Superelasticity Effect Research Articles

High-strength superelastic as-cast Ni50.9Ti49.1-TiB2 in-situ composites

In this study, as-cast Ni50.9Ti49.1−TiB2 in-situ composites were prepared by adding boron in the vacuum induction melting (VIM) process. With boron content of 1 at.%, TiB2 phases appear in the form of flakes at grain boundaries of the NiTi alloy matrix. These TiB2 flakes are two dimensional in geometry, featured with an ultrafine thickness less than 200 nm and two other dimensions up to ~20–60 μm. Increasing boron content to 2.4 at.% alters the morphology of TiB2 from discrete flakes to agglomerative clusters. This morphological dissimilarity has noticeable impact on resulting mechanical properties, superelasticity and shape memory effect. Overall, the NiTi-1at%B composite outperforms both the monolithic Ni50.9Ti49.1 alloy and the NiTi-2.4 at%B composite. More specifically, it exhibits complete superelasticity for pre-strains below 4% and up to 4.3% recoverable strain for 6% pre-strain, which is comparable to the pure as-cast Ni50.9Ti49.1 alloy. Meanwhile, the corresponding strength under 6% pre-strain reaches 1800 MPa, 20% higher than that of Ni50.9Ti49.1. In compression, the yield strength is 65% higher than that of pure Ni50.9Ti49.1. Moreover, a shape memory recovery ratio of 45.5%, slightly below 48.6% of the monolithic NiTi, was also achieved in NiTi-1at%B. This indicates that the addition of 1 at.% B can effectively improve the strength of NiTi while maintaining its superelasticity and shape memory effect.

Transformational, microstructural and superelasticity characteristics of Ti–V–Al high temperature shape memory alloys with Zr addition

This study analyzes the influences of Zr addition on the martensitic transformation behavior, microstructural evolution, mechanical properties and superelasticity effect of arc-melted Ti-12V-4Al-xZr (x = 0, 0.5, 1, 1.5 and 2 wt.%) high temperature shape memory alloys (HTSMA). The results revealed that the austenite transformation temperatures and activation energy values decreased linearly when the Zr addition was greater than 0.5 wt.%. The Ti-V-4Al (wt.%) and Ti12V-4Al-0.5Zr (wt.%) alloys were composed of α″ martensitic phase, while the others consisted of predominant α″ martensitic phase and a small amount of the β austenite phase. The thickness of martensitic plates in the alloys reduced with increased Zr addition. Hardness and reduced elastic modulus values calculated from load-depth curves of the alloys also decreased with increasing Zr addition. Along with the increase in the Zr addition, the alloys’ superelasticity behavior decreased at room temperature (24 °C), while this behavior increased at the high temperatures (450 °C).

Architected material analogs for shape memory alloys

Architected material analogs for shape memory alloys

Experimental investigation of the effects of super-elasticity on the machinability of NiTi alloys

Machining of NiTi alloys is a challenging task owing to their inherent material properties and unique phase transformation-based behaviors. In this study, the effects of super-elasticity on the machinability of room-temperature austenitic NiTi alloy were investigated through turning experiments conducted at various cutting speeds under the non-preheating and preheating conditions. As a result, the workpiece exhibited an unavoidable super-elastic shape recovery and underwent a partial phase transformation because the temperature of the workpiece had not exceeded the martensite desist temperature (Md) during machining under the non-preheating condition. The super-elastic recovery of the workpiece resulted in the deterioration of the machinability, through the decrease in the dimension accuracy, increase in the cutting resistance, and shortening of the tool life. The deterioration of the machinability caused by super-elasticity was intensified with the increase in the cutting speed. However, an excessively low cutting speed of 10 m/min induced severe built-up edge deposition on the machined surface. When the workpiece was preheated to a temperature above the Md, the dimension accuracy was enhanced, the cutting resistance decreased and the tool life was prolonged owing to the elimination of the effects of the super-elasticity. Therefore, machining at moderate cutting speeds in the range of 25 to 50 m/min after preheating the workpiece to a temperature above the Md can improves the machinability of room-temperature austenitic NiTi alloy.

Structure and Superelasticity of Novel Zr-Rich Ti-Zr–Nb Shape Memory Alloys

Two novel superelastic Ti-41Zr-8(10)Nb alloys were studied and compared to the reference Ti-18Zr-15Nb alloy (all in at. %) in terms of their microstructure and mechanical properties. A thermomechanical treatment consisting of cold rolling and post-deformation annealing was applied to all the alloys to create conditions for their superelastic behavior at room temperature. X-ray diffraction analysis demonstrated considerably larger lattice distortions of both parent and martensitic phases in Zr-rich alloys that resulted in significantly higher theoretical limits of recovery strain in these alloys (~ 8.0%) as compared to the reference alloy (~ 5.5%). However, during room temperature fatigue testing, the novel alloys accumulated considerable residual strains and showed a relatively weak fatigue resistance caused by the presence of notable quantities of α″-phase at this temperature. Conversely, the reference alloy containing only β-phase at the temperature of testing and therefore, a more favorable phase composition at the testing temperature manifested a better superelasticy, and therefore, a better fatigue resistance. Nonetheless, an excellent combined effect of shape memory and superelasticity in Zr-rich alloys indicates that their room temperature superelasticity could be improved via an additional adjustment of their chemical composition.

Behavior of Inelastic and Plastic Strains in Coarse-Grained Ti49.3Ni50.7(at%) Alloy Deformed in B2 States

The regularities of the change in inelastic strain in coarse-grained samples of the Ti49.3Ni50.7 (at%) alloy are studied when the samples are given torsional strain in the state of the high-temperature B2 phase. During cooling and heating, the investigated samples underwent the B2–B19′ martensite transformation (MT); the temperature of the end of the reverse MT was Af = 273 K. It was found that at the temperature of isothermal cycles “loading-unloading” Af + 8 K, when the specimen is assigned a strain of 4%, the effect of superelasticity is observed. With an increase in the torsional strain, the shape memory effect is clearly manifested. It is assumed that the stabilization of the B19′ phase in unloaded samples is due to the appearance of dislocations during deformation due to high internal stresses at the interphase boundaries of the B2 phase and the martensite phase during MT. The appearance of dislocations during the loading of samples near the temperatures of forward and reverse MT can also be facilitated by the “softening” of the elastic moduli of the alloy in this temperature range. At a test temperature above Af + 26 K, the superelasticity effect dominates in the studied samples.

Martensite Transformation and Superelasticity at High Temperature of (TiHfZr)74(NbTa)26 High-Entropy Shape Memory Alloy

In this work, a (TiHfZr)(NbTa)26 (%at) high-entropy quinary alloy has been developed especially for high-temperature superelastic applications and studied over a large range of temperatures. The mechanical properties of this new material were compared with those of other superelastic alloys. The different ingots have been made in a cold crucible from pure metals. Several thermomechanical treatments have been performed on the microstructure of four alloys among them (TiHfZr)(NbTa)26 alloy. The microstructure of each alloy has been characterized by differential scanning calorimetry (DSC), scanning electron microscopy (SEM), and x-ray diffraction technique (XRD) and the mechanical behavior was investigated through three-point bending tests between − 40 and 200 °C, in quasi-static monotonic and low cycle loading conditions. The effects of the thermomechanical treatments on the static and cyclic thermomechanical mechanical responses have been analyzed in combination with the microstructure investigations of the four studied alloys. It has been shown that the (TiHfZr)(NbTa)26 alloy presents a martensitic transformation and a superelastic effect over the studied range of temperatures, in the cold-worked state or after solution treatment. Finally, the obtained experimental results have been compared with those of other superelastic alloys demonstrating the features of the developed high-entropy high-temperature superelastic alloy.

Phase field simulation on the super-elasticity, elastocaloric and shape memory effect of geometrically graded nano-polycrystalline NiTi shape memory alloys

• A 2D non-isothermal phase field model considering multiple variants which can describe the thermo-mechanical deformation of NiTi SMAs is newly proposed. • The SE, ECE, OWSME and SATWSME of the geometrically graded nano-polycrystalline NiTi SMAs are simulated and predicted by phase field method for the first time. • The controllability of SE and SATWSME can be effectively improved by designing geometrically graded structures. • The design of geometrically graded structures has the potential to regulate ECE. A two-dimensional non-isothermal phase field model considering multiple martensite variants which can describe the thermo-mechanical deformation of polycrystalline NiTi shape memory alloys (SMAs) was newly proposed. The super-elasticity (SE), elastocaloric effect (ECE), one-way shape memory effect (OWSME) and stress-assisted two-way one (SATWSME) of the geometrically graded nano-polycrystalline NiTi SMA systems (for instances, the trapezoid, concave and convex ones) were simulated and predicted by phase field method for the first time. The simulated results show that: Due to the stress gradient caused by the heterogeneous geometry under an external applied load, the martensite transformation (MT) and its reverse in the geometrically graded systems propagate progressively along the direction with stress variation, resulting in an obviously wider transformation stress window or transformation temperature one (with the assistance of external applied load) there than that in the rectangular system (where no heterogeneous geometry is involved), thus, the controllability of the SE and SATWSME can be effectively improved; however, since the sensitivity of martensite reorientation to the external load is much higher than that of stress-induced MT, the stress gradient along the tensile direction of the geometrically graded systems cannot strictly control the gradual propagation of martensite reorientation, so the improvement of the controllability of OWSME is not considerable; moreover, the temperature field evolution is consistent with the progressively propagating MT within the geometrically graded systems in the SE process, and the variation rate of the average temperature is lower than that of the rectangular system, which means that designing the geometrically graded structure components of NiTi SMA systems has the potential to control their ECE. Phase-field simulated microstructure evolution in the processes reflecting super-elasticity (SE) and one-way shape memory effect (OWSME) of rectangular and geometrically graded nano-polycrystalline NiTi SMA systems.

Achieving Excellent Superelasticity and Extraordinary Elastocaloric Effect in a Directionally Solidified Co-V-Ga Alloy

We demonstrate excellent superelasticity and giant elastocaloric effect in a directionally solidified Co50.7V33.3Ga16 alloy. Owing to the formation of [001]A preferentially oriented columnar grains, the as-grown alloy displays nearly perfect superelastic behaviors with fully reversible compressive strain of up to 5.4%, narrow stress hysteresis of about 40 MPa and a wide operating temperature window from 258 to 373 K. Simultaneously, a giant adiabatic temperature change ΔTad of -18.7 K is obtained on removing a uniaxial stress of 400 MPa at room temperature, which is in good agreement with the theoretical values based on Maxwell relation (-19.0 K) and calorimetric measurements (-19.9 K). A large |ΔTad| of greater than 12.5 K is obtained over a wide temperature span of at least 115 K. Moreover, both superelasticity and elastocaloric effect remain good functional stability without any evident degradation for 100 cycles.

Atomic Layer Deposition of aTiO2 Layer on Nitinol and Its Corrosion Resistance in a Simulated Body Fluid

Nitinol is a group of nearly equiatomic alloys composed of nickel and titanium, which was developed in the 1970s. Its properties, such as superelasticity and Shape Memory Effect, have enabled its use, especially for biomedical purposes. Due to the fact that Nitinol exhibits good corrosion resistance in a chloride environment, an unusual combination of strength and ductility, a high tendency for self-passivation, high fatigue strength, low Young’s modulus and excellent biocompatibility, its use is still increasing. In this research, Atomic Layer Deposition (ALD) experiments were performed on a continuous vertical cast (CVC) NiTi rod (made in-house) and on commercial Nitinol as the control material, which was already in the rolled state. The ALD deposition of the TiO2 layer was accomplished in a Beneq TFS 200 system at 250 °C. The pulsing times for TiCl4 and H2O were 250 ms and 180 ms, followed by appropriate purge cycles with nitrogen (3 s after the TiCl4 and 2 s after the H2O pulses). After 1100 repeated cycles of ALD depositing, the average thickness of the TiO2 layer for the CVC NiTi rod was 52.2 nm and for the commercial Nitinol, it was 51.7 nm, which was confirmed by X-ray Photoelectron Spectroscopy (XPS) and Scanning Electron Microscope (SEM) using Energy-dispersive X-ray (EDX) spectroscopy. The behaviour of the CVC NiTi and commercial Nitinol with and without the TiO2 layer was investigated in a simulated body fluid at body temperature (37 °C) to explain their corrosion resistance. Potentiodynamic polarisation measurements showed that the lowest corrosion current density (0.16 μA/cm2) and the wider passive region were achieved by the commercial NiTi with TiO2. Electrochemical Impedance Spectroscopy measurements revealed that the CVC NiTi rod and the commercial Nitinol have, for the first 48 h of immersion, only resistance through the oxide layer, as a consequence of the thin and compact layer. On the other hand, the TiO2/CVC NiTi rod and TiO2/commercial Nitinol had resistances through the oxide and porous layers the entire immersion time since the TiO2 layer was formatted on the surfaces.

Superelasticity and shape memory effect in zirconia nanoparticles

Superelasticity and shape memory effect are two key properties of zirconia-based ceramics which are mediated by the reversible tetragonal to monoclinic phase transformation. Although experimental studies discovered that the shape memory behavior of ceramics can be improved by reducing the grain boundary density, degradation in shape memory response still happens after a few cycles of loading–unloading–heating–cooling. In this work, superelastic and shape memory properties of single crystalline and polycrystalline yttria stabilized tetragonal zirconia (YSTZ) nanoparticles are studied by atomistic simulations. Fully recoverable superelastic strain (8.3%) is observed in a single crystalline nanoparticle by merely removing the compressive load. However, the shape memory behavior degrades with the increase of loading–unloading–heating–cooling cycles. In other words, a higher temperature is required for an additional strain recovery, but certain residual displacement may still remain after few cyclic loadings. Amorphous phase formation and its accumulation around the applied load contact area, as well as an increase in surface roughness are responsible for the observed degradation of shape memory response. In a polycrystalline YSTZ nanoparticle, besides the regular reverse transformation at the end of cyclic loading, most amorphous phase transfers back to the original tetragonal structure. However, a small amount of monoclinic phase is restrained by the permanent amorphous phase and grain boundaries at the end of cyclic loading, causing degradation of shape memory behavior.

The influence mechanism of MoS2 and NiTi microparticles on the friction and wear properties of bearing steel

For improving the tribological properties of bearing steel, the friction structure of surface microcavities filled MoS2/NiTi was prepared, and the friction reduction mechanism of friction interface was analyzed. Results show that MoS2/NiTi in microcavities is towed to worn surfaces to form a lubricating layer. MoS2 can exert its interlayer sliding effect to achieve lubrication under lower loads, but the lubricating ability is greatly reduced at heavy load. NiTi microparticles are dispersed between MoS2 layers, which exerts the rolling effect to reduce the friction. More importantly, the superelastic effect of NiTi microparticles can withstand longitudinal load to protect the lubricating layer. The synergistic effect of MoS2 and NiTi makes bearing steel have excellent tribological properties in a wide load range.

PHASE FIELD SIMULATION ON THE SUPER-ELASTICITY AND SHAPE MEMORY EFFECT OF GRADIENT NANOCRYSTALLINE NiTi SHAPE MEMORY ALLOY1)

A two-dimensional phase field model was established to simulate and predict the super-elasticity, one-way and stress-assisted two-way shape memory effects of gradient nanocrystalline NiTi shape memory alloy system. The simulated results show that in the super-elastic process of gradient nanocrystalline NiTi alloy, the characteristics of martensite transformation and its reverse in the traditional coarse-grained NiTi alloy, i.e., nucleation-expansion and reduction-disappearance of local martensite band, are retained in the relatively coarse-grained region, but with the decrease of grain size, a homogeneous transformation mode, i.e., without the formation of local martensite band, is observed in the fine-grained region; moreover, in the super-elastic and shape memory processes, both the martensite transformation and reorientation originate from the relatively coarse-grained region and then propagate progressively to the fine-grained one, while the reverse transformation is opposite. The gradual propagations of martensite transformation and reorientation make the stress-strain and strain-temperature curves of gradient nanocrystalline NiTi alloy show a remarkable “hardening”, which can be attributed to the grain size-dependence of martensite transformation and re-orientation in nanocrystalline NiTi alloy, i.e., with decreasing the grain size, the transformation or re-orientation barrier increases gradually, and the nucleation and expansion of martensite transformation or re-orientation becomes more and more difficult. It is concluded that the gradient nanocrystalline structure has wider transformation stress, reorientation stress and transformation temperature windows than the traditional uniform-grained NiTi alloys, which means that the controllability of the inelastic deformation of such alloy is significantly improved.

Functionally graded NiTi shape memory alloy: Selective laser melting fabrication and multi-scale characterization

Besides the possibility of fabricating complex geometries, a major advantage of additive manufacturing is tailoring the microstructure and consequently properties of the parts. By changing the process parameters (PPs). Functionally graded systems can be achieved without the need for post-processing or joining. In this study, fabrication of a functionally graded NiTi using selective laser melting was investigated. Using two different sets of PPs resulted in a part with two different functional properties. Several characterization methods were employed to study microstructural distribution. Thermomechanical full-field strain tests showed a novel behavior, a combination of superelastic and shape memory effects at room temperature.

The influence of the crystallographic texture and phase composition of Ti-Nb-Zr alloys with shape memory and superelasticity on their functional properties

Currently, Ti-Zr-Nb-based biocompatible alloys are considered as prospective for medical applications. In this work, the mechanisms of the formation of the crystallographic texture of five alloys Ti-(17-19)Zr-(14-16)Nb (at.%) have been compared. The presence of martensitic transformations during the rolling process determines the features of the formed texture. At the initial stage of deformation (ε = 50%), the texture {112}<011> is formed in all five alloys. With an increase of the degree of deformation to 92%, the texture dissipates somewhat, which is due to an increase in the fraction of the martensitic phase in the foils. Recrystallization of rolled foils (ε = 92%) at 650°C leads to an exacerbation of the texture component close to {100}<011>, the blurring of which varies in different alloys. Cyclic tensile tests along three directions: the rolling direction (RD), the transverse direction, at an angle of 45° to the RD showed the presence of anisotropy in the samples. It was shown, that the effect of superelasticity is orientation-and structure-depending, and the anisotropy of mechanical properties.

Influence of Variant Electrolyte in Electrochemical Micromachining of Micro Holes in SMA Using Taguchi Optimization

Electrochemical micromachining (ECMM) is one of the commercially successful modern machining processes used in various manufacturing industries. Nitinol, a shape memory alloy (SMA) material used as micro and nano component in bio medical fields is difficult to machine using traditional methods due to its super elasticity and shape memory effect. This problem induced in studying the machining characteristics of NiTi alloy using ECMM Process. In this work, an effort is taken to produce micro holes in SMA—NiTi alloy of 0.25 mm thickness using variant electrolytes such as the passivating electrolyte (PE) and the non-passivating electrolyte (NPE) to study the machining characteristics. The variable input parameters considered are voltage, duty cycle and feed rate to study their effect on performance characteristics such as MRR, overcut, circularity and conicity. This study resulted in finding the suitable and optimized parameters for machining Nitinol with PE and NPE with better precision and good surface integrity. Optimization studies are also carried out by Taguchi method using minitab statistical software.

Finite element analysis of viscoelastic laminates embedded with shape-memory-alloy wires under low-velocity impact considering large deflection

This paper presents an experimentally validated finite element analysis of the low-velocity impact on viscoelastic laminates embedded with shape memory alloy (SMA) with consideration of large deflection effects. Substantial impact outputs, including contact force and deflection distribution, are then derived and a fully implicit numerical solution is utilized. The finite element model contains the following innovative items to develop more sophisticated results: (1) A user-defined (UMAT) is prepared in ABAQUS to simulate the viscoelasticity of composite laminate and superelasticity of SMA wires simultaneously. (2) The generalized Maxwell model consisting of 3 Mx elements is utilized for the viscoelastic feature of the composite laminate. (3) A modified phenomenological constitutive law, which is capable of modeling superelasticity and one-way shape memory effect, is used for SMA wires. (4) Large deflection effects on dynamical response are investigated for both conventional and SMA-reinforced composites. Many researchers have not regarded viscoelasticity in their calculation (especially polymeric composites), although the results show that the viscoelastic feature of laminates has a pronounced effect on the dynamical response of composite laminates embedded with SMA wires. It is also shown that large deflection effects have a significant influence on the results of SMA-reinforced composites. This model achieved good conformance with experimental results.

Shape memory alloy couplers applied for torsional vibration attenuation of drill-string systems

This paper investigates the addition of shape memory alloy (SMA) couplers as pipe joints in a drill-string, exploiting the hysteretic behavior related to the superelastic effect. The main idea is to attenuate critical torsional vibrations, especially stick-slip oscillations. A lumped parameter model is used to represent the torsional dynamics of the drill-string, with an imposed angular speed at the top and a nonlinear bit-rock interaction at the bottom. The thermomechanical behavior of the SMA is described using a constitutive model with internal variables. Numerical simulations are carried out comparing responses of the conventional system with the one with SMA couplers. It is shown that the drilling operating map (angular speed at the top vs. weight on bit) has a broader safe operating zone with the couplers. A parametric analysis is carried out establishing the influence of the SMA couplers’ in the drill-string dynamics. The influence of the material properties, the distribution along the drill-pipes, and the SMA coupler length, are investigated. All discussions compare the operating map in each new configuration showing that SMA couplers avoid critical operating conditions.

Cyclic pseudoelastic training and two-way shape memory behavior of a NiTi alloy with small irrecoverable plastic strains:numerical modeling

Novel applications of shape memory alloys (SMAs) in various fields of medicine and engineering necessitate theoretical models that can capture non-inherent material behaviors such as the two-way shape memory effect (TWSME). Most often, this requires the use of completely different sets of models or parameters to distinguish the “virgin” material exhibiting the fundamental behaviors like superelasticity (also known as pseudoelasticity), pseudoplasticity, and one-way shape memory effect, from the “trained” alloy showing the TWSME. Although using a single model (or set of parameters) can provide much insight on the changes within the material during various loading paths and histories from the initial virgin state to the trained state, it is quite challenging. In this study, an attempt was made to use a developed general multi-mechanism material model and a single set of parameters to predict both the inherent pseudoelastic behavior and non-intrinsic two-way shape memory effect characteristics of a NiTi shape memory alloy. To this end, several simulations were carried out to investigate the effect of pseudoelastic training conditions on the corresponding actuation strain, cyclic stability, and transformation temperatures of the NiTi under zero-load TWSME. Whilst previous studies considered external indicators such as the magnitude of plastic or irrecoverable strain as a prerequisite for TWSME, the present model focused primarily on internal stresses. Based on the magnitude of the zero-load actuation strain (measured from the trained material under no external load), the model results indicated that the magnitude of the pseudoelastic training strain (in comparison to the training temperature, and the number of training cycles) has a pronounced effect on the cyclic TWSME actuation characteristics. Moreover, observations from the internal state (hidden) variables of the model, representing the intrinsic residual stress state in the trained material (under zero observable/external stress) showed the possibility for an SMA to generate TWSME even if there is no significant accumulation of permanent strains.

Ultrafine shape memory alloys synthesized using a metastable metallic glass precursor with polymorphic crystallization

Metallic glasses have been considered as a precursor for developing nano- or ultrafine-structured alloys based on its crystallization behavior at large undercooled liquid state. However, the multicomponent system, requiring high glass-forming ability, makes it difficult to control the crystallization mechanism, and leads to the formation of brittle intermetallic compounds during crystallization. Here, we present an intermetallic-type metallic glass precursor designed via thermodynamic and physical considerations. The metallic glass precursor polymorphically crystallized into ultrafine-structured single-phase shape memory alloy. The crystallization kinetics and final grain-size strongly depend on isothermal annealing temperatures. The shape memory alloys fabricated using the metallic glass precursor revealed excellent superelastic and shape memory effects, which was affected by the critical stress for martensitic transformation depending on the grain size. Our approach and discovery provide a feasibility of the simple fabrication processing for ultrafine-structured shape memory alloys by one-step heat-treatment technique including thermoplastic forming of metallic glass precursor.