Accession Number : ADA330476

Title :   Tailoring of Grain Boundary Chemistry for the Development of Highly Creep Resistant Alumina

Descriptive Note : Final rept. 1 Jun 94-31 May 97

Corporate Author : LEHIGH UNIV BETHLEHEM PA WHITAKER LAB

Personal Author(s) : Harmer, M. P. ; Chan, H. M. ; Rickman, J. ; Bruley, J. ; Cho, J.

PDF Url : ADA330476

Report Date : 29 AUG 1997

Pagination or Media Count : 151

Abstract : Research has demonstrated that the controlled doping of ultra-high purity alumina with small amounts (<1000ppm) of rare earth elements, such as La and Y, dramatically lowers the sintering and creep rate. Due to the large ionic radius of the rare earth elements, rare earth elements have a low solubility in alumina and segregate strongly to the grain boundaries. Chemical composition (STEM) profiles indicate that segregation of Y and La is localized to within about 2nm of the grain boundary i.e. about a single unit cells width. The concentration is about 9 atom % within 1 run of the boundary. It is hypothesized that grain boundary segregation plays a key role in slowing down grain boundary diffusion and creep. Atomistic computer simulation has been utilized to predict the distribution of point defects and dopants near grain boundaries and free surfaces and will eventually permit a determination of the boundary diffusivity along segregated boundaries. Selective co-doping with 100ppm Zr and 100ppm Nd has been found to produce the largest reduction in creep rate, of about a factor 500. The creep kinetics suggest that, in the Nd/Zr co-doped alumina, grain boundary diffusion has been suppressed to such degree that lattice diffusion is rate controlling.

Descriptors :   *ALUMINUM OXIDES, *GRAIN BOUNDARIES, *CREEP STRENGTH, COMPUTERIZED SIMULATION, RESISTANCE, POINT DEFECTS, CHEMICAL COMPOSITION, SURFACES, CHEMISTRY, SEGREGATION(METALLURGY), ZIRCONIUM, DIFFUSION, DOPING, KINETICS, SOLUBILITY, NEODYMIUM, RARE EARTH ELEMENTS, LANTHANUM, YTTRIUM, SINTERING.

Subject Categories : Properties of Metals and Alloys
      Crystallography

Distribution Statement : APPROVED FOR PUBLIC RELEASE