Materials Science Foundations Vols. 81-82

Abstract: 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.

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: 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.

Abstract: 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.

Abstract: The results of experimental studies performed during the past decade at Saint-Petersburg State University in collaboration with the Mechanics and Materials Science Research Centre at Ningbo University (China) and Lobachevsky State University of Nizhny Novgorod (Russia) with the aim of investigating the basic regularities of the high rate straining of NiTi shape memory alloys are reviewed. The studies were concerned with the mechanical behaviour of these materials at high rate compression and tension, and the effect of high rate straining on the basic functional properties (shape memory effect and two-way shape memory). Special attention was given to the application of dynamic fracture theory to NiTi shock loading and to methods for obtaining experimental findings concerning the theoretical parameters involved in the criteria for determining the transition of these materials from an elastic to an inelastic state while high rate straining was applied. The effect of the quasi-equilibrium structure of NiTi on martensitic transformations and the role of this structure in the formation of more complicated effects than shape memory and superelasticity were studied. The results obtained are used to elaborate a method for the improvement of the functional properties of NiTi and a procedure for reversing two-way shape memory induction.

Abstract: 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.

Abstract: 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.

Abstract: The paper deals with the development of a structural composite material exhibiting two-way shape memory effect. A Ti-Ni-Cu alloy was produced by the melt spinning technique at different cooling rates in the form of a ribbon with a thickness of approximately 40 μm. Layered amorphous-crystalline structure of the ribbon was obtained by varying the alloy composition and the cooling rate and by modification of the alloy structure with the external extreme action (pulsed laser emission and periodical discharge in the liquid flow). The relation between thicknesses of the amorphous and crystalline layers was changed by the variation of parameters of the melt spinning and the external actions as well as with the aid of electrochemical polishing. The samples were characterized by means of inverted metallographic and scanning electron microscopy, X-ray diffraction, differential scanning calorimetry, energy dispersive X-ray analysis and microhardness measurements. It has been shown that the layered amorphous-crystalline composite material demonstrates clearly defined two-way shape memory behavior without any additional thermomechanical treatments and can be used to create micromechanical devices with a higher level of functional properties.

Abstract: 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.

Abstract: 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→ε→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 ε–martensite initial structure is obtained as a result of the retwinning of ε martensite during cold deformation and the subsequent ε→γ transformation during heating.