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Metastable Alumina Structures in Melt‐Extracted Alumina–25 wt% Zirconia and Alumina–42 wt% Zirconia Ceramics
Author(s) -
Ando Teiichi,
Shiohara Yuh
Publication year - 1991
Publication title -
journal of the american ceramic society
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.9
H-Index - 196
eISSN - 1551-2916
pISSN - 0002-7820
DOI - 10.1111/j.1151-2916.1991.tb06896.x
Subject(s) - eutectic system , materials science , lamellar structure , microstructure , cubic zirconia , transmission electron microscopy , dendrite (mathematics) , ceramic , amorphous solid , alloy , scanning electron microscope , metastability , phase (matter) , composite material , crystallography , metallurgy , chemical engineering , nanotechnology , chemistry , geometry , mathematics , organic chemistry , engineering
Microstructures are examined of rapidly solidified hypoeutectic Al 2 O 3 –25 wt% ZrO 2 and eutectic Al 2 O 3 –42 wt% ZrO 2 ceramic alloys by using transmission electron microscopy and scanning transmission electron microscopy. Structures observed in the hypoeutectic alloy were dendritic. Three different types of dendrite morphologies were observed. These are believed to be the stable α‐ and metastable γ‐ and δ‐ modifications of alumina. The cores of the γ‐alumina dendrites were somewhat richer in ZrO 2 than those of α‐alumina dendrites; δ‐alumina dendrites were substantially enriched in ZrO 2 . The lamellar structure of the eutectic in the Al 2 O 3 –42 wt% ZrO 2 alloy became increasingly finer with increasing cooling rate and at the highest cooling rates was replaced by a fully amorphous structure (except in some instances a ZrO 2 ‐rich needlelike phase identifiable as δ‐alumina was found in the amorphous matrix). An interpretation is given of the results obtained, based on assumed metastable free energy curves and dendrite growth theory.