Study of phase decomposition and coarsening of γ′ precipitates in Ni-12 at.% Ti alloy

The early stages of phase decomposition, morphological evolution of precipitates, coarsening kinetics of γ′ precipitates and micro-hardness in Ni-12 at.% Ti alloy are studied by transmission electron microscopy (TEM) and Vickers hardness tests (VHN). Disk-shaped specimens are solution treated at 1473 K (1200°C) and aged at 823, 923 and 1023 K (550, 650 and 750°C) during several periods of time. TEM results show that a conditional spinodal of order occurs at the beginning of the phase decomposition and exhibit the following decomposition sequence and morphological evolution of precipitates: α_sss → γ″ irregular-cuboidal + γ_s → γ′ cuboidal-parallelepiped + γ → η plates + γ. In general during the coarsening of γ′ precipitates, the experimental coarsening kinetics do not fit well to the LSW or TIDC (n = 2.281) theoretical models, however the activation energies determined using the TIDC and LSW theories (262.846 and 283.6075 kJ mol^{- 1}, respectively) are consistent with previously reported values. The highest hardness obtained at 823, 923 and 1023 K (550, 650 and 750°C) is associated with the presence of γ′ precipitates.