Elasticity Effects on the Microstructure Changes in Ordered Alloys Strengthened by Precipitation

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Microstructure changes in Ni-base γ and TiAl intermetallics were investigated with transmission electron microscopy and theoretical calculations. Main results obtained are as follows :1) Aγ phase of a Ni-Al-Ti alloy sometimes decomposes into a two-phase state of γ+γ, and spherical particles of disordered γ appear in the γ matrix. In the course of coarsening, γ particles clcange their shapes from spheres to plates parallel to {100}. Such a change in the equilibrium shape is a result of the elastic constraint and is quantitatively justified by theoretical calculations based on a microelasticity theory.2) The shape changes from spherical particles to plates and/or rods increase the probability that a particle contacts with the neighbouring particles, which results in the accelerated coarsening of γ particles.3) An α-Ti phase of a Ti-Al alloy sometimes decomposes into a two-phase state of Ti_3A1+α-Ti, and spherical ordered Ti_3Al particles appear in the disordered α-Ti matrix. In the course of coarsening, Ti_3Al particles change their shapes from spheres to rods, which is quantitatively justified by energy calculations based on a micrelasticity theory.4) An Al-rich TiAl intermetallic phase sometimes decomposes into a two-phase state of Ti_3Al_5+TiAl, and ordered Ti_3Al_5 particles appear in the ordered TiAl. At the early stage of decomposition, the two-phase structure sometimes exhibits the so-called tweed structure. At the later stage of decomposition, Ti_33Al_5 morphology exhibits a kind of rafted structure. The computer simulations based on a phase-field method successfully reproduce the morphological changes of Ti_3Al_5 in the TiAl matrix.