ELECTRON TRANSMISSION THROUGH THIN METAL SECTIONS 887 



low temperatures; i.e., of the order of that of liquid nitrogen (—196 

 °C). 



(5) The recovery domains do not show (at least not readily) on etched 

 surfaces. The overall rate of etching is much higher than in the an- 

 nealed state, however. 



(6) Deformation by simple extension does not produce the recovery 

 domains such as seen in Fig. 8. Neither domains nor slip bands are 

 visible. 



These conclusions and observations suggest explanations for several well 

 known phenomena in cold worked metals. One is the fact that slip lines are 

 not visible on a surface etched after cold working, which has always been 

 rather puzzHng. It seems clear now that this is simply due to an immediate 

 rearrangement giving rise to recovery so that the slip band exists as such 

 only during the actual deformation process. The traces left on polished sur- 

 faces are actually only traces of the displacements that occurred during 

 deformation and do not indicate where the energy of cold work resides 

 when slipping has stopped but only where the energy was introduced. The 

 energy would reside in the slip bands only if no recovery whatever took 

 place. 



Another point of interest is that of recrystalHzation. In one sense the re- 

 covery domains constitute recrystallization on a smaller scale than is usually 

 meant. However, in view of the fact that the domains do not etch preferen- 

 tially (probably due to internal strains) and that they disappear at the re- 

 crystallization temperature, it would seem more accurate to view the re- 

 covery domains as distinct from recrystallization. It would seem logical to 

 consider the recovery domains as embryos for recrystallization. When the 

 temperature is raised sufficiently those recovery domains which most rapidly 

 relieve their internal strains would serve as nuclei for new grains and con- 

 sume the surrounding embryos or domains. In a sense, then, it is the least 

 strained material from which new grains spring. However, the embryo for 

 the new grain very probably sprang from a region that was very highly 

 strained. Between the actual slip process and final recrystallization grains 

 there are actually two nucleation and growth processes. 



The author is grateful to Mr. W. T. Read and Dr. W. Shockley for 

 valuable criticisms and discussions of the subject matter presented in this 

 paper. 



