Transmission Electron Microscopy of Iron Metal in Almahata Sitta Ureilite

Almahata Sitta (AS) is a polymict breccia mainly composed of variable ureilite lithologies with small amounts of chondritic lithologies [1]. Fe metal is a common accessory phase in ureilites, but our earlier study on Fe metals in one of AS fragments (#44) revealed a unique mineralogy never seen in other ureilites [2,3]. In this abstract we report detailed transmission electron microscopy (TEM) on these metal grains to better understand the thermal history of ureilites. We prepared FIB sections of AS#44 by JEOL JIB-4000 from the PTS that was well characterized by SEM-EBSD in our earlier study [2]. The sections were then observed by STEM (JEOL JEM- 2100F). One of the FIB sections shows a submicron-sized symplectic intergrown texture composed of Fe metal (kamacite), Fe carbide (cohenite), Fe phosphide (schreibersite), and Fe sulfide (troilite). Each phase has an identical SAED pattern in spite of its complex texture, suggesting co-crystallization of all phases. This is probably caused by shock re-melting of pre-existing metal + graphite to form a eutectic-looking texture. The other FIB section is mostly composed of homogeneous Fe metal (93 wt% Fe, 5 wt% Ni, and 2 wt% Si), but BF-STEM images exhibited the presence of elongated lathy grains (approx. 2 microns long) embedded in the interstitial matrix. The SAED patterns from these lath grains could be indexed by alpha-Fe (bcc) while interstitial areas are gamma-Fe (fcc). The elongated alpha-Fe grains show tweed-like structures suggesting martensite transformation. Such a texture can be formed by rapid cooling from high temperature where gamma-Fe was stable. Subsequently alpha-Fe crystallized, but gamma-Fe remained in the interstitial matrix due to quenching from high temperature. This scenario is consistent with very rapid cooling history of ureilites suggested by silicate mineralogy

Zr60Al15(Ni,Cu)(25) noncrystalline alloys created by referring to ionic arrangements of a garnet structure with molecular dynamics simulations based on a plastic crystal model

International audienceThe Zr60Al15(Ni,Cu)(25) noncrystalline alloys with their initial crystalline states from the ionic arrangements of a Fe3Al2Si3O12 garnet structure were created with molecular dynamics simulations based on a plastic crystal model (MD-PCM). The analyses with pair-distribution function revealed that the randomly-rotated octahedral clusters around the Cu sites or tetrahedral clusters around the Ni sites and subsequent annealing with MD-PCM make the Zr60Al15(Ni,Cu)(25) crystalline alloys to vitrify. The interference function indicated that the Zr60Al15Ni25 noncrystalline alloy created through rotating operation for the octahedral clusters gives the best fit with the experimental data in an as-quenched state. Crystallographic analyses indicated that the prototype of the garnet structure, Weaire-Phelan (A15) structure, gives inhomogeneous Wigner-Seitz cell for the solute elements and an optimized heat of mixing for atomic pairs in the Zr60Al15(Ni,Cu)(25) noncrystalline alloys. These crystallographic features due to Weaire-Phelan structure are a reason for the Zr60Al15(Ni,Cu)(25) alloys to have high glass-forming ability

Structure of an Al-Fe-Ni Decagonal Quasicrystal Studied by Cs-Corrected STEM

The structure of an Al-Fe-Ni decagonal quasicrystal with two quasiperiodic planes along the periodic axis in an $Al_{72}Ni_{24}Fe_4$ alloy has been examined by spherical aberration (Cs)-corrected scanning transmission electron microscopy with high-angle annular dark-field and annular bright-field techniques. The transition-metal atoms and mixed sites (MSs) of Al and transition-metal atoms are represented as separated bright dots in the observed high-angle annular dark-field scanning transmission electron microscopy images, and consequently the arrangements of transition-metal atoms and mixed sites on the two quasiperiodic planes can be directly determined. The transition-metal atoms are arranged on a pentagonal tiling of an edge-length of 0.76 nm. The close examination of observed annular bright-field- and high-angle annular dark-field scanning transmission electron microscopy images indicates the existence of large decagonal columnar clusters with 3.2 nm diameter, and their arrangement on pentagonal, thin rhombic and squashed hexagonal tiles with an edge-length of 3.2 nm. The arrangements of transition-metal atoms in these three tiles are placed on an ideal pentagonal tiling with an edge-length of 3.2 nm, which is generated by the projection of a five-dimensional hyper-cubic lattice. The vertices are denoted by 5D hyper-cubic indices and then they are projected on the occupation domains in perpendicular space. The arrangement of Al atoms as well as transition-metal atoms and mixed sites in the large decagonal atom cluster with about 3.2 nm diameter is interpreted from the observed high-angle annular dark-field- and annular bright-field scanning transmission electron microscopy ABF-STEM images

Noncrystalline structure created through ensemble of clusters in metastable cubic Zr2Ni structure by their random rotations and subsequent annealing

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