Growth and morphological characterization of Al-Cr-Nb eutectic alloys

Directional solidification of eutectic alloys attracts considerable attention, when in situ composites are concerned. The eutectic alloys are regarded as presenting regular morphology (lamellar and fibrous structures). Besides, when directionally solidified they show high microstructure stability at high temperatures. This work reports a morphological study of an Al-Cr-Nb eutectic alloy. The solidification morphology of the alloys was studied both in the as-cast and in the directionally solidified conditions. The samples were first obtained in an arc furnace and then directionally solidified using Bridgman equipment. During the directional solidification process, the growth rates utilized varied from 5.0 to 30.0 mm/h. Optical (OM) and scanning electron microscopy (SEM) was used in order to determine the influence of the solidification conditions on the microstructure. The results obtained indicated that the eutectic transformation temperature is near 1347.9 °C with formation of Al3(Nb,Cr) and Cr(Al,Nb) phases. Also, it was noted that the Cr(Al,Nb) phase undergoes a polymorphic transformation (∼892.3 °C) forming the Al(Nb)Cr2 compound, followed by eutectoid decomposition Cr(Al,Nb) → Al(Nb)Cr2 + Al8Cr5.