New creep region and mechanism in hexagonal close-packed metals

Only hexagonal close-packed (h.c.p.) materials show creep behaviour significantly at ambient temperature or less even below their 0.2% proof stresses with their stress exponents of 3.0 and their apparent activation energies of 20 kJ/mol. Transmission electron microscopy revealed dislocation arrays as a planar slip without any tangled dislocations inside each grain. Atomic force microscopy and electron backscatter diffraction pattern analyses brought about the occurrence of grain boundary sliding. The grain-size exponent was evaluated as 1.0, which means grain boundaries work as the barrier of the dislocation motion. Ambient-temperature creep of h.c.p. materials is schematically illustrated as that lattice dislocations move inside each grain without any obstacles and then pile up at grain boundaries. To continue the creep deformation, these dislocations are absorbed by grain boundaries to accommodate the internal stress and lead to grain boundary sliding.