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11 records
All sourcesNASA NTRSDOE OSTI
DOE OSTI2020

Deformation-enhanced hierarchical multiscale structure heterogeneity in a Pd-Si bulk metallic glass

Here, the multiscale structures in a Pd 82 Si 18 binary bulk metallic glass before and after deformation were studied using electron microscopies, high-energy synchrotron X-ray diffraction, and small-angle scattering techniques. The experimental results revealed an enhancement of hierarchical structure heterogeneities on multiple length scales after deformation. Hierarchical multiple shear bands of high number density were observed after bending, introducing complex but periodically distributed residual strain. Pair distribution function analysis revealed that the connectivity of the short-range clusters on the medium-range scale determines the packing density difference between the tension side and the compression side in the sample after bending. In-situ synchrotron X-ray diffraction study also revealed a transformation of connection modes among short-range clusters under uniaxial tension and compression, which is consistent with those of triaxial tension/compression parts upon bending in Pd 82 Si 18 glassy alloys. The nanoscale heterogeneities for metallic glasses after deformation observed by small-angle scattering and transmission electron microscopy may be attributed to the nanoscale amorphous phase separation and interacting multiple shear bands enhanced by plastic deformation. Our findings suggested that the enhancement of hierarchical heterogeneous structure on multiple length scales may explain the excellent plasticity of Pd-Si glassy alloys, deepening the understanding of structure-property relation during plastic deformation in metallic glasses.

36 MATERIALS SCIENCE↗
NASA NTRS1982

Crystal nucleation in Pd-Si alloys

A study of the crystal phase nucleation in undercooled droplets of Pd-Si alloys with composition near the Pd(84.5)Si(15.5) eutectic composition is reported. Molten droplets are released at the top of a drop tube and solidify (to either a crystalline or glassy state) during descent. This provides a containerless (and nearly gravity free) environment so that nucleation due to container walls or vibrations is eliminated. It is found that crystallization, due to homogeneous nucleation, is bypassed in droplets of 1 mm diameter when cooled at 760 K/sec. From this an upper limit of the homogeneous nucleation rate is estimated. Results are compared with a previously published study of nucleation in 0.06 mm to 0.33 mm diameter droplets, which indicated that nucleation results from heterogeneous surface nucleation and that the number of these nuclei is dependent on the atmosphere in the drop tube.

Drehman, A. J.↗
NASA NTRS1978

Superconductivity in ternary molybdenum sulfides

Three research papers are presented: (1) Superconductivity in Th-Zr Alloys; (2) Superconductivity in Pd-Si-H(D) alloys; and (3) Low Temperature Specific Heat of Amorphous Pd-Si Alloys.

Luo, H. L.↗
NASA NTRS1982

The effect of solute on the homogeneous crystal nucleation frequency in metallic melts

A complete calculation that extends the classical theory for crystal nucleation in pure melts to binary alloys has been made. Using a regular solution model, approximate expressions have been developed for the free energy change upon crystallization as a function of solute concentration. They are used, together with model-based estimates of the interfacial tension, to calculate the nucleation frequency. The predictions of the theory for the maximum attainable undercooling are compared with existing experimental results for non-glass forming alloys. The theory is also applied to several easy glass-forming alloys (Pd-Si, Au-Si, Fe-B) for qualitative comparison with the present experimental experience on the ease of glass formation, and for assessment of the potential for formation of the glass in bulk.

Thompson, C. V.↗
NASA NTRS1984

Crystal nucleation and glass formation in metallic alloy melts

Homogeneous nucleation, containerless solidification, and bulk formation of metallic glasses are discussed. Homogeneous nucleation is not a limiting factor for metallic glass formation at slow cooling rates if the reduced glass transition temperature is high enough. Such glasses can be made in bulk if heterogeneous nucleants are removed. Containerless processing eleminates potential sources of nucleants, but as drop tube experiments on the Pd-Si alloys show, the free surface may still be a very effective heterogeneous nucleant. Combination of etching and heating in vacuum or fluxing can be effective for cleaning fairly large ingots of nucleants. Reduced gravity processing has a potentially useful role in the fluxing technique, for example to keep large metallic ingots surrounded by a low density, low fluidity flux if this proved difficult under ground conditions. For systems where heterogeneous nucleants in the bulk of the ingot need gravity to segregate to the flux-metal interface, reduced gravity processing may not be appropriate for bulk glass formation.

Spaepen, F.↗
DOE OSTI2020

Materials Data on SiPd3 by Materials Project

Pd3Si is Cementite structured and crystallizes in the orthorhombic Pnma space group. The structure is three-dimensional. there are two inequivalent Pd sites. In the first Pd site, Pd is bonded in a bent 120 degrees geometry to two equivalent Si atoms. There are one shorter (2.38 Å) and one longer (2.40 Å) Pd–Si bond lengths. In the second Pd site, Pd is bonded in a 3-coordinate geometry to three equivalent Si atoms. There are a spread of Pd–Si bond distances ranging from 2.38–2.60 Å. Si is bonded in a 8-coordinate geometry to eight Pd atoms.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Materials Data on SiPd by Materials Project

PdSi is Modderite structured and crystallizes in the orthorhombic Pnma space group. The structure is three-dimensional. Pd4+ is bonded to six equivalent Si4- atoms to form a mixture of distorted face, edge, and corner-sharing PdSi6 pentagonal pyramids. There are a spread of Pd–Si bond distances ranging from 2.46–2.60 Å. Si4- is bonded in a 6-coordinate geometry to six equivalent Pd4+ atoms.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Materials Data on SiPd2 by Materials Project

Pd2Si crystallizes in the hexagonal P-62m space group. The structure is two-dimensional and consists of one Pd3Si ribbon oriented in the (0, 0, 1) direction and one Pd3Si2 sheet oriented in the (0, 0, 1) direction. In the Pd3Si ribbon, Pd2+ is bonded in a 2-coordinate geometry to two equivalent Si4- atoms. Both Pd–Si bond lengths are 2.39 Å. Si4- is bonded in a 6-coordinate geometry to six equivalent Pd2+ atoms. In the Pd3Si2 sheet, Pd2+ is bonded in a bent 150 degrees geometry to two equivalent Si4- atoms. Both Pd–Si bond lengths are 2.37 Å. Si4- is bonded in a distorted trigonal planar geometry to three equivalent Pd2+ atoms.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Materials Data on SiPd2 by Materials Project

Pd2Si crystallizes in the hexagonal P-62m space group. The structure is three-dimensional. there are two inequivalent Pd2+ sites. In the first Pd2+ site, Pd2+ is bonded to four Si4- atoms to form PdSi4 tetrahedra that share corners with ten equivalent PdSi4 tetrahedra, corners with six equivalent PdSi5 trigonal bipyramids, edges with two equivalent PdSi4 tetrahedra, and edges with six equivalent PdSi5 trigonal bipyramids. There are two shorter (2.43 Å) and two longer (2.47 Å) Pd–Si bond lengths. In the second Pd2+ site, Pd2+ is bonded to five Si4- atoms to form distorted PdSi5 trigonal bipyramids that share corners with six equivalent PdSi4 tetrahedra, corners with ten equivalent PdSi5 trigonal bipyramids, edges with six equivalent PdSi4 tetrahedra, and edges with six equivalent PdSi5 trigonal bipyramids. There are one shorter (2.58 Å) and four longer (2.67 Å) Pd–Si bond lengths. There are two inequivalent Si4- sites. In the first Si4- site, Si4- is bonded in a 9-coordinate geometry to nine Pd2+ atoms. In the second Si4- site, Si4- is bonded in a 9-coordinate geometry to nine Pd2+ atoms.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Materials Data on SiPd4 by Materials Project

Pd4Si crystallizes in the trigonal R-3m space group. The structure is three-dimensional. there are two inequivalent Pd sites. In the first Pd site, Pd is bonded in a 12-coordinate geometry to three equivalent Pd and three equivalent Si atoms. All Pd–Pd bond lengths are 2.86 Å. All Pd–Si bond lengths are 2.46 Å. In the second Pd site, Pd is bonded to twelve Pd atoms to form a mixture of corner, edge, and face-sharing PdPd12 cuboctahedra. There are three shorter (2.79 Å) and six longer (2.87 Å) Pd–Pd bond lengths. Si is bonded in a 6-coordinate geometry to six equivalent Pd atoms.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Materials Data on SiPd by Materials Project

PdSi crystallizes in the tetragonal I4/mcm space group. The structure is three-dimensional. Pd4+ is bonded in a distorted pentagonal planar geometry to five equivalent Si4- atoms. There are four shorter (2.46 Å) and one longer (2.47 Å) Pd–Si bond lengths. Si4- is bonded in a 5-coordinate geometry to five equivalent Pd4+ atoms.

36 MATERIALS SCIENCE↗