High Martensitic Transformation Research Articles

Martensitic transformation and shape memory properties of Ti–Ta–Sn high temperature shape memory alloys

Abstract The effects of Ta and Sn contents on the martensitic transformation temperature, crystal structure and thermal stability of Ti–Ta–Sn alloys are investigated in order to develop novel high temperature shape memory alloys. The martensitic transformation temperature significantly decreases by aging or thermal cycling due to the formation of ω phase in the Ti–Ta binary alloys. The addition of Sn is effective for suppressing the formation of ω phase and improves stability of shape memory effect during thermal cycling. The amount of Sn content necessary for suppressing aging effect increases with decreasing Ta content. High martensitic transformation temperature with good thermal stability can be achieved by adjustment of the Ta and Sn contents. Furthermore, the addition of Sn as a substitute of Ta with keeping the transformation temperature same increases the transformation strain in the Ti–Ta–Sn alloys. A Ti–20Ta–3.5Sn alloy reveals stable shape memory effect with a martensitic transformation start temperature about 440 K and a larger recovery strain when compared with a Ti–Ta binary alloy showing similar martensitic transformation temperature.

Thermal behavior of Mullite–Zirconia–Zircon composites. Influence of Zirconia phase transformation

Mullite–Zirconia–Zircon composites have proved to be suitable for high-temperature structural applications, with good mechanical and fracture properties and good thermal shock resistance. In this paper, the special dilatometric behavior of a series of Mullite–Zirconia–Zircon (3–40 vol.% ZrO2) composites is evaluated and compared with that of a pure Zircon material and explained in terms of the high Zirconia linear thermal expansion coefficient (α) and Zirconia martensitic transformation. Linear thermal expansion (α) up to 1273 K is studied and correlated with the phase composition of the composites; a linear correlation was found with the m-ZrO2 content evaluated with the Rietveld method. Zirconia (m-ZrO2) dispersed grains containing ceramics material showed a hysteresis in a reversible dilatometric curve (DC). The martensitic transformation temperatures could be evaluated and then compared with the endothermic and exothermic peaks temperatures obtained from the differential thermal analysis (DTA). Furthermore, the hysteresis area was correlated with m-ZrO2 content, where composites with less than 10 vol.% ZrO2 did not show this behavior, and from this content up to 40 vol.% of ZrO2 a linear increase of the hysteresis area was found.

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.

Composition-dependent elastic properties and electronic structures of off-stoichiometric TiNi from first-principles calculations

The composition-dependent elastic properties and electronic structure of off-stoichiometric TiNi with a B2 structure are investigated by using the first-principles exact muffin-tin orbitals method in combination with coherent potential approximation and first-principles plane-wave pseudopotential method (for computing bonding charge densities). The Zener anisotropy, c 44/ c′, increases with increasing Ni contents, but is quite small, indicating a strong correlation between the softening of c 44 and c′ with decreasing temperature during martensitic transformation (MT). For the Ni-rich TiNi, c 44 increases with increasing Ni content whereas c′ decreases. On the Ti-rich side, both c 44 and c′ are insensitive to the composition. It was observed that larger c 44 corresponds to higher MT temperature, and the composition dependence of elastic modulus is discussed on the basis of the bonding charge densities and electronic density of states. We propose that the strong composition dependence of the elastic modulus of the Ni-rich TiNi can be attributed to the Coulomb static electronic repulsion between the antisite Ni atoms and their surroundings. The insensitivity of the elastic modulus of the Ti-rich TiNi to the composition is due to the absence of such repulsion between the Ti antisites and their nearest neighbors.

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]

Sputter alloying of Ni, Ti and Hf for fabrication of high temperature shape memory thin films

In the present paper, the fabrication and characterisation of typical high temperature Ni(Ti+Hf) alloyed thin films produced by simultaneous sputter deposition from separate elemental Ni, Ti and Hf targets are presented. Film composition, determined by energy dispersive X-ray spectroscopy, was controlled by adjusting the ratio of powers applied to each target. Films deposited at room temperature had an amorphous structure and subsequent annealing at 550°C was carried out in a high vacuum environment, based on crystallisation temperature evaluation by differential scanning calorimetry (DSC). High temperature martensitic transformation, confirmed by DSC and variable temperature X-ray diffraction (XRD), was achieved by deposition of (Ti+Hf) rich Ni–Ti–Hf films. Any slight change of composition towards Ni rich reduced the transformation temperature. Atomic force microscopy and XRD illustrated that the films had a fine grain structure (∼100 nm). One way shape memory effect was observed at ∼200°C in a film with composition of 15·6 at.-%Hf.

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.

Effect of long term, high temperature aging on luminescence from Eu-doped YSZ thermal barrier coatings

The use of the luminescence from europium doped yttria-stabilized zirconia thermal barrier coatings for non-contact thermometry has been previously studied up to 850 °C [Choy, K.L. et al., Surface Engineering, 16 [6] (2000) 496] and 1150 °C [M.M. Gentleman and D.R. Clarke, Surface and Coatings Technology, 188–189 (2004) 93]. In this contribution we examine the effect of long-term, high temperature aging and martensitic phase transformation on both the details of the luminescence spectrum and the temperature-dependent lifetime of the luminescence. It is found that even after prolonged aging, the wavelength of the 5D 0 → 7F 2 emission peak shifts only very slightly with increasing percentage of transformation to the monoclinic phase. Preliminary data suggests that the effect on the decay lifetime up to 1150 °C is very slight, expressed in a slight shift of the onset of thermal quenching to lower temperatures; the slope of the characteristic decay time-constant versus temperature appears relatively unaffected.

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.

Effect of atomic order on the martensitic transformation of Ni–Fe–Ga alloys

The effect of atomic order on martensitic transformation (MT) temperatures has been studied by differential scanning calorimetry in three polycrystalline alloys with compositions Ni53.5+x–Fe19.5−x–Ga27.0 (x = 0, 0.5 and 1.5). Samples water quenched from different temperatures (470–1070 K) exhibit higher MT temperatures than ones slow cooled from the same temperature. This effect has been ascribed to a decrease of the L21 degree of order of the austenitic phase, which promotes an increase in the MT temperatures in these Ni–Fe–Ga alloys. The differences in ordering with cooling rate have been qualitative confirmed by electron diffraction patterns.

Crystal structures and textures of hot forged Ni48Mn30Ga22 alloy investigated by neutron diffraction technique

A ferromagnetic shape memory alloy of Ni48Mn30Ga22 prepared by induction melting was successfully hot forged. Strong textures and a large anisotropy of in plane plastic flow were developed during the hot forging process. The crystal structures, both in austenitic and martensitic states, were investigated by means of neutron powder diffraction technique. The result suggests that Ni48Mn30Ga22 has a cubic L21 Heusler structure at room temperature, the same as that in the stoichiometric Ni2MnGa. When cooled to 243 K, the Ni48Mn30Ga22 alloy changes into a seven layered orthorhombic martensitic structure. No substantial change of the neutron diffraction pattern was observed upon further cooling to 19 K, indicating that there is no intermartensitic transformation in the investigated alloy, which is different from the transformation processes in the Ni–Mn–Ga alloys with higher martensitic transformation temperatures.