Study on the Precipitation Strengthening in Ni-Al-Cr Alloy System

Transmission electron microscopic investigation has been performed concerning the effect of precipitation on the mechanical properties in Ni3Al-and NiAl-based intermetallics containing various types of fine precipitates. In an L1(2)-ordered Ni3Al alloy with 4 mol. % of chromium and 0.2-3.0 mol % of carbon, fine octahedral precipitates of M23C6 type carbide, which has the cube-cube orientation relationship with the matrix, appear during aging. Typical Orowan loops are formed in Ni3Al containing fine dispersions of M23C6 particles. On the other hand, B2-ordered NiAl hardens appreciably by the precipitation of coherent alpha-Cr particles. By aging at temperatures around 973 K after solution annealing, fine spherical precipitates appeared homogeneously in the NiAl matrix. The selected area electron diffraction patterns have not revealed any additional extra-spots during aging, because the alpha-Cr particles have cube-cube orientation relationship and keep perfect coherency with the ordered matrix lattice. Dislocations bypass the particles during deformation. Although the dispersion of alpha-Cr particles increases the yield strength of NiAl at intermediate temperature, the strength decreases appreciably higher temperatures.