Abstract
Optimum alloy compositions and heat treatments to obtain fine dispersion of disordered γ particles in an L12 ordered γ′-Ni3(Al, Ti), and the morphology change of γ precipitates during aging have been investigated by means of transmission electron microscopy.
A uniform solid solution of γ′-L12 ordered phase supersaturated with Ni can be obtained by solution annealing in a suitable temperature range. The γ′ phase hardens appreciably by the fine precipitation of disordered γ. The shape of γ precipitates formed during aging is initially spherical or round-cubic and grow into platelets as aging proceeds. High resolution electron microscopy revealed that the γ precipitates are perfectly coherent with the matrix γ′ as long as the γ-precipitates are plates. The loss of coherency initiates by the introduction of dislocations at the γ⁄γ′ interface followed by the step formation at the dislocations. The γ precipitates become grobular after the loss of coherency.
The chemical compositions of the matrix and the precipitates have been determined by means of EDS microanalysis in an analytical electron microscope. The sense and magnitude of misfit between the γ′ matrix and γ precipitates are determined from the spacings of misfit dislocations and the chemical compositions of γ and γ′ phases.
