MICROSTRUCTURE AND EVOLUTION OF A MAGNESIUM LITHIUM ALUMINOSILICATE MATRIX COMPOSITE

The microstructure of a magnesium lithium aluminosilicate glass cerami c composite has been investigated by scanning electron microscopy and analytical transmission electron microscopy. Attention was focused on the as-received material, showing that there is a non-uniform distribu tion of the major silicate phases inside the matrix. The largest part is made of spodumene-type crystals containing more than 4wt% Mg. A min or part of the matrix is made of micrometre-sized crystallites of spod umene and cordierite. The spodumene is always sensitive to the electro n beam irradiation. The morphology of the amorphized spodumene areas i ndicates that it may have crystallized during a later stage of the mat rix formation, filling the gaps between cordierite crystallites. The t hird component of the matrix is made of carbon-rich areas. They can be as large as 10 mu m and they always include amorphous Mg-rich silicat es. However, they are mainly small (a few tens of nanometres in width) when located at grain boundaries of spodumene crystals. In this case the turbostratic carbon patches are also intimately mixed with an Mg-r ich amorphous silicate. The interface between the matrix and the fibre s has also been analysed, its thickness changes from one to the other, and it is sometimes empty due to decohesion. When it is filled, its o uter part contains mainly tubostratic carbon and the inner part is a m ixture of silicon oxide and probably carbon. After creep at 1373 K, th e spodumene-type crystals are larger and they are no longer sensitive to the electron beam. The cordierite areas appear to shrink and the am orphous patches which were mixed with carbon transform into small crys tallites (1-10 mu m). The areas next to the fibres are found to extend irregularly into the matrix, probably as a result of a chemical reduc tion.