Al based ultra-fine eutectic with high room temperature plasticity and elevated temperature strength

Developments of aluminum alloys that can retain strength at and above 250°C present a significant challenge. In this paper we report an ultrafine scale Al-Fe-Ni eutectic alloy with less than 3.5at% transition metals that exhibits room temperature ultimate tensile strength of ~400 MPa with a tensile ductility of 6-8%. The yield stress under compression at 300°C was found to be 150 MPa. We attribute it to the refinement of the microstructure that is achieved by suction casting in copper mold. The characterization using scanning and transmission electron microscopy (SEM and TEM) reveals an unique composite structure that contains the Al-Al₃Ni rod eutectic with spacing of ~90nm enveloped by a lamellar eutectic of Al-Al₉FeNi (~140nm). Observation of subsurface deformation under Vickers indentation using bonded interface technique reveals the presence of extensive shear banding during deformation that is responsible for the origin of ductility. The dislocation configuration in Al-Al₃Ni eutectic colony indicates accommodation of plasticity in α-Al with dislocation accumulation at the α-Al/Al₃Ni interface boundaries. In contrast the dislocation activities in the intermetallic lamellae are limited and contain set of planner dislocations across the plates. We present a detailed analysis of the fracture surface to rationalize the origin of the high strength and ductility in this class of potentially promising cast alloy.