On the Relation between the Internal Structure and the Corrodibility of Aluminium Light Alloys due to Mechanical Deformation and Heat-treatment (1942)

The effects of mechanical deformation and heat-treatment upon the corrodibility of pure aluminium and some aluminium alloys were examined in connection with the residual internal strain. Utilizing the test piece of specimens prepared under the different conditoins tabulated in Table 1, the measurement of their corrodibity was made on the one hand, to detect the change of the corrosion given rise to by the aforesaid procedures. While on the other hand, the general view, the macro- and micro-structures and the crystalline configuration of these specimens were also studied together with their lattice constant, to estimate the variation of the residual strain from the structural changes. As a consequence of the investigation thus advanced, it became clear that the chill-cast specimens of pure aluminium and Cu-Al alloys, which have been deformed by the external tension over their elastic limits, were apt appreciably to be more corrosive throughout the whole test piece than these unstrained. But, such a raising of the corrodibility appears unevenly in its amount, according to the part of the test piece; the nearer the part was situated to the areal contracted portion, the more the corrosion became notable, as represented in Fig. 2. With regard to the structural change, the macro- and micro-structures as well as the crystalline configurations revealed with X-rays by the Laue method, were also found to indicate the presence of the more severe residual effects of the external tension in the part nearer to the areal contracted portion, which had been rendered to be more corrosive, as may be seen in Photos. 2, 3, 5 and Photos. 8_??_12 respectively: i.e., the elongation of some of the crystal-grains or dentrites coming to appear in Photo. 2 or 5, and also the distruction of the micro-crystals or the formation of some fibrous-like structures deduced from Photos. 8_??_12 as in the previous researchlb, can be observed alike more clearly at the part nearer to the areal contracted portion of the test piece than those further to that portion. Moreover, the aforesaid parallelism between the change of the internal structure and that of the corrosion of these chill-cast specimens is also detectable by the displacement of lines in the “back-reflected” X-ray spectra reproduced in Photo 14, which shows the more perceptible expansion of the crystal lattice at the part where the corrodibility of the test piece increases the more remarkably. Thus, the corrosion of the chill-cast specimens of pure aluminium and Cu-Al alloys may be deemed to vary, in accordance with residual internal strain disclosed by the structural change due to the external tension. In addition to those which have hitherto been stated, it is noticed as a patent point of the consequence of the present experiment, that the aforesaid expansion of the crystal lattice was observed to take place not only parallel but perpendicular to the direction of the external tension, similarly as had been pointed out by Wood and Smith(2). By making use of the reflection from pure aluminium powder annealed as the reference line, the amount of the lateral expansion of the crystal lattice at the areal contracted portion, was estimate, from the displacement of (333) line, to correspond to the elongation percentage of the order 10-4 in pure aluminium, but sometimes to exceed the order 10-3 in 4% Cu-Al alloy. Here, it must be again postscribed that the facts, thus confirmed with various chill-cast specimens, could hardly be found with the specimens undergone some adding treatments besides the chill-casting, as those of the cold-rolled Cu-Al alloys and ESD alloys prepared by tempering at low temperature. This is probably due to the predominance of the residual strain already kept rather in these specimens, than those given rise to by the external tension.