Plastic instabilities explored via acoustic emission during spherical nanoindentation

The plastic instabilities of a silicon iron single crystal (Fe single crystal doped with 3 wt% of Si) were explored via spherical nanoindentation coupled with an acoustic emission detection at nano/micro-scale. Three possible scenarios were identified (gradually developing plasticity without detectable AE activity, sudden onset of plasticity with strong AE event, and an elastic response with frequent AE activity both during loading and unloading). Beside the irreversible dislocation slip, the reversible microstructural changes were identified that in turn provokes to extend the common definition of acoustic emission from a phenomenon caused by the sudden irreversible microstructural changes by including also reversible atomic reorganization. AE activity at nano/micro-scale was for the first time observed during both the loading and the unloading for purely elastic indentations suggesting an action of reversible twinning. Critical indentation stresses representing the onset of AE activity during loading and its termination during unloading were identified. The unambiguous classification of AE events and irregularities on the indentation curve showed a direct correlation between the depth excursion and AE hit parameters. Acoustic emissions proved to be a very powerful and complementary method for the examination and classification of different deformation mechanisms (slip and twinning) observed during indentation loading.