The interface between γ and γ' in Ni-Al alloys by HREM

Ni Base superalloys owe their excellent mechanical properties to the presence of particles of γ' phase (Ni₃Al with an L1₂ structure) in a γ matrix (Ni-Al solid solution with an fcc structure). Besides Al, other elements are used to impart either a higher strengthening or improved corrosion properties at high temperature. The interface between γ and γ' becomes of absolute importance for the resulting mechanical properties and technological application. Especially by considering the consequences that diffusion driven coarsening brings about to the particle distribution either with or without the influence of an applied stress or strain. In this work the interface between γ and γ' phases is characterized by means of measurements on phase images obtained from high resolution transmission electron microscopy images (HRTEM). Phase images represent the sample structure much more accurately than typical HREM experimental images and allow correction of spherical aberration and other residual aberrations. The investigation is performed by using a binary Ni-Al alloy as well as technical Ni base superalloys (MC2 and MCNG). While a sharp interface is developed during stress free coarsening in Ni-Al alloys, a wider volume needs to be considered when alloying elements are introduced. Measurements of lattice spacings on phase images and chemical composition from energy dispersive spectroscopy are used to show the interface characteristics in the alloys under consideration. The interface in the binary Ni-Al alloy can be described by micromechanics as a typical misfitting inclusion. In the technical alloys, the presence of concentration gradients changes the expected lattice strains in a given volume around the particles.