Brittle fracture and grain boundary chemistry of microalloyed NiAl

The room-temperature tensile properties, fracture mode, and grain boundary chemistry of undoped stoichiometric NiAl, as well as NiAl doped with boron, carbon, and beryllium, have been investigated. Pure, stoichiometric NiAl fractures with limited tensile ductility in a predominantly intergranular manner. Auger analyses revealed that the grain boundaries are clean and free of any segregated impurities, indicating that they are intrinsically brittle. Boron, when added at a bulk level of 300 wt. ppm, segregates to the grain boundaries and suppresses intergranular fracture. There is no improvement in tensile ductility because boron is an extremely potent solid solution strengthener in NiAl, more than doubling its yield strength. As a result, any potential benefit of improving grain boundary strength is more than offset by the increase in yield strength. Unlike boron, both carbon (300 ppm) and beryllium (500 ppm) are ineffective in suppressing intergranular fracture in NiAl. Auger analyses of the C-doped alloy revealed that carbon did not affect the fracture mode because it did not segregate to the grain boundaries. Although neither beryllium nor carbon suppressed grain boundary fracture, their effects on the tensile ductility of NiAl were different: ductility of the Be-doped alloy was higher than that of the B-doped alloy because beryllium has a modest strengthening effect, whereas the C-doped alloy was brittle like the B-doped alloy because carbon is a potent solid solution strengthener just like boron. These observations were rationalized by considering a hard-sphere model for interstitial and substitutional sites in NiAl.