Tensile tests on alloy crystals. Part I.—Solid solution alloys of aluminium and zinc

Abstract

In the ‘Proceedings of the Royal Society,’ 1925, some properties of crystals of an aluminium-zinc alloy were described and their method of extension and fracture compared with that of pure aluminium. This alloy contained 18.5 per cent, zinc and was just at the limit of solubility of zinc in aluminium. The crystals broke usually along a plane at, approximately, 45° to the axis and the position of this plane varied with regard to the crystal planes according to the orientation of the particular crystals. This type of fracture was so different to that of the pure metal that it appeared desirable to obtain alloys of intermediate compositions in order to study the effects of increasing amounts of zinc. Further, the small extension before fracture of the alloys containing higher percentages of zinc made it impossible to obtain reliable distortion measurements although measurements of the crystal axes, by means of X-rays, indicated that the distortion was probably of the same nature as aluminium. Through the courtesy of Mr. Murray Morrison, of the British Aluminium Company, a series of alloys were made in the form of 0.5 inch diameter bar. The number of the bar and the corresponding quantity of zinc is given in Table I. The bars will be referred to in future by the numbers in the first column. The bars were cut into lengths, annealed, strained, and further annealed in a way similar to that which had been successful in growing large aluminium crystals. It was found much more difficult to grow crystals right through the cross-section than in aluminium and the results were very variable. This was to be explained partly by the fact that having stretched a bar in the machine the required amount (1 to 2 per cent, on 3 inches), on removing the load there was sometimes as much as 1 per cent, contraction. This applied mostly to alloys containing from 10 per cent, zinc upwards. The bars, therefore, tended to be under strained. If allowance was made for subsequent shrinking by over-stretching, equally variable results were obtained as the bars did not all shrink to the same amount even after carefully regulating the time of application of the load. The alloys as a whole are more sensitive to straining and to heat treatment than the pure metal, and small variations which appeared to make little difference to aluminium doubled or halved the crystal size. An extension of 1 to 2 per cent, was the most successful treatment with a previous and subsequent annealing temperature of 500-550°C. Several crystals occupying the whole cross-section were usually obtained in one bar, and these were cut up and machined into small round test-pieces of 0.25 inches diameter, and of varying lengths according to the lengths of the crystal. The best crystals were put aside for machining square for distortion experiments. These will be described later.