188 
MR. A. A. GRIFFITH? ON 
hardens tool steel by preventing the separation of “ ferrite,” or iron containing no 
carbon. 
In a single crystal the molecules are presumably in an equilibrium configuration of 
maximum stability. In this event, the equilibrium of molecules at or near inter- 
crystalline boundaries, in a body composed of a large number of crystals, must, in 
general, be less stable than that of the molecules in the interior of the crystals. In 
fact, where the orientation of the component crystals is haphazard, the stability of 
the boundary molecules may be expected to range from the maximum of normal 
crystallisation down to zero, i.e., neutral equilibrium. If such a body be subjected 
to a shear stress, some of the molecules in or near neutral equilibrium must, in general, 
become unstable, and these will tend to rotate to new positions of equilibrium. This 
rotation, however, will be strongly resisted, as has been seen, by forces doubtless of 
a viscous nature, and its amount will accordingly depend on the time during which 
the stress is applied. If, therefore, the strain is observed it will be found to increase 
slowly as time goes on, but at a constantly decreasing rate, as the molecules concerned 
approach equilibrium. If now the load is removed, these molecules must rotate in 
order to regain their original positions of equilibrium, and this process in turn will be 
retarded by viscous forces. Hence a small part of the observed strain will remain 
after the removal of the load, and this mil gradually disappear as time goes on. These 
properties, known as “ elastic after-working,” are, of course, well known to belong 
to crystalline materials. Moreover, the theory shows that they should not be possessed 
by single crystals, and this has been demonstrated experimentally.* 
There is a special type of gliding or yield which may occur at stresses below the 
normal yield point. Consider a pair of adjacent crystals, separated by a plane boundary. 
If these crystals are thought of as sliding relatively to each other, it will be seen that 
only in a finite number of the positions so taken up can the two be in stable equilibrium. 
Between each pair of such positions there must in general be one of unstable equilibrium. 
Suppose that, while near such an unstable position, the two crystals are embedded in 
a number of others. Under these conditions the boundary molecules of the two crystals 
will be pulled over in the direction of one or other of the two adjoining stable positions, 
and they will strain the solid in the process. If now the body is subjected to a shearing 
stress tending to cause relative displacement of the two crystals towards the other 
stable position, then at a certain value of this stress the molecules on either side of the 
boundary mil be wrenched away, will pass through the position of instability, and will 
then take up a new position bearing the same relation to the second stable position 
as their original state did to the first. This new condition will, of course, persist after 
the removal of the load, as the original state cannot be regained without passing through 
unstable equilibrium, i.e., a condition of maximum potential energy. To cause the 
crystals to pass through this condition it would be necessary to apply a load of opposite 
sign, and in this way the process might be repeated indefinitely. In a body composed 
* H - v. Wartenberg, £ Deutsch. Phys. Gesell., Verb.,’ 20, pp. 113-122, August 30, 1918. 
