On the measurement of lattice mismatch between γ and γ′ phases in nickel-base superalloys by CBED technique

Abstract

Nickel base superalloys are strengthened by ordered {gamma}{prime} precipitates of L1{sub 2} structure, which are coherently embedded in the {gamma} matrix (fcc). The mismatch in lattice parameter ({delta}) between the {gamma} and the {gamma}{prime} phases influences the mechanical properties of these alloys. The value and the sign of {delta} also control the morphological changes in {gamma}{prime} (rafting), which occur during deformation at high temperatures and at low strain rates. Recently, Convergent Beam Electron Diffraction (CBED) has emerged as a powerful method for lattice parameter measurements and has been used to measure lattice constants in cubic systems including superalloys. In the CBED technique, the lattice constants are determined by analyzing the High Order Laue Zone (HOLZ) line patterns present in the transmitted beam of the CBED pattern. The symmetry and position of the HOLZ lines are very sensitively dependent on the lattice constants, the orientation and the thickness of the specimen as well as on the microscope operating conditions. By comparing the experimental HOLZ line patterns with computer simulated patterns from a known crystal, one can determine the lattice constants very accurately (comparable to X-ray measurements).