ELECTRON TRANSMISSION THllOtJGH THIN METAL SECTIONS 883 



As previously pointed out, the formation of recovery domains upon cold 

 working represents an early stage of polygonization with a much smaller 

 size than that observed at low deformation.^" This suggests a nucleation 

 and growth process^^ for recovery during and following plastic deformation. 

 Cahn^^ j^as pubHshed a detailed nucleation theory to account for recrystal- 

 Hzation grain size. He considers the nuclei to be regions in the crystal witfi 

 large curvature brought about by cold working. It would be expected that 

 both the nucleation and growth rates would be effected by changes in tem- 

 perature and by additions of alloying elements which reduce the rate of 

 diffusion of dislocations. The latter was tried first by adding about 4% 

 copper to the high purity aluminum and rolling the alloy to 0.005'' sheet. 

 A section prepared from the rolled sheet with no anneal gave the results 

 shown in Figure 11. It will be noted in Fig. 11a that the large, well defined 

 domains seen in Figs. 8 and 9 are not present in the alloy. The structure is 

 much smaller and is strung out in the direction of rolling. The electron dif- 

 fraction pattern, (Fig. 11 (b)) exhibits arced rings with the arcs being contin- 

 uous rather than showing the discrete spots seen for the pure metal (Fig. 

 8b). The number of recovery domains produced in the alloy is thus much 

 greater than in the base metal which checks with the much smaller recovery 

 exhibited by cold worked aluminum alloys as compared to pure aluminum. 

 It is concluded that the addition of copper has produced ''knots" in the 

 aluminum lattice which impede the rate of growth of domains. ^^ 



The effect of temperature is of much interest since nucleation processes 



A 

 generally involve a temperature dependent term of the form e — —- where 



KI 



A is an activation energy^^ A plot of nucleation rate against temperature 



should yield a curve exhibiting a maximum at some temperature Tc. For 



T > Tc, only a portion of the embryos are able to exceed the critical size, 



the smaller ones dissociating. For T< Tc, the thermal diffusion rates are 



sufficiently low to impede embryo formation. The maximum nucleation rate 



is thus a balance between the diffusion rate and the number of embryos 



able to exceed the critical size and grow. In order to investigate the effect 



of temperature, high purity aluminum specimens were cold worked by 



pounding at — 78°C (dry ice) and at — 196°C (liquid nitrogen) and then 



allowing the specimen to warm up slowly to room temperature. It was 



thought that if the working was done at a temperature below that at which 



10 A. Guinier and J. Tennevin, C. R. Acad, of Sci., Paris 226, 1530 (1948). 



11 R. D. Heidenreich, "Cold Working of Metals," page 57 (American Society for Metals, 

 Cleveland, 1949. 



12 R. W. Cahn, Froc. Phys. Soc. A 63, 323 (1950). 



1^ If this alloy is given a 10 min. anneal at 300°C, recovery domains very similar to Fig. 

 8 are obtained. 



"D, Tumbull, A.I.M.M.E. Gech. Pub. #2365 (1948). 



