The results of studies on the influence of ultrasonic vibrations (UVs) on shape memory effect (SME) in NiTi alloys are presented. The appearance of shape memory effect is due to changes in the temperature and mechanical stress. They can operate simultaneously under the ultrasonic treatment. It has been found that the ultrasonic vibrations can initiate shape recovery processes in a material with shape memory effect. The reversion of the accumulated deformation occurs at the direct and reverse martensite transformations under the influence of ultrasonic vibrations. Since the process is accompanied by ultrasonic heating, small thermal mechanical hysteresis loops have been observed during the direct transformation. The deformation reversion can be explained by ultrasonic heating of the sample due to the dissipation of vibrations and the formation of austenite.
This chapter proposes an efficient approach to thermomechanical constitutive modeling for shape memory alloys using a novel separation of a martensite internal variable. This approach assumes that the martensitic internal variable has two components corresponding to two contrary martensite ensembles, which are separated by their opposite signs of contributions to a quantity of transformation strain. The constitutive models, based on a new separating concept, are presented in this chapter for uniaxial and three-dimensional proportional loading. The kinetic relations of these models are constructed using the Brinson model ideas concerning the interphase transformation processes in shape memory alloys. A number of illustrative numerical examples are presented here for one-dimensional modeling. The new separation of the internal variable has prospects for the description of material behavior in cases when mechanical loading may change sign and the phenomena are caused by martensitic reorientation. The conceptual and methodological solutions stated in the present work may be useful for subsequent modeling.
It has been shown that the conversion of austenitic manganese steels – via their purposeful alloying – into the class of precipitation-hardening materials (when aging leads to the formation of VC carbides) provides for high values (2.5-2.7 %) of the shape memory effect (SME) and considerable improvement in strength characteristics of the steels after aging and implementation of γ→e→γ transformations. Strength characteristics and SME can be governed in wide limits by means of controlled changing of the amount, dispersity, and distribution of VC carbides during aging. The shape memory effect with a reversible deformation of 1.6—1.7% in the metastable steels such as 0.20C-20Mn-2Si-1V with an e–martensite initial structure is obtained as a result of the retwinning of e martensite during cold deformation and the subsequent e→γ transformation during heating.
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.
The phenomenological approach based on analysis the crystal-chemical parameters and their change depending on concentration of the alloying element for definition of concentration intervals of existence of stable phases with ordered structures in the ternary Titanium Nickelide based alloys is described. It was shown that a comparison of the data of change patterns in crystal-chemical parameters phenomenologically predicted and obtained from experimental data, allows to predict some of the conditions of stabilization in the examined phase, the solubility limits of alloying elements and, consequently, its homogeneity region, as well as formation conditions of multiphase systems. The analysis carried out allows us to make a conclusion regarding the issue that is often discussed in the analysis of phase diagrams of binary and multicomponent systems - what is the ratio of the size and electronic contributions to the stability of the phases. It was shown that the contribution of the size factor to the stabilization of B2 and B19(or В19′) phases is predominant and the role of this factor grows with the increase in the amount of the alloying element.
Recent studies in the field of thermomechanical treatment (TMT) of Ti-Ni shape memory alloys are presented and discussed. The main problems of structure and phase transformations, and their effect on the Ti-Ni functional properties are stated. The structure formation and phase transformations are studied using TEM, XRD and DSC analyses, and the specific features of the nanostructures formed as a result of TMT are described. Algorithms for the calculation of the theoretical limit of recovery strain under the single-crystal and poly-crystal approaches (with and without texture) are proposed and experimentally validated for nanostructured SMA. Static functional properties (recovery strain and stress, parameters of superelasticity) and dynamic (fatigue) functional properties (multiple realization of stress-free shape memory, shape memory under stress, recovery stress generation-relaxation, superelastic mechanocycling) of the thermomechanically-treated Ti-Ni shape memory alloys are discussed in detail. The main attention is paid to the interrelations between the microstructure and the functional properties of the thermomechanically-treated Ti-Ni shape memory alloys.
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.
In the work, we studied the regularities and mechanisms of microstructure formation in binary TiNi alloys with 50.2 and 50.8 at.% Ni under warm abc pressing with a stepped decrease in strain temperature (873, 673, 623, and 573 К) and isothermal (723 К) multipass caliber rolling. In the TiNi alloy with 50.2 at.% Ni at all abc pressing stages, microstructures inhomogeneous in grain size were formed due to faster dynamic recrystallization and hence faster formation of finer grains and subgrains in strain localization bands compared to microvolumes bounded by these bands. After final abs pressing at 573 К with a total true strain е = 7.7, a microstructure composed of submicro-and nanocrystalline grains and subgrains was found. In the TiNi alloy with 50.8 at. % Ni subjected to warm rolling, three stages of grain structure evolution were revealed. At the first stage with low strains, the average grain size ‹d› increased due to collective dynamic recrystallization. At the second and third rolling stage, the average grain size ‹d› decreased steeply due to discontinuous and continuous dynamic recrystallization. On rolling at е = 2.0, a microstructure composed of micro-and submicrocrystalline grains was formed. An algorithm was proposed for estimating the critical strain for the onset of dynamic recrystallization from dependences of grain sizes on true strain. The sequences and temperatures of martensite transformations from a cubic В2 phase to rhombohedral R and monoclinic В19′ martensite phases were studied depending on the strain accumulated in abc pressing and rolling.
The chapter is devoted to a study of the influence of neutron irradiation on the martensitic transformations and shape memory effects in TiNi-based shape memory alloys. Irradiation of the samples was carried out in the low-temperature helium loop of a WWR-M fusion reactor at Petersburg Nuclear Physics Institute in Gatchina (Russia). The experimental data showed that the variation in transformation temperatures depended on the irradiation temperature. The main factors influencing the variation in transformation temperatures during irradiation were disordering of the solid solution at low temperatures, radiation ordering at high temperatures, and thermally activated annealing of radiation damage. All of these mechanisms were taken into account in the differential equation given in the present work for description of the transformation temperature variation during irradiation at different temperatures. It was found that irradiation up to a fluence of 7⋅1018 cm-2 did not suppress the transformation plasticity and shape memory effects in TiNi alloy in spite of the variation in transformation temperatures. It was observed that the shape memory effect may be initiated by irradiation up to a fluence of 5⋅1020 cm-2 at a constant temperature (under isothermal conditions) due to a decrease in transformation temperatures.
Nowadays, ferromagnetic shape memory Heusler alloys are ones of famous multifunctional materials exhibiting many interesting features in the temperature interval of the martensitic transformation due to the strong interrelation between crystal structure and magnetic order. The multiferroic, magnetoresistive, martensitic and related magnetic shape-memory behavior as well as magnetocaloric properties are examples of these unique features. Generally, tuning of both structural and magnetic transition temperatures can be useful to achieve better functional properties. Today, the optimization problem of Heusler compounds is of a great importance. In this chapter, we review the most important features of ternary and quaternary ferromagnetic shape memory Ni-Mn-In and Ni-Mn-In-Z materials, which are experimentally and theoretically obtained in the last three years. We discuss the experiments devoted to the study of phase diagrams, thermomagnetizations, magnetic field and stress induced strains, magnetoresistance and magnetocaloric effects. The theoretical investigations of magnetic and structural properties are reviewed in the framework of the phenomenological approach, first-principles and Monte Carlo methods.
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.
Basic directions in the theory of martensitic transformations are briefly listed. Within the framework of the dynamic theory based on the synthesis of concepts of heterogeneous nucleation and wave growth of martensite crystals, the possibilities of description of morphological parameters during the В2→B19, В2→B19′, В2→R transformations are analyzed. It is demonstrated that the calculated and observed habit planes and orientation relationships can be matched.
The possibility of applying electropulse to increase the deformability, nanostructure formation and functional property enhancement of hard deformed TiNi-based alloys during severe plastic deformation is examined. It is shown that electroplastic rolling significantly increases total strain to failure. The formation of nanocrystalline structures by rolling with current depends on the strain degree and pulse current density, which are critical parameters of the new method. For the first time display of the electroplastic effect in coarse-grained and nanostructured TiNi alloys is investigated. It has been shown that the amplitude and direction of stress jumps on tensile stress-strain curves depends on grain size and pulse current regimes. An increase in the recovery strain and superelasticity of material processed by rolling with current is demonstrated.
This chapter is devoted to a study of structure, martensitic transformation and shape memory behaviour in TiNi foams produced by self-propagating high-temperature synthesis. The influence of the chemical composition of the Ti+Ni powder mixture as well as pre-heating temperature on the structure and properties of TiNi foams is studied. It is shown that the variation in Ni concentration in the powders mixture allows one to produce a porous TiNi alloy with properties close to the equiatomic cast Ti50Ni50 alloy or Ni-rich cast TiNi alloy. It is shown that the TiNi foams produced from a mixture where the Ni concentration is higher than 45 at.% should be subjected to post-production annealing to decrease the Ni concentration in the TiNi phase. The influence of annealing temperature and duration on the structure and martensitic transformation in TiNi foams produced by SHS is studied. The optimal conditions for annealing of the TiNi foams are found. It is shown that TiNi foams after optimal heat treatment demonstrate good shape memory properties.
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.
Nowadays, martensitic transformation and shape memory effect, superelasticity, high damping capacity and other effects associated with this type of structural phase transitions are still in the focus of scientists and engineers, especially once these phenomena are taking place at elevated temperatures. The list of the materials undergoing this kind of transformation is constantly widening. Yet, industrial application of these materials, called high temperature shape memory alloys, is still hindered due to the lack of understanding of the peculiarities of the high temperature martensitic transformation and shape memory effect. Present work summarizes results of scientific studies of these high temperature phenomena oriented onto the development of physical principles suitable for industrial high temperature shape memory alloys design.
The review on effect of a high strain rate on the properties of TiNi-shape memory alloys is presented. The study of thermo-mechanical and functional properties of SM_A after high strain rate loading was carried out. The object of study was an equiatomic TiNi shape memory alloy. The samples were tensioned at a strain rate of about 10(3) s(-1) at various temperatures in martensitic, austenitic, and two-phase state, using the Split Hopkinson Pressure Bar technique. Two-way shape memory effects were investigated. Two-way shape memory after high strain rate loading was less than after the quasi-static one for all cases, except for straining in martensitic state.
Recently we reported on the development of a composite material exhibiting reversible shape memory effect. A Ti–25Ni–25Cu (at.%) alloy was obtained by the melt spinning technique as amorphous–crystalline ribbons with a thickness of approximately 40 μm. The thickness of the amorphous and crystalline layers (d а and d c , respectively) was varied by electrochemical polishing. It has been ascertained that with varying the relationship d c /d а the martensite transformation and shape-recovery temperatures do not actually change, while the minimum radius of the ribbon bending decreases from 8.0 mm to 2.4 mm with increasing the relationship d c /d а from 0.33 to 1.40. The maximum reversible strain comprises 0.4% at d c /d а = 0.82. On the basis of experimental data obtained the phenomenological description, providing an explanation for nature of the phenomena taking place in the rapidly quenched amorphous-crystalline ribbon composite, has been proposed.
