Stress-assisted Two-way Memory Effect Research Articles

Grain size dependence of stress-assisted two-way memory effect in Ti-50.04 at.% Ni shape memory alloy

The grain size (GS) effects on stress-assisted two-way memory behavior of NiTi shape memory alloys (SMAs) were investigated by thermomechanical actuation tests over a wide GS range (from 10 nm to 230 nm). The actuation strain and transformation temperatures are observed to decrease monotonically as the GS is reduced, but the transition temperature window is enlarged. The irrecoverable strain decreases monotonically with GS reduction overall. The underlying mechanisms were investigated through microstructural and thermodynamic analysis. It is demonstrated that, as the GS is reduced, the increasing volume fraction of dislocations and grain boundaries (GB) occupying the space of austenite crystallite suppress the phase transition (PT) and lower the entropy change, leading to the decrease of transformation strain. GS reduction converts the PT from first-order type to a continuous one, and greatly expands the operating temperature range of actuation, tuning the discontinuous leap in strain changes into a continuous variation. When GS > 40 nm, transformation induced plasticity dominates the irrecoverable deformation and is monotonically affected by GS dependent hall-patch effect. Further modeling analysis reveals that, the increase in the volume of GB caused by GS reduction drastically modifies the free energy density convexity of the crystallite-GB composite system and leads to the monotonic decrease of transformation temperatures. The near-linear temperature dependency of actuation strain and wider transition temperature window of the nanostructured NiTi, compared to traditional coarse-grained NiTi, provide the opportunity of improved actuation control of SMA actuator and have the potential for some unique applications in the actuation area.

An Asymmetric Elasto-Plastic Phase-Field Model for Shape Memory Effect, Pseudoelasticity and Thermomechanical Training in Polycrystalline Shape Memory Alloys

We propose an elasto-plastic phase-field model (PFM) to conduct the first microscopic computational study of shape memory effect (SME), pseudoelasticity, stress assisted two-way memory effect (SATWME), and thermomechanical training of CuAlBe shape memory alloy (SMA). This non-isothermal PFM model considers the effects of temperature dependent properties, latent heat, grain boundaries, and asymmetric transformation and plasticity. PFM simulations demonstrate the capacity of our model to capture the thermomechanical and purely mechanical shape recovery in the SMA. When considering transforming grain boundaries, grain refinement generates a higher transformation stress, a steeper transformation hardening, and smaller hysteresis loops for both SME and pseudoelasticity. The results also point out slightly higher transformation stress when geometrical grain boundaries are used. The simulations of SATWME highlight an augmentation of the residual martensite as the hold stress increases, which is consistent with experimental observations. This is the first PFM that can mimic the thermal training within several SATWME cycles, showing an asymptotic increase of plastic strain and the related retained transformation strain until the fourth cycle, and their stabilization thereafter. The activation of plasticity occurs always after initiation of phase transformation. Although plasticity results in more stress relaxation, it deteriorates the shape recovery for both SME and pseudoelasticity by hindering the reverse transformation. Comparison between tension and compression demonstrates the capacity of this PFM to account for, for the first time, the nonsymmetrical transformation and plastic responses of SMAs.

Effect of Spun Velocities and Composition on the R‒phase and Thermomechanical Behavior in Ti‒Ni Ribbons Electrically Heated

This work deals with structural, electrical and mechanical characterization of Ti‒50.13Ni and Ti‒49.62Ni (at.%) shape memory alloys (SMAs) fabricated at different circumferential wheel velocities. The effect of wheel velocity, chemical composition and heat treatments are investigated. The characterization of crystallographic phases of the Ti‒Ni ribbons was carried out using X-ray diffraction. Electrical resistance variations as function of temperature (∆R/R %) were analyzed using a non-commercial technique, which consists in a thermal-adjustable bath apparatus revealing the temperatures of B2→R→B19´ two stage transformation, whereby the presence of R‒phase can be definitively confirmed. The Stress-Assisted Two-Way Memory Effect was measured by an own designed apparatus with an Linear Variable Differential Transformer captor and a current controlled heating, and results indicate that the as-spun condition, promotes the Stress-Assisted Two-Way Memory Effect. On the other hand, increments in Ni content tend to decrease transformation temperatures and high wheel velocities help to the R‒phase formation.

Partial Transformation and the Two-Way Shape Recovery Characteristics

It is widely known that two-way memory effect (TWME) is not an inherent property of shape memory alloy. The development of TWME requires thermomechanical training. Experimental study showed that undergoing partial reverse transformation in the course of training leads to the emergence of temporal two-step transformation, which was traditionally observed in the calorimetry measurement of an arrested stress-free heating cycle. The present work introduces a macromechanical approach to explain the mechanism of two-step transformation and its associated effects on stress-assisted two-way memory effect (SATWME) and TWME. The appearance of two-step transformation was observed to be a one-time only phenomenon and it clearly disappeared in the next full transformation. The disappearance of two-step transformation highlighted the occurrence of microstructural rearrangement driven by the internal stress field in the successive training cycles. A strain comparison demonstrated that the dominance of retransforming stress-assisted martensite (SAM) during cooling promoted the formation of internal back stress. This makes the accommodation process of deformation-induced martensite generated via pre-straining and SAM difficult, owing to which immobilizes the dislocations movement in the forward transformation direction, and causes detrimental effect on the TWME.

A Smart Polymer Composite Based on a NiTi Ribbon and a Magnetic Hybrid Material for Actuators with Multiphysic Transduction

A smart composite material constituted of a magnetic hybrid film and a NiTi shape memory alloy (SMA) ribbon was obtained and characterized. The magnetic hybrid film was joined to the NiTi ribbon in order to combine the properties of both materials. This new composite material combines magnetic properties of the hybrid film, (Fe2O3-CMC)/(polyvinyl butyral), and the shape memory properties of the NiTi ribbon, which has a chemical composition of Ti-50.13 at. % Ni. This smart composite material has a mass of 18.3% NiTi ribbon and 81.7% magnetic hybrid film. Results obtained by DSC show that the smart composite material presents a small delay of transformation during warming and cooling because the magnetic hybrid film acts like a thermal insulator. Thermomechanical results indicate that the hybrid material also acts as a mechanical reinforcement, since it is observed that the Stress-Assisted Two-Way Memory Effect (SATWME) of the smart composite is lower than the SATWME of the SMA ribbon. The density current values of phase transformations were clearly identified with a thermomechanical apparatus developed in our laboratory. Finally, displacements of the smart composite material in cantilever configuration are obtained by applying an external magnetic field. All these results demonstrate that the smart composite material can be activated by temperature, electrical current, stress, and/or magnetic field, offering good expectations for actuating applications with multiphysic transduction.

Influence of Annealing on the Stress‐Assisted Two‐Way Memory Effect in Cold‐Worked NiTi Wire

The magnitude of stress-assisted two-way memory effect (SATWME) in NiTi alloys is affected by many factors and the mechanism of the development of SATWME is not been explored in depth. The influence of annealing on SATWME is investigated in a cold-worked NiTi alloy by using a dynamic mechanical analyzer. Some new results are presented.

On the two-way shape memory behavior in NiTi alloy—An experimental analysis

The present research aims to understand the mechanism of the two-way memory effect (TWME) in NiTi alloy through experimentally investigating shape recovery in the course of various thermomechanical processes. Particular attention was paid to the effect of martensitic strain, either due to pre-deformation or constrained forward transformation, on the TWME. Attempts were made to clarify the correlation of the stress-assisted two-way memory effect (SATWME) and the TWME to martensitic strain resulting from pre-straining and/or constrained cooling. When the martensitic strain resulting from constrained forward transformation exceeds the initial pre-strain, it directly influences the SATWME and TWME. Partial reverse transformation through thermal arrest during heating leads to accommodation of stress-assisted and detwinned martensite variants that result in formation of internal forward and back stresses. TWME is promoted through dominant internal forward stress formation, while the dominance of internal back stress reduces the TWME by reducing the martensitic strain.

Thermomechanical training and the shape recovery characteristics of NiTi alloys

Thermomechanical training is an important process to develop a two-way memory effect (TWME) in shape memory alloys. The present study has been dedicated to understand the mechanisms responsible for the TWME by examining its correlation with a stress-assisted two-way memory effect (SATWME). It was observed that imposing partial transformation between A s and A f temperatures during training induces a two-step transformation. The appearance of two-step transformation was observed to be a one-time only phenomenon. The magnitudes of SATWME and TWME were both found to decrease with partial transformation, though their decreasing trend varied according to different cyclic conditions. The internal stress developed by the partial transformation was ineffective in promoting the SATWME and TWME. On the contrary, the internal stress developed by the full transformation during training is favorable to the development of SATWME and TWME.

On the mechanisms of two-way memory effect and stress-assisted two-way memory effect in NiTi shape memory alloy

This study aims at understanding the predominant mechanisms responsible for the maximum TWME and SATWME. It was found that partial reverse transformation initiated under constraint condition resulted in the generation of two-step phase transformations. The transformation occurring at higher temperature is attributed to the postponed memory effect of retained martensite. Although this retained martensite causes significant reduction in SATWME, it causes merely insignificant changes in TWME. These changes in memory effects are due to the generation of internal plastic deformation. The critical factor that determines the maximum memory effect therefore is attributable to the favorable dislocations and internal stress field created during martensite pre-deformation and under constrained thermal cycling.

Stress-assisted two-way memory effect electrically driven in 50 at.%Ti–45 at.%Ni–5 at.%Cu alloy

Electrical resistance ( R) and deformation ( ɛ) are studied in Ti–45 at.%Ni–5 at.%Cu alloy, electrically driven through the transformation range under different constant stress levels in the range 10–500 MPa. Specimens with increasing preliminary cold-work levels (13, 19, 28 and 45%) were selected, in order to single out the condition σ crss (critical resolved shear stress) > σ a (applied stress). In the following, attention is focussed on specimens with 45% cold work, which were submitted to 1000 cycles under constant stress levels of 50, 75 and 100 MPa. A linear relationship between electrical resistance change and strain was obtained under all such conditions, with no accumulated residual strain, for an applied stress of 50 MPa. For the higher stress levels investigated, a small decreasing residual strain appears during the first 500 cycles, which then levels off for a greater number of cycles.

Electrical resistance and deformation during the stress-assisted two-way memory effect in Ni 45Ti 50Cu 5 alloy

The electrical resistance detected during the stress-assisted two-way memory effect in Ni 45Ti 50Cu 5 alloy was investigated to discover the maximum stress levels required to avoid plastic deformation. Electrically driven P → M → P cycles were performed on Ni 45Ti 50Cu 5 specimens preliminarily submitted to different cold-work levels with the aim of modifying the critical resolved shear stress for plastic deformation. A linear relationship between the variation in electrical resistance and deformation across the stress-assisted transformation can be obtained at stress levels below 150 MPa. Results show that the electronic contribution to the variation in electrical resistance is definitely lower than the contribution related to oriented variants.

Shape recovery of NiTi shape memory alloy under various pre-strain and constraintconditions

The thermally induced shape recovery behavior of NiTi shape memory alloy (SMA) wireunder constraint conditions and variable pre-strain conditions was investigated. Two kindsof deformation procedures have been performed: test (1), fixed pre-strain with variableconstrained stress; and test (2), fixed constrained stress with variable pre-strain. Asstudied previously, when subjected to stress-free thermal cycling the maximumtwo-way memory effect (TWME) is generated at about 13% pre-strain, owing to theoptimum amount of dislocations and preferentially oriented martensite produced. Thepresent study showed that under test (1) conditions, the effect of constrained stresson the generation of the stress-assisted two-way memory effect (SATWME) isfound to be associated with the macroscopic increase in martensitic strain andinternal stress. The test (2) condition with fixed constrained stress of 200 MPashowed no significant change in SATWME with increasing pre-strain, and itsmaximum value is obtained at 13%. On the basis of these observations, it is concludedthat the development of maximum SATWME and TWME is independent ofdeformation procedures and is determined predominantly by the magnitude ofmartensitic strain generated during either pre-deformation or thermal cycling.The mechanism is further discussed in association with relevant thermodynamicproperties.

Factors affecting the generation of stress-assisted two-way memory effect in NiTi shape memory alloy

The combined effects of martensite pre-deformation and constrained stress on the generation of stress-assisted two-way memory effect (SATWME) were studied. Particular attention was paid to the generation of internal stress during thermal cycling under constant stress. The result highlights that the maximum SATWME is not determined solely by the maximum internal stress developed. The SATWME strain when subjected to 18% martensite pre-strain coincides, independently of constrained stress applied and internal stress developed. This study also provides experimental evidence that when under 400 MPa constrained stress the maximum SATWME shifts toward higher pre-deformation amplitude and this phenomenon is comparable to the increased cold-work. The results further suggest that the mechanism that associates with the change of M s temperature plays critical role in determining the SATWME.

Study of the stress-assisted two-way memory effect of a Ti–Ni–Cu alloy using resistivity and thermoelectric power techniques

The stress-assisted two-way memory effect has been studied for successive cycles in a Ti–Ni–Cu alloy for two different thermo-mechanical treatments: recovery annealed (TT425) and fully recrystallized. Simultaneous tensile strain and electrical resistance measurements have been performed as a function of temperature and for different applied stresses. The thermoelectric power (TEP) in the stressed martensitic state has also been measured. For the treatment TT425 corresponding to 30% cold drawing followed by 425 °C annealing, a stable reversible strain ε m-β of 4.5% at maximum is found. A relatively low plastic strain ε p occurs for stresses above 125 MPa, which increases with the number of thermal cycles. For the recrystallized treatment, the maximum amplitude of the reversible strain is almost the same but a quite large plastic strain occurs even at 50 MPa and strongly increases with the number of cycles, reaching 12% for 20 cycles at 200 MPa. It is shown that the electrical resistivity change, associated with the reverse transformation, increases linearly with the reversible strain ε m-β. This effect is related to the electrical resistivity anisotropy of the martensite, with the slope being slightly different for the two treatments. In addition, the resistivity is relatively insensitive to the plastic strain. The TEP and the resistivity are sensitive to the orientation of the martensite, but the TEP also strongly depends on the plastic strain.

The effect of prestrain on stress assisted two way shape memory effect in a Ni45Ti50Cu5 alloy

Stress Assisted Two Way Memory Effect (SATWME) and Electrical Resistance (ER) have been investigated on a Ni 45 Ti 50 Cu 5 alloy preliminarily prestrained in martensite to modify the critical stress for slip. Different prestrain levels have been applied. ER vs e during SATWME follows an almost linear relationship across the transformation range: the plastic deformation accumulated during cycling generally increases, but the figure of e p lowers with increasing the preliminary prestrain. Even the highest applied prestrain ( 15%) does not allow to eliminate the build up of plastic strain in each cycle. Different procedures to modify the critical stress for slip must be envisaged.

Influence of the mechanical loading history on the stress assisted two way memory effect in a Ti-Ni-Cu alloy

In this work, several tests of thermal cycling under constant load are carried out on Ti-45.0Ni-5.0Cu (at%) shape memory wires. The properties related to the Stress Assisted Two Way Memory Effect (SATWME) of the material are investigated as a function of the mechanical loading history for the same temperature range during cooling and heating. For this reason, two thermomechanical tests have been employed: tests I, where one sample is used for just one constant stress level test and tests II, where only one sample is employed for several constant stress level tests in sequence. The results obtained show that for loads applied below 150 MPa, the transformation temperatures and the thermal hysteresis associated with the transformation of the material are the same during the two tests. However, above 150 MPa in tests II transformation temperatures and thermal hysteresis are respectively higher and smaller than the ones obtained in tests I. On the other hand, transformation temperatures obtained from both tests are in good agreement with the ones measured by DSC and electrical resistance measurements. It is also observed that the SATWME obtained by tests II is smaller than the one measured during tests I. It is shown that these different behaviors are induced by accumulation of plastic strain in the sample during tests II.

Electro-thermomechanical behaviour of a Ti-45.0Ni-5.0Cu (at.%) alloy during shape memory cycling

In the present paper, electrical resistance (ER) changes are measured simultaneously with the stress-assisted two-way memory effect (SATWME) in Ti-45.0Ni-5.0Cu (at.%) wires during thermal cycling (max. 15 cycles) for several different stress levels. Interesting qualitative evolutions of the ε-ER-T loops during cycling are observed as a function of the applied stress. On cooling, for stresses higher than 175 MPa, a clear deviation of the ε-T curves is verified and the reversion of this anomaly is not observed during heating. After some cycles, serrations are frequently observed on the ER-T loops essentially below M f and above A f , indicating an interaction between the formation and reversion of oriented martensite variants with the defects introduced during the thermomechanical cycling. A linear relationship is observed between ER and ε for the direct and reverse transformation ranges. The characteristic slope d(Δ R / R )/dε is slightly dependent on the applied stress and on the number of thermal cycles.

Fatigue Behavior of Ti-Ni-Cu Thin Wires SME

Practical applications of the Shape Memory Effect (SME) involves frequently repetitive operations. Therefore, it's essential to have knowledge of the properties behavior related to SME under working conditions. The thermomechanical fatigue of Ti-Ni-Cu thin wires loaded by static uniaxial tensile stress and submitted to thermal cycling has been studied. The test samples were cycled until failure in oil bath, cooled by this one and heated by electrical current. One modified Wohler curve has been plotted and the influences of the applied stress and of the number of thermal cycles on the shape memory properties has been analyzed. It is shown that the martensitic transformation associated to SME changes according to stress level andlor number of thermal cycles. The degradation until failure of the Stress Assisted Two-way Memory Effect (SATWME) was set between 0 and 20% compared to initial value. The commercial wires (+=0.5mm) used in this study has a nominal chemical composition Ti-45.ONi- 5.OCu (%at). As received materials, with 35% of cold-work, were heat treated at 425OC for lh followed by cooling in air and slightly polished for to remove oxide layers. The transformation temperatures under stress-free conditions, measured by DSC (Mettler TA 3000) and according to French Norm (ll), were: MX90%)=27.3OC, MS(10%)=35.7OC, AS(90%)=42.7OC and AX10%)=53.3OC. Two views of the SME fatigue apparatus is shown in figure l(a). This apparatus allow to test two samples simultaneously. Thermal cycles are performed by forced convection in silicone oil (slow cycles) or by electrical heating and cooling by circulation of silicone oil at constant temperature below Mf (fast cycles).

Thermomechanical study of the stress assisted two way memory effect fatigue in TiNi and CuZnAl wires

Composites containing shape memory alloys in the shape of fibers are well-known for their use as actuators in adaptive structures. So the thermomechanical behavior of thin shape memory wires presents some interest. In the present study, the stress assisted two way memory effect (SATWME) is used: the specimens are thermally cycled under a constant load. This effect is similar to the shape memory effect but both the cooling and the heating are performed under a constant stress. During the cooling, the martensite is oriented by the stress and the strain is large. During the heating the sample recovers its original shape (elastic strain of the austenitic phase). In the present paper measurements of SATWME in two commercial wires 0.5 mm diameter are reported and some thermoelastic fatigue results are developed.