W. Sinkler et al b I cm Fig. 2. Aspect of the 1050C sintered(nitrogen atmosphere) YMAS glass-ceramic after a 1200 C annealing treatment (a) under argon,(b) under oxygen. The color changes from dark to clear indicating that the starting glass-ceramic material was under-stoichiometric with respect to the oxygen content As far as partial reduction of oxides can change optical absorption characteristics, it is thus likely that this unexpected color was due to such a partial reduction by the carbon of the furnace during the glass-ceramic preparation procedure. Support for a chemical reaction bertween YMaS and carbon was 盏200m found from thermogravimetric investigation. The YMAS glass-ceramic was subsequently treated for 3 h at 1200C either under oxygen or under argon (alumina furnace). This resulted in a change in color from black(YMAS glass-ceramic before and after 1200C annealing under argon)to white(after 4类A 1200.C annealing under oxygen), as illustrated in Fig 2(b), and a weight gain of 0.3%(compared to 0.1% weight gain under argon, presumably due to the me purities in the gas). This indicates that s glass-ceramic material was under-stoi- chiometric with respect to oxygen 盘 Correspondingly, Fig. 3(a) shows the 1050C sintered (nitrogen atmosphere) YMAs glass ceramic in OM using polarized light in transmis sion. The predominant color of the specimen is magenta, whichs indicates a predominantly amor phous state. Small inhomogeneities may be seen throughout the micrograph(often colored blue or yellow on the original slide) which indicates the A presence of small crystals within an amorphous Fig. 3. 1050C-sintered (nitrogen atmosphere)YMAs glass matrix. In addition, large opaque areas are present, ceramic. Black-and-white duplication of an OM image:(a) which are not voids or bubbles [since not visible in transmitted, polarised light. Black areas are assumed to be due reflected light, see Fig. 3(b)]. The strong light to oxygen-depleted zones: () same region using reflected, absorption in these regions suggests that conditions high concentrations of crystals, which are probably indialite are locally achieved which have causedan Needle-like crystals are probably corundum. enhancement of partial reduction. Figure 3(b) shows the same area using natural light in reflec- tion. Small needle-like crystals which intersect the pecimen surface are visible away from the large with the EDX measurements subsequently per opaque areas(which are no longer revealed).a formed on the matrix of the composite. The large SEM image using backscattered electrons Fig. 4 opaque areas [they can be located using Fig 3(a)] indicates that the needle-like crystals containcontain high concentrations of smaller, isometric nt elements(they appear dark), and therefore polyhedral crystals, which suggests that enhanced are presumably AlO3(corundum), consistent partial reduction is related to the phaseAs far as partial reduction of oxides can change optical absorption characteristics, it is thus likely that this unexpected color was due to such a partial reduction by the carbon of the furnace during the glass-ceramic preparation procedure. Support for a chemical reaction bertween YMAS and carbon was found from thermogravimetric investigation. The YMAS glass-ceramic was subsequently treated for 3 h at 1200C either under oxygen or under argon (alumina furnace). This resulted in a change in color from black (YMAS glass-ceramic before and after 1200C annealing under argon) to white (after 1200C annealing under oxygen), as illustrated in Fig. 2(b), and a weight gain of 0.3% (compared to 0.1% weight gain under argon, presumably due to oxygen impurities in the gas). This indicates that the YMAS glass-ceramic material was under±stoi￾chiometric with respect to oxygen. Correspondingly, Fig. 3(a) shows the 1050C￾sintered (nitrogen atmosphere) YMAS glass± ceramic in OM using polarized light in transmis￾sion. The predominant color of the specimen is magenta, whichs indicates a predominantly amor￾phous state. Small inhomogeneities may be seen throughout the micrograph (often colored blue or yellow on the original slide) which indicates the presence of small crystals within an amorphous matrix. In addition, large opaque areas are present, which are not voids or bubbles [since not visible in re¯ected light, see Fig. 3(b)]. The strong light absorption in these regions suggests that conditions are locally achieved which have caused an enhancement of partial reduction. Figure 3(b) shows the same area using natural light in re¯ec￾tion. Small needle-like crystals which intersect the specimen surface are visible away from the large opaque areas (which are no longer revealed). A SEM image using backscattered electrons Fig. 4 indicates that the needle-like crystals contain light elements (they appear dark), and therefore are presumably Al2O3 (corundum), consistent with the EDX measurements subsequently per￾formed on the matrix of the composite. The large opaque areas [they can be located using Fig. 3(a)] contain high concentrations of smaller, isometric polyhedral crystals, which suggests that enhanced partial reduction is related to the phase Fig. 2. Aspect of the 1050C sintered (nitrogen atmosphere) YMAS glass-ceramic after a 1200C annealing treatment (a) under argon, (b) under oxygen. The color changes from dark to clear indicating that the starting glass-ceramic material was under-stoichiometric with respect to the oxygen content. Fig. 3. 1050C-sintered (nitrogen atmosphere) YMAS glass￾ceramic. Black-and-white duplication of an OM image: (a) transmitted, polarised light. Black areas are assumed to be due to oxygen±depleted zones: (b) same region using re¯ected, natural light. The dark areas visible in (a) are associated with high concentrations of crystals, which are probably indialite. Needle-like crystals are probably corundum. 308 W. Sinkler et al