Characterization of Doped NiAl by Atom Probe Field Ion Microscopy

Abstract

The atom probe field ion microscope (APFIM) was used to characterize grain boundaries and matrix in NiAl doped with either boron, carbon or beryllium. Boron was observed to segregate to grain boundaries whereas carbon and beryllium did not. Atom probe analyses of the matrix revealed that the matrix was severely depleted of the solute in the boron- and carbon-doped alloys. Field ion imaging and matrix analyses also revealed ultrafine MB[sub 2] - and MC-type precipitates ranging in size between 2 run and 20 mm in diameter in the boron- and carbon-doped alloys. These precipitates occurred in significant number densities. Atom probe analyses of beryllium-doped NiAl did not reveal ultrafine precipitates and was consistent with the fact that almost all the beryllium was in solid solution. The enormous increase in yield stress in the boron- and carbon-doped alloys is predominantly due to a precipitation hardening effect. The small increase in yield stress in beryllium-doped NiAl is due to a mild substitutional solid solution hardening effect.