Amorphous Alloy

The density of amorphous materials is just a few per cent lower than that of crystals of the same composition the short-range order in both is expected to be comparable. An illustrative example of the occurrence of short-range order in an amorphous material is provided by silicate glasses. 

Amorphous solids can be characterised to some extent by means of the radial distribution function, RDF(r) (Luborsky, 1980), which describes the average number of atoms between r and (r -I- dr), with the origin taken at any particular atom. The average number of atoms is then 4nr P r) dr, where P(r) is the atomic distribution obtained from neutron or x-ray experiments  

A pair correlation function is sometimes used, which is related to the atomic distribution function by  

Interference and radial distribution functions for Fe8oPi3C7 amorphous alloys are shown in Fig. 6.16 (Note compositions in amorphous alloys are usually given as atom percentages). 

Amorphous materials are metastable phases, since the crystalline state corresponds to the thermodynamic equilibrium state. To obtain an amorphous material, a drastic cooling from the liquid or the gas phase down to a temperature where diffusion is negligible has to be performed to avoid crystallisation. Fig. 6.17. Experimentally, a number of curves such as (a) and (b) are obtained as a function of the cooling rate, which suggests that different disordered atomic arrangements may exist. 

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Ion implantation (qv) has a large (10 K/s) effective quench rate (64). This surface treatment technique allows a wide variety of atomic species to be introduced into the surface. Sputtering and evaporation methods are other very slow approaches to making amorphous films, atom by atom. The processes involve deposition of a vapor onto a cold substrate. The buildup rate (20 p.m/h) is also sensitive to deposition conditions, including the presence of impurity atoms which can faciUtate the formation of an amorphous stmcture. An approach used for metal—metalloid amorphous alloys is chemical deposition and electro deposition. 

Amorphous Silicon. Amorphous alloys made of thin films of hydrogenated siUcon (a-Si H) are an alternative to crystalline siUcon devices. Amorphous siUcon ahoy devices have demonstrated smah-area laboratory device efficiencies above 13%, but a-Si H materials exhibit an inherent dynamic effect cahed the Staebler-Wronski effect in which electron—hole recombination, via photogeneration or junction currents, creates electricahy active defects that reduce the light-to-electricity efficiency of a-Si H devices. Quasi-steady-state efficiencies are typicahy reached outdoors after a few weeks of exposure as photoinduced defect generation is balanced by thermally activated defect annihilation. Commercial single-junction devices have initial efficiencies of ca 7.5%, photoinduced losses of ca 20 rel %, and stabilized efficiencies of ca 6%. These stabilized efficiencies are approximately half those of commercial crystalline shicon PV modules. In the future, initial module efficiencies up to 12.5% and photoinduced losses of ca 10 rel % are projected, suggesting stabilized module aperture-area efficiencies above 11%. 

Only about 10 elements, ie, Cr, Ni, Zn, Sn, In, Ag, Cd, Au, Pb, and Rh, are commercially deposited from aqueous solutions, though alloy deposition such as Cu—Zn (brass), Cu—Sn (bronze), Pb—Sn (solder), Au—Co, Sn—Ni, and Ni—Fe (permalloy) raise this number somewhat. In addition, 10—15 other elements are electrodeposited ia small-scale specialty appHcations. Typically, electrodeposited materials are crystalline, but amorphous metal alloys may also be deposited. One such amorphous alloy is Ni—Cr—P. In some cases, chemical compounds can be electrodeposited at the cathode. For example, black chrome and black molybdenum electrodeposits, both metal oxide particles ia a metallic matrix, are used for decorative purposes and as selective solar thermal absorbers (19). 

T. Egami, Magnetic amorphous alloys. Report. Prog. Phys., 47 1601 (1984). 

An isotropic and non-magnetie amorphous alloy has only two independent second order elastic constants. The other elastic moduli are related through the equations [30] ... 

The Debye temperature of the bulk amorphous alloys was calculated from the relation ... 

Table 3 Room-temperature elastic constants, density, and the Debye temperature p of a number of Pd-Ni-P and Pd-Cu-P bulk amorphous alloys. The elastic moduli are in units of GPa and the density p is in units of g/cm. ...

Amorphous alloys stable at ambient and higher temperatures consist of at least two components without any long-range atomic order. They are produced by a variety of constituents from the gas, liquid and aqueous phases. Vitrification of metal surfaces is also caused by destruction of the long-range atomic order in the surfaces of solid metals. 

The corrosion behaviour of amorphous alloys has received particular attention since the extraordinarily high corrosion resistance of amorphous iron-chromium-metalloid alloys was reported. The majority of amorphous ferrous alloys contain large amounts of metalloids. The corrosion rate of amorphous iron-metalloid alloys decreases with the addition of most second metallic elements such as titanium, zirconium, vanadium, niobium, tantalum, chromium, molybdenum, tungsten, cobalt, nickel, copper, ruthenium, rhodium, palladium, iridium and platinum . The addition of chromium is particularly effective. For instance amorphous Fe-8Cr-13P-7C alloy passivates spontaneously even in 2 N HCl at ambient temperature ". (The number denoting the concentration of an alloy element in the amorphous alloy formulae is the atomic percent unless otherwise stated.)... 

The passive films formed by the addition of sufficient amounts of valve metals to amorphous nickel-valve-metal alloys are exclusively composed of valve-metal oxyhydroxides or oxides such as TaOjCOH) , Nb02(OH) or TajO,. Consequently, amorphous alloys containing strongly passivating elements, such as chromium, niobium and tantalum, have a very high ability... 

Table 3.S9 Concentration of chromic ion in passive films formed on amorphous alloys and stainless steels in I n HCI at ambient temperature...

The high corrosion resistance of amorphous alloys disappears on heat treatment that produces crystallisation . Figure 3.71 shows an example of the... 

Amorphous alloys are in a thermodynamically metastable state, and hence essentially they are chemically more reactive than corresponding thermodynamically stable crystalline alloyIf an amorphous alloy crystallises to a single phase having the same composition as the amorphous phase, crystallisation results in a decrease in the activity of the alloy related to the active dissolution rate of the alloy . 

Since amorphous alloys can be regarded as metallic solids with a frozen-in melt structure, the liquid structure freezes at different temperatures... 

As can be seen in Fig. 3.67, the corrosion resistance of amorphous alloys changes with the addition of metalloids, and the beneficial effect of a metaU loid in enhancing corrosion resistance based on passivation decreases in the order phosphorus, carbon, silicon, boron (Fig. 3.72). This is attributed partly to the difference in the speed of accumulation of passivating elements due to active dissolution prior to passivation... 

Stress-corrosion cracking based on active-path corrosion of amorphous alloys has so far only been found when alloys of very low corrosion resistance are corroded under very high applied stresses . However, when the corrosion resistance is sufficiently high, plastic deformation does not affect the passive current density or the pitting potential , and hence amorphous alloys are immune from stress-corrosion cracking. 

The oxidation behaviour of amorphous alloys studied below their crystallisation temperature is not greatly different from that of crystalline metals, although the presence of large amounts of metalloids complicates the situation . ... 

The thickness of amorphous alloys is dependent upon production methods. Rapid quenching from the liquid state, which is the most widely used method, produces generally thin amorphous alloy sheets of 10-30 tm thickness. This has been called melt spinning or the rotating wheel method. Amorphous alloy powder and wire are also produced by modifications of the melt spinning method. The corrosion behaviour of amorphous alloys has been studied mostly using melt-spun specimens. 

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