NiTi Shape Memory Research Articles

Design of SMA Actuator Based Access Device for Transanal Endoscopic Microsurgery

An access device using a combination of a compression coil spring made of NiTi shape memory alloy (SMA) and NiTi superelastic strips has been developed. Particular consideration is given to the application of the concept in tran- sanal endocopic microsurgery (TEM), but it could be used in other procedures where tissue expansion is required. In TEM, it could realize the gasless dilation of the rectum with safety and reliability, and facilitate surgeons to perform TEM. The theoretical design procedure to determine the specifications of the NiTi SMA spring and superelastic compo- nents is established. The feasibility and the possible problems associated with this application are discussed generally.

Influence of substrate temperature and deposition rate on structural and mechanical properties of shape memory NiTi films

NiTi thin films deposited by DC magnetron sputtering of an alloy (Ni/Ti:45/55) target at different deposition rates and substrate temperatures were analyzed for their structure and mechanical properties. The crystalline structure, phase-transformation and mechanical response were characterized by X-ray diffraction (XRD), Differential Scanning Calorimetry (DSC) and Nanoindentation techniques, respectively. The films were deposited on silicon substrates maintained at temperatures in the range 300 to 500∘C and post-annealed at 600∘C for four hours to ensure film crystallinity. Films deposited at 300∘C and annealed for 600∘C have exhibited crystalline behavior with Austenite phase as the prominent phase. Deposition onto substrates held at higher deposition temperatures (400 and 500∘C) resulted in the co-existence of Austenite phase along with Martensite phase. The increase in deposition rates corresponding to increase in cathode current from 250 to 350 mA has also resulted in the appearance of Martensite phase as well as improvement in crystallinity. XRD analysis revealed that the crystalline film structure is strongly influenced by process parameters such as substrate temperature and deposition rate. DSC results indicate that the film deposited at 300∘C had its crystallization temperature at 445∘C in the first thermal cycle, which is further confirmed by stress temperature response. In the second thermal cycle the Austenite and Martensite transitions were observed at 75 and 60∘C respectively. However, the films deposited at 500∘C had the Austenite and Martensite transitions at 73 and 58∘C, respectively. Elastic modulus and hardness values increased from 93 to 145 GPa and 7.2 to 12.6 GPa, respectively, with increase in deposition rates. These results are explained on the basis of change in film composition and crystallization.

Clinical trial of the treatment using superelastic shape-memory Ti-Ni alloy intramedullary nails for metastatic pathologic fractures

Clinical trial of the treatment using superelastic shape-memory Ti-Ni alloy intramedullary nails for metastatic pathologic fractures

Structure Refinement and Electropulse Current Effect on Mechanical Properties of Shape Memory TiNi Alloy

The paper presents data on structure refinement and electropulse current effects on deformability, strength and ductility of shape memory TiNi alloy, processed different severe plastic deformation techniques including equal channel angular pressing and electroplastic rolling. It was found that deformability and elongation to failure decreases with grain refinement but they can be enhanced by electropulse current.

Measurement of the lattice plane strain and phase fraction evolution during heating and cooling in shape memory NiTi

We report on in situ neutron diffraction measurements during heating and cooling through the phase transformation in shape memory NiTi. The lattice plane specific strain evolution remains linear with temperature and is not influenced by intergranular stresses, enabling the determination of the thermal expansion tensor of B19′ NiTi. The neutron measurements are consistent with macroscopic dilatometric measurements and a 30 000 grain polycrystalline self-consistent model. The accommodative nature of B19′ NiTi results in macroscopic shape changes being offset (with temperature) from the start and finish of the transformation. The texture does not evolve in the absence of biasing stresses.

Debonding initiation in a NiTi shape memory wire–epoxy matrix composite. Influence of martensitic transformation

The effect of martensitic transformation on the debonding initiation in single shape memory NiTi wire–epoxy matrix composite was studied. Three different heat treatment cycles were performed in order to obtain wires with different transformation characteristics. The composite samples were prepared using NiTi wires and steel wire for comparison. The single-wire specimens were subjected to the pull-out test. The in situ observations of the interfacial debonding and sliding behaviour during the test were carried out. In the pull-out tests, the embedded length is usually small and the interfacial shear strength (IFSS) can be estimated by dividing the maximum force to the lateral surface. But for evaluating the phase transformation effect, a long embedded length is required. In this case, a new method must be developed to estimate the IFSS. In the present study, a new method has been developed for the long embedded wire length samples using an analytical model. The IFSS was determined by implementing the experimental results into the model. It has been observed that the debonding stress for NiTi wire is significantly less than that for steel wire. It can be associated with the difference between the elastic modulus of the wires. The interfacial shear strength was then determined for these systems. It has been experimentally found that, when there is no phase transformation the IFSS is about 9 MPa while it is about 14 MPa when the martensitic transformation/reorientation occurs in the reinforced wire.

Anomaly in Electrical Resistivity with Thermoelastic Martensitic Transformation of Shape Memory NiTi Alloy

Anomaly in Electrical Resistivity with Thermoelastic Martensitic Transformation of Shape Memory NiTi Alloy

Constitutive model for uniaxial transformation ratchetting of super-elastic NiTi shape memory alloy at room temperature

The transformation ratchetting of super-elastic NiTi shape memory alloy was observed by the uniaxial stress-controlled cyclic tests [Kang, G.Z., Kan, Q.H., Qian, L.M., Liu, Y.J, 2009a. Ratchetting deformation of super-elastic and shape memory NiTi Alloys. Mech. Mater. 41, 139–153]. It is concluded that the NiTi alloy presents apparent ratchetting behaviour, and the ratchetting is collectively caused by the cyclic accumulation of residual induced-martensite and the transformation-induced plastic deformation (i.e., namely transformation ratchetting). Based on the experimental results, a cyclic constitutive model was constructed in the framework of generalized plasticity [Lubliner, J., Auricchio, F., 1996. Generalized plasticity and shape memory alloys. Int. J. Solids Struct. 33, 991–1003] to describe the transformation ratchetting of super-elastic NiTi alloy. The proposed model simultaneously accounts for the evolutions of residual induced-martensite and transformation-induced plastic strain during the stress-controlled cyclic loading by introducing an internal variable z c , i.e., cumulated induced-martensite volume fraction. The dependence of transformation ratchetting on the applied stress levels and the phase transformation hardening behaviour of the NiTi alloy are also considered in the developed model. The anisotropic phase transformation behaviours of the alloy presented in the tension and compression cases are described by employing a Drucker–Prager-typed transformation surface. It is shown that the simulated results of transformation ratchetting obtained by the proposed model are in good agreement with the corresponding experiments, since the typical features of transformation ratchetting are reasonably captured by the proposed model.

Characterization and properties of NiTi(W) and CuAlNi shape memory alloys

Abstract Shape memory wires were trained under constant stress in order to introduce a uniaxial shape memory effect by a thermomechanical treatment. These investigations were carried out on three different alloy systems (NiTi, NiTiW and CuAlNi) with different microstructures (cold-worked and annealed condition, with and without particles, large and small grain size). Several thousand thermal cycles were performed on the trained shape memory elements, continuously observing the changes in the deformation behaviour. The influence of the microstructure on development and stability of the intrinsic two-way shape memory effect has been discussed. Furthermore, this work deals with the production of thin specimens of shape memory alloys by melt-spinning and splat-cooling, the training procedure and their characterization with respect to microstructure and functional properties.

An ultrastructure study on the palatomaxillary suture of dog expanded by NiTi-SMA

To explore the ultrastructure of the palate-maxillary sutures under tensile forces by transmission electron microscope (TEM). The Suture expanders were made in NiTi-Shape memory alloy (NiTi-SMA). The maximum force was 3.5 N. Fourteen 8-month old mongrel dogs were used in the study. They were divided into three groups, (1) experimental group, (2) control group, (3) sham group. In the experiment and control groups, an 8 mm wide cleft was made by surgery. The suture expanders were fixed onto the palatine bones of the experimental group. The dogs of the experimental group were executed after 3, 7, 14, 28, 56 days of suture expansion. The change of suture tissue was examined by TEM. The cleft of the experiment group were closed at the ninth day of expansion. At the beginning, tissue rupture, exudation, death of fibroblasts, disruption of collagen and tear vessels were seen at the early stage of suture expansion. Then highly active functional manifestations were seen in both osteocytic and fibrocytic populations. At last, normal structure restored. Cell types and functional condition could be distinguished clearly by TEM. It suggests that the suture expansion should be the process of tissue repair and regeneration. The suture cells response, especially, the osteogenic response were the major factor of increasing suture width.

Fabrication of porous NiTi-shape memory alloy objects by partially hydrided titanium powder for biomedical applications

Porous NiTi-shape memory alloy (SMA) is a promising biomaterial with desirable mechanical property and appropriate biocompatibility for human implant manufacturing. In this research, porous NiTi-SMAs have been successfully produced by using thermohydrogen process (THP). This process has capability of production of homogenous structures, appropriate pore-size distributions and short sintering times. The THP-SMA samples produced in this research have a low Young’s modulus (19.8 GPa) and a high tensile strength of 255 MPa. These properties are close to those of the natural bone and can meet the mechanical property demands of the hard-tissue implants for heavy load-bearing applications. The samples produced exhibit sufficient thermoelastic effect distinguished by a 1.2% mean recoverable strain.

Shape memory Ni-Ti alloy swan-like bone connector for treatment of humeral shaft nonunion

From August 1990 to December 2007, 156 patients with humeral shaft nonunion were treated with our patented Ni-Ti shape memory alloy swan-like memory pressure connector (SMC). The SMC device cooled with ice before implantation was warmed to 40-50 degrees C after implantation to produce balanced axial and compression forces to stabilise the fracture three-dimensionally. This combined with autologous bone grafting achieved bone tissue regeneration in the fracture and promoted smooth recovery of joint function, with a nonunion healing rate of 98.7% after a single SMC implantation. Failure of nonunion healing occurred in only two cases but was successfully managed by a further operation. Complications were not found in any of these patients apart from four with pre-existing radial nerve injuries. These results demonstrate the effectiveness of the SMC device for the management of humeral shaft nonunion. The device provides continuous compression of the fracture with minimal trauma to the local blood supply.

Energy landscape for martensitic phase transformation in shape memory NiTi

First-principles calculations are presented for parent B2 phase and martensitic B19 and B19′ phases in NiTi. The results indicate that both B19 and B19′ are energetically more stable than the parent B2 phase. By means of ab initio density functional theory, the complete distortion–shuffle energy landscape associated with B2 → B19 transformation in NiTi is then determined. In addition to accounting for the Bain-type deformation through the Cauchy–Born rule, the study explicitly accounts for the shuffle displacements experienced by the internal ions in NiTi. The energy landscape allows the energy barrier associated with the B2 → B19 transformation pathway to be identified. The results indicate that a barrier of 0.48 mRyd atom −1 (relative to the B2 phase) must be overcome to transform the parent B2 NiTi to orthorhombic B19 martensite.

Fabrication and static characterization of carbon-fiber-reinforced polymers with embeddedNiTi shape memory wire actuators

In this work, unidirectional carbon-fiber-reinforced polymers (CFRP) with embedded NiTishape memory alloy (SMA) wire actuators were manufactured using a universal testingmachine equipped with a thermally controlled chamber. Beam specimens containingcold-worked, annealed and trained NiTi SMA wires distributed along their neutral planewere fabricated. Several tests in a three-point bending mode at different constanttemperatures were performed. To verify thermal buckling effects, electrical activation ofthe specimens was realized in a cantilevered beam mode and the influence ofthe SMA wire actuators on the tip deflection of the composite is demonstrated.

Ratchetting deformation of super-elastic and shape-memory NiTi alloys

The ratchetting deformation of super-elastic NiTi alloy was first observed by uniaxial stress-controlled cyclic tests, and the dependence of ratchetting upon the applied stress and loading type was discussed. The evolutions of responded peak/valley strain, nominal elastic modulus and transformation stress, as well as dissipation energy of the alloy during the stress-controlled cyclic loading were investigated. It is shown that the super-elastic NiTi alloy presents significant “transformation ratchetting” which is mainly caused by the cyclic accumulation of remained martensite due to the incomplete reverse transformation from the stress-induced martensite to original austenite, and the transformation ratchetting and its evolution depend greatly upon the applied stress amplitude, mean stress and loading chart. For comparison, the ratchetting deformation of shape-memory NiTi alloy and its dependence upon the loading condition were also observed. It is seen that the ratchetting deformation of shape-memory NiTi alloy under the stress-controlled cyclic loading differs greatly from that of super-elastic NiTi alloy, since no reversible transformation from the austenite to the stress-induced martensite occurs in the shape-memory NiTi alloy during the stress-controlled cyclic loading at room temperature. It means that no transformation ratchetting occurs in the shape-memory NiTi alloy, and the ratchetting deformation of the alloy occurred during the asymmetrical stress-controlled cyclic loading is mainly caused by the cyclic accumulation of visco-plastic deformation of re-oriented martensite, which is similar to the ratchetting deformation of ordinary metals. For both the super-elastic and shape-memory NiTi alloys, a nearly stable stress–strain response with small dissipation energy occurs after certain cycles. Some significant conclusions are obtained, which are useful to establish a constitutive model describing the ratchetting deformation of the NiTi alloys.

The Effect of Chemical Composition on Microstructure and Transformation Behavior of NiTi Shape Memory Alloy Prepared by Vacuum Arc Melting

NiTi shape memory alloys are a group of materials which have a lot of applications especially in aerospace industries and medical equipments because of their excellent properties. Shape memory effect (SME), pseudo-elasticity (PE), high corrosion resistance and biocompatibility is special properties of these alloys which lead to their extensive applications. The superior behavior of NiTi alloy is due to thermoelastic martensitic phase transformation. In the present paper, two NiTi shape memory alloys were prepared by non-consumable vacuum arc melting technique in copper water cooled crucible. One of them had commercial elements and the other had high purity elements. Metallographic investigation, chemical analysis, XRD and DSC were carried out on two alloys. Metallographic observation and XRD shows that structure at ambient temperature consists of austenite phase besides Ti2Ni, Ni3Ti intermetallic compounds and martensite phase. Transformation investigation determines that the impurity such as iron in commercial alloy causes two stage phase transformation B2→R→B19′.

Nanostructured Shape Memory TiNi Alloy Processed by Severe Electroplastic Deformation

Electropulse current effect during cold rolling on deformability, nanostructure formation and mechanical behavior in coarse-grained (CG) and ultrafine-grained (UFG) TiNi alloys enriched by nickel is investigated. The UFG sample subjected to cold rolling with current has a fracture strain (е = 1.91) which is higher than that without a current (е = 0.59). As a result of cold rolling with a current and a subsequent annealing at 400-450 °C, nanostructure is formed in both alloys, which leads to a significant enhancement of yield and ultimate stresses. It has been shown that the efficiency of electroplastic rolling depends on the purity of the alloys.

Powder metallurgy technology of NiTi shape memory alloy

Powder metallurgy technology was elaborated for consolidation of shape memory NiTi powders. The shape memory alloy was compacted from the prealloyed powder delivered by Memry SA. The powder shows Ms = 10°C and As = -34°C as results from DSC measurements. The samples were hot pressed in the as delivered spherical particle's state. The hot compaction was performed in a specially constructed vacuum press, at temperature of 680°C and pressure of 400 MPa. The alloy powder was encapsulated in copper capsules prior to hot pressing to avoid oxidation or carbides formation. The alloy after hot vacuum compaction at 680°C (i.e. within the B2 NiTi stability range) has shown similar transformation range as the powder. The porosity of samples compacted in the as delivered state was only 1%. The samples tested in compression up to e = 0.06 have shown partial superelastic effect due to martensitic reversible transform- ation which started at the stress above 300 MPa and returned back to e = 0.015 after unloading. They have shown also a high ultimate compression strength of 1600 MPa. Measurements of the samples temperature changes during the process allowed to detect the temperature increase above 12°C for the strain rate 10-2 s-1 accompanied the exothermic martensite transformation during loading and the temperature decrease related to the reverse endothermic transformation during unloading.

A Study on Fatigue Properties of a TiNi/Al Shape Memory Alloy

Shape memory alloy (SMA) has been used to improve the tensile strength of composite materials. Because it produces compressive residual stress in the matrix by using its shape memory effect. In order to fabricate a shape memory alloy composite, TiNi alloy and Al2024, Al6061 were used as reinforcing material and matrix, respectively. In this study, TiNi/Al2024 and TiNi/Al6061 shape memory alloy composites were made by the hot press method. The fatigue limit of a shape memory alloy composite determined the volume ratio and reduction ratio. The probabilistic stresslife curve (P-S-N curve) about the shape memory alloy composite makes up using statistical method.

Chemical and mechanical treatments to improve the surface properties of shape memory NiTi wires

In this paper the results of an experimental study concerning the effect of different surface treatments on NiTi shape memory alloy wires are presented. These treatments were conducted in order to improve the adhesion properties between the NiTi wires and an epoxy resin, acting as the matrix of a composite material. Mechanical and chemical surface treatments (immersion in acid and alkaline solutions), and different combinations of the above surface preparation procedures were studied. For the characterisation of the resulting alloy surface conditions electrochemical impedance spectroscopy, polarisation curves and potential versus time measurements were carried out. The alloy wire/epoxy matrix adhesion was characterised through pull out tests. The results proved that all adopted treatments can remarkably influence the electrochemical properties of the wires. The acid treatments favour the formation of a surface passivation layer, while the alkaline treatments are effective in producing a rougher surface morphology. Moreover, these basic treatments significantly reduce corrosion resistance of the alloys, another material property that has been incidentally investigated in the present context. The main effect of the mechanical surface treatment, consisting in abrading the alloy wires using an emery paper, was to increase the homogeneity of surface roughness. From the experimental results clear indications on the most promising surface treatments can be inferred.