ADDRESSING THE CHALLENGE OF CHARACTERISATION ACROSS THE LENGTH SCALES
One of the biggest issues facing the microscopy of
materials is the length scale issue. Components that are
used in every day life are typically on the length scale of
millimetres to several meters. For example, the
aluminium alloy wing skin on a modern passenger
aircraft is tens of meters long. The high strength steel
bars used for impact resistance to protect passengers in
cars are typically around a meter in size. There are, of
course, many examples of finer scale artefacts of huge
technological importance, including among many,
computer chips and catalysts. Irrespective of which
artefact is considered, the over-riding conclusion is that
the properties are determined by the microstructure; the
microstructure that is important is at the atomic or near
atomic dimensions. Taking the example of the
aluminium wing skin, the strength is derived from
strengthening precipitates typically 10-100nm in size.
The exact strength obtained is a direct function of the
interface between the precipitates and the matrix- i.e.
the local atomic structure. So, there is clearly a problem:
to understand the properties of the component (say 10
meters long) we need to understand the atomic scale
microstructure (10-10 m), i.e. 100 billion times
difference in length scale! Is what we examine at 10-10
m in any way representative of all areas of the
component? In the absence of a technique that will
really allow an answer to this problem, microscopists
have had to ?do the best you can?. We remain some
way off a complete solution (although techniques such
as synchrotron radiation and neutron diffraction help).