Anisotropy of etching behaviour in b.c.c. tungsten and the problem of edge-screw dislocation distributions

Abstract

Experiments of previous workers on b.c.c. tungsten had shown that several etchants revealed etch-pits over the entire range of surface orientations which were not more than 35° from the (001) axis. This fact has been exploited to investigate the etch-pit distributions of polycrystals and single crystals of several axial orientations, using for each sample many etchable surfaces comprising a large angular spread of crystal planes. The density of etch-pits is not homogeneous for a given sample, but depends on the particular surface being etched. Etch-pit densities have a systematic dependence on etch-plane orientation and are consistently 10² to 10³ smaller when the four 〈111〉 crystal directions make nearly equal angles with the surface of observation. Use of the Scanning Electron Microscope permitted measurements of the pit densities to higher values than are usually associated with etch-pit methods. The results indicate either that tungsten is not reliably etchable over the range of surface planes usually assumed, or that the distributions of edge and screw are markedly different in the crystals examined.