The anisotropy of thermal expansion as a cause of deformation in metals and alloys

Abstract

Plastic deformation produced by repeated heating and cooling of certain non-cubic metals has been studied over a temperature range from ─ 190 to 250° C. The extent of the deformation which results from the anisotropy of thermal expansion of individual crystals becomes greater as the temperature range is increased. In the case of cadmium, plastic deformation was detected even after 20 cycles over the temperature range 30 to 75° C; in cyclic treatment to higher temperatures the deformation was associated with extensive grain-boundary migration. Cyclic treatment of tin, cadmium and zinc between room temperature and that of liquid air resulted in complex slipping and twinning, but grain-boundary migration was practically absent. Cooling from the liquid state which represents half a thermal cycle sets up stresses in metals possessing anisotropy of thermal expansion, and leads to plastic deformation. This is shown by the detection of slip lines and grain -boundary accentuation in certain cast noncubic metals. In cadmium, subsequent annealing results in marked grain growth. In order to investigate the interaction between the crystals of the two phases of a duplex alloy during cyclic thermal treatment, experiments were carried out with a series of tin-rich tin-antimony alloys. The majority of the alloys consisted of the tin-rich matrix in which particles of a hard second phase of cubic crystal structure were embedded. It was found that the deformation in the region of the boundaries between crystals of the two phases was considerably smaller than that in the region of the crystal boundaries of the anisotropic matrix. Similar results were obtained with tin-base bearing alloys.