Geology Reference
In-Depth Information
Chapter 5
Deformation Mechanisms: Atomic
Transfer Flow
5.1 A General Model
In this category of deformation mechanisms we are concerned with processes in
which individual atoms or small groups of associated atoms are removed from
certain interfaces or discontinuities within the structure of the body (sources) and
are transferred to other interfaces or discontinuities (sinks) in such a way that the
overall shape of the body is changed, that is, the body undergoes macroscopic
strain. The sources and sinks may be dislocation cores, planar crystal defects, grain
boundaries or free internal or external surfaces, and the transfer may take place by
a variety of mechanisms, including solid state diffusion (intra- or intergranular)
and transfer via a fluid phase (in case of a porous or partially melted body). The
overall kinetics may be controlled by the kinetics of the transfer process or by the
kinetics of the detachment and re-attachment processes. We have used the term
''atomic transfer flow'' in introducing this class of mechanisms in order to
emphasize that the transfer occurs more or less atom by atom rather than by the
movement of relatively large blocks of atoms; however, the term ''diffusion creep''
is commonly used in the same sense, especially when it is wished to emphasize
diffusion as the transfer process or as being rate controlling.
Atomic transfer flow is essentially a solid state process in that the atoms
detached or re-attached belong to solid grains, even though the transfer may in
some cases occur via a fluid medium. This type of flow is therefore to be distin-
guished fundamentally from viscous flow in fluids where, although individual
atom movement is again involved, the atoms do not have defined sites in sources
and sinks and the resistance to flow derives from the process of momentum
transfer as the atoms move about. In Newtonian viscous flow in a fluid, the flux of
stream momentum is proportional to the gradient in stream velocity of the particles
and so the stress, deriving from the rate of change of momentum, is proportional to
the strain rate (Cottrell 1964 , p. 26). The viscosity associated with momentum
transfer in the diffusive movement of atoms in a solid would be enormous and so
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