Investigation of small-scale plasticity/fatigue mechanisms and size effects using advanced transmission electron microscopy

Abstract

In the present work, the fundamental plasticity/fatigue mechanisms operating at interfaces in micro/nano-scale Ni samples have been investigated. in-situ SEM fatigue tests have been performed on FIB prepared single and bi-crystal micropillars with well-known orientations as revealed by electron backscatter diffraction (EBSD). Careful characterizations of the nature and the distribution of deformation dislocations, the character and the local structure of the interface as well as the mechanisms controlling the interaction between these defects under cyclic loads were performed using ex-situ TEM techniques including diffraction contrast imaging, automated crystallographic orientation and nanostrain mapping in TEM (ACOM-TEM) as well as electron diffraction tomography. Furthermore, quantified in-situ TEM nanotensile tests were performed on both single and bi-crystal samples in order to directly observe the plasticity mechanisms. Recently, an original method combining the measurement of dislocation mobility using commercial in-situ TEM nanomechanical testing and dislocation dynamic (DD) simulations has been used to revisit the plasticity of olivine single crystals at low temperature [1]. Cyclic deformation was applied in the load control mode. Load was increased to a given value, which is maintained constant for several minutes before unloading. During the plateau, dislocation motion is observed and characterized (hence, under a known and constant applied stress). Using this method, we found that the intrinsic ductility of olivine at low temperature is significantly lower than previously reported values which were obtained under strain-hardened laboratory conditions. More generally, we demonstrated the possibility of characterizing the mechanical properties of specimens which could be available in the form of sub-millimetre sized particles only. References [1] H. Idrissi, C. Bollinger, F. Boioli, D. Schryvers, P. Cordier, Low-temperature plasticity of olivine revisited with in situ TEM nanomechanical testing. Science Advances. 2 (2016) e1501671