Undercooling And X-Ray Structural Studies Of Ti-Zr-Ni Liquids
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CC10.5.1
UNDERCOOLING AND X-RAY STRUCTURAL STUDIES OF Ti-Zr-Ni LIQUIDS G.W. Lee *, A.K. Gangopadhyay *, K.F. Kelton *, * Physics Department, Washington University, St. Louis, MO, USA; R.W. Hyers ***, T.J. Rathz **, M.B. Robinson ***, J. R. Rogers ***, **University of Alabama, Huntsville, AL, *** NASA/ George C. Marshall Space Flight Center, Huntsville, AL, USA; L. Hennet ****, **** CNRS-CRMHT, Orleans Cedex 2, France, S. Krishnan *****, *****KLA-Tencor Film and Surface Technology, 160 Rio Robles, San Jose, CA 95134, USA.
ABSTRACT Maximum undercooling results for the icosahedral phase (i-phase), polytetrahedral C14 Laves phase, and solid solution phases are presented as a function of composition in Ti-Zr-Ni liquids. Containerless processing was achieved using the electrostatic levitation facility located at NASA/Marshall Space Flight Center. The maximum reduced undercooling decreases with increasing icosahedral short-range order in the ordered phase. The first synchrotron x-ray diffraction data from aerodynamically levitated liquids of Ti-Zr-Ni alloys suggest an icosahedral short-range order in the liquids, supporting Frank’s hypothesis, correlating icosahedral order in the liquid with the nucleation barrier to the crystal phase. The strong negative heats of mixing between Ti/Zr and Ni and their relative atomic sizes likely favor the formation of this local icosahedral order. INTRODUCTION Turnbull demonstrated that most liquid metals can be significantly undercooled below their equilibrium melting temperature without crystallizing [1], indicating the presence of a large barrier to the nucleation of the crystal phases. Frank hypothesized that this arises because the liquid contains significant icosahedral short range order (ISRO) [2]. ISRO is energetically favorable; for a Lennard-Jones potential, the energy for an icosahedral packing of 13 atoms is 8.4 % lower than for the close-packed fcc or hcp crystalline structures. While incompatible with crystalline periodicity, an amorphous structure might prefer this packing. The nucleation of the crystal phase would require that the ISRO be transformed to a crystallographic local order, giving rise to the observed nucleation barrier. Frank’s hypothesis has been examined by computer simulation, but no direct experimental proof exists. This would require a correlation between the nucleation barrier and a direct observation of icosahedral order in a liquid or glass. A shoulder on the high angle side of the second peak in the x-ray structure factor, S(q), is an indication of ISRO [3, 4]. The relative intensity of the shoulder has been reported to increase with undercooling in simulation study, implying a developing ISRO in the liquid [5]. Icosahedral quasicrystals (i-phase) have extended icosahedral order and, correspondingly, have a lower nucleation barrier than crystal phases. By correlating a decrease in this barrier with an increase in measured ISRO, Frank’s hypothesis can be explored.
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Here, we contrast undercooling data for simple crystal phases, polytetrahedr
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