AbstractPrevious reports on the solubility of magnesium in •-sialons have been conflicting. The present work shows conclusively that metastable crystalline B11 - magnesium sialons do exist and can be formed by two distinct crystallization mechanisms:
1.Low temperature annealing of quenched MgSiAlON (3M/4X) glasses containing precipitated B- S i3 N4 / B 1 s i a l on nuclei which must be present for epitaxial nucleation and growth to proceed. The degree of B11 crystallization which can be achieved is sensitive to both composition and impurity levels and complete crystallization (>90%) can be achieved only for compositions within the 3M/4X plane using high purity starting materials.
2.Coupled growth with forsterite from MgSiAlON (3M/4X) liquids producing low magnesium content B11 in a 'eutectic' microstructure.
B11-magnesium sialons have a range of compositions within the 3M/4X plane of the Mg-Si-Al-0-N system, with compositions of high magnesium content tending to exist in a limited composition range indicative of a 1 phenacite type metal atom ordering between (Mg,Al) and Si, as confirmed by the existence of additional superlattice reflection in electron diffraction photographs.The metastable nature of B11-magnesium sialans means that they tend to transform to stable crystalline products at high temperatures. The range of B11 composition gives a corresponding range of thermal stability, with high magnesium content B11 stable only up to 1120 °c, while B11 crystals with lower magnesium levels are able to exist up to 135o0 c.
A previously unknown MgSiAlON phase, termed Q-phase, has been crystallized as a secondary phase from MgSiAlON liquids in the 3M/4X plane in association with forsterite.
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