Mechanical properties and microstructures after abnormal grain growth in electrodeposited Ni-W alloys

The thermal stability of electrodeposited Ni-W alloys has been identified to be due to W segregation. Although a kinetic contribution has been recently proposed, namely, the formation of W-oxides at grain boundaries (GBs) by annealing, the formation mechanism is unclear. In this study, we annealed electrodeposited Ni-W alloys up to 600 degrees C; then, we examined the mechanical properties and microstructures to deeply understand the thermal stability of Ni-W alloys. Mechanical testing of annealed alloys demonstrated that the proof stress and hardness increased against the increase in annealing temperature in the range of 380-450 degrees C. At the mesoscale, microstructure observations revealed that the grain size was reduced with increasing temperature from 390 to 450 degrees C. Finer-grained microstructures were maintained up to the annealing temperature of 520 degrees C. At the nanoscale, grain boundary structures were directly observed, and the chemistry was also demonstrated. These nanoscale analyses detected S segregation at GBs and triple junctions, whereas oxides and precipitates likely to affect grain growth were not identified. Our results and discussion indicate that transition in the grain size after annealing can be explained by the conventional relationship between the annealing temperature and frequency of abnormal growth grains.