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Mobility of <c+a> dislocations in zirconium

Plasticity in hexagonal close-packed zirconium is mainly controlled by the glide of dislocations with 1/3<1-210> Burgers vectors. As these dislocations cannot accommodate deformation in the [0001] direction , twinning or glide of <c+a> dislocations, i.e. dislocations with 1/3<1-213> Burgers vector, have to be activated. We have performed in situ straining experiments in a transmission electron microscope to study the glide of <c+a> dislocations in two different zirconium samples, pure zirconium and Zircaloy-4, at room temperature. These experiments show that <c+a> dislocations exclusively glide in first-order pyramidal planes with cross-slip being activated. A much stronger lattice friction is opposing the glide of <c+a> dislocations when their orientation corresponds to the <a> direction defined by the intersection of their glide plane with the basal plane. This results in long dislocations straightened along <a> which glide either viscously or jerkily. This <a> direction governs the motion of segments with other orientations, whose shape is merely driven by the minimization of the line tension. The friction due to solute atoms is also discussed.