Geology Reference
In-Depth Information
so intense that it contributes significantly to the strain itself, as a diffusion creep
mechanism (
Chap. 5
). However, in other cases, diffusion is thought to be inhibited
in the neighborhood of particles (Martin
1980
, pp. 167-180). In any case, the
activation energy may be relatively high because of the additional need to elim-
inate attractive junctions (Guyot
1980
).
6.7 Dynamics of Mechanical Twinning
Mechanical twinning is generally viewed as taking place by the orderly propa-
gation of a partial dislocation, with some mechanism such as the pole mechanism
of Cottrell and Bilby (
1951
) and Thompson and Millard (
1952
) for ensuring that
the twinning dislocation progresses from plane to adjacent plane with each sweep.
The motion of the twinning dislocation will be impeded by similar interactions to
those involved for slip dislocations but, as (Friedel
1964
, p. 176) has pointed out,
these interactions can be much weaker than for slip dislocations because of the
smaller Burgers vector of a partial dislocation, resulting in lower Peierls potential,
lower energies of interaction with solutes and other obstacles, etc.; also the
twinning dislocation is not subject to strain-hardening interactions with a growing
dislocation density. Consequently, it is often observed that twinning can proceed
rapidly at relatively low stresses, as, for example, in calcite at room temperature.
For a general discussion of mechanical twinning in terms of dislocations, see Hirth
and Lothe (
1982
, Chap. 23).
Although twinning can propagate easily, there can be considerable nucleation
barriers against its initiation. Friedel (
1964
, p. 176) suggests that the initiation
stress needed should be of the order of c
=
b, where c is the energy per unit area of
the stacking fault generated by the twinning dislocation and b its Burgers vector.
For metals, this quantity is often of the order of 1 % of the shear modulus. Hirth
and Lothe (
1982
, pp. 757, 825) deduce similar or somewhat higher nucleation
barriers from a consideration of the thermally activated nucleation of partial dis-
locations. Therefore, local stress concentrations can be expected to play an
important part in the initiation of twinning. The existence of an initiation stress
that is high relative to the propagation stress would provide an explanation of
various phenomena that are common in twinning, such as sharp stress drops or
serrated stress-strain curves and acoustic effects (''twinning cry'', as observed in
tin). Some particular dislocation processes that may be involved in the nucleation
of twins in the common metals are discussed by Mahajan (
1981
) and references
given to others.
Mechanical twinning is more widely observed at relatively low temperatures.
This observation is consistent with twinning being essentially an athermal process
in most respects, initiated more readily when the thermal activation of slip is at a
minimum and applied stress at a maximum. However, the sensitivity of the
nucleation to structural imperfections probably rules out there being a well-defined
critical shear stress for twinning. As a result of the generally athermal character,
