SFB 459 - A Research Center for Progress in Understanding and New Applications of Shape Memory Alloys (SMAs)

摘要

In the present Special Issue of Zeitschrift Materialwis-senschaft und Werkstofftechnik - Materials Science and Engineering Technology, researchers from the Ruhr-Universitaet Bochum (RUB) and their colleagues from Juelich, Bonn and Dortmund provide an insight into the work which they perform within the Research Center SFB 459 - Shape Memory Technology (Sonderforschungsbereich 459 - Formgedaecht-nistechnik) funded by the Deutsche Forschungsgemeinschaft (DFG) and supported by the Land Nordrhein Westfalen (NRW). Our SFB 459 has three long term objectives: 1. To increase the level of understanding of all aspects regarding the fascinating properties of shape memory alloys (SMAs). 2. To stimulate interest in SMAs and to assisst design engineers and product developers in this respect. 3. And to improve existing and develop new processing and manufacturing techniques for SMAs. The SFB 459 focusses on NiTi and NiTi-X systems (binary and ternary alloys). NiTi based alloys are the most successfull shape memory materials today, because they combine good functional properties with good mechanical strength. Moreover, a dense oxide layer provides corrosion protection and accounts for good bio compatibility. The exploitable functional properties can be attributed to a thermal (1 way effect, 2 way effect) and mechanical (pseudo elasticity) memory which both rely on a diffusionless ("martensitic") phase transformation. Thermal memory means, that one can deform a SMA to high strains (as large as 8 %), and that then, on heating, the material returns into its original shape. And mechanical memory allows the material to be pseudo elastically deformed to strains of the same order of magnitude; and on unloading, its original shape is recovered. Both effects are fascinating; and both effects are not as much exploited as one should expect, because a number of technological problems need to be overcome. Shape memory properties are strongly affected by temperature and the characteristic temperatures which govern whether the material behaves in one way or another strongly depend on alloy composition and microstructture. Thermal memory and mechanical memory are associated with different temperature ranges and phase transition temperatures therefore must be precisely adjusted to match the requirements of specific applications. There is a need to work on appropriate constitutive equations which capture the essence of the material behavior and can be implemented into finite element codes to calculate stresses, displacements and strains. Surface effects need to be understood and coating procedures need to be developed. And it is important to work on a better understanding of the martensitic transformation on atomistic, mesoscopic and macroscopic length scales.