A study was made of the plastic deformation of germanium single crystals. Specimens oriented for single slip were strained in tension at elevated temperatures (< 400°C). Deformation at a constant strain rate is characterised by a pronounced yield point, followed by an extensive easy glide region. The size and nature of the yield point depend sensitively on the initial dislocation density and orientation of the specimen, strain rate and temperature, but are not significantly affected by either the surface condition of the specimen or the presence of small amounts of impurity.

A yield point is not observed on unloading and reloading in the easy glide region. It is found that a return of the yield point on loading is only effected by reducing the dislocation density of the deformed specimen to a level approaching that of the undeformed specimen.

The yield point phenomenon is explained in terms of the number and velocity of dislocations. It is concluded that grown in dislocations provide the operative dislocation sources.

The flow stress in the easy glide region is found to be a sensitive function of the dislocation density, strain rate and temperature. From these results a physical model of flow stress is developed, in which the contributions are the internal stress field and the stress to move certain dislocation "dragging points".

Some preliminary observations on the nature of work-hardening are also made.