Precipitation of binary quasicrystals along dislocations

Abstract

Dislocations in crystals naturally break the symmetry of the bulk, introducing local atomic configurations with symmetries such as fivefold rings. But dislocations do not usually nucleate aperiodic structure along their length. Here we demonstrate the formation of extended binary quasicrystalline precipitates with Penrose-like random-tiling structures, beginning with chemical ordering within the pentagonal structure at cores of prismatic dislocations in Mg–Zn alloys. Atomic resolution observations indicate that icosahedral chains centered along [0001] pillars of Zn interstitial atoms are formed templated by the fivefold rings at dislocation cores. They subsequently form columns of rhombic and elongated hexagonal tiles parallel to the dislocation lines. Quasicrystalline precipitates are formed by random tiling of these rhombic and hexagonal tiles. Such precipitation may impact dislocation glide and alloy strength.

A decagonal quasicrystal with rhombic and hexagonal tiles decorated with icosahedral structural units

“…The intensities of image spots located at the vertices are about twice those at the mid-edges. These characteristic parameters of fat rhombic and flattened hexagonal tiles in the present Zn-Mg-Y quasicrystal are as the same as those in the F-K Mg 4 Zn 7 crystal (Abe et al, 1999;Yang et al, 2018). These brighter dots at the vertices correspond to central Zn atomic columns of icosahedral chains projected along their pseudo-fivefold axis in F-K compound crystals ( five fat rhombic tiles can be observed, while a small number of flattened hexagonal tiles are also grouped in fives, forming star motifs in the Zn-Mg-Y quasicrystal.…”

“…Short 'zigzag' packing of several fat rhombic tiles is present randomly in the Zn-Mg-Y quasicrystal, in addition to the star motifs, as shown in Figs. 3(b) and 3(c), which is similar to Mg-Zn nanoprecipitates (Yang et al, 2018). In addition to fat rhombic and flattened hexagonal tiles, one boat tile can be observed, as indicated by the green square in Fig.…”

“…However, a very short distance of the short diagonal (a/ < a) of skinny rhombic tiles will induce overlap of two icosahedrons, which has not been observed in the F-K compounds. Triangular face sharing of icosahedrons in fat rhombi and flattened hexagons is observed frequently in the Mg-Zn Laves phase (Frank & Kasper, 1958;Wang et al, 1986;Ye et al, 1985;Kuo et al, 1986;Yang et al, 2018. In addition, calculations also indicate that fat rhombic and flattened hexagonal tiles with icosahedrons located at vertices have energies lower than other arrangements of icosahedrons (Mihalkovič et al, 2014).…”

“…In the case of Mg-Zn Laves phases, atomic columns within fat rhombic and flattened hexagonal units consist purely of Mg atoms; therefore, they could not produce image spots showing intensity high enough in the HAADF-STEM image, due to the low atomic number of Mg (Yang et al, 2018;Zhang et al, 2019). Interestingly, an intensity spot can be observed at one of the two sites which is expected to be occupied by Mg in Laves Mg-Zn crystals in some of the fat rhombic tiles, indicating the occurrence of substitution of heavy elements for Mg at such lattice sites.…”

“…For Al-TM decagonal quasicrystals, their structure, physical properties, thermodynamic stability and chemical bonding have long been investigated (Steurer & Kuo, 1990;Hiraga et al, 1991Hiraga et al, , 1993Hiraga et al, , 1994Abe et al, 2003). Icosahedral clusters are ubiquitous in Mg-Zn Laves phases (such as MgZn 2 and Mg 4 Zn 7 ), and juxtaposed packing of icosahedral chains is a characteristic feature of F-K intermetallic compounds (Frank & Kasper, 1958;Wang et al, 1986;Ye et al, 1985;Kuo et al, 1986;Yang et al, 2018;Zhang et al, 2018). However, studies on the F-K class of decagonal quasicrystals have far fewer outputs than the Al-TM group so far.…”