Papers by Keyword: Superelasticity

Abstract: The effect of heat treatment temperature from 1173 K to 1373 K for 3.6 ks on mechanical and superelastic properties of an Ni-free Au-51Ti-18Co alloy (mol%) was investigated. The stress for inducing martensitic transformation (SIMT) and the critical stress for slip deformation (CSS) slightly decrease with increasing the heat–treatment temperature. Regardless of heat–treatment temperature, good superelasticity was definitely recognized with the maximum shape recovery ratio up to 95 % and 4 % superelastic shape recovery strain. As the mentioned reasons, the Au-51Ti-18Co alloy is promising for practical biomedical applications.

Abstract: Nickel-titanium (NiTi) alloys have been largely applied in biomedical devices due to their special properties of superelasticity and shape memory. Even though NiTi generally displays good corrosion resistance and biocompatibility, mechanical fatigue and fretting-corrosion resistance remain important challenges in a number of applications, since it can accelerate nickel ions releasing, that have been reported as cytotoxic, mutagenic, and allergenic. This study aims to develop an appropriate coating to help delaying crack nucleation and corrosion in NiTi alloys. Zirconia (ZrO₂) coating stands as a good candidate to improve the corrosion and wear resistance of metallic substrates and, in this work, it was obtained by electrodeposition on NiTi superelastic and shape memory orthodontic wires. The surface morphology and the chemical composition of the coated samples were evaluated using scanning electron microscopy (SEM) with energy dispersive X-ray spectrometry (EDS), X-ray diffraction (XRD), and atomic force microscopy (AFM). The mechanical response was evaluated by three-point bending tests. The results showed that the ZrO₂ layer was uniform and well adhered to the NiTi subtract. Additionally, it was observed that this coating was capable of undergoing severe deformation without cracking, indicating a potential increase in fatigue resistance of the conjugate.

Abstract: Shape memory alloy ribbons in austenitic state were studied in a tensile testing machine in order to assess their superelastic behavior. They were compared with conventional materials and hair wire. The shape memoy alloy ribbon shows a particular behavior, with an ultimate tensile stress of about 1450 MPa reached at 9.5 % strain. The superelastic plateau was recorded around 590 MPa on loading and around 350 MPa on the unloading branch. Following multiple loading and unloading cycles, the superelastic behavior was not affected, nor was affected the ultimate tensile strength, that remained in the same range as for the one tested before cycling. The advantages of the superelastic ribbon compared to the ones of conventional materials are discussed. The superelastic properties of the ribbon are in the range of single crystals on what concerns the recoverable strains. This is attributed to the particular fine microstructure of the NiTi ribbon.

Abstract: TiNi shape-memory properties are successfully used today for the fabrication of various technical devices. The limited machinability and high cost of TiNi encourage the use of near-net shape production techniques such as metal injection moulding. In this work TiNi alloys tensile test specimens are produced by metal injection moulding from pre-alloyed powders. A binder based on a mixture of polyethylene, paraffin wax and stearic acid is used. Parts with a density of about 96.6% of theoretical density are obtained. Scanning electron microscopy coupled with EDX measurements reveals a microstructure consisting of a TiNi matrix with small Ti₄Ni₂O_x and TiC inclusions. DSC and X-ray diffraction observations indicate the presence of additional Ni₄Ti₃ precipitates. The parts exhibit full superelasticity at room temperature even for strains of up to 4%, without the need for additional thermal post-treatments. Ultimate tensile strengths up to 980 MPa are obtained.

Abstract: 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 using in-vitro simulation. For this reason, we developed two prototype devices for in-situ simulation 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.

Abstract: Cu-12.5 wt.% Al-5 wt.% Mn-(0.05 to 0.2) wt.% B alloys were prepared by ingot metallurgy. The Aluminum and Manganese contents of the alloys were maintained constant, while that of Boron was varied. The alloys were then characterized by subjecting them to compositional analysis, Differential Scanning Caloriemeter and microstructural examination. The shape memory effect and superelasticity of the alloys were determined by bend and tensile tests on the alloy specimens. The investigation reveals that Boron acts as a good grain refiner, resulting in a reduction of about 80% in grain size. The addition of Boron also increases the transformation temperatures by ~ 10^oC, while at the same time decreasing the strain recovery by shape memory effect by 4%, and that by superelasticity by ~ 2%.

Abstract: The thermoelastic martensitic transformations, shape memory effect and superelasticity in high-strength single crystals of ferromagnetic FeNiCoAlX (X = Ta, Nb, Ti), CoNiGa, NiFeGaCo alloys and TiNi alloy in monophase and heterophase states with nanoscale dispersed particles are investigated. The dependences of the thermal and stress hysteresis, superelasticity temperature range, reversible transformation strain on the size of the dispersed particles, crystal orientation, stress state, level of applied stress and test temperature are obtained. The criteria of high-temperature superelasticity and the conditions for narrow thermal and stress hysteresis, large value of reversible transformation strain, which exceeds the theoretical lattice strain, are established. The thermodynamic description of the effect of particles on the stress-induced martenstic transformation in single crystals of new high-ferromagnetic alloys are elaborated.

Abstract: The main fields of the practical application of Ti-Ni-based alloys, with shape memory and superelasticity effects, in engineering and medicine, have been identified in the past decade. There are temperature-sensitive elements for the actuators, damping devices, fasteners, medical instruments and implants (correctors, clamps, stents), for trauma, spine, dentistry, soft tissues and vessels. The development of science and high technologies to produce semi-finished methods (thin-walled tubes, tapes and thin wire), as well as processing methods (laser cutting and welding) of Ti-Ni-based shape memory alloys (SMA) over the last 10 years has contributed to the creation and implementation into practice of more complicated and advanced devices, based on solid and porous shape memory materials. New technologies require not only the creation of fundamentally new shape-memory devices, but also, more importantly, the achievement of the highest possible functional properties (FP) of the SMA, by creating an optimal type of structure by thermomechanical treatment. Techniques for the regulation of FP are different for Ti-Ni SMA of different compositions. For the non-ageing equiatomic and near-equiatomic Ti-Ni SMA, the basic method of FP control is thermomechanical treatment (ТМT), including severe plastic deformation (SPD), forming various structures: from a well-developed recovered and polygonized dislocation substructure to a nanocrystalline structure. In the framework of the scientific direction, fundamental and applied research in the field of SMA thermomechanical treatment (TMT) has been carried out since 1977, by the Shape Memory Alloys Research Group of the National University of Science and Technology MISIS.The present review provides a brief description of the devices running on the shape-memory effect and superelasticity, developed jointly by NUST "MISIS" and various companies: Globetek 2000 Ltd (Melbourne, Australia); Semashko Central Clinical Hospital of Ministry of Railway Communications; Closed Joint-Stock Company ARMGAS-NT; Scientific-production Enterprise AVTOMATPROM (Moscow, Russia), et al. In addition, it presents the analysis of medical problems that can be solved using data devices, including work items of thermomechanically treated Ti-Ni SMA.

Abstract: This Chapter is focused on the Ti-Nb-based shape memory alloys for biomedical applications; the principal objective being to understand interrelations between structure and transformation features, static and dynamic functional properties, and conditions of their thermomechanical treatment. This Chapter includes also preliminary study of the surface characteristics of Ti-Nb-based alloys, including their elemental and phase compositions, tribological characteristics, wettability, electrochemical behaviour, and in vitro biocompatibility. The results obtained make it possible to conclude that Ti-Nb-based shape memory alloys represent one of the strongest candidates for a new generation of load-bearing orthopaedic or dental implants with improved biocompatibility, since they combine high biomechanical compatibility of Ti-Ni shape memory alloys with excellent biochemical compatibility of pure titanium.

Abstract: In the article there is presented a brief overview of combined systematic investigations of the alloys exhibiting thermoelastic martensitic transformations (TMTs). As is known, such alloys are distinguished by a whole number of specific one-and multi-fold-reversibility shape-memory (SM) effects occurring upon changing temperature, pressure, magnetic field, at the background of superplasticity and highly reversible deformation of transformation under a load or its relieving.There have been considered the alloys classification, the thermodynamical and kinetic aspects of TMTs, the origin and structural mechanisms of realization of multivarious pre-transition phenomena and TMTs proper, the influence of complex alloying, the peculiar features of a structure, the physico-mechanical properties and methods of production of the alloys most promising from the viewpoint of application. For this purpose there were employed structural methods of the X-ray diffraction (XRD) structure-phase analysis, neutron diffraction analysis, transmission and scanning electron microscopy techniques of high resolution, together with studying texture and atomic composition, as well as measuring a number of physical properties and SM-effect characteristics.The effect of the alloying and external actions of different origin on the magnetic and structural phase transformations and properties of the studied alloys with the magnetically, thermally, and mechanically controlled manifestations of the shape memory is discussed. A sequence of the TMTs with the formation of different martensitic phases, as well as the crystallographic structure and crystal-geometry specific features of the formation of these phases is described. The phase diagrams of the magnetic and martensitic transformations in a number of binary, ternary, and quaternary alloys synthesized via different schemes of alloying are presented.On the example of the binary and doped alloys TiNi it has been shown that an employment of thermo-mechanical treatments of the alloys via multiple or repeated torsion under pressure, rolling or drawing leads to their high-level strengthening and grain refinement (up to amorphization). In this case the use of low-temperature annealing provides for both the creation in the alloys of the homogeneous nanostructured state with a controllable grain size already in the interval of 50–200 nm and efficient regulation of physico-mechanical properties with retaining comparably high values of the parameters of SM, including deformation-, temperature-, and force-related.There has been revealed the effect of the grain size on the critical temperatures of TMTs and, as a consequence, on the TMT-stipulated SMEs. On this basement, in dependence of the grain size, the poly-packet, mono-packet twinned or single-crystal structures of martensite can be realized.There are discussed an experimentally revealed deformation-induced atomic disordering in Heusler alloys with self-forming a nanocrystalline fcc (A1) structure and amorphization in the alloys of titanium nickelide, as well as long-range order recovery in them taking place in the course of low-temperature annealing in conditions of retaining of the nanostructured state of austenite and at feasibility of cascade occurrence of TMTs and SMEs. It has been shown that the use of super-rapid quenching (SRQ) via employment of a melt spinning technique makes it possible to produce ductile submicrocrystalline ribbons of the Heusler Ni₂MnGa-based alloys with magnetically controllable TMTs. For a number of alloys based on titanium nickelide the effect of SRQ on the internal structure, grain refinement, and amorphization has been considered. The reasons of the amorphization and ways of subsequent nanostructurization of the alloys under investigation are discussed.