Structure and polygonization of bent zinc monocrystals

Abstract

The geometry of bent zinc crystals is discussed. It is found that the slip planes in bent zinc crystals have the shape of involutes with the hexagonal axes of the lamellae parallel to tangents of the evolute. This concurs with other investigators' results for bent corundum-crystals and with theory.Evidence is presented to show that bent crystals, especially sharply bent ones, are not necessarily polygonized immediately after bending. They sometimes contain low-angle grain-boundaries which may or may not form during the bending process. The normal structure of a homogeneously bent zinc crystal is believed to consist of a network of meandering dislocation lines connecting short segments of dislocation walls.Quantitative data on the kinetics of polygonization in cylindrically bent zinc crystals are presented. Polygonization, in the absence of inhomogeneous stresses, does not occur below about 170°C in “chemicallypure” zinc and this temperature is raised somewhat by contamination with 0.1 at. % Cd. At a given temperature, the polygon angles increase in proportion to the logarithm of the time. The temperature dependence of the growth rate, dθ/dt, at constant angle, varies with temperature. If an Arrhenius equation is used to describe it at temperatures below 250°C, Q is ~60 kcal/mol and at temperatures from 250 to 325°C, Q is ~20 kcal/mol.The effect of several variables on the polygon growth rate were investigated: 1.1. Stress after annealing: causes boundaries to coalesce.2.2. Twins: on compression side they markedly inhibit polygon growth.3.3. Bending temperature: variations from −196°C to 225°C had no marked effect.4.4. Surface condition: no effect as long as surface was macroscopically smooth.5.5. Intermittent versus continuous anneals: no effect.6.6. Skew bending: no effect for small amounts of skewness.7.7. Radius of curvature: polygon size is roughly inversely proportional to radius of curvature at constant time and temperature.The theory of polygonization is discussed and it is pointed out that the rate of climb of unit dislocations does not seem to be the only factor that controls the growth rate.