Recently, the development of a new generation of advanced instruments for in-situ TEM nanomechanical testing has allowed establishing a one-to-one relationship between load-displacement characteristics and stress-induced microstructure evolution in the transmission electron microscope (TEM). In the present work, it will be demonstrated that a step forward in the investigation of structural defects and small-scale plasticity mechanisms can be made by combining commercial [1] and in-house developed lab-on-chip [2] nanomechanical testing techniques with advanced TEM techniques.

High resolution aberration corrected TEM and STEM as well as automated crystallographic orientation and phase mapping in TEM (ACOM-TEM) have been used to reveal the nanoscale plasticity mechanisms controlling the mechanical responses of nanocrystalline (nc) metallic Al and Pd freestanding thin films. Special attention was paid to the strain rate sensitivity of nanostructured metallic materials involving thermally activated plasticity mechanisms [3] (Figure 1). Furthermore, the microstructure of nc Pd thin films subjected to hydriding cycles has been investigated in order to unravel the interaction mechanisms of hydrogen with dislocations and interfaces [4] as well as the influence of hydrogen cycling on the mechanical properties of the Pd films. An original method combining the measurement of dislocation mobility using in-situ TEM nanomechanical testing and dislocation dynamic (DD) simulations has also been used to investigate the plasticity of olivine small-sized crystals at low temperature. It demonstrates for the first time the possibility of characterizing the mechanical properties of specimens, which could be available in the form of micron-sized samples only [5].

More recently, quantitative nanobeam electron diffraction (NBED) was used to investigate the relationship between the local atomic order and the activation of shear transformation zones (STZs) in nanostructured ZrNi metallic glasses freestanding thin films. These films exhibit outstanding mechanical properties involving very large homogenous plastic deformation and giant ductility without the observation of mature shear bands until fracture. The basic principle of NBED is shown in Figure 2, consisting of a coherent electron beam with diameter of around 0.4 nm in order to produce two-dimensional diffraction patterns from atomic clusters with comparable size. Furthermore, high resolution HAADF-STEM and EELS revealed a heterogeneous microstructure with Ni-rich and Zr-rich regions exhibiting different atomic densities with characteristic length of 2-3 nm. The role of such behaviour in the absence of shear bands and the delay of fracture in the ZrNi thin films is discussed.

References

[1] H. Idrissi, A. Kobler, B. Amin-Ahmadi, M. Coulombier, M. Galceran, J-P Raskin, S.Godet, C. Kübel, T. Pardoen, D. Schryvers. Applied Physics Letters. 104 (2014) 101903

[2] H. Idrissi, B. Wang, M.S. Colla, J.P. Raskin, D. Schryvers, T. Pardoen. Advanced Materials. 23 (2011) 2119

[3] M.S. Colla, B. Amin-Ahmadi, H. Idrissi, L. Malet, S. Godet, J.P. Raskin, D. Schryvers, T. Pardoen. Nature communications. 6 (2015) 5922

[4] B. Amin-Ahmadi, D. Connetable, M. Fivel, D. Tanguy, R. Delmelle, S. Turner, L. Malet, S. Godet, T. Pardoen, J. Proost, D. Schryvers, H. Idrissi. Acta Materialia. 111 (2016) 253.

[5] H. Idrissi, C. Bollinger, F. Boioli, D. Schryvers, P. Cordier. Science Advances. 2 (2016) e1501671.

Figures:

Figure1: (a) The lab-on-chip technique devoted to in-situ TEM uniaxial testing of submicron freestanding thin films. (b-d) Filtered HRTEM images showing the continuous change of the dislocation positions with time upon relaxation of nanocrystalline Pd thin films. Note the formation and destruction of Lomer-Cottrell dislocations indicated by L-C in c. The perfect dislocations are indicated by ‘T’ symbols [3].

Figure 2: (a) Principle of the NBED technique used to probe nanosized ordered regions in Zr65Ni35 thin films metallic glasses. Convergence angle α = 3.8 mrad and an electron beam with diameter of 0.4 nm have been used. (b-c) NBED patterns with strong Bragg reflections obtained on atomic clusters aligned in on-axis, three-beam and two-beam condition, respectively. (e) NBED pattern without Bragg reflections._x000D_

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