Type of presentation: Oral

MS-11-O-1794 Systematic mapping of icosahedral short-range order in a melt-spun Zr36Cu64 metallic glass using scanning electron nano-diffraction

Liu A. C.1,2, Neish M. J.2,3, Bourgeois L.4,1, Ott R. T.5, Kramer M. J.5,6, de Jonge M. D.7
1Monash Centre for Electron Microscopy, Monash University, Clayton, Australia, 2School of Physics, Monash University, Clayton, Australia, 3School of Physics, University of Melbourne, Parkville, Australia, 4Department of Materials Engineering, Monash University, Clayton, Australia, 5Division of Materials Science and Engineering, Ames Laboratory, Ames, USA, 6Department of Materials Science and Engineering, Iowa State University, Ames, US, 7Australian Synchrotron, Clayton, Australia
amelia.liu@monash.edu

Understanding the structure of disordered materials remains a challenge. In the aberration-corrected scanning transmission electron microscope (STEM) intense, coherent, quasi-parallel and nanometre-sized beams may be achieved allowing diffracted information to be obtained from small volumes.  Electron nano-diffraction (END) patterns from thin metallic glass specimens contain angular correlations related to symmetries in nearest-neighbour polyhedral clusters [1]. Examining the persistence of these angular correlations in a scanned array of END patterns allows a measure of the medium-range order in the material [1]. We apply this novel technique to understanding the excellent glass formability of ZrxCu100-x glasses [1].

Scanned arrays of END patterns from a melt-spun Zr36Cu64 glass were obtained in a Titan3 80-300 FEGTEM (Fig. 1 a) and b)). Subtle angular symmetries in the END patterns were detected by calculating the angular autocorrelation function (Fig. 1 c) and d)). The autocorrelation function was decomposed into a Fourier Cosine series at each scattering vector magnitude and the symmetry intensities in each pattern were measured (Fig. 1 e)) and mapped as a function of scanned distances to examine the extent of any order (Fig. 2).

We statistically analysed the incidence of two-, six- and ten-fold symmetries in the SEND patterns and found that these compare favourably to those expected for a random ensemble of icosahedra, consistent with many modeling studies.

Fig 2. shows the 2-, 6- and 10-fold symmetry maps for both the experimental glass and a model glass structure. The only correlation length that extends beyond the probe diameter is the experimental two-fold map, demonstrating that the glass has extended order. The MRO consistent with this trend is face-sharing or interpenetrating icosahedral clusters in which the 2-fold symmetry axes align, but the 5- and 6-fold do not. The correlation length in the 2-fold map corresponds to four face-sharing or seven interpenetrating icosahedra.

Using scanning END and a novel analysis of the angular correlations in END patterns we determine that the S-MRO in Zr36Cu64 is consistent with efficiently packed icosahedral clusters [1], suggesting a structural basis for glass formability.

[1] A. C. Y. Liu, M. J. Neish, G. Stokol, G. A. Buckley, L. A. Smillie, M. D. de Jonge, R. T. Ott, M. J. Kramer, and L. Bourgeois, Phys. Rev. Lett., 110, 205505 (2013).

ACL acknowledges the Science Faculty, Monash University. TEM was performed in the MCEM.  Specimens were prepared at Ames Laboratory, funded by the US DoE (Office of Science–Basic Energy Sciences) Contract No. DE-AC02-07CH11358.

Fig. 1: a) HAADF reference image of glass with scanned area b) END pattern c) angular cross-correlation function and d) profile at 4 nm-1 e) magnitude of 0-12-fold symmetry intensities

Fig. 2: Maps of symmetry intensity from array of END patterns for model glass (a),c),e)) and experimental glass (b),d),f)) and corresponding radially averaged, two-dimensional autocorrelation functions.