Deformation structures in zone-melted molybdenum

Abstract

Abstract Glide dislocation configurations in zone-melted single crystals of molybdenum, deformed in tension at temperatures in the range 4·2–300·K, have been examined by thin film electron transmission microscopy. A majority of the crystals were oriented for single slip on the (011) [111] system, and were sliced and thinned parallel to (011). The slip band structure is characterized by long primary screw dislocations having a Burgers vector α/2[111], and by secondary glide dislocations parallel to [111] or [111]. Consistent with the presence of secondary slip, the bulk stress–strain curves do not exhibit a region of easy glide. The primary screws have a high jog density (∼5 × 104 cm−1) which can be accounted for by the cutting of secondary dislocations intersecting the (011) glide plane. The jogs lower the velocity of the screw components relative to that of the edges, and from dislocation mobility data the calculated velocity ratio of edge: screw is ≈ 40: 1; this appears to be a realistic ratio in terms of the lengths of screw and edge components seen in the micrographs. At low temperatures (≾200°K) the screw dislocations are straight and uniformly distributed on the slip plane. Glide configurations at 300°K are, in general, similar to those at the lower temperatures except that the screw dislocations are wavy, the distribution is less uniform and, surprisingly, the number of secondary slip dislocations is lower. Dislocation tangling is restricted to the immediate vicinity of inclusions or grown-in dislocations. The observations are consistent with a picture of yielding in which the true elastic limit is largely independent of temperature and corresponds to the movement of edge segments. The large temperature dependence commonly associated with the macroscopic yield point is, in reality, a flow stress measured in a region of extremely high work-hardening. It is suggested that movement of the screw dislocations is controlled by a thermally-activated conservative movement of jogs.