| It has become clear that, for a given undercooling DT0, various combinations of
interlamellar spacing l and growth velocity V are possible. However, there is a particular
spacing which gives a maximum velocity. This can be found by evaluating dV/dl and setting it
equal to zero. |
|
| When a spacing is adopted which gives this maximum velocity, growth is
said to be occurring under extremum conditions. It can be seen that the extremun spacing
is given by le = 2lmin |
| This is also the spacing of which, for a fixed
velocity, growth would be taking place with the minimum interfacial undercooling. |
|
|
| In practice, most eutectic systems tend to solidify at the extremum.
Substitution of the above expression for l into the equation
relating V to l
now gives |
|
| which, using the relationship between lmin and DT0 can be written as |
|
| or, expressed in terms of the expected spacing l
(=le) |
|
| The RHS of this equation is a constant for a given system. It
typically has a value of about 10-16 m3 s-1. Clearly
faster growth leads to finer lamellar spacings. |
