Part IV – April 1968 - Papers - The Influence of Plastic Deformation on the Theta Prime to Theta Transformation in AI-4 pct Cu

Cold rolling by 50 pct a A1-4 pct Cu alloy containing many 0' plates has been found to accelerate markedly the transformation of 8' to 0 upon further aging. Although the basic mechanism of this transformation remains unchanged by plastic deformation, the nuclea-tion kinetics of 8 become appreciably more rapid and also less dependent upon temperature. Particularly at lower aging temperatures, rounded idiomorphs replace several faceted rnorphologfes. The rates of growth of 9 idiomorphs in deformed specimens aged at 300°C are appreciably more rapid than simple volume diffusion control allows. This observation is shown to be explained by an excess vacancy, rather than by a dislocation Piping mechanism of growth. ThE rather complex phenomenology of the 8' —8 transformation in Al-Cu alloys has been recently elucidated in considerable detail by means of transmission electron microscopy.'-4 This technique was also employed in the present investigation in order to examine the effects of plastic deformation upon the transformation of 8' to 8. The mechanism of the transformation in undeformed Al-Cu alloys has been established as the following sequence of events.1"4 The first step is the nucleation of 9 at (Y:8' boundaries. Nucleation of a 0 crystal takes place, however, at only a very small proportion of the 8' plates present. Each 0 crystal simultaneously consumes the host 8' plate and the a! in the immediate vicinity, with growth into a being the rate-controlling process. The remaining 8' plates are then dissolved as they fall under the concentration shadow of the growing 8 crystals. The evidence on the effects of plastic deformation upon these processes is intriguing, but much less detailed. The careful X-ray studies of Graf and Guinier have shown that the aging times required to form detectable amounts of 8' and 8 are reduced by prior cold working. Thomas and Nutting confirmed these results by replication electron microscopy. Nile-shwar,* on the other hand, reported that creep deformation decreases the rate of formation of 0. While this difference may have been caused by the use of concurrent rather than successive deformation and ag- ing, the X-ray technique which he used was able to monitor only the 0 phase. It is thus possible that the times needed to form 8' were increased moreg than the kinetics of 8' — 8 transformation were accelerated. At 90 pct deformation by cold rolling, Graf and Guinier found that the usual 0' —0 8 sequence is reversed: 0 forms first. von Heimendahl 10 confirmed this result with transmission electron microscopy. Pollard and Nutting, 11 and more recently Driver and Barrand, 9 having used the same technique, report that 9 also forms first during creep deformation. In the present study, the maximum amount of deformation was limited to 50 pct reduction by cold rolling in order to insure that 8 formed principally by the decomposition of . I) EXPERIMENTAL PROCEDURES This investigation was conducted on an Al- 3.93 pct Cu alloy, prepared as previously described. 4 Morphological and crystallographic effects of plastic deformation were studied in specimens annealed 6 hr at 548°C, quenched, and then aged for either 3 or 8 hr at 250°C to form large numbers of 0' plates. After cold rolling to a 50 pct reduction in thickness, the