ETCH-PITTING CHARACTERISTICS OF HIGH-PURITY MOLYBDENUM.

The etch-pitting behavior of single and polycrystalline molybdenum was examined using various etching solutions and prior heat treatments. Murakamis solution, Wolffs solution, oxalic acid and sulphuric acid in methanol are found effective on planes in a region 30 degrees about 001, however it is shown that a one to one correspondence between etch-pits and dislocations does not necessarily exist. A plane about 11 degrees from 112 towards 012, which is to be the observation plane in the dislocation velocity studies, was examined in detail. Reliable and reproducible pitting of dislocation sites occurs on this plane with a diluted Murakamis solution. A one to one correspondence between etch-pits and dislocations was positively established by transmission electron microscopy of thin foils of this orientation etched prior to examination. The etch-pit shape depends critically on the crystal orientation but is independent of the nature or Burgers vectors of the dislocations. Annealing of molybdenum single crystals at about 1900 degrees C in hydrogen for 8 hours lowers the dislocation density from about 10 to the 8th powersq cm to 1,000,000sq cm as determined from both etch-pitting and transmission electron microscopy. The dislocation density is reduced by i complete removal of carbides in the metal matrix the carbides being responsible for the high in-grown dislocation density and ii re-arrangement of the dislocations into sub-grain boundaries. Author