Investigation of plasticity/fatigue mechanisms at interfaces in N i using ex‐situ and in‐situ SEM / TEM micro/nano‐mechanical testing

Abstract

The present work focuses on the fundamental plasticity/fatigue mechanisms operating at interfaces in micro/nano scale Ni samples. In‐situ SEM fatigue tests have been performed on FIB prepared single and bi‐crystal micropillars with well‐known orientations as revealed by 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 tomography. The primary TEM results obtained on single crystal micropillars after fatigue tests revealed the presence of dislocation walls structure as shown in Figure 1(a) and (b). ACOM‐TEM revealed local changes of crystal orientation around 1–2 degrees at the position of the dislocation walls. Furthermore, systematic contrast analysis of dislocations in these areas confirmed that only slip systems with the highest Schmid factor have been activated. The analysis of micropillars with GBs subjected to fatigue tests has shown the accumulation of dislocations at the GBs (Figure 1(c) and (d)) without slip transfer or localized plasticity. Electron tomography has been used to investigate the 3D distribution and the interaction of deformation dislocations within the dislocation walls. In order to directly observe the plasticity mechanisms, quantified in‐situ TEM tensile tests were performed on both single and bi‐crystal samples using the Pi 95 picoIndenter instrument and a MEMS device called ‘’Push‐to‐Pull’’ (PTP) from Hysitron.Inc (Fig 2). In order to minimize the effect of FIB on the in‐situ tensile samples, an original sample preparation method combining twin jet electro‐polishing and FIB was used, see figure (2). Nucleation‐controlled‐plasticity has been observed with defects induced by FIB at the edges of the sample acting as preferential sources for the nucleation of deformation dislocations. Furthermore, the in‐situ TEM nanotensile experiments revealed the elementary mechanisms controlling the interaction between dislocations and pre‐selected GBs from the electro‐polished thin foils, as the nucleation of dislocations from GB, see figure (3). These results indirectly shed light on the micropillar's behaviour as the root cause of the deformation is connected to the FIB preparation and the dislocation/GB interaction mechanisms.