Transmission Electron Microscopy Studies of Grain Boundary Precipitation in Spinodally Decomposed Cu-Ni-Fe Alloys
In many alloy systems the grain boundaries play an important role in controlling properties but little is known about the mechanism of grain boundary precipitation reactions. Transmission electron microscopy and diffraction are necessary to distinguish reactions such as discontinuous precipitation and preferential coarsening. The present paper describes current progress on the Cu-Ni-Fe system which is known to be embrittled at the grain boundaries. Two alloys are considered, having their compositions on a binary section through the Cu-Ni-Fe ternary with terminal values at the Cu corner and the point Ni0.7Fe0.3. Alloy A(51.5 at % Cu-33.5 at % Ni-15 at % Fe) lies very near to the center of the miscibility gap. Phase identification in the electron micrographs is facilitated by comparison with alloy B (69.3 at % Cu-19.4 at X Ni-11.1 at % Fe) of asymmetrical composition. The microstructure shown in Fig. 1 results from aging alloy B within the spinodal (650°C) for 10 hrs.