High-temperature relaxations

In 1947, Kê observed a large relaxation peak around 0.46Tm in polycrystalline aluminum. This peak being absent in single crystal, Kê concluded that this relaxation effect was due to grain boundary sliding (GBS). In the 1970s, Woirgard, in the same temperature range, observed a peak in single crystal. Later on Rivière, Esnouf, and No systematically studied different relaxation effects in single and polycrystals and they concluded that the Kê peaks, as well as the other relaxations observed in this temperature range, are due to dislocation motion, the mechanism being probably the climb of jogs. More recently, in Ni–Cr, Cao clearly observed the presence of a large relaxation peak in polycrystals, which is absent in single crystal. He showed that the peak is due to grain boundary sliding, the mechanism of which is the climb of dislocations in the grain boundaries. Does a relaxation effect due to grain boundary sliding exist in metals? The question is still open and will be discussed in this paper considering recent results of the Chinese, French, Spanish and Swiss school.