II. Microscopic mechanism

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Abstract
In part I of this study, extensive evidence for glide on the non-compact plane (001) was obtained in [112] aluminium single crystals under creep conditions at high temperatures. In this part II the same single crystals are used to measure the critical resolved shear stress for (001) glide as a function of strain rate and temperature in constant-strain-rate tests. The observed dependence is in good agreement with the kink-pair mechanism, initially proposed for prismatic glide in hexagonal closepacked metals. The apparent activation enthalpy ΔH 001 for the dislocation glide on (001) is equal to 1.74eV. This model also applies to {110} glide with an apparent activation enthalpy of 1.40eV. The screw dislocation motion predicted by this model in the face-centred cubic structure is in good agreement with the slip-line observations and in situ experiments reported in part I. This model also agrees well with the other non-compact glide characteristics available, such as the activation temperatures of {110} glide in different face-centred cubic metals or the influence of (001) and {110} glide on the shape of the stress-strain curves. In addition, it is shown that, at very high temperatures (about 400°C), the different types of non-compact system obey the Schmid law. The creep activation enthalpy ΔH creep has been measured in these [112] aluminium single crystals between 200 and 320°C, where (001) glide is activated, but still difficult. The value of ΔH creep is found to be 1·57 ± 0·1 eV, which is close to ΔH 001 and suggests that dislocation glide on the non-compact plane is rate controlling during creep in this temperature range. The same model seems to apply to other single-crystal orientations and to polycrystals as well, with (eventually) other types of non-compact planes. It also plays an important role in constant-strain-rate tests and rolling operation and could explain some high values of creep activation enthalpies measured at high temperatures in silver and copper.