Recrystallization of Lead

WHILE the recrystallization properties of most of the practically important metals are known in considerable detail, those of lead are still relatively little known in spite of some valuable contributions in recent years. This condition probably is due to the difficulties involved in applying the usual methods of investigation. The microscopic method of discriminating between the elongated deformed grains of the familiar lenticular shape and the equiaxed recrys-tallized grains is not successful with fully refined lead: lenticular grains cannot be seen under the microscope with any amount of deformation whatever. The probable cause of this phenomenon was recognized long ago. With large deformation the time of recrystallization at room tem-perature is shorter than the period of preparation of the specimen for microscopic examination; with small deformation the deformed grains are not noticeably elongated in the first place. However, with small deformation the migration of the grain boundaries can be observed. The only systematic investigations yet published on the recrystallization of lead, those by R. S. Russell,1-3 make use of the conditions just described. Russell considered recrystallization completed when no further migration of boundaries could be observed. A serious objection to this method arises from the fact that there is no evidence to support the view that grain-boundary migration results only from recrystallization. If such migration does result from other sources after recrystallization is com-pleted-after the deformed grains have been replaced by new unde-formed grains-the "time of recrystallization" as determined by Russell may be too long. Furthermore, the method is subject to the above-mentioned limitation because of the unavoidable period of preparation of the sample. According to Russell, even with the fastest technique 5 min. elapsed before the deformed samples were ready for examination, and consequently no recrystallization time shorter than 5 min. could be measured.