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Synthesis of Zr-Zn alloys from elemental powders by severe plastic deformation under pressure
Materials Characterization 156 (2019) 109848
Contents lists available at ScienceDirect
Materials Characterization journal homepage: www.elsevier.com/locate/matchar
Synthesis of Zr-Zn alloys from elemental powders by severe plastic deformation under pressure A.V. Dobromyslov, N.I. Taluts
T
⁎
Mikheev Institute of Metal Physics, Ural Branch, Russian Academy of Sciences, Ekaterinburg 620108, Russia
A R T I C LE I N FO
A B S T R A C T
Keywords: Zr–Zn alloys Mechanical alloying Structure Pressure
Zr–Zn alloys with a zinc content to 50 at.% have been prepared from elemental powders by severe plastic deformation under pressure. Synthesized alloys have been studied by X-ray diffraction analysis and transmission electron microscopy. The study suggests that both the plastic deformation degree and the zinc content are responsible for the grain size. High pressure torsion at a pressure of 8 GPa results in intensive α → ω transformation in all the alloys. It forms an amorphous structure in alloys with 40 and 50 at.% zinc. Aging of a Zr–20 at. % Zn alloy at 500 °C for 15 min causes transformation of an ω phase into a zinc supersaturated α phase, which subsequently decomposes to an equilibrium α phase and a ZrZn intermetallic compound.
1. Introduction Synthesis of alloys from various elemental metal powders by severe plastic deformation under pressure belongs to promising techniques for producing materials with new functional and mechanical properties. The advantage of this technique as compared to mechanical alloying in ball mills is the high synthesizing rate, no contaminating impurities, and a resulting bulk product. Deformation of powders in contrast to deformation of solid samples forms material with a fundamentally refined structure, accelerates mechanodiffusion kinetics due to an increase in the common area of interacting surfaces, facilitates the solubility of components into each other, and forms metastable phases, which are thermodynamically unstable under normal conditions [1–14]. For synthesizing titanium and zirconium alloys with various d metals, alloying metals with melting temperatures close enough to titanium or zirconium those were mainly used. The investigation of the phase composition and the structure in alloys with a large difference in the melting points of initial components, the production of which by traditional techniques is very difficult is of great interest. Among such alloys are Zr–Zn alloys. The melting point of zirconium is 1855 °С, and that of zinc is 419.6 °С. The phase diagram of this system has not been completed yet on the zirconium side [15,16]. A high-temperature zirconium β phase is assumed to decompose during eutectoid reaction β → α + ZrZn at a temperature of 750 °C. The α phase has a hcp structure with lattice parameters а = 0.32312 nm, с = 0.51477 nm, and с/а = 1.593; and the ZrZn intermetallic compound has a CsCl (В2)
⁎
structure with the lattice parameter а = 0.3336 nm. Zirconium does virtually not dissolve in zinc and zinc in zirconium. An ω phase having a hexagonal structure forms in zirconium under pressure and it retains after the pressure is removed. We obtained the following lattice parameters of the ω phase: а = 0.5036 ± 0.0003 nm, с = 0.3130 ± 0.0003 nm, and с/а = 0.622. Theses parameters were the same for both ω phase formed by quasihydrostatic pressure and ω phase formed by shock waves [17]. No pressure-induced phase transitions were detected in zinc to a pressure of ~126 GPa [18]. Currently the study of structure of Zr–Zn alloys is strongly limited due to difficulty of their preparation. There are only data on structure of these alloys at very small content of zirconium [19] as well as data on structure of the alloys received by quenching from a vapor state [20]. Nowadays these alloys are generally used only as a master alloy for receiving high-strength alloys on a magnesium basis. However, Zr–Zn alloys are potentially perspective for development of materials with new functional properties, as they have invar properties and become magnetic at low temperatures [21–23]. The aim of the work is to synthesize Zr–Zn alloys from elemental powders by severe plastic deformation, in which the zinc content is below or equal to 50 at.%, and to study the phase composition and the structure of the alloys. 2. Experimental procedure Zirconium and zinc powder mixtures with 5, 10, 20, 30, 40, and 50 at.% zinc were taken for synthesizing the alloys. Zirconium powder and
Corresponding author. E-mail addresses: [email protected] (A.V. Dobromyslov), [email protected] (N.I. Taluts).
https://doi.org/10.1016/j.matchar.2019.109848 Received 29 May 2019; Received in revised form 2 August 2019; Accepted 2 August 2019 Available online 03 August 2019 1044-5803/ © 2019 Elsevier Inc. All rights reserved.
Materials Characterization 156 (2019) 109848
A.V. Dobromyslov and N.I. Taluts
Fig. 1. XRD patterns of the (a) initial Zr–40 at.% Zn powder mixture and (b, c) Zr–40 at.% Zn alloy synthesized by HPT at n = 5 at various pressures: (b) 3 and (c) 8 GPa.
zinc powder were prepared from iodide zirconium (99.9 wt%) and granulated zinc of an ChDA grade (99.95 wt%), respectively. The size of the powder particles was 10–30 μm. The alloys were synthesized by high pressure torsion (HPT) of powder mixtures between cylindrical Bridgman anvils at a pressure (P) of 3 and 8 GPa at room temperature. The rate of anvil rotation was 0.3 revolution per minute (rpm). The number of anvil revolutions n was 5 and 10. The average thickness of the obtained samples was 0.05 mm, and their diameter was 5 mm. The true strain was calculated according to [24,25] from the formula Fig. 2. XRD patterns of the alloys with various zinc contents, synthesized by HPT at a pressure of 8 GPa and n = 10: (a) Zr–5 at.% Zn, (b) Zr–10 at.% Zn, (c) Zr–20 at.% Zn, (d) Zr–30 at.% Zn, (e) Zr–40 at.% Zn, and (f) Zr–50 at.% Zn.
ε = εsh + εup = ln(1 + (2πnr / h)2)1/2 + ln(h 0 / h), where εsh is the shear strain, εup is the upsetting strain, n is the number of anvil revolution, r is the distance from the center of the sample and h0, h are the thicknesses of the sample before and after deformation, respectively. The true strain for r = 2.5 mm after deformation at n = 5 ε = 8.05, and after deformation at n = 10 ε = 8.74. The Zr–20 at.% Zn alloy was aged in a vacuum at 3 × 10−3 Pa and a temperature of 500 °C for 15 min. The structure of the synthesized alloys were analyzed by X-ray diffraction (XRD) and examined by transmission electron microscopy (TEM). XRD analysis was performed using a DRON-3 diffractometer with a diffracted-beam graphite monochromator in Cu Кα radiation. TEM examination was carried out using a JEM-200CX transmission electron microscope. Thin foils were prepared by chemical etching in a mixture of fluorhydric and nitric acids.
3. Results and discussion All the alloys prepared from the elemental powders exhibit a monolithic structure; however, their phase compositions and structures depend on the zinc content in the initial mixture, the pressure, and the deformation degree. The pressure effect on the phase composition and structure was studied only for 40 and 50 at.% Zn alloys. High pressure torsion at 3 GPa and n = 5 changes the XRD pattern in the following way. The ratio of the diffraction peaks intensities of the α zirconium phase changes, indicating the formation of a deformation-induced texture. The diffraction peaks width slightly increases due to the grain refinement (Fig. 1a, b). The intensity of all diffraction peaks of zinc decreases. A weak (110) ω-phase diffraction peak appears between (002) and (101) peaks of the α zirconium phase. The formation of a small amount 2
Materials Characterization 156 (2019) 109848
A.V. Dobromyslov and N.I. Taluts
Fig. 2 shows the XRD patterns of the zinc-bearing alloys synthesized at P = 8 GPa and n = 10. The figure suggests that the ω phase forms in all the alloys. Only the strongest (101) diffraction peak of the α phase is retained in XRD patterns. The intensity of the ω phase peaks decreases significantly with increasing zinc content. The diffraction pattern profoundly changes near the closely located (101) and (110) ω phase diffraction peaks and the (101) α phase peak (35–37°). The width of this scattering region increases continuously. The XRD patterns of the Zr–40 at.% Zn and Zr–50 at.% Zn alloys show its maximum width (Fig. 2e, f). The center of this region is approximately at the (011) diffraction peak of the ZrZn intermetallic compound. No peaks of zinc are observed in the XRD patterns. Diffraction peaks of the ω phase are shifted toward large angles θ, as compared to their position in diffraction patterns of pure zirconium. This indicates that ω phase lattice parameters decreases and, hence, zinc atoms dissolve in the zirconium crystal lattice. However, the very low intensity of the peaks at large angles and the absence of necessary peaks for calculating a and c make the precise determination of the ω phase lattice parameters impossible. A comparison of Figs. 1c and 2e suggests that the intensity of the diffraction peaks becomes lower, their width increases, and the area of diffuse scattering becomes more pronounced when the plastic deformation degree increases. In order to determine the reaction whereby the decomposition takes place in the synthesized alloys, the Zr–20 at.% Zn alloy was aged at 500 °C for 15 min. After this annealing, pronounced peaks of the α phase and no ω phase diffraction peaks are observed. In addition, the XRD patterns exhibit peaks of the ZrZn intermetallic compound (Fig. 3). The TEM examination has revealed that the structure of the synthesized samples depends on both the zinc content in the initial powder mixture and the deformation degree. Fig. 4 shows the structure in the Zr–10 at.% Zn alloy synthesized from elemental powders at P = 8 GPa and n = 5. Selected-area electron diffraction (SAED) patterns show pronounced textural maxima, which are related mainly to the ω phase (Fig. 4b). The textural maxima are strongly smeared around a ring. In some cases, electron diffraction patterns can also include individual zinc reflections. Dark-field images taken in ω phase reflections show regions of various shapes, which form clusters in some places. The
Fig. 3. XRD patterns of the Zr–20 at.% Zn alloy synthesized by HPT at 8 GPa and n = 10 and aged at 500 °C for 15 min.
of the ω phase at a pressure of 3 GPa can be explained by the fact that, regardless the critical pressure of α → ω transformation is 5–6 GPa [26], plastic deformation is able to decrease the transformation pressure to 2.8–3 GPa [27]. According to [28], the equilibrium pressure of the α and ω phases at room temperature is 2.2 GPa for zirconium. This is confirmed by the data of [29], in which, after HPT at 2 GPa, the ω phase was not observed in zirconium. HPT at a pressure of 8 GPa to 5 revolutions changes the diffraction pattern significantly. The diffraction peaks of zinc completely disappear and the intensity of the α-phase peaks is greatly decreases; and intensive ω-phase peaks appear in the XRD patterns of the synthesized alloys instead (Fig. 1c). The ω phase peaks are broadened substantially. An extended diffuse scattering region is observed in the 2θ angle region from 32° to 44°, reflecting the dominance of an amorphous phase in this alloy.
Fig. 4. Microstructure of the Zr–10 at.% Zn alloy synthesized by HPT at 8 GPa and n = 5: (а) bright-field image and (b) SAED pattern corresponding to (a), (c) darkfield images taken in the (001) reflection of the ω phase. 3
Materials Characterization 156 (2019) 109848
A.V. Dobromyslov and N.I. Taluts
Fig. 5. Microstructure of the Zr–20 at.% Zn alloy synthesized by HPT at 8 GPa and n = 5: (а) bright-field image, (b) SAED pattern corresponding to (a), (c) dark-field images taken in the (001) reflection of the ω phase, and (d) SAED pattern.
Fig. 6. Microstructure of the Zr–40 at.% Zn alloy synthesized by HPT at 8 GPa and n = (a) 5 and (b, c, d, e) 10: (а, b, e) dark-field images, (c) bright-field image, and (d) SAED pattern corresponding to (c).
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Materials Characterization 156 (2019) 109848
A.V. Dobromyslov and N.I. Taluts
Fig. 7. Microstructure of the Zr–50 at.% Zn alloy synthesized by HPT at 8 GPa and n = 5: (а, d, e) dark-field images, (b) bright-field image, and (c) SAED pattern corresponding to (b).
Fig. 8. Microstructure of the Zr–20 at.% Zn alloy synthesized by HPT at 8 GPa and n = 5 after aging at 500 °C for 15 min: (а) bright-field image and (b) SAED pattern corresponding to (a), (c) dark-field images taken in the group of closely located (011) α-phase and (011) ZrZn reflections.
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Materials Characterization 156 (2019) 109848
A.V. Dobromyslov and N.I. Taluts
average grain size in this sample is ~60 nm. The image of the ω phase (Fig. 4c) demonstrates a specific banding, which is induced by stacking faults located along planes {2110} in the ω phase structure [17]. An increase in the zinc content to 20 at.% results in a decrease in the average grain size to 42 nm and a more uniform structure in the alloy (Fig. 5). Textural maxima still exist in electron diffraction patterns, however they are smeared a lot. There are completely formed diffraction rings in SAED patterns in some cases (Fig. 5d), which testifies a large number of highly misoriented grains in the structure. A structure in alloys with 40 and 50 at.% Zn changes most significantly. There are regions with a crystalline structure and regions with an amorphous structure (Figs. 6, 7). The grain size of the ω phase in regions with the crystalline structure in the Zr–40 at.% alloy after deformation at n = 5 is 5–25 nm, whereas that in the Zr–50 at.% Zn alloy after deformation at the same n = 5 is 5–15 nm. An increase in the number of revolutions to n = 10 causes further structural refinement and an increase in the number of regions with an amorphous structure (Fig. 6b, c). The SAED patterns taken from amorphous regions in Zr–40 at.% Zn and Zr–50 at.% Zn alloys exhibit a diffuse halo. In some cases, it is also possible to detect two diffuse halos of different diameters (Figs. 6d, 7c). Dark-field images taken in the halo regions show the amorphous regions consisting of fine particles or clusters ~1–3 nm in size (Figs. 6e, 7d, f). In some places, such particles are arranged in regular rows. Their crystal structure cannot be determined, because of very small particle size. We can assume that such particles (clusters) have a ZrZn intermetallic structure, since the center of the diffuse scattering region in the diffraction patterns is in the (011)ZrZn diffraction peak. The peculiar crystal structure of the ω phase is responsible for the formation of such clusters. According to [30], the structure of the ω phase in zirconium and titanium, except for fine details, can be described by a lattice consisting of three bcc crystal lattices. Therefore, the introduction of a single zinc atom into the ω phase crystal lattice will immediately lead to the formation of a CsCl nucleus. The structure of the Zr–20 at.% Zn alloy after aging at 500 °С for 15 min consists of α-phase grains with equilibrium boundaries (Fig. 8a). In addition to α-phase reflections, electron diffraction patterns also include ZrZn intermetallic reflections (Fig. 8b). Dark-field images show ZrZn intermetallic particles 10–20 nm in size inside α phase grains (Fig. 8c). This indicates that the initial ω phase was saturated with zinc and its decomposition proceeded according to the following scheme:
zinc begins earlier than in zirconium layers, and the grain size in zinc is significantly less than in zirconium. Besides, zinc atoms partially pass into a crystal lattice of ω-phase of zirconium. These two factors explain the disappearance of diffraction peaks of zinc in the XRD patterns and its reflexes in SAED patterns of the studied alloys. The study showed that the necessary condition of appearance of an amorphous state in zirconium is the formation of supersaturated solid solution of zinc in zirconium. 4. Conclusions The phase composition and the structure of the Zr–Zn alloys with zinc content below and equal to 50 at.% and prepared from elemental powders by severe plastic deformation at a pressure of 3 and 8 GPa were investigated. It was established that the structure of the synthesized alloys depends on both the zinc content and the plastic deformation degree and pressure. Deformation at a pressure of 3 GPa just initiates the α → ω transformation, whereas deformation at 8 GPa intensively continues this transformation. The structure formed at small deformation degrees and low zinc content in the synthesized alloys is nonuniform. The grain size decreases and the structure uniformity increases with increasing the deformation degree and the zinc content. An amorphous layer formed in the Zr–40 at.% Zn and Zr–50 at.% Zn alloys. Aging of the Zr–20 at.% Zn alloy at 500 °C for 15 min caused the ω phase to transform into an α phase supersaturated with zinc, which subsequently decomposed to form an equilibrium α phase and a ZrZn intermetallic compound. Acknowledgements This work was performed in terms of the state assignment (theme “Pressure” No. АААА-А18-118020190104-3) and supported in part by the Presidium of the Ural Branch, Russian Academy of Sciences (project no. 18-10-2-24). The electron microscopic studies were performed at the Center of the Collaborative Access “Test Center of Nanotechnologies and Advanced Materials,” Institute of Metal Physics, Ural Branch, Russian Academy of Sciences. References
ωsat → α sat → α + ZrZn. As previous studies show, at the initial stage of alloy synthesis produced by the treatment in ball mills as well as by torsion in Bridgman anvils, there is a formation of the layered structure consisting of the alternating layers (lamellae) of initial metals [10,13,31,32]. Emergence of extended layers is connected with the fact that the plasticity of materials under pressure significantly increases [33]. So, for example, formation of lamellar structure from alternating thin layers of aluminum and iron was observed by TEM study of structure of the Al–Fe alloys obtained from element powders by intensive plastic deformation under pressure in Bridgman anvils [10]. It was shown that at increase in shear deformation the structure of alloys becomes more uniform, the thickness of lamellae decreases, and the lamellae begin to break into numerous fragments. Formation of nanocrystal structure happens both on the boundary of such fragments and inside them. In our case, at the initial stage of synthesis there is also a formation of the structure consisting of layers of zirconium and zinc. Affected by pressure and plastic deformation, zirconium undergoes the α – ω transformation. The mechanical properties of zirconium and zinc strongly differ, and this difference becomes greater after formation of the ω-phase. Because the tensile strength and yield strength of zirconium is significantly greater than the same values of zinc, at under the same loading conditions zinc is deformed much stronger than zirconium. As a result, the formation of nanocrystal structure in layers of
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[23] S. Ogawa, N. Sakamoto, Magnetic properties of ZrZn2–itinerant electron ferromagnet, J. Phys. Soc. Jpn. 22 (1967) 1214–1221. [24] M.V. Degtyarev, T.I. Chashchukhina, L.M. Voronova, A.M. Patselov, V.P. Pilyugin, Influence of the relaxation processes on the structure formation in pure metals and alloys under high-pressure torsion, Acta Mater. 55 (2007) 6039–6050. [25] A.P. Zhilyaev, T.G. Langdon, Using high-pressure torsion for metal processing: fundamentals and applications, Prog. Mater. Sci. 53 (2008) 893–979. [26] A. Jayaraman, W. Klement, G.C. Kennedy, Solid-solid transitions in titanium and zirconium at high pressure, Phys. Rev. 131 (1963) 644–649. [27] V.D. Blank, M.E. Weller, Yu.S. Konyaev, E.I. Estrin, α–ω transformation in zirconium upon deformation under high pressure, Fiz. Met. Metalloved. 47 (1979) 1109–1111. [28] V.A. Zilbershtein, N.P. Chistotina, A.A. Zharov, N.S. Grishina, E.I. Estrin, Alphaomega transformation in titanium and zirconium during shear deformation under pressure, Fiz. Met. Metalloved. 39 (1975) 445–447. [29] R. Haraguchi, Yu. Yoshimatsu, T. Nagaoka, M. Arita, Ka. Edalati, Z. Horita, Electrical resistivity mapping of titanium and zirconium discs processed by highpressure torsion for homogeneity and phase transformation evaluation, J. Mater. Sci. 52 (2017) 6778–6788. [30] U. Zwicker, Titan und Titanlegirungen, Springer-Verlag Berlin-Heidelberg, New York, 1974. [31] C. Suryanarayana, Mechanical alloying and milling, Prog. Mater. Sci. 46 (2001) 1–147. [32] I.A. Ditenberg, K.I. Denisov, A.N Tyumentsev., M.A. Korchagin, A.V. Korznikov Nanostructural states in Nb–Al mechanocomposite after combined deformation treatment, AIP Conf. Proc. 1683 (2015) 020041–1–020041–4. [33] P.W. Bridgman. Studies in large plastic flow and fracture, With Special Emphasis on the Effects of Hydrostatic Pressure, McGraw-Hill, New York-London, 1952.
deformation under pressure, Phys. Met. Metallogr. 117 (2016) 135–142. [12] A.V. Dobromyslov, N.I. Taluts, Structure of Al–Fe alloys prepared by different methods after severe plastic deformation under pressure, Phys. Met. Metallogr. 118 (2017) 564–571. [13] K.S. Kormout, P. Ghosh, V. Maier-Kiener, R. Pippan, Deformation mechanisms during severe plastic deformation of a Cu–Ag composite, J. Alloys Compd. 695 (2017) 2285–2294. [14] A.V. Dobromyslov, N.I. Taluts, Mechanical alloying of Ti–Fe alloys using severe plastic deformation by high-pressure torsion, Phys. Met. Metallogr. 119 (2018) 1127–1132. [15] J. Dutkiewicz, The Zn-Zr (zinc-zirconium) system, J. Phase Equil. 13 (1992) 430–433. [16] Phase Diagrams of Binary Metal Systems: A Handbook, Ed. by N.P. Lyakishev, vol. 3, Book 2, Mashinostroenie, Moscow, 2000. [17] N. Taluts, A. Dobromyslov, E. Kozlov, Features of the ω-phase formation in zirconium and its alloys under quasihydrostatic pressure and dynamic loading, High Pressure Res. 30 (2010) 35–38. [18] K. Takemura, Absence of the c/a anomaly in Zn under high pressure with a heliumpressure medium, Phys. Rev. B 60 (1999) 6171–6174. [19] M. Wątrobaa, W. Bednarczyka, J. Kawałkob, P. Bałaa, Effect of zirconium microaddition on the microstructure and mechanical properties of Zn–Zr alloys, Mater. Charact. 142 (2018) 187–194. [20] H. Yasuda, K. Sumiyama, Y. Nakamura, Nonequilibrium crystalline and amorphous Zr–Zn alloys produced by vapor quenching, Trans. Jpn. Inst. Metals 28 (9) (1987) 692–698. [21] O.A. Khomenko, Origin and specific features of invar anomalies of physical properties: Fe–Ni alloys with an FCC lattice, Phys. Met. Metallogr. 104 (2007) 146–156. [22] B.T. Matthias, R.M. Bozorth, Ferromagnetism of a zirconium-zinc compound, Phys. Rev. 109 (1958) 604.
7
Contents lists available at ScienceDirect
Materials Characterization journal homepage: www.elsevier.com/locate/matchar
Synthesis of Zr-Zn alloys from elemental powders by severe plastic deformation under pressure A.V. Dobromyslov, N.I. Taluts
T
⁎
Mikheev Institute of Metal Physics, Ural Branch, Russian Academy of Sciences, Ekaterinburg 620108, Russia
A R T I C LE I N FO
A B S T R A C T
Keywords: Zr–Zn alloys Mechanical alloying Structure Pressure
Zr–Zn alloys with a zinc content to 50 at.% have been prepared from elemental powders by severe plastic deformation under pressure. Synthesized alloys have been studied by X-ray diffraction analysis and transmission electron microscopy. The study suggests that both the plastic deformation degree and the zinc content are responsible for the grain size. High pressure torsion at a pressure of 8 GPa results in intensive α → ω transformation in all the alloys. It forms an amorphous structure in alloys with 40 and 50 at.% zinc. Aging of a Zr–20 at. % Zn alloy at 500 °C for 15 min causes transformation of an ω phase into a zinc supersaturated α phase, which subsequently decomposes to an equilibrium α phase and a ZrZn intermetallic compound.
1. Introduction Synthesis of alloys from various elemental metal powders by severe plastic deformation under pressure belongs to promising techniques for producing materials with new functional and mechanical properties. The advantage of this technique as compared to mechanical alloying in ball mills is the high synthesizing rate, no contaminating impurities, and a resulting bulk product. Deformation of powders in contrast to deformation of solid samples forms material with a fundamentally refined structure, accelerates mechanodiffusion kinetics due to an increase in the common area of interacting surfaces, facilitates the solubility of components into each other, and forms metastable phases, which are thermodynamically unstable under normal conditions [1–14]. For synthesizing titanium and zirconium alloys with various d metals, alloying metals with melting temperatures close enough to titanium or zirconium those were mainly used. The investigation of the phase composition and the structure in alloys with a large difference in the melting points of initial components, the production of which by traditional techniques is very difficult is of great interest. Among such alloys are Zr–Zn alloys. The melting point of zirconium is 1855 °С, and that of zinc is 419.6 °С. The phase diagram of this system has not been completed yet on the zirconium side [15,16]. A high-temperature zirconium β phase is assumed to decompose during eutectoid reaction β → α + ZrZn at a temperature of 750 °C. The α phase has a hcp structure with lattice parameters а = 0.32312 nm, с = 0.51477 nm, and с/а = 1.593; and the ZrZn intermetallic compound has a CsCl (В2)
⁎
structure with the lattice parameter а = 0.3336 nm. Zirconium does virtually not dissolve in zinc and zinc in zirconium. An ω phase having a hexagonal structure forms in zirconium under pressure and it retains after the pressure is removed. We obtained the following lattice parameters of the ω phase: а = 0.5036 ± 0.0003 nm, с = 0.3130 ± 0.0003 nm, and с/а = 0.622. Theses parameters were the same for both ω phase formed by quasihydrostatic pressure and ω phase formed by shock waves [17]. No pressure-induced phase transitions were detected in zinc to a pressure of ~126 GPa [18]. Currently the study of structure of Zr–Zn alloys is strongly limited due to difficulty of their preparation. There are only data on structure of these alloys at very small content of zirconium [19] as well as data on structure of the alloys received by quenching from a vapor state [20]. Nowadays these alloys are generally used only as a master alloy for receiving high-strength alloys on a magnesium basis. However, Zr–Zn alloys are potentially perspective for development of materials with new functional properties, as they have invar properties and become magnetic at low temperatures [21–23]. The aim of the work is to synthesize Zr–Zn alloys from elemental powders by severe plastic deformation, in which the zinc content is below or equal to 50 at.%, and to study the phase composition and the structure of the alloys. 2. Experimental procedure Zirconium and zinc powder mixtures with 5, 10, 20, 30, 40, and 50 at.% zinc were taken for synthesizing the alloys. Zirconium powder and
Corresponding author. E-mail addresses: [email protected] (A.V. Dobromyslov), [email protected] (N.I. Taluts).
https://doi.org/10.1016/j.matchar.2019.109848 Received 29 May 2019; Received in revised form 2 August 2019; Accepted 2 August 2019 Available online 03 August 2019 1044-5803/ © 2019 Elsevier Inc. All rights reserved.
Materials Characterization 156 (2019) 109848
A.V. Dobromyslov and N.I. Taluts
Fig. 1. XRD patterns of the (a) initial Zr–40 at.% Zn powder mixture and (b, c) Zr–40 at.% Zn alloy synthesized by HPT at n = 5 at various pressures: (b) 3 and (c) 8 GPa.
zinc powder were prepared from iodide zirconium (99.9 wt%) and granulated zinc of an ChDA grade (99.95 wt%), respectively. The size of the powder particles was 10–30 μm. The alloys were synthesized by high pressure torsion (HPT) of powder mixtures between cylindrical Bridgman anvils at a pressure (P) of 3 and 8 GPa at room temperature. The rate of anvil rotation was 0.3 revolution per minute (rpm). The number of anvil revolutions n was 5 and 10. The average thickness of the obtained samples was 0.05 mm, and their diameter was 5 mm. The true strain was calculated according to [24,25] from the formula Fig. 2. XRD patterns of the alloys with various zinc contents, synthesized by HPT at a pressure of 8 GPa and n = 10: (a) Zr–5 at.% Zn, (b) Zr–10 at.% Zn, (c) Zr–20 at.% Zn, (d) Zr–30 at.% Zn, (e) Zr–40 at.% Zn, and (f) Zr–50 at.% Zn.
ε = εsh + εup = ln(1 + (2πnr / h)2)1/2 + ln(h 0 / h), where εsh is the shear strain, εup is the upsetting strain, n is the number of anvil revolution, r is the distance from the center of the sample and h0, h are the thicknesses of the sample before and after deformation, respectively. The true strain for r = 2.5 mm after deformation at n = 5 ε = 8.05, and after deformation at n = 10 ε = 8.74. The Zr–20 at.% Zn alloy was aged in a vacuum at 3 × 10−3 Pa and a temperature of 500 °C for 15 min. The structure of the synthesized alloys were analyzed by X-ray diffraction (XRD) and examined by transmission electron microscopy (TEM). XRD analysis was performed using a DRON-3 diffractometer with a diffracted-beam graphite monochromator in Cu Кα radiation. TEM examination was carried out using a JEM-200CX transmission electron microscope. Thin foils were prepared by chemical etching in a mixture of fluorhydric and nitric acids.
3. Results and discussion All the alloys prepared from the elemental powders exhibit a monolithic structure; however, their phase compositions and structures depend on the zinc content in the initial mixture, the pressure, and the deformation degree. The pressure effect on the phase composition and structure was studied only for 40 and 50 at.% Zn alloys. High pressure torsion at 3 GPa and n = 5 changes the XRD pattern in the following way. The ratio of the diffraction peaks intensities of the α zirconium phase changes, indicating the formation of a deformation-induced texture. The diffraction peaks width slightly increases due to the grain refinement (Fig. 1a, b). The intensity of all diffraction peaks of zinc decreases. A weak (110) ω-phase diffraction peak appears between (002) and (101) peaks of the α zirconium phase. The formation of a small amount 2
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Fig. 2 shows the XRD patterns of the zinc-bearing alloys synthesized at P = 8 GPa and n = 10. The figure suggests that the ω phase forms in all the alloys. Only the strongest (101) diffraction peak of the α phase is retained in XRD patterns. The intensity of the ω phase peaks decreases significantly with increasing zinc content. The diffraction pattern profoundly changes near the closely located (101) and (110) ω phase diffraction peaks and the (101) α phase peak (35–37°). The width of this scattering region increases continuously. The XRD patterns of the Zr–40 at.% Zn and Zr–50 at.% Zn alloys show its maximum width (Fig. 2e, f). The center of this region is approximately at the (011) diffraction peak of the ZrZn intermetallic compound. No peaks of zinc are observed in the XRD patterns. Diffraction peaks of the ω phase are shifted toward large angles θ, as compared to their position in diffraction patterns of pure zirconium. This indicates that ω phase lattice parameters decreases and, hence, zinc atoms dissolve in the zirconium crystal lattice. However, the very low intensity of the peaks at large angles and the absence of necessary peaks for calculating a and c make the precise determination of the ω phase lattice parameters impossible. A comparison of Figs. 1c and 2e suggests that the intensity of the diffraction peaks becomes lower, their width increases, and the area of diffuse scattering becomes more pronounced when the plastic deformation degree increases. In order to determine the reaction whereby the decomposition takes place in the synthesized alloys, the Zr–20 at.% Zn alloy was aged at 500 °C for 15 min. After this annealing, pronounced peaks of the α phase and no ω phase diffraction peaks are observed. In addition, the XRD patterns exhibit peaks of the ZrZn intermetallic compound (Fig. 3). The TEM examination has revealed that the structure of the synthesized samples depends on both the zinc content in the initial powder mixture and the deformation degree. Fig. 4 shows the structure in the Zr–10 at.% Zn alloy synthesized from elemental powders at P = 8 GPa and n = 5. Selected-area electron diffraction (SAED) patterns show pronounced textural maxima, which are related mainly to the ω phase (Fig. 4b). The textural maxima are strongly smeared around a ring. In some cases, electron diffraction patterns can also include individual zinc reflections. Dark-field images taken in ω phase reflections show regions of various shapes, which form clusters in some places. The
Fig. 3. XRD patterns of the Zr–20 at.% Zn alloy synthesized by HPT at 8 GPa and n = 10 and aged at 500 °C for 15 min.
of the ω phase at a pressure of 3 GPa can be explained by the fact that, regardless the critical pressure of α → ω transformation is 5–6 GPa [26], plastic deformation is able to decrease the transformation pressure to 2.8–3 GPa [27]. According to [28], the equilibrium pressure of the α and ω phases at room temperature is 2.2 GPa for zirconium. This is confirmed by the data of [29], in which, after HPT at 2 GPa, the ω phase was not observed in zirconium. HPT at a pressure of 8 GPa to 5 revolutions changes the diffraction pattern significantly. The diffraction peaks of zinc completely disappear and the intensity of the α-phase peaks is greatly decreases; and intensive ω-phase peaks appear in the XRD patterns of the synthesized alloys instead (Fig. 1c). The ω phase peaks are broadened substantially. An extended diffuse scattering region is observed in the 2θ angle region from 32° to 44°, reflecting the dominance of an amorphous phase in this alloy.
Fig. 4. Microstructure of the Zr–10 at.% Zn alloy synthesized by HPT at 8 GPa and n = 5: (а) bright-field image and (b) SAED pattern corresponding to (a), (c) darkfield images taken in the (001) reflection of the ω phase. 3
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Fig. 5. Microstructure of the Zr–20 at.% Zn alloy synthesized by HPT at 8 GPa and n = 5: (а) bright-field image, (b) SAED pattern corresponding to (a), (c) dark-field images taken in the (001) reflection of the ω phase, and (d) SAED pattern.
Fig. 6. Microstructure of the Zr–40 at.% Zn alloy synthesized by HPT at 8 GPa and n = (a) 5 and (b, c, d, e) 10: (а, b, e) dark-field images, (c) bright-field image, and (d) SAED pattern corresponding to (c).
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Fig. 7. Microstructure of the Zr–50 at.% Zn alloy synthesized by HPT at 8 GPa and n = 5: (а, d, e) dark-field images, (b) bright-field image, and (c) SAED pattern corresponding to (b).
Fig. 8. Microstructure of the Zr–20 at.% Zn alloy synthesized by HPT at 8 GPa and n = 5 after aging at 500 °C for 15 min: (а) bright-field image and (b) SAED pattern corresponding to (a), (c) dark-field images taken in the group of closely located (011) α-phase and (011) ZrZn reflections.
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average grain size in this sample is ~60 nm. The image of the ω phase (Fig. 4c) demonstrates a specific banding, which is induced by stacking faults located along planes {2110} in the ω phase structure [17]. An increase in the zinc content to 20 at.% results in a decrease in the average grain size to 42 nm and a more uniform structure in the alloy (Fig. 5). Textural maxima still exist in electron diffraction patterns, however they are smeared a lot. There are completely formed diffraction rings in SAED patterns in some cases (Fig. 5d), which testifies a large number of highly misoriented grains in the structure. A structure in alloys with 40 and 50 at.% Zn changes most significantly. There are regions with a crystalline structure and regions with an amorphous structure (Figs. 6, 7). The grain size of the ω phase in regions with the crystalline structure in the Zr–40 at.% alloy after deformation at n = 5 is 5–25 nm, whereas that in the Zr–50 at.% Zn alloy after deformation at the same n = 5 is 5–15 nm. An increase in the number of revolutions to n = 10 causes further structural refinement and an increase in the number of regions with an amorphous structure (Fig. 6b, c). The SAED patterns taken from amorphous regions in Zr–40 at.% Zn and Zr–50 at.% Zn alloys exhibit a diffuse halo. In some cases, it is also possible to detect two diffuse halos of different diameters (Figs. 6d, 7c). Dark-field images taken in the halo regions show the amorphous regions consisting of fine particles or clusters ~1–3 nm in size (Figs. 6e, 7d, f). In some places, such particles are arranged in regular rows. Their crystal structure cannot be determined, because of very small particle size. We can assume that such particles (clusters) have a ZrZn intermetallic structure, since the center of the diffuse scattering region in the diffraction patterns is in the (011)ZrZn diffraction peak. The peculiar crystal structure of the ω phase is responsible for the formation of such clusters. According to [30], the structure of the ω phase in zirconium and titanium, except for fine details, can be described by a lattice consisting of three bcc crystal lattices. Therefore, the introduction of a single zinc atom into the ω phase crystal lattice will immediately lead to the formation of a CsCl nucleus. The structure of the Zr–20 at.% Zn alloy after aging at 500 °С for 15 min consists of α-phase grains with equilibrium boundaries (Fig. 8a). In addition to α-phase reflections, electron diffraction patterns also include ZrZn intermetallic reflections (Fig. 8b). Dark-field images show ZrZn intermetallic particles 10–20 nm in size inside α phase grains (Fig. 8c). This indicates that the initial ω phase was saturated with zinc and its decomposition proceeded according to the following scheme:
zinc begins earlier than in zirconium layers, and the grain size in zinc is significantly less than in zirconium. Besides, zinc atoms partially pass into a crystal lattice of ω-phase of zirconium. These two factors explain the disappearance of diffraction peaks of zinc in the XRD patterns and its reflexes in SAED patterns of the studied alloys. The study showed that the necessary condition of appearance of an amorphous state in zirconium is the formation of supersaturated solid solution of zinc in zirconium. 4. Conclusions The phase composition and the structure of the Zr–Zn alloys with zinc content below and equal to 50 at.% and prepared from elemental powders by severe plastic deformation at a pressure of 3 and 8 GPa were investigated. It was established that the structure of the synthesized alloys depends on both the zinc content and the plastic deformation degree and pressure. Deformation at a pressure of 3 GPa just initiates the α → ω transformation, whereas deformation at 8 GPa intensively continues this transformation. The structure formed at small deformation degrees and low zinc content in the synthesized alloys is nonuniform. The grain size decreases and the structure uniformity increases with increasing the deformation degree and the zinc content. An amorphous layer formed in the Zr–40 at.% Zn and Zr–50 at.% Zn alloys. Aging of the Zr–20 at.% Zn alloy at 500 °C for 15 min caused the ω phase to transform into an α phase supersaturated with zinc, which subsequently decomposed to form an equilibrium α phase and a ZrZn intermetallic compound. Acknowledgements This work was performed in terms of the state assignment (theme “Pressure” No. АААА-А18-118020190104-3) and supported in part by the Presidium of the Ural Branch, Russian Academy of Sciences (project no. 18-10-2-24). The electron microscopic studies were performed at the Center of the Collaborative Access “Test Center of Nanotechnologies and Advanced Materials,” Institute of Metal Physics, Ural Branch, Russian Academy of Sciences. References
ωsat → α sat → α + ZrZn. As previous studies show, at the initial stage of alloy synthesis produced by the treatment in ball mills as well as by torsion in Bridgman anvils, there is a formation of the layered structure consisting of the alternating layers (lamellae) of initial metals [10,13,31,32]. Emergence of extended layers is connected with the fact that the plasticity of materials under pressure significantly increases [33]. So, for example, formation of lamellar structure from alternating thin layers of aluminum and iron was observed by TEM study of structure of the Al–Fe alloys obtained from element powders by intensive plastic deformation under pressure in Bridgman anvils [10]. It was shown that at increase in shear deformation the structure of alloys becomes more uniform, the thickness of lamellae decreases, and the lamellae begin to break into numerous fragments. Formation of nanocrystal structure happens both on the boundary of such fragments and inside them. In our case, at the initial stage of synthesis there is also a formation of the structure consisting of layers of zirconium and zinc. Affected by pressure and plastic deformation, zirconium undergoes the α – ω transformation. The mechanical properties of zirconium and zinc strongly differ, and this difference becomes greater after formation of the ω-phase. Because the tensile strength and yield strength of zirconium is significantly greater than the same values of zinc, at under the same loading conditions zinc is deformed much stronger than zirconium. As a result, the formation of nanocrystal structure in layers of
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