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 ∼ T o α 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 1–1000 s, much of the non-elastic strain is recoverable – consistent with substantial contributions from elastically accommodated and diffusionally assisted grain-boundary sliding.