Investigation of microstructure and chemical stability in composites of Ni3Al reinforced by alumina–silica fibers

An investigation was carried out on the composite of Ni3Al reinforced by alumina–silica fibers (Sumitomo fiber). Electron microscopy (TEM, SEM) and microanalysis (XEDS, EELS and electron diffraction) were employed to study the microstructure development and the chemical stability of the components and their interfaces during the diffusion bonding (1200–1300 °C/10–30 MPa/2 h) in vacuum. Two different types of chemical reaction occurred in the fiber during hot pressing. The reaction between the fiber components, nanocrystalline γ-Al2O3 and amorphous SiO2 (intrinsic reaction), generated mullite (Al6Si2O13). No crystallographic texture developed in the newly formed mullite grains. Other reactions (extrinsic reactions) were the formation of nickel oxide particles in the fiber and long shaped α-Al2O3 in the interface. The former occurred by a chemical reaction of the diffusing Ni and free oxygen in the fiber, and the latter was confirmed to be the product of the reaction between free oxygen and Ni3Al in the interface. The required free oxygen for the extrinsic reactions was inferred to stem from the reduction of silica at the hot pressing temperatures.