In situ deformation of b.c.c. crystals at low temperatures in a high-voltage electron microscope Dislocation mechanisms and strain-rate equation

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Abstract
In situ deformation of thin foils of niobium, molybdenum and α-iron has been performed in a high-voltage electron microscope, at temperatures ranging from 70 to 400 K. The observations of dislocation mobility, interaction with obstacles and several multiplication mechanisms are discussed. The ratio of screw to non-screw dislocation velocities is strongly temperature dependent and it is shown that a transition occurs in a narrow temperature range, which is characteristic for a given metal. It has been found that the different behaviour of the three metals investigated is greatly influenced by the different cross-slip abilities of the screw dislocations. Simple equations are derived to describe the simultaneous interactions of screw dislocations with localized obstacles and lattice friction. Only a few and measurable parameters are used, and therefore a rate equation can be derived in order to allow a comparison with the results of conventional tests to be made. In particular, the transition temperature and its dependence on obstacle density can be calculated.