Metallic glasses are promising materials for application as structural materials. An important aspect is measuring the response of metallic glasses to external stresses and the corresponding atomic displacements. Up to now such measurements have been performed using synchrotron radiation only. It is the aim of the present study to show that atomic-level elastic strain of amorphous materials can be determined by quantifying the distortion of the first diffraction halo in selected area electron diffraction (SAD) images during in-situ deformation in the transmission electron microscope (TEM).
MEMS based tensile testing stages with freestanding thin film of Ti45Al55 facilitates macroscopic strain and stress measurements during in-situ TEM tensile deformation. The tensile tests were carried out in a Philips CM200 microscope at an accelerating voltage of 200kV. The samples were uniaxially strained in steps of 150 nm and bright-field images and SAD patterns were recorded. Microscopic strain tensor on atomic level was obtained from SAD images by tracing the shift of the maximum of the first broad diffraction halo during tensile loading .
Figure 1 shows a characteristic diffraction pattern of amorphous TiAl from a selected area of 1.2 micrometer in diameter. The evaluation procedure to obtain the maximum position q1 of the first broad ring as a function of the angle χ is described in detail in . The 2D strain tensor is calculated from the relative change of q1(σ, χ) at a given stress with respect to the unloaded position q1(0,χ). From a series of SAD patterns recorded from the same area at different stress levels the principal strains e11 (parallel) and e22 (perpendicular to the loading direction) are calculated and plotted as a function of stress (Fig. 2). From the linear fits the Young's modulus and the Poisson's ratio are obtained. The systematic difference of e11 to the macroscopic strain can be attributed to topological rearrangements of atoms in metallic glasses.