ALUMINA YTTRIUM-ALUMINUM-GARNET CRYSTALLOGRAPHIC ORIENTATION RELATIONSHIPS AND INTERPHASE BOUNDARIES - OBSERVATIONS AND INTERPRETATION BY GEOMETRIC CRITERIA

An orientation relationship of (111)(112BAR) [110BAR]a parallel-to (112)(111BAR)[110BAR]Y was observed by TEM in sol derived and DS eutectic YAG-alumina composites. Facetting on (111)a(112)Y was observed in both materials. The SIGMA-YAG = 12 NCSL of 2[111BAR], [110BAR], [112] may be the preferred structural unit. Dislocations with b = 1/3[111]a and b = 1/2[110BAR]a were observed and inferred to accommodate deviation from the structural unit, respectively. Clear distinctions between merit of real and reciprocal-space NCSL descriptions could not be made. The direction [110]a,Y met some of the criteria for an invariant line. Although the OR was explained by geometric criteria, a priori prediction of it with those criteria would have been difficult. YAG-alumina composites were also made by hot-pressing, and a predominant OR was not observed in these samples. The ratios of the interfacial energies were estimated to be 1.00-gamma-a-a approximately 1.35-gamma-a-Y approximately 1.70-gamma-Y-Y from triple junction angles.