A fine-grained polycrystalline metal was creep tested at a low stress at about 0.5 times its melting point (in absolute temperature). The steady-state creep rate was nearly proportional to stress (stress exponent n ≈ 1) and nearly inversely proportional to the cube of the grain size. Which creep mechanism is most likely to be operating?

Answer: B. Coble creep by diffusion of atoms along grain boundaries

In diffusion creep, atoms diffuse from grain boundaries under tension to those under compression, elongating the grains, and the rate is proportional to stress (n = 1). When the diffusion path runs along the grain boundaries it is called Coble creep, and the rate is inversely proportional to the cube of the grain size. When it is lattice diffusion through the grain interiors it is called Nabarro–Herring creep, and the rate is inversely proportional to the square of the grain size. Grain boundary diffusion has a lower activation energy than lattice diffusion, so Coble creep dominates at relatively low temperatures and fine grain sizes, while the contribution of Nabarro–Herring creep grows at high temperatures near the melting point. Power-law creep by dislocations has a large n of 3 to 8 and hardly depends on grain size.