EBSD Analysis of Grain Boundary Characteristics of Abnormally Grain Grown Alumina

Abstract

Electron Back Scattered Diffraction(EBSD) analysis was carried out to investigate boundary characteristics of Abnormally Grain Grown(AGG) Alumina. To obtain whole orientation information from AGG Alumina, separately measured EBSD mapping data was combined together to be satisfied with condition of minimum misorientation, which is called Montage technique. The orientation information from 0.5 x 3.5mm and 0.35 x 1.00 mm sizes of AGG alumina was obtained. Misorientation distribution in Normally Grain Grown(NGG) Alumina was very close to the random misorientation boundary. Misorientation distribution between AGG and NGG alumina showed random boundary characteristics in 0.5 x 3.5 mm AGG alumina. Large part of captured grain inside AGG grain has high angle boundary of 90. Introduction Microstructures of polycrystalline alumina and their related properties often depend critically on both the presence of dopants and of residual impurities. It is well known that small amounts of liquid forming additives such as CaO or SiO2 can induce AGG during sintering of alumina.[1-2] On the other hand, the addition of MgO can prevent AGG[3], however, there exist critical concentration for the prevention of AGG in alumina.[4] During AGG, large grains develop and grow fairly rapidly at the expense of the fine matrix materials. Abnormally large alumina grains are usually elongated with long basal planes. An amorphous(liquid, glass) phase was observed to be present as a thin film at the flat basal planes.[5,6] AGG in the presence of a liquid is a phenomenon observed only when the grains are angular with flat interface such as Al2O3[7] TaC-TiC-Ni[8], and BaTiO3.[9] In order to understand the growth mechanism of AGG, the investigation of orientation relationships between AGG and matrix grain boundaries are essential including identification of solid/liquid interface structures. Recently orientation of crystalline material and misorientation between adjacent grains can be easily obtained EBSD equipment installed in SEM. Observation of AGG grain by EBSD was restricted due to its large geometrical features of AGG grains, typically mm in size. In this study, to get whole orientation data from AGG alumina, montage technique was used, which can join separately measured fairly small size map into whole area map data. To get optimum montage condition, a concept of minimum misorientation was used. The misorientation between AGG and NGG alumina was analyzed and compared with random misorientation. Random Misorientation of Hexagonal Crystal 1700 Texture of Materials Among the reported crystal structure of alumina, hexagonal crystal information was used to indexing Kikuchi pattern during EBSD mapping. The misorientation distribution in randomly distributed hexagonal crystal was obtained by using quaternion representation of orientation and integrated over fundamental zone. The obtained results are as follows. P(φ) is probability and φ is misorientation. The calculated results shown in following figures comparing with measured data. Average value of misorientation is theoretically 60.07. The maximum misorientation angle is 93.84 1) o o 30 ) 3 2 ( tan 2 0 1 = − ≤ ≤ − φ : ) cos 1 ( 15 1 ) ( φ φ − = p 2) o o 90 ) 1 ( tan 2 30 1 = ≤ ≤ − φ : φ φ sin 15 ) 3 2 ( ) ( − = p 3) o o 98 . 91 )) 1 3 ( 2 ( tan 2 90 1 = − ≤ ≤ − φ : )] cos 1 ( 6 sin ) 3 8 [( 15 1 ) ( φ φ φ − − − = p 4) o o 84 . 93 ) 3 8 15 ( tan 2 98 . 91 1 = − ≤ ≤ − φ )] cos 1 }( 6 ) 6 , , ( 12 ) 2 , , ( 24 { sin ) 3 8 [( 15 1 ) ( φ π π φ φ − − + + − = b b S b a S p }] sin sin cos cos cos { cos cos } sin sin cos cos cos { cos cos } sin sin cos cos cos { [cos 2 1 ) , , (