Elastically accommodated grain-boundary sliding: New insights from experiment and modeling

Substantial progress is reported towards a reconciliation of experimental observations of high-temperature viscoelastic behaviour of fine-grained materials with the micromechanical theory of grain-boundary sliding. The classic Raj-Ashby theory of grain boundary sliding has recently been revisited - confirming the presence of the following features: (i) at a characteristic period τ_e much less than the Maxwell relaxation time τ_d, a dissipation peak of amplitude ~10^-2 and associated shear modulus relaxation resulting from elastically accommodated sliding on grain boundaries of relatively low viscosity; (ii) at intermediate periods, a broad regime of diffusionally-assisted grain-boundary sliding within which the dissipation varies with period as Q-1~Toα with α~1/3, sliding being limited by stress concentrations at grain corners, that are progressively eroded with increasing period and diffusion distance; and (iii) for periods longer than the Maxwell relaxation time τ_d, diffusionally accommodated grain-boundary sliding with Q^-1~T_o. For periods T_o≫τ_e, laboratory dissipation data may be adequately described as a function of a single master variable, namely the normalised period T_o/τ_d. However, it is becoming increasingly clear that the lower levels of dissipation measured at shorter periods deviate from such a master curve - consistent with the existence of the two characteristic timescales, τ_e and τ_d, for grain-boundary sliding, with distinct grain-size sensitivities. New forced-oscillation data at moderate temperatures (short normalised periods) provide tentative evidence of the dissipation peak of elastically accommodated sliding. Complementary torsional microcreep data indicate that, at seismic periods of 1-1000s, much of the non-elastic strain is recoverable - consistent with substantial contributions from elastically accommodated and diffusionally assisted grain-boundary sliding.