Plastic Flow in Solid Helium

The plastic deformation of solid helium 4 is investigated by measuring the force required to move steel ball in solid helium at a nearly constant speed. Any force-displacement curve reveals marked yield drop and gradually reaches to a constant force, which is of the order of 2×10 5 dyn. It is concluded that the plastic flow in solid helium takes place primarily by the motion of dislocations. The force required for steady motion is proportional to the cubic root of the velocity of the steel ball at 2.1 K, but the force increases by about 10 5 dyn irrespectively of the speed of the steel ball below 1.6 K. Shear stress-shear strain curves of helium crystals are estimated from the force-displacement curves of the steel ball, and are discussed in terms of multiplication and motion of dislocations.