Papers by Keyword: Martensitic Transformation (MT)

Abstract: We report on the influence of short annealing treatments at 923 K and 1073 K during 10min on both martensitic transformation and exchange bias effect for the Ni_50.0Mn_36.5Sn_13.5 Heusler alloy ribbon by means of magnetic measurements. We have observed that the martensitic transformation is shifted towards higher temperatures with increasing annealing temperature. Furthermore, isothermal M(H) hysteresis loops performed under field-cooling protocol show an exchange bias effect for as-quenched and two annealed ribbons, which indicates the existence of ferromagnetic-antiferromagnetic interactions at low temperatures. In particular, we observe that H_C diminishes with the increasing of the annealing temperature, but H_E is not affected by the heat treatment.

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: 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: 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 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: 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.

Abstract: We reviewed structural and magnetic properties of Ni-Mn-Al Heusler alloys which are a representative of ferromagnetic shape memory alloy system. Special attention was given to a variety of crystal structures of the low-temperature martensitic phase formed in this alloys and to the influence of disorder and Co alloying on magnetic properties of Ni-Mn-Al. Magnetic properties strongly depends on the chemical composition, crystal structure and heat treatment. In present review works concerning magnetic properties dependence on treatment are made out. A two way shape memory effect observed recently in Ni-Mn-Al melt spun ribbons is briefly described. The aim of the review is to sum up the information about Ni-Mn-Al family.

Abstract: 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.