Anneal hardening and high temperature strain rate sensitivity of nanostructured metals and their relation to intergranular dislocation accommodationThursday (27.09.2018) 10:15 - 10:30 S1/01 - A03 Part of:
Nanocrystalline materials show various properties or phenomena not appearing in the conventional grain size regime, such as increased strain rate sensitivity or strength increase during recovery annealing treatments. Many of these properties are associated with the enhanced confinement of plasticity and accordingly the interaction of dislocations with the numerous grain boundaries, the boundary state as well as its local chemistry. Due to these various influencing factors, determination of the dominant and rate controlling processes remains challenging. Here we present a study on selected nanostructured fcc materials for which dislocation-grain boundary interactions have been studied earlier in their coarse grained counterparts. High temperature nanoindentation revealed for all materials above a certain test temperature a pronounced increase of the strain rate sensitivity, peaking at temperatures where significant grain growth occurs. Static annealing close to these temperatures, restricting the recovery regime, leads for samples with sufficiently small grain sizes to a maximum hardness increase. Interestingly, despite the significantly smaller grain size, these critical temperatures perfectly agree with those obtained earlier for annihilation of lattice dislocations at grain boundaries in coarser grained samples. This indicates that at elevated temperatures the rate controlling mechanism is the thermally activated annihilation of lattice dislocations, provoking loss of mobile dislocations and boundary relaxation during static annealing, thus resulting in strengthening.