ISSN 1027-4510, Journal of Surface Investigation: X-ray, Synchrotron and Neutron Techniques, 2017, Vol. 11, No. 3, pp. 544–548. © Pleiades Publishing, Ltd., 2017. Original Russian Text © J.V. Osinskaya, A.V. Pokoev, 2017, published in Poverkhnost’, 2017, No. 5, pp. 71–75.

Effect of a Constant Magnetic Field on the Structure and Physical-Mechanical Properties of Cu57Be43 Alloy J. V. Osinskaya* and A. V. Pokoev** Samara State University, Samara, 443011 Russia *e-mail: [email protected] **e-mail: [email protected] Received June 30, 2016

Abstract—The work presents data on the microhardness, average grain size, lattice parameters, and phase composition of Cu57Be43 metal alloy, annealed at a temperature of ~350°С for 1 h in a constant magnetic field with the intensity ranging from 80.0 to 557.0 kA/m. The main observed regularities of changes in the structure and properties of the material during annealing in a constant magnetic field and without it are formulated. Keywords: constant magnetic field, Cu57Be43 alloy, magnetoplastic effect DOI: 10.1134/S1027451017030144

INTRODUCTION An improvement in the mechanical strength of an aging metal alloy is known to be due to the interaction of dislocations with phases released during the decomposition of a preliminarily quenched supersaturated solid solution. As a result of decomposition, phase segregations form in the crystal lattice of the alloy. The growth rate of nuclei is determined by diffusion, external impacts, and in particular, can depend on the applied constant magnetic field (CMF). Growing phase segregations greatly affect the motion of dislocations and thus, cause changes in the physicalmechanical properties of aged alloys. In [1] it is found that a CMF with an intensity of 557.0 kA/m, applied to the aging process of BrB-2 beryllium bronze, increases its microhardness to 30%, alters the alloy microstructure, the size and amount of released γCuBe phase being different in samples aged in a CMF and without it [2]. The mechanisms of the CMF effect on this alloy have not been revealed completely so far. Therefore, it is relevant and practically important to study changes in the structural and structural-energy properties of the γ-CuBe-phase itself, including the magnetic and structural ordering caused by the application of a CMF and at elevated temperatures in the absence of a CMF. EXPERIMENTAL Cu57Be43 metal alloy with a copper concentration of 57.0 at % and a beryllium concentration of 43.0 at %, i.e., a composition close to that of the γ-Cu50Be50

intermetallic phase, was chosen as the object of study. The material of samples having a cubic shape with an edge of 10 mm was obtained from high-purity copper and beryllium at the Federal State Unitary Enterprise, Scientific Research Institute, Scientific Industrial Organization “Luch” (Podolsk, Moscow region). The sample composition was determined at the Analytical Certification Center “GIREDMET”. The samples were quenched by rapid submergence into water at a temperature of 20°C after maintaining a temperature of 800°C for 20 min in air. Thermal-magnetic treatment modes were chosen based on published data [3] and previously performed experiments with BrB-2 beryllium bronze [1, 2]: the aging temperature was 350°C, aging time was 1 h in a CMF with an intensity ranging from 80.0 to 557.0 kA/m and without it in a vacuum chamber at a residual vapor pressure of 10–3 Pa. The studies were carried out by metallography, microhardness and X-ray analyses. Metallographic measurements were carried out using an optical MIM-8M metallographic microscope. The grain size was determined using the VideoTestSize-5.0 program. The relative measurement error in the average grain size was 10–15%. The microhardness was measured by a HAUSER microhardness meter at a load of 100 g and a load time of 7 s. Each microhardness value was obtained by averaging 10 measurements. The relative error in the average microhardness value was 1–3%. X-ray diffraction analysis was conducted using a DRON-2 diffractometer (СоKα radiation) equipped

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with a hardware-software complex to control, measure, and process the measurement results. The X-ray analysis modes were: anode current of 20 mA; X-ray tube voltage of 30 kV; counter movement speed of 0.2 and 0.4°/min; and a slit: 0.5–0.4–0.5. The relative error of separate measurement of the lattice parameter was 0.03%. RESULTS AND DISCUSSION Metallographic studies were performed on samples of Cu57Be43 alloy in the initial state (as-received condition), after quenching and annealing. Fig. 1 shows typical images of the sample surfaces and Table 1 lists the average grain sizes in the samples studied. In the initial state (as-received condition) (Fig. 1a) the structure of the studied alloy is revealed as roundish grains differently oriented in relation to the sample surface. At the grain boundaries triple junctions at an angle close to 120° are observed, which indicates the single-phase state of the alloy under study. Estimation of the grain size in the sample in the initial state shows that the average size is 416 ± 62 μm (Table 1). After quenching the sample structure barely changed (Fig. 1b), with the grain size decreasing almost 1.5 times and being 290 ± 43 μm (Table 1). After annealing the material with the application of a CMF and its absence the microstructure and the average grain size do not undergo significant changes (Figs. 1c–1j). The grain size varies in the range from 235 ± 30 to 335 ± 43 μm (Table 1). In the initial state the average microhardness of the alloy was 3300 MPa (Fig. 2). After quenching the microhardness value drastically increased more than 1.5 fold and reached 5700 MPa. This increase in the microhardness is due to a dramatic change in the grain size which, according to the above metallographic data, also decreased approximately 1.5 times. Annealing of the alloy for 1 h at a temperature of 350°C resulted in an insignificant increase in the microhardness whose value reached 6050 MPa, which is also explained by a decrease in the grain size (Table 1). The application of CMF with an intensity from 80.0 to 557.0 kA/m to the annealed material under study almost always results in a decrease in the alloy microhardness to 6%. The so-called positive magnetoplastic effect (MPE) is observed [4, 5]. From published data it is known [6] that the main reason for an increase in the microhardness of the annealed alloy is the deceleration of moving dislocations by the phases arisen upon the decomposition of the supersaturated solid solution. Therefore, it is possible to suppose that during annealing in a CMF the structural state of the alloy changed, which is con-

545

Table 1. Measurement results of the average grain size of the Cu57Be43 alloy Intensity Н, kA/m

dav, μm

Initial state

–

416 ± 48

Quenching 800 → 20°С

–

290 ± 44

0

273 ± 34

80.0

278 ± 56

159.0

323 ± 53

239.0

285 ± 42

318.0

311 ± 44

399.0

348 ± 77

477.0

235 ± 30

557.0

335 ± 43

Annealing time t, h

1

firmed by the results of powder X-ray diffraction (XRD) analysis, and the phases arisen here hinder the moving dislocations not so efficiently, which causes a decrease in the microhardness. We note also the explanation of a decrease in the formation energy of a critical size nucleus of the γ-CuBe phase upon aging in a CMF, which was proposed in [2]. Furthermore, when a CMF is applied the alloy structure is subjected to magnetic structural ordering which in turn can also affect changes in the microhardness. By means of powder XRD the lattice parameter of the alloy annealed in a CMF with different intensities and without it was measured. In the initial state the lattice parameter of the alloy was 2.709 Å. This lattice parameter is consistent with the reference data [7, 8], which indicates the reliability of the results obtained. After quenching from 800°C in water, beryllium atoms having a smaller atomic size were evenly distributed in the copper crystal lattice, which led to a decrease in the lattice parameter to 2.705 Å (Fig. 3). From Fig. 3 it is seen that annealing of the alloy with the application of a CMF and in its absence almost always causes a decrease in the measured lattice parameter by a value ranging from 0.001 to 0.01 Å. This fact, in combination with the powder XRF results, gives evidence of the formation of a more ordered structure of the phase, its approximation to the stoichiometric composition and the equilibrium state.

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(a)

100 μm (b)

100 μm (c)

100 μm

(d)

100 μm (e)

100 μm (f)

100 μm

(g)

100 μm (h)

100 μm (i)

100 μm

(j)

100 μm

Fig. 1. Metallographic images of the sample surface: initial state (a); quenching from 800 to 20°C (water) (b); annealing 1 h, 0 kA/m (c); annealing 1 h, 80.0 kA/m (d); annealing 1 h, 159.0 kA/m (e); annealing 1 h, 239.0 kA/m (f); annealing 1 h, 318.0 kA/m (g); annealing 1 h, 399.0 kA/m (h); annealing 1 h, 477.0 kA/m (i); annealing 1 h, 557.0 kA/m (j).

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shifted to larger angles relative to the lines of pure copper. This shift of the lines (0.10°–0.41°) is due to the presence of beryllium atoms, which are a substitutional impurity, in the copper solid solution. The size of beryllium atoms is 11.5% smaller [9] than that of copper atoms, which leads to a decrease in the crystal lattice parameter of the material being studied, and as a consequence, to a shift of the lines in the diffraction patterns. Lines corresponding to the γ-CuBe phase are also observed, which indicates its presence in the material under study and agrees with the Cu–Be state diagram [7], according to which at a beryllium content of 43 at % the phase composition of the material is governed by the γ-CuBe phase and a small volume content of the copper-based α-solid solution.

7000 6000 Hμ, МPа

5000 4000 Cu Initial state Quenching Field dependence H = 0 kА/m

3000 2000 1000 0 100

200

300

400

500 600 H, kА/m

Fig. 2. Field dependence of the microhardness of the Cu57Be43 alloy at an annealing temperature of 350°C for 1 h.

а, 10–2 Å 273

After quenching of the material only lines corresponding to the γ-CuBe phase are observed in the diffraction patterns (Table 2). They are shifted to smaller angles as compared to the initial state, which evidences the formation process of the phase structure.

Field dependence Initial state Quenching H = 0 kА/m

272

547

271

Annealing for 1 h without a CMF results in a decrease in the intensity and the broadening of lines corresponding to the γ-CuBe phase. This fact indicates greater distortion of the crystal lattice as compared to the quenched state, which seems to be due to the onset of the alloy transition to a more equilibrium state.

270 269 268 267 0

100

200

300

400

500 600 H, kA/m

Fig. 3. Dependence of the lattice parameter of the Cu57Be43 alloy on the CMF intensity.

As a result of the performed powder XRD analysis, from the diffraction patterns of the studied samples, data on the intensities of diffraction lines, interplanar distances, and the line half-width were obtained for each sample. From the data obtained the phases arisen during ageing in CMF and without it were identified. The powder XRD results show that the diffraction patterns of the initial sample (Table 2) contain lines of the copper-based α-solid solution (α-Cu), which are

When a CMF with an intensity ranging from 80.0 to 557.0 kA/m is applied at the same time as heat treatment, an increase in the intensity and a decrease in the line half-width of the γ-CuBe phase are observed (Fig. 4), which evidences the formation of a more ordered structure of this phase due to atomic ordering processes in a CMF. With an increase in the CMF intensity the half-width of lines of the γ-CuBe phase decreases. Moreover, from Table 2 it is seen that CMF application during alloy annealing almost always causes the formation of the same number of lines corresponding to the γ-CuBe phase as after annealing without CMF, i.e., within the sensitivity limit of the applied method the volume content of the phase in the alloy remains unchanged.

Table 2. Composite table of the interpretation of diffraction patterns CMF intensity, kA/m

0

80.0

159.0

239.0

318.0

399.0

477.0

557.0

Initial state Quenching

Number of lines of γ-CuBe

4

5

4

5

5

4

4

4

3

5

Number of lines of α-Cu

–

1

–

–

–

1

1

1

5

–

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(100) 38.75 γ-CuBe

80 70 60 50 40 30 20 10 0 30 40

(210) 95.61 γ-CuBe

β

50

60

(211) 108.65 γ-CuBe β

70 80 90 100 110 120 2θ, deg

(b)

(100) 38.80 γ-CuBe

I, pulse/s

I, pulse/s

(а) (110) 55.91 γ-CuBe

500 400 300 200 β 100 0 30 40

β

(110) 55.96 γ-CuBe

50

60

(211) 108.60 γ-CuBe

(200) 83.12 γ-CuBe

70 80 90 100 110 120 2θ, deg

Fig. 4. Diffraction pattern of the sample aged without CMF (a) and in CMF with an intensity of 557.0 kA/m (b).

CONCLUSIONS A combined analysis of the experimental data obtained when CMF with an intensity ranging from 80.0 to 557.0 kA/m is applied during isothermal treatment of the Cu57Be43 alloy at a temperature of 3501°C for 1 h shows that positive MPE is observed, which causes a decrease in the microhardness to 6%. By metallography it is found that alloy quenching results in an almost 1.5 fold decrease in the average grain size in comparison with the initial state. However, in the presence and absence of CMF the microstructure and the average grain size do not undergo significant changes. The results of powder XRD analysis show that CMF application causes an increase in the intensity and a decrease in the half-width of the characteristic diffraction lines corresponding to the γ-CuBe phase, which indicates the formation of a more perfect and homogeneous structure of the phase due to atomic and magnetic ordering processes. The results obtained stimulate interest in the attraction of neutron and magnetic methods for study-

ing magnetic ordering in the phase under study and its effect on the MPE value. REFERENCES 1. Yu. V. Osinskaya and A. V. Pokoev, Fiz. Khim. Obrab. Mater., No. 3, 12 (2003). 2. Yu. V. Osinskaya, S. S. Petrov, A. V. Pokoev, and V. V. Runov, Phys. Solid State 52, 523 (2010). 3. R. L. Tofpenets, Softening Processes in Ageing Alloys (Nauka i Tekhnika, Minsk, 1979) [in Russian]. 4. V. I. Alshits, E. V. Darinskaya, M. V. Koldaeva, and E. A. Petrzhik, Crystallogr. Rep. 48, 768 (2003). 5. Yu. I. Golovin, Phys. Solid State 46, 789 (2004). 6. K. P. Bunin, and A. A. Baranov, Metallography (Metallurgiya, Moscow, 1970) [in Russian]. 7. State Diagrams of Binary Metal Systems, Ed. by N. P. Lyakishev (Mashinostroenie, Moscow, 1996), Vol. 1 [in Russian]. 8. M. I. Gitgarts and A. V. Tolstoi, Fiz. Met. Metalloved. 67, 547 (1989). 9. Physical Metallurgy, Ed. by R. W. Cahn and P. Haasen (North-Holland Physics Publ., Amsterdam, Oxford, New York, Tokyo, 1983), Vol. 2.

Translated by L. Chernikova

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2017