Varied linear phason strain and its induced domain structure in quasicrystalline precipitates of Zr-Al-Ni-Cu-Nb bulk met
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un Department of Materials Science and Engineering, Research Center for Materials, Zhengzhou University, Zhengzhou 450002, China (Received 9 June 2012; accepted 1 October 2012)
Quasicrystalline precipitates in ZrAlNiCuNb alloy were systematically studied by transmission electron microscopy. It was found that precipitates always contain various linear phason strains. By electron diffraction analysis, two types of linear phason strain with two different directions perpendicular to the incident beam described by strain matrices with only one nonzero element were identified. After measuring the deviations of diffraction spots and quantitatively fitting against their perpendicular components of the reciprocal lattice vectors, the phason strain matrices were obtained. Domain structures formed as a result of linear phason strain variants along directions with equal probability. Electron diffraction and high-resolution electron imaging provide supportive evidence of this result.
I. INTRODUCTION
High strength and plasticity cannot hold concurrently in metals and alloys; plasticity is normally reduced with increasing strength. Nevertheless, this dilemma has been overturned by ZrCuNiAl, a bulk metallic glass (BMG) with disordered atomic structure, exhibiting superior strength combined with improved plasticity.1–3 Structural heterogeneity, for instance, the short-/middle-range order structure, is considered to be related to the plasticity of BMG.4–8 The existence of strong short-/middle-range order, indicated by the initial formation of icosahedral quasicrystals (IQCs) during the appropriate annealing process,9–11 causes inhomogeneous distribution of free volume and eventually leads to the enhancement of plasticity.12,13 Hence, the fundamental investigation on the detailed microstructure of the IQC precipitates is helpful to understand the relation between the icosahedral short-/middle-range order and the plasticity. ZrAlNiCuNb alloy was chosen for this research owing to the enhanced formation of IQC structure by adding additional elements, such as Nb, Ta, V, Mo, etc.14–18 During our transmission electron microscopy (TEM) study of ZrAlNiCuNb alloy, icosahedral quasicrystalline precipitates with distorted diffraction patterns breaking
a)
These authors contributed equally to this work. Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/jmr.2012.351 b)
J. Mater. Res., Vol. 27, No. 23, Dec 14, 2012
http://journals.cambridge.org
Downloaded: 18 Mar 2015
the icosahedral symmetry and the golden ratio were widely observed. Such distortion of diffraction patterns can be explained in terms of phason strain, a characteristic defect in quasicrystal.19–22 The phason strain associates with discrete atomic displacements, and the relaxation time is estimated to be extremely long.23,24 This suggests that the phason strain will be frozen in IQC even after the heat treatment. It is characterized by subtle deviation of diffraction peaks from their standard positions, for instance, the broadening of x-ray diffracti
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