Copper-based Shape Memory Alloys Research Articles

Premartensitic and martensitic transitions and crystallography in copper based shape memory alloys

Copper-based beta-phase alloys constitute a class of materials which exhibit shape memory effect within a certain range of composition. This effect in beta-phases of ternary alloys based on noble metal copper is evaluated by the structural changes in microscopic scale with a displacive transformation called martensitic transition which is first-order transformation, due to the metastable character and change their internal crystalline structures with changing temperature. Beta phases of copper based alloys have the disordered bcc structure, (β)-phase, at high temperatures and undergo two types of ordered reactions called premartensitic transitions with cooling. The first transition is a firstnearest neighbour (nn) ordering reaction and results in a B2-type superlattice. However, bcc to DO3 transition induces the next-nearest neighbour (nnn) ordering reaction. The martensitic transition occurs from the B2(CsCl) or DO3(Fe3Al) type ordered structures to the layered structures on further cooling. Martensitic transformations in these alloys occur occurs in a few steps. . In this kind of transformation, internal stress is effective and one of the steps is type shear mode occuring on close-packed planes of matrix phase called basal plane of martensite. Product martensite phase has the long period layered structures which consist of an array of close-packed planes with complicated stacking sequences called as 3R, 9R or 18R structures depending on the stacking sequences. It is determined that the basal plane of 9R (or 18R) martensites originates from one of the {110}planes of the parent, and a homogenous shear occurs on this plane in two opposite directions during the transformation. Also, the basal plane is subjected to the hexagonal distortion with martensite formation on which atom sizes have important effect. In case the atoms occupying the lattice sites have the same size, the hexagon becomes regular hexagon otherwise Otherwise deviations occur from the hexagonal arrangement. The knowledge of the associated distortion allows us to obtain information on the ordering degree in the martensite. In the present contribution, x-ray diffraction and transmission electron microscopy (TEM) studies were carried out on two copper based ternary alloys aged for long term, nearly 20 years.

Smart materials and the influence of atom sizes on martensite microstructures in copper-based shape memory alloys

A series of alloy systems exhibit a peculiar property which involves the repeated recovery of macroscopic shape of material at different temperatures. The study of such materials, which are often called smart materials due to their capacity of responding to changes in the environment, is a field of research in rapid evolution. The origin of this phenomenon lies in the fact that the material changes its internal crystalline structure with changing temperature. Copper-based β-phase alloys are widely used as a shape memory component in devices. On cooling from high temperature, these alloys undergo a displacive transition which has a close packed-structure following two ordering transitions. This transition is called martensitic transition and responsible for the shape memory effect. The martensitic structures in β-phase alloys are closely related to the austenitic structures and inherit the order in the parent phase due to the displacive character of transition. In case these alloys are deformed in a temperature range in martensitic condition they change in shape and recover the undeformed original austenitic shape on heating over the reverse transition temperature after removing the strain. These materials regain the deformed shape on cooling to the martensitic state and cycle between deformed and undeformed shapes on cooling and heating. Therefore, this property is called reversible shape memory effect. The {1 1 0} β type plane of parent which is the basal plane for martensite is subjected to the hexagonal distortion with martensite formation on which atom sizes have important effect. In case the atoms occupying the lattice sites have the same size, the hexagon becomes regular hexagon otherwise the hexagon undergoes a distortion in case atom sizes are different. Due to this distortion, the spacing differences, Δ d, between particularly selected pairs of diffraction planes providing a special relation between miller indices become different zero and can be a measure of the ordering degree in martensite. The decrease in spacing difference leads to disordering in martensite.

The effect of rapid solidification and grain size on the transformation temperatures of Cu–Al–Be melt spun alloys

Copper base shape memory alloys were prepared by melt spinning technique at different wheel speed. The study was focused to investigate the effect of rapid solidification and grain size on characteristic M S, M f, A S and A f transformation temperatures. Changes on martensitic transformation temperatures in Cu–11.83 wt.%Al–0.48 wt.%Be melt spun ribbons were observed as grain size is reduced. A linear behavior curve of transformation temperatures as a function of grain size was observed. The transformation hysteresis grows as cooling rate increase. Results of optical microscopy and electrical resistivity were used to associate grain size with transformation temperatures.

X-ray analysis of some shape memory CuZnAl alloys due to the cooling rate effect

Copper-based shape memory alloys are very sensitive to the thermal effects before and after the transformation, and these effects may cause important changes on crystallographic properties or the other transformation parameters of the alloys. In this study, the changes which occurred on some transformation parameters of two shape memory CuZnAl alloys due to the effects of various thermal procedures have been investigated by using X-ray diffraction. At the specimens cooled from high temperature β phase region, the L2 1→M18R martensitic transformation was observed. The lattice parameters of M18R monoclinic martensitic structure, tetragonality ( Ψ) and the changes occurred by cooling rate on lattice parameters were defined. In addition, spacing differences (Δ d) between some selected pairs of diffraction planes with ( h 1 k 1 l) ve ( h 2 k 2 l) Miller indices satisfying the relation ( h 1 2 -h 2 2) / 3 = ( k 2 2 -k 1 2) / n ( n = 1 for 9R and n = 4 for 18 R structure) were examined due to the cooling conditions.

Photoacoustic detection of phase transitions with a resonant piezoelectric scheme with extreme sensitivity to small volume changes

Resonant piezoelectric photoacoustic detection is demonstrated to be a sensitive tool for the determination of phase transitions. A model is presented that describes the changes in the signal expected during phase transitions when resonant detection is used. The technique is applied to the study of first-order martensitic diffusionless transformations in copper-based shape-memory alloys. The model takes into account the signal changes arising from two sources. One behaves like an effective change in the heat capacity, and arises due to the enthalpy of the reaction, and the other can be described as an effective change in the thermal expansion coefficient, and arises from the volume change during the transformation. Due to the relative high frequency used (around 20 kHz), the transformation lags behind the temperature oscillations, yielding a phase shift in the acoustic signal as the transformation temperature is passed. The relative sign of the phase angle and amplitude as the transformation proceeds is an indication as to whether the signal arises from volume changes or heat exchange (enthalpy). Huge signals from very small volume changes (smaller than 0.5%) were observed.

Acoustic Study of Martensitic-Phase Aging in Copper-Based Shape Memory Alloys

An acoustic technique was applied to study aging of the β 1 ′ martensitic phase in a number of copper-based shape memory alloys (Cu-Zn-Al, Cu-Al-Ni, Cu-Al-Be) characterized by various degrees of martensitic-phase stabilization. The nonlinear anelasticity of the martensitic phase was studied in wide ranges of temperature (7–300 K) and vibrational strain amplitude (2 × 10−7 −2 × 10−4) at vibrational-loading frequencies of ∼100 kHz. It was shown that aging effects of the martensitic phase can have homogeneous and heterogeneous components. The homogeneous component is associated with a change in the degree of atomic order in the crystal volume. The basic heterogeneous mechanisms of martensitic-phase aging are associated with the formation of atmospheres of point defects and local changes (which are greater than those in the crystal volume) in the degree of atomic order in the vicinity of partial dislocations and the boundaries between martensite variants. It is concluded that various stabilization properties of the alloys at hand result not only from the different diffusion properties of quenching point defects but also from the different influence of these defects on the degree of atomic order and the different features of their interaction with partial dislocations and intervariant boundaries.

Crystallography of microstructural transitions in copper-based shape memory alloys

Crystallography of microstructural transitions in copper-based shape memory alloys

Coupled stress-strain and electrical resistivity measurements on copper based shape memory single crystals

Recently, electrical resistivity (ER) measurements have been done during some thermomechanical tests in copper based shape memory alloys (SMA's). In this work, single crystals of Cu-based SMA's have been studied at different temperatures to analyse the relationship between stress (s) and ER changes as a function of the strain (e). A good consistency between ER change values is observed in different experiments: thermal martensitic transformation, stress induced martensitic transformation and stress induced reorientation of martensite variants. During stress induced martensitic transformation (superelastic behaviour) and stress induced reorientation of martensite variants, a linear relationship is obtained between ER and strain as well as the absence of hys teresis. In conclusion, the present results show a direct evidence of martensite electrical resistivity anisotropy.

Study of martensitic stabilisation under stress in Cu–Al–Be shape memory alloy single crystal

Abstract Martensitic stabilisation phenomenon in the copper-based shape memory alloys has been invariably related to thermomechanical treatments and post-quench history. This phenomenon degrades shape memory properties and reduces their potential for industrial applications. In the present work, superelastic tests in a single crystal of Cu–Al–Be alloy were realised to study the martensitic stabilisation processes. Direct quenched samples have been studied in a special apparatus which allows to measure simultaneously stress–strain and electrical resistance. Two experimental procedures were used: superelastic cycles (8% strain) and ageing in mixed state. In the second test, the sample is submitted to a 4% strain by superelasticity which leads to a mixture of β-phase and single variants of martensite separated by parallel interfaces. After 24 h, the sample is strained up to 8% and finally the stress is removed leading to a total strain recovery. The stress–strain as well as the resistivity curves show a very different behaviour between the volume fraction which have been aged in the martensitic state and the one aged in the β-phase state. The results are discussed in terms of thermal treatment influence and stabilisation processes.

Oxidation behaviour and kinetic properties of shape memory CuAl xNi 4 ( x=13.0 and 13.5) alloys

Although the copper-based shape memory alloys (SMA) have some important problems such as controlling of the kinetic properties in the shape memory ability, they have relatively more advantages when compared to nitinol, such as lower price and simpler production technology. In order to determine the kinetic properties and oxidation rates of shape memory CuAl x Ni 4 ( x=13 and 13.5) alloys with polycrystalline forms, the alloys have been homogenized in β-phase field at 930 °C for 30 min and immediately quenched in iced-brine water at −3 °C. The transformation temperatures in a period of three thermal cycles which include heating and cooling processes have been determined through Shimadzu DSC-50 differential scanning calorimeter. Activation energies of forward and reverse martensitic transformations have been calculated by using the Kissenger method. Thermogravimetric analysis with Shimadzu TGA-50 have been carried out for the determination of mass changes of alloys during heating and cooling cycles with two temperature rates selected as 10 and 30 °C/min up to 900 °C. It has been shown that increasing aluminium content reduces the oxidation rates of the alloys. It has also been established that CuAlNi shape memory alloys have a good stabilization in martensitic phase.

High temperature copper-based shape memory alloys with primary particles

In order to lower the transition temperatures and increase the resistivity against thermal decomposition of the Cu-Al-Nb alloys, the effect of Ni, Co and Cr addition on this behaviour was studied. The fourth element in the ternary alloy changes the chemical composition of the matrix and the particles of the Nb(Cu,Al) 2 phase and decreases the plasticity of the quaternary alloys. To explain this behaviour the nano-indentation measurements have been carried out and changes of the elastic modulus and nano-hardness across the interface have been evaluated.

A one-cycle training technique for copper-based shape memory alloys

Shape memory alloys have been used in the industrial and medical fields due to their shape memory effect (SME), pseudoelasticity and two-way shape memory effect (TWSME). With TWSME, the shape memory alloy (SMA) can changes its shape during heating and recover its shape during cooling. This recoverable property can be thermally cycled over 10 5 times, but TWSME is not an intrinsic property of shape memory alloys: it should be trained by multiple thermomechanical cycling. The present authors have developed a one-cycle training technique to obtain TWSME for Cu-based alloys. This method is usually called the stabilized stress-induced martensite (SSIM) training method. With the SSIM training in Cu-based alloys, many small γ precipitates were induced with a large strain field oriented in a same direction in the austenite. It was considered that these oriented γ precipitates in the austenite contribute their strain field to thermoelastic martensite transformation during cooling. When the alloy is heated again, during reverse transformation the strain field around the γ precipitates can be recovered again. In this way, with only one-cycle training, the Cu-based SMA can obtain TWSME.

Interaction of twin boundaries with stacking faults in 2H martensite: A high-resolution electron microscopy study

Twinned 2H martensite is observed in copper-based shape memory alloys together with basal and non-basal plane stacking faults. Knowledge of the twin-boundary structure and its interaction with the faults is important to understand the deformation mechanism by twin coalescence. High-resolution electron microscopy coupled with image simulations have been used for these studies. The interface of the type I twinning in 2H martensite shows an atomic configuration with mirror antisymmetry. An atomic plane is shared by the twinned variants in such a way that the distance between the planes, parallel to the interface, is unchanged at the boundary and the mean atomic volume is conserved. The interaction of the boundary with a basal plane fault generates a shift in the interface. A mirror antisymmetrical boundary is maintained by introducing an imperfect interface dislocation.

Copper-base shape memory alloys

Copper-base shape memory alloys

Effects of thermal treatments on transformation behaviour in shape memory Cu–Al–Ni alloys

The martensitic transformation behaviour, morphology and transition temperatures in copper-based shape memory alloys are strongly influenced by heat treatments. The effects of various quenching, up-quenching and step-quenching in a water bath 100 °C and in an oil bath at 200 °C have been studied on two shape memory Cu–Al–Ni alloys. The changes at entropy and enthalpy at the martensitic transition have been determined by means of differential scanning calorimeter (DSC) measurements. It was observed that quenching has a marked influence on the transformation temperatures.

Copper-based shape memory alloy

Copper-based shape memory alloy

Studies of copper-based shape memory alloys

Studies of copper-based shape memory alloys

Computing dynamics of copper-based SMA via centre manifold reduction of 3D models

In this work, we present a reduction procedure for 3D models describing phase transformations in copper-based shape memory alloys (SMAs) and develop a robust numerical algorithm for the computational analysis of thin single-crystal slabs of these alloys. Starting from a general Landau-type 3D model for the SMA dynamics we have developed a new mathematical “slow manifold” model that allows us to describe effectively the main features of the thermomechanical behaviour of CuAlNi alloys. Results of the mathematical modelling of the thermomechanical fields in CuAlNi SMAs are discussed with numerical examples.

Two types of the first cycle effects in copper based shape memory alloys

Two types of the first cycle effects in copper based shape memory alloys

Ausageing and Ausforming of a Copper Based Shape Memory Alloy with High Transformation Temperatures

Ausageing and Ausforming of a Copper Based Shape Memory Alloy with High Transformation Temperatures