Structure, strength and fracture resistance of interfaces in NiAl/Mo model laminates

Intermetallic matrix composites with ductile phase reinforcement are being considered for high-temperature structural applications. Increases in room temperature toughness over that of monolithic NiAl have been achieved in directionally-solidified NiAl/Mo eutectic composites. Since the nature of interface fracture is critical to understanding toughening effects in these composites, NiAl/Mo model laminates were bonded at high temperature to create interfaces for structural examination and mechanical testing. In the slightly off-stoichiometric NiAl, a reaction layer identified as Mo3Al forms at the interface and varies in thickness with bonding conditions. Microscopic features on either side of the interface fracture surface show excellent matching and some evidence of plastic deformation on the Mo side of the interface. Mechanical response of the interfaces was studied under various loading conditions using several methods, including a novel technique in which one of the adjoining phases was loaded in compression, producing a stable interface crack. The NiAl/Mo interface was also fractured in three- and four-point bending. Interface fracture resistances were determined using finite element analysis to model the experimental tests. A small amount of plasticity was found to play a large role in crack initiation.