Impurity modification of aluminium-silicon eutectic alloys

Abstract

Interface undercooling and interfibre spacing measurements and observations of the Si phase structure are presented for Al-12.7wt%Si-0.04wt%Sr alloys directionally solidified over the growth velocity range 20–820/μms⁻¹ and with intermediate temperatures gradients in the liquid of 32 and 82 Ccm⁻¹ These measurements complement previous measurements at high and low temperature gradients and provide the first measurements at high growth velocities where chill modification occurs in untreated alloys. Sr modification results in decreased Si spacing and increased undercooling at all growth velocities studied. Multiple twinning was observed in the Si fibres at all growth velocities. These observations are reconciled with a modification mechanism in which Sr absorbs at the Si growth front, poisoning growth steps operative during flake growth and creating lattice instabilities which result in multiple twinning. The multiply twinned Si phase grows with a higher interface undercooling, which decreases the lead distance of the Si phase over the Al phase allowing growth at a more planar interface. The total interface undercooling can be related to the rod eutectic growth curve of the Jackson and Hunt analysis provided an extra undercooling contribution associated with the surface energy of the high twin density of the Si phase is introduced. An increase in the amount of primary Al phase in the modified structure is confirmed and it is shown that the relative amounts of Al primary phase in differently treated alloys can be explained by a competitive growth model using the present undercooling measurements.