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The evolution of / ' microstructure in elastically constrained .%Ti alloy was investigated by means of transmission electron microscopy and the energy calculations based on a microelasticity theory. When the homogenized Ni-AlTi alloy is aged at 1213 K, cuboidal ' precipitate particles appear throughout the matrix. When the / ' two-phase microstructure is aged at 1023 K or 973 K, the phase separation takes place in each ' precipitate and a number of particles newly appear in the cuboidal ' precipitate. In the course of further ageing at 1023 K or 973 K, the particles coarsen and become plates with surfaces parallel to {100}, then some plates extend along one of {100} to become flatter, and finally reaches the matrix surrounding the ' precipitate. Not only the elastic energies but also the chemical free energy plays an essential role in this type of split as the driving force. The split takes place throughout the microstructure although the volume fraction is high, which results in the occurrence of the decelerated coarsening of / ' two-phase microstructure in an elastically constrained Ni-Al-Ti alloy.

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