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Structural and phase transformations in a new eutectic Al-Ca-Mn-Fe-Zr-Sc alloy induced by high pressure torsion
Accepted Manuscript Structural and phase transformations in a new eutectic Al-Ca-Mn-Fe-Zr-Sc alloy induced by high pressure torsion S.O. Rogachev, E.A. Naumova, R.V. Sundeev, N.Yu. Tabachkova PII: DOI: Reference:
S0167-577X(19)30267-8 https://doi.org/10.1016/j.matlet.2019.02.043 MLBLUE 25748
To appear in:
Materials Letters
Received Date: Revised Date: Accepted Date:
11 January 2019 31 January 2019 5 February 2019
Please cite this article as: S.O. Rogachev, E.A. Naumova, R.V. Sundeev, N.Yu. Tabachkova, Structural and phase transformations in a new eutectic Al-Ca-Mn-Fe-Zr-Sc alloy induced by high pressure torsion, Materials Letters (2019), doi: https://doi.org/10.1016/j.matlet.2019.02.043
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Structural and phase transformations in a new eutectic Al-Ca-Mn-Fe-Zr-Sc alloy induced by high pressure torsion S.O. Rogachev1,*, E.A. Naumova1,2, R.V. Sundeev3, N.Yu. Tabachkova1 1
The National University of Science and Technology “MISIS”, 4 Leninsky pr., Moscow,
119049, Russia 2
MSTU “STANKIN”, 3a Vadkovskiy Pereulok, Moscow, 127055, Russia
3
Moscow Technological University “MIREA”, 78 Vernadskogo av., Moscow, 119454,
Russia *Corresponding author; e-mail: [email protected]
Abstract. The effect of high-pressure torsion (HPT) on the structural and phase transformations in a new eutectic aluminium alloy (94.9%Al; 3.5%Ca; 0.9%Mn; 0.5%Fe; 0.1%Zr; 0.1%Sc) was studied. HPT leads to the formation of a nanocrystalline structure with a predominant grain size of 11–34 nm. HPT causes the refining of eutectic [(Al)+Al4Ca] and the formation of nanoclusters and segregations. The phase composition of the alloy after HPT tends to the phase composition at elevated temperatures. Keywords: eutectic Al-Ca alloy; high-pressure torsion; nanocrystalline materials; phase transformation; microstructure.
1. Introduction. Recently, there have been studies of multiphase eutectic alloys based on the Al-Ca system with high technological properties both after casting and plastic deformation [15]. The structure of these alloys consists of an aluminum solid solution (Al) and eutectic [(Al)+Al4Ca], which has a very thin structure. Due to the low density of the Al4Ca intermetallic compound (2.33 g/cm3), these alloys have a low density. In work [1], high corrosion resistance of the aluminum-calcium alloys was mentioned, and these data confirmed recent studies [2-5]. The authors of [2] showed that the addition of iron in an amount of up to 1% does not adversely affect the structure and properties of the Al-Ca alloys, since iron is part of the Al10CaFe2 ternary eutectic intermetallic compound. Manganese, zirconium and scandium form a supersaturated solid solution in aluminum
already during the casting process, which makes it possible to harden these alloys by annealing, without using the quenching operation [5]. Despite a number of the advantages described above, this Al-Ca alloy has medium strength. Finding the possibility of improving its mechanical properties is of undoubted interest. The use of severe plastic deformations (SPD) is a promising and rather welldeveloped method of refining the grain structure and improving the complex properties of various metallic materials [6]. SPD can significantly increase the strength of pure aluminum and aluminum alloys [7-9]. The greatest refining of the grain structure and the simultaneous extreme increase in strength is typical for the processing of aluminum alloys by high pressure torsion (HPT) [10, 11]. At the same time, aluminum alloys with a single-phase structure obtained after solid solution processing and quenching, or alloys containing a small amount of intermetallic phases, were mainly subjected to SPD. The number of works devoted to the study of the effect of SPD on changes in the structure and phase composition of multiphase aluminum alloys and especially eutectic is relatively small [12-14]. Therefore, the purpose of this work is to study of the effect of HPT on the structural and phase transformations of the eutectic Al-Ca type alloy.
2. Materials, experiment and methods. As the material for investigation, we used a cast eutectic aluminum alloy based on the Al-Ca system of the following chemical composition: 94.9%Al; 3.5%Ca; 0.9%Mn; 0.5%Fe; 0.1%Zr; 0.1%Sc. The melting was carried out in an electric resistance furnace in a graphite-chased crucible based on high purity aluminium (99.99%). Calcium, manganese, iron, zirconium and scandium were introduced into the aluminium melt in the form of binary master alloys (Al-15% Ca, Al-10% Mn, Al-10% Fe, Al-15% Zr and Al-2% Sc, respectively). The casting was carried out in a graphite mould at a temperature of ~780°C to obtain flat ingots with dimensions of 15×30×180 mm (the cooling rate during solidification was ~10 K/s). HPT was performed at room temperature, pressure of 6 GPa by one and five revolutions using samples 8 mm in diameter and 0.7 mm in initial thickness. HPT were carried out under constrained conditions, i.e. the sample was placed in a 0.3 mm deep
shaped hole located in the lower rotating anvil. After HPT, the thickness of the samples was about 0.4 mm. The Vickers microhardness was measured using a Micromet 5101 Buehler tester (0.5-N load and 10-s loading time). The microstructure of the samples was studied using a TESCAN VEGA ans JSMIT500 JEOL scanning electron microscopes (SEM) with EDS and a Axio Observer D1m Carl Zeiss optical microscope. Electron-microscopic studies of thin foils were performed using a JEM-2100 JEOL transmission electron microscope (TEM) with EDS. The studies were performed at the sample mid-radius. The transverse size of the structural elements was calculated from the dark-field image using the ImageExpert software. X-ray diffraction phase analysis was performed using a RIGAKU Ultima IV diffractometer and monochromatized CoKα radiation.
3. Results and discussion. The HPT deformation leds to an increase in the microhardness of the alloy from 0.58±0.02 MPa to 2.4–2.7 MPa (at the sample mid-radius), i.e. by 4.1–4.6 times. According to the optical microscopy data, the structure of the alloy in the initial state consisted of the large dendrites of the solid solution (Al) with an average size of 16±2 μm and eutectic [(Al)+Al4Ca] (Figure 1a). SEM analysis revealed a small amount of the iron and manganese enriched large particles in the initial alloy structure. According to the TEM data, an ultrafine-grained structure was formed in the alloy (at the sample mid-radius) after 1 revolution of HPT (Figure 1b, c). Both large fragments (or subgrains) with a size of 300–450 nm and recrystallized fine grains with a size of 50–300 nm are present. The TEM photographs of the microstructure of the sample subjected to HPT by one revolution show areas containing the ex-eutectic phase, while the Al4Ca particle size in these areas decreased from 1–3 μm to 5–10 nm or less (Figure 1c). EDS analysis of the alloy structure showed the presence of calcium-rich (2–10%) aluminum matrix areas and the presence of areas in which calcium was absent. An increase in the number of revolutions to five leds to the formation of a nanocrystalline structure in the alloy with a predominant grain size of 11–34 nm (Figure
1e, f). EDS microanalysis showed that the aluminum matrix of the alloy is rich in calcium (from 0.8 to 3%). At the same time, there are only rings from aluminum on the electron diffraction pattern. This may indicate the Al4Ca partial dissolution in the aluminum matrix. The iron and manganese enriched particles 0.5–2 µm in size (after 1 revolution) and 40–500 nm in size (after 5 revolutions) were observed on the TEM images of the HPT-alloy structure. According to the SEM data and EDS microanalysis, a contrast of dark gray and light gray areas is observed in the alloy structure after HPT by one revolution (Figure 2a). The calcium concentration in the dark gray areas varies from 0.4 to 2%, and in light gray areas – about 8%. This suggests that the light gray areas are ex-eutectic. An increase in the number of revolutions from one to five leads to a decrease in the contrast of the structure and the formation of a more homogeneous structure. According to the X-ray phase analysis data, the structure of the alloy in the initial state consisted of a solid solution (Al) and eutectic [(Al)+Al4Ca] with a volume fraction of the Al4Ca phase about 12%. The intensity of the Al4Ca X-ray lines substantially decreases already after one revolution of HPT. HPT by five revolutions resulted in the complete disappearance of the Al4Ca X-ray lines. At the same time, the appearance of the Al6(Mn, Fe) type phase was observed. Thus, HPT leads to the refining of the original large Al6(Mn, Fe) particles and, possibly, to the release of new Al6(Mn, Fe) particles. Based on the data of TEM and SEM analysis, the eutectic disappearance occurs due to the substantial refinement of the Al4Ca particles and the formation of nanoclusters and segregations instead the formation of a solid solution. Hovewer their partial dissolution in the aluminum matrix is possible. According to the equilibrium Al-Ca phase diagram, calcium is not soluble in a solid aluminum [15]. Considering the chemical composition of our alloy, calcium begins to dissolve in the aluminum matrix only at a temperature of about 615ºС (Figure 3) [4]. At these temperatures, the Al6(Mn, Fe) phase is present in the phase diagram. Thus, we can conclude that the phase composition of the alloy after HPT tends to the phase composition at a temperature above 615ºС, i.e. above the solidus line.
It is known that HPT can shift the temperature of phase transformations, and therefore a number of researchers have introduced the concept of effective temperature [16]. Its meaning is the fact that HPT can transfer the material to a state equivalent to its state at a certain (higher) temperature. This is explained by the fact that high pressures slow down the diffusion processes, and the forced motion of atoms during HPT transfers the system to a state equivalent to an increased (effective) temperature. Apparently this process contributes to the dissipation of excess energy introduced into the material during large plastic deformations [17]. The effective temperature will obviously depend, among other things, on the nature of the material itself. Either fragmentation or dissolution of the second phases was observed for many materials during severe plastic deformations: fragmentation of nitrides in AISI439 steel [18], fragmentation of Si-eutectic in Al-Si alloys [13], dissolution of cementite in perlitic steels [19], dissolution of intermetallic compounds in zirconium alloys [20] and aluminum alloys [14], etc., which can be associated with different effective temperature. However, as noted above, the disappearance of the phases may be associated with their substantial refining and formation of nanoclusters and segregations but not with the direct dissolution of the phases and the formation of a solid solution. The transformation of Al4Ca particles into nanoclusters and segregations may be due to the phenomenon of intense mass transfer in the material caused by HPT. This process is associated with inhomogeneous (turbulent) plastic flow during HPT, due to the stress gradients and different shear strain rates, as noted in works [21, 22]. The intensity of this process is probably related to the alloying elements, defects, heterogeneity of the material itself, and other factors.
4. Conclusions (1) The HPT deformation of a new eutectic aluminium alloy (94.9%Al; 3.5%Ca; 0.9%Mn; 0.5%Fe; 0.1%Zr; 0.1%Sc) leads to the formation of a nanocrystalline structure with a predominant grain size of 11–34 nm. (2) HPT causes the refining of eutectics [(Al)+Al4Ca] and the formation of nanoclusters and segregations. The phase composition of the alloy after HPT tends to the phase composition at elevated temperatures.
(3) HPT leds to an increase in the microhardness of the alloy from 0.58±0.02 MPa to 2.4–2.7 MPa, i.e. 4.1–4.6 times.
Acknowledgment The work was carried out with financial support from the Ministry of Education and Science of the Russian Federation in the framework of Increase Competitiveness Program of MISiS. The authors greatly thank V.M. Khatkevich and M.Yu. Magurina for the help with obtaining the results and their discussion.
References [1] Swaminathan K., Padmanabhan K.A., J. Mater. Sci. 25 (1990) 4579–4586. [2] Belov N.A., Naumova E.A., Ilyukhin V.D., Doroshenko V.V., Tsvetnye Metally. 3 (2017) 69–75. [3] Belov N.A., Naumova E.A., Akopyan T.K., Mater. Res. 18 (2015) 1384–1391. [4] Belov N.A., Batyshev K.A., Doroshenko V.V., Non-ferrous Metals. 2 (2017) 49–54. [5] Belov N.A., Naumova E.A., Alabin A.N., Matveeva I.A., J. Alloys Compd. 646 (2015) 741–747. [6] Valiev R.Z., Estrin Y., Horita Z., Langdon T.G., Zehetbauer M.J., Zhu Y.T., Mater. Res. Lett. 4 (2016) 1–21. [7] Orlov D., Beygelzimer Y., Synkov S., Varyukhin V., Tsuji N., Horita Z., Mater. Sci. Eng. A. 519 (2009) 105–111. [8] Nikulin S.A., Dobatkin S.V., Khanzhin V.G., Rogachev S.O., Chakushin S.A., Met. Sci. Heat Treat. 51 (2009) 208–217. [9] Horita Z, Fujinami T, Nemoto M, Langdon TG. Metall. Mater. Trans. A. 31 (2000) 691–701. [10] Loucif A., Figueiredo R.B., Baudin T., Brisset F., Langdon T.G., Mater. Sci. Eng. A. 527 (2010) 4864–4869. [11] Lee H.-J., Han J.-K., Janakiraman S., Ahn B., Kawasakia M., Langdond T.G., J. Alloys Compd. 686 (2016) 998–1007. [12] Kawasaki M., Foissey J., Langdon T.G., Mater. Sci. Eng. A. 561 (2013) 118– 125.
[13] Cepeda-Jiménez C.M., García-Infanta J.M., Zhilyaev A.P., Ruano O.A., Carreño F., J. Alloys Compd. 509 (2011) 636–643. [14] Cerri E, Leo P., Mater Sci Eng A. 410–411 (2005) 226–229. [15] Anglezio J.C., Servant C., Ansara I., Calphad. 18 (1994) 273–309. [16] Straumal B., Korneva A., Zieba P., Arch. Civ. Mech. Eng. 14 (2014) 242-249. [17] Glezer A.M., Metlov L.S., Phys. Solid State. 52 (2010) 1162–1169. [18] Rogachev S.O., Nikulin S.A., Khatkevich V.M., Steel Res. Int. 88 (2017) 1700070. [19] Ivanisenko Yu., Lojkowski W., Valiev R.Z., Fecht H.-J., Acta Mater. 51 (2003) 5555–5570. [20] Rogachev S.O., Nikulin S.A., Rozhnov A.B., Gorshenkov M.V., Adv. Eng. Mater. 20 (2018) 1800151. [21] Kulagin R., Beygelzimer Y., Ivanisenko Yu., Mazilkin A., Straumal B., Hahn H., Mater. Lett. 222 (2018) 172–175. [22] Kulagin R., Beygelzimer Y., Ivanisenko Y., Mazilkin A., Hahn H., IOP Conf. Ser.: Mater. Sci. Eng. 194 (2017) 012045.
Figure Captions
Figure 1. The microstructure of the aluminum alloy before (a) and after HPT by one (b-d) and five (e, f) revolutions: a – optical microscopy; b, d, e – the light field of TEM; c, f – the dark field of TEM Figure 2. The SEM-structure of the aluminum alloy after HPT by one (a) and five (b) revolutions Figure 3. Vertical section of the Al-Ca-Mn-Fe-Zr-Sc system with the content of 4%Ca, 0.7%Mn, 0.4%Fe and 0.1%Sc (the dotted arrow indicates the approximate chemical composition of the studied alloy) [4]
Highlights 1. A new eutectic Al-Ca-Mn-Fe-Zr-Sc aluminium alloy was subjected to HPT. 2. HPT leads to the formation of a nanocrystalline structure with a predominant grain size of 11–34 nm. 3. HPT causes the refining of the Al4Ca phase and the formation of nanoclusters and segregations. 4. HPT leds to an increase in the microhardness of the alloy by 4.1–4.6 times.
S0167-577X(19)30267-8 https://doi.org/10.1016/j.matlet.2019.02.043 MLBLUE 25748
To appear in:
Materials Letters
Received Date: Revised Date: Accepted Date:
11 January 2019 31 January 2019 5 February 2019
Please cite this article as: S.O. Rogachev, E.A. Naumova, R.V. Sundeev, N.Yu. Tabachkova, Structural and phase transformations in a new eutectic Al-Ca-Mn-Fe-Zr-Sc alloy induced by high pressure torsion, Materials Letters (2019), doi: https://doi.org/10.1016/j.matlet.2019.02.043
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Structural and phase transformations in a new eutectic Al-Ca-Mn-Fe-Zr-Sc alloy induced by high pressure torsion S.O. Rogachev1,*, E.A. Naumova1,2, R.V. Sundeev3, N.Yu. Tabachkova1 1
The National University of Science and Technology “MISIS”, 4 Leninsky pr., Moscow,
119049, Russia 2
MSTU “STANKIN”, 3a Vadkovskiy Pereulok, Moscow, 127055, Russia
3
Moscow Technological University “MIREA”, 78 Vernadskogo av., Moscow, 119454,
Russia *Corresponding author; e-mail: [email protected]
Abstract. The effect of high-pressure torsion (HPT) on the structural and phase transformations in a new eutectic aluminium alloy (94.9%Al; 3.5%Ca; 0.9%Mn; 0.5%Fe; 0.1%Zr; 0.1%Sc) was studied. HPT leads to the formation of a nanocrystalline structure with a predominant grain size of 11–34 nm. HPT causes the refining of eutectic [(Al)+Al4Ca] and the formation of nanoclusters and segregations. The phase composition of the alloy after HPT tends to the phase composition at elevated temperatures. Keywords: eutectic Al-Ca alloy; high-pressure torsion; nanocrystalline materials; phase transformation; microstructure.
1. Introduction. Recently, there have been studies of multiphase eutectic alloys based on the Al-Ca system with high technological properties both after casting and plastic deformation [15]. The structure of these alloys consists of an aluminum solid solution (Al) and eutectic [(Al)+Al4Ca], which has a very thin structure. Due to the low density of the Al4Ca intermetallic compound (2.33 g/cm3), these alloys have a low density. In work [1], high corrosion resistance of the aluminum-calcium alloys was mentioned, and these data confirmed recent studies [2-5]. The authors of [2] showed that the addition of iron in an amount of up to 1% does not adversely affect the structure and properties of the Al-Ca alloys, since iron is part of the Al10CaFe2 ternary eutectic intermetallic compound. Manganese, zirconium and scandium form a supersaturated solid solution in aluminum
already during the casting process, which makes it possible to harden these alloys by annealing, without using the quenching operation [5]. Despite a number of the advantages described above, this Al-Ca alloy has medium strength. Finding the possibility of improving its mechanical properties is of undoubted interest. The use of severe plastic deformations (SPD) is a promising and rather welldeveloped method of refining the grain structure and improving the complex properties of various metallic materials [6]. SPD can significantly increase the strength of pure aluminum and aluminum alloys [7-9]. The greatest refining of the grain structure and the simultaneous extreme increase in strength is typical for the processing of aluminum alloys by high pressure torsion (HPT) [10, 11]. At the same time, aluminum alloys with a single-phase structure obtained after solid solution processing and quenching, or alloys containing a small amount of intermetallic phases, were mainly subjected to SPD. The number of works devoted to the study of the effect of SPD on changes in the structure and phase composition of multiphase aluminum alloys and especially eutectic is relatively small [12-14]. Therefore, the purpose of this work is to study of the effect of HPT on the structural and phase transformations of the eutectic Al-Ca type alloy.
2. Materials, experiment and methods. As the material for investigation, we used a cast eutectic aluminum alloy based on the Al-Ca system of the following chemical composition: 94.9%Al; 3.5%Ca; 0.9%Mn; 0.5%Fe; 0.1%Zr; 0.1%Sc. The melting was carried out in an electric resistance furnace in a graphite-chased crucible based on high purity aluminium (99.99%). Calcium, manganese, iron, zirconium and scandium were introduced into the aluminium melt in the form of binary master alloys (Al-15% Ca, Al-10% Mn, Al-10% Fe, Al-15% Zr and Al-2% Sc, respectively). The casting was carried out in a graphite mould at a temperature of ~780°C to obtain flat ingots with dimensions of 15×30×180 mm (the cooling rate during solidification was ~10 K/s). HPT was performed at room temperature, pressure of 6 GPa by one and five revolutions using samples 8 mm in diameter and 0.7 mm in initial thickness. HPT were carried out under constrained conditions, i.e. the sample was placed in a 0.3 mm deep
shaped hole located in the lower rotating anvil. After HPT, the thickness of the samples was about 0.4 mm. The Vickers microhardness was measured using a Micromet 5101 Buehler tester (0.5-N load and 10-s loading time). The microstructure of the samples was studied using a TESCAN VEGA ans JSMIT500 JEOL scanning electron microscopes (SEM) with EDS and a Axio Observer D1m Carl Zeiss optical microscope. Electron-microscopic studies of thin foils were performed using a JEM-2100 JEOL transmission electron microscope (TEM) with EDS. The studies were performed at the sample mid-radius. The transverse size of the structural elements was calculated from the dark-field image using the ImageExpert software. X-ray diffraction phase analysis was performed using a RIGAKU Ultima IV diffractometer and monochromatized CoKα radiation.
3. Results and discussion. The HPT deformation leds to an increase in the microhardness of the alloy from 0.58±0.02 MPa to 2.4–2.7 MPa (at the sample mid-radius), i.e. by 4.1–4.6 times. According to the optical microscopy data, the structure of the alloy in the initial state consisted of the large dendrites of the solid solution (Al) with an average size of 16±2 μm and eutectic [(Al)+Al4Ca] (Figure 1a). SEM analysis revealed a small amount of the iron and manganese enriched large particles in the initial alloy structure. According to the TEM data, an ultrafine-grained structure was formed in the alloy (at the sample mid-radius) after 1 revolution of HPT (Figure 1b, c). Both large fragments (or subgrains) with a size of 300–450 nm and recrystallized fine grains with a size of 50–300 nm are present. The TEM photographs of the microstructure of the sample subjected to HPT by one revolution show areas containing the ex-eutectic phase, while the Al4Ca particle size in these areas decreased from 1–3 μm to 5–10 nm or less (Figure 1c). EDS analysis of the alloy structure showed the presence of calcium-rich (2–10%) aluminum matrix areas and the presence of areas in which calcium was absent. An increase in the number of revolutions to five leds to the formation of a nanocrystalline structure in the alloy with a predominant grain size of 11–34 nm (Figure
1e, f). EDS microanalysis showed that the aluminum matrix of the alloy is rich in calcium (from 0.8 to 3%). At the same time, there are only rings from aluminum on the electron diffraction pattern. This may indicate the Al4Ca partial dissolution in the aluminum matrix. The iron and manganese enriched particles 0.5–2 µm in size (after 1 revolution) and 40–500 nm in size (after 5 revolutions) were observed on the TEM images of the HPT-alloy structure. According to the SEM data and EDS microanalysis, a contrast of dark gray and light gray areas is observed in the alloy structure after HPT by one revolution (Figure 2a). The calcium concentration in the dark gray areas varies from 0.4 to 2%, and in light gray areas – about 8%. This suggests that the light gray areas are ex-eutectic. An increase in the number of revolutions from one to five leads to a decrease in the contrast of the structure and the formation of a more homogeneous structure. According to the X-ray phase analysis data, the structure of the alloy in the initial state consisted of a solid solution (Al) and eutectic [(Al)+Al4Ca] with a volume fraction of the Al4Ca phase about 12%. The intensity of the Al4Ca X-ray lines substantially decreases already after one revolution of HPT. HPT by five revolutions resulted in the complete disappearance of the Al4Ca X-ray lines. At the same time, the appearance of the Al6(Mn, Fe) type phase was observed. Thus, HPT leads to the refining of the original large Al6(Mn, Fe) particles and, possibly, to the release of new Al6(Mn, Fe) particles. Based on the data of TEM and SEM analysis, the eutectic disappearance occurs due to the substantial refinement of the Al4Ca particles and the formation of nanoclusters and segregations instead the formation of a solid solution. Hovewer their partial dissolution in the aluminum matrix is possible. According to the equilibrium Al-Ca phase diagram, calcium is not soluble in a solid aluminum [15]. Considering the chemical composition of our alloy, calcium begins to dissolve in the aluminum matrix only at a temperature of about 615ºС (Figure 3) [4]. At these temperatures, the Al6(Mn, Fe) phase is present in the phase diagram. Thus, we can conclude that the phase composition of the alloy after HPT tends to the phase composition at a temperature above 615ºС, i.e. above the solidus line.
It is known that HPT can shift the temperature of phase transformations, and therefore a number of researchers have introduced the concept of effective temperature [16]. Its meaning is the fact that HPT can transfer the material to a state equivalent to its state at a certain (higher) temperature. This is explained by the fact that high pressures slow down the diffusion processes, and the forced motion of atoms during HPT transfers the system to a state equivalent to an increased (effective) temperature. Apparently this process contributes to the dissipation of excess energy introduced into the material during large plastic deformations [17]. The effective temperature will obviously depend, among other things, on the nature of the material itself. Either fragmentation or dissolution of the second phases was observed for many materials during severe plastic deformations: fragmentation of nitrides in AISI439 steel [18], fragmentation of Si-eutectic in Al-Si alloys [13], dissolution of cementite in perlitic steels [19], dissolution of intermetallic compounds in zirconium alloys [20] and aluminum alloys [14], etc., which can be associated with different effective temperature. However, as noted above, the disappearance of the phases may be associated with their substantial refining and formation of nanoclusters and segregations but not with the direct dissolution of the phases and the formation of a solid solution. The transformation of Al4Ca particles into nanoclusters and segregations may be due to the phenomenon of intense mass transfer in the material caused by HPT. This process is associated with inhomogeneous (turbulent) plastic flow during HPT, due to the stress gradients and different shear strain rates, as noted in works [21, 22]. The intensity of this process is probably related to the alloying elements, defects, heterogeneity of the material itself, and other factors.
4. Conclusions (1) The HPT deformation of a new eutectic aluminium alloy (94.9%Al; 3.5%Ca; 0.9%Mn; 0.5%Fe; 0.1%Zr; 0.1%Sc) leads to the formation of a nanocrystalline structure with a predominant grain size of 11–34 nm. (2) HPT causes the refining of eutectics [(Al)+Al4Ca] and the formation of nanoclusters and segregations. The phase composition of the alloy after HPT tends to the phase composition at elevated temperatures.
(3) HPT leds to an increase in the microhardness of the alloy from 0.58±0.02 MPa to 2.4–2.7 MPa, i.e. 4.1–4.6 times.
Acknowledgment The work was carried out with financial support from the Ministry of Education and Science of the Russian Federation in the framework of Increase Competitiveness Program of MISiS. The authors greatly thank V.M. Khatkevich and M.Yu. Magurina for the help with obtaining the results and their discussion.
References [1] Swaminathan K., Padmanabhan K.A., J. Mater. Sci. 25 (1990) 4579–4586. [2] Belov N.A., Naumova E.A., Ilyukhin V.D., Doroshenko V.V., Tsvetnye Metally. 3 (2017) 69–75. [3] Belov N.A., Naumova E.A., Akopyan T.K., Mater. Res. 18 (2015) 1384–1391. [4] Belov N.A., Batyshev K.A., Doroshenko V.V., Non-ferrous Metals. 2 (2017) 49–54. [5] Belov N.A., Naumova E.A., Alabin A.N., Matveeva I.A., J. Alloys Compd. 646 (2015) 741–747. [6] Valiev R.Z., Estrin Y., Horita Z., Langdon T.G., Zehetbauer M.J., Zhu Y.T., Mater. Res. Lett. 4 (2016) 1–21. [7] Orlov D., Beygelzimer Y., Synkov S., Varyukhin V., Tsuji N., Horita Z., Mater. Sci. Eng. A. 519 (2009) 105–111. [8] Nikulin S.A., Dobatkin S.V., Khanzhin V.G., Rogachev S.O., Chakushin S.A., Met. Sci. Heat Treat. 51 (2009) 208–217. [9] Horita Z, Fujinami T, Nemoto M, Langdon TG. Metall. Mater. Trans. A. 31 (2000) 691–701. [10] Loucif A., Figueiredo R.B., Baudin T., Brisset F., Langdon T.G., Mater. Sci. Eng. A. 527 (2010) 4864–4869. [11] Lee H.-J., Han J.-K., Janakiraman S., Ahn B., Kawasakia M., Langdond T.G., J. Alloys Compd. 686 (2016) 998–1007. [12] Kawasaki M., Foissey J., Langdon T.G., Mater. Sci. Eng. A. 561 (2013) 118– 125.
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Figure Captions
Figure 1. The microstructure of the aluminum alloy before (a) and after HPT by one (b-d) and five (e, f) revolutions: a – optical microscopy; b, d, e – the light field of TEM; c, f – the dark field of TEM Figure 2. The SEM-structure of the aluminum alloy after HPT by one (a) and five (b) revolutions Figure 3. Vertical section of the Al-Ca-Mn-Fe-Zr-Sc system with the content of 4%Ca, 0.7%Mn, 0.4%Fe and 0.1%Sc (the dotted arrow indicates the approximate chemical composition of the studied alloy) [4]
Highlights 1. A new eutectic Al-Ca-Mn-Fe-Zr-Sc aluminium alloy was subjected to HPT. 2. HPT leads to the formation of a nanocrystalline structure with a predominant grain size of 11–34 nm. 3. HPT causes the refining of the Al4Ca phase and the formation of nanoclusters and segregations. 4. HPT leds to an increase in the microhardness of the alloy by 4.1–4.6 times.