AuCd alloy is a typical martensitic alloy. The crystal structural of the ζ2' (trogonal) martensite was recently determined and the analysis indicated the possibility of phonon softening. Despite a large absorption coefficient of Cd phonon softening was actually observed by using an isotope 114Cd. In the present study, precise x-ray scattering studies were performed with a rotating anode x-ray source and synchrotron radiation at Photon Factory in KEK, using small crystals. The structure factors of the parent phase and the diffraction profiles along <110> direction were measured with four circle diffractometers. The structure factors decrease when approaching the transformation temperature. Superstructure reflections were observed prior to the onset of the transformation. They are very weak and rather sharp and appear at q = (1/3,1/3,0) within the experimental error, i. e. at the commensurate position.

The origin of the rubber-like behavior in mono-domain Au-Cd martensite was explained in terms of a new model that focused attention on the change of long-range elastic interaction energy among vacancies during a domain reversion. Vacancies in martensite, the lower-symmetry phase, produce stress fields with lower symmetry. During martensite aging, vacancies tend to rearrange themselves to lower elastic interaction energy. The low-symmetry elastic field results in a low-symmetry vacancy configuration. When a stabilized martensite domain reverts to a new domain (twin) under external stress, the original vacancy configuration is inherited to the new domain, but such a configuration becomes a high energy configuration because of the lower symmetry of elastic field, and thus it tends to restore the original configuration by reverse twinning. The above vacancy reconfiguration model is consistent with the fact that the rubber-like behavior is closely related to vacancies.

Shape-memory alloys have the largest energy output per unit volume per cycle of known actuator systems [1]. Unfortunately, they are temperature activated and hence, their frequency is limited in bulk specimens. However, this is overcome in thin films; and hence shape-memory alloys are ideal actuator materials in micromachines[l]. The heart of the shape-memory effect lies in a martensitic phase transformation and the resulting microstructure. It is well-known that microstructure can be significantly different in thin films as compared to bulk materials. In this paper, we report on a theory of single crystal martensitic this films. We show that single crystal films of shape memory material offer interesting possibilities for producing very large deformations, at small scales.

During the superelastic deformation process, because of the involvement of the austenite-martensite phase transformation, a superelastic wire will experience a self-heating or self-cooling process due primarily to the latent heat of the material. As the strain rate increases, and conditions become more adiabatic, the self-heating and self-cooling will cause a temperature rise upon loading and will drop upon unloading. As a consequence, an apparent effect of the strain rate on the superelastic behavior in the shape-memory alloys with a large diameter or more adiabatic conditions will be noticed. In the present paper, a constitutive stress-strain-strain rate equation is proposed to describe the self-heating behavior. In order to verify the model, a series of experiments have been conducted to study the effect of the strain rate, wire diameter, and adiabatic condition on the superelastic behavior of the shape-memory alloy wire. As will be shown later, the proposed equation can predict the behavior of the superelastic wire accurately, and the prediction is in good agreement with the experimental data.

The 45Ni50Ti5Cu shape memory alloy exhibits one single thermoelastic martensitic transformation from the parent B2 phase to monoclinic B19’and looks as an appealing system to be used for the two way memory effect (TWME). First results, appeared in literature, on the TWME in 45Ni50Ti5Cu indicate that in this alloy high figures for the two way strain can be obtained. The prestrain method is here investigated to imprint the TWME in a 45Ni50Ti5Cu alloy: a pseudoplastic deformation higher than the reorientation strain is applied in martensite, followed, after unloading, by a recovery of the one-way strain by heating to 110°C. The two way memory strain is detected in the following thermal cycle. Results show the two way memory strain can be imprinted in just one cycle, but it can even considerably be increased by repeating the procedure: the highest figure for the two way memory strain is however obtained whenever a plastic unrecoverable strain is accumulated in the specimen in some prestraining cycles. Often the maximum two way strain is of the same order as the plastic strain and the former decreases with the increase of the latter.

Thin films of thermoelastic titanium-nickel are of interest as a material basis for force-producing elements in microelectromechanical systems, and for active phases in mechanically-adaptive composite materials. The successful introduction of this material system into such application areas will depend on development of reliable thin film deposition protocols, together with the refinement of analytical models which successfully predict the response of active microstructures to a variety of dynamic thermal and mechanical stimuli. In the present paper we review some of our recent experimental and theoretical work which bear on these problems. With respect to thin film fabrication techniques we focus on problems of composition control and the manipulation of microstructure, with particular emphasis on opportunities afforded by amorphous precursor phases formed during low temperature processing, and the fine-grained, thermally stable crystalline microstructures obtainable using hot-substrate deposition. The films resulting from either approach retain the important thermomechanical response features of the well-known bulk-alloy system: shape memory and transformational superelasticity. The response can be modeled in terms of a continuum description augmented with internal variables that track fractional partitioning of the material between austenite and variants of the martensite.

We discuss a comprehensive design approach of Ti-Ni alloy coil springs and introduce a new application of the R-phase transformation. In order to attain high cyclic performance, one must understand the two relationships between design parameters and material characteristics and between material characteristics and cyclic performance. Metallurgical parameters and coil spring dimensions play an important role as design parameters in the former relationship. High cyclic performance of an actuator is closely related to the suppression of the monoclinic martensite. Transformation temperatures and their stress dependence is of primary importance as material characteristics in the latter relationship. A thermostatic mixing valve, which is the latest application of the R-phase transformation in Japan is then discussed as a new type of a shape memory alloy actuator. The R-phase transformation is employed to achieve not only a long cycle life but a linear operation with the set temperature to continuously control the mixing ratio of hot and cold water. This is achieved by changing the total length of the two-way actuator in a linear manner with the set temperature. The linear characteristic is satisfied between 35–50°C by optimizing thermomechanical treatment and the dimensions of Ti-Ni and biasing coil springs.

The composition of an Fe-Mn-Si-Cr-Ni stainless steel with shape memory effect was altered in this work in order to increase the strength of the alloy. The alloy possessed a low yield strength which is a major draw back for structural applications.

Nitrogen alloying, using nitrogen pressurized melting (P=l–10 atm), was employed to introduce a nitrogen concentration of up to 0.36 wt%. The effect of nitrogen alloying on shape memory effect was studied through mechanical testing. It was found that nitrogen alloying increased the hardness; however, nitrogen as an interstitial alloying element suppressed the γ⇒ε transformation and therefore decreased the shape memory effect.

Introducing small amount of Nb (e. g., 0.36 wt%) to the nitrogen containing alloys caused formation of NbN. The NbN compound was in the form of globular dispersed particles (200 nm) which increased the strength of the alloy without significantly changing the shape memory effect.

Because of its optimal radiopacity, superelasticity and shape memory properties Nickel-Titanium (NiTi) is an ideal material for the fabrication of stents. Indeed, these properties can facilitate the implantation and precise positioning of those devices. However, in vitro studies on NiTi report the dependency of the alloy biocompatibility and corrosion behavior to surface treatments. Oxidation of the surface seems to be very promising to improve both the corrosion resistance and the biocompatibility of NiTi. The present study investigate the effect of electropolishing, heat treatment (in air and in a salt bath) and nitric acid passivation to modify the oxide layer on NiTi stents. Techniques such as potentiodynamic polarization tests, Scanning Electron Microscopy (SEM) and Auger Electron Spectroscopy (AES) have been used to develop relationships between corrosion behavior, surface characteristics and surface treatment. Results show that all surface treatments improve the corrosion behavior of the alloy. SEM results indicate that treated stents which exhibit a smooth and uniform surface show a higher corrosion resistance than non treated stents which possess a very porous oxide layer. AES results, indicate that the best corrosion behavior was observed for the stents which exhibit the thinnest oxide layer (electropolished and passivated samples).

In the present work the use of single-crystals of Cu-Zn-Al Shape Memory Alloys (SMA) in actuator applications is analyzed. 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 SMA coupled to a conventional spring that represents the load to be displaced. A special experimental stage was designed for performing controlled thermal cycles under load. In this way the effects of different parameters (cycle number, friction, temperature range, load level) on the actuator behavior can be studied. From the results obtained, the use of a single-crystal of an SMA in a thermostatic device is analyzed and compared with the commercial wax actuator performance.

Phase transformations and crystallography of twins in near-equiatomic Ti-Pd alloys have been studied. In the first half we have found that the transformation temperature decreases with decreasing the Pd contents and the successive transformations take place in the Ti-rich alloys. While the transformation temperature is nearly constant with the composition and a single transformation takes place in the equiatomic and Pd-rich alloys. The solubility limit of TiPd compound in Ti-rich side is extended to about 55 at%Ti at 900°C and abruptly decreased with decreasing temperature. In the latter half we have found three twinning modes, i.e., {111} Type I, <121> Type II and {101} compound twins, in the martensite. The {111} Type I and <121> Type II twinnings which are conjugate to each other coexist in the same variant. The {111} Type I twins are dominantly observed and the <121> Type II twins are less frequently observed. The former twinning is considered to be a lattice invariant shear. There is no martensite variant consisting wholly of the <121> Type II twins throughout the present observation. The <121> Type II twinning is considered to be a deformation twin due to the elastic interaction during the transformation. The {101} compound twinning is also considered to be a deformation twin, since the twin has an isolated fashion in the martensite variant. The boundary structure of the above three twinning modes was also discussed on the basis of lattice image.

Since the fatigue life time is a serious problem of the Cu-Al-Ni alloy, it is difficult to apply for practical use. The authors have investigated the influence of the cooling condition of liquid-quenching on fatigue property. The liquid-quenching is performed by a piston-anvil apparatus. It is easy to control the solidification condition. The fatigue life time increases with increasing the cooling rate. The long life time is due to the small grain size, the small volume of γ1' phase, the low density of lattice defects and the low transformation temperature.

On the Ti-50.8%Ni, Ti-51%Ni and Ti-51.3%Ni (at.%) single crystals, the investigations of the shape memory effects (SME) and superelasticity (SE) have been carried out in order to find out the dependence on a size and a volume fraction of the Ti3Ni4 dispersive particles, a crystal orientation and a sign of applied stress. It is shown, that in crystals without particles the strong dependence of SME on an orientation and a sign of applied stress takes place and SE is not observed. Precipitation of the coherent particles of Ti3Ni4 of 50–100 nm in size leads to the reduction of SME, weakening of orientation dependence of σcr interval of stress-induced martensitic transformations (MT).

It was experimentally shown that the achievement of a high deforming stress level due to dispersion hardening and solid solution strengthening of FCC single crystals with a low stacking-fault energy leads to the deformation mechanism changing from slip to twinning, the dependence of mechanical properties on a crystal orientation and a sign of applied stresses. During deformation by twinning at T<150–300K effects of pseudoelasticity associated with elastic twinning is observed.

The dependence of shape memory effect (SME) and pseudoelasticity (PE) on crystal orientation and on sign of applied stresses (tension/compression) has been investigated on Ti-40%Ni-10%Cu (at.%) single crystals. It have been shown that both SME and PE depend on an orientation and an applied stress sign. Asymmetry effects were found at deformation in high-temperature B2 phase and in the range of stress-induced martensite transformations.

Acoustic emission (AE) signals generated during the martensitic transformation in an Fe-24at%Pt alloy with various ordering treatments have been measured, and the AE parameters have been correlated with phase transformation events. AE events are generated during cooling of specimens, which is the first detection of the AE events associated with the martensitic transformation in Fe-Pt alloys. In all specimens, AE events were observed at higher temperature than the Ms temperatures determined by electrical resistivity measurement. AE events started to increase abruptly for the weakly ordered specimen with decreasing temperature. In contrast, AE events increased gradually for the highly ordered specimen with decreasing temperature. This is consistent with the nature of the transformation behavior in this alloy. The critical defect size of weakly ordered and highly ordered specimens were calculated using Ms temperature determined by AE technique. Results obtained in this investigation strongly suggests that the AE technique is useful for analysis of transformation kinetics in this alloy.

The effect of martensitic reverse transformation on recovery • recrystallization process in cold rolled Ti-Pd-Ni high temperature shape memory alloys has been investigated systematically by flash heating treatment, micro-Vickers hardness test, differential scanning calorimetry and transmission electron microscopy. It was found that the temperatures of softening in hardness after flash heating treatments agree well with the reverse transformation temperatures in the present alloys, and most of the softening occurs within 60 seconds when annealing temperature is raised to above the reverse transformation temperature. We conclude that the recovery • recrystallization process is controlled by the reverse transformation. The reasons are considered based on the large difference in atomic diffusion rate in the parent phase and in the martensite.

6061 Al-matrix composite with TiNi shape memory fiber as reinforcement has been fabricated by vacuum hot pressing to investigate the microstructure and mechanical properties. The yield stress of this composite increases with increasing amount of prestrain, and it also depends on the volume fraction of fiber and heat treatment. The smartness of the composite is given due to the shape memory effect of the TiNi fiber which generates compressive residual stresses in the matrix material when heated after being prestrained. Microstructual observations have revealed that interfacial reactions occur between the matrix and fiber, creating two intermetallic layers. The flow strength of the composite at elevated temperatures is significantly higher than that of the matrix alloy without TiNi fiber.

New smart hybrid composite with TiNi shape memory alloy (SMA) fiber as reinforcement is designed to be composed of TiNi fiber and carbon/epoxy (CFRP) composite. One-dimensional composite modeling is utilized to evaluate residual stresses of the smart composite generated by shape memory effects. The modeling indicated that the tensile strength of TiNi/CFRP composite is improved due to the shape memory effect of TiNi fiber for both parallel and perpendicular configurations of TiNi fiber with respect to the direction of carbon fiber. Modeling also shows the tensile strength is increased with increasing both volume fraction of TiNi fiber and amount of prestrain.

Both austenite/martensite transformations, and martensite/martensite variant reorientation are central to superelasticity and shape memory actuation. Here we discuss a simple model for tracking these transformations that is appropriate for device modeling. The approach is to augment conventional continuum mechanical descriptions with internal variables that track fractional partitioning of the material between austenite and the various martensite variants. A three-species model involving austenite and two complementary martensite variants provides sufficient generality to capture the martensite variant distributions that underlie shape memory, and the strain-accommodation associated with superelasticity. Transformations between all of these species can be tracked on the basis of triggering algorithms that reflect both tran sformation hysteresis and the variation of triggering stress with temperature. The algorithm described here is for temperature-dependent response resolved in a single direction. It requires only the following experimentally determined parameters: the four transformation temperatures Mf, Ms, As, Af, the transformation strain, the Young's modulus, and the transformation latent heat.