Metallic Liquids and Glasses: Atomic Order and Global Packing

Abstract

In this Letter, we have revealed the common structural behavior of metallic glasses through scrutinizing the evolution of pair distribution functions from metallic liquids to glasses and statistically analyzing pair distribution functions of 64 metallic glasses. It is found that the complex atomic configuration in metallic glasses can be interpreted globally as a combination of the spherical-periodic order and local translational symmetry. The implications of our study suggest that the glass transition could be visualized mainly as a process involving in local translational symmetry increased from the liquid to glassy states. DOI: 10.1103/PhysRevLett.105.155501 PACS numbers: 61.43.Dq, 61.43.Bn The atomic structure of metallic glasses (MGs) is a fundamental and intriguing issue in condensed-matter physics, due to its importance in understanding the glassforming mechanism and unique properties [1][2][3][4]. Many structural models have been proposed over the years, such as ''dense random packing'' of hard spheres [5] and the ''stereochemically designed '' model [6]. Although these early models significantly improved our understanding of atomic structures in MGs, especially for short-range order (SRO), they still have their own insufficiencies [7]. Recent studies confirm that the characteristics of medium-range order (MRO) clusters dictate the stability and mechanical properties of monolithic MGs [8][9][10][11], which have shed new light on our understanding of structural features at this length scale [7,[12][13][14]. By taking the earlier models further with advances in relevant experimental techniques and computational capacity, the idealized cluster packing schemes, such as efficient cluster packing on a facecentered cubic (fcc) lattice [12] and quasiequivalent clusters on an icosahedral packing [7], have been proposed and furthered our comprehension of the MRO. However, the latest finding indicated that the MRO in MGs can be described by packing of quasiequivalent clusters on a fractal network with a dimension (D f ) of 2.31 [14], implying that it is impossible to fill the real space in these amor phous solids by building blocks in a form of crystal (D f ¼ 3:0) or icosahedral-quasicrystal (D f ¼ 2:72) symmetry. The nature of atomic packing at the MRO scale or larger in MGs still remains mysterious [15].Glassy alloys are commonly considered as the state of ''frozen liquids''; nevertheless, their structure difference is obvious: A split in the second peak of the atomic-pair distribution function (PDF) curve, gðrÞ, for MGs, occurs with respect to that for liquids. Actually, the splitting of the second maximum of the gðrÞ curve into two subpeaks is recognized as a characteristic indication of amorphous solids [16], and its origin has been investigated extensively [17][18][19]. For example, Bennett proposed that the first subpeak at R 2 =R 1 ¼ ffiffiffi 3 p results from a continuum of configuration of two coplanar equilateral triangles sharing a common side, and the second one at R 3 =R 1 ¼ ffiffiffi 4 p originat...

Hidden topological order and its correlation with glass-forming ability in metallic glasses

“…It can be seen that at least five pronounced peaks are present in the range of about 1.2 nm, which indicates that there exists a structural order from short to medium range in glasses. We analysed the scaled peak positions R i /R 1 (i ¼ 1,2,3,4,5) in PCFs by fitting each peak with a Gaussian function 9 It has been demonstrated that the constants associated with the values of R i /R 1 (i ¼ 2,3,4,5) in glassy Ni correspond to some relative atomic positions in fcc lattice structure 9 , so that we naturally examine the relative atomic positions of R 0 i =R 0 1 (i ¼ 1-14) for fcc and bcc lattice structures, respectively, and the data are summarized in Table 2. Here, R 0 i (i ¼ 1-14) denotes the distance of the ith nearest neighbour atoms.…”

“…Although significant progress has been made and provided insights into the atomic-level structure and short-to-mediumrange order in glasses [3][4][5][6][7][8][9][10][11][12][13][14] , the way of leading to the mediumrange order is still unclear [5][6][7][8][9][10] . Does a universal rule exist that nature follows to construct a glass structure as what has been discovered for crystals?…”

“…However, the diffraction data such as structure factors or pair correlation functions (PCFs) are not as trivial as they look like. On the contrary, some studies have demonstrated that plenty of structural information is hidden behind the data of structure factors or PCFs, for example, a global packing containing both spherical-periodic order and local translational symmetry has been revealed by analysing PCFs of many metallic glasses 9 . A fractal nature of medium-range order in metallic glasses was also found by examining the relationship between the first peak position in structure factors and atomic molar volumes in many metallic glasses 7 .…”

Impact of medium-range order on the glass transition in liquid Ni-Si alloys

“…Recent work on the structure of metallic glasses represents the MRO in the metallic glass as a combination of spherical periodic order and translational symmetry. 24 They achieved the global structural information by calculating the position ratios between the peaks related to the MRO and the first peak in RDF curves, and found that the ratio values are well consistent with the atomic arrangement in fcc crystals along the dense-packing direction. This is a simple and reasonable method to study the local order structure of liquids or metallic glasses from a global viewpoint.…”

“…24 They represented the MRO structure as a combination of spherical periodic order and local translational symmetry. The atomic arrangement within the MRO prefers the dense-packed positions in fcc or hexagonal close packing (hcp) lattices.…”