Microstructural characteristics of the directionally-solidified Al-Al₃Ni eutectic and their influence on creep fracture

Abstract

The microstructure and fracture mechanisms of the directionally solidified Al-Al₃Ni eutectic were studied in depth in this investigation. Solidification experiments conducted at 3.5 cm per hr resulted in the formation of a dual-orientation microstructure in which portions of the whiskers were misaligned from the normal growth direction. This structure was found to be extremely detrimental to creep behavior. The modes by which premature specimen failure occurred were identified and the detailed fracture mechanisms were determined. Microstructures of ingots solidified nominally at rates from 3.5 to 35 cm per hr were characterized. The conditions leading to the formation of the dual-orientation microstructure were established and the microstructural variations with solidification rate and with distance along the directionally solidified ingots were observed. It was found that, even with a simple shape, care must be taken in order to produce uniform controlled microstructures and the accompanying desirable properties.