Interpreting slip transmission through mechanically induced interface energies: a Fe–3%Si case study

Nanoindentation experiments are performed at the vicinity of grain boundaries, in Fe–Si tricrystals, to illustrate the existence of a critical stress at which slip transmission occurs across grain boundaries. Such a critical stress can be considered as a grain boundary yield stress and can be quantified within the framework of conventional gradient plasticity theory, enhanced by introducing a new mechanically induced “interface energy” term. The present study takes a first step in trying to provide a physical interpretation for this “far from thermodynamic equilibrium” interface energy term by conducting nanoindentation tests in three Fe–3wt%Si tricrystals, each of which had three distinct types of grain boundary misorientations, namely 22.5°, 42.0° and 44.6°. By relating the experimentally measured grain boundary yield stress to the predictions of interfacial gradient plasticity, it is possible to determine the interface parameter (ξ), which provides a measure of the resistance to slip transmission for each grain boundary examined. In particular, microscopic arguments from standard dislocation theory reveal that ξ depends on both the grain interior properties and the grain boundary structure. The internal length is shown to depend on multiple characteristic lengths of the microstructure, while a new expression is deduced for relating the Hall-Petch slope to both the interface parameter and internal length.