The contribution of mechanical spectroscopy to understanding the problem of grain boundary sliding

Reference Speaker Authors
Daniele Mari Mari, D.(Ecole Polytechnique Fédérale de Lausanne); Grain boundary sliding is widely observed in ceramics and even constitutes the main mechanism of plastic deformation at high temperature. Grain boundary sliding relaxation peaks were observed in some oxide ceramics and nitrides. In most cases, the presence of such peaks is related to the presence and stability of an amorphous phase at grain boundaries. In the case of alumina, that does not show such amorphous phase, grain boundary sliding has been related to grain boundary dislocations. In all cases the activation energy is apparent. The presence of a disordered or amorphous phase justifies the use of a coupling model that considers that molecular motions are hierarchically correlated. In metals, grain boundary sliding is effectively observed in some cases but the correlation of such phenomenon with internal friction peaks has been much more controversial. The case of a relaxation mechanism that could be related to grain boundary sliding (GBS) is one of the oldest examples of relaxation phenomena, already described by Zener. In 1947, Kê observed a large relaxation peak around 0.46Tm in polycrystalline aluminum. This peak being absent in single crystal, he concluded that this relaxation effect was due to grain boundary sliding (GBS). However, since dislocation motion related peaks were also observed in single crystals in the same temperature range, doubts were raised about the true origin of the Kê peak. On the other hand, some systems such as NiCr alloys are known to show a precipitation at grain boundaries that undoubtedly blocks grain boundary sliding leading to intergranular fracture. A peak that only appears when grain boundaries are precipitate free could be attributed to grain boundary sliding. Even if we have man examples of relaxations that can be attributed to grain boundaries the microscopic mechanism that are at the origin of grain boundary relaxations are still a matter of debate. However, targeted experiments have recently given a deeper insight into such mechanism. On one hand, it seems that the topological Zener model perfectly represents the behavior of polycrystalline materials as a function of grain size. Moreover, a grain boundary peak is only related to a polycrystalline grain structure and not to dislocations or dislocation walls. In fact, accurate experiments on bi-crystals clearly show that the nature of the peak depends on the boundary type and orientation. Finally, the problem of apparent activation energies might be conductible to the same approach as in ceramics. In fact, molecular dynamics simulations show that a phase transition and a thin amorphous phase could occur in most cases when a grain boundary relaxation is observed.
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