Grain growth behavior with a roughening of faceted boundaries in sintered ultra-fine Ni

Abstract

In polycrystalline, a grain growth occurs during annealing to reduce grain boundary energy by decreasing the grain boundary area. In Ultra-Fine Grained (UFG) metals, the phenomenon usually occurs at ‘lower temperature’ and with ‘more abnormal’ manner. For examples, in UFG Pd, and Cu, abnormal grain growth (AGG) occurs even at ambient temperature. And in various UFG metals, AGG occurs at relatively lower temperatures than that of counterpart metals (i.e. coarse-grained metals). It is suggested that the two features on growth behavior in UFG metals are attributed to the high driving force for grain growth that is resultant from the large amount of grain boundary energy. But it had pointed out that the suggestion only can explain why grain growth occurs rapidly in the UFG metals; as leading to increased migration rate of grain boundaries, cannot explain why grains grow selectively (i.e. why grain growth occurs abnormally). UFG Ni is characterized to have superplasticity at very low temperatures compared with microcrystalline Ni. But their technological application is often limited by occurrence of AGG. Therefore, a lot of groups have carry out research on thermal stability in UFG Ni. In UFG Ni, two consecutive (i.e. Initial Stage and Late Stage-) AGG occurs below 550 ℃. IS-AGG is occurred with increasing grain size from several tens of nanometers to several hun-dred nanometers. LS-AGG is occurred with cubic shape from several hundred nanometers to above several micrometers. The occurrence of the consecutive AGG in UFG Ni system has been explained as initial texture mechanism or S phase wetting mechanism. In chapter 3, the mechanism of cube-shaped AGG in UFG Ni was investigated To this end, ultra-fine Ni powder compacts were used, unlike in previous studies in which ED Ni sheets were usually used. The Ni powder used in this experiment had a very low S concentration of 0.001 wt%, unlike in the ED Ni with a relatively high S of 0.085 wt%. Thus, the possible effects ...