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 gamma' 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 degrees C) and aged at 823,923 and 1023 K (550, 650 and 750 degrees 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: alpha(sss) -> gamma '' irregular cuboidal + gamma(s) -> gamma' cuboidal parallelepiped + gamma -> eta plates + gamma. In general during the coarsening of gamma' 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 degrees C) is associated with the presence of gamma' precipitates. (C) 2013 Elsevier Inc. All rights reserved.