particles, the ingots were remelted and held for 6 h at
750°C after addition of TiBz particles. The particle
number density remained almost the same after a
holding period of 6 h and no change in particle
chemistry was observed (by EPMA), confirming that
TiB2 particles once introduced into the AI-S1 melts
were stable, or at the most had very low solubility.
Grain refinement of hypoeutectic AI-Si alloy by
Al-Ti-B master alloy was simulated by adding
various amounts of silicon and solute Ti to
commercial purity Al melts already containing
synthetic TiB2 crystals. Silicon addition was achieved
by adding synthetic binary Al-14%Si master alloys.
Figures 4(a) and (b) show the microstructure of alloys
containing 0.5 and 3% Si, respectively, and 0.5% Ti.
The behaviour of TiB2 has changed dramatically with
the particles now present within the cc-phase in
association with aluminide crystals. It is evident from
Fig. 4(b) that the aluminide crystals preferentially
nucleate on the TiB, crystals. At 0.5% Ti ( >>Tipentectlc)
the aluminide phase is thermodynamically feasible in
the melt, but its preferential association with TiB,
suggests that it nucleates heterogeneously on TiB,.
Subsequent nucleation of x-aluminium then occurs
on the aluminide.
Electron microprobe analysis revealed that these
aluminides are essentially a ternary compound of