Name: Dr. Ayan Bhowmik

Supervisor: Dr. Finn Giuliani

Sponsor: EPSRC

            Developing a good understanding of the response of brittle materials, like ceramics and intermetallics under loading, especially during the initial stages of plastic deformation before failure, is crucial for the designing of engineering components out of such materials. This work investigates the deformation mechanisms in a [001]-oriented single crystal MgO.

            To measure this we have carried out in-situ white beam micro-Laue diffraction while nanoindentation within synchrotron facilities in Diamond* and Advanced Photon Source†. This provides a unique protocol to study the fundamental operating mechanisms of deformation of material in small scale. Using a micro-focussed beam, both time and spatially resolved deformation map was obtained during nano-indentation. The Laue spots showed both rotation and streaking upon indentation that is typically indicative of both elastic lattice rotation and induced plastic strain in the material. Multiple facets of streaking of the Laue peaks suggested plastic slip occurring on almost all the {101}-type slip planes oriented 45˚ to the sample surface with no indication of slip on the 90˚ {110} planes. Owing to asymmetric slip beneath the indenter, as predicted by modelling results and observed through Laue analysis, sub-grains were found to nucleate with distinct misorientation. Tracking the Laue peak movement, a higher degree of lattice rotation was seen to occur in the material under the indent as compared to far field. Crystal plasticity finite element modelling was also applied to validate the experimental observations of plasticity.

* Diamond Light Source, Rutherford Appleton Laboratory, Didcot, UK

† Advanced Photon Source, Argonne National Laboratory, Argonne, Chicago, USA

snapshot

 

Snapshot of the (1-13) peak obtained from the different locations of the pillar, as shown on the right with an exposure time of 3s. Also superimposed is the location of the indent with respect to the pillar axis based on the observation of the streaking of the peaks.

Streaked Reflections

The streaked reflections obtained from adjacent positions in the pillar under the indenter on which is superimposed the vectors denoting the direction along which the spot would streak given slip on various {110} planes.