Conspectus
Refining grains of metals and alloys is an effective
approach to
tailor their properties and performance. Plastic deformation is routinely
used for structure refinement, with which microstructures down to
the submicron scale can be accessed in most metals and alloys. However,
further reducing grain sizes below the submicron scale is challenging.
Grain refinement induced by deformation is basically a process of
generating grain boundaries (GBs) through interaction of dislocations
and/or other defects. During plastic straining, however, the opposite
processes may be triggered simultaneously: dislocation annihilation
via their interactions and grain coarsening due to GB migration. These
processes may intensify with decreasing grain sizes. The refinement
may cease at a certain size (so-called “saturation grain size”)
when a balance is reached between the generation and annihilation
processes.
To refine grains further below the saturation size,
one has to
break the balance, either to suppress the GB annihilation or to enhance
the GB generation, or both. In the past decades, several techniques
have been developed by optimizing the processing parameters for promoting
grain refinement. By increasing strain rates, decreasing deformation
temperature, and enlarging strain gradient, grains of pure FCC metals
like Cu, Ni, and Ag can be refined down into a few tens of nanometers.
Within this grain size regime, an important phenomenon was discovered
that structure relaxation of GBs is triggered through interactions
of GB with partial dislocations. The relaxed GBs further stabilize
the nanostructures, yielding an extraordinary stability of the nanograined
metals under thermal and mechanical activations, even beyond that
of their coarse-grained counterparts.
In this Account, we summarize
the development of plastic deformation
induced grain refinement in metals and alloys in recent years, including
the grain refinement mechanisms and advancement of the processing
techniques. Then, GB relaxation in several nanograined FCC metals
below the critical grain size induced by plastic deformation will
be introduced. Changes of GB structures and the grain size effect,
as well as the effect on thermal and mechanical stabilities of the
nanograined metals will be emphasized. In addition, GB relaxation
in nanograined and submicron-grained Cu induced by rapidly heating
will be discussed. The effect of the GB relaxation, induced by either
deformation or rapid heating, on stabilities of the nanograined samples
will be analyzed in comparison with that of their coarse-grained counterparts.
We conclude with a perspective on the future studies on this topic.
On one hand, possibility of even further structure refinement in metals
will be addressed. On the other hand, a novel strategy of materials
development in contrast to traditional alloy is proposed, namely,
material plainification by architecting stable boundaries. Aiming
to promote the sustainability of materials, the “material plainification”
strategy enables specific properties of met...