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 τe much less than the Maxwell relaxation time τd, 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 τd, diffusionally accommodated grain-boundary sliding with Q-1~To. For periods To≫τe, laboratory dissipation data may be adequately described as a function of a single master variable, namely the normalised period To/τd. 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, τe and τd, 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