Secondary recrystallization effects in nanostructured elemental metals

The thermal stability of ultrafine grained, elemental metal powder compacts (Cu, Ag, Pd) with low impurity content and high mass density were investigated by DSC and TEM measurements. In all our samples the crystallites grow abnormally. Grain growth is found to set in at temperatures below the recrystallization temperature known from coarse grained metals that have been heavily cold worked. For Cu and Pd even at ambient conditions abnormal grain growth occurs. Porosity up to 15% does not change grain growth temperature and abnormal grain growth behaviour. Gaseous impurities (oxygen) tend to stabilize the grain boundaries and enhance the effective activation energy for the grain growth process