Changes in Electrical Strength, Microhardness, Flow Pressure, and Heat of Formation of Solid Solutions of Some Alkali-Halide Salts during Their Aging

A. N. Kislina

Academician N. S. Kurnakov and his collaborators established that the physicochemical properties of solid solutions depend on the heat absorbed during their formation [1]. By the heat of formation of a solid solution they took the difference between the heats absorbed in the dissolution in water of a mechanical mixture of salts and of a solid solution of the same weight and composition. According to the data of N. S. Kurnakov and S. F. Zhemchuzhnyi, when a solid solution of 1 gram-molecule of KBr and 1 gram-molecule of NaBr is dissolved in 100 gram-molecules of water, 3823 calories are absorbed, while when a mechanical mixture of the same weight and composition is dissolved, 5428 calories are absorbed. The difference between these quantities, equal to 1425 calories, according to the authors’ definition, is the heat of formation of the solid solution. The heat of formation characterizes the internal energy of the system. According to numerous measurements it has been established that the heat of formation of most binary alloys possessing complete mutual solubility depends on chemical composition according to a parabolic law. The maximum value of the heat of formation corresponds approximately to the equimolar proportion of the mixture, 1 : 1. Solid solutions of alkali-halide salts formed with absorption of heat are unstable at low temperatures and decompose with time with the release of heat [2]. The stability of the system is the greater, the less heat was absorbed during its formation. The rate of decomposition of solid solutions can be followed from the release of heat and from changes in density, specific resistance, and other characteristics. The change in the internal energy of solid solutions is caused by changes in the order of arrangement of ions in the lattice. W. Hovi [3] describes the process of transformations in the distribution of atoms or ions in homogeneous solid solutions with time as follows: “Owing to annealing, usually carried out after growing a crystal of a solid solution from the melt, the ions are compelled to pass more and more into an order corresponding to the high temperature. If, after annealing, the crystal is rapidly cooled, then the ordered state of the ions corresponding to the annealing temperature is preserved. At room temperature such a crystal will no longer be in an equilibrium state and will show a tendency to reach the equilibrium state corresponding to this new temperature.”

In many cases these processes (aging processes) require an extremely long time, so that they have no practical significance; but sometimes the aging process proceeds so rapidly that it is possible to follow it. Aging is accompanied by a change in the mechanical and physical properties of the solid solution. It is well known that many technical alloys acquire the desired properties owing to quenching and subsequent aging at room temperature (natural

aging). The aging process can be accelerated by the application of heat treatment—artificial aging. The most significant effect is the strengthening of the alloy in the initial stages of aging. Microscopic investigation shows that the strength of a decomposing solid solution increases only so long as the phase precipitating during decomposition is extremely dispersed (dispersion hardening), while coagulation of the precipitating phase always causes a decrease in the hardness of the alloy [4].

Aging phenomena, previously considered characteristic only of certain aluminum alloys, are becoming the subject of extensive investigations in all countries of the world [5]. The change with time in the physicochemical properties of solid solutions of alkali-halide salts has been studied by N. S. Kurnakov, S. F. Zhemchuzhnyi and F. Rambach, Wrzesniewski, Hovi, and others.

S. F. Zhemchuzhnyi and F. Rambach [2] experimentally established the fact that the heat of formation changes with time for single crystals of equimolar solid solutions KBr—NaBr and KJ—NaJ. Hovi [3] traced the change in the heat of formation of single crystals of solid solutions of the KCl—KBr system as a function of annealing temperature and time, and of single crystals of equimolar KCl—KBr solid solutions as a function of storage time at room temperature. Wrzesniewski [6] measured the change with time in the flow pressure of KCl—KBr and KJ—KBr alloys.

The electrical properties of solid solutions of alkali-halide salts have been studied by Hippel, Li [7], Bogdanova [8], Krivov [9], and others.

We determined the change in the electrical strength of single crystals of solid solutions of the KCl—KBr and KBr—NaBr systems during aging and compared these changes in electrical strength with the change in microhardness, flow pressure, and heat of formation with time. The microhardness was determined on the same specimens as the electrical strength, after their breakdown. Changes in the heat of formation and flow pressure with time were taken from the data of other authors.

Change in the Electrical Strength of Single Crystals of Solid Solutions during Aging

The electrical strength was determined for freshly grown single crystals of solid solutions of the systems KCl—KBr, KBr—NaBr and for single crystals of these systems that had been stored for a long time at room temperature. Single crystals of solid solutions of the KCl—KBr system were stored in desiccators, and single crystals of the KBr—NaBr system—in dried transformer oil. Specimens were then prepared from the single crystals, and the impulse electrical strength was determined on these specimens. The conditions for growing the crystals, the method of preparing the specimens, and the determination of the impulse electrical strength are described in [8]. The change in the impulse electrical strength of single crystals of fresh solid solutions of the KBr—NaBr system as a function of chemical composition is given in the above-cited article [8], and analogous data for the KCl—KBr system were taken from the work of A. M. Trubitsyn [9].

In Fig. 1, the curves are compared for the change in electrical strength (maximum and arithmetic-mean values) of single crystals of solid solutions of the KCl—KBr system: fresh, after 3 months of storage, and after 2 years of storage from the moment of growth, as a function of chemical composition. In the dependence of electrical strength on chemical composition for fresh specimens, a minimum is observed in the region of approximately equal contents of the components in the solid solution. The electrical strength of single crystals of solid solutions of composition 30, 40, 50, 60, 70, 80 mol % KBr in KCl after 3 months of storage increased in comparison with the electrical strength of fresh crystals. This increase proved

proved to be the most significant (if compared by average values) for the equimolar solid solution, for which the electrical strength exceeded the electrical strength of the individual components KCl and KBr, and for solid solutions with concentrations close to equimolar. Thus, the values of the electrical strength of these 6 compositions, after 3 months of storage, lay on a curve with a maximum. For the maximum values of the electrical strength of specimens stored for 3 months, the law of distribution with chemical composition was different than for the average values. In this case the maximum value of the electrical strength corresponds to a single crystal of composition 70 mol % KBr and 30 mol % KCl.

Fig. 1. Change in electrical strength \((E_{\mathrm{br}})\) under pulses as a function of the chemical composition of single crystals of KCl—KBr solid solutions.
× — \(E_{\mathrm{br}}\), maximum for fresh specimens.
○ — \(E_{\mathrm{br}}\), average for fresh specimens.
□ — \(E_{\mathrm{br}}\), maximum after 3 months of storage of specimens.
▲ — \(E_{\mathrm{br}}\), average after 3 months of storage of specimens.
⊕ — \(E_{\mathrm{br}}\), maximum after 2 years of storage of specimens.

For single crystals of solid solutions of composition 5 and 10 mol % KBr in KCl, stored for 2 years, the average values of the electrical strength agree well with the average electrical strength of fresh specimens of analogous concentrations. For the single crystal of the equimolar KCl—KBr solid solution, the electrical strength is considerably higher than for fresh crystals, but, however, lower than for the equimolar crystal stored for 3 months.

Figure 2 shows the change in the average values of the electrical strength of single crystals of solid solutions of the KCl—KBr system: fresh, after 3 months of storage, after 6 months of storage, annealed at a temperature of \(500^\circ\mathrm{C}\) for 6 hours and at \(300^\circ\mathrm{C}\) for 40 hours in [[unclear: continuation cut off on next page]].

Fig. 2. Change in the electrical strength \((E_{\mathrm{br}})\) of single crystals of solid solutions with aging.

1. ○ — \(E_{\mathrm{br}}\) after 3 months of storage of the specimens.
2. ▲ — \(E_{\mathrm{br}}\) after annealing at \(500^\circ\mathrm{C}\) for 6 hours.
3. □ — \(E_{\mathrm{br}}\) after 6 months of storage of the specimens.
4. ✱ — \(E_{\mathrm{br}}\) after annealing at \(300^\circ\mathrm{C}\) for 40 hours.
5. × — \(E_{\mathrm{br}}\) of fresh specimens.

Fig. 3. Change in the electrical strength \((E_{\mathrm{br}})\) under pulses as a function of the chemical composition of KBr--NaBr solid solutions.

□ — \(E_{\mathrm{br}}\), maximum after 3 months of storage of the specimens.
▲ — \(E_{\mathrm{br}}\), average after 3 months of storage of the specimens.
× — \(E_{\mathrm{br}}\), maximum for fresh specimens.
○ — \(E_{\mathrm{br}}\), average for fresh specimens.

dependence on chemical composition. A comparison of the electrical strength of crystals stored before breakdown for 6 months and 3 months with the electrical strength of fresh specimens shows that, for small impurity concentrations within the range of 10–20 mole %, the electrical strength changes only insignificantly during aging. For equimolar solid solutions, a considerable increase in electrical strength during aging is observed. Annealing of the crystals is likewise accompanied by an increase in their electrical strength. The effect of aging after 3 months of storage proved to be more substantial than annealing of the crystals at temperatures of 300 and 500°C.

Fig. 3 shows the change in the electrical strength of single crystals of solid solutions of the KBr—NaBr system, fresh and after 3 months of storage, as a function of chemical composition. During aging the electrical strength increases for the equimolar solid solution and for solid solutions close in composition to the equimolar one. The maximum values of electrical strength for this system, it seems to us, are not characteristic because of the great sensitivity of specimens with a high NaBr content to the action of moisture, as a result of which a very hard enamel-like layer forms on the surface of the specimens. The electrical strength of such specimens increases. Comparing in Fig. 3 the maximum values of electrical strength with the mean values for the region close to the NaBr component, one may note that specimens with high electrical strength are evidently accidental.

Change in the Microhardness and Flow Pressure of Single Crystals of Solid Solutions of the KCl—KBr System during Aging

Systematic studies of the relationship between the composition and hardness of alloys, carried out by N. S. Kurnakov and S. F. Zhemchuzhnyi [10], made it possible to establish the principal types of composition—hardness diagrams. It was found that a continuous solid solution corresponds to continuous curves with a maximum for the composition—hardness and composition—flow-pressure diagrams. Hardness proved to be an extremely sensitive property for detecting solid solutions of the smallest concentrations [5]. In this connection, changes in the hardness of solid solutions during aging should be expected.

It is known, for example, that the mechanical properties of an aluminum alloy with copper increase not only as a result of tempering at a temperature of 100–300° after quenching from 500–540°, but also when the quenched alloys remain for a long time at room temperature. The change in the hardness of alloys during isothermal tempering [11] is characterized by the curves shown in Fig. 4. According to modern views, the presence of a maximum is due to the fact that, in binary alloys during aging

Fig. 4. Change in the hardness of duralumin during aging.

In parallel with the precipitation of a dispersed second phase, which causes an increase in hardness, coagulation of the precipitated crystallites also occurs, reducing the hardness. The higher the temperature, the more rapidly the effect of coagulation manifests itself. In the initial stage of aging, when the precipitating

Fig. 5. Change in microhardness \((1, 1', 1'')\) and flow pressure \((2 \text{ and } 2')\) of solid solutions of the KCl—KBr system with aging; for specimens stored for 3 months \((1')\), 6 months \((1'')\), and fresh specimens \((1)\). Curves 2 and \(2'\) were plotted from the data of I. V. Vrzhesnevsky for fresh specimens \((2)\) and after 5 weeks of storage \((2')\).

phase is highly dispersed, the thickness of its plate-like crystals should not exceed several tens of atomic layers [12]. Such extremely thin oriented inclusions especially strongly impede slip in the solid-solution crystal, sharply increasing the hardness of the latter.

Fig. 6. Change in the heat of formation of a single crystal of a solid solution of composition 50 mol% KCl—50 mol% KBr with time. The single crystal was annealed at \(600^\circ\mathrm{C}\) for 200 hours and rapidly cooled to room temperature. The curve was plotted from Hovi’s data.

For solid solutions of alkali-halide salts, there are no data in the literature on changes in hardness during aging. There are data by I. B. Vrzhesnevsky [6] on the change in flow pressure of alloys of the KJ—KBr and KCl—KBr systems as a function of chemical composition for fresh specimens and for specimens stored for 5 weeks. Figure 5 gives diagrams of flow pressure versus composition for fresh KCl—KBr alloys and after 5 weeks. A comparison of these curves shows that the flow pressures of the alloys decreased after 5 weeks, although the form of the flow-pressure—composition curve remained the same. An analogous phenomenon also occurs for KJ—KBr alloys.

We measured the microhardness of single crystals of solid solutions KCl—KBr, both fresh and aged at room temperature for 3 and 6 months, and, for the equimolar solid solution, for 2.5 years (Fig. 6). The method for determining microhardness is described in [8]. As can be seen from Fig. 5, the microhardness of both fresh KCl—KBr specimens and specimens subjected to aging varies with chemical composition along a curve with a maximum. However, after 3 months of storage of the crystals the microhardness rose sharply in comparison with fresh crystals, while after 6 months a decrease is again observed.

Change in the Heat of Formation of Single Crystals of Equimolar Solid Solutions KCl—KBr and KBr—NaBr during Aging

The change in the physical properties (in our case—the electrical strength, microhardness, and flow pressure) of solid solutions during aging is evidently connected with a rearrangement of their crystal lattices, with a change in the positions of the ions. The latter causes a change in the internal energy of the solid solutions. For solid solutions formed with absorption of heat, the internal energy is characterized by the heat of formation. Knowing the rate of heat evolution, one can judge how rapidly the equilibrium state is established in the solid solution at a given temperature. Consequently, we can judge the rate of the aging process. Therefore we consider it appropriate to present the results of other authors on the change in the heat of formation of equimolar solid solutions KCl—KBr and KBr—NaBr with time of storage.

S. F. Zhemchuzhnyi and F. Rambach [2] established that the heat of formation of a freshly prepared equimolar solid solution KBr—NaBr is 712 cal/mole, whereas after 2.5 months it is only 250 cal/mole; i.e., in 2.5 months the solid solution, upon decomposition, releases about 60% of the heat absorbed in its formation. According to our data, the electrical strength of these crystals increased in approximately the same time from \(0.54 \cdot 10^6\) to \(0.9 \cdot 10^6\) V/cm.

V. Hovi [3] investigated the change in the heat of formation of solid solutions of the KCl—KBr system as a function of annealing temperature and time, and that of the equimolar solid solution as a function of aging time at room temperature.

Figure 6 presents the change in the heat of formation of a single crystal of the equimolar solid solution KCl—KBr as a function of aging time at room temperature, according to Hovi’s data. The single crystal was grown from the melt, annealed for 200 hours at a temperature of 600°C, and rapidly cooled.

The graph of the change in heat of formation shows that the most substantial decrease in the heat of formation of the single crystal occurs within the first 2 months; thereafter, from 2 to 8 months, this change is very small. Comparing the change in electrical strength and microhardness (Figs. 2 and 5) with the change in heat of formation (Fig. 6) for single crystals of equimolar solid solutions KCl—KBr, one can see that the electrical strength, microhardness, and heat of formation change most significantly by 2–3 months of natural aging.

The effect of increasing electrical strength in the process of natural aging exceeds the effect of annealing (artificial aging, Fig. 2), just as “the decrease in energy in the aging process exceeds the effect caused by annealing” [3].

Conclusions

1. In the course of natural aging of single crystals of solid solutions of the alkali-halide salts KCl—KBr and KBr—NaBr, an increase in electrical strength and microhardness and a decrease in the heat of formation are observed. For equimolar solid solutions and those close to them in composition, at a certain stage of aging the electrical strength exceeds the strength of crystals of the pure components.

2. The electrical strength and microhardness of single crystals of KCl—KBr solid solutions, in the course of aging, first increase sharply and then begin to decrease.

3. Annealing of mixed KCl—KBr crystals at temperatures of 300 and 500°C also leads to an increase in electrical strength. However, the effect of aging under natural conditions exceeds the effect of aging during annealing (artificial aging).

Taking this opportunity, I express my deep gratitude to A. A. Vorob’ev for supervising the work.

References

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Tomsk Polytechnic Institute.