Experiments on the deformation of niobium single crystals

Abstract

Thin foils were prepared from slices taken in selected orientations from crystals deformed in compression by single glide on (101) or (211) or by double glide on (211) and (211). At room temperature the structures observed after single glide were quite similar to those which have been reported for copper. In stage I, braids in the primary slip plane consisted mainly of edge dislocation multipoles, but more complex tangles containing many secondary dislocations were formed in stage II, and were joined into the continuous walls of a cell structure. The main new feature observed in crystals orientated for double slip was the formation of high angle boundaries after large strains. Few screw dislocations were retained in the room temperature structures, but foils from crystals deformed at 158°K contained fairly uniform distributions of long primary screw dislocations, together with many small prismatic loops. It is believed that screw dislocations from different sources cross-slip and mutually annihilate at room temperature, but are prevented from doing this by the frictional stress at low temperatures.