Structure of amorphous alloy

INVESTIGATION OF HYDROGEN INFLUENCE ON STRUCTURE OF AMORPHOUS ALLOYS Mg-Cu-Y AND Mg-Ni-Y BY NEW METHOD OF DIFFRACTION FROM ATOMIC COORDINATE SPHERES... 

X-ray electron and neutron diffraction have all been employed in studying the structure of amorphous alloys. Although the concepts and procedures outlined below are made with reference to X-ray diffraction, they can easily be extended to neutron diffraction and electron diffraction. 

A substantial amount of effort has been spent on finding model descriptions of the atomic scale structure of amorphous alloys. Such three-dimensional models have attempted to provide concrete though idealized pictures of the arrangements of the atoms that go beyond the information that can usually be obtained from experimental radial distribution functions. The most prominent among them are microcrystalline and cluster models, and models based on the dense random packing of hard spheres (DRPHS). 

Amorphous NiP alloys with > 10% P (generally obtained by deposition from acidic electrolytes) are non-magnetic (see [66] and references therein), as required of the underlayer for thin-film media. Although the structure of these alloys is generally assumed to be a solid solution of P in Ni, a recent report [67] has suggested that NiP with 7.4-10% P deposited from acid sulfate electrolytes is better represented by a microcrystalline structure composed of 4-5 nm fee NiP solid-solution grains. 

Another characteristic point is the special attention that in intermetallic science, as in several fields of chemistry, needs to be dedicated to the structural aspects and to the description of the phases. The structure of intermetallic alloys in their different states, liquid, amorphous (glassy), quasi-crystalline and fully, three-dimensionally (3D) periodic crystalline are closely related to the different properties shown by these substances. Two chapters are therefore dedicated to selected aspects of intermetallic structural chemistry. Particular attention is dedicated to the solid state, in which a very large variety of properties and structures can be found. Solid intermetallic phases, generally non-molecular by nature, are characterized by their 3D crystal (or quasicrystal) structure. A great many crystal structures (often complex or very complex) have been elucidated, and intermetallic crystallochemistry is a fundamental topic of reference. A great number of papers have been published containing results obtained by powder and single crystal X-ray diffractometry and by neutron and electron diffraction methods. A characteristic nomenclature and several symbols and representations have been developed for the description, classification and identification of these phases. 

Tsu DV, Chao BS, Ovshinsky SR, Guha S, Yang J (1997). Effect of hydrogen dilution on the structure of amorphous silicon alloy. Appl Phys Lett 71 1317-1319... 

Amorphous Ni-(40-x) at% Zr-x at% RE (x = 0, 1, 5 and 10 RE = Y, Ce and Sm) alloy ribbons of about 1 mm width and about 20 pm thickness were prepared by a single-roUer melt spinning method. The structure of the alloys prepared was confirmed by X-ray diffraction with Cu K radiation. The amorphous alloy ribbons were oxidized at 773 K in air for 5 hours and then reduced at 573 K imder flowing hydrogen for 5 hours. During this treatment the amorphous aUoys transformed to nickel catalysts supported on zirconia or zirconia-rare earth element oxides. 

CH1/SHE] Chizhevskaya, S. N., Shelimova, L. E., Se-Te phase diagram and structures of amorphous and crystalline Sej. Tex alloys A critical review, Russ. J. Inorg. Chem., 42, (1997), 741-750. Cited on page 183. 

The structure of amorphous metals, quasicrystals, and crystalline inter-metallic compounds can be modelled by atom clusters with icosahedral arrangement [3.113-117]. The differences between the various phases result from a different arrangement of the individual atom clusters. Therefore, it is evident that there exists a close relation between the different states of matter, and that the different phases corresponding to minima of the free enthalpy can be quite easily transformed into each other. For example, rapid cooling from the melt results in an amorphous alloy for high quenching rates, and a quasicrystalline... 

Differential thermal analysis (DTA) and differential scanning calorimetry (DSC) are also useful methods for structure determination. These methods can detect crystallization of amorphous alloy catalysts as a result of heat treatment (21, 23, 41-44) or as a result of the action of reacting gases, such as in the case of hydrogenation of carbon monoxide (53) or ammonia synthesis (22). 

Beside the beneficial effect of the addition alloying metallic elements that contribute to the increased corrosion resistance, the amorphous structure itself is also responsible for the very low corrosion. For example, crystalline alloys with the same composition exhibit high rates of dissolution. The chemically homogeneous, single-phase nature of amorphous alloys is believed to account for their corrosion resistance (8, 100, 101). This also allows for the formation of a uniform, protective film on the surface of amorphous alloy electrodes. 

The demands of practical applications led to attempts to overcome the high electric resistance of thin ribbons by a new technical solution of laser-induced surface vitrification (105, 106). First an amorphous alloy ribbon was adhered uniformly to a nickel plate by heat treatment. Subsequently, this surface alloy layer was transformed to the amorphous structure by laser surface melting and self-quenching (107). A sample consisting of Pd56Rh23P oSi9 adhered to bulk crystalline nickel exhibited anodic characteristics very similar to those of the melt-spun amorphous ribbon (102). Clearly, similar improvements forced by practical demands will be a part of the future use of amorphous alloys. 

A.M. Gleser and B.V. Molotilov, Structure and Mechanical Properties of Amorphous Alloys, Metallurgiya, Moscow, Russia, 1992. [In Russian]... 

Results of investigations by Favaron et al. (1980) make it clear that the system is less favourable with regard to formation of amorphous alloys. The amorphous state can be obtained free from crystalline material only in a narrow range close to a = 0.30. At other concentrations the amorphous phase is contaminated with crystalline material of the fee structure (ySj) or with the crystalhne phases of the composition LaCu (C,). As indicated in the top part of fig. 79, no amorphous phase is present when x > 0.40. The concentration dependence of Tq and the corresponding transition widths are shown in the main part of fig. 79. Note that there is a similar correspondence between these widths and the microstructural composition of the samples, as was also found for La. Au (fig. 77). 

The photoemission studies so far reported in the literature comprise crystalline and amorphous alloys of early (Tg) and late (Tl) transition metals with either Zr (Amamou, 1980 Oelhafen et al., 1979, 1980) or Gd (Giintherodt and Shevchik, 1975 Shen et al., 1981). These studies have contributed significantly to the understanding of the electronic structure of amorphous as well as crystalline alloys, especially now that the results of band structure calculations have become available for most of the materials investigated (Kiibler et al., 1981 Oelhafen et al. (1982), Moruzzi et al., 1983). 

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