Coarsening behaviours of coherent γ′ and γ precipitates in elastically constrained Ni–Al–Ti alloys

The coarsening behaviours of γ′ and γ precipitates in elastically constrained Ni–Al–Ti alloys were investigated by means of transmission electron microscopy. When the Ni–8 at.% Al–6 at.% Ti alloy is aged at 1023 K, coherent γ′ particles having L12 structure appear and coarsen in the γ matrix having disordered A1 structure. At first the mean particle size 〈r〉 increases in proportion to the cube root of ageing time t (〈r〉 ∝ t1/3), and then the coarsening remarkably decelerates. The shape of γ′ precipitate changes from the sphere to the cube as the coarsening progresses. When the Ni–13 at.% Al–9 at.% Ti alloy is aged at 973 K, coherent γ particles appear and coarsen in the γ′ matrix. At first the relation of 〈r〉 ∝ t1/3 holds good, and then the coarsening accelerates, so that the 〈r〉 increases in proportion to the square root of t (〈r〉 ∝ t1/2). The shape of γ precipitate changes to the plate having {1 0 0} planes as the coarsening progresses. Such coarsening behaviours of γ′ and γ precipitates are good examples of the elasticity effects in elastically constrained systems.