Contrasting viscoelastic behavior of melt-free and melt-bearing olivine: Implications for the nature of grain-boundary sliding

Melt-free and basaltic (complex alumino-silicate) melt-bearing specimens of fine-grained polycrystalline olivine (Mg0.9Fe0.1)2SiO4, tested at high temperature and low frequency in torsional forced oscillation and microcreep, display markedly different behavior. For the melt-bearing materials, superimposed upon the high-temperature background is a dissipation peak whose height varies systematically with melt fraction that is attributed to elastically accommodated grain-boundary sliding facilitated by the rounding of grain edges at melt-filled triple junctions. The melt-free materials display only the high-temperature background dissipation associated with transient diffusional creep—elastically accommodated sliding evidently being inhibited by their tight grain-edge intersections. These and similar observations for other ceramic materials require that the classic theory of grain-boundary sliding be revisited and suitably modified.