UNCONSTRAINED AND CONSTRAINED TENSILE FLOW AND FRACTURE-BEHAVIOR OF AN NB-1.24 AT PCT SI ALLOY

The unnotched and notched tensile behavior of the beta-phase constituent (Nb with Si in solid solution) of the (Nb)/Nb5Si3 composite has been investigated at room temperature and -196 degrees C. At room temperature, the unnotched tensile behavior comprises significant strengthening due to Si, low strain-rate sensitivity, low strain hardening, extensive ductility, and ductile microvoid coalescence fracture, even at strain rates as high as 1.1 s(-1). At -196 degrees C, the unnotched alloy exhibited much higher strength, good ductility, and cleavage fracture. At room temperature, the notched specimens exhibited cleavagelike fracture with significant plasticity, and at -196 degrees C, they exhibited cleavagelike fracture with much lower plasticity at the notch. A finite-element analysis (FEA) of stress and strain fields in the vicinity of the notch root, together with unnotched tensile behavior, indicates that plasticity plays an important role in nucleating cracks, while the high-axial tensile stress component governs crack propagation. These results are used to rationalize the observed toughening and fracture behavior of a (Nb)/Nb5Si3 composite.