Anomalous hardening and microstructural evolution accompanied by reordering and restoring of plastically deformed Co₃Ti

The anomalous hardening and microstructural evolution accompanied by the reordering and restoring of cold-rolled Co₃Ti alloy annealed at 573-773K were investigated by hardness testing, SEM and TEM observations, XRD analysis and DSC measurements. The hardening observed in the heavily cold-rolled samples was similar to that observed in precipitation reactions, with the hardness increasing with increasing annealing time to a point and then decreasing. The hardness peak occurred more rapidly at higher annealing temperatures, and the annealing time leading to this peak was insensitive to the degree of cold-rolling reduction. The stored lattice strain evaluated from the half-width of the X-ray reflection peak steadily decreased with increasing annealing time but remained relatively high regardless of the degree of cold-rolling reduction or annealing temperature. The TEM deformation microstructures showed that a high density of dislocations was attained, even in an over-aged state. DSC measurements showed two exothermic peaks at low and high temperatures, which were attributed to the annihilation of point defects and/or reordering and to the rearrangement of dislocations, respectively. The observed anomalous hardening was discussed in association with the reordering of the mechanically disordered structure and the restoring of the plastically deformed microstructure.