Structural hierarchy as a key to complex phase selection in Al-Sm

Investigating the unknown structure of the complex cubic phase, previously observed to crystallize from meltspun amorphous Al–10 at. % Sm alloy, we determine the structure in full site-occupancy detail, highlighting several critical structural features that govern the far-from-equilibrium phase selection pathway. Using an efficient genetic algorithm combining molecular dynamics, density functional theory, and x-ray diffraction, the structure is clearly identified as body-centered cubic Im ̄3m (No. 229) with ∼140 atoms per cubic unit cell and a lattice parameter of 1.4 nm. The complex structure is further refined to elucidate the detailed site occupancy, revealing full Sm occupancy on 6b sites and split Sm/Al occupancy on 16f sites. Based on the refined site occupancy associated with the experimentally observed phase, we term this phase ɛ−Al60Sm11(bcc), corresponding to the limiting situation when all 16f sites are occupied by Sm. However, it should be recognized that the range of solubility enabled by split occupancy at Sm sites is an important feature in phase selection under experimental conditions, permitting an avenue for transition with little or no chemical partitioning. Our analysis shows that the ɛ−Al60Sm11(bcc) exhibits a “3-6-6-1” first-shell packing around Sm centers on 16f sites, the same dominant motif exhibited by the undercooled liquid. The coincident motif supports the notion that liquid/glass ordering at high undercooling may give rise to topological invariants between the noncrystalline and crystalline states that provide kinetic pathways to metastable phases that are not accessible during near-equilibrium processing. Disciplines Condensed Matter Physics | Materials Science and Engineering Authors Zhuo Ye, Yang Sun, Manh Cuong Nguyen, Shihuai Zhou, Lin Zhou, Fanqiang Meng, Ryan T. Ott, E. Park, Matthew Besser, Matthew J. Kramer, Z. J. Ding, Mikhail I. Mendelev, Cai-Zhuang Wang, Ralph E. Napolitano, and Kai-Ming Ho This article is available at Iowa State University Digital Repository: http://lib.dr.iastate.edu/ameslab_manuscripts/30 PHYSICAL REVIEW MATERIALS 1, 055601 (2017) Structural hierarchy as a key to complex phase selection in Al-Sm Z. Ye,1,* F. Zhang,1,† Y. Sun,1,2 M. C. Nguyen,1 S. H. Zhou,1 L. Zhou,1 F. Meng,1 R. T. Ott,1 E. Park,1 M. F. Besser,1 M. J. Kramer,1 Z. J. Ding,2 M. I. Mendelev,1 C. Z. Wang,1 R. E. Napolitano,1,3 and K. M. Ho1,4,2,‡ 1Ames Laboratory, US Department of Energy, Ames, Iowa 50011, USA 2Hefei National Laboratory for Physical Sciences at the Microscale and Department of Physics, University of Science and Technology of China, Hefei, Anhui 230026, China 3Department of Materials Science and Engineering, Iowa State University, Ames, Iowa 50011, USA 4Department of Physics, Iowa State University, Ames, Iowa 50011, USA (Received 4 June 2017; published 12 October 2017) Investigating the unknown structure of the complex cubic phase, previously observed to crystallize from melt-spun amorphous Al–10 at. % Sm alloy, we determine the structure in full site-occupancy detail, highlighting several critical structural features that govern the far-from-equilibrium phase selection pathway. Using an efficient genetic algorithm combining molecular dynamics, density functional theory, and x-ray diffraction, the structure is clearly identified as body-centered cubic Im3̄m (No. 229) with ∼140 atoms per cubic unit cell and a lattice parameter of 1.4 nm. The complex structure is further refined to elucidate the detailed site occupancy, revealing full Sm occupancy on 6b sites and split Sm/Al occupancy on 16f sites. Based on the refined site occupancy associated with the experimentally observed phase, we term this phase ε-Al60Sm11(bcc), corresponding to the limiting situation when all 16f sites are occupied by Sm. However, it should be recognized that the range of solubility enabled by split occupancy at Sm sites is an important feature in phase selection under experimental conditions, permitting an avenue for transition with little or no chemical partitioning. Our analysis shows that the ε-Al60Sm11(bcc) exhibits a “3-6-6-1” first-shell packing around Sm centers on 16f sites, the same dominant motif exhibited by the undercooled liquid. The coincident motif supports the notion that liquid/glass ordering at high undercooling may give rise to topological invariants between the noncrystalline and crystalline states that provide kinetic pathways to metastable phases that are not accessible during near-equilibrium