Figures 1 through 8 show the microstructure of the bolt in an area where the brass is in the annealed state. The bolt was manufactured from an alpha (fcc) - beta (bcc) brass which has a nominal composition of 60% Cu - 40% Zn. The grains of alpha phase can be recognized by the presence of annealing twins and the grains of beta phase tend to have a yellow color. Figure 8 and 9 are SEM micrographs of the same structure.
Figures 10 through 16 are optical micrographs of the threaded section of the bolt. The threads were formed by a cold-forming process known as thread rolling and the cold-worked structure of the threads is readily apparent.