Abstract
Nanocrystalline thin films are widely applicable in various micro-electro-mechanical systems. Restricted grain sizes and film dimensions cause activation of deformation mechanisms different from the bulk materials and change of mechanical properties. In-situ deformation in a transmission electron microscope (TEM) allows direct observation of the sample and measurement of mechanical properties. This experimental method was combined with results of simulations by molecular dynamics (MD). 150 nm thick films were prepared by a DC magnetron sputtering from Al-3wt%Mg alloy. The film was sputtered onto a polymer tape, which was then dissolved. The film was in-situ annealed up to 400 °C in transmission electron microscope. A dog-bone shape specimen was cut from the film in a scanning electron microscope using focused ion beam and fixed onto a Push-to-Pull device with stiffness 150 N/m by a layer of Pt deposited by the gas injection system. The deformation was realised by a Hysitron PI 95 TEM PicoIndenter. During straining, rapid contrast changes in bright field (BF) (Figs. 1a and 1b) and very low dislocation density, both suggesting grain boundary-related deformation mechanisms were observed. ASTAR orientation maps were taken before the deformation and after the failure (Figs. 1c and 1d). The maps confirmed the possibility of grain rotations during the deformation. Moreover, the intergranular character of the crack propagation can be deduced from ASTAR images taken after the failure (Figs. 1b and 1d).
For MD simulations using ATOMSK software, a polycrystal structure was constructed in an orthogonal box with dimensions x = 400 nm, y = 400 nm, z = 200 nm. Twelve hexagonal grains with [110] parallel to z-direction and random orientation in x and y were created in the box. Periodic boundary conditions were employed in x- and y-directions. The MD simulations were performed using a large-scale atomic/molecular massively parallel simulator (LAMMPS). The polycrystal was deformed in the y-direction at a strain rate of 2.109 s-1 and a temperature of 300 K up to 20 % strain. Open visualisation tool (OVITO) was employed to visualise the atomic structure. Visualisation of simulation results is shown in Fig. 2. Common features with experimental results are visible: Restricted dislocation activity, deformation by grain boundary mechanisms, namely dislocations inside grain boundaries, and intergranular crack propagation. However, measurements of grain rotations showed no significant changes in misorientations at the beginning and the end of the simulation.