松永 哲也

The suppressing effect of ambient-temperature creep of CP-Ti by cold-rolling was reported. Annealed plates of CP-Ti grade 2 were cold-rolled with thickness reductions, and then creep tests under the applied stresses of 0.6-0-9 sigma(0.2) were performed at ambient temperature. With increasing the thickness reduction, the twin, dislocation density and sigma(0.2) were found to increase. At the same time, the steady-state creep rates under the applied stress for constant sigma/sigma(0.2) were decreased. The cold-rolled sample with 20% thickness reduction was then annealed at 813 K for 2400 s to decrease only the dislocation density. After the annealing, the steady-state creep rate remained constant, suggesting that the reduction of the steady-state creep is associated with the increasing twin density. (C) 2008 Elsevier B.V. All rights reserved.

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: (I) 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. (C) 2008 Elsevier B.V. All rights reserved.