Evolution of the microstructure during annealing of ultrafine-grained Ni with different Mo contents

Research was undertaken to evaluate the thermal stability of ultrafine-grained (UFG) Ni alloys with two different molybdenum (Mo) contents. The UFG microstructures in Ni samples with low (~ 0.28 at.%) and high (~ 5.04 at.%) Mo concentrations were achieved by the consecutive application of cryorolling and high-pressure torsion (HPT). The evolution of the microstructure during annealing up to ~ 1000 K was studied by differential scanning calorimetry (DSC) at the peripheral regions of the disks processed by 20 turns of HPT. The grain size and the fraction of low-angle grain boundaries (LAGBs) were monitored by electron backscatter diffraction while the dislocation density was determined by X-ray line profile analysis as a function of temperature. It was found that the recovery of the UFG microstructures started at ~ 400 K irrespective of the Mo content but recrystallization occurred at a much higher temperature for the Ni alloy with higher Mo content. During recovery, the LAGB fraction increased due to the arrangement of dislocations into low energy configurations. In the recrystallization process, the fraction of LAGBs decreased. After annealing up to ~ 1000 K, the grain size remained much smaller for the sample with higher Mo concentration. Moreover, the larger Mo content yielded a separation of recovery and recrystallization processes in the DSC thermogram. It is concluded that the higher Mo concentration has a more pronounced hindering effect on recrystallization than on recovery.