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Coherent Precipitation in the System MgO‐Al 2 O 3 ‐Cr 2 O 3
Author(s) -
GRESKOVICH C.,
STUBICAN V. S.
Publication year - 1968
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.1151-2916.1968.tb11826.x
Subject(s) - corundum , spinel , crystallography , monoclinic crystal system , precipitation , phase (matter) , materials science , metastability , ternary operation , solid solution , isothermal process , crystal structure , chemistry , thermodynamics , metallurgy , physics , organic chemistry , meteorology , computer science , programming language
An isothermal section of the ternary system MgO–Al 2 O 3 ‐Cr 2 O 3 was determined at 1700°± 15°C to delineate the stability field for spinel crystalline solutions (cs). Crystalline solutions were found between the pseudobinary joins MgAl 2 O 4 –Cr 2 O 3 and MgCr 2 O 4 ‐Al 2 O 3 , and the binary join MgAl 2 O 4 ‐MgO. The first two crystalline solutions exhibit cation vacancy models while the latter can probably be designated as a cation interstitial model. Precipitation from spinel cs may proceed directly to an equilibrium phase, (Al 1‐x Cr x ) 2 O 3 , with the corundum structure or through a metastable phase of the probable composition Mg(Al 1‐x C r ) 26 O 40 . The composition and temperature limits were defined where the precipitation occurs via metastable monoclinic phases. The coherency of the metastable monoclinic phase with the spinel cs matrix can be understood by considering volume changes with equivalent numbers of oxygens and known crystallographic orientation relations. Electron probe and metallographic microscope investigations showed no preferential grain boundary precipitation.