S. Mao, X. Han, Institute of Microstructure and Property of Advanced Materials, Beijing, China; M. H. Wu, Advanced Materials Technology Edwards Lifesciences LLC, Irvine, CA; Z. Zhang, Zhejiang University, Hangzhou, China
Electron back-scattered diffraction (EBSD) in scanning electron microscope has been very active in geology, metallurgy and materials science for more than a decade since the automatic EBSD system was commercialized in 1995. Automatic EBSD system enables mapping the grain boundary distributions, grain size, grain orientation distributions in high spatial resolution and fast speed for bulk materials. Comparing to the transmission electron microscope, it has the advantage of providing large quantity of statistical information and no worries about the artificial thin film effects.
In this study, we demonstrate the effectiveness and usefulness of in situ EBSD method in the fields of martensitic transformations and super-elasticity based on our recent progress in revealing the strain induced martensitic transformation cooperative collective effects in super-elastic polycrystalline TiNi shape memory alloys with a scanning electron microscope (SEM). We were able to collect the electron diffraction Kikuchi patterns for both of B2 parent phase and B19’ martensite, therefore, we were able to determine the lattice correspondence of these two phases instantly. By single surface analysis method, the crystallographic characters of the martensitic platelets can be determined among neighboring grains in the bulk specimen. Tracking a large amount of grains, we demonstrated the cooperative collective effects of strain-induced martensitic variant accommodation in nano- and micron-grained polycrystalline TiNi shape memory alloys in uni-axial tensile experiments. Using the martensitic transformation cooperative collective effects in polycrystalline system by tracking the individual grain’s orientation and the corresponding martensitic platelets, the orientation of the shearing bands in polycrystalline TiNi alloys and the bending induced martensitic transformation can be interpreted well. The crystallographic characters of the strain induced martensitic platelets in the polycrystalline system can be well interpreted using phenomenological crystallographic theory and Schmidt's law with a strong correlation to texture
Summary: Electron back-scattered diffraction (EBSD) was employed to investigate the strain-induced martensitic transformation in TiNi shape memory alloys in this study. The nucleation and formation of martensite variants in grains of a polycrystalline system satisfy the cooperative collective effect. The prior nucleated martensite variants will supply nucleation energy which makes the nucleation of martensite in the neighboring grains easier and martensite variants chains will be formed in polycrystalline TiNi shape memory alloys.
