Formation of Twins and Stacking Faults during the Primary Recrystallization of Pure Nickel

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The formation of annealing twins has been examined by observing 96% rolled and electrolytically thinned high-purity nickel foils in a heating stage of a 500 kV electron microscope.<BR>The thickness dependence of the recrystallization temperature is found to be large; 500°C in specimens 0.2 μ thick, 350°C in those 0.7 μ thick as compared to 250°C in bulk ones. In specimens 0.2 μ thick which are transparent for 100 kV electrons, no annealing twins ars formed, whereas specimens 0.7 μ thick can generate many twins similar to those in bulk-annealed specimens.<BR>The nucleation sites of the annealing twins are always high-angle boundaries in motion, and the twins are divided into two classes from a morphorogical point of view ; parallel-sided twins and thin twins, respectively. The parallel-sided twins result if the twinning plane is nearly parallel to the recrystallized grain boundary. These twins can grow to an extensive size through the migration of the high-angle boundaries, as in the case of usual recrystallized grain growth. On the other hand, the twinning planes of the thin twins are almost perpendicular to the boundary. The thin twins grow through a successive pile-up of stacking faults, probably by the pole mechanism, during dragging by the migrating boundary. The formation of twins is largely dependent on the local deformation texture at the site of twin nucleation.<BR>A mechanism which conforms to the ‘stimulation theory’ proposed by Burgers has been considered to be the most plausible explanation of forming the parallel-sided twins.

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