Atomic Distance Tuning Effect for Nucleation in Liquid Iron
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Turnbull[1] proposed that nucleation behavior of crystals in an undercooled liquid is related to the catalyzer it contains, which can be estimated by lattice disregistry between the catalyzer and the nucleated solid. However, even under the same thermal and substrate conditions, a nucleation system can exhibit distinct undercooling values.[2–4] This phenomenon suggests that there are unknown factors affecting nucleation process. To comprehend this puzzle, liquid structure was proposed as an important clue responsible for the variations observed in undercooling.[5,6] Frank[7] hypothesized a theory to explain the origin of undercooling by assuming liquid is composed of icosahedral short-range orders (ISROs). The proposed ISRO has six fivefold symmetry axes,[7] which is incompatible with the crystal translational periodicity. This structural dissimilarity is regarded
M. XU, X. GE, W. YAO, S. TANG, W. LU, Q. HU, J. LI, and M. XIA are with the School of Materials Science and Engineering, Shanghai Jiao Tong University, Shanghai 200240, China. Contact e-mail: [email protected] M. QIAN is with the Centre for Additive Manufacturing, School of Engineering, RMIT University, Melbourne, VIC 3000, Australia. Y. FU, H. XIE, and T. XIAO are with the Shanghai Synchrotron Radiation Facility, Shanghai Institute of Applied Physics, Shanghai 215600. Manuscript submitted April 19, 2018.
METALLURGICAL AND MATERIALS TRANSACTIONS A
as the atomic structural origin of the detected undercooling (nucleation energy barrier). Many experimental and simulation evidences have verified the Frank’s hypothesis, where ISRO was identified in metallic liquids, especially in liquid iron.[8–10] The ISRO can be a perfect or distorted fivefold symmetry structure,[11] both can decrease the barrier to nucleation of the metastable i-phase[12] or metallic glass.[13] More recently, the concept of ISRO was considered as the leading structure for icosahedral quasicrystals (iQC) to act as a template for nucleation of crystalline solids, e.g., face-centered-cubic (fcc)-Al in Al-Zn-Cr alloys.[14] Thus, the obtained results suggest that ISRO might be a key intermediate structure for crystal nucleation, i-phase formation, and glass transition during liquid-to-solid transition process. The earlier study on the effect of the substrate on atomic ordering in liquid was observed through high-resolution transmission electron microscopy (HRTEM) by Howe[15] and Oh et al.[16] Layer ordering and in-plane ordering were identified simultaneously in the first few layers adjacent to the liquid/substrate interface.[15–18] Greer[19] pointed out that such ordering could affect the nucleation either positively or negatively, depending on substrate template effect. Stateof-art experimental studies[20–23] have shown that indeed the substrate can affect the nucleation behavior of new crystals but the ordering effect on the liquid is missed in these studies. For instance, there is little information about how the liquid ordering takes part in the nucleation process and how the ordering itself is a
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