Amorphous atomic diffusivity

SSAR is observed when the binary diffusion couples listed in Table 2.4 are heated to an appropriate reaction temperature, TR. Examples of typical values of JR are given in Table 2.4. It is well known that amorphous metallic alloys tend to crystallize in laboratory timescales upon heating to temperatures close to their glass-transition temperature, T% [2.16]. For a typical practical timescale (e.g., minutes), one can define crystallization temperature as the temperature at which a significant fraction of an amorphous sample undergoes crystallization in the specified time. The time required for an amorphous phase to crystallize can be identified with t 2 of Fig. 2.6 (see discussion in Sect 2.1.3). In the low temperature regime (well below Tg), atomic diffusion in amorphous alloys is... 

Transition metals in amorphous samples exhibit a direct interstitial diffusion behavior which was retarded by temporary trapping at defects that were intrinsic to the amorphous structure. Diffusion was investigated here by means of Rutherford back-scattering spectrometry. It was found that the data could be fitted by using foreign-atom interstitial diffusion coefficients for crystaHine Si modified by the presence of... 

As silicon nitride has a higher density than amorphous silicon oxide, it will serve as a better barrier to metal atom diffusion, and consequently it is used in VLSI production to prevent the cross-penetration of dopant atoms from one region of the device to another. During the manufacture of devices based on gallium arsenide, silicon nitride coatings will prevent the evaporation of (toxic) arsenic. Further information on the use of silicon nitride in semiconductor processing is available elsewhere (Belyi et al., 1988). 

The discussion up to this point has focused on the role of free surfaces and internal interfaces, such as grain boundaries, in mass diffusion. Surfaces produced internally in the material as a consequence of permanent deformation and damage induced by stress can also serve, in some cases, as paths along which enhanced atomic diffusion may occur. In amorphous solids undergoing active plastic flow, such increased atomic mobility along shear bands can result in the formation of nanocrystalline particles locally at the bands. An example of such crystallization process is illustrated in this section for the case of a bulk amorphous metallic alloy subjected to quasi-static nanoindentation at room temperature. 

Point defects have a significant role in intermetallic compounds, as they control many properties of technological importance, such as atomic diffusion, high-temperature creep and other mechanical properties, sintering, behavior under irradiation, and in particular irradiation-induced crystalline-to-amorphous transitions. Introduction of point defects by irradiation has even allowed one to obtain an ordered phase (FeNi) in a system where it was hindered by the low atomic mobility (Neel et al., 1964 Koczak et ai, 1971). 

The possibility to prepare amorphous alloys via charging with hydrogen gas was discussed by Buschow and Beckmans (1979). These authors argued that the disappearance of sharp reflection lines after charging is due to the formation of microcrystalline decomposition products rather than to the formation of amorphous alloys. Diffusion of metal atoms is a step necessary to bring about phase separation after, or better, during charging. The possibility of metal atom diffusion would, however, lead to spontaneous crystallization if the alloys were amorphous. 

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