Geology Reference
In-Depth Information
Chapter 6
Deformation Mechanisms: Crystal
Plasticity
6.1 Basic Geometry of Slip and Twinning
The deformation mechanisms of the greatest importance in the intragranular
plastic deformation of crystalline materials are slip and twinning. In these
mechanisms, the strain or change of shape is achieved by the relative movement of
blocks of atoms rather than by the more or less independent movement of indi-
vidual atoms that characterizes the atomic transfer mechanisms considered in the
previous chapter. Deformation of means of the slip and twinning mechanisms is
commonly referred to as crystal plasticity. It can be effective at all temperatures
but is of overriding importance in the athermal regime (
Sect. 6.6.1
).
Macroscopically, the basic process in both slip and twinning consists of a more
or less uniform simple shear (in the continuum mechanics sense, Means
1976
,
p. 146). The shearing occurs parallel to a well-defined crystallographic direction
and, in most cases, on a well-defined crystallographic plane (Fig.
6.1
). When the
crystallographic orientation remains unchanged relative to the direction and plane
of shearing, the process is known as crystallographic slip or translation gliding.
When the orientation changes to a crystallographic twin orientation, the process is
known as mechanical twinning or twin gliding. For brevity, we refer to the two
processes as slip and twinning, respectively.
Slip. Close examination of the slip process shows that it is not homogeneous on
the microscopic scale. The blocks that slide over each other can often be distin-
guished through the steps produced in a bounding surface at the loci of sliding.
These steps may be visible microscopically as fine lines if the surface has been
previously polished (Fig.
6.2
). Such lines, which can be seen in the optical
microscope, are known as ''slip bands''. The electron microscope (Heidenreich
and Shockley
1948
) reveals that they tend to be of multiple structure, consisting at
the near atomic scale of finer steps called ''slip lines'', although this term is
