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Characterization of nanostructured CuCr bulk composites prepared by high-energy mechanical alloying
Materials Chemistry and Physics xxx (2016) 1e7
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Characterization of nanostructured CueCr bulk composites prepared by high-energy mechanical alloying A.S. Prosviryakov*, A.I. Bazlov National University of Science and Technology “MISiS”, Leninskiy Prospect 4, Moscow 119049, Russia
h i g h l i g h t s Copper chips were used as raw material for production of Cu-(20e50%)Cr composites. Microstructure and properties of hot-pressed samples were studied. After 5 h milling the hardness of the composite with 50%Cr was increased to 600 HV. With the increase of milling time, the wear resistance was raised by 30 times.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 13 October 2015 Received in revised form 9 March 2016 Accepted 31 March 2016 Available online xxx
In this paper, the structure and properties of metal matrix composite materials on the basis of copper reinforced with chromium (20e50 wt%) were investigated. Raw materials consisted of grinded copper chips 5000 mm in size and chromium particles with an initial size of 10 mm. The composites were produced by mechanical alloying in a high-energy planetary mill with a quasi-cylindrical grinding body followed by hot pressing at 650 С. The milling duration was 1e10 h. It was shown that microstructure refinement occurs in the composites with an increase in chromium content and milling time. The structure of Cu-50 wt% Cr bulk materials after mechanical alloying for 5 h followed by hot pressing consists of the Cu and Cr-rich phases with average crystallite sizes of 37 and 28 nm, respectively. During short time milling the hardness of this composite increases to 600 HV due to the high intensity of mechanical alloying. At the same time, its wear resistance increases almost by 30 times in comparison with 1 h milling. On the contrary, electrical conductivity decreases after mechanical alloying. © 2016 Elsevier B.V. All rights reserved.
Keywords: Composite materials Powder metallurgy Powder diffraction Electron microscopy Hardness Electrical properties
1. Introduction CueCr composites produced using the conventional powder metallurgical technique are well-known and widely used as electrical switches due to their high arc resistance [1,2]. Another method of producing this material is mechanical alloying. Mechanical alloying is an advanced and efficient method of producing nanostructured composite materials with extreme properties [3e6]. The unique feature of this method is that the process occurs in the solid state and is accompanied by refinement of all the structural components. Mechanical alloying of copper with insoluble chromium produces a combination of good strength and electrical conductivity [7e11]. In all works that dealt with CueCr composites, mechanical alloying was carried out with milling balls,
* Corresponding author. E-mail address: [email protected] (A.S. Prosviryakov).
and copper powder was used as a raw material. To reduce the production costs of the materials prepared by mechanical alloying it would be desirable to use cheap scrap materials, e. g. chips, instead of copper powder. Mechanical milling of large-sized metallic particles is most efficient if a non-typical quasi-cylindrical grinding body is used instead of conventional grinding balls. During milling with a quasicylindrical grinding body the mechanical energy delivered per unit volume of the processed material is dozens of times higher than the mechanical energy achieved when grinding balls are used, and therefore the efficiency of processing is much higher [12]. Moreover, mechanical alloying with a quasi-cylinder grinding body occurs at a substantially lower average temperature. The efficiency of quasi-cylindrical grinding bodies for the production of mechanically alloyed composite materials from large-sized particles of copper was shown earlier [13,14]. In the Author's recent work [13] the thermal stability of mechanically alloyed Cu50Cr composite was
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estimated, and this material showed good heat resistance. The aim of the present paper is to analyze the effect of chromium content and milling time on the structure and basic properties (hardness, electrical conductivity and wear resistance) of CueCr composites produced by high-energy mechanical alloying from copper chips as raw material. 2. Materials and methods Grinded chips of 99.95% pure copper less than 5000 mm in size and chromium powder with an average particle size of 10 mm were used as the initial materials for composite material preparation. Mechanical alloying of CueCr powders with 20, 30, 40 and 50 wt% chromium content was performed in a “Gefest 11-3” planetary mill with a single quasi-cylindrical grinding body [15] in an argon atmosphere. The cross-section of this body is an equilateral triangle with convex sides obtained by the intersection of three 80 mm diameter circles inscribed in a 61 mm diameter circle. The body weighs about 1 kg and is 50 mm in height. The grinding medium and jars with a volume of 800 cm3 used for mechanical alloying were made of hardened chromium steel. The weight ratio between the grinding body and the powder was 4:1, and the milling speed was 600 rpm. The milling time was 1e10 h. The mechanically alloyed powders were compacted by uniaxial cold pressing followed by hot pressing at 650 C for 20 min. The cylindrical bulk specimens that were produced had the following sizes and were processed at the following pressures: the specimen for studying the structure and properties (hardness, electrical conductivity and density) was 10 mm in diameter (d) and 5 mm in height (h) and compacted at a pressure of 760 MPa, and the specimen for studying wear resistance had sizes 25 10 (d h) and compacted at a pressure of 200 MPa,. The structure of the compacted composites was examined by scanning electron microscopy (SEM), transmission electron microscopy (TEM) and X-ray diffraction (XRD). Xray diffraction analysis was performed on a D8 Bruker diffractometer using Cu Ka radiation. Lattice parameter, crystallite size and lattice microstrain of the specimens were determined by Rietveld analysis with TOPAS v 4.2 (Bruker-AXS) software. TEM analysis was carried out on a JEM-2000EX. Thin foils for electron microscopy were prepared using the Gatan Precision Ion Polishing System (PIPS). The hardness of the specimens was measured using a Vickers tester with a 5 kg load for 10 s. Electrical conductivity measurements were performed using the eddy-current method. The density of the specimens was examined using the hydrostatic weighing method. The wear resistance of the specimens was tested using a Tribometer, CSM Instr. with a 5 N load and at a 100 mm/s velocity using a hardened steel ball 3 mm in diameter. The depth of the wear track was measured using an Alpha-Step200 profilometer. 3. Results and discussion The initial stage of mechanical alloying with copper chips that were used as the raw material causes bending and twisting of the matrix particles followed by trapping of the free reinforcing particles in the resultant voids and welding of adjacent layers with each other [16]. At the same time, in the case of a binary metal system where the elements are ductile the formation of composite particles and the refinement of the structure occur predominantly by cold welding and fracturing of both metallic components [3,17]. The ductile particles get flattened due to a micro-forging process. At the next stage, these flattened particles are cold welded together and form a composite with a lamellar structure of the constituent metals as can be seen in Fig. 1. Here, the microstructure of Cue50% Cr composite powder particles after 1 h milling is presented in which copper lamellas randomly alternate with chromium
Fig. 1. Layered structure of Cue50%Cr composite powder particles after 1 h milling (light microscopy).
lamellas. Similar structures were obtained in other metal-metal systems [18,19]. Fig. 2 shows the structure of hot pressed CueCr specimens containing 30 to 50 wt% Cr examined using SEM in reflected electron mode. It can be seen that after 1 h processing in a planetary mill (Fig. 2(a), (b) and (c)) the structure of the materials consists of two components appearing as light and grey regions. Comparison of the atomic numbers of copper (Z ¼ 29) and chromium (Z ¼ 24) suggests that the light regions are Cu-rich ones while the grey regions contain almost pure chromium. With an increase in the chromium content from 30 to 40% (Fig. 2(a) and (b)) the structure of the material becomes more fragmented, and the Cu and Cr regions decrease in size. Higher chromium alloying (to 50%) affects the structural pattern only slightly (Fig. 2(c)). An increase in the time of mechanical alloying from 1 to 5 h results in structural refinement and significant fragmentation of chromium (Fig. 2 (c)-(d)). The Cr fragments become so fine that the clear separation into two structural components disappears. Further milling leads to the formation of an even more homogeneous structure (Fig. 2(e)). The absence of new phase formation after mechanical alloying followed by hot pressing was proven using X-ray analysis. The diffraction patterns obtained for the compact Cue50Cr composite specimens contain only the copper and chromium lines (Fig. 3). An increase in the time of processing caused a significant broadening of the X-ray lines indicating a decrease in the sizes of the crystallites and an increase in the degree of microstrain as a result of intense plastic strain. X-ray line broadening due to the abovementioned processes after mechanical alloying of CueCr powders was demonstrated earlier [7,9,10,20e22]. Results of X-ray analysis of the compact Cue50Cr specimens are summarized in Table 1. These results suggest that the broadening of X-ray lines after hot pressing is mainly caused by a decrease of the crystallite sizes occurring with an increase in the time of mechanical alloying. For example, the average size of the copper crystallites after 5 h milling and compaction is 37 nm. However, the degree of microstrain changes only slightly with an increase in processing time, its level being by more than one order of magnitude lower than that for mechanically alloyed powders. This can be caused by recovery processes occurring during hot pressing. Thus, the structure of the hot pressed material remains nanocrystalline, and this is confirmed by TEM data. Fig. 4(a) shows a light-field image of the structure of the compacted specimen after 5 h processing where it can be seen that the structure of the material is fine-grained and consists of crystallites with sizes of within 100 nm. The average crystallite size (about 40 nm) observed in the TEM image is in a good agreement with the values calculated on the
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Fig. 2. SEM micrographs of hot-pressed CueCr composites after different milling times: (a) Cue30%Cr, 1 h milling; (b) Cue40%Cr, 1 h; (c) Cue50%Cr, 1 h; (d) Cue50%Cr, 3 h; (e) Cue50%Cr, 5 h.
Cu
Cr
Intensity (a.u.)
5h
3h
1h 40
50
60
70 2θ (Degree)
80
90
100
Fig. 3. XRD patterns of Cue50%Cr composite after different milling times and subsequent hot pressing.
basis of XRD data. This image also shows dislocation clusters, the density of which for copper is 1.5,1014 m2 according to X-ray data. The dislocation density r was calculated on the basis of the crystallite sizes D and the values microstrain hε2 i1=2 (Table 1) using pffiffiffi of 2 1=2 i the formula [20]: r ¼ 2pffiffiffi3 hεDb , where b is the Burgers vector which for FCC metals is a 2 2 (a is lattice parameter). Fig. 4(b) shows a Cu-rich region, where there are fine grinded chromium particles (dark) with sizes of 20e30 nm embedded into copper as a result of mechanical alloying; they were not resolved by SEM. The structure and composition of materials are known to determine their properties. Fig. 5 illustrates the effect of chromium content on the hardness and electrical conductivity of CueCr specimens after 1 h milling followed by hot pressing in comparison with the hardness and electrical conductivity of pure copper (0 wt% Cr). It can be seen that the hardness of the composites grows linearly with an increase in the content of chromium which is harder than copper. The extrapolated value for 0% Cr (130 HV) is closely to the value for the nanocrystalline copper with a grain size of about 50 nm [23]. However, the electrical conductivity of the material
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Table 1 Lattice parameter, crystallite size and microstrain of copper and chromium in Cue50%Cr bulk composites for different milling times. Milling time (h)
Lattice parameter (nm)
1 3 5
Crystallite size (nm)
Lattice microstrain (%)
Cu
Cr
Cu
Cr
Cu
Cr
3.6152 ± 0.0002 3.6158 ± 0.0002 3.6158 ± 0.0002
2.8851 ± 0.0003 2.8856 ± 0.0003 2.8847 ± 0.0003
48 ± 3 40 ± 2 37 ± 2
39 ± 2 33 ± 2 28 ± 3
0.041 ± 0.004 0.042 ± 0.003 0.042 ± 0.005
0.028 ± 0.003 0.070 ± 0.005 0.050 ± 0.008
(a)
35
Electrical conductivity (%IACS)
decreases linearly with an increase in the chromium content (see Fig. 5) because the electrical conductivity of chromium is substantially lower than that of copper. The electrical conductivity of
30
25
20
15
10 0
1
2
3 Milling time (h)
4
5
6
(b) 7.97
100
7.94
99
7.88 7.85 7.82
98
Density (%)
Density (g·cm-3)
7.91
7.79 7.76 Fig. 4. TEM images of Cue50%Cr composite after 5 h milling and hot pressing: (a) nanocrystalline structure; (b) chromium nanoparticles.
7.73
(c) 100 Hardness
90
Elec. conductivity
80
250
70 60
200 50 150
40 30
100
1
2
3 4 Milling time (h)
5
6
700 600
Hardness HV (kg·mm-2)
Hardness HV (kg·mm-2)
300
Electrical conductivity (%IACS)
350
97 0
500 400 300 200
20 50
10 0
10
20 30 40 Content of Cr (wt%)
50
60
Fig. 5. Effect of chromium content on the hardness and electrical conductivity of hotpressed CueCr composites milled for 1 h.
100 0
1
2
3
4 5 6 Milling time (h)
7
8
9
10
Fig. 6. Effect of milling time on the electrical conductivity (a), density (b) and hardness (c) of hot-pressed Cue50%Cr composite.
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pure copper shown in the graph is higher than the value obtained by extrapolation. Furthermore, the electrical conductivity decreases with an increase in milling time, as can be seen in Fig. 6(a). This can be caused by several factors, e.g. the formation of a supersaturated chromium solid solution in copper, grain refinement during mechanical alloying, the formation of residual pores after hot pressing and, finally, contamination of the material with iron delivered from the milling body and the container. In this work we observed an increase in the mutual solubility of chromium and copper indicated by the shift of the diffraction lines in the diffraction patterns and an increase in the lattice period of both copper and chromium (Table 1). An increase in the solubility of chromium in copper was also observed elsewhere [10]. After 5 h milling followed by compaction, the lattice parameter of copper was 0.36158 nm compared to 0.36150 for pure copper. The solubility of chromium in copper increases due to an increase in the diffusion rate during intense plastic deformation caused by mechanical alloying which is accompanied by the formation of a large number of crystal defects [3,24]. Based on the commonly available data on the lattice parameter of the supersaturated copper solid solution [25] we assessed chromium solubility in copper for 5 h milling using Vegard's rule [26]. Its value was approx. 0.2 wt% whereas at temperatures below 400 C the equilibrium chromium content in the copper solid solution is less than 0.03 wt% [27]. It should be emphasized that the supersaturated solid solution formed during mechanical alloying remains in the structure after hot pressing. The electrical conductivity of the material decreases with an increase in the nonequilibrium content of chromium due to electron scattering caused by an elastic strain in the copper lattice [28]. Electron scattering also occurs at grain and phase boundaries the area of which increases with a decrease in the size of copper and chromium crystallites during mechanical alloying (see Table 1). This is probably the main cause of the decrease in the electrical conductivity with an increase in the milling time. Another cause of this decrease is as follows. After hot pressing the density of the test
5
specimens varied from 97% to 99% of the theoretical one and decreased with an increase in the processing time as illustrated in Fig. 6(b). The 1e3% decrease was caused by residual pores, i.e. discontinuities at the boundaries of former powder granules; they are caused by incomplete consolidation during compaction. The decrease in the density of the compacted specimens with an increase in the time of milling is caused by a decrease in the compressibility of the material due to its higher hardness. The pores do not conduct electricity, and therefore collisions of electrons with a large number of pores reduce the electrical conductivity of the material. The conductivity can also decrease because of iron impurities delivered to the material due to the abrasion of the steel milling bodies and the containers during milling. Energy dispersion data suggest that the content of iron in the test materials may be as high as 1 wt %. Fig 6(c) illustrates the hardness of the compacted Cue50%Cr specimens as a function of the time of mechanical alloying. It can be seen that increasing the time of milling to 5 h leads to an increase in the hardness of the material to almost 600 HV. This value is far greater (by almost three times) than that obtained in other works [7,8,29]. After longer processing the hardness does not change; this indicates that a steady-state stage of mechanical alloying is achieved. The hardening of the material with an increase in the time of milling is generally associated with the homogenization and refinement of the structure (Fig. 2(c)-(e)) and is caused by the HallPetch and Orowan effects [28]. As can be seen from Table 1, the size of the crystallites of both copper and chromium decreases during mechanical alloying. According to the well-known Hall-Petch equation, with a decrease in the grain size, the mechanical properties such as yield stress and hardness of materials are improved because the grain boundaries are efficient barriers for migrating dislocations [30]. The smaller the grains, the greater the number of these barriers on the path of the sliding dislocations and the higher the stress required for maintaining plastic strain at its early stages. Herewith, the hardening potency caused by grain refinement is higher in chromium than in copper [31]. Additional cause of
Fig. 7. Cross-sectional profile of wear track and images of steel ball for Cue50%Cr after different milling times: (a) wear track and (b) steel ball after 1 h milling; (c) wear track and (d) steel ball after 5 h milling.
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strengthening is the presence of fine particles. Mechanical alloying introduces isolated chromium particles into copper and further leads to their refinement, and therefore the spacing between the particles decreases. This in turn increases the stress required for forcing the dislocations between the particles and the dislocation loops formed around them. An increase in the hardness of materials is favorable for their wear resistance [32]. Wear tests showed that with an increase in milling time from 1 to 5 h the wear rate of the materials decreased from 3.01$103 to 1.07$104 mm3/Nm. In the latter case the wear rate was lower than that of the counter-body, i.e. the steel ball. Fig. 7 shows the wear track profiles of the hot pressed specimens after 1 and 5 h processing and the respective light microscopy images of the steel balls. After 5 h processing the depth of the wear track was within 2.5 mm (Fig. 7(c)) whereas after 1 h processing it was more than 40 mm (Fig. 7(a)). However, in the latter case testing was stopped after a 55 m run instead of 100 m due to catastrophic wear. The material removed from the wear track transferred to the steel ball as can be seen from the image of the ball with the adhered material (Fig. 7(b)). After 5 h processing, no adhered material was observed while the ball had a wear spot after the test (Fig. 7(d)). Because of intense plastic strain the specimen material was not removed but transferred along the wear surface. The wear track had deposits of the specimen material on both the inner and outer sides (Fig. 7(c)) indicating hardening of the surface layers during testing. Moreover, the coefficient of friction of the material was 0.47 which is sufficient to provide good antifriction properties. 4. Conclusions The effect of chromium content and milling time on the structure and basic properties of Cu-based bulk composites with 20e50 wt%Cr produced by high-energy mechanical alloying using copper chips as a raw material has been studied. We show that microstructure refinement occurs in the composites with an increase in the content of chromium and in milling time. The structure of Cue50Cr materials after mechanical alloying and subsequent hot pressing consists of the nanocrystalline Cu and Crrich phases; the size of the individual chromium particles embedded into copper being up to 20 nm. Because of the very intense structure refinement, when milling time is increased from 1 to 5 h, the hardness of the hot-pressed composites increases to 600 HV. At the same time, the wear resistance of the Cue50Cr composite increases almost by 30 times. On the contrary, its electrical conductivity decreases after mechanical alloying, as well as with an increase in the content of chromium particles. Acknowledgements The work was carried out on equipment of Joint Use Center “Materials Science and Metallurgy” with partial financial support from the Ministry of Education and Science of the Russian Federation in the framework of the Program aimed at increasing the competitiveness of the National university of Science and Technology “MISiS” (N К1-2015-026) and in the framework of the State Assignment to the Universities of the Russian Federation (Project No.11.1760.2014/K). The Authors are grateful to Kirill Shcherbachev for carrying out Rietveld analysis. References [1] S. Tsuneyo, Y. Atsushi, K. Takashi, O. Tsutomu, Contact material for vacuum valve, US Patent 5972068, 1999. [2] G. Renner, U. Siefken, Powder-metallurgically produced composite material and method for its production, US Patent 6350294, 2002. [3] C. Suryanarayana, Mechanical alloying and milling, Prog. Mater. Sci. 46 (2001)
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Characterization of nanostructured CueCr bulk composites prepared by high-energy mechanical alloying A.S. Prosviryakov*, A.I. Bazlov National University of Science and Technology “MISiS”, Leninskiy Prospect 4, Moscow 119049, Russia
h i g h l i g h t s Copper chips were used as raw material for production of Cu-(20e50%)Cr composites. Microstructure and properties of hot-pressed samples were studied. After 5 h milling the hardness of the composite with 50%Cr was increased to 600 HV. With the increase of milling time, the wear resistance was raised by 30 times.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 13 October 2015 Received in revised form 9 March 2016 Accepted 31 March 2016 Available online xxx
In this paper, the structure and properties of metal matrix composite materials on the basis of copper reinforced with chromium (20e50 wt%) were investigated. Raw materials consisted of grinded copper chips 5000 mm in size and chromium particles with an initial size of 10 mm. The composites were produced by mechanical alloying in a high-energy planetary mill with a quasi-cylindrical grinding body followed by hot pressing at 650 С. The milling duration was 1e10 h. It was shown that microstructure refinement occurs in the composites with an increase in chromium content and milling time. The structure of Cu-50 wt% Cr bulk materials after mechanical alloying for 5 h followed by hot pressing consists of the Cu and Cr-rich phases with average crystallite sizes of 37 and 28 nm, respectively. During short time milling the hardness of this composite increases to 600 HV due to the high intensity of mechanical alloying. At the same time, its wear resistance increases almost by 30 times in comparison with 1 h milling. On the contrary, electrical conductivity decreases after mechanical alloying. © 2016 Elsevier B.V. All rights reserved.
Keywords: Composite materials Powder metallurgy Powder diffraction Electron microscopy Hardness Electrical properties
1. Introduction CueCr composites produced using the conventional powder metallurgical technique are well-known and widely used as electrical switches due to their high arc resistance [1,2]. Another method of producing this material is mechanical alloying. Mechanical alloying is an advanced and efficient method of producing nanostructured composite materials with extreme properties [3e6]. The unique feature of this method is that the process occurs in the solid state and is accompanied by refinement of all the structural components. Mechanical alloying of copper with insoluble chromium produces a combination of good strength and electrical conductivity [7e11]. In all works that dealt with CueCr composites, mechanical alloying was carried out with milling balls,
* Corresponding author. E-mail address: [email protected] (A.S. Prosviryakov).
and copper powder was used as a raw material. To reduce the production costs of the materials prepared by mechanical alloying it would be desirable to use cheap scrap materials, e. g. chips, instead of copper powder. Mechanical milling of large-sized metallic particles is most efficient if a non-typical quasi-cylindrical grinding body is used instead of conventional grinding balls. During milling with a quasicylindrical grinding body the mechanical energy delivered per unit volume of the processed material is dozens of times higher than the mechanical energy achieved when grinding balls are used, and therefore the efficiency of processing is much higher [12]. Moreover, mechanical alloying with a quasi-cylinder grinding body occurs at a substantially lower average temperature. The efficiency of quasi-cylindrical grinding bodies for the production of mechanically alloyed composite materials from large-sized particles of copper was shown earlier [13,14]. In the Author's recent work [13] the thermal stability of mechanically alloyed Cu50Cr composite was
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estimated, and this material showed good heat resistance. The aim of the present paper is to analyze the effect of chromium content and milling time on the structure and basic properties (hardness, electrical conductivity and wear resistance) of CueCr composites produced by high-energy mechanical alloying from copper chips as raw material. 2. Materials and methods Grinded chips of 99.95% pure copper less than 5000 mm in size and chromium powder with an average particle size of 10 mm were used as the initial materials for composite material preparation. Mechanical alloying of CueCr powders with 20, 30, 40 and 50 wt% chromium content was performed in a “Gefest 11-3” planetary mill with a single quasi-cylindrical grinding body [15] in an argon atmosphere. The cross-section of this body is an equilateral triangle with convex sides obtained by the intersection of three 80 mm diameter circles inscribed in a 61 mm diameter circle. The body weighs about 1 kg and is 50 mm in height. The grinding medium and jars with a volume of 800 cm3 used for mechanical alloying were made of hardened chromium steel. The weight ratio between the grinding body and the powder was 4:1, and the milling speed was 600 rpm. The milling time was 1e10 h. The mechanically alloyed powders were compacted by uniaxial cold pressing followed by hot pressing at 650 C for 20 min. The cylindrical bulk specimens that were produced had the following sizes and were processed at the following pressures: the specimen for studying the structure and properties (hardness, electrical conductivity and density) was 10 mm in diameter (d) and 5 mm in height (h) and compacted at a pressure of 760 MPa, and the specimen for studying wear resistance had sizes 25 10 (d h) and compacted at a pressure of 200 MPa,. The structure of the compacted composites was examined by scanning electron microscopy (SEM), transmission electron microscopy (TEM) and X-ray diffraction (XRD). Xray diffraction analysis was performed on a D8 Bruker diffractometer using Cu Ka radiation. Lattice parameter, crystallite size and lattice microstrain of the specimens were determined by Rietveld analysis with TOPAS v 4.2 (Bruker-AXS) software. TEM analysis was carried out on a JEM-2000EX. Thin foils for electron microscopy were prepared using the Gatan Precision Ion Polishing System (PIPS). The hardness of the specimens was measured using a Vickers tester with a 5 kg load for 10 s. Electrical conductivity measurements were performed using the eddy-current method. The density of the specimens was examined using the hydrostatic weighing method. The wear resistance of the specimens was tested using a Tribometer, CSM Instr. with a 5 N load and at a 100 mm/s velocity using a hardened steel ball 3 mm in diameter. The depth of the wear track was measured using an Alpha-Step200 profilometer. 3. Results and discussion The initial stage of mechanical alloying with copper chips that were used as the raw material causes bending and twisting of the matrix particles followed by trapping of the free reinforcing particles in the resultant voids and welding of adjacent layers with each other [16]. At the same time, in the case of a binary metal system where the elements are ductile the formation of composite particles and the refinement of the structure occur predominantly by cold welding and fracturing of both metallic components [3,17]. The ductile particles get flattened due to a micro-forging process. At the next stage, these flattened particles are cold welded together and form a composite with a lamellar structure of the constituent metals as can be seen in Fig. 1. Here, the microstructure of Cue50% Cr composite powder particles after 1 h milling is presented in which copper lamellas randomly alternate with chromium
Fig. 1. Layered structure of Cue50%Cr composite powder particles after 1 h milling (light microscopy).
lamellas. Similar structures were obtained in other metal-metal systems [18,19]. Fig. 2 shows the structure of hot pressed CueCr specimens containing 30 to 50 wt% Cr examined using SEM in reflected electron mode. It can be seen that after 1 h processing in a planetary mill (Fig. 2(a), (b) and (c)) the structure of the materials consists of two components appearing as light and grey regions. Comparison of the atomic numbers of copper (Z ¼ 29) and chromium (Z ¼ 24) suggests that the light regions are Cu-rich ones while the grey regions contain almost pure chromium. With an increase in the chromium content from 30 to 40% (Fig. 2(a) and (b)) the structure of the material becomes more fragmented, and the Cu and Cr regions decrease in size. Higher chromium alloying (to 50%) affects the structural pattern only slightly (Fig. 2(c)). An increase in the time of mechanical alloying from 1 to 5 h results in structural refinement and significant fragmentation of chromium (Fig. 2 (c)-(d)). The Cr fragments become so fine that the clear separation into two structural components disappears. Further milling leads to the formation of an even more homogeneous structure (Fig. 2(e)). The absence of new phase formation after mechanical alloying followed by hot pressing was proven using X-ray analysis. The diffraction patterns obtained for the compact Cue50Cr composite specimens contain only the copper and chromium lines (Fig. 3). An increase in the time of processing caused a significant broadening of the X-ray lines indicating a decrease in the sizes of the crystallites and an increase in the degree of microstrain as a result of intense plastic strain. X-ray line broadening due to the abovementioned processes after mechanical alloying of CueCr powders was demonstrated earlier [7,9,10,20e22]. Results of X-ray analysis of the compact Cue50Cr specimens are summarized in Table 1. These results suggest that the broadening of X-ray lines after hot pressing is mainly caused by a decrease of the crystallite sizes occurring with an increase in the time of mechanical alloying. For example, the average size of the copper crystallites after 5 h milling and compaction is 37 nm. However, the degree of microstrain changes only slightly with an increase in processing time, its level being by more than one order of magnitude lower than that for mechanically alloyed powders. This can be caused by recovery processes occurring during hot pressing. Thus, the structure of the hot pressed material remains nanocrystalline, and this is confirmed by TEM data. Fig. 4(a) shows a light-field image of the structure of the compacted specimen after 5 h processing where it can be seen that the structure of the material is fine-grained and consists of crystallites with sizes of within 100 nm. The average crystallite size (about 40 nm) observed in the TEM image is in a good agreement with the values calculated on the
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A.S. Prosviryakov, A.I. Bazlov / Materials Chemistry and Physics xxx (2016) 1e7
3
Fig. 2. SEM micrographs of hot-pressed CueCr composites after different milling times: (a) Cue30%Cr, 1 h milling; (b) Cue40%Cr, 1 h; (c) Cue50%Cr, 1 h; (d) Cue50%Cr, 3 h; (e) Cue50%Cr, 5 h.
Cu
Cr
Intensity (a.u.)
5h
3h
1h 40
50
60
70 2θ (Degree)
80
90
100
Fig. 3. XRD patterns of Cue50%Cr composite after different milling times and subsequent hot pressing.
basis of XRD data. This image also shows dislocation clusters, the density of which for copper is 1.5,1014 m2 according to X-ray data. The dislocation density r was calculated on the basis of the crystallite sizes D and the values microstrain hε2 i1=2 (Table 1) using pffiffiffi of 2 1=2 i the formula [20]: r ¼ 2pffiffiffi3 hεDb , where b is the Burgers vector which for FCC metals is a 2 2 (a is lattice parameter). Fig. 4(b) shows a Cu-rich region, where there are fine grinded chromium particles (dark) with sizes of 20e30 nm embedded into copper as a result of mechanical alloying; they were not resolved by SEM. The structure and composition of materials are known to determine their properties. Fig. 5 illustrates the effect of chromium content on the hardness and electrical conductivity of CueCr specimens after 1 h milling followed by hot pressing in comparison with the hardness and electrical conductivity of pure copper (0 wt% Cr). It can be seen that the hardness of the composites grows linearly with an increase in the content of chromium which is harder than copper. The extrapolated value for 0% Cr (130 HV) is closely to the value for the nanocrystalline copper with a grain size of about 50 nm [23]. However, the electrical conductivity of the material
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Table 1 Lattice parameter, crystallite size and microstrain of copper and chromium in Cue50%Cr bulk composites for different milling times. Milling time (h)
Lattice parameter (nm)
1 3 5
Crystallite size (nm)
Lattice microstrain (%)
Cu
Cr
Cu
Cr
Cu
Cr
3.6152 ± 0.0002 3.6158 ± 0.0002 3.6158 ± 0.0002
2.8851 ± 0.0003 2.8856 ± 0.0003 2.8847 ± 0.0003
48 ± 3 40 ± 2 37 ± 2
39 ± 2 33 ± 2 28 ± 3
0.041 ± 0.004 0.042 ± 0.003 0.042 ± 0.005
0.028 ± 0.003 0.070 ± 0.005 0.050 ± 0.008
(a)
35
Electrical conductivity (%IACS)
decreases linearly with an increase in the chromium content (see Fig. 5) because the electrical conductivity of chromium is substantially lower than that of copper. The electrical conductivity of
30
25
20
15
10 0
1
2
3 Milling time (h)
4
5
6
(b) 7.97
100
7.94
99
7.88 7.85 7.82
98
Density (%)
Density (g·cm-3)
7.91
7.79 7.76 Fig. 4. TEM images of Cue50%Cr composite after 5 h milling and hot pressing: (a) nanocrystalline structure; (b) chromium nanoparticles.
7.73
(c) 100 Hardness
90
Elec. conductivity
80
250
70 60
200 50 150
40 30
100
1
2
3 4 Milling time (h)
5
6
700 600
Hardness HV (kg·mm-2)
Hardness HV (kg·mm-2)
300
Electrical conductivity (%IACS)
350
97 0
500 400 300 200
20 50
10 0
10
20 30 40 Content of Cr (wt%)
50
60
Fig. 5. Effect of chromium content on the hardness and electrical conductivity of hotpressed CueCr composites milled for 1 h.
100 0
1
2
3
4 5 6 Milling time (h)
7
8
9
10
Fig. 6. Effect of milling time on the electrical conductivity (a), density (b) and hardness (c) of hot-pressed Cue50%Cr composite.
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pure copper shown in the graph is higher than the value obtained by extrapolation. Furthermore, the electrical conductivity decreases with an increase in milling time, as can be seen in Fig. 6(a). This can be caused by several factors, e.g. the formation of a supersaturated chromium solid solution in copper, grain refinement during mechanical alloying, the formation of residual pores after hot pressing and, finally, contamination of the material with iron delivered from the milling body and the container. In this work we observed an increase in the mutual solubility of chromium and copper indicated by the shift of the diffraction lines in the diffraction patterns and an increase in the lattice period of both copper and chromium (Table 1). An increase in the solubility of chromium in copper was also observed elsewhere [10]. After 5 h milling followed by compaction, the lattice parameter of copper was 0.36158 nm compared to 0.36150 for pure copper. The solubility of chromium in copper increases due to an increase in the diffusion rate during intense plastic deformation caused by mechanical alloying which is accompanied by the formation of a large number of crystal defects [3,24]. Based on the commonly available data on the lattice parameter of the supersaturated copper solid solution [25] we assessed chromium solubility in copper for 5 h milling using Vegard's rule [26]. Its value was approx. 0.2 wt% whereas at temperatures below 400 C the equilibrium chromium content in the copper solid solution is less than 0.03 wt% [27]. It should be emphasized that the supersaturated solid solution formed during mechanical alloying remains in the structure after hot pressing. The electrical conductivity of the material decreases with an increase in the nonequilibrium content of chromium due to electron scattering caused by an elastic strain in the copper lattice [28]. Electron scattering also occurs at grain and phase boundaries the area of which increases with a decrease in the size of copper and chromium crystallites during mechanical alloying (see Table 1). This is probably the main cause of the decrease in the electrical conductivity with an increase in the milling time. Another cause of this decrease is as follows. After hot pressing the density of the test
5
specimens varied from 97% to 99% of the theoretical one and decreased with an increase in the processing time as illustrated in Fig. 6(b). The 1e3% decrease was caused by residual pores, i.e. discontinuities at the boundaries of former powder granules; they are caused by incomplete consolidation during compaction. The decrease in the density of the compacted specimens with an increase in the time of milling is caused by a decrease in the compressibility of the material due to its higher hardness. The pores do not conduct electricity, and therefore collisions of electrons with a large number of pores reduce the electrical conductivity of the material. The conductivity can also decrease because of iron impurities delivered to the material due to the abrasion of the steel milling bodies and the containers during milling. Energy dispersion data suggest that the content of iron in the test materials may be as high as 1 wt %. Fig 6(c) illustrates the hardness of the compacted Cue50%Cr specimens as a function of the time of mechanical alloying. It can be seen that increasing the time of milling to 5 h leads to an increase in the hardness of the material to almost 600 HV. This value is far greater (by almost three times) than that obtained in other works [7,8,29]. After longer processing the hardness does not change; this indicates that a steady-state stage of mechanical alloying is achieved. The hardening of the material with an increase in the time of milling is generally associated with the homogenization and refinement of the structure (Fig. 2(c)-(e)) and is caused by the HallPetch and Orowan effects [28]. As can be seen from Table 1, the size of the crystallites of both copper and chromium decreases during mechanical alloying. According to the well-known Hall-Petch equation, with a decrease in the grain size, the mechanical properties such as yield stress and hardness of materials are improved because the grain boundaries are efficient barriers for migrating dislocations [30]. The smaller the grains, the greater the number of these barriers on the path of the sliding dislocations and the higher the stress required for maintaining plastic strain at its early stages. Herewith, the hardening potency caused by grain refinement is higher in chromium than in copper [31]. Additional cause of
Fig. 7. Cross-sectional profile of wear track and images of steel ball for Cue50%Cr after different milling times: (a) wear track and (b) steel ball after 1 h milling; (c) wear track and (d) steel ball after 5 h milling.
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strengthening is the presence of fine particles. Mechanical alloying introduces isolated chromium particles into copper and further leads to their refinement, and therefore the spacing between the particles decreases. This in turn increases the stress required for forcing the dislocations between the particles and the dislocation loops formed around them. An increase in the hardness of materials is favorable for their wear resistance [32]. Wear tests showed that with an increase in milling time from 1 to 5 h the wear rate of the materials decreased from 3.01$103 to 1.07$104 mm3/Nm. In the latter case the wear rate was lower than that of the counter-body, i.e. the steel ball. Fig. 7 shows the wear track profiles of the hot pressed specimens after 1 and 5 h processing and the respective light microscopy images of the steel balls. After 5 h processing the depth of the wear track was within 2.5 mm (Fig. 7(c)) whereas after 1 h processing it was more than 40 mm (Fig. 7(a)). However, in the latter case testing was stopped after a 55 m run instead of 100 m due to catastrophic wear. The material removed from the wear track transferred to the steel ball as can be seen from the image of the ball with the adhered material (Fig. 7(b)). After 5 h processing, no adhered material was observed while the ball had a wear spot after the test (Fig. 7(d)). Because of intense plastic strain the specimen material was not removed but transferred along the wear surface. The wear track had deposits of the specimen material on both the inner and outer sides (Fig. 7(c)) indicating hardening of the surface layers during testing. Moreover, the coefficient of friction of the material was 0.47 which is sufficient to provide good antifriction properties. 4. Conclusions The effect of chromium content and milling time on the structure and basic properties of Cu-based bulk composites with 20e50 wt%Cr produced by high-energy mechanical alloying using copper chips as a raw material has been studied. We show that microstructure refinement occurs in the composites with an increase in the content of chromium and in milling time. The structure of Cue50Cr materials after mechanical alloying and subsequent hot pressing consists of the nanocrystalline Cu and Crrich phases; the size of the individual chromium particles embedded into copper being up to 20 nm. Because of the very intense structure refinement, when milling time is increased from 1 to 5 h, the hardness of the hot-pressed composites increases to 600 HV. At the same time, the wear resistance of the Cue50Cr composite increases almost by 30 times. On the contrary, its electrical conductivity decreases after mechanical alloying, as well as with an increase in the content of chromium particles. Acknowledgements The work was carried out on equipment of Joint Use Center “Materials Science and Metallurgy” with partial financial support from the Ministry of Education and Science of the Russian Federation in the framework of the Program aimed at increasing the competitiveness of the National university of Science and Technology “MISiS” (N К1-2015-026) and in the framework of the State Assignment to the Universities of the Russian Federation (Project No.11.1760.2014/K). The Authors are grateful to Kirill Shcherbachev for carrying out Rietveld analysis. References [1] S. Tsuneyo, Y. Atsushi, K. Takashi, O. Tsutomu, Contact material for vacuum valve, US Patent 5972068, 1999. [2] G. Renner, U. Siefken, Powder-metallurgically produced composite material and method for its production, US Patent 6350294, 2002. [3] C. Suryanarayana, Mechanical alloying and milling, Prog. Mater. Sci. 46 (2001)
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Please cite this article in press as: A.S. Prosviryakov, A.I. Bazlov, Characterization of nanostructured CueCr bulk composites prepared by highenergy mechanical alloying, Materials Chemistry and Physics (2016), http://dx.doi.org/10.1016/j.matchemphys.2016.03.047