Ti Shape Memory Alloy Research Articles

Martensite transformation of epitaxial Ni–Ti films

The structure and phase transformations of thin Ni–Ti shape memory alloy films grown by molecular beam epitaxy are investigated for compositions from 43 to 56 at. % Ti. Despite the substrate constraint, temperature dependent x-ray diffraction and resistivity measurements reveal reversible, martensitic phase transformations. The results suggest that these occur by an in-plane shear which does not disturb the lattice coherence at interfaces.

Effect of Solution Treatment on Thermal Conductivity of Porous NiTi Shape Memory Alloy

In this study, a new solution treatment “solution treatment under loading” was applied to a porous Ni–50 at.%Ti shape memory alloy (SMA), which was fabricated by self-propagating high-temperature synthesis (SHS), to explore the microstructural improvement regarding single-phase NiTi. The effects of solution treatment under loading and without loading on the phase constituent and thermal conductivity were investigated and discussed. The phase constituent and thermal conductivity of the specimens considerably changed with solution treatment under loading, but they were not affected significantly with solution treatment without loading. Intermetallic phases such as B19′(NiTi), Ti2Ni, and Ni4Ti3 disappeared, the density of the B2(NiTi) phase increased with solution treatment under loading, and thus the thermal conductivity was increased. It was also seen that the thermal conductivity of porous NiTi was less than that of solid NiTi.

The optimization of annealing and cold-drawing in the manufacture of the Ni–Ti shape memory alloy ultra-thin wire

In this paper, the mechanical properties of the Ni–50.5 at.%–Ti alloy super-elastic wires manufactured by a conditioned multi-passed process of annealing and cold-drawing have been studied. The annealing temperature of 450~800°C, time of 20 min~3 h and the cold-drawing amount of 6.9%~39% were chosen. Their effects on the thermo, mechanical, and surface morphology of the Ni–Ti wires have been studied. The differential scanning calorimetry and tensile-recovery tests were adopted to obtain the phase transformation temperatures and mechanical hysteresis of the Ni–Ti SMA wires with different treatment conditions. The results show that the phase transition temperature of Ni–Ti wire can be changed by varying the annealing temperature and time; cold-drawing deformation and subsequent annealing have a great influence on the super-elasticity. The process with 39% cold-drawing amount, 600°C and 20 min annealing is shown to be effective in the manufacturing.

The influence of imposed strain on the development of microstructure and transformation characteristics of Ni–Ti shape memory alloys

Presented paper describes the influence of imposed strain on the development of microstructure (substructure) in Ni–Ti materials during the application of ECAE (equal channel angular extrusion). The interrelationship between imposed strain and resulting transformation characteristics is in the main point of interest while the chemical composition, namely the occurrence of secondary phases such as TiC and Ti 4Ni 2O resulting from the technology used for the sample alloy preparation is also investigated. The alloy Ni50.6–Ti (at.%) was prepared by melting in the HF vacuum induction furnace using a graphite crucible. The deformation was carried out via ECAE combined with prior rotary forging/swaging. ECAE was carried out at 280 °C. The imposed strain had the value ∼2. The alloy was processed by deformation route Bc. It was established the high dislocation density generated by the imposed strain stabilized the B2 and R phases and shifted the transformation R ↔ B19′ towards the lower temperature region. Direct effect of the deformation stress was demonstrated in samples after the second pass when the occurrence of the deformation induced transformation B2 → B19′ was confirmed. Deformation induced transformation also represents contribution to the overall deformation of the samples during the process.

A novel low-profile shape memory alloy torsional actuator

This paper presents low-profile torsional actuators applicable for mesoscale and microscalerobots. The primary actuator material is thermally activated Ni–Ti shape memory alloy(SMA), which exhibits remarkably high torque density. Despite the advantages of SMAs foractuator applications—high strain, silent operation, and mechanical simplicity—theresponse time and energy efficiency limit overall performance. As an alternative to SMAwires, thin SMA sheets are used to fabricate effective yet compact torsional actuators. Also,instead of using conventional Joule heating, an external Ni–Cr heating element isutilized to focus heat on the regions of highest required strain. Various designparameters and fabrication variants are described and experimentally explored inactuator prototypes. Controlled current profiles and discrete heating produces a20% faster responsetime with 40% less power consumption as compared to Joule heating in a low-profile (sub-millimeter) torsional actuatorcapable of 180° motion.

A Strain Rate Effect of Ni–Ti Shape Memory Alloy Wire

This paper presents a strain-rate thermo-mechanical description of stress-induced martensite phase transformation and the associated shape memory effect on shape memory alloys (SMAs). The main difficulty in describing the thermo-mechanical behavior of SMA lies in the modeling of the transformation stresses. Since there is a conflict between the strain hardening associated with the stress and the re-orientational hardening due to temperature variation according to the strain-rates of SMA, modeling the stress of SMA is not trivial. In this paper, a linear evolution of the temperature with deformation rate is experimentally and theoretically validated for low and medium strain rates. The transition stress increases during forward transformation and decreases during reverse transformation at a certain environmental temperature in the pseudo-elastic range. This variation of the transition stress is attributed to the temperature rise and fall related to the strain rate during the phase transformation.

Effect of Ni 4Ti 3 precipitation on martensitic transformation in Ti–Ni

Precipitation of Ni 4Ti 3 plays a critical role in determining the martensitic transformation path and temperature in Ni–Ti shape memory alloys. In this study, the equilibrium shape of a coherent Ni 4Ti 3 precipitate and the concentration and stress fields around it are determined quantitatively using the phase field method. Most recent experimental data on lattice parameters, elastic constants, precipitate–matrix orientation relationship and thermodynamic database are used as model inputs. The effects of the concentration and stress fields on subsequent martensitic transformations are analyzed through interaction energy between a nucleating martensitic particle and the existing microstructure. Results indicate that R-phase formation prior to B19′ phase could be attributed to both direct elastic interaction and stress-induced spatial variation in concentration near Ni 4Ti 3 precipitates. The preferred nucleation sites for the R-phase are close to the broad side of the lenticular-shaped Ni 4Ti 3 precipitates, where tension normal to the habit plane is highest, and Ni concentration is lowest.

Isothermal and athermal martensitic transformations in the B2–R–B19′ sequence in Ni–Ti shape memory alloys

The kinetics of accumulation of martensite/austenite during the direct/reverse two-stage B2–R–B19′ martensitic transformation in NiTi has been studied by means of resistance measurements during interruptions of cooling/heating scans. Experimental results indicate that the R–B19′ transformation is isothermal, whereas both direct and reverse B2–R and R–B2 transformations are perfectly athermal. The isothermal accumulation of the R-phase is also detected during B19′–R transformation. A good correspondence exists between the rate of the isothermal transformation and the transformation rate in an uninterrupted cooling cycle.

Texture development, microstructure and phase transformation characteristics of sputtered Ni–Ti Shape Memory Alloy films grown on TiN

Near equiatomic Ni–Ti films have been deposited by magnetron co-sputtering on TiN films with a topmost layer formed by < 111> oriented grains (TiN/SiO 2/Si(100) substrate) in a chamber installed at a synchrotron radiation beamline. In- situ X-ray diffraction during Ni–Ti film growth and their complementary ex-situ characterization by Auger electron spectroscopy, scanning electron microscopy and electrical resistivity measurements during temperature cycling have allowed us to establish a relationship between the structure and processing parameters. A preferential development of < 110> oriented grains of the B2 phase since the beginning of the deposition has been observed (without and with the application of a substrate bias voltage of −45 and −90 V). The biaxial stress state is considerably influenced by the energy of the bombarding ions, which is dependent on the substrate bias voltage value applied during the growth of the Ni–Ti film. Furthermore, the present work reveals that the control of the energy of the bombarding ions is a promising tool to vary the transformation characteristics of Ni–Ti films, as shown by electrical resistivity measurements during temperature cycling. The in-situ study of the structural evolution of the growing Ni–Ti film as a consequence of changing the Ti:Ni ratio during deposition (on a TiN<111> layer) has also been performed. The preferential growth of < 110> oriented grains of the Ni–Ti B2 phase has been as well observed despite the precipitation of Ti 2Ni during the deposition of a Ti-rich Ni–Ti film fraction. Functionally graded Ni–Ti films should lead to an intrinsic “two-way” shape memory effect which is a plus for the miniaturization of Ni–Ti films based devices in the field of micro-electro-mechanical systems.

Thermomechanical behaviors of Ni–Ti shape memory alloy ribbons and their numerical modeling

The thermomechanical behaviors of Ni–Ti shape memory alloy (SMA) ribbons, associated with stress and temperature-induced transformations, are experimentally and numerically investigated. A 2-D incremental formulation of an SMA model is proposed and implemented in an ABAQUS finite element program through a user-defined material (UMAT) subroutine for the purpose of predicting the thermomechanical characteristics of SMA films. The evolution of stress–strain curves and phase transformation temperatures is examined with various annealing temperatures. The rhombohedral phase (R-phase) transformation appears in the specimen of an Ni–Ti SMA ribbon. The uncommon behaviors of permanent elongation and partial pseudoelastic behavior are observed. The 2-D SMA model with a new transformation function shows a good prediction for the hysteresis of the strain–stress curve and the recovery stress. For application of the developed numerical model of 2-D SMAs, the feasibility of the gripper actuator with the SMA ribbon is numerically demonstrated.

A comparative study on titania layers formed on Ti, Ti-6Al-4V and NiTi shape memory alloy through a low temperature oxidation process

For improving the bioactivity and biocompatibility of metals for medical applications, anatase titania layers were synthesized on Ti, Ti-6Al-4V and NiTi shape memory alloy (SMA) using the H 2 O 2 -oxidation and hot water aging treatment method at 80 °C. The thickness of the titania layers on Ti, Ti-6Al-4V and NiTi SMA was 7.43 ± 0.93 μm, 3.14 ± 0.38 μm and 4.04 ± 0.25 μm, respectively. X-ray diffraction (XRD) and transmission electron microscopic (TEM) analysis indicated that the titania layers formed were poorly crystalline anatase. Fourier transform infrared spectroscopy (FTIR) suggested that abundant Ti–OH functional groups were produced on titania, which could improve bioactivity of the metals. In addition, the titania layer formed on Ti substrate was shown to contain more molecularly chemisorbed water and Ti–OH functional groups than those on Ti-6Al-4V and NiTi SMA. Atomic force microscopic (AFM) results showed that the surface roughness values of metal samples depended on the scanning size and that surface roughness of samples significantly increased after the H 2 O 2 -oxidation and hot water aging treatment for all three metals. Compared to Ti-6Al-4V and NiTi SMA, the H 2 O 2 -treated and aged Ti samples exhibited the roughest surface. The wettability of samples was evaluated through water contact angle measurements. After the H 2 O 2 -oxidation treatment, the three metals exhibited high hydrophilicity. The bonding strength of titania layers on Ti, Ti-6Al-4V and NiTi was also investigated. Potentiodynamic polarization tests indicated that the corrosion resistance of H 2 O 2 -treated and aged Ti, Ti-6Al-4V and NiTi SMA was significantly improved due to the titania layer formation.

Development of corrosion-free concrete beam–column joint with adequate seismic energy dissipation

The use of Fibre-Reinforced Polymer (FRP) as reinforcement in concrete structures has received much attention owing to its higher resistance to corrosion compared to that of regular steel reinforcement. Since FRP is a brittle material, its use in seismic resisting structural elements has been a concern. FRP RC structures can be made ductile by utilizing a ductile material such as steel at the plastic hinge regions. However, the use of steel negates the corrosion resistance purpose of FRP. On the other hand, Nickel–Titanium (Ni–Ti) shape memory alloy (SMA) is highly resistant to corrosion. It also brings about an added advantage in seismic regions since it has the unique ability to undergo large deformation, but can regain its undeformed shape through stress removal. In this study, a SMA–FRP hybrid RC beam–column joint has been proposed to address not only corrosion resistance, but also seismic related problems. This joint is reinforced with a super-elastic Ni–Ti SMA bar at the plastic hinge regions of the beam and FRP in the other regions of the beam and column. To validate the proposed joint, an experimental investigation has been carried out to develop such a joint and test it under reversed cyclic loading. The results are compared in terms of load–storey drift, moment–rotation and energy dissipation capacity to those of a similar RC beam–column joint specimen reinforced with conventional steel. The SMA–FRP beam–column joint proved to have adequate energy dissipation under earthquake type loading.

Fabrication of a smart air intake structure using shape memory alloy wire embedded composite

Shape memory alloys (SMAs) have been actively studied in many fields utilizing their high energy density. Applying SMA wire-embedded composite to aerospace structures, such as air intake of jet engines and guided missiles, is attracting significant attention because it could generate a comparatively large actuating force. In this research, a scaled structure of SMA wire-embedded composite was fabricated for the air intake of aircraft. The structure was composed of several prestrained Nitinol (Ni–Ti) SMA wires embedded in ∩-shape glass fabric reinforced plastic (GFRP), and it was cured at room temperature for 72 h. The SMA wire-embedded GFRP could be actuated by applying electric current through the embedded SMA wires. The activation angle generated from the composite structure was large enough to make a smart air intake structure.

Superelastic cycling and room temperature recovery of Ti74Nb26 shape memory alloy

The superelastic cyclic response of Ti74Nb26 shape memory alloy (SMA), and the nature of cyclic evolution of its superelastic properties and their unexpected static recovery process after cycling, were investigated at room temperature. The critical stress for stress-induced martensitic transformation (σSIM) and stress hysteresis (Δσ) continuously decrease with increasing number of superelastic cycles. However, cumulative irrecoverable strain during cycling in samples of particular processing conditions increases only up to a certain number of cycles before decreasing with further cycling. Stress-free aging at room temperature after cycling was shown to increase σSIM and Δσ. The unexpected room temperature recovery is attributed to the recovery of retained martensite and point defects. Similar experiments conducted on conventional Ni-rich Ni–Ti SMAs also show static recovery at room temperature, indicating that the recovery process is not unique to Ti–Nb SMAs.

Thermal characteristics of Ni–Ti SMA (shape memory alloy) actuators

For two typical actuators of intelligent systems (Ni–Ti SMA cantilever and SMA helical spring), the evaluation of their thermal characteristics is presented. In order to determine the transformation temperatures and other thermal parameters of the two studied elements, the attention was concentrated on thermal analysis experiments. For each actuator configuration, comprehensive graphical interfaces have been developed, to run in Visual Basic, with respect to the results of performed thermal analyses.

Unnotched fatigue behavior of an austenitic Ni–Ti shape memory alloy

Constant stress amplitude fatigue life of an austenitic Ni (55.88 wt.%)–Ti shape memory alloy (SMA) within the stress amplitude range of 180–450 MPa was evaluated. The stress–strain hysteresis loops were monitored throughout the fatigue loading. They reveal that with the increasing number of fatigue cycles, the critical stress required for the stress-induced martensitic transformation, width of the hysteresis loop, recoverable and frictional energies of each cycle, all decrease while accumulated plastic strain increases. Post-mortem characterization of the fatigued specimens by employing differential scanning calorimetry (DSC), X-ray diffraction (XRD), and fractography were carried out, in order to understand the fatigue micromechanisms. Results indicate that the progressive accumulation of stress-induced martensite in the alloy is the source for the fatigue failure. Implications of these observations are discussed within the context of fatigue performance of SMAs and other materials that undergo stress-induced transformations.

Mechanical performance of apatite/TiO 2 composite coatings formed on Ti and NiTi shape memory alloy

To make metals bioactive for orthopaedic applications, apatite/TiO 2 composite coatings were formed on Ti and NiTi shape memory alloy (SMA) using a H 2O 2-oxidation and hot water aging technique and the subsequent accelerated biomimetic process. In the current investigation, nanoindentation, scratch testing and frictional testing were employed to assess mechanical properties and the adhesion of apatite/TiO 2 composite coatings formed on Ti and NiTi SMA. Nanoindentation testing conducted on cross-sections of composite coatings indicated that there was no significant difference in nanohardness and elastic modulus between apatite/TiO 2 composite coatings formed on Ti and NiTi SMA samples. The enhancement of the adhesion between the apatite layer and the metal substrates arose from the TiO 2 intermediate layer in the composite coating. The highest values of coating adhesion strength for Ti and NiTi SMA samples, as measured by scratch tests, were 22.58 N and 19.07 N, respectively. However, compared to corresponding Ti samples, NiTi SMA samples had better tribological properties.

Low-temperature biomimetic formation of apatite/TiO 2 composite coatings on Ti and NiTi shape memory alloy and their characterization

Through a low temperature process, a bilayer composite coating was formed on Ti and NiTi shape memory alloy (SMA). The composite coating consisted of a layer of titania, which was formed using a H 2O 2-oxidation and hot water aging technique, and a layer of apatite, which was formed through an accelerated biomimetic process by immersing as-oxidized metals in a high-strength simulated body fluid (5SBF). Various techniques including X-ray diffraction, scanning electron microscopy and energy dispersive X-ray spectroscopy were used to characterize the surfaces of samples at different stages of coating formation and the coatings formed. Bioactive apatite/TiO 2 coatings could be formed on NiTi SMA and firmly bonded to the metal substrate. But there were differences for the formation of the composite coating on Ti and NiTi SMA substrates. The composite coatings formed will render both metals bioactive and hence bone-bonding.

Characteristics and Chemical Stability of the Bioactive Titania Layer Formed on Ti, Ti-6Al-4V and NiTi SMA through a Low Temperature Oxidation Process

To improve the biocompatibility and bioactivity of titanium and titanium alloys, a titanium oxide layer was synthesized on Ti, Ti-6Al-4V and NiTi shape memory alloy (SMA) using a H2O2-oxidation and hot water aging technique. The surface of these metals before and after the oxidation treatment was characterized using scanning electron microscopy and energy dispersive X-ray spectroscopy. Because of the synthetic titanium oxide surface layer, the Al and V contents on the surface of as-oxidized Ti-6Al-4V decreased significantly. Similarly, the Ni content on the surface of as-oxidized NiTi SMA was also significantly reduced. Potentiodynamic polarization curves indicated that the synthetic titania layer was more chemically stable than the spontaneous titania film on the metals. Among the three metals, the oxide layer on Ti was the most stable chemically. The in vitro bioactivity of as-oxidized metals was assessed through incubation in simulated body fluid (SBF). Compared to as-oxidized Ti-6Al-4V and NiTi SMA, as-oxidized Ti was the most bioactive.

Mechanical characterization of NiTi SMA wires using a dynamic mechanical analyzer

An investigation into the thermo-mechanical response of trained Ni–Ti shape memory alloy (SMA) wires with two-way shape memory effect (TWSME) induced into them was conducted in order to examine the influence of the cold work on quasi-static and fatigue behavior of SMAs. Use of dynamic mechanical analyzer (DMA) for such tests on thin wires was examined and successfully utilized. Quasi-static stress–strain responses of the wires at different temperatures were obtained to determine the critical stresses for forward as well as reverse transformations. Mechanical stress-controlled fatigue tests were conducted on the wires in the austenitic state (130°C). They showed a two-stage deterioration of fatigue life—a rapid decay when the maximum stress of the fatigue cycle is higher than the critical transformation stress, and a more gradual deterioration at stress levels considerably lower than the critical stress. The fatigue life of the wires was found to increase with the load ratio, R, whereas the frequency of cyclic loading had only a marginal effect. Differential scanning calorimetry (DSC) of the fatigued austenite specimens indicated the presence of residual stress-induced martensite in the austenitic state. The high concentration of the austenite–martensite interfaces act as potential sites for stress-concentration and are the main source of the drastic decrease of the fatigue life in the two-phase region.