Interpretation of Alumina/Yttrium-Aluminum Garnet Orientation Relationships by Geometric Criteria
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INTERPRETATION OF ALUMINA/YTTRIUM-ALUMINUM GARNET ORIENTATION RELATIONSHIPS BY GEOMETRIC CRITERIA R.S. HAY AND L.E. MATSON Wright Research and Development Center, WPAFB OH 45433 ABSTRACT for the Geometric criteria for low interface energy and interface structure were tested Orientation relationships near (11 1 )[11 01a // cubic-rhombohedral system YAG/alumina. 2 sol-gel derived composites (112)[1101y and facets on (11 1)a(1 1 )y were observed in both and directionally solidified eutectic composites. The 1yA;=12 near-CSL of 21111j, (110], with b=1 /3(1-11] and unit. Dislocations structural [112] was inferred to be the preferred b=1/21 10]a were observed and inferred to accomodate deviation from the structural unit, respectively. The [11 0]a y direction met some of the criteria for an invariant line. Although the OR was explained bý geometric criteria it would have been difficult to predict it with such criteria. INTRODUCTION A recent review concluded that general geometric criteria for low energy do 1not exist structure must be considered . Other and that details of interface bonding and atomic studies concluded that invariant line analysis 2 - and O-lattice theorys may predict orientation in non-cubic materials low 1 near-CSL's (NCSL's) relationships (OR's) in precipitates. Even frequently 3occur6. Parallelism of close-packed planes is a general trend for phase boundaries . Observations of OR's and interphase boundaries in YAG (yttrium-aluminum garnet)/alumina composites have not been reported. YAG0 is cubic with a=1.201 nm7 and alumina is rhombohedral with a=0.514 nm and a=55.26 . YAG and alumina have small differences in thermal expansion and do not react with each other; less than 0.1 mol% yttria is soluble in alumina at 1650°Cs. A rhombohedral-cubic system could be a good test of eometnc criteria because real and reciprocal-space directions are in general not the same. his work reports YAG/alumina OR's and interface structure observed in both sol-gel derived and DS eutectic material, and attempts to interpret the observations by geometric criteria. EXPERIMENT TECHNIQUE Yttria and alumina sols were made by published techniques 9 -12. The sols were mixed to yield a YAG:alumina ratio of 1:1, 1:2, and 1:4 by volume. Drops were placed on platinum films over the grid TEM grids, dried to a gel, and aged. This resulted in 0electron transparent 1200 C to 1570 0C for 1 to 12 hours. The spaces. The grids were heat-treated in air at 13 procedure was similar to that used for YAG films . Yttrium and aluminum isopropoxides were mixed to the eutectic composition 14 and hydrolyzed, dried, calcined, cold pressed and then sintered at 1700 0C for 5 hours 0in air. in a Bridgman-type furnace with a 200 C/cm The pellets were charged into a Mo crucible 15 gradient, melted, and then solidified at 6 cm/h . Some were annealed in air for 24 hours at 1650 0C or 5 hours at 1700 0C. The sol-gel samples were initially examined by TEM without ion-milling or carbon sometimes caused difficulty with microscope alignment. coating, although chargin
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