Investigation of anomalous grains in polynanocrystalline nickel

Polynanocrystalline nickel, with an as received grain size distribution in the range 30 to 80 nm, when heat treated at a temperature of 500 degrees C causes overall coarsening of the grains and anomalous growth of some grains. This leads to a bimodal grain size distribution. The anomalous large grains, typically 75-100 μ m, have an angular morphology that is not typical of the polyhedral grains within the overall distribution. Using a combination of focus ion beam milling and imaging, and electron backscatter diffraction it has been established that the interfacial planes forming the overall grain boundaries for these anomalous large grains include 110, 210, 112 and 310 planes. In addition a substructure is produced within the large grains interrelated in size with the subsumed nano-scale grains. The results are discussed briefly with respect to the more energetically favourable 111 boundary planes and the overall mobility.