Microstructure and crystallographic orientation relationship in directionally solidified Mg-Mg sub 17 Al sub 12 -eutectic

The microstructure and crystallographic orientation relationship of the Mg- and the Mg sub 17 Al sub 12 -phase in a directionally solidified Mg-Al-eutectic have been study in the light microscope and in the scanning electron microscope equipped with an electron back scattering facility. The effects of the addition of small amounts of Sr on the microstructure were also studied. The Mg-Al-eutectic solidified with a lamellar morphology where the relationship lambda exp 2 V=constant was obeyed for growth rates V < 1.8x10 exp -3 cm/s and lamellar spacings lambda > =2.5 mu m. The crystallographic orientation relationship is: (1101) Mg(parallel to)(101) Mg sub 17 Al sub 2 , [1120] Mg(parallel to)[111] Mg sub 17 Al sub 12 . The interface plane was found to lie approx10 deg from the (101) plane. The two phases of the eutectic grew with the angular discrepancy ( theta sub ND ) to the [1120]- and the [110]-directions. The angle theta sub ND was found to decrease with increasing growth rate. The microstructure and crystallographic orientation relationship seemed unaffected by Sr addition.