Mechanical properties, fracture behavior, and grain-boundary chemistry of B-doped NiAl.

摘要

This paper summarizes the results of our work aimed at overcoming the intrinsic grain-boundary weakness of NiAl by microalloying with boron. In previous work we have shown that 300 wppm boron is very effective in suppressing intergranular fracture in NiAl. It does this by segregating strongly to the grain boundaries and strengthening them. Despite this dramatic effect on the fracture mode, however, boron is unable to improve ductility because it is a potent solid solution strengthener, more than doubling the yield strength relative to that of undoped NiAl. The present work attempts to decrease this deleterious hardening effect by lowering the bulk concentration of boron in NiAl. Our results show that if the boron concentration in the bulk is lowered to 30 wppm, the yield strength of boron-doped NiAl is only about 30% higher than that if undoped NiAl. In addition, there is enough boron at the grain boundaries of this alloy to suppress intergranular fracture. Under these conditions, boron-doped NiAl has a tensile ductility of 2%, which is essentially identical to that of undoped NiAl. This result, namely that the strengthening of grain boundaries might well be the weakest links in NiAl, cleavage planes are not much stronger. In other words, even though boron additions serve to strengthen the grain boundaries and suppress intergranular fracture, ductility is note improved, because the next brittle fracture mode, namely transgranular cleavage, takes over before significant plastic deformation can occur. 18 refs., 2 figs., 2 tabs.