Abstract :
As a tribute to the scientific work of Professor
David Brandon, this paper delineates the possibilities
of utilizing in situ transmission electron
microscopy to unravel dislocation-grain boundary
interactions. In particular, we have focused on the
deformation characteristics of Al–Mg films. To this
end, in situ nanoindentation experiments have been
conducted in TEM on ultrafine-grained Al and Al–Mg
films with varying Mg contents. The observed propagation
of dislocations is markedly different between Al
and Al–Mg films, i.e. the presence of solute Mg results
in solute drag, evidenced by a jerky-type dislocation
motion with a mean jump distance that compares well
to earlier theoretical and experimental results. It is
proposed that this solute drag accounts for the difference
between the load-controlled indentation responses of
Al and Al–Mg alloys. In contrast to Al–Mg alloys,
several yield excursions are observed during initial
indentation of pure Al, which are commonly attributed
to the collective motion of dislocations nucleated
under the indenter. Displacement-controlled indentation
does not result in a qualitative difference between
Al and Al–Mg, which can be explained by the specific
feedback characteristics providing a more sensitive
detection of plastic instabilities and allowing the natural
process of load relaxation to occur. The in situ
indentation measurements confirm grain boundary
motion as an important deformation mechanism in
ultrafine-grained Al when it is subjected to a highly
inhomogeneous stress field as produced by a Berkovich
indenter. It is found that solute Mg effectively pins
high-angle grain boundaries during such deformation.
The mobility of low-angle boundaries is not affected by
the presence of Mg.
