Topic：Basics of Shape Memory Alloys and the Role of Fatigue in NiTi Shape Memory Technology

Speaker: Prof. Dr.-Ing. Gunther Eggeler,

　　　　 Chair for Materials Science, Ruhr-Universität Bochum

Abstract：

Cyclic loading is one of the generic features of many of the present and potential future applications of NiTi shape memory alloys (SMAs), no matter whether they exploit mechanical (pseudo-elasticity, PE) or thermal shape memory (one way effect, 1WE). Both fascinating shape memory properties rely on the martensitic transformation between a high and a low temperature phase (austenite and martensite). The presentation starts with a brief introduction into thermodynamic, atomistic/crystallographic and micromechanic aspects of the martensitic transformation in SMAs. Then the importance of fatigue in shape memory technology is highlighted. Both, functional and structural fatigue can limit the service life of shape memory components [1]. The term “functional fatigue” indicates that shape memory effects like the exploitable stroke in a 1WE actuator application or the dissipated energy (area of the stress strain hysteresis) in a loading–unloading cycle of a pseudo-elastic (PE) damping application decrease with increasing cycle numbers. By “structural fatigue” we mean the microstructural damage that accumulates during cyclic loading and eventually leads to fatigue failure. In both cases it is important to know how fatigue cycling affects microstructure and shape memory properties. Examples of recent research results on the influence of lattice misfit on the functional fatigue of shape memory actuators [2] and on the influence on impurity levels on the structural fatigue of PE NiTi SMAs [3] are presented. Areas in need of further work are pointed out.

[1] G.Eggeler, E.Hornbogen, A.Yawny, A.Heckmann, M.Wagner, On functional and structural fatigue of shape memory alloys, Mat. Sci. Eng., A 378 (2004) p. 24

[2] Ch. Grossmann, J. Frenzel, V. Sampath, T. Depka, G.Eggeler, Elementary transformationa and deformation processes and the cyclic stability of NiTi and NiTiCu shape memory spring actuators, Met. Mat. Trans., 40 A (2009) p. 2530

[3] M. Rahim, J. Frenzel, M. Frotscher, J. Pfetzing-Micklich, R. Steegmüller, M. Wohlschlögel, H. Mughrabi, G. Eggeler, Impurity levels and fatigue lives of pseudoelastic NiTi shape memory alloys, accepted for publication in Acta Mater., to appear 2013

CV-Prof. Dr.-Ing. Gunther Eggeler