CHEMICAL-COMPOSITION MEASUREMENTS OF A NICKEL-BASE SUPERALLOY BY ATOM-PROBE FIELD-ION MICROSCOPY

Single crystals of the rhenium-containing nickel-base superalloy CMSX- 4 were investigated using a time of flight atom probe. The distributio n of the alloying elements in the two phases, matrix (gamma phase) and precipitate (gamma' phase), was studied. The high spatial resolution of the atom probe allows to analyse the gamma-gamma' interface. The tr ansition from matrix to gamma' phase takes place within one atomic lay er. The boundary is enriched with titanium. Layer-by-layer analysis of (001) planes of the L1(2) ordered gamma' phase reveals alternating Ni - and Al-rich planes. The probability of site occupation by different elements could be evaluated. An approximate formula of the gamma' phas e is given by (Ni-81 Ti-3 Ta-2 Cr-5 Co-7 W-2)(3) (Ni-20 Al-70 Ta-6 Co- 2 W-2). Rhenium is found mainly in the matrix and has a tendency to bu ild clusters.