Effect of Cr and Mn on the microstructure of spray-formed Al–25Si–5Fe–3Cu alloy

► Cr is more effective than Mn in changing rod-like β phase to particulate α-phase ► The modification effect of Mn plus Cr on β phase is superior to that of Mn or Cr. ► Mn or Cr can greatly change the onset temperatures of Si, δ/β phase and α phase. The hypereutectic Al–Si–Fe alloys with different Mn and Cr addition were synthesized by spray forming technique, and the microstructural evolutions induced by Mn and Cr have been investigated using field emission scanning electron microscopy (FE-SEM), X-ray diffraction (XRD), transmission electron microscopy (TEM) and differential scanning calorimetry (DSC). The results show that the modification effect of co-addition of Mn and Cr is more effective than the individual Mn or Cr addition in transforming harmful rod-like Fe-bearing β phase to particulate α-phase. Meanwhile, the modification effect of Cr is superior to that induced by Mn. The Mn/Cr ratio plays an important role in the combined addition. The addition of 2 wt% Mn + 1 wt% Cr to spray formed alloy has induced an ideal microstructure with entirely fine granular intermetallic compounds dispersed in Al matrix. DSC analyses indicate that both Mn and Cr can greatly influence the onset temperatures and the region widths of primary Si, δ phase and α phase, but bring little changes on multi-phase reaction, which would contribute to the modification effects of Mn, Cr and Mn + Cr.