Most eutectic systems grow with uniform lamellar spacings which correspond to extremum
conditions. However, some eutectics do not do this but show a wide range of spacings
within each specimen and an average spacing which is considerably higher than that
corresponding to extremum growth. This is due to at least one of the two phases having
preferred crystallographic growth directions (and tending to exhibit facets
). Such systems cannot readily
adjust their interlamellar spacings in response to changing growth conditions. Observed
spacings tend to vary locally, typically between a lower limit close to the extremum value
and an upper limit of lb
at which the facetted phase branches - usually via an offshoot in an equivalent
crystallographic direction.
| A well-known example of an anomalous eutectic
is Al-Si. Such systems can often be modified by melt additions, so as to promote
solidification like a regular eutectic. Such additions usually poison the preferred atomic
addition sites on the facetted phase, so that growth occurs more readily in different
directions. |
