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 influence of processing rate and method on the microstructure was determined. For a given composition and processing method, the processing rate was varied. The influence of processing rate as a function of composition was studied. After directional processing, the microstructure of the eutectic alloys was composed of a rod Fe(Al,Ta) and Fe2Ta(Al) composite with no evidence of a dendritic structure. The composite spacing followed the lambda(2)R relationship. The processed Ta-rich alloys were found to have finer microstructures at the higher velocities; All Ta-rich alloys, except one, were composed of a mixture of primary Laves dendrites and eutectic. One off-eutectic alloy was completely composed 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 more uniform, better aligned, and finer when processed in the Bridgman unit. A periodic defect was observed in all levitation processed alloys. This defect was not observed in the Bridgman processed alloys. The results of this work showed for this alloy system, the Bridgman unit produced superior microstructures when compared to those produced during levitation processing. (C) 1997 Elsevier Science Ltd.