Memory Alloy NiTi Research Articles

Multi-phase intermetallic mixture structure effect on the ductility of Al3Ti alloy

In this work, to improve the ductility of Al3Ti alloy, a multi-phase intermetallic mixture structure was achieved by the post annealing treatments (AT) on the as-fabricated continuous shape memory alloy NiTi fiber reinforced Al3Ti composite (CSMAR-Al3Ti). Experimental results revealed that the multi-phase intermetallic mixture structure displays a multi-layer characteristic, including the newly formed intermetallic layer and the eutectic area. Microstructure characterization and formation mechanism confirmed that the newly formed intermetallic layer consists of NiAl, Al3Ni2 and Al3Ti phases, and the eutectic area still consists of Al3Ni and Al3Ti phases. The results of compression and tensile tests indicated that the multi-phase intermetallic mixture structure can simultaneously improve the maximum strength and the failure strain of the monolith Al3Ti alloy. Based on the systematic investigations, it is found that the toughening mechanism is related to the multi-layer characteristic of the multi-phase intermetallic mixture structure, which is beneficial to the crack blunting, crack deflecting and load transformation during the deformation process. Furthermore, the multi-phase intermetallic mixture structure achieved by the 48 h post annealing treatment is the most effective method to improve the ductility of Al3Ti alloy.

Interfacial microstructure characterization and mechanical behavior of NiTi fiber reinforced Al3Ti composite

To improve the ductility of Al3Ti alloy, the continuous shape memory alloy NiTi fiber (CSMAR) was introduced into intermetallic Al3Ti matrix for fabricating the novel CSMAR-Al3Ti composite in this work. Microstructure characterizations demonstrated that the CSMAR-Al3Ti composite mainly consists of Al3Ti layer, NiTi fiber, eutectic area and interfacial reaction layer. EBSD results indicated that the eutectic area is made up of Al3Ti and Al3Ni phases, the Al3Ti phase shows a strong [001] crystallographic oriented structure, while the Al3Ni phase has a non-textured structure. TEM results showed that the interfacial reaction layer between NiTi fiber and eutectic area is a multiple phase mixture, including various Ti-Al and Ni-Al intermetallics. Furthermore, TEM and HRTEM analyses revealed a newly formed Ti2Ni layer between NiTi fiber and interfacial reaction layer. Tensile test results confirmed that the CSMAR-Al3Ti composite could effectively improve the ductility of the Al3Ti alloy. Based on the systematic investigations of interfacial microstructure characterization, mechanical behavior and fracture morphology observation, it is found that the toughening mechanism of CSMAR-Al3Ti composite is related to the interfacial fine grain strengthening effect with the gradual distribution characteristic. In addition, the excellent metallurgical bonding between fiber reinforcement and matrix is also beneficial to the mechanical properties.

Microstructure and mechanical properties of continuous ceramic SiC and shape memory alloy NiTi hybrid fibers reinforced Ti-Al metal-intermetallic laminated composite

An optimized continuous ceramic SiC and shape memory alloy NiTi hybrid fibers reinforced Ti-Al metal-intermetallic laminated composite (as-optimized CSMAFR-MIL) was designed and fabricated using vacuum hot pressing sintering technique. The microstructure characterization of the composite was performed by scanning electron microscopy (SEM), energy-dispersive spectroscopy (EDS) and X-ray diffractometer (XRD). The formation mechanisms of the intermetallic layer were discussed. Furthermore, the mechanical properties of the as-optimized CSMAFR-MIL composite were measured via tensile tests. The experimental results indicated that the NiTi fibers were exhausted, and the inhomogeneous intermetallic layer was formed containing Al3Ni, Al3Ti, Al3Ti0.8V0.2 intermetallics due to the reactions of Al with NiTi and Ti-6Al-4V alloy. Meanwhile, the intermetallic centerline, which usually appeared in Ti-Al laminated composite, was significantly eliminated in the as-optimized CSMAFR-MIL composite due to the diffusion reaction between NiTi fiber and Al. In addition, the SiC fibers combined well with the intermetallics, and the interfacial phases TiC and Al4C3 were formed around the SiC fiber. Moreover, compared to continuous SiC fiber reinforced Ti-Al metal-intermetallic laminated (CFR-MIL) composite, the as-optimized CSMAFR-MIL composite possessed a good combination of high strength and superior ductility owing to the optimized microstructure of the composite and the mixed fracture mode of intermetallics.

Elastic deformation of twinned microstructures.

Many crystalline materials exhibit twinned microstructures, where well-defined orientation relationships define the special symmetry between different, elastically anisotropic twin variants. When such twins are subjected to external loading, additional internal stresses necessarily occur at the twin boundaries in order to maintain compatibility. These compatibility stresses are constant inside each variant in repeating stacks of twins and considerably affect the local stress state. In this paper, we use anisotropic linear elasticity to derive general analytical solutions for compatibility stresses in a stack of twin variants in arbitrary materials, for arbitrary variant volume fractions and twin types, subjected to arbitrary applied stresses. By considering two examples, growth twins in electrodeposited Cu and B19′ martensite twins in the shape memory alloy NiTi, we further demonstrate that compatibility stresses can considerably alter the preferred slip systems for dislocation plasticity as well as the effective macroscopic behaviour of twinned microstructures.

Dislocation core effects on slip response of NiTi- a key to understanding shape memory

The understanding of the non-Schmid behavior in shape memory alloy NiTi is considered a significant breakthrough because this alloy still remains enigmatic. We utilize an Eshelby-Stroh formalism in conjunction with Molecular Statics-Peierls stress calculation to predict the experimental CRSS results for three different crystal orientations and tension-compression cases. These combination of tools incorporating elastic anisotropy and core displacements are necessary to develop a precise understanding of non-Schmid behavior. We note the different extents of core spreading produce a highly asymmetric tension-compression behavior and strong orientation dependence governed by the non-glide shear and normal stress components. The experimental results and empiricism free modeling produces excellent agreement, and most importantly enlisting scientific insight into the shape memory response.

A study of shape memory alloy NiTi fiber/plate reinforced (SMAFR/SMAPR) Ti-Al laminated composites

In this work, shape memory alloy NiTi fiber reinforced (SMAFR) Ti-Al laminated composites containing ∼41 vol%Ti and ∼2.5 vol%NiTi was fabricated via vacuum hot pressing method. In order to accurately determine the reaction products of the fabrication process, a new kind of laminated composites i.e. shape memory alloy NiTi plate-reinforced (SMAPR) Ti-Al laminated composites was successfully prepared using the fabrication process that is similar to SMAFR laminated composites. Results showed that Al3Ti and Al3Ni are formed during fabrication process, and residual Al existed in the intermetallic layer. In addition, the quasi-static tensile test indicates that the tensile property of SMAFR laminated composites is superior to that of Ti/Al3Ti composites. The fracture process of SMAFR laminated composite includes three basic stages: (I) delamination of interface of Ti/Al3Ti; (II) fracture of Al3Ti (and other products) and (III) fracture of NiTi fiber. Moreover, the novel SMAPR laminated composite possessed a good combination of high strength (compressive strength: 1105.4 MPa) and high plasticity (strain to failure: 5.1%), indicating that this kind of laminated composites is a new class of structural materials.

Experimental and computational investigation of the effect of phase transformation on fracture parameters of an SMA

A comprehensive, multi-method experimental characterization of fracture is conducted on shape memory alloy NiTi that exhibits superelasticity due to austenite-to-martensite stress induced phase transformation. This characterization includes (i) load-based measurement of critical stress intensity factor (Kmax) using ASTM standard E399, (ii) measurement of crack tip opening displacement (CTOD) per ASTM standard E1290, (iii) the digital image correlation (DIC) characterization of the transformation zone as well as the displacement-field based measurement of Kmax from the DIC data. Samples have also been tested at T = 100 °C to suppress the martensitic transformation to investigate transformation toughening. The experimental investigation is complemented with finite element (FE) analysis that uses Auricchio–Taylor–Lubliner constitutive model. A direct observation with DIC revealed a small scale transformation (K-dominance). Kmax of the transforming material is higher than that of the transformation-suppressed material tested at 100 °C, suggesting transformation toughening. At 100 °C, the material becomes quite brittle with a very small crack-tip plastic zone when the transformation mechanism is blocked. By measures of critical CTOD, the gap widens even more between the superelastic and transformation-suppressed cases, particularly because of the side effect that, in this very interesting material, material modulus increases with temperature. Evaluating the transformation zone from the DIC strains with reference to the uniaxial stress–strain curve, an equivalent strain form is proposed in conjunction with the plane stress FE prediction.

Fabrication, mechanical properties and damping capacity of shape memory alloy NiTi fiber-reinforced metal–intermetallic–laminate (SMAFR-MIL) composite

A novel Shape Memory Alloy NiTi fiber-reinforced metal-intermetallic–laminate (SMAFR-MIL) composite with a volume fraction of ~3.5% NiTi was fabricated using vacuum hot pressing method in this paper. The microstructure characterization of this laminate composite was performed by scanning electron microscope (SEM), energy dispersive spectrometer (EDS) and X-ray diffractometer (XRD), and the mechanical property of SMAFR-MIL composite was determined by compression test. In addition, damping capacity was measured using Dynamic Mechanical Analyzer (DMA) in a temperature range from 30°C to 50°C. The experimental results indicated that the average compressive strength and the strain to failure are ~1208MPa and ~4.4% for loading perpendicular to the layers, and ~962MPa and ~4.8% for loading parallel to the layers, respectively. The loss modulus of SMAFR-MIL composite is ~3.7GPa, which is five times greater than that of Ti/Al3Ti laminate composite without NiTi fiber reinforcement, while the damping factor of SMAFR-MIL composite is ~4.5%. Both the mechanical and damping tests demonstrated that SMAFR-MIL composite is a new class of structural and functional composite with superior mechanical and damping properties.

Development of Patient-specific Orbital Floor Implants Made of Shape Memory Alloys

The shape memory alloy NiTi has successfully proved in many areas of medical technology. This paper describes the innovative idea and development of orbital floor implants made of NiTi. Pseudoelastic NiTi has the peculiarity to withstand large mechanical deformations of up to 8% without being plastically deformed, i.e. it returns to its original shape with relief of the strain. This material property can be used to produce flexible, yet stable implants. The NiTi implants can be implanted into the body in compressed form through relatively small approaches. Arriving at the fracture position, they develop self-reliantly in the previously memorized shape. The perioperative trauma is reduced. The treatment of orbital floor fractures is often done surgically by use of implants made of rigid materials such as titanium, which are inserted in the eye socket through an open approach on the lower eyelid. Implants made of a pseudoelastic shape memory alloys can help to reduce the required incision length of the approach. It is even possible to insert the implant in a minimally invasive procedure via endoscopic access routes and to completely avoid open approach. In this paper first design variants of pseudoelastic orbital floor implants are presented and considerations on the manufacturing process are made.

Treatment of Aseptic Necrosis of the Lunate Bone (Kienböck Disease) Using a Nickel–Titanium Memory Alloy Arthrodesis Concentrator

Avascular necrosis of the lunate bone (Kienböck disease) is caused by loss of blood supply of the bone. This study aimed to evaluate the efficacy and safety of a novel nickel–titanium (Ni–Ti) memory alloy arthrodesis concentrator in the treatment of this disease.A consecutive 24 patients with stage IIIb aseptic lunate necrosis were treated with scapho-trapezio-trapezoeid (STT) arthrodesis using a Ni–Ti arthrodesis concentrator from August 2008 to December 2012. Wrist pain, grip strength, carpal height, and scapholunate angle were measured and compared before and after the surgery. The wrist functions were evaluated using the Mayo scale.Patients were followed up for a mean of 12 months (range, 6–24 months). Grip strength of the affected side was significantly improved after the surgery (18 ± 4.74 kg vs. 30.21 ± 7.14 kg, P < 0.0001). Wrist pain score was significantly decreased from 5.88 ± 0.9 to 0.5 ± 0.51 (P < 0.0001). Carpal height and Mayo score were also significantly increased after the surgery (P < 0.0001). Scapholunate angle was significantly decreased after the surgery (68.38 ± 7.28° vs. 49.91 ± 4.28°, P < 0.0001). No implant breakage, loose implant, wound infection, or nonunion occurred.STT arthrodesis is effective for the treatment of stage IIIb lunate necrosis. The Ni–Ti memory alloy arthrodesis concentrator is a convenient tool for STT arthrodesis with excellent and reliable results.

Infrared thermography videos of the elastocaloric effect for shape memory alloys NiTi and Ni2FeGa.

Infrared thermogrpahy was utilized to record the temperature change during tensile loading cycles of two shape memory alloy single crystals with pseudoelastic behavior. During unloading, a giant temperature drop was measured in the gage section due to the elastocaloric effect. This data article provides a video of a [001] oriented Ni2FeGa single crystal, including the corresponding stress–strain curve, shows the temperature drop over one cycle. The second video of a [148] oriented NiTi single crystal depicts the repeatability of the elastocaloric effect by showing two consecutive cycles. The videos are supplied in this paper. For further analysis and enhanced discussion of large temperature change in shape memory alloys, see Pataky et al. [1]

Observation of Phase Transformation in Shape Memory Alloy NiTi Wire during &lt;i&gt;In&lt;/i&gt; &lt;i&gt;Vitro&lt;/i&gt; Simulation of Orthodontic Treatment

The aim of this study was to simulate the different loadings on NiTi Shape Memory Alloy in the form of wire that occurs in orthodontic praxis. Our main goals were to detect and determine the beginning of the stress-induced transformation from the austenite to martensite phases and to indicate the transformation plateau at different deformations during orthodontic treatment usingin-vitrosimulation. For this reason, we developed two prototype devices forin-situsimulation of orthodontic treatment by which we measured the electrical resistance and hardness. Accompanying these two properties and the microstructure observations of loaded wires by Transmission Electron Microscopy enabled us to detect successfully the stabilized stress-induced martensite due phase transformation. We found out that transformation depends strongly on the type of loading.

Mechanical behaviour of Ni-Ti memory alloy femoral head support device during implant operation: a finite element analysis study

Objective To explore dynamic simulation of Ni-Ti memory alloy umbrella necrosis of the femoral head in an open process by the finite element method,which may provide the theoretical support for use of individualized treatment of early osteonecrosis memory alloys.Methods By using fast individual establishment method,the early stage osteonecrosis model of the femoral head was constructed based on data from multi-slice spiral CT with Mimics 10.0,Geomagic Studio 11.0 and Hypermesh 11.0 softwares.According to various parameters of shape memory alloy umbrella,we established a three-dimensional finite element model of Ni-Ti memory alloy umbrella.We simulated memory alloy umbrella commence within the femoral head.Lsdyna was used to analyze Ni-Ti memory alloy position after opening in the femoral head and the destruction of the femoral head.Results The top of umbrella should be fully constrained to avoid the umbrella opening in the unexpected location.The device had an efficacy rate of 82.4％.The patient's bone mineral density (BMD) directly impacted on effect of memory alloy umbrella.The patients with osteoporosis had the high risks that memory alloy umbrella could penetrate the femoral head.With the sleeve placed in the middle,there was none extra damage to the decompression channel.Conclusion BMD is an important factor for success of umbrella memory alloy Ni-Ti implant surgery.It can be used to to asses the risks of the surgery.The proposed finite element analysis method could be used as pre-operation phase of individual treatment for individual patient. Key words: Osteonecrosis of the femoral head ; Ni-Ti memory alloy ; Bone mineral density

Nonequiatomic NiTi Alloy Produced by Self Propagating High Temperature Synthesis

Shape memory alloy NiTi in porous form is of high interest as implantable material, as low apparent elastic modulus, comparable to that of bone, can be achieved. This condition, combined with proper pore size, allows good osteointegration. Porous NiTi can be produced by self propagating high temperature synthesis (SHS), starting from mixed powders of pure Ni and Ti. Process parameters, among which powder compaction degree and preheating temperature, strongly influence the reaction temperature and the resulting product: at low reaction temperatures, high quantity of secondary phases are formed, which are generally considered detrimental for biocompatibility. On the contrary, at higher reaction temperatures, the powders melt and crystallize in ingots. The porous structure is lost and huge pores are formed. Mechanical activation of powders through ball milling and addition of TiHx are investigated as means to reduce reaction temperature and overheating, in order to preserve high porosity and limit secondary phases content. Both processes affect SHS reaction, and require adjustment of parameters such as heating rate. Changes in porous shape and size were observed especially for TiHx additions: the latter could be a promising route to obtain shaped porous products of improved quality.

Mechanical behaviour of umbrella-shaped, Ni-Ti memory alloy femoral head support device during implant operation: a finite element analysis study.

A new instrument used for treating femoral head osteonecrosis was recently proposed: the umbrella-shaped, Ni-Ti memory femoral head support device. The device has an efficacy rate of 82.35%. Traditional radiographic study provides limited information about the mechanical behaviour of the support device during an implant operation. Thus, this study proposes a finite element analysis method, which includes a 3-step formal head model construction scheme and a unique material assignment strategy for evaluating mechanical behaviour during an implant operation. Four different scenarios with different constraints, initial positions and bone qualities are analyzed using the simulation method. The max radium of the implanted device was consistent with observation data, which confirms the accuracy of the proposed method. To ensure that the device does not unexpectedly open and puncture the femoral head, the constraint on the impact device should be strong. The initial position of sleeve should be in the middle to reduce the damage to the decompression channel. The operation may fail because of poor bone quality caused by severe osteoporosis. The proposed finite element analysis method has proven to be an accurate tool for studying the mechanical behaviour of umbrella-shaped, Ni-Ti memory alloy femoral head support device during an implant operation. The 3-step construct scheme can be implemented with any kind of bone structure meshed with multiple element types.

NiTiおよびNiフリー生体用形状記憶Ti合金の腐食挙動

ステントやコイルなどの体内留置医療機材として形状記憶合金の利用が拡大している．本論文では生体用機器に実用されているNiTi と，およびさらなる安全性の面から，今後NiTiに代わる材料として注目されているNiなどの生体アレルギー元素を含まないNiフリー生体用形状記憶Ti合金の生体環境での腐食挙動について，とくに1990年以降に公表された論文のデータを総括することとした．現状として，どちらの合金も生体用として耐食性に大きな問題は無いが，両者を比べるとNiフリー生体用Ti基形状記憶合金の耐食性方がNiTiのそれよりやや良いと判断できる．これより，耐食性の面でもより安全なNiフリー生体用形状記憶チタン合金のさらなる開発が望まれる．

胸骨骨折治疗方法的临床研究

目的：探讨胸骨骨折的外科治疗方法。方法：回顾分析我院于2000年1月至2014年4月收治的35例胸骨骨折，将33例手术治疗的胸骨骨折分为3组：钢丝固定组(包括钢子加钢丝固定)、钢板固定组和记忆合金固定组(镍钛记忆合金胸骨环抱固定)，比较其手术时间、术中出血、术后疼痛时间，并比较其术后并发症。结果：记忆合金固定组手术时间为31 ± 7.6 min，明显少于钢丝固定组(52 ± 5.4 min)和钢板固定组(74 ± 5.1 min)，P < 0.05；记忆合金固定组术中出血为7.0 ± 4.6 ml，明显少于钢丝固定组(11.0 ± 3.4 ml)和钢板固定组(16.0 ± 4.1 ml)，P < 0.05；记忆合金固定组术后疼痛时间为1.0 ± 0.5 d，明显少于钢丝固定组(3.5 ± 0.5 d)和钢板固定组(3.0 ± 1.0 d)，P < 0.05；记忆合金固定组术后并发症发生率明显低于钢丝固定组和钢板固定组，P < 0.05。结论：镍钛记忆合金胸骨环抱固定是目前治疗胸骨骨折的最佳方法。 Objective: To compare the efficacies of treatment for sternal fracture with steel wire, steel plate, and memory alloy. Methods: 35 cases with sternal fracture surgical treated from Jan. 2000 to April 2014 were followed up. Among them, 15 cases were surgical treated with memory alloy, 8 cases were surgical treated with steel plate, 10 cases were surgical treated with steel wire. The condi- tions before, during and after operation and complications were compared. Result: The operation time of memory alloy group was 31 ± 7.6 min, less than steel plate group (74 ± 5.1 min) and steel wire group (52 ± 5.4 min), P < 0.05; the blood loss of memory alloy group was 7.0 ± 4.6 ml, less than steel plate group (16.0 ± 4.1 ml) and steel wire group (11.0 ± 3.4 ml), P < 0.05; the duration of pain of memory alloy group was 1.0 ± 0.5 d, less than steel plate group (3.0 ± 1.0 d) and steel wire group (3.5 ± 0.5 d), P < 0.05. The postoperative complication incidence rate of memory alloy group was less than steel plate group and steel wire group, P < 0.05. Conclusion: The best way of treat- ment for sternal fracture is to fasten with Ni-Ti memory alloy surrounding.

Stress dependent electrical resistivity of orthodontic wire from the shape memory alloy NiTi

The aim of this study was to compare and evaluate the stress induced martensitic phase under different loading conditions in orthodontic wire from the Shape Memory Alloy Nickel–Titanium (NiTi). For this purpose we investigated the phase transformation from austenite to martensitic due to the different loading conditions by measuring the electrical resistance which could show the type of deformation which occurs at the beginning of phase transformations. In this framework we developed two special devices for measurements of electrical resistance in different types of load and their combination on the orthodontic wire. These results were compared with the analytically calculated stresses in the orthodontic wire. It was shown that they caused complex or multi-axial stress state phase transformation rather than other more simple load such as uniaxial loading. Finally, the article presents the deformation which occurs at the change of phase that is nearly connected to the useful superelasticity effect of the Shape Memory Alloy NiTi.

Polymer (PTFE) and shape memory alloy (NiTi) intercalated nano-biocomposites

Engineering on a nano-scale has been undertaken to mimic a biomaterial by forming an intercalated nano-composite structure by PVD sputtering of a polymer with a nickel-titanium (NiTi) shape memory alloy (SMA). A PTFE polymer has been selected due to its elastic properties, low interactions with water, optimum surface energies, stability and chemical resistance. NiTi SMAs allow the coatings to be energy absorbent and thus suitable in load bearing situations. The coatings are aimed to constantly withstand variable adverse biological environments whilst maintaining their characteristics. The nano-intercalated structures have been characterised for their wettability, friction coefficients, chemical composition, and morphology. Intercalation of a polymer with energy-absorbing alloys uncovers a set of material systems that will offer characteristics such as self-healing of hierarchal tissue in the body. The reformation of PTFE following sputter deposition was confirmed by FTIR spectra. According to SEM analysis PTFE shows a promising surface interaction with NiTi, forming stable coatings. Surface interactions are evident by the hydrophobic behaviour of films as the composite's water contact angle is around 86° which lies in-between that of PTFE and NiTi. The nano composite films are lubricious and have a measured CoF below 0.2 which does not vary with layer thickness.

Constitutive model for localized Lüders-like stress-induced martensitic transformation and super-elastic behaviors of laser-welded NiTi wires

The application of the shape memory alloy NiTi in micro-electro-mechanical-systems (MEMSs) is extensive nowadays. In MEMS, complex while precise motion control is always vital. This makes the degradation of the functional properties of NiTi during cycling loading such as the appearance of residual strain become a serious problem to study, in particular for laser micro-welded NiTi in real applications. Although many experimental efforts have been put to study the mechanical properties of laser welded NiTi, surprisingly, up to the best of our understanding, there has not been attempts to quantitatively model the laser-welded NiTi under mechanical cycling in spite of the accurate prediction required in applications and the large number of constitutive models to quantify the thermo-mechanical behavior of shape memory alloys. As the first attempt to fill the gap, we employ a recent constitutive model, which describes the localized SIMT in NiTi under cyclic deformation; with suitable modifications to model the mechanical behavior of the laser welded NiTi under cyclic tension. The simulation of the model on a range of tensile cyclic deformation is consistent with the results of a series of experiments. From this, we conclude that the plastic deformation localized in the welded regions (WZ and HAZs) of the NiTi weldment can explain most of the extra amount of residual strain appearing in welded NiTi compared to the bare one. Meanwhile, contrary to common belief, we find that the ability of the weldment to memorize its transformation history, sometimes known as ‘return point memory’, still remains unchanged basically though the effective working limit of this ability reduces to within 6% deformation.