The possibility of having Sr as an interstitial metal cation in α‐SiAION has been investigated in two systems: a single‐cation system (Si3N4‐SrO‐AlN) and a multication system (Si3N4‐(Y2O3/SrO/CaO)‐AlN). It was found that Sr alone does not form α‐SiAlON and that Sr could only be accommodated in α ‐SiAION in conjunction with Y and Ca. The Sr content of α‐SiAION increased as the total content of (Y + Ca) increased and appeared to reach a limit at 0.5 at.%, or 0.15 atom per α‐SiAlON. Unexpectedly, some of the α‐SiAlON that contained (Sr + Y + Ca) was present as laths or fibers with the c‐axis perpendicular to the hot‐press direction.

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A small addition of CuO to 2-mol%-Yz03-stabilized tetragonal zirconia polycrystals significantly enhances superplasticity by forming an amor hous grain-boundary phase containing primarily C u ' , Y", Zr", and 0'-. This phase apparently melts at around 1130°C, but it already provides a fast diffusion path even below the melting temperature. There are abrupt changes in stress exponent, activation energy, and grain size exponent across the melting temperature. Superplasticity is diffusion-controlled below the melting temperature and is interfaced-controlled above that. [

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