Microstructural evolution during annealing of the melt-spun ternary hypoeutectic Al-7.6 Si-3.3 Fe (in wt.%) alloy

A ternary hypoeutectic Al-7.6 wt.%Si-3.3 wt.%Fe alloy was rapidly quenched at cooling rates between 10⁶ and 10⁷ K/s using the melt-spinning technique. Differential scanning calorimetry (DSC) scan of the as-quenched ribbon revealed a small exothermic peak at 348 °C, attributed to the silicon particle coarsening and two endothermic peaks: one at 585 °C and the other at 600 °C corresponding to ternary eutectic and liquidus transformation, respectively. On the basis of the DSC analyses, the as-quenched ribbons were annealed at T=390, 560, 600 and 632 °C. The annealed ribbons were characterized using X-ray diffraction (XRD) and transmission electron microscopy (TEM) techniques. From the XRD scans, a solid solubility extension value of 1.75 at.% Si in α-Al was measured for the as-quenched ribbon and the β-Al₅FeSi intermetallic phase start temperature was determined as 560°C. TEM investigations revealed the coarsening of Si particles in the α-Al matrix and the increase of the β-Al₅FeSi intermetallic phase particle size from 80 to 100 nm at the annealing temperature of 560 °C to about 700 nm at the annealing temperature of 632 °C. The microhardness value of the as-quenched hypoeutectic alloy was measured as 146 kg/mm² which is 2.5 times more than that of the conventionally cast counterpart. The microhardness values decreased gradually with increase in annealing temperatures.