INSIDES OF METALS — ZAPFFE 255 



guished. Pecularities existing within the grain itself, however, be- 

 come for the most part unobservable. 



About 10 years ago, the centuries-old method of de Reaumur and 

 Swedenborg was tried again, this time with a fresh attack and with 

 the benefit of modern improvements in the construction of the micro- 

 scope. A special fractographic stage was designed which allowed the 

 investigator to study nascent fracture surfaces, although this time not 

 by exploring tlie general appearance of the fracture, but by exploring 

 detail within the individual fractured grain. Metals are vast com- 

 posites of minute crystals, called grains, and the older technique had 

 done little other than view the surface of the entire assemblage. With 

 modern f ractography it is not the forest but the individual tree that 

 is being observed. 



For the past 4 years fractography has been the subject of a special 

 study in the author's laboratory, principally under the sponsorship of 

 the Office of Naval Research, and the research from which this review 

 stems has been largely conducted by F. K. Landgraf and C. O. Worden. 

 Many fascinating new features of metals, also other crystals, have been 

 discovered. Just a few of these will now be given to show the astonish- 

 ing elaboration to be found within the boundaries of the microscopic 

 grain itself, and the many significant research fields inviting further 

 exploration with this new tool. 



In plate 1, figure 2, a fractograph of pure metallic bismuth is shown. 

 The entire field of the photogi-aph belongs to a single grain, as is 

 proved by the fact that its markings have a common geometric rela- 

 tionship. If this specimen had been polished and etched, nothing 

 would appear but a more or less blank surface, the grain boundary 

 lying outside the field of observation. 



On the other hand, one finds in the fractograph a wide assortment 

 of markings. The most prominent of these are bands which are placed 

 at exactly 60° with one another, forming equilateral triangles where 

 all three directions appear. These are now known to be "twins," 

 which means that the atoms throughout the region of the twin band 

 have been forced into a certain special relationship with one another 

 by the impact which fractured the metal. The fact that these twin 

 bands lie at exactly 60° to one another is highly significant, for it re- 

 veals that the fracture has traveled along a special plane in the bismuth 

 crystal— a crystal face that is the weakest link. This plane is the 

 so-called basal plane, and is similar to the prominent cleavage plane 

 that characterizes crystalline graphite, also mica. It has further 

 been determined that the twin bands are intersections of three sloping 

 crystallographic planes that form a low pyramid on the basal cleavage 

 plane. 



