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Localization Phenomena in NiTi Shape Memory Alloys - Experimental Analysis of Lueders-like Shear Bands in One- and Two-dimensional Loading Scenarios
|Andreas Schäfer 1,2, Christian Großmann 1,2, Gunther Eggeler 1, Martin F. Wagner 1,2|
1. Ruhr-University Bochum, Universitaetsstrasse 150, Bochum 44801, Germany
Deformation of pseudo elastic NiTi shape memory alloys is often associated with the formation of macroscopically observable Lueders-like shear/transformation bands. Within these bands, the stress-induced martensitic transformation occurs almost completely. Simultaneously, the adjacent material remains in the austenitic state. Macroscopic straining of a specimen proceeds via the movement of the martensite/austenite interfaces between transformation bands and (yet) untransformed regions. During cyclic loading, functional fatigue (i.e., microstructural changes that alter macroscopic mechanical behavior) occurs only in those regions subjected to the forward and reverse transformation. In consequence, deformation associated with transformation bands leads to a localization of functional fatigue and partitions a specimen (or an engineering component) into regions of different mechanical properties.
While these phenomena are in principle well-documented, the fundamental mechanisms during the formation and growth of Lueders-like transformation bands are not understood in detail. In this contribution, we present experimental results on the deformation behavior of pseudo elastic NiTi wires, ribbons and sheet material during quasi-static and cyclic loading. The formation and propagation of transformation regions is characterized by infrared thermal imaging, in-situ optical microscopy and Speckle interferometry for different specimen geometries (one-dimensional loading: tensile specimens; two-dimensional deformation: plate specimens with and without notches/holes). The combination of these experimental approaches, supplemented by detailed microstructural characterization of initial and deformed microstructures (grain size distributions, textures) allows a detailed analysis of microstructural, mesoscopic and macroscopic factors contributing to the formation of transformation bands with a particular emphasis on non-uniaxial load cases.
Presentation: Oral at E-MRS Fall Meeting 2007, Symposium E, by Andreas Schäfer
See On-line Journal of E-MRS Fall Meeting 2007
Submitted: 2007-05-14 22:17 Revised: 2009-06-07 00:44