Shape Memory Effect In Cu Research Articles

Microstructure, martensitic transformation and shape memory effect of novel Cu–Al-Ga based shape memory single crystals

In this study, the shape memory effect of novel iron-alloyed Cu–Al-Ga shape memory single crystals were reported for the first time. The microstructure, crystal structure, martensitic transformation and shape memory effect of Cu–10Al-6Ga (master alloy), Cu–10Al-6Ga-3Fe, Cu–12Al-4Ga-3Fe and Cu–14Al-4Ga-3Fe (wt.%) were investigated. All studied alloys consist of dominant martensite and fine bcc nanoparticles with richer Fe content and poorer Cu content except master alloy. The martensitic transformation start temperatures are 241.3 °C, 251.3 °C and 71.9 °C for Cu–10Al-6Ga-3Fe, Cu–12Al-4Ga-3Fe and for Cu–14Al-4Ga-3Fe alloys, respectively. For master alloy, the martensitic transformation disappears resulted from the precipitation of a large amount of α-Cu during heating. In addition, the shape memory effects clearly increase as a result of increase of the pre-strains. The Cu–14Al-4Ga-3Fe single crystal obtains a maximum shape memory effect of 6.8% with 100% strain recovery when 10% pre-strain is applied.

Enhancing the shape memory effect of Cu–Al–Ni alloys via partial reinforcement by alumina through selective laser melting

Cu–Al–Ni shape memory alloys (SMAs) have high thermal stability and various potential applications. The most significant factors limiting their application are their relatively low elastic strain and poor shape memory effect known as recoverable strain. In addition, the incorporation of new elements for reinforcement usually results in a polycrystalline structure with a larger grain size, which is not suitable for shape memory applications. To overcome these drawbacks, a new method is introduced in this study to increase the shape memory effect and its stability in Cu–Al–Ni SMAs through partial reinforcement with alumina (Al2O3) by selective laser melting. In this approach, the reinforcing lines are deposited in-plane at the mid-height of the printed part, i.e., in the neutral plane. To characterize the effect of partial reinforcement, partially reinforced parts with 0.3 wt% and 0.9 wt% Al2O3 are compared to fully reinforced parts. The results show that the accumulation of localized residual compressive stresses above the reinforcing lines in the neutral plane, induced during selective laser melting, increases the shape memory effect as a result of the increase in mechanical anisotropy in the building direction. Moreover, a new phenomenon named the “waistband effect” is observed in cyclic compressive operation, which results in incremental penetration of the reinforcing particles into the neutral plane during compression. This facilitates a higher strain recovery in the partially reinforced part. These results clearly confirm that partial reinforcement with Al2O3 significantly enhances the shape memory effect of Cu–Al–Ni SMAs.

Structural and Phase Transformations and Physical and Mechanical Properties of Cu-Al-Ni Shape Memory Alloys Subjected to Severe Plastic Deformation and Annealing.

Using the methods of electron microscopy and X-ray analysis in combination with measurements of the electrical resistance and magnetic susceptibility, the authors have obtained data on the peculiar features of pre-martensitic states and martensitic transformations, as well as subsequent decomposition, in the alloys with shape memory effect of Cu–14wt%Al–3wt%Ni and Cu–13.5wt%Al–3.5wt%Ni. For the first time, we established the microstructure, phase composition, mechanical properties, and microhardness of the alloys obtained in the nanocrystalline state as a result of severe plastic deformation under high pressure torsion and subsequent annealing. A crystallographic model of the martensite nucleation and the rearrangements β1→β1′ and β1→γ1′ are proposed based on the analysis of the observed tweed contrast and diffuse scattering in the austenite and the internal defects in the substructure of the martensite.

Microstructure, mechanical properties and shape memory effect of Cu–Hf–Al–Ni alloys

ABSTRACTThe influence of hafnium element’s incorporation on a Cu–xHf–13.0Al–4.0Ni (wt-%) (x = 0.5, 1.0 and 2.0) high-temperature shape memory alloy was investigated systematically. The results show that the matrix of Cu–xHf–13.0Al–4.0Ni (x = 0.5, 1.0 and 2.0) alloys is 18R martensite, and an orthorhombic-structured Cu8Hf3 phase is formed and distributed at the grain boundaries. The grain size is significantly reduced with increasing Hf content. The mechanical properties of Cu–xHf–13.0Al–4.0Ni (x = 0.5, 1.0 and 2.0) alloys are improved by Hf doping due to the combination of refinement strengthening, solid solution strengthening and second phase strengthening. After heating under pre-strain of 10%, the shape memory effect of the Cu–1.0Hf–13.0Al–4.0Ni alloy reaches 5.6%, which is obviously higher than that of the Cu–13.0Al–4.0Ni alloy.

Study of Cu–Al–Ni–Ga as high-temperature shape memory alloys

The effect of Ga element on the microstructure, mechanical properties and shape memory effect of Cu–13.0Al–4.0Ni–xGa (wt%) high-temperature shape memory alloy was investigated by optical microscopy, SEM, XRD and compression test. The microstructure observation results showed that the Cu–13.0Al–4.0Ni–xGa (x = 0.5 and 1.0) alloys displayed dual-phase morphology which consisted of 18R martensite and (Al, Ga)Cu phase, and their grain size was about several hundred microns, smaller than that of Cu–13.0Al–4.0Ni alloy. The compression test results proved that the mechanical properties of Cu–13.0Al–4.0Ni–xGa alloys were improved by addition of Ga element owing to the grain refinement and solid solution strengthening, and the compressive fracture strains were 11.5% for x = 0.5 and 14.9% for x = 1.0, respectively. When the pre-strain was 8%, the shape memory effect of 4.2 and 4.6% were obtained for Cu–13.0Al–4.0Ni–0.5 Ga and Cu–13.0Al–4.0Ni–1.0 Ga alloys after being heated to 400 °C for 1 min.

Microstructure characterization, stress–strain behavior, superelasticity and shape memory effect of Cu–Al–Mn–Cr shape memory alloys

In this study, the Cr was added into Cu–Al–Mn alloys for replacing Cu and Mn, and the microstructure, martensitic transformation, stress–strain behavior, superelasticity and shape memory effect of quaternary Cu–Al–Mn–Cr shape memory alloys were investigated. All the studied alloys exhibit a mixed microstructure consisted of dominant L21 parent, small amounts of A2(Cr) and 2H(γ1′) martensite, as well as a reversible martensitic transformation. Although the alloys are main L21 parent before deformation, partial stress-induced 2H(γ1′) martensite can be stabilized and retained after unloading. Therefore, the same alloy under a certain deformation temperature not only exhibits superelasticity property during deformation, but also the deformed alloy also shows shape memory effect when heated. The results further show that Cu–12.8Al–7.5Mn–2.5Cr alloy has a good superelasticity strain of 2.9% as well as a shape memory effect of 1.5%. Cu–12.7Al–6.9Mn–1.8Cr alloy possesses much the best superelasticity strain close to 5.0% under a pre-deformation of 10% and a shape memory effect of 2.0%. The best shape memory effect up to 2.5% with 10% of pre-deformation and a superelasticity strain of 2.8% are obtained in Cu–12.5Al–5.8Mn–4.1Cr alloy.

Superelasticity and shape memory effect in Cu–Al–Mn–V shape memory alloys

In this study, Cu–Al–Mn–V alloys with bcc phase separation were designed through changing V and Mn contents, resulting in complex microstructure consisted of L21 parent, small amounts of δ(V, Mn) and 2H(γ'1) martensite. The amounts of δ(V, Mn) phase and 2H(γ'1) martensite increase with the increase of V content. The results further show that the stabilization of stress-induced 2H(γ'1) martensite from L21 parent occurs among all Cu–Al–Mn–V alloys during deformation. Thus the same alloy not only exhibits superelasticity, but also shape memory effect after unloading when heated. The superelasticity and shape memory effect are summarized to three situations. Situation I includes Cu63.7Al25.3V1.9Mn9.1 and Cu63.7Al25.8V2.2Mn8.3 alloys (at.%), which have excellent superelasticity strains being up to 5.4% and 3.9% respectively. Cu63.8Al25.4V4.2Mn6.6 and Cu63.0Al26.2V4.7Mn6.1 alloys are situation II, and exhibit good shape memory effects being up to 3.5% and 3.9% respectively. The superelasticity and shape memory effect of Cu62.8Al26.2V2.6Mn8.4 alloy that is situation III are between the above two situations. The obtained results may open an avenue to design copper-based shape memory alloys simultaneously have superelasticity and shape memory effect under the same composition and deformation temperature through applying stress.

Influence of Alloying Element Addition on Cu–Al–Ni High-Temperature Shape Memory Alloy without Second Phase Formation

In this study, the effects of rare earth Gd and Fe elements on the microstructure, the mechanical properties and the shape memory effect of Cu–11.9Al–3.8Ni high-temperature shape memory alloy were investigated by optical microscopy, scanning electron microscopy, X-ray diffraction and compression test. The microstructure observation results showed that both Cu–11.9Al–3.8Ni–0.2Gd and Cu–11.9Al–3.8Ni–2.0Fe–0.2Gd alloys displayed the fine grain and single-phase $$\beta^{\prime}_{1}$$ martensite, and their grain size was about several hundred microns, one order of the magnitude smaller than that of Cu–11.9Al–3.8Ni alloy. The compression test results proved that the mechanical properties of Cu–11.9Al–3.8Ni alloy were dramatically improved by alloying element additions due to grain refinement and solid solution strengthening, and the compressive fracture strains of Cu–11.9Al–3.8Ni–0.2Gd and Cu–11.9Al–3.8Ni–2.0Fe–0.2Gd were 12.0% and 17.8%, respectively. When the pre-strain was 10%, the reversible strains of 5.4% and 5.9% were obtained for Cu–11.9Al–3.8Ni–0.2Gd and Cu–11.9Al–3.8Ni–2.0Fe–0.2Gd alloys after being heated to 500 °C for 1 min, and the obvious two-way shape memory effect was also observed.

The Investigation of Thermal and Magnetic Properties and Microstructure Analysis of Cu–Al–Mn Shape Memory Alloys

In this study, the transformation temperatures and shape memory effect of Cu–Al–Mn shape memory alloys were investigated by calorimetric measurements. Cu–Al–Mn alloys in the range of 10–15 wt% of aluminum and 0–10 wt% of manganese; exhibiting β-phase at high temperatures, were prepared. With an increase in the aluminum and manganese concentrations of the alloy, the martensite morphology is modified and the transformation temperatures are decreased. The activation energies of the alloys were calculated according to the Ozawa and Kissinger methods. Magnetic properties of the alloys were studied and it was seen that the alloys show low magnetization. The microstructures of the alloys were investigated by optical micrographs where the martensite variants with different orientations can be observed from the micrographs.

Tuning of the martensitic transformation temperature in Cu–Zn thin films by control of zinc vapor pressure during annealing

We report a shape memory effect in Cu–Zn thin films grown by electrodeposition on pyrolytic graphite from pyrophosphate-based electrolytes. Cu–Zn films showing a martensitic transformation could only be obtained after the optimization of thermal annealing parameters such as annealing temperature, Zn vapour pressure and fast quenching. By means of controlling the Zn vapour pressure during annealing using a bulk reference Cu–Zn alloy, the chemical composition of the films could be adjusted and the martensitic transformation temperatures of the films could be tuned.

Effects of Gd addition on microstructure and shape memory effect of Cu–Zn–Al alloy

As modification of Cu–Zn–Al shape memory alloys (SMAs), the characteristics of Cu–Zn–Al alloys containing different amount of rare earth Gd addition are investigated by optical microscopy, scanning electron microscopy (SEM), X-ray diffraction (XRD) and the bending method. The results show that Gd addition refines the grains of Cu–Zn–Al alloys obviously, and there are small spherical Gd-rich phases precipitated in the matrix of the alloys. There is no effect of Gd addition on the martensitic transformation type martensitic transformation temperatures of Cu–Zn–Al alloys. The shape recovery ratio is evidently improved with Gd addition, and the shape memory effect of Cu–Zn–Al alloy is the best when the content of Gd is between 0.08 and 0.12 wt.%.

A study of the two-way shape memory effect in Cu–Zn–Al alloys by the thermomechanical cycling method

We have examined the two-way shape memory effect (TWME) in Cu–Zn–Al alloys with a novel thermomechanical cycling method: bending the alloy strips around a cylinder mould and by using a constrained heating/cooling technique. The strength of TWME was affected by the number of training cycles and by the constrained heating temperature. We found that the presence of the β-phase is a crucial factor in generating TWME. After the thermomechanical cycling treatment, we observed the new martensite in addition to the existing martensite in Cu–Zn–Al alloys.

Effect of small γ-precipitates on the two-way shape memory effect in Cu–Zn–Al alloys

The γ-precipitates in Cu–Zn–Al alloys, trained by the stabilization of the stress induced martensite (SSIM) method, have been studied. After the SSIM treatment, it was found that small γ-precipitates in the β-austenite are ellipsoidal, with a large strain field oriented in the same direction; while in the martensite the γ-precipitates changed their shape from ellipsoid to spheroid, and relaxed their strain fields. In order to check whether the strain field of the γ-precipitates is capable of producing a thermoelastic martensitic transformation, an in-situ observation, by heating a sample holder in TEM, was performed. It was found that during heating over a temperature A s, the γ-precipitates with a spherical shape in the martensite recovered their strain field and elliptical shape. During cooling, the strain field of the γ-precipitates disappeared again. It was proposed that the strain field of the γ-precipitates, trained by the SSIM method, plays an important part in the thermoelastic martensitic transformation, and presents two-way shape memory effects.

On the origin of the two way shape memory effect in Cu–Zn–Al single crystals

Abstract The two way shape memory effect (TWME) can be obtained in single crystals of Cu–Zn–Al, Cu–Al–Ni and Cu–Al–Be alloys simply by a diffusion controlled stabilization of the martensite phase. After the retransformation to the austenite on heating some nuclei are retained which are responsible for the formation of preferential martensite variants on cooling without a stress. In this paper it is attempted to contribute to the clarification of the nature of these nuclei in Cu–Zn–Al single crystals. In the single crystals with martensite start temperatures between 0 and 20°C the martensite was stress induced and aged at 60°C. Samples which were retransformed on heating were studied by transmission electron microscopy. Thin martensite plates were observed to remain even after the bulk had retransformed owing to a diffusion controlled lowering of their free energy and probably to pinning effects. They acted as nuclei which on cooling first expanded laterally and then thickened, their high density leading to a perfect TWME.