STUDY OF THE PLASTICITY AND STRENGTH OF THE YuNDK24 ALLOY

V. S. Golovenko, L. M. Sedokov, D. Ya. Chigirinskii

(Presented by the scientific seminar of the Department of Strength of Materials)

In modern electrical machines and instruments, and in radio-engineering and electronic devices, permanent magnets have become widespread; various iron-based magnetic alloys are used for their manufacture.

Such alloys include the YuNDK24 alloy, whose chemical composition is characterized by the following data: Al—8%, Ni—14%, Cu—3%, Co—24%, the balance—Fe.

The YuNDK24 alloy and its modifications are distinguished by high strength, low ductility, increased anisotropy of structure, and good magnetic properties. The magnetic properties of this alloy can be substantially improved by applying, for example, preliminary plastic deformation. Therefore, the need arose to develop a method that would make it possible to ensure increased plasticity of an extremely brittle alloy without disrupting the continuity of its structure.

To study the mechanical properties of hardened steels and to increase their plasticity, B. D. Grozin [1] used all-round nonuniform compression. A specimen of hardened steel, according to this method, was inserted into a holder made of a plastic material. When the specimen was compressed in the holder, it was possible to record a substantial increase in the plasticity of the hardened steel.

In work [2], an increase in the plastic properties of gray cast iron was also obtained. We took this method as the basis for a way of obtaining enhanced magnetic properties in alloys of the YuNDK24 type. To study the mechanical properties of the YuNDK24 alloy, the following modifications were selected: the YuNDK24T alloy, alloyed with 0.5% titanium, and YuNDK24 itself.

Alloying the alloy with titanium was prompted by the need to obtain an initially homogeneous structure and to improve the conditions for studying recrystallization of the alloy. For the same purpose, before deformation the alloys were heat-treated to the \(\gamma\)-phase.

It should be noted that the study of the mechanical properties of the YuNDK24 alloy as a typical representative of highly brittle materials is of definite theoretical and practical interest from the standpoint of the theory of plasticity and strength, methods for determining mechanical characteristics, the study of the fracture process, the accumulation of damage, and the capacity of brittle materials for strengthening.

For the experimental solution of the indicated questions, several series of tests with the YuNDK24 alloy were carried out in the laboratory of the Department of Strength of Materials of the Tomsk Polytechnic Institute: determination of mechanical characteristics under linear tension and compression, and study of plasticity and strength under all-around nonuniform compression.

As a result of carefully conducted linear-tension tests on ten prismatic specimens \((17.5 \times 18 \times 200\ \text{mm})\) and ten round specimens (diameter \(8\ \text{mm}\), gauge length \(40\ \text{mm}\)), the following mechanical characteristics of the YuNDK24 alloy were obtained: ultimate tensile strength \(\sigma_{\mathrm{p}} = 11\ \text{kg}/\text{mm}^2\), maximum axial strain \(\varepsilon = 0.065\%\), Poisson’s ratio \(\mu = 0.27\), and modulus of normal elasticity \(E = 2.04 \cdot 10^6\ \text{kg}/\text{cm}^2\).

In the axial-compression tests, cylindrical specimens were used having the following dimensions: diameter \(10\ \text{mm}\), height \(15\ \text{mm}\). With this ratio of height to diameter, the influence of friction at the specimen ends was insignificant [3].

From the results of linear-compression tests of twenty-one specimens, curve (1) was constructed in the coordinates \(\sigma_i\)--\(\varepsilon_i\), as presented in the proposed figure.

The maximum stress in compression was \(\sigma_c = 163 \pm 8.1\ \text{kg}/\text{mm}^2\); the strain corresponding to this stress was \(\varepsilon = 2.3\%\), while the strain at the moment of fracture reached \(\varepsilon = 3\%\).

The results of tests under all-around nonuniform compression are presented in the figure by curve (2) for the YuNDK24 alloy and by curve (3) for the YuNDK24T alloy. The curves were obtained after processing the results of compression of specimens of these alloys in holders made of steel 10, with height \(15\ \text{mm}\) and diameter \(60\ \text{mm}\).

From comparison of the indicated curves 1, 2, 3, it is seen that the YuNDK24 alloys compressed in a holder have increased strength and plasticity. Thus, for the YuNDK24 alloy the strength limit was \(\sigma_{\mathrm{B}} = 680\ \text{kg}/\text{mm}^2\), and the plasticity corresponding to this limit was \(\varepsilon = 8\%\); for the YuNDK24T alloy, respectively, \(\sigma_{\mathrm{B}} = 705\ \text{kg}/\text{mm}^2\) and \(\varepsilon = 6.5\%\), which constitutes an increase of the strength limit, compared with linear compression, by 4.2 times, and of the corresponding plastic strain by 2.8–3.5 times.

Fig. 1. Deformation curves of the YuNDK24 and YuNDK24T alloys under uniaxial and all-around compression

In the figure:
1 — linear compression, YuNDK24
2 — upsetting in a holder, YuNDK24
3 — upsetting in a holder, YuNDK24T

Thus, the strengthening of materials in this case is confirmed if, by the loading conditions, it is possible to increase the magnitude of the relative plastic deformation. After the strength limit is reached under linear and all-around compression, a decrease in stress is observed, which corresponds to the beginning of the fracture process, accompanied by an intensive increase in the number of microcracks and growth of their sizes. This process is clearly observed in studying the structure of the corresponding specimens.

After tests of the YuNDK24 alloy under nonuniform all-around compression, special studies were carried out to determine the influ-

of preliminary plastic deformation on the recrystallization process of the alloy and the improvement of its magnetic properties.

Specimens of the YuNDK24 alloy, compressed in holders to various degrees of reduction (from 1 to 10%, approximately in one-percent increments), were annealed for one hour at temperatures of 1250°, 1300°, and 1350°. A recrystallization diagram was then constructed by a metallographic method, based on the static change in grain size, which made it possible to find the “critical” degree of deformation and the corresponding recrystallization temperature.

At a temperature of 1250°C, the optimal preliminary plastic deformation is 6.22%.

If the average statistical grain size in the initial state is 0.08 mm², then at a recrystallization temperature of 1350°C and a preliminary plastic deformation of about 4% it was 1.21 mm², i.e., a 10–15-fold increase takes place. Individual grains in this case increased by 150–200 times [4].

Thus, all-round nonuniform compression of the YuNDK24 and YuNDK24T alloys not only sharply changes their strength and plasticity, but also substantially improves their recrystallization properties, which is of definite practical interest.

REFERENCES

1. B. A. Grozin. Mechanical Properties of Hardened Steels. Mashgiz, 1951.

2. L. M. Sedokov. Study of the plasticity and strength of gray cast iron. “Izvestiya vysshikh uchebnykh zavedenii,” Physics, No. 5, 1959.

3. L. M. Sedokov. Static tests of brittle materials. “Standartizatsiya,” No. 1, 1961.

4. D. Ya. Chigirinskii, V. S. Golovenko. “Mechanism and kinetics of recrystallization of the YuNDK24 alloy.” In: Proceedings of the All-Union Symposium on Ferromagnetic Materials, Lvov, November, 1969.