Precipitation and grain boundary segregation in molybdenum-doped NiAl

An atom probe field ion microscopy characterization of the precipitate and grain boundary composition of an NiAl alloy doped with 0.7 at% Mo has been performed. The microstructure of this alloy consisted of a low number density (∼ 1 to 2 × 1021 m−3) of 10 to 50 nm diameter spherical Mo-(3.3 ± 0.6)at%Al precipitates embedded in the NiAl matrix. These precipitates generate an increase in yield stress of between 82 and 210 MPa which is in good agreement with the differences in the yield stresses of the molybdenum-doped (254 MPa) and undoped alloys (154 MPa). Atom probe analysis of the B2-ordered matrix revealed extremely limited solute content of Mo and other impurities: Ni-(50.4 ± 1.2)%Al-(0.005 ± 0.001)%Mo, 0.007 ± 0.002% Fe, 0.005 ± 0.001% Si and N and 00.0004 ± 0.0004% V. A detailed examination (> 115000 total ions) yielded grain boundary concentrations of 0.7% Mo, 0.4% Si and N, 0.09% C, 0.04% B, and 0.02% Fe. Despite these relatively low values, the molybdenum, silicon and nitrogen, boron, and iron levels represent enrichments of ∼ 140, ∼ 80, ∼ 85 and ∼ 3 times the matrix level, respectively. The carbon level at the boundary was also significantly enriched.