The use of convergent-beam electron diffraction to determine local lattice distortions in nickel-base superalloys

Abstract

Convergent-beam electron diffraction has been used to study local lattice distortions of γ′ (the ordered intermetallic compound based on Ni₃Al) in nickel-base alloys. Specifically, the patterns of higher-order Laue zone lines exhibited in central discs of convergent-beam patterns recorded with the beam accurately aligned parallel to given zone axes have been used to detect small changes in the lattice parameters of γ′ in two nickel-base superalloys, one the ternary eutectic Ni-12·8Al-22·2Mo alloy (where the composition is in atomic per cent), the other Udimet 720 in the commercially heat-treated or partially recrystallized condition. The various results are interpreted on the basis of the observed distortions being caused by stresses which may be introduced by effects such as differing thermal expansion or contraction coefficients of the component phases and lattice mismatch. The effect of relaxations of these stresses by the proximity of the foil surfaces has been demonstrated. It is concluded that extreme caution should be exercised when this technique is used to make determinations of quantities such as lattice mismatch and coherency strains.