The effect of annealing electrodeposited nanocrystalline nickel at 250 deg C was studied by microhardness testing, XRD, TEM and three-dimensional atom probe (3DAP) analysis. TEM micrographs revealed that the type of growth is abnormal, i.e. a composite structure is formed with some large (>1 mum) grains embedded in a matrix of nanometre sized grains. 3DAP studies showed no grain boundary segregation in as deposited materials and only low levels in material annealed for 1 h. Upon annealing, a 10 percent increase in hardness was observed within the first five minutes, after which the hardness was maintained for long times, despite the high volume fraction of large grains. Only after extensive annealing, when the majority of the material was made up of large grains, did the hardness decrease rapidly. The results demonstrate that the hardness is not determined by a law of mixtures but by percolation of the large grains.
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