Solid solubility of aluminum in silicon carbide was studied by the powder X‐ray diffraction technique (Debye‐Scherrer method). At 2200° C the solubility limit was 1.0 wt%. Measurements of the lattice parameter of Al‐saturated samples as a function of temperature gave a heat of solution of 147 kJ/mol. The change in the lattice parameter indicates that Al substitutes for Si. Literature data are interpreted as indicating that B replaces both Si and C.

SOLID state reactions' and phase relations' in the system Y20,-TiO, were studied in the range below 1600°C. Four compounds were found: a-Y,TiO, (orthorhombic structure, ao= 10.50 8,, b,= 11.4 A, and c,=3.68 P-Y,TiO, (hexagonal structure, a,=3.614 A, co= 11.84 a pyrochlore-type solid solution of Y2Ti,O7 (cubic structure, a,= 10.09 &,Ia6,' and a fluorite-type solid solution of Y zTi 1-x02-s/2Ux~2, with an oxygen deficiency (x=0.62 to 0.80, cubic structure, a,=5.097 to 5.215 8,).la8Phase relations are presented for the system Y ,O,-TiO, in air in the range 800" to 1900°C.The starting materials were coprecipitates of Ti (IV) and Y (111) hydroxides for the studies in the range below 1200°C and mixtures of Y,O, and TiO, powders (99.99% pure) above 1200°C. The coprecipitates were prepared by adding NHIOH to mixtures of TiCll and YCI, aqueous solutions, and were dried in air at 25°C for 30 days then heated to 500°C for 48 h. Subsequently, Y,O, and TiO, contents were determined by chemical analysis.Samples were heat-treated below 1600°C in a sintered-Sic electric furnace; the sample holders were Pt which had been heated for 5 to 30 days in air and quenched in liquid N,. Above 16OO0C, an electric furnace* with sintered-LaCrO, heating element and Rh-Pt or Ir sample holders were used.Furnace temperatures were controlled to within 2 5°C by a PIDtype controllert and were measured with Pt-Pt13Rh and Pt20Rh-Pt40Rh thermocouples. The temperature-measuring system for each furnace was calibrated against the melting points of Au (lO8O0C), Pd (1540°C), and Pt (1780°C).An X-ray diffractometert (at room temperature) and a hightemperature X-ray diffractometer' (below 1600°C) were used for phase investigations. The degree of melting was determined by the physical appearance of the quenched samples. The phase diagram for the system Y,O,-TiO, is presented in Fig. l . Compositions and temperatures of the examined points are indicated by dots on the diagram.The a-Y,TiO, has an orthorhombic lattice and is stable below 1330°C. The PY,TiO,, which has a hexagonal lattice and is stable between 1330" and 1520"C, changes to a fluorite-type solid solution at higher temperatures. Neither TiO, nor Y,O, is soluble in aor PY,TiO,. Only a small amount of TiO, and YzO, are soluble in the pyrochlore-type solid solution below 1450°C, whereas above 1450"C, Y,O, is markedly soluble in it and the structure of the solid solution gradually changes from pyrochlore to fluorite type.The structure of the pyrochlore is similar to that of the fluorite, and its lattice parameter is about twice that of the fluorite. The lattice parameter of these solid solutions changed continuously as a function of composition (between 35 and 68 mol% TiO, at 1800°C); however, no 2-phase region could be detected from X-ray powder diffraction patterns and lattice parameter changes. Therefore the dashed line in this region in Fig. 1 indicates an undetermined boundary line.The lowest temperature at which the fluorite phase is stable is 1390°C, where the solid solution contains 43.7 mol% Ti...

Progress in sintering process and improvement of mechanical properties of silicon nitride ceramics are reviewed. Emphases are placed on contributions of advanced sintering techniques and better understanding of sintering additives and microstructure-properties relations. Current applications as engine components and cutting tools are described, and future prospect is considered.

No abstract