An atom probe study of grain boundary and matrix chemistry in microalloyed NiAl

Abstract

The atom probe field ion microscope (APFIM) has been used to perform a quantitative study of the grain boundary and matrix chemistry in NiAl microalloyed with either boron, carbon or beryllium. Boron was observed to segregate to grain boundaries whereas carbon and beryllium did not. Atom probe measurements of the matrix chemistry revealed severe depletion of the solute in the boron- and carbon-doped alloys. Field ion imaging, atom probe analyses and transmission electron microscopy (TEM) of the matrix revealed ultrafine MB 2- and MC-type precipitates ranging in size between 2 and 20 nm in diameter in the boron- and carbon-doped NiAl. The number densities of these precipitates were estimated by atom probe analyses to range between 1 × 10 20 m −3 and 1 × 10 23 m −3 for precipitates of 20 and 2 nm diameter, respectively. Atom probe analyses of the matrix in beryllium-doped NiAl revealed that almost all the beryllium was in solid solution. This was consistent with the fact that ultrafine beryllide precipitates were not detected in the atom probe. The enormous increase in yield stress in the boron- and carbon-doped alloys is predominantly due to a precipitation hardening effect. The relatively small increase in yield stress in beryllium-doped NiAl is due to a weak substitutional solid solution hardening effect.