Microstructure control and ductility improvement of La-Al-(Cu, Ni) composites by Bridgman solidification

Microstructure evolutions were experimentally studied as a function of growth velocity and alloy composition in La_xAl₁₄(Cu, Ni)_{86 - x} alloys (x = 57-74). The composition-velocity ranges were determined for the formation of eutectic, amorphous, eutectic + compound, amorphous + dendrite, and eutectic + dendrite. A skewed eutectic coupled zone was found towards the compound phase. A composite structure composed of ductile La dendrites in a matrix of either eutectic or amorphous phase can be produced in hypoeutectic alloys. Compressive tests were performed on amorphous/dendrite and eutectic/dendrite composites. As the volume fraction of dendrites increases, fracture strength linearly decreases and obeys a rule-of-mixtures relationship, whereas plastic strain exhibits an exponential increase. Optimised mechanical properties can be realized through both alloy composition design and well-controlled solidification processing.