High Martensitic Transformation Temperature Research Articles

Development of Polyimide/SMA Thin-Film Actuator

Ti-Ni-Cu shape memory alloy (SMA) thin films were sputter-deposited on heated polyimide substrates. The (Ni,Cu) rich Ti-Ni-Cu films deposited at a substrate temperature of 543 K were found to possess a high martensitic transformation temperature above room temperature over a wide range of Cu content from 7 to 23 at%, which allows stable production of actuators that operate at room temperature. Additional deposition of a Cu film onto the Ti-Ni-Cu films facilitated the soldering of wires onto the actuators and also decreased the power consumption and response time of the actuator. The force of a polyimide/Ti-Ni-Cu SMA actuator could be increased merely by increasing the thickness of the polyimide film. An actuator composed of a 125 m thick polyimide film and an 8 m thick TiNiCu film was able to lift a 13.5 g weight. Furthermore, a Ti-Ni-Cu film could be pattern etched on a polyimide film to produce a circuit. The results indicate that a polyimide/SMA film actuator is a promising simple actuator that can be produced by simply cutting out an appropriately shaped piece with scissors or by punching and then connecting the two edges to a battery by soldering.

Structural ordering tendencies in the new ferromagnetic Ni–Co–Fe–Ga–Zn Heusler alloys

In search for new ferromagnetic shape memory alloys (FSMA) we have calculated structural energy differences, magnetic exchange interaction constants and mixing energies of quaternary (X1X2)YZ Heusler alloys with X1,X2,Y =Ni,Co,Fe and Z=Ga, Zn using density functional theory. The comparison of the energy profiles of (NiCo)FeZ, (FeNi)CoZ, and (FeCo)NiZ with Z=Ga and Zn as a function of the tetragonal distortion c/a reveals that the energetically preferred ordering type is (NiCo)FeGa and (NiCo)FeZn which shows that Fe prefers to occupy the same cubic sublattice as Ga or Zn what implies that Fe favors Co and Ni as nearest neighbors, respectively. The Curie temperatures of (NiCo)FeGa and (NiCo)FeZn are high of the order of 600K. (NiCo)FeGa, which has the same valence electron concentration (e/a=7.5) as Ni2MnGa and also possesses a high martensitic transformation temperature (>500K), is of interest for future magnetic shape memory devices.

Evaluation of Magnetostriction of the Single-Variant Ni-Mn-Ga Martensite

The temperature dependence of ordinary magnetostriction of the axially compressed Ni–Mn–Ga alloy with the low values of shear elastic modulus C'(T) ~ 1 – 10 GPa has been evaluated theoretically in the framework of Landau theory. The computations showed that the compression with 50 MPa stress reduces the ordinary magnetostriction by factor 3 at room temperature. Nevertheless, the magnetostriction of compressed alloy exceeds the value of 10–4 in the whole temperature range of martensitic phase stability, strongly depends on the temperature in the vicinity of martensitic transformation (MT), and is practically constant well below MT temperature. Therefore, the purposeful search for the alloy with the low value of shear elastic modulus and high MT temperature (well above 300 K) may result in the discovery of good magnetostrictive material. This material will posses the temperature-independent magnetostriction value about of 10–4 –10–3 and rather low electric conductivity enabling the technical applications of this material in dynamic regimes.

Internal friction of Cu–13.5Al–4Ni shape memory alloy measured by dynamic mechanical analysis under isothermal conditions

Cu–13.5Al–4Ni shape memory alloy (SMA) exhibits a β 1(DO 3) → β 1′ (18R) internal friction peak with high damping capacity and elevated martensitic transformation temperature in a dynamic mechanical analysis tan δ cooling curve. When the specimen is isothermally maintained at peak temperature, the damping capacity decreases significantly and reaches a steady value. The inherent and intrinsic internal frictions of Cu–13.5Al–4Ni SMA are extremely low because the β 1′ (18R) martensite has an ordered 9R structure with stacking faults rather than twinning with movable twin boundaries.

Martensitic transformation of TiNiPd high-temperature shape memory alloys: A first-principles study

Heat of formation, elastic property and electronic structure of TiNiPd high-temperature shape memory alloys have been investigated by first-principles calculations using the pseudopotentials plane-wave method. The results show that the heat of formation difference between austenite and martensite plays an important role in the martensitic transformation. The effect of Pd content on the martensitic transformation temperature and transformation type is clarified based on the elastic constants of the B2 phases. High martensitic transformation temperature can be attributed to a low shear resistance C ′. Furthermore, the mechanism of the effect of Pd addition on elastic constants is explained on the basis of the electronic structure.

Giant magnetic-field-induced strain in NiMnGaSi magnetic shape memory alloy prepared by diffusion-reduction

NiMnGaSi alloys were prepared by diffusion-reduction reacting which is a new and simple method. The effect of small mount of Si addition on the martensitic transformation temperature (Tm) and strain was investigated. It was found that the addition of Si caused a stronger magnetic exchange interaction leading to higher Curie temperature and giant magnetic field-induced strain. The typical Ni43.03Mn32.07Ga18.48Si6.43 alloy shows high martensitic transformation temperature (Tm) of 300K and a 0.56% giant strain measure at a magnetic field of 0.7 T.

Shape memory behaviour of annealed Ti48.5Ni(51.5− x )Cu x (x = 6.2–33.5) thin films

The shape memory behaviour of (Ni,Cu)-rich Ti–Ni–Cu thin films (Ti48.9Ni44.9Cu6.2, Ti48.5Ni40Cu11.5, Ti48.6Ni35.9Cu15.5, Ti48.3Ni28.4Cu23.3, Ti48.3Ni23.9Cu27.8 and Ti48.5Ni18Cu33.5) annealed at 773, 873 and 973 K for 1 h was investigated. The films with 6.2, 11.5–15.5 and 23.3–33.5 at% Cu showed a single-stage deformation due to a B2 ↔ B19′ transformation, a two-stage deformation due to the B2 ↔ B19 ↔ B19′ transformation and a single-stage deformation due to the B2 ↔ B19′ transformation, respectively. The martensitic transformation start temperature (M s) increased with increasing Cu content and then levelled off for more than 15 at% Cu, indicating a high Ms temperature of 345 K. Temperature hystereses were almost 15 K for all films with more than 10 at% Cu. The critical stress for slip increased with increasing Cu content and increased significantly for the Ti48.5Ni18Cu33.5 film, whereas the maximum recoverable strain significantly decreased for the Ti48.5Ni18Cu33.5 film. With decreasing annealing temperature, the critical stress for slip increased, but the M s temperature decreased. It was found that films with 11.5 at% Cu or more, annealed at 873 K, showed a high martensitic transformation temperature and a high critical stress for slip.

Elastic property and electronic structure of TiNiPt high-temperature shape memory alloys

Elastic property and electronic structure of TiNiPt high-temperature shape memory alloys have been investigated by first-principles calculations using the pseudopotentials plane wave method. The effect of Pt content on the martensitic transformation temperature and transformation type is clarified based on the elastic constants of the B2 phases. It is found that high martensitic transformation temperature is due to a low shear resistance C ′ . Furthermore, the electronic structure mechanisms behind the elastic properties are discussed based on the density of states.

Ti–Ni–Cu shape-memory alloy thin film formed on polyimide substrate

Ti–Ni–Cu shape-memory alloy (SMA) thin films were sputter-deposited on heated polyimide substrates. Ti–Ni–Cu films deposited at substrate temperatures of 543 and 583 K were found to be crystalline. Especially, a Ti 48Ni 29Cu 23 film deposited at 583 K exhibited a high martensitic transformation temperature above room temperature and a narrow transformation temperature range, which enable the film to be used at room temperature. Double-beam cantilevers made of 8 μm thick Ti 48Ni 29Cu 23 films deposited on 12.5 and 25 μm thick polyimide substrates displayed a repeatable shape-memory effect by a battery of 1.5 V and it was verified that the composite film consisting of an 8 μm thick Ti 48Ni 29Cu 23 film and a 25 μm thick polyimide film is capable of moving 0.18 g wings of a dragonfly toy up and down. These results offer the prospect for using an SMA/polyimide actuator as a convenient small actuator, which will find wide-ranging applications.

Diffusion Bonding of Co to TiAu High Temperature Shape Memory Alloy

We propose a high-temperature actuator composite material composed of a high temperature shape memory alloy (HTSMA) TiAu with a high martensitic transformation temperature (Ms ¼ 880 K) and ferromagnetic cobalt with a high Curie temperature (TC ¼ 1388 K). This actuator material can be driven by magnetic field in a bending mode due to ferromagnetic force acting on the Co-layer and generates a large actuation strain which originates from the HTSMA. The purposes of this work are (1) to fabricate the composite materials laminated as TiAu/Co/TiAu by a diffusion bonding method through hot pressing, (2) to characterize the microstructure near the bonding interface between TiAu and Co layers, (3) to evaluate growth behavior of the diffusion layer, and (4) to determine the optimum condition for the fabrication. The composite materials were fabricated by hot pressing at 1073, 1173 and 1273 K for 10 h. The bonding interface between TiAu and Co was observed by a scanning electron microscope and concentration profiles were measured by an energy-dispersive X-ray spectroscopy. In order to evaluate the growth behavior of the diffusion layer, the TiAu/Co composites were aged at 773, 1073, 1173 and 1273 K for 24 h. It was found that, after the hot pressing, TiAu and Co layers were successfully bonded, and that two reactant intermetallic compounds were formed near the TiAu/Co interface. The intermetallic compounds were identified to be C11b Ti(Au,Co)2 and C36 (Ti,Au)Co2. As for the growth behavior, the thickness of the diffusion layer was not changed by aging at 773 K. However, the thickness was increased by increasing the aging temperature above 1073 K. The apparent activation energy for the growth of the diffusion layer was estimated to be 280 � 20 kJ/mol in a temperature range of 1073–1273 K. Using the values of the activation energy and the diffusion constant, the thickness of the diffusion layer was predicted to be sufficiently thin: 12 mm by the hot pressing at 1073 K for 10 h. This predicted value was in good agreement with the experimental result of 7 mm. [doi:10.2320/matertrans.MAW200862]

High-temperature shape memory alloys based on the RuNb system

Many applications of shape memory alloys (SMAs) require the development of alloys with high martensitic transformation (MT) temperatures. Among the different systems for high temperature SMAs, equiatomic RuNb alloys demonstrate both shape memory effect (SME) and MT temperatures above 800 °C. This work investigates Ru 50− x Nb 50+ x (at.%) alloys and shows that Nb content significantly affects the MT behavior. Alloys near the equiatomic composition ( x = 0, 2, 4) undergo two displacive transformations on cooling: β (B2) → β′ (body centered tetragonal) → β′′ (monoclinic). The Ru 45Nb 55 alloy exhibits a single transition from cubic to tetragonal on cooling. This MT gives rise to a highly twinned microstructure with a (0 1 1) compound-twinning mode and is considered to be responsible for the SME in both types of alloys. The reorientation of martensite variants during deformation has been confirmed through scanning electron microscopy of compression specimens. A promising shape memory behavior is obtained through three-point bend tests performed both in the β′ and β′′ phases.

Thermal stability of the martensitic transformation of Cu–Al–Ni–Mn–Ti

The development of new shape memory alloys with high martensitic transformation temperature increases the potential for applications. The development and use of these new alloys depends on the stability of the structure during cycling at high temperatures. If it is possible to guarantee that on alloys keeps the structure during cycling, then the alloy can be used because of the shape memory properties. The aim of this work is to obtain a kinetic model of the forward and backward martensitic transformation of two Cu–Al–Ni–Mn–Ti alloys. Differential scanning calorimetry has been performed in order to establish the kinetic stability of the martensite and the beta transformation.

First-principles study on alloying effect on martensitic transformation behavior of TiNi alloy

The alloying effect on the martensitic transformation behavior of TiNi alloy is investigated for the first time from the elastic properties and electronic structure by the plane-wave pseudopotential method within the local density approximation. The elastic constants and density of states are calculated. The results show that the alloying effect on the martensitic transformation behavior of TiNi alloy is close related to its elastic properties. The Ti d density of states at Fermi level is mainly responsible for the B2 phase stability of TiNi alloy with the low addition of the third element. As for TiNiPd ternary alloy, the high martensitic transformation temperature stems from the stabilization of B19 phase TiNiPd by the high addition of Pd content.

High temperature Cu-Al-Nb - based shape memory alloys

New type of Cu-Al-Nb - base alloys containing nickel, cobalt and chromium with high martensitic transformation temperatures have been developed. In this paper the mechanical properties, shape memory characteristics, the temperatures of reversible martensitic transformation and thermal stability of these alloys are presented. Introducing chromium and specially nickel and cobalt to the Cu-Al-Nb alloys gives a possibility to decrease the characteristic temperatures. However, these elements create in the martensitic matrix primary particles of intermetallic phases such as AlCo and NiAl and change the chemical composition of the Nb(,Al,Cu) 2 - the main phase in the ternary Cu-Al-Nb alloy. Presence of precipitates results in decrease of the elongation of these alloys. Nickel and cobalt improve the thermal stability and resistance to decomposition.

Martensite aging effect in a Ti 50Pd 50 high temperature shape memory alloy

Ti-Pd alloy system is one of the potential high temperature shape memory alloys due to its high martensitic transformation temperatures. Thus, many researches including shape memory characteristics, martensitic transformations and mechanical behavior of the alloys have been done in recent yeas. However, martensite aging effect in the alloy, which is an important issue as to the stability of martensite at high temperature, has not been reported yet. Ti{sub 50}Pd{sub 50} transforms from B2 parent phase to B19 martensite upon cooling, and M{sub s} is 823 K (25) and T{sub m} is 1,673 K (26). Thus M{sub s}/T{sub m} ratio of the alloy is about 0.49, and the alloy may show strong martensite aging effect according to the above proposal. It is now of interest to examine whether the Ti{sub 50}Pd{sub 50} alloy show martensite aging effect. As will be shown, the Ti{sub 50}Pd{sub 50} alloy indeed shows the aging effect, as expected; however, the aging effect of this alloy exhibits a unique feature, which is not found in other shape memory alloys.

Relación estructura-propiedades en aleaciones inteligentes con memoria de forma Cu-Al-Ag de alta temperatura de transformación martensítica termoelástica

New shape memory alloys have been developed in the last few years with high martensitic transformation temperatures. However, the highest martensitic transformation temperatures are lower than 200°C and they are achieved with very complex alloys. New Cu-Al-Ag shape memory alloys have been developed with martensitic transformation temperatures ranging from 250 to 400°C. It is really interesting to increase the knowledge of the phase stability behaviour when cycling the alloy above or below the martensitic transformation temperature because of the high temperatures involved. Two Cu-Al-Ag shape memory alloys with very different composition have been thermal cycled in a DSC following the amount of energy release during the forward and reverse transformation and the changes of the martensitic transformation temperatures in order to follow the phase changes that takes place during cycling.

A New Cu-Based SMA with Extremely High Martensitic Transformation Temperatures

A new series of high temperature copper based shape memory alloys has recently been patented. These alloys contain 8-20 wt% Al, 1-20 wt% Ag, 0-2 wt% of a minor element (preferably Co), balance copper. The martensitic start transformation temperatures of these alloys are above 200 o C and, in some cases, they have good high temperature stability and may be useful in commercial applications where higher operating temperatures than those obtained from Cu-Zn-Al and Cu-Al-Ni memory alloys are required