The vertical offset between the center of the GP zone
and the initial dislocation slip plane did have an effect on
the mechanism and the applied shear stress at which the
dislocation ultimately overcame the GP zone, sc. When
the approaching dislocation slip plane was above the
center point of the 4.4 nm GP zone (relative to
Fig. 2a) the dislocation did not cross-slip, but rather it
Orowan looped around the GP zone, with sc ranging between
270 and 310 MPa. When the intersection point
was below the center point, cross-slip was always observed,
with sc near 50 MPa.
Cross-slip also occurred for the 0 screw dislocation–
GP zone interaction. Figure 2b details the series of
events that occur as the applied loading is increased at
0 K for a 4.4 nm diameter GP zone centered on the dislocation
glide plane. At syz < 30 MPa ð0:053sOrowanÞ, the
equilibrium position of the leading partial is 7 nm from
the GP zone and no noticeable contraction of the core is
evident. Between syz ¼ 30 and 40 MPa the dislocation
cross-slips from the (111) to the ð111Þ plane, with the
leading partial remaining stationary at the surface of
the GP zone. Although the dislocation cross-slips to
the ð111Þ plane, its forward motion is still obstructed
by the GP zone. Furthermore, the applied force acting
on the dislocation is now reduced by a factor of
pffi3ffiffi=ffiffi2ffiffiffi. The dislocation remains in this state until
syz ¼ 520 MPa ð0:912sOrowanÞ, at which point it crossslips
back onto a (111) plane and overcomes the GP
zone via Orowan looping [4]. The (111) plane that the
dislocation ultimately resides on is Dy ¼ ffi3ffiffi
p
a0 above
the original (11 1) plane on which it started. Screw dislocation
cross-slip in this case, in contrast to the 60 interaction,
acts as a hardening mechanism. In a random
field of obstacles where multiple mechanisms can occur
along the dislocation line, only segments of the dislocation
may cross-slip consistently with the early stages of
the Hirsch mechanism for screw dislocations [11,12].
For the 0 interaction orientation, GP zone size was
found to have an appreciable effect. For small sizes with
diameters