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Amorphous materials are also called amorphous or glassy materials, which are a large class of rigid solids with high hardness and high viscosity comparable to crystalline materials (generally at 10 poise, that is, above 1 Pa·s, is 10 times the viscosity coefficient of a typical fluid). However, the spatial arrangement of its constituent atoms and molecules does not show periodicity and translational symmetry, and the long-term program of the crystal state is destroyed; it is only due to the interconnection between atoms that it is within a small area of several atomic (or molecular) diameters. Has a short program. Since there is no effective experimental method to accurately determine the atomic structure of amorphous materials, the above definitions are relative.
Amorphous materials have three basic characteristics.
① There are only short programs in small intervals, without any long programs; the wave vector k is no longer a good quantum number to describe the state of motion (see energy bands of solids).
② Its electron diffraction, neutron diffraction and X-ray diffraction patterns are composed of wider halos and diffuse rings; no diffraction contrast formed by grain boundaries, crystal defects, etc. can be seen with an electron microscope.
③ The amorphous solid of any system is metastable compared with its corresponding crystalline material. When the temperature is continuously raised, in a narrow temperature range, there will be obvious structural changes, from amorphous to crystalline. This crystallization process mainly depends on the atomic diffusion coefficient, interface energy and melting entropy of the material.
There are many methods for preparing amorphous materials, the most common are melt quenching and deposition from the vapor phase (such as evaporation, ion sputtering, glow discharge, etc.). In recent years, new technologies such as ion bombardment, strong laser radiation and high-temperature detonation have been developed, and large-scale continuous production has been possible.
Some liquids of sufficient viscosity obtain their glassy state upon rapid cooling. In 1960, P. Duweis and others used a high cooling rate to develop the traditional glass technology to metals and alloys to make corresponding amorphous materials, which are called metallic glasses or glassy metals. When the RF heating coil melts the sample, the valve is opened, and the pressurized air flow (such as He, N, Ar, etc.) breaks through the polyester diaphragm, so that the sample is rapidly sprayed from the nozzle at the lower end of the quartz crucible to the cooling copper block, and the cooling speed can reach Above 10K/s to obtain its amorphous state. Except for a few alloys that are relatively easy to form a glass state (such as Pd-Cu-Si, Pd-Ni-P, Pt-Ni-P, etc.), the cooling rate of most metallic glasses is quite high, generally 10~10K/s , the thickness is within 50 μm, and some amorphous fine particles within tens of microns are first made, and then pressed into a bulk amorphous alloy. It is generally believed that pure metals cannot be rapidly cooled from a liquid state to a glass state at the current cooling rate of 10-10K/s. Therefore, all glassy metals currently contain two or more components. Most glassy alloys have two components, one part is a strong metallic element, such as Cu, Ag, Au or transition metal Fe, Co, Ni, Pd, Pt; the other part is a non-metallic, metalloid element, such as trivalent B, 4-valent C, Si, Ge, 5-valent P. The sum of the former accounts for about 70~80at% (atomic percentage), and the latter accounts for about 20at%. Such a composition ratio can be explained by the Bernard polyhedron model of an amorphous solid. The most easily amorphous component is near the eutectic point of the alloy phase diagram, and its corresponding melting temperature is low.