Martensitic NiTi Shape Memory Alloy Research Articles

Laser deposition of titanium layer to enhance two-way shape memory behavior of Nitinol sheet

One of the long-standing mechanical challenges associated with shape memory alloys (SMAs) is achieving and stabilizing an evolving two-way shape memory effect (TWSME) through thermomechanical training/cycling. In most martensitic NiTi SMAs that exhibit zero TWSME at room temperature, the TWSMEs typically require hundreds of cycling repetitions to track trivial values. This demonstrates a promising solution to overcome this challenge. The surface of NiTi SMA sheets was treated by depositing a certain number of titanium (Ti) powder layers on the NiTi surface using a laser beam. The results showed that a unique TWSME of approximately 12% with good stability was achieved after 100 cycles when the optimum number of five Ti layers was deposited, which was much higher than that of the NiTi sheet (0.3% TWSME) and the sample with a single-time laser path irradiation (1.8%). This was attributed to the low coefficient of thermal expansion of the Ti vs. NiTi sheet, which prevented reverse bending during heating, and the formation of Ti-rich intergranular Ti2Ni(Ox) precipitates, which hindered dislocation movement. Meanwhile, in this sample, owing to the generation of a large residual tensile stress and a large heat-affected zone that led to a reduction in plastic zones, the one-way shape memory effect (OWSME) was retained at values above 90% with good stability over cycling. However, the introduction of additional Ti layers can cause problems in terms of oxygen accumulation within the NiTi sheet, which should be considered when designing a product.

Self-centering capacity of RC columns with smart plastic hinges of martensitic NiTi SMA bars

The objective of this study was to examine the self-centering capacity of reinforced concrete (RC) columns with martensitic shape memory alloy (SMA) bars in the plastic hinge region. To achieve this, three RC columns were prepared: one was conventional and the other two had SMA bars in the plastic hinge zone. The columns had a diameter of 400 mm, height of 1400 mm, and aspect ratio of 3.5. The 400 mm long SMA bars were connected to the steel rebars using specially devised couplers. A cyclic lateral load was applied at the top of the columns, and the lateral displacements along the height of the columns and the corresponding forces were measured during cyclic lateral loading. The strain in the SMA bars was also measured. The RC column with SMA bars exhibited a plastic hinge around the couplers, thus concrete damage from cracking was concentrated at this site. This was completely different from the failure mode of the conventional RC column. The SMA bars in the column were in an elastic state, whereas the steel rebars yielded inside the couplers. The RC column with SMA bars exhibited excellent self-centering capacity, and its displacement recovery ratio was approximately 95% at a drift of 5%. However, its energy-dissipation capacity was lower than that of the conventional RC column. The equivalent damping ratio of the RC column with SMA bars was approximately 3% after 1.75% drift. It is conjectured that the excellent self-centering of the RC column with SMA bars was caused by the elastic behavior of the martensitic SMA bars.

Comparative analysis of process-induced strain glass states in austenitic and martensitic NiTi shape memory alloy plates

Strain glass alloys (SGAs) are metallic alloys with glassy martensitic nanodomains within a crystalline material that occur from compositionally or processing-induced strain. SGAs originate from shape memory alloys (SMAs) and exhibit similar shape memory properties and high actuation densities. The transition from SMA to SGA is relatively unexplored, and although there are similarities to amorphous SMAs and cold-worked SMAs, SGAs should be distinguished as a separate grouping. The transition occurs by interrupting the long-range martensitic order, which in turn disrupts the martensitic transformation, resulting in short-range martensitic order. A glassy martensitic phase is produced that exhibits enhanced structural and load-bearing abilities, functional stresses, and recoverability. In this study, the transformation from SMA to SGA is explored in two common commercially available SMAs, Ni49.5Ti50.5 and Ni50.8Ti49.2 (at. %), to compare martensitic versus austenitic SGAs, respectively. SMA plates were cold worked in 5% increments until a strain glass transition occurred. Characterizations of the samples at various stages of cold work were examined via differential scanning calorimetry (DSC), Vickers hardness, transmission electron microscopy (TEM), and synchrotron radiation X-ray diffraction (SR-XRD). Some prominent characteristics between the two plates, such as enthalpy peaks, twin size reduction, and crystallographic structure, were examined and compared to improve the understanding of the SMA to SGA transition.

Interactions between martensitic NiTi shape memory alloy and Nb nanowires in composite wire during tensile deformation

This study investigated the interactions between martensitic NiTi shape memory alloy and Nb nanowires in composite wire during tensile loading by means of in-situ high-energy X-ray diffraction. Nb nanowires have significant influence on the transformation behavior, annealed texture, and martensite variant reorientation behavior including texture evolution and load transfer among martensite variants of NiTi matrix. Meanwhile, the existence of densely distributed Nb nanowires increases the internal stress in NiTi matrix generated during martensite reorientation process, and causes the negative axial lattice stresses of all unfavored NiTi martensite variants after reorientation when the composite wire is under axial tensile stress. In addition, an exceptionally large elastic strain of 4.7% was produced over Nb nanowires after tensile deformation . Meanwhile, heterogeneous strain distribution exists in Nb nanowires, and it is associated with heterogeneous martensite reorientation strain of NiTi matrix. • Interactions between martensitic NiTi alloy and Nb nanowires is investigated. • Nanowires inhibit NiTi martensitic transformation and deformation twinning. • Nanowires induce the negative lattice stresses of all unfavored NiTi variants. • Inhomogeneous strain distribution exists in nanowires.

Mechanical properties and fracture mechanisms of martensitic NiTi shape memory alloy based on various thermomechanical-processing microstructures

Mechanical properties and fracture mechanisms of martensitic NiTi shape memory alloy (SMA) were investigated based on various thermomechanical-processing microstructures, where the thermomechanical processing was realized via local canning compression (LCC) and post-deformation annealing at 450 °C. Severe plastic deformation (SPD) occurs when the solution-treated martensitic NiTi SMA is compressed to 50% deformation by LCC, where amorphous structure (AMS) embedded with nano-sized crystalline inclusions (NCIs) and nanocrystalline structures (NCS) involving nanograined structures (NGS) and nanosubgrained structures (NSS) are dominant. Post-deformation annealing at 450 °C leads to the nanocrystallization of AMS, the formation of NSS as well as the growth of NCIs and NGS. The microstructure difference aroused by thermomechanical processing leads to the results that the alloy annealed at 450 °C presents much lower plasticity but higher fracture strength compared with the solution-treated one, while the as-compressed alloy exhibits no plasticity but extremely high fracture strength. In addition, the microstructure difference induced by thermomechanical processing also results in various fracture characteristics. The significant decrease of plasticity in the annealed alloy lies in two aspects, where the first one is that the decrease of reorientation+detwinning zone and plastic zone weakens their shielding effect of preventing the crack growth, and the second one is that crack-path configuration difference leads to larger proportion of fracture mode Ⅰ in the annealed alloy. The river pattern on the brittle fracture surface of the as-compressed alloy is induced by the convergence of cracks which propagate on a serial of parallel interfaces between shear bands and NCS regions.

Heterogeneity and inelasticity of deformation in a notched martensitic NiTi shape memory alloy specimen

The low symmetry martensite phase in Nickel-Titanium (NiTi) shape memory alloy (SMA) has a hierarchical microstructure with micro-scale crystallites, which themselves consist of nano-laminates with an internal twin structure. The martensite phase deforms inelastically though reorientation of the twin structure. Furthermore, structural features in the specimens such as cracks or notches interact with the microstructure, influencing the deformation of martensite. In this work, using multi-scale experiments and macro-scale modeling, we demonstrate that the deformation of martensite in NiTi at the macro scale in a notched specimen is largely determined by the structural features, and the hierarchical nature of the microstructure plays a smaller role. At the micro-scale, in-situ measurements indicate that the martensite microstructure continuously evolves during tensile loading. However, the nano-scale twin laminate structure persists despite the large imposed deformation and the presence of stress concentrating features such as a notch. This observation is counter to the intuition of martensite twins reorienting to large domains of the most favored crystallographic variant at larger loads. This persistence of a nano-laminate structure rather than coarsening into large domains of individual variants indicates that the deformation field around cracks/notches in martensite may be modeled for fracture-related phenomena with adequate accuracy using coarse-grained models, without a compelling need for martensite single-variant-scale models.

Low-Pressure and Low-Temperature Hydriding–Pulverization–Dehydriding Method for Producing Shape Memory Alloy Powders

Shape memory alloys (SMAs) are of high interest as active, adaptive “smart” materials for applications such as sensors and actuators due to their unique properties, including the shape memory effect and pseudoelasticity. Binary NiTi SMAs have shown the most desirable properties, and consequently have generated the most commercial success. A major challenge for SMAs, in particular, is their well-known compositional sensitivity. Therefore, it is critical to control the powder composition and morphology. In this study, a low-pressure, low-temperature hydriding–pulverization–dehydriding method for preparing well-controlled compositions, size, and size distributions of SMA powders from wires is presented. Starting with three different diameters of as-drawn martensitic NiTi SMA wires, pre-alloyed NiTi powders of various well-controlled sizes are produced by hydrogen charging the wires in a heated H3PO4 solution. After hydrogen charging for different charging times, the wires are pulverized and subsequently dehydrided. The wires and the resulting powders are characterized using scanning electron microscopy, differential scanning calorimetry, and X-ray diffraction. The relationship between the wire diameter and powder size is investigated as a function of hydrogen charging time. The rate of diameter reduction after hydrogen charging of wire is also examined. Finally, the recovery behavior due to the shape memory effect is investigated after dehydriding.

Effects of Hydrogen Charging on the Phase Transformation of Martensitic NiTi Shape Memory Alloy Wires

Ti-rich martensitic NiTi shape memory alloy (SMA) wires of 0.5 mm diameter were tested under hydrogen-charging conditions to reveal the effects on phase transformation. Hydrogen charging was performed by immersion testing for several durations. The SMA wires were characterized by differential scanning calorimetry (DSC), scanning electron microscopy with energy dispersive spectroscopy, and synchrotron radiation X-ray diffraction (SR-XRD) for the the as-received, polished, and hydrogen-charged conditions. The DSC revealed the phase-transformation behavior of the NiTi SMA wires. Single and triple heating/cooling cycles in the DSC show the relationship between hydrogen and temperature on the material. Five distinct peaks (peaks I–V) are observed during heating/cooling in the DSC. Peak I corresponds to the martensite-to-austenite (M → A) transformation. Peaks II, III, and IV are related to hydrogen charging. Peak II appears at about 210–230 °C, while peaks III and IV appear at about 350 and 440 °C, respectively. These higher temperature peaks, peaks II–IV, were observed for the first time for a martensitic NiTi SMA due to the large temperature range covered using the DSC. Only one peak (peak V) appears during cooling and corresponds to the austenite-to-martensite transformation peak. Ex situ and in situ SR-XRD revealed the phases and the crystallographic relationship to peaks I–V in the DSC.

High Strain Rate Compression of Martensitic NiTi Shape Memory Alloy at Different Temperatures

The compressive response of martensitic NiTi shape memory alloy (SMA) rods has been investigated using a modified Kolsky compression bar at various strain rates (400, 800, and 1200 s−1) and temperatures [room temperature and 373 K (100 °C)], i.e., in the martensitic state and in the austenitic state. SEM, DSC, and XRD were performed on NiTi SMA rod samples after high strain rate compression in order to reveal the influence of strain rate and temperature on the microstructural evolution, phase transformation, and crystal structure. It is found that at room temperature, the critical stress increases slightly as strain rate increases, whereas the strain-hardening rate decreases. However, the critical stress under high strain rate compression at 373 K (100 °C) increase first and then decrease due to competing strain hardening and thermal softening effects. After high rate compression, the microstructure of both martensitic and austenitic NiTi SMAs changes as a function of increasing strain rate, while the phase transformation after deformation is independent of the strain rate at room temperature and 373 K (100 °C). The preferred crystal plane of the martensitic NiTi SMA changes from (\( 1\bar{1}1 \))M before compression to (111)M after compression, while the preferred plane remains the same for austenitic NiTi SMA before and after compression. Additionally, dynamic recovery and recrystallization are also observed to occur after deformation of the austenitic NiTi SMA at 373 K (100 °C). The findings presented here extend the basic understanding of the deformation behavior of NiTi SMAs and its relation to microstructure, phase transformation, and crystal structure, especially at high strain rates.

High Strain Rate Compression of Martensitic NiTi Shape Memory Alloys

The compressive response of martensitic NiTi shape memory alloys (SMAs) under high strain rate (1200 s−1) was investigated on a modified Kolsky (Split Hopkinson) compression bar. The single-loading momentum trapping system ensures precise deformation control (1.4, 1.8, 3.0, 4.8, and 9.6 % strain) and single loading during dynamic compression. With increasing strain, the phase transformation peaks shift toward lower temperatures, while the intensities of these peaks decrease and eventually disappear completely at strains above ~7 %, where the onset of plastic deformation of reoriented martensite occurs. All transformation peaks are recoverable after deformation simply by annealing at 873 K (600 °C) for 30 min, except those peaks corresponding to strains above ~7 % (e.g., 9.6 %) which return upon annealing, but at a lower temperature. XRD results showed the variation of the strongest diffraction peak from (1 $$\bar{1}$$ 1) to (111) crystal plane before and after high strain rate compression.

Influence of Dynamic Compression on Phase Transformation of Martensitic NiTi Shape Memory Alloys

Shape memory alloys (SMAs) exhibit high damping capacity in both austenitic and martensitic phases, due to either a stress-induced martensite phase transformation or a stress-induced martensite variant reorientation, making them ideal candidates for vibration suppression devices to protect structural components from damage due to external forces. In this study, both quasi-static and dynamic compression was conducted on a martensitic NiTi SMA using a mechanical loading frame and on a Kolsky compression bar, respectively, to examine the relationship between microstructure and phase transformation characteristics of martensitic NiTi SMAs. Both endothermic and exothermic peaks disappear completely after experiencing deformation at a strain rate of 103 s−1 and to a strain of about 10 pct. The phase transformation peaks reappear after the deformed specimens were annealed at 873 K (600 °C) for 30 minutes. As compared to samples from quasi-static loading, where a large amount of twinning is observed with a small amount of grain distortion and fracture, samples from dynamic loading show much less twinning with a larger amount of grain distortion and fracture.

Annealing behavior and shape memory effect in NiTi alloy processed by equal-channel angular pressing at room temperature

A martensitic NiTi shape memory alloy was processed successfully by equal-channel angular pressing (ECAP) for one pass at room temperature using a core–sheath billet design. The annealing behavior and shape memory effect of the ECAP specimens were studied followed by post-deformation annealing (PDA) at 673K for various times. The recrystallization and structural evolution during annealing were investigated by differential scanning calorimetry, dilatometry, X-ray diffraction, transmission electron microscopy and microhardness measurements. The results indicate that the shape memory effect improves by PDA after ECAP processing. Annealing for 10min gives a good shape memory effect which leads to a maximum in recoverable strain of 6.9pct upon heating where this is more than a 25pct improvement compared with the initial state.

Wear behaviour of martensitic NiTi shape memory alloy under ball-on-disk sliding tests

Ball-on-disk sliding wear tests were conducted on martensitic NiTi shape memory alloy against an alumina counter ball. The wear features and coefficients of friction of the samples subjected to different loads and sliding cycles were investigated to reveal the degradation process of the NiTi surface. Three stages of degradation were identified, namely, a near-zero wear stage, a transition wear stage and an abrasive wear stage. A special wear feature, herein referred to as the “crown-like structure”, was produced under near-zero and transition wear conditions and was observed for the first time in this study. The mechanisms of the surface degradation process are further discussed with respect to the deformation distribution in the vicinity of the contact area under different loads.

Surface form memory by indentation and planarization of NiTi: displacements and mechanical energy density during constrained recovery

Indentation-induced two-way shape memory leads to pronounced temperature dependence of the depth of spherical indents made in martensitic NiTi shape-memory alloys. They are shallower when austenitic, and depth varies during both M → A and A → M transformations. If the impression is planarized, by metallographic grinding at T 107 J/m3, fully ~10% of the M → A enthalpy.

Engineering applications of NiTi shape memory alloys

This paper considers the use of martensitic NiTi shape memory alloys (SMAs) for technical applications in drive technology with a focus on bearing and gear technology. Within bearing technology, SMAs can be applied for the clearance adjustment of taper roller bearings using the extrinsic two-way effect. First constructive solutions show the axial motion of the outer ring to compensate the decreasing axial clearance as a result of different temperatures between shaft and housing. To compensate different thermal expansions SMAs offer improvements using the extrinsic two-way effect. With the help of steel springs the external load the SMA is working against can be realized. With given information about the stress–strain behavior for NiTi SMA under axial compression the necessary forces and compressions to reach the demanded elongations can be calculated.

In situTEM study of the shape recovery process in predeformed NiTi shape memory alloy

Shape recovery of pre-deformed martensitic NiTi shape memory alloy was studied by TEM during in-situ heating up to 300°C, a temperature significantly higher than the At temperature of the same material in bulk. Special attention has been paid to the microstructure variation upon subsequent heating and cooling. It is found that, although both forward and reverse phase transformations have taken place, the morphology of pre-deformed microstructure remains unchanged. The subsequently formed martensite upon re-cooling to room temperature consists of no lattice twins.

Rate dependence of deformation mechanisms in a shape memory alloy

Abstract The rate-dependent tensile deformations of twinned martensitic NiTi shape memory alloy (SMA) are studied for strain rates as high as 300s−1. The results show that the stress plateau associated with the martensite detwinning process still exists under high-strain-rate deformations. The results further suggest that the detwinning process taking place at the initial stage of the tension deformation is not very sensitive to the strain rate at least within the studied range (10−3 -3 × 102 s−1). Beyond the stress-plateau region where dislocation mechanisms are expected to dominate the deformation processes, the stress level increases more strongly with increasing strain rate. The transformation characteristics and shape recovery processes of the dynamically deformed samples provide further evidence that the detwinning process is well developed in the NiTi SMA under dynamic tension deformation.

High strain rate deformation of martensitic NiTi shape memory alloy

Shape memory alloys possess three fundamental properties due to their unique deformation mechanisms: shape memory effect, superelasticity and high damping capacity. The first two properties have already shown significant prospects for application, while the third one has not attracted significant attention. Recently, however, these materials appear promising for civil engineering applications due to the high damping capacity, coupling with good strength, ductility and very good corrosion resistance. Previous research on the deformation behavior of SMAs has been focused on low strain rates. Recently, some preliminary results have been reported on stress-induced martensite formation with austenitic SMAs subjected to impact. However, the high strain rate deformation behavior of SMAs in their martensitic state has not been reported. In the present work, the mechanical behavior of a martensitic NiTi SMA under very high state rate (3 {times} 10{sup 3} s{sup {minus}1}) compression is obtained and is primarily compared to its low rate (3 {times} 10{sup {minus}4} s{sup {minus}1}) deformation behavior.

Asymmetry of stress–strain curves under tension and compression for NiTi shape memory alloys

The stress–strain curves of polycrystalline martensitic NiTi shape memory alloys are often different for loading under tension and compression. Under tension, a flat stress-plateau occurs, while under compression, the material is quickly strain hardened and no flat stress-plateau is observed. Cyclic deformation under tension–compression also shows that it is more difficult to deform the material during compression than during tension, where an asymmetric stress–strain loop is obtained. TEM observations show that, under tension to 4% strain, martensite variants are partially reoriented via migration of variant interfaces with formation of dislocation networks mainly along the junction plane areas, and no significantly plastic deformation has been observed inside the martensite twin bands. While under compression to 4% strain, a high density of dislocations has been generated in both the martensite twin bands and the variant accommodation area, and no significant martensite reorientation via variant interfacial migration has been observed. This shows that the deformation mechanism of martensitic polycrystalline NiTi SMAs under tension is different from that under compression.