Al Shape Memory Alloys Research Articles

Conceptual design of actuator applications with Cu–Zn–Al single crystals

Abstract A systematic approach to the conceptual design of actuator applications with single-crystals of Cu–Zn–Al shape memory alloys (SMA) is presented. The actuator considered here, a device capable of doing work in response to temperature changes, is based on a single-crystal nucleus of a Cu–Zn–Al alloy coupled to a conventional spring that represents the load to be displaced. The different steps and factors involved in the conceptual design are described. In this stage, a full transformation cycle approach was adopted. In this way the effects of different parameters (cycle number, friction, temperature range, load level) on the actuator behavior can be studied.

Marforming and martempering of a Cu–Zn–Al shape memory alloy

A Cu–Zn–Al alloy with Ms=−25°C has been quenched in liquid nitrogen. A subsequent rolling of the thermally induced martensite (marforming (MF) at −196°C) by an amount of ϕ=20% provides oriented and high defect domains. This results in a defect stabilized structure which can not retransform diffusionless into austenite. Tempering (martempering (MT)) of this martensite is conducted in two ways: (1) Isochronal (1 h) at different temperatures 150°C<TMT≤800°C. The material is subsequently water quenched. (2) Heat-up treatments in a differential scanning calorimeter (DSC) device with different heating rates (5–50 K min−1). The microstructural changes caused by these MT treatments are investigated by optical and transmission electron microscopy. The course of different precipitation processes (γ-, β- and α-brass) is confirmed by the DSC heat-up treatments. This results in a time–temperature reaction diagram. Recovery of the diffusionless transformability of the defect martensite caused by isochronal MT and its effect on the non conventional yielding of the material is investigated by stress–strain experiments.

Substructure and boundary structure of deformed 18R martensite in a Cu–Zn–Al alloy

The substructure of martensite and the intervariant boundary structure in the deformed 18R martensite in a Cu–Zn–Al shape memory alloy have been investigated in detail by a high resolution electron microscope (HREM). It is found that the deformation-produced dislocations, which mainly appear in the (0018) plane, extend into partial dislocations with a spacing of 5–8 atomic layers. In addition to (0018)[010] slipping, dislocations in the ( 1 ̄ 28) plane are also found. To accommodate strain, lattice rotation takes place near the boundary plane to bring close packed planes of the related variants into parallelism, reducing the resistance for the boundary to move. The intervariant boundaries in the same plate group or between different groups develop into stepped boundary structure due to deformation. These steps correspond to twinning dislocations whose movement results in the migration of the boundary perpendicular to itself.

Low-lying phonon dispersion curves ofD03Cu3Al(+Be)

The low-lying phonon dispersion curves of a D0{sub 3} ordered Cu{sub 3}Al(+Be) shape-memory alloy were measured, at room temperature, along the [{xi}00], [{xi}{xi}0], [{xi}{xi}{xi}], [{xi}{xi}1], and [ (1) /(2) , (1) /(2) , {xi}] symmetry directions. As in other bcc metals and alloys, we find that the frequencies of the TA{sub 2}[{xi}{xi}0] modes are relatively low and that the L[{xi}{xi}{xi}] branch exhibits a pronounced dip at {xi}= (2) /(3) . The measured low-lying phonon dispersion curves are quite similar to those of a bcc structure. Actually, a fifth-nearest-neighbor force constant bcc model provides a satisfactory fit to the low-lying phonon dispersion curves of this alloy. This model was used to evaluate the elastic constants, the phonon density of states, and the lattice specific heat. The results of these calculations are in good agreement with macroscopic measurements of the elastic constants and specific heat of this alloy. {copyright} {ital 1999} {ital The American Physical Society}

Hydrogen in Ti–Ni–Cu and Cu–Zn–Al shape memory alloys

The influence of the absorption of hydrogen on the martensitic transformation of polycrystalline wires of a Ti–49.83at%Ni–0.08at%Cu alloy was studied using electrical resistivity. It was found that the stability of the austenite and the martensite is affected in the same way, resulting in no change of the transformation temperatures for H concentrations as high as 12 at%. No indication for hydrogen desorption was found below 423 K. Hydrogen has also been charged in single crystalline samples of three Cu–Zn–Al alloys at room temperature, presenting the β, 18R or 2H phases. The maximum concentration reached was around 0.04 at% and no influence was found on several properties of the martensitic transformation.

A micromechanical modelling of the hysteretic behaviour in thermally induced martensitic phase transitions: application to Cu–Zn–Al shape memory alloys

The hysteretic behaviour of Cu–Zn–Al shape memory alloys (SMAs) in thermally induced martensitic phase transition is dealt with. The problem is studied by means of a kinematic analysis where the internal variables describing the material's microstructure are regarded as implicit functions of the applied thermomechanical loading parameters (Σ ij , T). On the other hand, a thermodynamic approach is used in which the local balance formalism is based on the thermoelastic equilibrium concept. Considering that thermoelastic equilibrium temperatures between phases, in the forward and reverse transformation, are dependent on the location in the transformation path enables the hysteretic behaviour to be determined. Hence, a set of non-linear equations is deduced simulating the complete and partial cycles. Results obtained in this way, in the thermally induced phase transition with no applied stress, are in good agreement with experimental observations performed on Cu-based SMAs.

Transformation relaxation and aging in a CuZnAl shape-memory alloy studied by modulated differential scanning calorimetry

The reverse martensitic transformation and aging processes in a polycrystalline Cu-23.52 at. pct Zn-9.65 at. pct Al shape-memory alloy have been studied using the recently developed modulated differential scanning calorimetry (MDSC) technique, and some new findings are obtained. By separating the nonreversing heat flow from the reversing heat flow, MDSC can better characterize the thermodynamic, kinetic, and hysteretic features of thermoelastic martensitic transformations. Two kinds of exothermal relaxation peaks have been identified and separated from the endothermal reverse martensitic transformation: one is associated with the movement of twin interfaces or martensite-parent interfaces, and another is due to the atomic reordering in the parent phase via a vacancy mechanism. The martensite aging processes have been examined, and two stages of the aging process been distinguished: the first stage of aging is characterized by the stabilization of martensite, as manifested in the increase in the reversing enthalpy of the reverse martensitic transformation and in the transformation temperatures, and the second stage is, in fact, the decomposition of the martensite on prolonged aging, accompanied by a decrease in the transformation enthalpy. The results suggest that the mechanisms of the relaxation in the martensite and in the parent phase may be quite different.

Effect of ageing in the two-phase region in a Cu–Zn–Al shape memory alloy

A plateau in the biased displacement versus temperature curves was observed for Cu–Zn–Al based shape memory alloys (SMAs) which were aged at the two-phase region for a long time. This corresponds to the plateau in the reversely transformed quantities of martensite versus temperature curves, which has been confirmed by the in-situ X-ray diffraction study. These plateaus are caused by the stabilized martensite which is aged at the two-phase region and undergoes structural changes.

Magnetic and martensitic transformations in Ni 50Al xMn 50− x alloys

The Ni–Al shape memory alloy system is well known by the unique characteristics of phonon-softening and tweed, which are closely related to the B2-14 M[(5 2) 2] martensitic transformation. If Mn is added to the alloy, additional martensite appears such as 10 M[(3 2) 2] long period stacking order structure. The magnetic and martensitic transformations in the Ni 50Al x Mn 50− x alloys were investigated in order to understand their relations. This was accomplished by extensively utilizing various techniques such as magnetic susceptibility and magnetization measurements, transmission electron microscopy, differential scanning calorimetry (DSC) and electrical resistivity measurements. These investigations uncovered strange successive magnetic transformations (antiferromagnetic to spin-glass state), which were examined further in relation to their martensitic counterparts.

Incommensurate modulated structure study of a Cu–Zn–Al–Zr phase

Zr was added to Cu–Zn–Al shape memory alloy as a grain refinement element. There are two new phases, Cu 50.2Zr 24.6Al 17.3Zn 7.9 (at%) ( Z 1 phase) and Cu 57.4Zr 20.4Zn 10.3Al 11.9 (at%) ( Z 2 phase), in the Cu–19.0Zn–13.1Al–1.1Mn–0.3Zr (at%) alloy. The structure of the Cu 57.4Zr 20.4Zn 10.3Al 11.9 (at%) ( Z 2) phase is studied in detail in the present paper. The results of electron diffraction and high-resolution electron microscopy (HREM) investigations indicated a one-dimensional incommensurate modulated structure in the Z 2 phase. The average structure of the phase is base-centred orthorhombic. The systematic reflection conditions associated with the main and satellite reflections demonstrate that the Bravais class of the Z 2 phase is P Cmmm 11 1 type in a (3+1) dimensional space for the incommensurate modulated structure. The substitution modulation effects of the Al and Zr elements are revealed by energy-filtered transmission electron microscopy (EFTEM). { 1 3 ̄ 2 ̄ } compound-type twins are observed in the Z 2 phase. The relationship between the Z 1 and Z 2 phases in the alloy is discussed.

A comparative study of the post-quench behaviour of Cu–Al–Be and Cu–Zn–Al shape memory alloys

This paper reports a comparative investigation of the effect of quenching on the Cu–Al–Be and Cu–Zn–Al shape memory alloys by the use of several experimental techniques. In a first stage, the order–disorder transitions in these alloys have been characterized by means of modulated calorimetry. Results have proved that the A2⇋ DO 3 transition in Cu–Al–Be is first order with a latent heat of 1160 J/mol; the B2⇋ L2 1 transition in Cu–Zn–Al is second order, and a peak in the specific-heat vs temperature curve has been observed. Secondly, the post-quench behaviour of these alloys, when subjected to some of the typical heat treatments used to stabilize the β phase, has also been studied by means of neutron diffraction, positron annihilation and highly sensitive calorimetry. A different post-quench time evolution of the martensitic transition temperatures has been found for the two alloys. For Cu–Al–Be, this evolution has been shown to be correlated with positron annihilation data, while, for Cu–Zn–Al, a correlation with neutron diffraction data has been established. These results show that the measured shifts in the transition temperatures induced by a quench are mostly due to an excess of vacancies in the case of Cu–Al–Be, and to an incomplete degree of L2 1 atomic order in Cu–Zn–Al.

Effect of cobalt addition on grain growth kinetics in CuZnAl shape memory alloys

Abstract β-phase in copper alloys present a rapid growth. Coarse grain size materials should be avoided since they decrease the uniformity and reproducibility of properties, they can cause pseudoelastic brittleness and they decrease the stresscorrosion resistance. The grain size produces changes in the transformation temperatures and in the stress required to induce the martensitic phase. The purpose of this work is to show the effect of cobalt addition on grain growth kinetic in Cu-ZnAl shape memory alloys. The cobalt was added with the purpose of refining the grains. Grain growth kinetics have been evaluated in Cu-Zn-Al shape memory alloys with different cobalt contents by the calculation of several grainsize parameters (perimeter, area and diameter) at different temperatures and heat treatment times. The growth exponent values have been determined and the activation energies estimated.

Accommodation of γ-phase precipitates in CuZnAl shape memory alloys studied by high resolution electron microscopy

The accommodation of small γ-phase precipitates within the surrounding matrix in CuZnAl single crystals has been studied using High Resolution Electron Microscopy (HREM). When embedded in a β-phase matrix, the HREM images at atomic resolution show that the precipitates are fully coherent with the matrix. The images formed only with superlattice spots show that the precipitates are always located at the antiphase boundaries of the β matrix. Considering the correspondences between the HREM image and the atomic positions, analysis of the boundaries indicates that they are next-nearest neighbor antiphase boundaries (APB), with two possible shift vectors. Inside the precipitates, small microdomains can also be observed. The boundaries between them are not antiphase boundaries, but planar defects breaking the translational symmetry of the lattice. When the precipitates are embedded in a martensitic matrix, the accommodation of the precipitates becomes difficult due to the intrinsic deformation associated with the transformation. Relaxational processes take place near the interface, such as the formation of small self-accommodating plates around the precipitates as well as dislocations.

A study on the nickel-rich ternary Ti–Ni–Al shape memory alloys

The transformation sequence and hardening effects of 400 °C aged Ti47.5Ni50.65Al1.85 and Ti49.5Ni50.13Al0.37 shape memory alloys have been investigated by electrical resistivity tests, internal friction measurements, hardness tests and TEM observations. Both solution hardening and precipitation hardening are found to occur in these alloys. The hardening effects of Ti47.5Ni50.65Al1.85 alloy are obvious and much higher than those of Ti49.5Ni50.13Al0.37 alloy due to the former having the larger Ni/Ti ratio and a higher Al solute content in its matrix. The transformation sequence of 400 °C aged Ti47.5Ni50.65Al1.85 alloy shows B2↔R-phase only for an ageing time of more than 10 h and that of 400°C aged Ti49.5Ni50.13Al0.37 alloy shows the sequence B2↔R-phase↔B19′ or B2↔R-phase with different ageing times. All of these characteristics are associated with Ti11Ni14 precipitates during the ageing process. These aged Ti–Ni–Al alloys exhibit very good shape memory effects, in which the maximal shape recovery occurs at the peak of hardness.

Effects of additions of small amounts of fourth elements on structure, crystal structure and shape recovery of CuZnAl shape memory alloys

Cu-based shape memory alloys (SMAs) are particularly interesting, compared to Ni-Ti SMAs because of their low cost and relatively ease process. However, there are important problems to be solved, such as intergranular fracture due to large grain size, stabilization of martensite, etc. In the present work, the influences of additions only less than 1 mass% of several fourth elements, such as Mn, Fe, Co, Ni, and Y, to two kinds of Cu-Zn-Al SMAs, i.e., Cu-30Zn-4Al and Cu-25Zn-7Al on their structure, crystal structure and shape recovery have been examined in order to know what elements are the most effective for the thermal stability of the parent and martensite phases and the shape memory capacity.

Characterization, thermomechanical behaviour and micromechanical-based constitutive model of shape-memory CuZnAl single crystals

CuZnAl shape memory alloys are grown as single crystals by the Bridgman technique with a final shape directly suitable for thermomechanical tests (cylinders with tapered heads: 25 mm gauge length, 4 mm in diameter). The four classical transformation temperatures are checked by Differential Scanning Calorimetry and resistivity. The orientation of crystal structure is investigated by X-ray diffraction. Isothermal pseudoelastic tensile tests show that the width of the hysteresis loops and the slope of the stress-strain curves during phase transformation increase as the applied stress rate increases. A micromechanical-based constitutive model allows us to describe this single crystal behaviour.

Thermodynamical model of cyclic behaviour of TiNi and CuZnAl shape memory alloys under isothermal undulated tensile tests

Abstract The thermodynamical description of observed evolution of mechanical properties of shape memory alloys during isothermal simple tension cyclic loading (in pseudoelastic temperature range) is presented. To define the new internal state parameter, the concept of instantaneous residual martensite is introduced. The known form of the free energy function is suitably modified, and the appropriate kinetic equation for the new state variable is proposed. All constants occurring in constitutive equations are determined for special NiTi and CuZnAl alloys. The satisfactory agreement between predicted and experimental stress-strain hysteresis loops is shown.

Recent improvements in conduction calorimetry. Application to the martensitic transformations in shape memory alloys

A critical reading of the work of Tian and Calvet and the actual state of the art in thermostats, temperature control and the modelling of differential conduction calorimeters suggest a back-wards evolution to non-differential calorimeters. An elementary design shows increased performances at constant temperature; the sensitivity is 700 mV W −1 at room temperature, the short-term noise around 200 nV ≈ 0.3 μW, and the baseline fluctuations around 1 μV ≈ 1 μW. Measurements of the dissipated heat power, indicative of the internal processes after a heat treatment (different types of quench) were realised for a CuZnAl shape memory alloy. The experimental results established that the dissipated heat evolves in time for at least 2 or 3 days. At least two time constants were detected and, as expected, the values are dependent on the quench type and sample mass.

Austenitic—martensitic interface damping measured in shape memory alloys by a cyclic tensile machine

A tensile machine has been constructed to test shape memory alloys. With this machine it is possible to measure the internal friction anywhere in the stress—strain plot. The minimum strain amplitude for the measurements is near 10 −4 and the maximum frequency is 0.6 Hx. In this paper we present some preliminary results obtained on single crystals of CuZnAl shape memory alloys. The internal friction vs. temperature has been measured at various cooling—heating rates (from 0 to 1 K min −1). We have also measured the internal friction vs. deformation during a superelastic test (at T above A f ). This last experiment shows that the internal friction depends on the number of interfaces between the martensitic and austenitic phases.

From adapted and computerized thermomechanical equipments to modelling and the time-evolution behaviour in Cu−Zn−Al shape memory alloys

Using two similar high resolution computer controlled stress-strain-temperature set-up of equivalent resolution (1 mN, 0.1 μm, 5 mK) the detailed study of the martensitic transformation in single crystals of the Cu−Zn−Al shape memory alloys is realized. The devices can obtain 20 or 150 N in applied force, 2 or 4 mm in length and can be operated near room temperature (between 280 and 360 K). The analysis of the hysteresis domain in single crystals clearly visualises the intrinsic characteristics of the material (pseudoelasticity, nucleation, interface friction) and enables the obtenton of parameters for physical models of the hysteretic behaviour in force—lengthening—temperature and, eventually, time-dependent processes. The observation of time evolution shows the ‘recoverable martensite creep’ associated to a microstabilization process.