| Electrodeposited nanocrystalline materials offer a unique opportunity to study the microstructural evolution in systems having very high driving forces for grain growth. In addition, these materials are not complicated by the residual porosity, untransformed amorphous regions, or non-equilibrium grain boundary structures that may be inherent to nanostructured materials produced by other synthesis methods. There have been numerous studies of grain growth in electrodeposited nanocrystalline materials. While there is general agreement between calorimetric observations, there is little agreement in terms of transformation mechanisms. One of the principle objectives of this study was to unify previous observations of grain growth in nanostructured electrodeposits. This was achieved by an extensive study of the structural evolution in nanostructured nickel during isothermal annealing. Four distinct growth stages were observed—stage 1 (abnormal), stage 2 (normal), stage 3 (normal and abnormal), and stage 4 (normal). This multi-staged growth process can explain the disparate previous observations of grain growth.; While 4 stages of growth were observed, it is the initial sequence of structural evolution (stage 1) that is the key to understanding grain growth in nanostructured materials. In this growth sequence, the majority of grains retain essentially the same size until they are consumed by migrating abnormal growth fronts (AGFs). Indirect evidence suggests that the velocity of the AGFs decreases with increasing migration distance into the matrix. This was attributed to the collection of solute atoms (primarily sulfur) by dynamic segregation (i.e. solute sweeping by the migrating AGFs). The initial sequence of grain growth was also studied in nanocrystalline Co and Ni-Co alloys—materials systems that had not previously been investigated. The effect of starting structure (e.g. grain size, sulfur concentration, crystallographic texture, alloy composition, and crystal structure) on the initial sequence of structural evolution was examined. In all cases, qualitatively the same sequence of initial structural evolution was observed. These observations, in combination with the strong correlation found between bulk sulfur concentration and thermal stability in nanocrystalline Co, suggest that dynamic segregation may be a general rate limiting mechanism for grain growth in nanostructured electrodeposits. |
