The crystalline-to-amorphous transition in shock-loaded mullite Al2VI(Al2+2x Si2-2x)IVO10-x in the light of shear modulus anisotropy

Kurzfassung

We present experimental evidence for shock-wave induced amorphization in polycrystalline and single crystal mullite, Al2VI(Al2+2x Si2-2x)IVO10-x, at peak pressures above 35 GPa . The transition proceeds along a network of very thin glass lamellae (“planar deformation features” (PDFs)) of mullite-normative composition extending parallel to low-index crystallographic planes including {120}, {230} and {110}. Cumulative microstructural evidence from the PDFs derived via analytical transmission electron microscopy suggests a shear-induced for-mation mechanism. Experimental PDFs match the relative minima of the calculated represen-tation surfaces of the shear modulus suggesting that suitable PDF orientations can be derived from the elastic anisotropy of mullite. PDFs in mullite are in good agreement with those re-ported for naturally shocked sillimanite. Unlike the formation of shear-induced PDF-type glass lamellae in shocked mullite, the ther-mal decomposition of mullite following high post-shock temperatures results in a fine-grained phase assemblage consisting of corundum plus amorphous silica, and represents the most abundant transformation mechanism in the shock regime investigated (20-40 GPa). No stishovite was observed. At shock levels beyond 35 GPa thermal decomposition of mullite may occur along with PDFs within the same specimen.