TRANSPARENT METAL

AND NEW POSSIBILITIES FOR OPTICAL

STRESS ANALYSIS

One of the most powerful methods for studying stresses arising in various structural components is the optical method, which consists in observing the optical activity of a specimen subjected to an external action. Because metals are opaque, such studies are inevitably carried out on models made of a transparent material. Since amorphous materials are used for modeling (glass, plastics, etc.), the range of questions accessible to study by this method is limited to problems in which the structure of the metal can be neglected and the metal can be regarded as an isotropic homogeneous body. Thus such very important phenomena as hardening, fatigue, creep, residual stresses, recrystallization, and also all questions of metalworking, both hot and cold, fall outside consideration. Therefore, the most urgent task, from the standpoint of the further development of the method toward directly satisfying the needs of production, is the search for transparent materials possessing a polycrystalline structure similar to the structure of metals, mechanical properties close to those of metals. This task has been successfully solved at the Leningrad Physicotechnical Institute of the Academy of Sciences.

Fig. 1. A rod of silver chloride tied by hand into a knot.

From Current Literature

USSR by A. V. Stepanov*). As early as 1933 he established that the halide salts of silver and thallium and various alloys based on them fully satisfy these requirements.

From the standpoint of microstructure and mechanical properties, these substances—of which silver chloride, AgCl, is the typical representative—completely imitate metals and, being transparent, may with full justification be called “transparent metals.”

According to the investigations of A. V. Stepanov, silver chloride, crystallizing in the cubic system, forms homogeneous transparent polycrystalline specimens that readily lend themselves to all types of hot and cold working used for metals, including casting, stamping, annealing, cutting, etc. Likewise, as in the case of metals, by varying the methods of treatment one can visibly alter the crystalline structure of a specimen made of AgCl and thereby its mechanical properties.

A detailed study of the mechanical properties of silver chloride showed that at room temperature they are analogous to the properties of copper weakened by approximately a factor of ten. Correspondingly, in outward behavior it resembles lead—it bends easily, is scratched with a fingernail, and so on. Fig. 1 shows a photograph of an annealed polycrystalline rod of silver chloride, 3 mm in diameter, tied by hand into a knot at room temperature. Like metals, silver chloride also has the ability to harden as a result of plastic deformation (work hardening), which can be removed either by annealing, or as a result of simple stress relief (recovery), or as a result of recrystallization. At room temperature, the processes leading to the removal of work hardening are practically absent; preservation of work hardening was observed over the course of two years.

The Young’s modulus of a pressed specimen in the work-hardened state is about 4500 kg/mm².

In the absence of stresses, crystals of silver chloride are optically isotropic. Under the action of external loads, double refraction arises in them, disappearing upon removal of the load. Residual internal stresses in the specimen are accompanied by residual double refraction, which disappears (together with the stresses) as a result of the corresponding heat treatment.

Fig. 2. Photograph of a large-crystalline specimen under load, obtained with crossed nicols.

Since silver chloride decomposes in light, articles made from it usually have a yellowish or violet color with an oily sheen.

*) ZhTF 19, 205 (1949).

The use of light filters that cut off the actinic part of the spectrum makes it possible to avoid coloring the specimens.

The fact that silver chloride and other “transparent metals” possess a structure, mechanical properties, and a mechanism of stress redistribution under the action of external factors that are qualitatively similar to those possessed by metals makes them indispensable for purposes of modeling, and opens up entirely new possibilities for optical methods of studying stresses in metals. With their aid it is possible to investigate such previously inaccessible processes as the interaction between grains, the behavior of individual grains and the boundaries between them, relaxation processes, recrystallization, recovery, the kinetics of forming processes such as forging, rolling, annealing, and cutting out from a “transparent metal” of greater hardness parts made of a “transparent metal” of lower hardness, residual stresses, etc.

As examples of the applicability of a “transparent metal” for investigations of this kind, the author presents a number of results from his study of residual stresses arising as a result of pressing and their variation with time, of stresses under tension and circular bending of a polycrystalline specimen, and also of the behavior of individual grains in a coarse-crystalline specimen under compression and tension. In the latter case he notes that, despite the homogeneity of the externally applied stress, the stresses inside the specimen vary from point to point both in magnitude and in direction, and not only from grain to grain but also within a single grain.

In Fig. 2 the author gives a photograph (with a light filter) of a coarse-crystalline specimen under load, obtained between crossed nicols (stress \(820\ \mathrm{g/mm^2}\), area \(8 \times 2\ \mathrm{mm^2}\); the axes of the nicols are at an angle of \(45^\circ\) to the direction of tension). This photograph, in which the boundaries between grains are clearly visible, gives an idea of the stressed states of individual regions of the specimen, determined not only by external forces but also by the interaction of the grains (anisotropy of the elastic constants of the crystal). This picture appears especially vivid in the color photograph presented in the paper.

The author indicates that at small loads, removal of the load leads to disappearance of the picture. But if the load exceeds a certain (small) value, residual stresses are observed, localized mainly along grain boundaries and slip lines, if the stress was sufficient for their formation.

The author points out that further study of the properties of “transparent metals” and the development of the laws of modeling and similitude are necessary. There is no doubt that in the very near future the use of “transparent metals” for the purpose of studying processes occurring in real metals will make it possible to solve a number of specific problems of enormous practical importance.

G. Rozenberg