Microstructure and crystallographic orientation relationship in directionally solidified Mg–Mg17Al12-eutectic

The microstructure and crystallographic orientation relationship of the Mg- and the Mg17Al12-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 λ2V=constant was obeyed for growth rates V<1.8×10−3 cm/s and lamellar spacings λ≥2.5 μm. The crystallographic orientation relationship is: (11̄01) Mg‖(101̄) Mg17Al12, [112̄0] Mg‖[111] Mg17Al12. The interface plane was found to lie ∼10° from the (101̄)plane. The two phases of the eutectic grew with the angular discrepancy (θND) to the [112̄0]- and the [110]-directions. The angle θND was found to decrease with increasing growth rate. The microstructure and crystallographic orientation relationship seemed unaffected by Sr addition.