MICROSTRUCTURE OF DIRECTIONALLY SOLIDIFIED EUTECTIC BASED FE(AL,TA) FE2TA(AL) ALLOYS AS A FUNCTION OF PROCESSING CONDITIONS/

Three compositions based on the eutectic between the Fe (Al,Ta) phase and the Fe2Ta(Al) phase - the eutectic and two hypereutectic alloys - were directionally processed in levitation and Bridgman units. The inf luence of processing rate and method on the microstructure was determi ned. For a given composition and processing method, the processing rat e was varied. The influence of processing rate as a function of compos ition was studied. After directional processing, the microstructure of the eutectic alloys was composed of a rod Fe(Al,Ta) and Fe2Ta(Al) com posite with no evidence of a dendritic structure. The composite spacin g followed the lambda(2)R relationship. The processed Ta-rich alloys w ere found to have finer microstructures at the higher velocities; All Ta-rich alloys, except one, were composed of a mixture of primary Lave s dendrites and eutectic. One off-eutectic alloy was completely compos ed of a eutectic structure grown under planar interface conditions, (i .e. a coupled microstructure). This alloy was processed at 30 cm/h in the Bridgman unit. In general, the microstructures were found to be mo re uniform, better aligned, and finer when processed in the Bridgman u nit. A periodic defect was observed in all levitation processed alloys . This defect was not observed in the Bridgman processed alloys. The r esults of this work showed for this alloy system, the Bridgman unit pr oduced superior microstructures when compared to those produced during levitation processing. (C) 1997 Elsevier Science Ltd.