High-purity single crystals which had orientations that were in the central part of the stereographic triangle were deformed by tension, compression, and cyclic deformation at temperatures below ambient. The deformation behavior was monitored by using slip-line, Laue X-ray back-reflection, X-ray topographic, and transmission electron microscopic techniques. Anomalous (011) slip was observed in Nb crystals at temperatures below about 200K, and in Ta crystals at liquid N temperatures. The effect of specimen purity upon anomalous slip was investigated by means of etching experiments. Thus, etching in an acidic solution suppressed anomalous slip in Ta specimens. This was attributed to the effect of H interstitials. Pre-straining of the specimens had differing effects upon the appearance of anomalous slip; depending upon the temperature and the pre-deformation method. Anomalous slip was usually suppressed by pre-straining, unless the specimens were pre-deformed into stage-I of work-hardening at the given temperature. Transmission electron microscopic investigations revealed that the dislocation arrangement consisted of regular networks of screw dislocations with primary and conjugate Burgers vectors lying in the anomalous slip plane. The X-ray topographic investigations demonstrated that twist walls of alternating sign built up during anomalous slip. The separation of 2 adjacent twist walls of alternating sign corresponded to the separation of the anomalous slip lines at the crystal surfaces. The density of excess dislocations in the twist walls was estimated, from the angle of twist, to be equal to about 3% of the total dislocation density.

W.Wasserbaech: Physica Status Solidi A, 1995, 147[2], 417-46