DEFORMATION AND FRACTURE-BEHAVIOR OF NB IN NB5SI3/NB LAMINATES AND ITS EFFECT ON LAMINATE TOUGHNESS

The mechanical behaviour of the Nb reinforcement (250 mu m thick foils) in Nb5Si3/Nb/Nb5Si3 laminates were investigated under a variety of stress states and test temperatures. Tests were conducted at room temperature (298 K) and 77 K on as-received Nb foils, on Nb5Si3/Nb/Nb5Si3 laminates, on Nb foils extracts from the hot pressed laminates, and on Nb foils simply exposed to vacuum at high temperature. It is shown that Nb grain size, solid solution of Si in Nb, degree of constraint, and test temperature significantly affect the mechanical behavior of Nb. All unconstrained Nb foils failed via ductile rupture at room temperature, while a mixture of cleavage and ductile fracture was exhibited by the constrained Nb present in the laminates. A similar ductile-to-brittle transition in fracture appearance was observed for some foils tested at 77 K, accompanied by a reduction in total energy to fracture. The peak toughness exhibited by the laminates tested at 77 K was only reduced 28% from that exhibited at 298 K, despite this drastic change in fracture mode from ductile to brittle. The effects of composition, interface strength and test conditions on the mechanical behavior of the Nb and its subsequent role on the toughness of the laminates are discussed.