68 
NATURE 
[May 18, 1899 
Fig. 4 is another sample of iron strained by pull. The 
specimen in this case was a bar of Swedish iron, in which a 
comparatively large crystalline structure had been developed by 
annealing for some hours at 700° C. The photograph was 
taken after the bar had been broken in the testing machine, and 
shows with a magnification of 400 diameters a portion of the 
surface not far from the place of fracture, 
The slip-bands are developed by compression as well as by 
extension. The bands developed by compression have appar- 
ently all the characteristics which they present in stretched 
pieces, and we could not, by microscopic examination of the 
surface, distinguish in this respect between the effects of com- 
pression and extension. 
By twisting an iron bar well beyond the elastic limit the slip- 
bands are made to appear, for the most part, in directions 
parallel and perpendicular to the axis of twist. 
A strip of sheet metal, such as iron or copper, in the soft 
state, when bent and unbent in the fingers, shows them well 
developed by the extension and compression of the surface. 
These experiments throw what appears to us to be new light 
| on the character of plastic strain in metals and other irregular 
| crystalline aggregates. Plasticity is due to slip on the part of 
the crystals along cleavage or gliding surfaces. Each crystalline 
grain is deformed by numerous internal slips occurring at intervals 
throughout its mass. In general these slips no doubt occur in three 
planes, or possibly more, and the combination of the three allows 
the grain to accommodate itself to its envelope of neighbouring 
grains as the strain proceeds. The action is discontinuous : it 
is. not a homogeneous shear but a series of finite slips, the 
portion of the crystal between one slip and the next behaving 
like a rigid solid. The process of slipping is one which takes 
time, and in this respect the aggregate effect is not easily dis- 
tinguishable from the deformation of a viscous liquid. 
We infer from the experiments that ‘‘ flow” or non-elastic 
deformation in metals occurs through slip within each crystalline 
grain of portions of the crystal on one another along surfaces of 
cleavage or gliding surfaces. There is no need to suppose the 
portions which slip to be other than perfectly elastic. The slip, 
Fic. 4.—Swedish iron, much strained. 
We have developed the slip-bands in iron, steel, copper, 
silver, gold, nickel, bismuth, tin, gun-metal, and brass. In 
gold.and silver they show particularly well, the crystalline struc- 
ture tbeing large and the lines straight. In copper also the 
lines are straighter and more regularly spaced than is general in 
iron. Most of these metals have been tested in the form of 
blocks under compression. A beautiful development of slip- 
bands may readily be produced by pinching a button of polished 
silver or copper in a vice, or by bending a strip of sheet metal. 
In carbon steels we have found the slip-bands considerably 
more difficult to observe than in wrought iron. The smaller 
granular structure of steel apparently makes the slip-bands 
correspondingly minute. In mild steel they are seen readily 
enough, but in a rather high carbon steel we succeeded in seeing 
them only with difficulty in the ‘‘ferrite” areas under a magni- 
fication of 1000 diameters. A cast piece of the nearly pure iron 
used for dynamo magnets showed a relatively very large granular 
structure and well marked slip-bands. 
NO. 1542, VOL. 60] 
400 diameters. 
when it occurs, involves the expenditure of work in an irre- 
versible manner. 
It is because the metal is an aggregate of irregular crystals that 
it is plastic as a whole, and is able to be deformed in any manner 
as a result of the slips occurring in individual crystals. Plasticity 
requires that each portion should be able to change its shape and 
its position. Each crystalline grain changes its shape through 
slips occurring within itself, and its position through slips 
occurring in other grains.! 
The experiments were made in the engineering laboratory at 
Cambridge, and are being continued. The authors express their 
indebtedness to Sir W. Roberts-Austen and Mr. T. Andrews 
for advice as to the preparation of specimens of metals for 
microscopic examination. 
1 Attention should be called in this connection to the experiments of 
Messrs. McConnel and Kidd on the plasticity of glacier ice (Ray. Soc. 
Proc., vol. xliv. p. 331). They found that bars cut from glacier ice which 
is an aggregate of irregular crystals are plastic. 
