DUCTILE-BRITTLE TRANSITION IN REFRACTORY METALS.

Zone-refined tungsten crystals were compressed at 298 K up to a residual strain of 0.45 and at 77 K up to a residual strain of 0.35. Compression at 298 K resulted in the formation of a plastically deformed surface layer which confined the bulk of the elastic strains in the crystal and which was released when the surface layer was removed by etching. By contrast, compression at 77 K gave rise to the generation of dislocations, which were distributed throughout the crystal and which were predominantly confined to sites of ingrown dislocations. The lattice of the subgrains appeared to become elastically strained and not plastically deformed by slip dislocations. With increased compression a preferred directionality of the maximum normal elastic strain parallel to the 100 plane began to develop, which is interpreted as an indication for the tendency of the crystal to cleave.