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Keynote Lecture

Mechanical Properties of Materials and Devices at the Microscale under Extreme Conditions of High and Low Temperatures and High Strain Rates

Thursday (27.09.2018)
15:45 - 16:15 S1/01 - A1
Part of:

Several recently developed techniques used to measure mechanical properties at small length scales will be presented. One of the techniques allows for variable temperature and variable strain rate testing of micron sized samples within a scanning electron microscope for in-situ observation. By utilizing a displacement-controlled micro-compression setup, which applies displacement using a miniaturized piezo-actuator, the attainable range of strain rates has been recently extended up to~ 10000 s−1 as well as cyclic loading up to 10.000.000 cycles. With this system it is also possible to achieve stable variable temperature indentation and micro-compression experiments at temperatures as low as -150°C and up to 700°C. Additionally, recent developments have allowed for the capability to conduct electron backscatter diffraction (EBSD) during micro-mechanical testing. Combined with the cross-correlation technique (HR-EBSD), the strain/stress field and the GNDs distribution can be mapped at several steps during progressive deformation.

Using these new capabilities, the size dependent plasticity and fracture mechanics of a number of materials have been explored:

a) The loading rate effect on the brittle-ductile transition temperatures of tungsten single crystals at the micro-scale was investigated by microcantilevers with a (100)[001] crack system. At low temperatures (-90 to -25 °C) the samples failed by brittle cleavage fracture irrespective of the applied loading rate at a fracture toughness of 3.2 MPa·m1/2. With increasing temperatures up to 500 °C and depending on the applied loading rate the fracture toughness increased and significant crack tip plasticity and dislocation-controlled microcleavage were observed by means of high resolution electron backscatter diffraction measurements performed after testing.

b) Nanocrystalline Palladium-Gold alloys were investigated by means of elevated & low temperature micropillar compression experiments. A series of strain rate jump tests, where the strain rates were varied from 10−4 s−1 up to 10−2 s−1, allowed us to extract strain rate sensitivity and average activation volumes in the order of 8 b3. Repeated load relaxation tests yielded lower activation volumes in the range of 4 b3. From the variation of test temperatures up to 125 °C, we extracted an apparent activation energy of ∼0.77 eV.

Dr. Johann Michler
Swiss Federal Laboratories for Materials Science and Technology (EMPA)