The thermal cycling of cadmium and dilute Cd-Pb alloys was studied by measuring the length change of a specimen as a function of the number of cycles a specimen had undergone. The majority of the experiments were performed using material which had been swaged to 90% reduction in area followed by various amounts of drawing reduction, and which were cycled from 0°C to -190°C. Other areas of interest were the effect of temperature range and grain size on the growth rate, and the effect of cycling on the microstructure and on the axial thermal expansion coefficient. It was found that swaged-and-drawn material cycled from 0 to -190°C, exhibited a three-stage growth curve. The growth rates in Stages 2 and 3 were constant; Stage 1 was ill-defined and was not investigated to any extent. The transition from stage to stage was abrupt and was accompanied by a decrease in growth rate, a lower incidence of non-basal slip and a change in the expansion coefficient. Both the Stage 2 and the Stage 3 growth rates showed a (grain size)¯ ½ dependence and reacted in approximately the same way to alloying in that maxima in both growth rates were indicated for alloys containing between 10 and 50 ppm Pb. The results were interpreted in terms of an idealized model of a textured specimen. The model consisted of a bicrystal representing all grains possessing a given orientation, surrounded by a matrix representing all other grains in the specimen. Stresses are set up in the specimen since the expansion of the textured grains and the matrix grains differ in a given reference direction. These expansion stresses are then relaxed at the upper temperature by creep processes. Stage 3 was identified with the overall cycling mechanism. Stage 2 was considered to result from the interaction of the swaged and the drawn portions of the swaged-and-drawn texture. Stage 1 was believed to result from boundary stresses arising from the differential expansion of adjacent grains. Some experiments were carried out using rolled, drawn and swaged material. The results of these experiments as well as those results obtained by changing the temperature range were used primarily for purposes of comparison to the results obtained from cycling swaged-and-drawn specimens from 0 to -190° C. / Applied Science, Faculty of / Materials Engineering, Department of / Graduate

This work was undertaken to study the nature of the deformation mechanisms in polycrystalline zinc and cadmium over a temperature range from 77°K to 300°K. It has been observed that the only non basal slip system which is observed under normal light microscopy is that of second order pyramidal [ll22] <ll23>. At temperature above [formula omitted], the amount of non basal slip is greater in zinc than in cadmium. The amount of twinning, substructure formation and grain boundary migration is comparable in both systems. Negative work hardening beyond the U.T.S. at temperatures above Tн = .4 is associated with recrystallization. In both systems at temperatures below Tн = .26 a region of temperature and strain rate independent linear work hardening occurs. The extent of linear hardening increases with decreasing temperature below Tн = .26. Above Tн = .26, polycrystalline hardening in both systems is parabolic from yield on and the rate of hardening at a given value of strain decreases with increasing temperature. Cadmium single crystals showed a similar trend in that below .26 both [formula omitted] remained constant. However above .26 there was a steady decrease in the shear hardening rates. It was observed that the Cottrell-Stokes law is obeyed only in the linear hardening regions of polycrystals and in Stage II hardening of single crystals below .26. When dynamic recovery occurs [formula omitted] increases with increasing strain. It has been observed that below .26 the linear hardening rate in cadmium decreased with increasing grain size ( constant specimen dimensions) so that [formula omitted] The value of [formula omitted] was shown to correspond to the tensile hardening rate during Stage II single crystal deformation. The tensile hardening rate was used because of the extensive twinning found to be associated with Stage II hardening. The grain size dependence of 0 has been interpreted in terms of a grain size dependence of the extent of [ll22] <1123> slip. It was found that during linear hardening in both zinc and cadmium the difference in flow stress at two different temperatures is a reversible difference implying that the dislocation configurations produced with increasing strain do not vary in nature or extent with temperature. Under such conditions it is possible to formulate a mechanical equation of state. Extensive rate theory measurements have been made in both systems in order to attempt an evaluation of the rate controlling mechanisms both during linear hardening and.during dynamic recovery. The former has tentatively been associated with intersection. Dynamic recovery on the other hand has been linked to the loop annealing observations of Price. / Applied Science, Faculty of / Materials Engineering, Department of / Graduate