Geology Reference
In-Depth Information
½
in the
ð
111
Þ
plane (the notation 1
=
n uv
½
denotes the vector
in the uv
½
direction having the length 1/nth of the vector u
a
þ
v
b
þ
w
c
where
a
;
b
;
c
are the unit vectors defining the unit cell, and 11
ð ½
011
is the usual slip
system for f.c.c. crystals such as copper). These dislocations can dissociate
according to
Burgers vector
2
101
1
2
½
011
¼
1
6
½
121
þ
1
6
½
112
1
2
½
101
¼
1
6
½
211
þ
1
6
½
112
leading to two pairs of partial dislocations with fault ribbons between each pair. If
the two extended dislocations are then moved toward the intersection of their slip
planes, the leading partial dislocations can react according to
1
6
þ
1
6
¼
1
6
121
211
½
½
½
110
with reduction in total elastic strain energy. The product is a partial dislocation
with Burgers vector
6
11
½
and line direction
½
110
, which together define a slip
plane (001) that is not a normal slip plane in f.c.c. crystals. The 1
=
6 11
½
dislo-
cation is also linked by two inclined fault ribbons to the 1
=
211
½
and 1
=
6 11
½
partial dislocations in the (111) and
ð
111
Þ
planes, respectively, and hence is
sometimes called a stair-rod dislocation. Its sessile character follows from the
immobility of these fault ribbons in the (001) slip plane.
1
6.2.7 Electric Charge on Dislocations
Dislocations in an insulator can, like other crystal defects, be electrically charged;
see review by Whitworth (
1975
), and sections in Hirth and Lothe (
1982
, Chaps. 12
and 14). This effect has potential implications for the mobility of the dislocations
and for the transport of charge during deformation. There are two ways of viewing
the charging, depending on whether the covalent or the ionic aspect of the bonding
is being emphasized.
In the first case, the charging can be viewed as arising electronically by the
transfer of electrons or holes to or from electron energy levels associated with the
dislocation or with kinks or jogs in the dislocation. In the case of edge dislocations
the extra levels can be associated with ''dangling bonds'', but additional energy
levels can also possibly be associated with distortions in the crystal field in screw
dislocations and so these can also, in principle, be charged. This view of dislo-
cation charging has been developed for the semiconductors (Haasen and Schröter
1970
; Labusch and Schröter
1975
; Mataré
1971
).
In the second case, the charging of dislocations can be viewed as arising ion-
ically from a lack of charge balance among ions located within the dislocation
