Papers by Keyword: Shape Memory

Abstract: Since the matrix phase is transformed to martensitic phase in shape memory alloys (SMAs) during plastic deformation, complicated residual stresses may arise during deformation, and they may affect the shape recovery ability of the alloys. Thus, it is important to be able to characterize the residual stresses formed in SMAs during plastic deformation and annealing. In this study, X-ray diffraction was used to characterize the residual stress formed in a Fe-Mn-Si-Cr SMA, which was deformed in the tensile direction and subsequently annealed. The results showed that the compressive stress persisted in the tensile direction of the face-centered cubic (fcc) matrix upon tensile deformation and unloading. Compressive stress is believed to result from the hexagonal close-packed (hcp) phase formed during stress-induced martensitic transformation. After the deformed samples were annealed to recover their shapes, the residual stress was considerably reduced. This is believed to be due to the decrease in the formation of the hcp phase or to the recovery of their shapes during annealing. Our results indicated that residual stress in the fcc matrix phase is associated with the shape recovery characteristics of the alloys after martensitic and reverse martensitic transformations.

Abstract: In the present contribution several advanced electron microscopy techniques are employed in order to describe chemical and structural features of the nano- and microstructure of a Ni_45.5Ti_45.5Nb₉ alloy. A line-up of Nb-rich nano-precipitates is found in the Ni-Ti-rich austenite of as-cast material. Concentration changes of the matrix after annealing are correlated with changes in the transformation temperatures. The formation of rows and plates of larger Nb-rich precipitates and particles is described. The interaction of a twinned martensite plate with a Nb-rich nano-precipitate is discussed and the substitution of Nb atoms on the Ti-sublattice in the matrix is confirmed.

Abstract: In the present study we investigate the effect of ultrasonic treatment on the functional properties of Ti 50,4at.%Ni alloy after the processing in free and pre-deformed conditions. It is shown that the shape memory effect and its realization peculiarities depend on the parameters and conditions of ultrasonic influence. Ultrasonic treatment of preliminary deformed TiNi alloy in pre-deforming conditions promotes the reduction of strain restoring at the subsequent heating through the temperature interval of reverse martensite transformation. I. e. some setting of induced strain occurs during the ultrasonic treatment of material in pre-deforming conditions. In the specimens after ultrasonic treatment, it is observed the extension of shape recovery temperature range in the first thermal cycle, as well as the dependence of strain recovery temperature dynamics on the power of ultrasonic influence. Also it is established that ultrasonic treatment helps to increase the shape memory effect.

Abstract: The article represents results of influence of different severe plastic deformation (SPD) techniques on TiNi alloys. It is demonstrated that strength and shape memory effect (SME) of TiNi can be significantly enhanced due to formation of ultrafine-grained (UFG) and nanocrystalline (NC) structures by SPD. Influence of equal channel angular pressing (ECAP), high pressure torsion (HPT), multi-step SPD deformations (ECAP plus cold rolling) on structure, mechanical and functional properties of TiNi alloys is considered. There are represented first results of influence of equal channel angular pressure-Conform (ECAP-C) on TiNi alloys, which is a perspective technology for industrial fabrication of UFG metals and alloys.

Abstract: A vibroisolation of a payload connected to a vibrating housing by two helical shape memory alloy (SMA) elements is considered. A microstructural theory is used for the simulation of the mechanical behavior of the SMA. The simulations have shown that the resonant frequency and the mitigation of the external vibrations depend on the shape memory alloy state. The maximum reduction of the acceleration amplitude for harmonic excitation is reached when the SMA is in the martensitic pseudoplastic state or in the two-phase state. Variation of temperature allows changing the resonance frequency and thus escaping from resonance. The acceleration of the payload at impact is the smallest when the SMA elements are in the pseudoelastic state.

Abstract: At room temperature, Fe-15Mn-5Si-9Cr-5Ni alloys are usually austenitic and the application of a stress induces a reversible martensitic transformation leading to a shape memory effect (SME). However, when a ribbon of this material is obtained by melt-spinning, the rapid solidification stabilizes a high-temperature ferritic phase. The goals of this work were to find the appropriate heat treatment in order to recover the equilibrium austenitic phase, characterize the ribbon form of this material and evaluate its shape memory behaviour. We found that annealing at 1050°C for 60 min, under a protective argon atmosphere, followed by a water quenching stabilizes the austenite to room temperature. The yield stress, measured by tensile tests, is 250 MPa. Shape-memory tests show that a strain recovery of 55% can be obtained, which is enough for certain applications.

Abstract: Kevlar fiber and carbon fiber reinforced shape memory epoxy composites were prepared respectively. The fold-deploy shape memory properties of the composite system were studied at the temperature which was 30°C higher than the glass transition temperature of resin. The results shows that shape fixed rate and shape recovered rate of the composites are decreased with the increase of Kevlar and carbon fiber volume. The shape fixed rate of Kevlar fiber reinforced shape memory composite is higher than that of carbon fiber reinforced shape memory composite, but the shape recovered rate of the former is much lower than the latter.

Abstract: Using our local equilibrium model of the martensitic transformation [ the elastic energy contributions, as the function of martensite volume fraction, ξ, in the phase transformation of single crystalline Cu-11.5wt%Al-5.0wt%Ni shape memory alloy were calculated from our measurements published earlier [. The derivative of the elastic energy δE/δξ=e (E is the total elastic energy stored/released during the austenite to martensite (AM) as well as MA transformation) could be calculated only irrespectively of the ST₀ term (T₀ is the equilibrium transformation temperature and S is the entropy change of phase transformation). But, since ST₀ is independent of ξ, the functions obtained reflect the ξ dependence of e as well as E quantities. From the DSC curves measured at zero uniaxial stress (σ = 0) [, the ξ-T hysteric loop was constructed. Then the e (ξ) curves at fix σ as well as fix T were calculated. The E values obtained from the integral of e (ξ), fit well to the E(σ) as well as E(T) curves calculated from the strain-temperature and stress-temperature curves measured in [.

Abstract: Thermoplastic polyurethane (TPU) nanocomposites were prepared using polycaprolactonediol as the soft segment, 4,4’-diphenylmethane diisocyanate as the hard segment, 1,4-butanediol and palm oil polyol. Nanoclay with certain weight percent (wt%) was reinforced as filler to improve both mechanical and shape memory behavior of the nanocomposites. Palm oil polyol was introduced in order to provide hyperbranched structure for better dispersion of filler in the matrix as well as aiding the crosslinking process. The experimental results showed that the mechanical and shape memory behavior of clay reinforced polyurethane nanocomposites were influenced by clay weight percent in the polymer matrix. TPU with 3 wt% clay showed optimum values of mechanical properties while the shape memory behavior decreases with increasing clay content.

Abstract: Recent research is showing that Shape Memory Alloys (SMA) can be advantageously employed for a number of applications in Rehabilitation Medicine and the related field of Neuroscience. This innovative use of SMA was investigated with the specific aim of improving the treatment approach to neurological patients with sequelae from stroke, traumatic brain injury, cerebral palsy, etc. Several examples of devices built for this purpose will be presented together with an outline of the reasons why the shape-memory and pseudoelastic effects can be regarded as interesting resources on account of scientific, technical and clinical reasons. In particular the design and functioning of an SMA-based ankle exerciser and pseudoelastic repositioning splints for the upper and lower limbs will be discussed in relation with results of neurophysiologic and clinical tests. The main observations so far suggest that this type of devices is able to support patients’ physical rehabilitation by adapting to changing conditions and needs during functional recovery. Furthermore, due to their improved tolerability relative to traditional treatments SMA devices can be used for longer times and tend to produce interesting effects in the control of spastic syndromes.