Interface shear properties and toughness of NiAl/Mo laminates

Abstract

Mechanical properties at the interface between NiAl intermetallic and Mo ductile phase in sandwich laminates have been studied under a variety of stress states involving primarily shear. A shear/compression test for laminates of NiAl/Mo/NiAl has been used in which a double-notched specimen with cut-through metal interlayer is loaded to failure in compression. Because of the large elastic mismatch between NiAl and Mo, a primarily shear loading generates a significant amount of normal stress in this test. Interface shear response and toughness obtained using this technique are compared for NiAl/Mo laminates with interfacial segregant, Cr, Re and C layers fabricated by high temperature diffusion bonding. A microstructural investigation of the interfacial reaction zone and the shear failure path is conducted to identify failure mechanisms. Failure strength and related interfacial toughness is examined using an FEM model of local stress state close to the notch in the bimaterial shear/compression specimen geometry. A trend of increasing bond strength with reducing notch spacing is identified and analyzed in terms of local stress state, the onset of microyielding and crack shielding. Thus, a new fracture stress locus has been developed for bimaterial interfaces. Residual stress due to CTE thermal expansion mismatch is measured and the contribution of thermal residual stress to the failure behavior of the laminate is explored. The combined effect of stress concentration at crack tip, residual stress and slip discontinuity across interface have been used to explain the apparently brittle loading response and easy debonding of Mo fibers in eutectic NiAl/Mo alloys.