886 THE BELL SYSTEM TECHNICAL JOURNAL, OCTOBER 1951 



the nucleation rate is a maximum, then as the specimen warmed slowly it 

 would pass through the maximum and result in a larger number of nuclei. 

 The time for recovery at the low temperature was varied from 15 minutes 

 to several hours before bringing the specimen to room temperature but the 

 results did not reflect any dependence on the time at low temperature. Even 

 so, a primary weakness in the experiment lies in the fact that the structure 

 could not be observed at the temperature of working. The results of cold 

 working at — 78°C and — 196°C are shown in Fig. 12. Surprisingly enough, 

 the domain size after cold working at — 78°C is practically the same as at 

 room temperature as seen in Fig. 12a. It was thought that this simply 

 meant that the structure had reverted to the room temperature configura- 

 tion until the results were obtained at — 196°. The domain size resulting 

 from the — 196°C treatment is slightly less than half that obtained from the 

 — 76°C treatment, indicating that the effect of recovery temperature can be 

 seen after bringing the specimen up to room temperature.^^ This is consistent 

 with low temperature rolling experiments^^ on pure copper performed by 

 W. C. EUis and E. Greiner of Bell Telephone Laboratories in which the 

 amount of work hardening was considerably increased over that obtained 

 by rolling at room temperature. Thus, at present it appears that the re- 

 covery domains seen in Fig. 12 are a fair approximation to those produced 

 at the low temperatures. More work on low temperatures is certainly justi- 

 fied since it appears that the recovery mechanism involves a process with a 

 very low activation energy, at least in the case of pure aluminum. 



General Remarks 



The conclusions drawn from the electron images of cold worked alumi- 

 num are, in review, 



(1) During and immediately following cold working of pure aluminum 

 self -recovery takes place by the formation of recovery domains about 

 2jLi in size. 



(2) The recovery domains produced at room temperature and below at 

 first possess sufficient internal strains to prohibit extinction contours. 

 These strains are slowly relieved at room temperature. 



(3) The addition of copper to the aluminum inhibits the growth of re- 

 covery domains resulting in a recovery domain size much smaller 

 than for the pure metal. Thus, aluminum-copper work hardens to a 

 far greater extent than does pure aluminum. 



(4) Recovery in pure aluminum is reduced only by going to relatively 



" The microbeam x-ray technique (reference 9) should be invaluable in checking this 

 point since the entire experiment could be done at the low temperature. 

 "To be published. 



