Showing papers on "Shape-memory alloy published in 2002"

Magnetic and martensitic phase transitions in ferromagnetic Ni–Ga–Fe shape memory alloys

Abstract: Ferromagnetic shape memory alloys with a body-centered-cubic ordered structure in a Ni–Ga–Fe system have been developed. The alloys with the composition range of Ni 27 at. % Ga (20–22 at. %)Fe exhibit a thermoelastic martensitic transformation from a B2 and/or an L21 parent to a martensite phase, with a seven-layer modulated (14M) and a five-layer modulated (10M) structure, in the ferromagnetic state. The parent phase transforms from the B2 to the L21 structure at about 970 K during cooling, and the degree of the L21 order in the parent phase is increased by annealing at 773 K, resulting in the increase of both the martensite starting and the Curie temperatures. The ductility of these alloys is improved by introducing of a small amount of a γ-phase solid solution. Consequently, we can conclude that the present alloys are promising for ferromagnetic shape memory alloys.

Magnetic and Martensitic Phase Transformations in a Ni54Ga27Fe19 Alloy

Abstract: The martensitic and magnetic transitions of Ni 54 Ga 27 Fe 19 alloy were investigated by differential scanning calorimetry and X-ray powder diffraction and with a vibrating sample magnetometer. The alloy is martensitically transformed from a L2 1 to a martensite phase with a 14M (7R) structure. The ferromagnetic transition is also accompanied by the martensitic transformation from a paramagnetic parent phase to a ferromagnetic martensite phase in the temperature interval between M s (= 293 K) and M f (= 274 K). The Ni-Ga-Fe system is promising as a ferromagnetic shape memory alloy.

Recovery of microindents in a nickel-titanium shape-memory alloy: A ``self-healing'' effect

Abstract: The thermally induced recovery of microscopic deformation in a nickel–titanium shape-memory alloy was examined. Surface deformation was simulated by indenting the alloy in the martensite phase at room temperature using both spherical and pyramidal indenters. We show that deformation in spherical microindents can be almost completely reversed by moderate heating. Partial recovery was observed for pyramidal impressions formed by a Vickers indenter and the recovery ratio was independent of the indentation depth. The observations were rationalized using the concept of representative strain and maximum stress under the spherical and pyramidal indenters.

Cyclic thermomechanical behavior of a polycrystalline pseudoelastic shape memory alloy

Abstract: In this work, 3D finite element modeling is employed to examine the thermomechanical behavior of a polycrystalline Ni–Ti shape memory alloy in the pseudoelastic regime. It is shown that the tension-compression asymmetry during uniaxial cyclic loading is due to a preferred orientation of the crystallographic texture. In pure shear loading, the thermomechanical behavior exhibits symmetry in both senses of shear, due to the fiber texture of the specimen bar stock. It is also shown that the apparent strain rate-dependence is due to thermomechanical coupling with latent heat generation/absorption during phase transformation.

Hysteresis in shape-memory alloys

Abstract: We present an overview of hysteresis phenomena in the martensitic transformation, and their relevance in the thermomechanical behaviour of shape-memory alloys. The first part of the paper introduces the concept of hysteresis, and the related phenomena of branching, dissipation and memory. The second part deals with revising some aspects of the thermomechanical behaviour of shape-memory alloys, emphasizing the hysteretic behaviour of single crystals and polycrystals under different driving conditions. The last part of the work is dedicated to the problem of modelling hysteresis phenomena in shape-memory alloys. Our focus is on phenomenological approaches which, as shown in the paper, account for the memory properties observed in hysteretic trajectories and open the possibility of deriving a generic energy balance for systems with hysteresis.

Mechanical Behavior of a Cu-Al-Be Shape Memory Alloy Under Multiaxial Proportional and Nonproportional Loadings

Abstract: This paper is concerned with the mechanical behavior of a Cu-Al-Be Shape Memory Alloy (SMA). A large experimental database, made of tension-internal pressure tests and biaxial compressive tests, is reported. Particular attention is paid to the behavior of the material under multiaxial proportional and nonproportional loadings. Moreover, these two types of complementary tests allow for the determination, on the one hand, of the shape of the initial surface of transformation onset and, on the other hand, of the initial direction of the transformation strain rate. A generalized macroscopic J 2 -J 3 criterion to describe the transformation onset is proposed and identified.

Phase Change Behavior of Nitinol Shape Memory Alloys

Abstract: Among other special characteristics Shape Memory Alloys (SMAs) have the ability to return to a predetermined shape when heated. In fact, the phase change of an existing element can strongly be influenced by thermal and thermo-mechanical treatments. Up to now, SMEs have been discovered in various materials, which can generally be classified into noble-metal based, Cu-based, Fe-based, Ni-Ti-based alloy systems and non-metallic SMAs. In this paper a general overview of the Ni-Ti system and a detailed review over the Ni-Ti-Cu system will be given with special regard to the influence of heat treatments upon the phase change behavior.

Thermal post-buckling and aeroelastic behaviour of shape memory alloy reinforced plates

Abstract: A novel concept is proposed: the use of shape memory alloy (SMA) to reduce panel thermal deflection and flutter responses. SMA has a unique ability to recover large pre-strains completely when the alloy is heated above the austenite finish temperature Af. The transformation austenite start temperature As for nitinol can be anywhere between -60 °F (-50 °C) and +340 °F (+170 °C) by varying the nickel content. During the recovery process, a large tensile recovery stress occurs if the SMA is restrained. The shape memory effect phenomenon is attributed to a change in crystal structure known as a reversible austenite to martensite phase transformation. This solid-solid phase transformation also gives a large increase in Young's modulus and yield stress. In this paper, a panel subject to the combined aerodynamic and thermal loading is investigated. A nonlinear finite element model based on the von Karman strain-displacement relation is utilized to study the effectiveness of an SMA-embedded panel on the flutter boundary, critical buckling temperature, post-buckling deflection and free vibration. The study is performed on an isotropic panel with embedded SMA. The aerodynamic model is based on the first-order quasi-steady piston theory. The dynamic pressure effect on the buckling and post-buckling behaviour of the panel is investigated by introducing the aerodynamic stiffness term, which changes the critical buckling temperature. Panels with SMA embedded in either the longer or shorter direction and either fully or partially embedded are investigated for post-buckling behaviour. Similarly, the influence of temperature elevation on the flutter boundary and vibration frequencies is investigated.

Modelling of laminated microstructures in stress-induced martensitic transformations

Abstract: This paper is concerned with micromechanical modelling of stress-induced martensitic transformations in crystalline solids, with the focus on distinct elastic anisotropy of the phases and the associated redistribution of internal stresses. Micro–macro transition in stresses and strains is analysed for a laminated microstructure of austenite and martensite phases. Propagation of a phase transformation front is governed by a time-independent thermodynamic criterion. Plasticity-like macroscopic constitutive rate equations are derived in which the transformed volume fraction is incrementally related to the overall strain or stress. As an application, numerical simulations are performed for cubic β1 (austenite) to orthorhombic γ1′ (martensite) phase transformation in a single crystal of Cu–Al–Ni shape memory alloy. The pseudoelasticity effect in tension and compression is investigated along with the corresponding evolution of internal stresses and microstructure.

An Active Guide Wire With Shape Memory Alloy Bending Actuator Fabricated By Room Temperature Process

Abstract: An active guide wire of 0.5mm diameter has been developed. The guide wire has a simple and flexible structure that consists of a NiTi shape memory alloy (SMA) bending actuator with meandering shape and a bias coil that is covered with thin polyurethane tube. It could bend to one-direction by heating with electric current and restore its initial shape by the bias coil. The SMA actuator was photo-fabricated from a flatten SMA sheet which was spread flatly after it was memorized in curve-shape.

Modeling the dynamic behavior of shape memory alloys

Abstract: The paper studies the single degree of freedom vibration of a rigid mass suspended by a thin-walled shape memory alloy tube under torsional loading. The behavior is analyzed for the cases of quasiplasticity (low temperatures) and pseudoelasticity (high temperatures) on the basis of an improved version of the Muller–Achenbach model. To illustrate the strong hysteresis-induced damping capacity and the non-linear vibration characteristics, both, free and forced vibrations are considered in the first part of the paper. This is done on the basis of an isothermal version of the model, while the second part of the paper focuses on the effect of non-constant temperature caused by the rate-dependent release and absorption of latent heats.

Effect of pre-deformation of austenite on shape memory properties in Fe-Mn-Si-based alloys containing Nb and C

Abstract: The shape memory properties of Fe-Mn-Si-based alloys containing Nb and C are further improved by pre-rolling of the solution-treated austenite and the subsequent ageing treatment. For an Fe-28Mn-6Si-5Cr-0.53Nb-0.06C (mass%) alloy, 90% of an initial 4% strain is recovered on heating without any previous training treatment, if the alloy in austenitic state is rolled by 6-14% at 870 K and aged at 1070 K for 10 min to produce NbC precipitates. In the same condition, the alloy develops shape recovery stresses of 255 MPa and 295 MPa for 6% and 14% pre-rolling, respectively, when initially deformed by 4.5%. TEM observations indicate that these improved shape memory characteristics are related to a fine distribution of NbC precipitates in the fcc matrix and their interaction with stacking faults.

Phenomenological Modeling and Numerical Simulation of Shape Memory Alloys: A Thermo-Plastic-Phase Transformation Coupled Model

Abstract: The thermomechanical behavior of shape memory alloys (SMAs) may be modeled either by microscopic or macroscopic point of view. Shape memory, pseudoelasticity and thermal expansion are phenomena related to the SMA behavior. Constitutive models consider phenomenological aspects of these phenomena. The present contribution considers a one-dimensional constitutive model with internal constraint to describe SMA behavior including the effect of plastic strains. The proposed theory contemplates four phases: three variants of martensite and an austenitic phase. Different material parameters for austenitic and martensitic phases are considered. Thermal expansion phenomenon and plastic effects are also contemplated. Hardening effect is represented by a combination of kinematic and isotropic behaviors. A plastic-phase transformation coupling is incorporated into the model. A numerical procedure is developed and numerical results show that the proposed model is capable to capture the general thermomechanical behavior of shape memory alloys.

Shape memory behavior of FeNiCoTi single and polycrystals

Abstract: We present experimental and theoretical evidence of thermoelastic martensites in Fe29Ni18Co4Ti alloys. In this class of alloys, the high strength in the austenite domains limits the slip deformation as verified with transmission electron microscopy. The restriction of slip permits a higher degree of recoverability of the transformation. Using both single crystals with [123] orientation and polycrystals, the appearance of martensite plates upon deformation, and their reversion back to austenite upon heating (the shape memory effect), is revealed with in-situ optical microscopy. Theoretical results for the transformation strains and the detwinning of martensite are presented, which demonstrate convincingly the potential of these classes of alloys. Electrical resistance measurements identified the stress and temperature levels at the onset of forward and reverse transformations in isothermal deformation and thermal cycling experiments, respectively. The return of the electrical resistance to its reference value, upon austenite to martensite followed by martensite to austenite transformation, verified the recovery in the transformation strains measured in the experiments.

Electrical resistivity and strain recovery studies on the effect of thermal cycling under constant stress on R-phase in NiTi shape memory alloy

Abstract: In this paper, the results of electrical resistivity and strain recovery measurements involved in the study of the stability of R-phase in NiTi shape memory alloy upon thermal cycling under a constant tensile stress of 100 and 200 MPa are presented. Two wire samples are chosen such that the one heat-treated at 560°C exhibits a pure martensitic phase and the other heat-treated at 380°C consists of a mixture of R-phase and martensitic phase with residual austenites at ambient temperature. In both cases, the applied stress has been found to promote the R→A transformation during the heating part of the thermal cycling unlike the case of stress free condition in which R-phase is found to exist only in the cooling part of thermal cycling. R-phase is found to become more prominent with increasing applied stress. It is also found that the applied tensile stress enables the development of R-phase in the cooling part of the first thermal cycle itself in the sample heat-treated at 560°C whereas under stress-free condition it requires about 15 thermal cycles to develop R-phase. The NiTi wire heat-treated at 560°C exhibits more recoverable strain in the initial cycles than the wire heat-treated at 380°C, but after a large number of thermal cycles of the order of 1000 the recoverable strain in both samples is found to be almost the same. Under tensile stress, the sample heat-treated at 380°C is found to be more stable against plastic deformation with thermal cycling and hence can be preferred over the sample heat-treated at 560°C for two-way applications of SMA.