Even at ambient temperature or less, below their 0.2% proof stresses all hexagonal close-packed metals and alloys show creep behaviour because they have dislocation arrays lying on a single slip system with no tangled dislocation inside each grain. In this case, lattice dislocations move without obstacles and pile-up in front of a grain boundary. Then these dislocations must be accommodated at the grain boundary to continue creep deformation. Atomic force microscopy revealed the occurrence of grain boundary sliding in the ambient-temperature creep region. Lattice rotation of 5° was observed near grain boundaries by electron backscatter diffraction pattern analyses. Because of an extra low apparent activation energy of 20kJ/mol, conventional diffusion processes were not activated. To accommodate these piled-up dislocations without diffusion processes, lattice dislocations must be absorbed by grain boundaries through a slip-induced grain boundary sliding mechanism.

Grain Boundary Sliding during Ambient-Temperature Creep in Hexagonal Close-Packed Metals. T.Matsunaga, T.Kameyama, S.Ueda, E.Sato: Philosophical Magazine, 2010, 90[30], 4041-54