  Dislocation interactions at the crack nucleation site were investigated in
near-alpha titanium alloy Ti-6242Si subjected to low cycle fatigue. Cyclic
plastic strain in the alloy resulted in dislocation pile-ups in the primary
alpha grains, nucleated at the boundaries between the primary alpha and the
two-phase regions. These two phase regions provided a barrier to slip transfer
between primary alpha grains. We suggest that crack nucleation occurred near
the basal plane of primary alpha grains by the subsurface double-ended pile-up
mechanism first conceived by Tanaka and Mura. Superjogs on the basal <a>
dislocations were observed near the crack nucleation location. The two phase
regions showed direct transmission of a3 dislocations between secondary alpha
plates, transmitted through the beta ligaments as a[010], which then decompose
into (a/2)<111> dislocation networks in the beta. The beta ligaments themselves
do not appear to form an especially impenetrable barrier to slip, in agreement
with the micropillar and crystal plasticity investigations of Zhang et al.
