Structural response to shape variations in the AI-AI₃Ni eutectic composite

Abstract

Structural irregularities that arise due to shape variations in Al-Al₃Ni directionally solidified eutectic composites have been analyzed. The shape variations studied are axisymmetric increases in the cross-sectional area of cylindrical rods, and divergence. The results show that larger angles of divergence cause more fiber misorientation, increase the curvature of the solid-liquid interface, and result in more pronounced changes of the local growth rate in the divergence area. The disturbances were shown to be accompanied by changes in the number of fibers in a growing composite. Extra fibers should be generated when the cross sectional area increases, the solid-liquid interface curvature increases, or the growth rate is accelerated. The extra fibers were found to be mostly supplied by fiber branching mechanism rather than by individual fiber nucleation which was not observed frequently. In cases of new areas of growth, fiber branching is too slow to supply the required extra fibers. Under these conditions Al₃Ni phase nucleates in the lamellar morphology and then changes to fibers by repeated branching. In the present work Al₃Ni lamellae were nucleated at the beginning of directional solidification, areas above oxide inclusions and secondary arms of aluminum dendrites, and at the shoulders of large angles of divergence.