Relaxation mechanisms at grain boundaries for ambient-temperature creep of h.c.p. metals

The role of grain boundaries (GBs) for ambient-temperature creep of h.c.p. metals was investigated using pure Zn with several grain sizes. To reveal the relaxation mechanism of ambient-temperature creep, scanning electron microscopy and atomic force microscopy were performed to evaluate the amount of grain boundary sliding. Grain orientation variations were then measured using electron backscatter diffraction to investigate a lattice rotation after ambient-temperature creep. The results obtained by these experiments are as follows: (1) Strong grain size dependency, i.e. larger grain size showed lower total true strain. This is different from high temperature dislocation creep. (2) Grain boundary steps of a few tenths of a micrometer gave evidence of grain boundary sliding during ambient-temperature creep. (3) Lattice rotation of a few degrees was observed near GBs, which indicates that dislocations piled up at GBs. (4) Grain boundary sliding is considered as accommodation process of piled-up dislocations with an apparent activation energy of 18 kJ/mol.