochronously annealed specimen was largest at 773 K and increased with increasing stress. Such behavior provides superior controllability for actuation applications.

Effects of thermo-mechanical parameters on microstructure and mechanical properties of Ti–50at.%Ni shape memory alloy produced by VAR method

► The martensite content and tensile strength increased by increasing cold reduction. ► During annealing, softening was occurred by recrystallization at high temperature. ► Thermo-mechanically treated specimens showed a pseudo-yield plateau region. ► The thermo-mechanical treatment led to an increase in shape recovery of the alloy. ► Higher cold reduction and lower annealing temperature resulted more shape recovery. In this work, effects of a thermo-mechanical treatment including cold rolling and annealing on microstructure, mechanical properties and shape memory characteristics of Ti–50 at.%Ni alloy were studied. The vacuum arc remelted ingot was first homogenized followed by hot rolling and annealing to prepare the initial microstructure. The annealed specimens were then cold rolled to 10–40% thickness reduction at room temperature. Post deformation annealing was conducted at 400, 500 and 600 °C for 1 h. Phase transformation and microstructural evolution was studied by X-ray diffraction and optical and scanning electron microscopes. Tensile, hardness and three-point bending tests were conducted to determine mechanical and shape memory properties. Experimental results showed that volume fraction of martensite, hardness and tensile strength were increased by increasing cold reduction. More recrystallization was occurred at 40% reduction followed by annealing at 600 °C, leading to a significant decrease in hardness and the amount of remained martensite. Better shape memory characteristics were achieved by applying the thermo-mechanical treatment. Both a higher cold reduction and a lower annealing temperature caused more shape recovery. The specimen annealed at 400 °C after 20% cold rolling showed full shape recovery.

Synthesis and properties of Cu–Al–Ni shape memory alloy strips prepared via hot densification rolling of powder preforms

Cu–Al–Ni shape memory alloy strips were successfully prepared by a powder metallurgy route consisting of preparing powder preforms from premixed Cu, Al and Ni powders by cold compaction, stepwise sintering in the range 873–1273 K, followed by unsheathed multipass hot rolling at 1273 K in protective atmosphere. The densification behaviour of the sintered powder preforms during hot rolling has been discussed. Homogenisation of the hot rolled strips was carried out at 1173 K for 4 h. It has been shown that the finished Cu–Al–Ni alloy strip consisted of self-accommodated plates ofβ' and γ' martensites together with a small amount of nanocrystalline Cu9Al4 phase. The finished hot rolled Cu–Al–Ni strips had fracture strength of 476 MPa, coupled with 2·5% elongation. The shape memory tests showed almost 100% recovery after 10 thermomechanical cycles in the hot rolled strips at 1 and 2% applied prestrain.

Surface acoustic waves and elastic constants of Cu14%Al4%Ni shape memory alloys studied by Brillouin light scattering

A single crystal Cu14%Al4%Ni shape memory alloy in austenitic phase was studied by Brillouin spectroscopy to analyse angular dispersion of Rayleigh surface waves in the planes (0 0 1) and (1 1 0). Measurements were made at room temperature using a tandem-type Fabry–Perot interferometer (JRS Scientific Instruments). Experimental results permitted determination of three independent elastic constants and anisotropic Young's modulus. In the (1 1 0) plane the pseudo-surface waves were found to occur, which—in the context of the past literature data—is unusual for cubic crystals of anisotropy coefficient higher than one. The elastic tensor components determined were used for simulation of surface phonons by the finite element method. The surface phonon velocities obtained were compared with the experimental results and the results of calculations based on the available analytical expressions for the phonon velocities for high-symmetry directions. Particular attention was paid to the relation between the elastic constants (anisotropy coefficient) and the depth of surface phonon penetration.

Thermomechanical behavior at the nanoscale and size effects in shape memory alloys

Abstract

Phase stability and hardness of some ternary Ti–Zr based shape memory alloys

The phase stability and hardness of Ti–30Zr–M (M = Nb, V, Cr, Mo, Fe, Ni and Al) shape memory alloys were investigated by structural observations, d-electron alloy design and microhardness tests. Optical microscopy and X-ray diffraction results show that Nb, V, Cr, Mo and Fe are all β-stability elements in Ti–30Zr–M alloys with the effect enhanced by the presence of Zr. The composition of the least stable β-phase alloy values of Nb, V, Cr, Mo and Fe in Ti–30Zr alloys are 12%, 9%, 5%, 3% and 2%, respectively, indicating an increasing sequence of the β-stability ability. Ni and Al show α-stability or moderate effects and the addition of Ni induces the appearance of (Ti, Zr)2Ni phase in Ti–30Zr–5/15Ni alloys. Based on the d-electron alloy design method, the bond order of Ti and alloying atoms (Bo) and the metal d-orbital energy level (Md) are calculated to build the – diagram of Ti–30Zr–M alloys associated with the phase structure identification. The microhardness of Ti–30Zr alloy is increased by addition of a small amount of the alloying element, except for Nb. There is a drop of the microhardness of each Ti–30Zr–M alloys when β phase appears by the addition of more alloying elements.

Microstructure and properties of Cu–Al–Ni shape memory alloy strips prepared via hot densification rolling of argon atomized powder preforms

Abstract In the present work, Cu–Al–Ni SMA strips with grain size less than 100 μm were successfully prepared via a novel powder metallurgy route involving hot densification rolling of unsheathed sintered powder preforms prepared from argon atomized pre-alloyed Cu–Al–Ni powder. It has been shown that the hot rolled Cu–Al–Ni strips exhibited a very small grain growth during heat treatment at 950 °C up to 4 h. This behavior was attributed to the pinning effect of nano-sized alumina segregated on the grain boundaries. It was established that the grain size of the finished Cu–Al–Ni strips is governed by the starting powder particle size, and the mechanism of this aspect has been discussed. The finished heat-treated Cu–Al–Ni alloy strips consisted of elongated shape grains, with average grain size approximately 27 μm. The heat treated strips were fully martensitic in nature consisting of self-accommodated β ′ 1 martensite along with small amount of γ ′ 1 martensite. The heat treated strips had an extremely good combination of average fracture strength (∼626 MPa) and fracture strain (∼13.5%). The two-way shape memory strain was found approximately 0.23% after 25 training cycles at 4% applied training strain.

Avoiding error of determining the martensite finish temperature due to thermal inertia in differential scanning calorimetry: model and experiment of Ni–Ti and Cu–Al–Ni shape memory alloys

It is known that the thermal effect (thermal inertia) in differential scanning calorimetry causes significant error of measuring the martensite finish temperature (Mf) in shape memory alloy, while the start temperature (Ms) is virtually unaffected. This article shows that the error can be avoided by accounting for the thermal effect quantitatively by mathematical modeling, if the kinetics of the martensitic transformation is properly prescribed. In common with two representative shape memory alloys, Cu–Al–Ni and Ni–Ti alloys, exponential decay is appropriate for expressing the kinetics. The model analysis is extended to the two methods of extrapolation which aims at excluding the thermal effect from DSC data. One is the extrapolation of the cooling rate to zero, and the other is that of the mass of sample to zero. It is shown that both extrapolations construct a temperature between the Ms and Mf. Typically, the temperature is below the Ms by one-third of the interval between the two temperatures.

Structure and properties of nanostructured Cu-13.2Al-5.1Ni shape memory alloy produced by melt spinning

The microstructure and properties of nanostructured Cu-13.2Al-5.1Ni shape memory alloy (SMA) were compared with those of initial coarse structure. The nanostructured Cu-Al-Ni ribbons were produced via rapid solidification using melt spinning technique. The structure and properties of both nanostructured and coarse-grain specimens were then characterized using XRD, SEM, AFM and DSC techniques. According to the obtained results, the nanostructured ribbons show one way shape memory effect. Besides, the formation of nanoparticles of γ 2 (Cu 9Al 4) and the nanograins results in a significant decrease in the martensite↔austenite transformation temperature. The produced nanostructure not only leads to a considerable increase in the recovered deformation but also results in the structure stability when it is subjected to deformation-recovery cycles.

The Effect of Heat Treatment on the Shape Memory Effect of Ni-Ti Alloy Ultra-Thin Wires

Various heat treatment conditions with temperature scans of 300-700 °C, and annealing time scans of 1-4 hours were adopted in the annealing of Ti-49.5at.%-Ni shape memory alloy ultra thin wires. The mechanical hysteretic curves and the crystal morphology were obtained by the tensile-recovery testing and scan electric microscopes (SEM). The result shows that the shape memory effect of Ni-Ti alloy first decline after rising with the heat treatment temperature increases. Alloy with heat treatment effect in 500°C, presents the best. Time is not a significant factor for heat treatment.

The effect of aging on the B2-R transformation behaviors in Ti-51at%Ni alloy

Aging is an effective way to control the transformation behaviors of Ti–Ni shape memory alloys. In the present study, the transformation behaviors of Ti-51at%Ni polycrystals after aging at various temperatures (250 °C–500 °C) for different time were investigated by DSC measurement. It is found that B2-R transformation temperature (RS) of the aged samples is uniquely determined by the aging temperature for aging above 400 °C, being almost independent on aging time. Higher aging temperature causes a lowering of RS temperature. At lower aging temperatures (e.g., 250 °C, 300 °C), RS gradually increases with aging time and finally saturates. The saturated RS value is also a unique function of the aging temperature. These facts can be explained by the phase equilibrium between Ti–Ni matrix (B2) and Ti3Ni4 precipitates. The Ti3Ni4 precipitates formed at different aging temperatures alter the Ni concentration in B2 matrix and consequently cause an aging temperature dependence of the R-phase transformation of B2 matrix. By considering the kinetics of precipitation reaction, the “time-independence” of RS for high temperature aging and the “time-dependence” of RS for low temperature aging can also be reasonably understood. Besides, the R–B19′ transformation temperature (MS) gradually increases with aging time as well, which can also be understood by considering the growth of precipitates during aging.

Electrochemical and microstructural study of Cu–Al–Ni shape memory alloy

The corrosive behaviour of Cu–Al–Ni shape memory alloy in deaerated 0.5 M NaCl solution at 20 °C was studied by means of open circuit potential measurements, linear polarization, potentiodynamic polarization measurements, cyclic voltammetry and electrochemical impedance spectroscopy measurements. The electrode surface was examined by light microscope, SEM, XRD and EDX methods. The polarization measurements have shown that corrosive behaviour of Cu–Al–Ni alloy in NaCl solution was dominated by the Cu component. The results of impedance measurements at open circuit potential have shown that the overall impedance of the system increases with immersion time due to continuous growth of the passive film on the alloy surface. The XRD and EDX analysis showed the presence of copper, aluminium and nickel compounds, Cu-oxides and Cu-chlorides on alloy surface.

Modelling Residual Strains During Cycling of Ti–Ni and Ti–Ni–Cu Shape Memory Alloys in a Pseudoelastic Range of Behaviour Conditions

Abstract: Pseudoelastic behaviour of three types of Ti–Ni shape memory alloys in a pseudoelastic state has been studied under conditions of maximum strain‐ and maximum stress‐controlled cycling. Experimental results proved that residual deformation after unloading increases with the number of cycles; however, critical stress for the induction of martensite and the energy dissipated in one cycle decline during cycling. A higher critical stress for slip, and more intense cyclic dislocation hardening promoted by greater maximum deformation and greater maximum applied stresses, generally reduce the rate at which residual elongation grows with the number of cycles, and tend to stabilise the cyclic stress‐elongation diagrams. The small magnitude of critical stress for slip in low‐nickel alloys, and also cyclic strain hardening, induce greater internal stresses and a more marked decrease in critical stress for the induction of martensite as cycling progresses. Detailed analysis of plastic deformation propagation in cyclically loaded specimen helped develop a model of dependence of residual elongation on the number of cycles. This model enables identification of three main factors that govern the magnitude of residual elongation: one residual plastic elongation caused by dislocation hardening after the alloy is heat treated, and two cyclic strain hardening parameters describing how residual elongation grows with number of cycles, and how this residual elongation is reduced, as cycles increase, by the rising critical stress level for slip. The model has proved to yield very close agreement with experimental findings.

A finite element analysis of the morphology of the twinned-to-detwinned interface observed in microstructure of the Cu–Al–Ni shape memory alloy

A detailed morphology of the twinned-to-detwinned interface in microstructure of 2H-martensite phase of the Cu–Al–Ni shape memory alloy is observed by optical methods (optical microscopy, white-light interferometry). Based on these observations, a finite element model of the transition layer is constructed and applied to calculate the elastic stress distribution inside the observed microstructure. The results show that the real micromorphology does not correspond to the minimum of the sum of the elastic and surface energy, and that the maxima of the stress field necessary for the existence of this morphology are comparable to the elasticity limits of the 2H-martensite.

Effect of Heat Treatment Temperature on Functional Degradation Properties of Ti-50.3at%Ni Shape Memory Alloy

Effect of Heat Treatment Temperature on Functional Degradation Properties of Ti-50.3at%Ni Shape Memory Alloy

J044073 TiNi SMA素材の現在の状況と加工研究の取り組み([J04407]知的材料・構造システム(7))

For about a half century up to today since the invention of Ti-Ni shape memory alloy, diverse products have been developed. However, compared with iron, copper, aluminum, and other metals and alloys, types of Ti Ni shape memory alloy materials and products made of them are still unpopular. This is because only a small variety of types of Ti-Ni shape memory alloy materials are available, they are difficult to process, material costs are high, and it is difficult for ordinary people to understand the fact from a technological standpoint that they cause phase transformation due to a temperature change, unlike other metals. Both materials and products cannot make advances unless the development of materials and the practical use of processed products harmonize with each other. We wrote this paper with the aim of providing helpful information for the future diffusion of Ti-Ni shape memory alloy materials on the market and further sophisticating technology by introducing some of our products manufactured by making use of the materials and processing technology our group has developed to date over difficulties, and as a reference document to show the development steps of Ti-Ni shape memory alloy products.

Nanoscale Pseudoelastic Behavior of Cu–Al–Ni Shape-Memory Alloy Induced by Partial Indentation Unloading

Nanoscale Pseudoelastic Behavior of Cu–Al–Ni Shape-Memory Alloy Induced by Partial Indentation Unloading

Influence of chemical composition on the damping characteristics of Cu–Al–Ni shape memory alloys

Cu– XAl–4Ni shape memory alloys (SMAs) are capable of martensitic transformation across a wide temperature range through the precise adjustment of their chemical composition from X = 13.0 to 14.5. In addition, the variations in chemical composition significantly influence the internal friction characteristics of Cu– XAl–4Ni SMAs. Cu– XAl–4Ni SMAs with a higher content of Al exhibit lower internal friction peaks due to decreases in the amount of transformed martensite and the formation of γ 2 phase precipitates. The damping capacity of the inherent and intrinsic internal friction for Cu–13.5Al–4Ni SMA is extremely low due to the fact that the transformed β ′ 1 ( 18 R ) martensite has only an ordered 9R structure with stacking faults. The Cu–14.0Al–4Ni SMA exhibits a relative increase in the inherent and intrinsic damping capacity because the transformed γ ′ 1 ( 2 H ) martensite exhibits twinning with abundant moveable twin boundaries.