K. Mehrabi

Investigation of Bending Trainings, Transformation Temperatures, and Stability of Two-Way Shape Memory Effect in NiTi-Based Ribbons

This paper discusses the application of rapid solidification by the melt-spinning method for the preparation of thin NiTi-based ribbons. Generally, the application of rapid solidification via melt-spinning can change the microstructure, improving the ductility and shape memory characteristics and lead to small-dimensioned samples. Several thousand thermal cycles were performed on the trained ribbons using bending deformation procedure, continuously observing the changes in the shape memory and transformation behaviors. These changes are due to the appearance of an intermediate phase which was stabilized probably by the accumulation of defects introduced by thermomechanical training. The influence of training and thermal cycling on characteristics of ribbons was studied by x-ray diffraction and transmission electron microscopy and differential scanning calorimetry. The results displayed that bending training methods were useful in developing a two-way shape memory effect (TWSME). All samples show a shape memory effect immediately after processing without further heat treatment. The addition of copper in NiTi alloys was effective to narrow the transformation hysteresis. The W addition has improved the stability of the TWSME and mechanical properties. The TWSME of ribbons and its stability are well suited for important applications such as microsensors and microactuators.

The response of peripheral blood mononuclear cells to shape memory alloys

The aim of this work was to study the response of human peripheral blood mononuclear cells (PB-MNCs) to Cu-Al-Ni and Ni-Ti shape memory alloys (SMAs). The alloys were prepared as rapidly solidified thin ribbons via melt spinning. It has been shown that the ribbons were non cytotoxic for PB-MNCs as determined by the viability and apoptosis assay. In 18 cultures out of 20 cultures of individual donor PB-MNCs cultivated with Cu-Al-Ni SMAs, no significant changes in the production of Th1 (IFN-γ), Th2 (IL-5 and IL-10) and proinflammatory (IL-1s, TNF-α, IL-6 and IL-8) cytokines were detected compared to controls. In two cultures out of 20, a significant increase in the production of Th1 and proinflammatory cytokines was observed. Microstructural analyses confirmed the formation of thin oxide on the surface of both SMA ribbons.

Influence of Tensile Training Conditions on Shape-Memory Properties of NiTi-Based Ribbons

This work presents a study of the mechanical properties and influence of thermomechanical training conditions on the two-way shape-memory effect (TWSME) (magnitude and stability) in NiTi and NiTiCu alloys. Correlation with a stress-assisted two-way shape-memory effect (SATWSME) was also investigated. A rapid solidification technique was employed for preparation of small-dimension NiTi samples with homogeneous microstructure. Samples were fabricated by melt spinning. The melt-spun ribbons were trained under tensile deformation by thermal cycling through the phase transformation temperature range. Results showed that the tensile trainings under both constant strain and constant stress were effective in developing a TWSME. The SATWSME was comparable in magnitude to the TWSME. Addition of copper was effective in narrowing the transformation hysteresis and transformation temperature dependence on composition.

Characterization of Oxidation Resistance of Stainless Steels at High Temperature by Metallographic Examinations and In-Situ Electrical Resistance Measurements

Abstract Practically all metals and alloys survive high-temperature exposure by growing oxide scales and/or by precipitation of the oxide particles in the matrix. Formed products can grow in shape of external oxide layers on surfaces, or as discrete oxide particles precipitated in a metal matrix. The first case represents external oxidation, and the other case is called internal oxidation. These processes are very important, because they determine the properties and applicability of metallic materials. Generally, they are undesired, because they cause deterioration of the mechanical properties and decomposition of metallic material. On the other side, the controlled process of external oxidation could be used for formation of protective coatings and the internal oxidation for dispersion strengthening of materials. In this paper we present monitoring of high-temperature oxidation of X12Cr13 stainless steel by in-situ electrical resistance measurements at different annealing temperatures in the air atmosphere. We determined the kinetics of oxide scale formation and its morphology with additional metallographic examination made by optical and scanning electron microscopy. The results of this research work show that in-situ monitoring and characterization of high-temperature oxidation present a strong tool that will contribute to a better fundamental understanding of the phenomena that occur during high-temperature oxidation of metallic materials.

Microstructural investigation of melt-spun NiTi based ribbons

Abstract In order to get small-dimensioned shape memory alloys (SMAs) with good functional and mechanical properties, a rapid solidification technique was employed. This paper discusses the use of different melt-spinning procedures for the preparation of proper thin NiTi ribbons. Metallographic characterization of SMAs is an important way to evaluate material properties, therefore the effect of alloy composition and different conditions of melt-spinning process (crucible, melting time and temperature) on the various inclusions and microstructure of NiTi alloys was investigated. Specimens for metallographic studies were cut from the longitudinal cross sections of melt-spun ribbons. The microstructures and different phases were examined by optical, scanning and transmission electron microscopy and the influence of the processing methods on microstructures of ribbons will be discussed.

Characterisation of melt spun Ni-Ti shape memory Ribbons’ microstructure

NiTi alloys are the most technologically important medical Shape Memory Alloys in a wide range of applications used in Orthopaedics, Neurology, Cardiology and interventional Radiology as guide-wires, self-expandable stents, stent grafts, inferior vena cava filters and clinical instruments. This paper discusses the use of rapid solidification by the melt spinning method for the preparation of thin NiTi ribbons for medical uses. Generally, the application of rapid solidification via melt-spinning can change the microstructure drastically, which improves ductility and shape memory characteristics and leads to samples with small dimensions. As the increase in the wheel speed led to a reduced ribbon thickness, the cooling rate increased and, therefore, the martensitic substructure became finer. Furthermore, no transition from the crystalline phase to the amorphous phase was obtained by increasing the cooling rate, even at a wheel speed of 30 m/s. Specimens for our metallographic investigation were cut from the longitudinal cross sections of melt-spun ribbons. Conventional TEM studies were carried out with an acceleration voltage of 120 kV. Additionally, the chemical composition of the samples was examined with a TEM equipped with an EDX analyser. The crystallographic structure was determined using Bragg-Brentano x-ray diffraction with Cu-Kα radiation at room temperature.