Influence of the Manufacturing Process On Workhardening of Nitinol

It is well known in the industry, that during solidification of a metal different phases may be formed in dependency of the elements used as well as their ratio. Furthermore the presence of impurities like oxygen and carbon during the melting process may generate inclusions in the formed alloy.  Phases and inclusions have an impact on strength and ductility of the material. Thus the controlled formation of both concomitants during the melting process can lead to predictable and designated material properties. In dependency of different kinds of melting principles for Nitinol Vacuum-Induction-Melting (VIM), Vacuum-Arc-Remelting (VAR), Electron-Beam-Remelting (EBR) and their combinations, the mode of solidification and the cleanliness of the used primary metals, the formation of phases and inclusions were studied. Beside the intermetallic compound NiTi, a variety of binary, ternary and quaternary phases are formed in the solidification process with different sizes and distributions. This paper demonstrates the influence of phases, sizes and distributions on the workhardening process of Nitinol during deformation and thus on the final mechanical properties of semi-finished products like wires and tubes. Mechanical properties were measured with tension testing and microindenter methods. Metallurgical examinations were done with optical microscopes, scanning electron microscopes and an energy dispersive X-ray device. DSC (Differential Scanning Calorimeter) and BFR (Bend-and-Free Recovery) methods were used to determine the influence on transformation temperatures A_s and A_f.