Effect of minor Sc and Zr addition on grain refinement of as-cast Al-Zn-Mg-Cu alloys

The optical microscopy and scanning electron microscopy as well as energy dispersive spectroscopy were employed to investigate the influence of joint addition of Sc and Zr on grain refinement of Al-Zn-Mg-Cu alloys. Results show that the addition of 0.20% Sc has a little effect on grain refinement because Sc is mainly dissolved into the matrix and hardly any primary Al3Sc particles are precipitated. The alloy with addition of 0.30% Sc and 0.16% Zr has more equiaxed grains than that of others,giving cast grain sizes as fine as 13 μm. This is because the Sc substitutes for Al atom in the Al-Zr crystal cell and forms Al-Sc-Zr unit cell,which grows and becomes Al3(Scx,Zr1-x) particle,acting as a nucleus for the formation of α-Al. The addition of 0.04% Ti and 0.008% B makes the grain size drop from 250 μm to 50 μm. Its refinement effect is less than the 13 μm achieved by the alloy including 0.30% Sc and 0.16% Zr.