Abstract
Bicrystals with (110) tilt boundaries and (100) twist boundaries were prepared by joining two single crystals of Mgo, using a graphite heating element at 2480°C without pressure. The joined boundaries were observed with an optical microscope and TEM. The evaporation/condensation mechanism was operative in joining. The boundary energies were measured at 1500°C by the thermal grooving method. The development of boundary grooves was controlled by surface diffusion. The ratio of the boundary energy to the surface energy (σb/σs) for small angle (110) tilt boundaries below 17° varies according to the Read-Shockley model, which suggests the boundary being constructed by edge dislocation arrays. TEM observation suggested that the small angle twist boundaries below 10° were formed by screw dislocation arrays. The (110) tilt boundaries had low σb/σs at about 40°, 70° and 130° which correspond to twin boundaries of (221), (111) and (113), respectively. σb/σs for high angle boundaries at 1500°C was around 0.4, lower than that of (100) tilt boundaries. Twist boundaries above 10° had also low energies at 22°, 29° and 37° which correspond to Σ of 13, 17 and 15 of the CSL theory, respectively. Hence, it is concluded that the high angle twist boundaries on (100) are explained by the CSL theory.
