An experimental programme has been undertaken to determine which of the gra
in formation mechanisms of equiaxed crystals are dominant in the solidifica
tion of Al-Si foundry alloys. Small ingots were cast from alloys of varying
silicon concentration with and without gauze barriers, using different typ
es of mould materials and different mould preheats. The results show that t
wo mechanisms of grain nucleation are operating. The first is a wall mechan
ism where crystals are nucleated either on or near the mould wall owing to
thermal undercooling. The second is a constitutional supercooling mechanism
where nucleants are activated in the constitutionally undercooled zone ahe
ad of the advancing interface. As a consequence, the grain size decreases w
ith increasing silicon content. However a transition in the growth mode occ
urs once a critical degree of constitutional undercooling is exceeded. This
change in growth is accompanied by an increase in grain size. The transiti
on point can be shifted with respect to solute content by changing the cast
ing conditions, and a mechanism is proposed to explain this effect. MST/410
9