A previous paper described the preparation and tensile properties of brass crystals containing 25 to 30 per cent. zinc. Similar experiments have now been carried out with a copper-aluminium alloy containing 5 per cent, aluminium, which was kindly prepared by the Broughton Copper Company. Crystals of this alloy were easier to prepare than the brass crystals and the composition was more uniform, as unlike zinc, aluminium is not volatile. On the other hand, a very resistant oxide film forms on the surface, and the prepared surfaces of the crystals tarnish very readily. The bars were sound, except near the top, where very well-developed dendrites were formed. It was found on etching that the crystals were not always of uniform orientation throughout; and were very much cored. Bands of very slightly differing orientation occurred, of which fig. 1 (Plate 20) is an example. This has almost the appearance of lamellar twinning. Actually, X-ray measurements showed that both parts had the same relationship to the vertical axis, but that one part was rotated 7° with regard to the other, about their plane of union, which in this case was nearly perpendicular to the axis. Figs. 2 and 3 (Plate 20) are photographs of the cross-section and vertical section of the same crystal. The scratch in fig. 2 corresponds with the plane of the section shown in fig. 3. The cores are not developed uniformly, nor are they evenly spaced. In this particular crystal those perpendicular to the axis were most complete. X-ray measurements showed that both sections were {100} planes, indicating that the cores followed the cubic axes of the crystal, but as the photographs show, they were not equally developed along the three possible directions. This is in accordance with previous observations on the crystallization of metals.
Effects of laser paint stripping on oxide film damage of 2024 aluminium alloy aircraft skin
