Rethinking atomic packing and cluster formation in metallic liquids and glasses

Abstract

In traditional view, atomic packing is random in glasses made of metallic elements with non-directional interactions as the glass-forming liquid needs to be excited to remain in liquid state before being cooled sufficiently fast to a glass. Locally ordered packing however is possible if certain conditions are favorable, such as a strong bonding between elements, or low configuration energy of a cluster of atoms as suggested by Frank. In alloy systems made of different metallic elements, we show that Frank's criterion alone does not necessarily lead to certain specific local ordered packing or cluster formation such as icosahedral packing. In this context, we revisit the issue of atomic packing and cluster formation, and show that an alloy system with fairly random liquid configuration could be sufficient to produce a variety of noticeable locally ordered packing with low energy, albeit largely statistical in nature. Therefore, we emphasize the importance of the system parameters such as the atomic size, alloy concentration, and interaction potential in their collective contribution to local atomic packing. The question of whether or not a liquid should have a locally ordered structure has a significant bearing on many issues in materials science and condensed matter physics. To some extent, stability of a liquid depends on whether or not such an ordered structure exists, so deep undercooling could be achieved [1]. A glass-forming liquid with extended stability is a precursor for good glass forming ability as the cooling rate q = T/t is determined by the temperature gap T= T l T g between the liquidus line T l and the glass transition temperature T g , and   t which is the time taken during cooling across the gap. A critical cooling rate is represented by the slowest q for a system. Obviously, a system with the smallest T would have the slowest cooling rate at a sufficiently long cooling time to avoid crystallization, thus more stable [2]. When local atomic packing with certain noncrystalline order is present, the liquid would have lower configuration energy and therefore a low T l . So the liquid becomes more stable and could be made relatively easily into a glass [1,2]. One such locally ordered packing is icosahedral cluster made of 13 atoms. As proposed by Frank [1], by considering the pairwise interactomic interactions, i.e. the LennardJones (LJ) interaction,  LJ (r), between atoms separated by a distance r, the potential energy of the central atom interacting with the 12 nearest neighbor atoms would be about 8.4% lower if they are packed as an icosahedron than that if they are packed as the face-centered-cubic (fcc) structure. An icosahedron packing requires that all 12 neighboring atoms are located at the vertex points of 3 perpendicular rectangles with the golden ratio. In contrast, an fcc packing has all 12 neighboring atoms sitting at the vertices of 3 perpendicular squares. As a result, the ratio between the nearest neighbor distances in an icosahedron is shorter than that in an fcc by

Structural evolution and kinetics in Cu-Zr metallic liquids from molecular dynamics simulations

“…Furthermore, recent work has shown icosahedral clusters are not even the most energetically stable cluster in metallic glasses. 26 Taken together, these arguments suggest that there is no unique advantage in energetic stability or slowed dynamics specific to the formation of icosahedral order in metallic glasses.…”

Processing dependence of mechanical properties of metallic glass nanowires

“…2 is the variation of the nearest atomic packing with 12 nearest neighbors with five-fold symmetry, or the icosahedral cluster h0,0,12,0i described by Voronoi index. Due to the favorable atomic size ratio, 23 when Cu and Zr are mixed a large number of Cu atoms with surrounding neighbor atoms form h0,0,12,0i cluster, over 10% (see Fig. 2).…”

Chemical composition dependence of atomic oxygen erosion resistance in Cu-rich bulk metallic glasses

“…In order to avoid forming the autoxidation passive film on the test samples in air and mislead the analysis of test results, all the samples have been carefully grinded with fine sandpaper prior to AO exposure. The experimental materials used in this work with the compositions of C 22 …”

“…So far, the field of AO erosion study is largely focused on crystalline materials. Recent decades, bulk metallic glasses (BMGs), new comers of metallic materials, have attracted lots of attention [10][11][12][13][14] due to excellent mechanical and functional properties [15][16][17][18][19][20][21], which are believed to be resulted by their unique structure [22,23]. Since high performance of BMGs make them potential candidates of space materials, their AO erosion resistance is of interest to be investigated.…”