CuZnAl Shape Memory Alloys Research Articles

Preparation, characterization and dynamic mechanical analysis of CuZnAl shape memory alloys

Shape-memory alloys (SMAs) are metallic materials that, after undergoing temperature or stress cycling, regain their original shape. Although Ni-Ti is widely commercialized, it is seen as a costly alternative. Therefore, cost-effective and easy-to-process alloys such as Fe- and Cu-based alloys are being explored nowadays. Fe-based alloys have serious processing difficulties and have texture-dependent properties. Therefore, Copper based SMAs are seen as potential alternatives. Cu-based SMAs are High-Temperature SMAs, Cu-Zn-Al, in particular, operates at higher temperature while showing good SME properties. In this present study, Cu-Zn-Al were manufactured by induction melting process, hot rolled, and then homogenized. The phase transformation behavior was investigated by Differential Scanning Calorimetry (DSC) and their phase composition was investigated by X-ray Diffraction (XRD). Dynamic mechanical analysis (DMA) was performed to study the damping behavior of the alloy. The findings revealed that Cu-Zn-Al has the highest damping capacity, which makes them desirable for particular applications.

Effect of element evaporation on the microstructure and properties of CuZnAl shape memory alloys prepared by selective laser melting

Cu-based shape memory alloys are promising materials for functional or intelligent applications because of its good shape memory properties and low cost. As an additive manufacturing technique, selective laser melting (SLM) is easy to produce such complex intelligent components, moreover, the rapid solidification rate can obtain fine microstructure and get good properties. In this study, CuZnAl shape memory alloys were prepared by SLM for the first time to study the effect of the process parameters on the relative density, phase constitution, microstructure, phase transformation behavior, microhardness and superelastic response. Results show that the intrinsic reason for the differences in the microstructure and the macroscopic properties is the different Zn content in samples fabricated with different energy densities. The lower energy densities lead to the lower amount of Zn evaporation and thus the higher remained Zn content in the fabricated samples. In terms of phase constitution, samples with higher Zn content mainly consist of martensite (β’) and present needle-like shape microstructure morphology. On the contrary, samples with lower Zn content are predominated by α phase and have rod-like or even equiaxed shape microstructure. In addition, higher amount of martensite in the samples results in the higher peak intensity in the DSC curves, the higher microhardness and the lower irrecoverable strain.

Concurrent tracking of strain and noise bursts at ferroelastic phase fronts

Many technological applications are based on functional materials that exhibit reversible first-order ferroelastic transitions, including elastocaloric refrigeration, energy harvesting, and sensing and actuation. During these phase changes inhomogeneous microstructures are formed which fit together different crystalline phases, and evolve abruptly through strain bursts related to domain nucleation and the propagation of phase fronts, accompanied by acoustic emission. Mechanical performance is strongly affected by such microstructure formation and evolution, yet visualisation of these processes remains challenging. Here we report a detailed study of the bursty dynamics during a reversible stress-induced martensitic transformation in a CuZnAl shape-memory alloy. We combine full-field strain-burst detection, performed by means of an optical grid method, with the acoustic tracking of martensitic strain avalanches using two transducers, which allows for the location of the acoustic-emission events to be determined and the measurement of their energies. The matching of these two techniques reveals interface formation, advancement, jamming and arrest at pinning points within the transforming crystal.

Comparative Studies of Work-developing Capacity of Shape Memory Alloys (SMA) and Polymers

The thermodynamic and work-generating behaviours of a Cu-14.86 Zn-5.81 Al (mass. %) shape memory alloy (SMA) and a three-layer PET heat shrinkable sheath were comparatively discussed. The experimental CuZnAl SMA under study experienced a reversible martensitic transformation and was able to lift over 4.8 mm, during heating-cooling cycles, a load which was 463 times heavier than the specimen. During heating, the PET fragments underwent up to three glass transitions and the PET ring-shaped specimens were able to lift over 3 mm, a load which was 1571 times heavier. By comparing the work generating capacity of CuZnAl SMA and PET heat shrinkable sheath, defined as the ratio between total absorbed enthalpy and specific work output, it was found out that the former has been ten times less effective than the latter. Keywords: shape memory alloy, heat-shrinkable sheath, work-generating shape memory effect, martensitic transformation, glass transition

Preliminary Results on Thermal Shock Behavior of CuZnAl Shape Memory Alloy Using a Solar Concentrator as Heating Source

It is highly accepted that martensitic transformation can be induced by temperature variation and by stress solicitation. Using a solar concentrator, we manage to increase the material surface temperature (till 573 respectively 873 K) in very short periods of time in order to analyze the material behavior under thermal shocks. The heating/cooling process was registered and analyzed during the experiments. Material surface was analyzed before and after thermal shocks by microstructure point of view using scanning electron microscopy (SEM) and atomic force microscopy (AFM). The experiments follow the material behavior during fast heating and propose the possibility of activating smart materials using the sun heat for aerospace applications.

The complexity of non-Schmid behavior in the CuZnAl shape memory alloy

The paper addresses one of the most important yet overlooked phenomenon in shape memory research- the plastic slip response. We show that the slip response is highly crystal orientation dependent and we demonstrate the precise reasons behind such complex response. The fractional dislocations on <111> {112} or <111> {011} systems can be activated depending on the sample orientation and solutions are derived for the variations in disregistries and dislocation core spreadings. This leads to the calculation of critical resolved shear stress in close agreement with experimental trends. The results show considerable dependence of the flow behavior on the non-Schmid stress components and the proposed yield criterion captures the role of stress tensor components.

Functional Performances of CuZnAl Shape Memory Alloy Open-Cell Foams

Shape memory alloys (SMAs) with cellular structure offer a unique mixture of thermo-physical–mechanical properties. These characteristics can be tuned by changing the pore size and make the shape memory metallic foams very attractive for developing new devices for structural and functional applications. In this work, CuZnAl SMA foams were produced through the liquid infiltration of space holder method. In comparison, a conventional CuZn brass alloy was foamed trough the same method. Functional performances were studied on both bulk and foamed SMA specimens. Calorimetric response shows similar martensitic transformation (MT) below 0 °C. Compressive response of CuZnAl revealed that mechanical behavior is strongly affected by sample morphology and that damping capacity of metallic foam is increased above the MT temperatures. The shape memory effect was detected in the CuZnAl foams. The conventional brass shows a compressive response similar to that of the martensitic CuZnAl, in which plastic deformation accumulation occurs up to the cellular structure densification after few thermal cycles.

A short review on the interaction of precipitates and martensitic transitions in CuZnAl shape memory alloys

The main effects of [Formula: see text] non-equilibrium nanoprecipitates in CuZnAl shape memory alloys are briefly reviewed. Aspects related to the nucleation and growth of precipitates are commented on and their effect on stress induced martensitic transitions is analyzed. Results concerning the relationship between the size of precipitates and the hysteresis of the stress induced [Formula: see text]-18R transition are studied. The improvement of the two-way shape-memory effect after the introduction of precipitates is shortly commented on. The 18R–6R transition is also analyzed and recent findings on the optimization of the mechanical reversibility associated to the 18R–6R transformation in a matrix with a high density of nano precipitates are reviewed.

Microstructure and calorimetric behavior of laser welded open cell foams in CuZnAl shape memory alloy

Cellular shape memory alloys (SMAs) are very promising smart materials able to combine functional properties of the material with lightness, stiffness, and damping capacity of the cellular structure. Their processing with low modification of the material properties remains an open question. In this work, the laser weldability of CuZnAl SMA in the form of open cell foams was studied. The cellular structure was proved to be successfully welded in lap joint configuration by using a thin plate of the same alloy. Softening was seen in the welded bead in all the investigated ranges of process speed as well as a double stage heat affected zone was identified due to different microstructures; the martensitic transformation was shifted to higher temperatures and the corresponding peaks were sharper with respect to the base material due to the rapid solidification of the material. Anyways, no compositional variations were detected in the joints.

Numerical simulations of the pseudoelastic effect in CuZnAl shape-memory single crystals considering two successive martensitic transitions

Shape memory alloys (SMAs) with double martensitic transitions are potential candidates for highly effective, nonstandard mechanical damping systems. This paper presents a numerical model that can be used to simulate pseudoelasticity in systems with two successive martensitic transformations, such as adequately oriented CuZnAl, CuAlNi and CuAlBe single crystals. The model is based on stress versus strain data obtained from the tensile test of a CuZnAl single crystal and is able to simulate the mechanical damping of SMAs with two successive martensitic transitions. The numerical model has been implemented as an algorithm and used to assess the mechanical damping capacity of a system based on CuZnAl SMA single crystals, considering the complete β-18R–6R cycle. A numerical model with a single degree of freedom is used and the behavior of the SMA-based damper is analyzed both under free and forced oscillation conditions. The results obtained indicate that the alloy studied is a very effective mechanical damper.

Use of Decomposition of Non-Thermoelastic Martensite for Structural Changes of CuZnAl Shape Memory Alloys

Cu22Zn4.6Al shape memory alloy shows memory phenomenon after quenching from dual phase region. Amount of shape memory effect continuously decreases with raised quenching temperature. Decrease is caused by increasing of amount of non-thermoelastic martensite in structure [. Approximately above 700°C there is no effect and martensite is non-thermoleastic. Non-thermoelastic martesite decomposes to alpha and gamma/beta phases at heating at elevated temperatures.[ This effect can be used for modification of structure and thus mechanical and shape memory properties. Some various types of structures were obtained and mechanical and shape memory properties were evaluated. Using of decomposition of non-thermoelastic martensite allows to obtain fine structure with the same amount of alpha phase and martensite as in basic state. This type of structure has better shape memory properties and higher mechanical properties.

Mechanical behavior under cyclic loading of the 18R-6R high-hysteresis martensitic transformation in Cu-Zn-Al alloys with nanoprecipitates

Mechanical damping applications could benefit from the large hysteresis, large pseudoelastic strain and the fact that the transformation stresses of the 18R↔6R martensite–martensite transformation depend very little on temperature in Cu-based alloys. This work presents the 18R↔6R mechanical cycling behavior of CuZnAl shape-memory alloy single crystals with electronic concentration e/a=1.48. A fine distribution of gamma phase nanoprecipitates is introduced to prevent plastic deformation of the 6R phase. Results show that, although significant 6R stabilization is observed at very low frequencies (below 10−2Hz), it is possible to obtain more than 1000 stable pseudoelastic cycles with only minor changes in transformation stresses and hysteresis width at frequencies above 10−1Hz. A more pronounced decrease in transformation stresses is observed after 1000 cycles. Nevertheless, the decrease in hysteresis is small up to 2000 cycles. Reported and present results indicate that pair interchange of atoms can explain the stabilization of 6R under quasistatic experimental conditions. However, at higher frequencies of cycling, stabilization of this martensite shows additional features, leading to a dynamic stabilization with slight effects on the mechanical behavior at the required frequency and number of cycles. On the whole, the behavior of this transformation is unique and very promising.

Thermoelastic transition kinetics of a gamma irradiated CuZnAl shape memory alloy

Many types of shape memory alloys have been used in nuclear reactors and aerospace applications where they are exposed to high levels of various kinds of radiation. The effect of gamma irradiation on the transformation kinetics of thermoelastic transformations in a shape memory CuZnAl alloy was investigated by differential scanning calorimetry (DSC) with heating/cooling rates of 10, 20, 30 and 40 °C/min. Irradiation doses of 10, 20, 30 and 40 kGy were applied to the samples of the alloy. Changes in Gibbs free energies, entropies and elastic energies were calculated. Reverse transition temperatures A s and A f systematically decreased with increasing doses, although forward transition temperatures M s and M f underwent a minimum value at a dose of 20 kGy. Hysteresis in the transition temperatures changed as an inverse parabolic function of the irradiation dose. The activation energies of transformations were calculated by using Kissinger and Osawa methods.

Nanoparticles from Cu-Zn-Al shape memory alloys physically synthesized by ion milling deposition

In this research, an ion milling equipment was used to elaborate nanoparticles from Cu-Zn-Al alloys with shape memory effect. Two different compositions were used, target A: 75.22Cu-17.12Zn-7.66Al at % with an Ms of -9 °C and target B: 76.18Cu-15.84Zn-7.98Al with an Ms of 20 °C. Nanoparticles were characterized by High Resolution Transmission Electron Microscopy, Electron Diffraction and Energy Dispersive X-ray Spectroscopy. The obtained nanoparticles showed a small dispersion, with a size range of 3.2-3.5 nm. Their crystal structure is in good agreement with the bulk martensitic structure of the targets. In this sense, results on morphology, composition and crystal structure have indicated that it is possible to produce nanoparticles of CuZnAl shape memory alloys with martensitic structure in a single process using Ion Milling.

FLEXURAL VIBRATION SUPPRESSION OF GLASS FIBER/CuZnAl SMA COMPOSITE

This work proposes the functional characterization of a composite material, suitable for passive suppression of flexural vibration of beams and shells. Two patterned thin sheets of CuZnAl Shape Memory Alloy (SMA) are embedded into a layered beam of glass fiber. The composite combines the density and stiffness of the glass fiber with high damping properties of SMA in martensitic state. Properly shaped patterning of the SMA sheets, for improving adhesion between the SMA and glass fiber, is performed by means of laser technology. The effect of the laser micromachining on transformation temperatures and internal friction properties of the SMA elements are analyzed. Finally, measurements of the structural damping of the layered glass fiber/SMA composite are reported and the flexural vibration suppression, due to the embedded CuZnAl sheets, is shown.

Improvements in the mechanical properties of the 18R↔6R high-hysteresis martensitic transformation by nanoprecipitates in CuZnAl alloys

Abstract The 18R ↔ 6R martensite–martensite transformation in Cu-based alloys exhibits large hysteresis, large pseudoelastic strain and weak transformation stress dependence on temperature. However, concomitant plastic deformation taking place in the 6R phase inhibits the use of these properties for applications. A novel approach to minimizing or even suppressing 6R plastic deformation during the 18R–6R transformation in CuZnAl shape-memory alloy single crystals with electronic concentration e/a = 1.48 is presented. The method is based on a thermal treatment that introduces nanoprecipitates in the alloy. Results suggest that the role of CuZnAl shape-memory alloys in engineering should be reconsidered, as many energy damping applications could benefit from the huge hysteresis associated with the 18R–6R transformation, once the 6R plastic deformation is suppressed.

Instability of equilibrium of evolving laminates in pseudo-elastic solids

This study is concerned with isothermal stability of equilibrium of evolving laminated microstructures in pseudo-elastic solids with a multi-well free energy function. Several possible modes of instability associated with phase transition between energy wells are analysed. The related rate-independent dissipation is included by imposing a threshold value on the thermodynamic driving force. For a homogenized phase-transforming laminate with no length scale it is shown that localization instability is a rule in case of a non-zero interfacial jump of a directional nominal stress, irrespectively of actual boundary conditions. A stabilizing effect of elastic micro-strain energy at the boundary of the localization zone is demonstrated for laminates of finite spacing. Illustrative numerical examples are given for an evolving austenite–martensite laminate in a crystal of CuZnAl shape memory alloy.

Functional Characterization of Shape Memory CuZnAl Open-Cell Foams by Molten Metal Infiltration

In the recent years, the research for novel materials with tailored mechanical properties, as well as functional properties, has encouraged the study of porous and cellular materials. Our previous work proposed and reported about the possibility to manufacture open-cell metal foams of CuZnAl shape memory alloy by liquid infiltration in a leachable bed of silica-gel particles. This innovative methodology is based on cheap commercial consumables and a simple technology, focusing on intermediate-density low-cost foams with interesting cost/benefits ratio. Microstructural analyses on foamed specimens showed uniform microstructure of ligaments and a very regular and well reproducible open-cell morphology. Moreover, calorimetric analysis detected a thermo-elastic martensitic transformation in the foamed material. In this study, a CuZnAl shape memory alloy was considered and tested to clarify possible effects of the foaming process on the functional properties of the material. Morphological, calorimetric, and thermo-mechanical analyses were carried out. The results show that it is possible to produce metal foams of CuZnAl shape memory alloy with different functional properties and able to recover mono-axial compressive strains up to 3%.

Effect of the deposition rate on thin films of CuZnAl obtained by thermal evaporation

AbstractThermal evaporation is used to deposit thin films of CuZnAl on silicon substrates. For this purpose, a CuZnAl shape memory alloy is used as evaporation source. The chemical composition and the phases present in the films are evaluated at two different deposition rates: 7 and 0.2 Å/s. The thin films are heat treated to promote the diffusion of the elements and characterized by X-ray Diffraction, Energy Dispersive X-ray Spectroscopy and Scanning Transmission Electron Microscopy (STEM). It is shown that the chemical composition of the thin films is significantly different to that of the CuZnAl alloy used as evaporation source. Moreover, the films produced at 7 Å/s show a significant loss of Zn, contrary to the results obtained using a deposition rate of 0.2 Å/s. It is also observed that the composition varies across the thickness of the film, suggesting that the various alloying elements are evaporated at different rates during the deposition process. Finally the predominant phases present in the films belong to the AlxCuy family.

Atomic ordering in CuZnAl shape memory alloys investigated via x-ray absorption and diffraction

We investigate the structure of the austenite phase in CuZnAl shape memory alloys by a combined x-ray absorption and diffraction analysis. Ab initio simulations of the near Zn-edge x-ray absorption coefficient allow us to directly discard the hypothesis of a DO3 superstructure. At the same time, we give evidence of the existence of an ordered structure (B2-like) different from the L21 one recently proposed by neutron diffraction. However, some partial L21 ordering is present at room temperature. This superstructure develops and recovers order when increasing the temperature above 400K.