Chong-Min Wang

A study of grain-boundary structure in rare-earth doped aluminas using an EBSD technique

Oversized rare-earth dopant ions such as Y3+, Nd3+, and La3+ segregate to grain boundaries and reduce the tensile creep rate of α-Al2O3 by 2-3 orders of magnitude. It has been speculated that these dopant ions can modify the grain boundary structure in alumina by promoting the formation of special grain boundaries. If this were indeed the case, it would provide a possible explanation for the aforementioned creep rate retardation. In order to test this hypothesis, electron backscatter diffraction (EBSD) has been used to assess both the proportion of coincidence-site lattice boundaries, and the grain boundary misorientation distribution, in aluminas doped with various ions (Zr, Y, Nd, La, Nd/Zr). The results show that the grain boundary structure in alumina is not significantly altered by the addition of the above dopants, implying that the change in grain boundary chemistry is primarily responsible for the observed creep behavior.

Microstructure of liquid phase sintered superplastic silicon carbide ceramics

Superplastic silicon carbide ceramics were fabricated at low temperatures by a liquid phase sintering very fine β–SiC powder. The microstructural features of this material, of both before and after the superplastic deformation, have been investigated by transmission electron microscopy. Evaluated from the point of view of phase transformation, dislocation motion, and dynamic grain growth, the materials show very stable microstructures, indicating that the superplastic deformation process was dominated by the liquid phase assisted grain boundary sliding. In addition, the material is also characterized by the formation of clusters of fine SiC particles (5–20 nm) encapsulated in layered graphitized carbon.

Atomic structural environment of Y in the residual glass phase of silicon nitride and α-sialon

Abstract The atomic structural environments around Y ions in the residual glass phase of Si 3 N 4 and α-sialon densified with Y 2 O 3 and Al 2 O 3 have been investigated using extended X-ray absorption fine structure (EXAFS). In the residual glass phase, on average, the Y–O/N nearest neighbor bond length is determined to be 2.30 A, which corresponds to the Y–O bond length in cubic Y 2 O 3 , indicating that O is the dominant nearest neighbor of Y. The fitted apparent Y–O/N nearest neighbor coordination number is ~4, which is apparently lower than that when Y is dissolved in a bulk glass, indicating that Y occupies substitutional Si sites. It is also further suggested that the Y–O/N polyhedron in the residual glass phase in silicon nitride and α-sialon is more distorted in comparison with that when Y is dissolved in a bulk glass phase.

Effects of Y and Zr Dopants on Grain Boundary Structure in Creep Resistant Polycrystalline Alumina

Dopants Y and Zr at 100 ppm levels in high purity, micron grain-size polycrystalline alumina are mainly segregated to the alumina grain boundaries and strongly reduce high temperature creep. Information about this segregation has come from high resolution STEM composition mapping experiments. Information about local structural surroundings of the dopant atoms has come from EXAFS experiments, and information about local bonding of the dopant atoms has come from XANES experiments. Structural models for dopant grain boundary segregation provide a context for these experimental results and for effects of dopant incorporation on grain boundary mediated transport. Recent experimental and theoretical results are discussed in this paper.