Microstructure of Y 3 Al 5 O 12 garnet solidified from the melt undercooled beyond the hypercooling limit
- PDF / 7,560,782 Bytes
- 7 Pages / 612 x 792 pts (letter) Page_size
- 47 Downloads / 237 Views
RODUCTION
THE transition from cellular-to-planar morphology at high-growth velocities was predicted by the linear-stability analysis of Mullins and Sekerka.[1] Recently, Ludwig and Kurz observed this transition in situ in the directional solidification of succinonitrile-argon alloys.[2] The dendrite-to-cell transition at high-growth velocity for constrained growth has been roughly characterized by the simple formula, lD ⫽ ␣d0, where lD , ␣, and d0 are the solute-diffusion length, a constant, and the capillarity length, respectively.[3] The conditions required for the dendrite-to-cell transition at highgrowth velocity for free growth, however, are not yet identified. Very recently, we achieved the undercooling more than the hypercooling limit (⌬Thyp ⫽ ⌬Hf /CpL ⫽ 769 K) of Y3Al5O12 garnet (YAG) using an aeroacoustic levitator (AAL).[4] The YAG material is a stoichiometric compound oxide with a congruent melting point of 2213 K, whose phase diagram has been reported by Caslavsky and Viechnicki.[5] Here, hypercooling conditions allow complete solidification of the entire liquid during recalescence without extracting any additional heat. The original microstructure formed adiabatically can be retained by avoiding remelting after recalescence. Glicksman and Schaefer reported the solidification from hypercooled white-phosphorus (alpha P4) melt for the first time.[6,7] Other researchers also found hypercooling of an organic alloy[8] and metallic alloys.[9,10,11] The microstructure, however, was not examined in detail in spite of the interest. In our previous article,[4] a cellular microstructure was observed at undercooling more than ⌬Thyp, while dendritic microstructure was observed at undercooling less than ⌬Thyp. K. NAGASHIO, formerly Postdoctoral Candidate, Department of Materials Science, The University of Tokyo, Bunkyo-ku, Tokyo 113-8656, Japan, is Postdoctoral Student, Center for Materials Research, Stanford University, Stanford, CA 94305. Contact e-mail: [email protected] K. KURIBAYASHI, Professor, is with the Institute of Space and Astronautical Science, Sagamihara, Kanagawa 229-8510, Japan. Manuscript submitted October 10, 2001. METALLURGICAL AND MATERIALS TRANSACTIONS A
That is, hypercooling may provide some information concerning the dendrite-to-cell transition at high-growth velocity for free growth. However, it was difficult to observe the microstructure in detail because YAG is so thermally and chemically stable that the microstructure cannot be etched. In the present study, we observed the microstructure in the wide undercooling range beyond ⌬Thyp by fabricating thin films for transmission stereomicroscopy and by breaking the processed samples to elucidate the dendrite-to-cell transition at high-growth velocity. Drastic transitions from coarse-grained dendritic to grainrefined equiaxed microstructures are commonly observed in undercooled metallic melts. Walker reported that a marked decrease of the grain size of Ni occurred in the undercooling range of 140 to 150 K.[12] However, it is pointed out that W
Data Loading...
