This is commonly added to FeSiMg a nodulariser used in manufacturing ductile iron. It improves the shape of graphite nodules and negates the effect of tramp element and sulphur.
In steel castings we add it to obtain finer grain size ,hence better mechanical properties.

Cerium mischmetal is also used as an alloying addition for magnesium alloys.  Magnesium forms solid solutions with a number of rare earth elements, so that at fairly low alloying levels (< 5 %), the resulting alloys are simple eutectics (low-melting point, good for casting).

The rare earth elements also form intermetallic phases with Mg and Al, which serves to pin grain boundaries, and improve the high temperature creep properties (relatively speaking-- on the order of 150-250 C).