FABRICATION AND CHARACTERIZATION OF NANOSTRUCTURED CU-15WT.% MO COMPOUND BY MECHANICAL ALLOYING

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1 2nd International Conference on Ultrafine Grained & Nanostructured Materials (UFGNSM) International Journal of Modern Physics: Conference Series Vol. 5 (2012) World Scientific Publishing Company DOI: /S FABRICATION AND CHARACTERIZATION OF NANOSTRUCTURED CU-15WT.% MO COMPOUND BY MECHANICAL ALLOYING Soheil Sabooni Department of Materials engineering, Isfahan university of technology, Isfahan, Iran s.sabooni@ma.iut.ac.ir Tayebeh Mousavi Department of Materials, University of Oxford, Parks Road, Oxford, 0X 13 PH, UK tayebeh.mousavi@materials.ox.ac.uk Fathallah Karimzadeh Department of Materials engineering, Isfahan university of technology, Isfahan, Iran karimzadeh_f@cc.iut.ac.ir In the present study nanostructured Cu(Mo) compound with 15 weight percent Mo was produced by mechanical alloying using a planetary ball mill. The milling operation was carried out in hardened chromium steel vial and balls under argon atmosphere with a constant ball to powder ratio of 10:1. The structural evolution and characterization of powder particles after different milling times were studied by X-Ray Diffraction, SEM observation and Microhardness measurements. The results showed the displacement of broadened Cu peaks to lower angles, because of dissolving Mo in Cu. The final product was a nanocomposite contains nanocrystalline Cu(Mo) supersaturated solid solution matrix and dispersion of nanometric Mo reinforcements. The microhardness of formed nanocomposite increased to 350HV because of grain refinement, formation of solid solution and dispersion hardening. Keywords: Mechanical alloying, Nanostructured materials, Supersaturated Solid solution, Cu-Mo Compound. 1. Introduction Copper and Copper alloys are widely used in engineering because of their excellent electrical and thermal conductivities, Outstanding resistance to corrosion, Ease of fabrication and good fatigue resistance 1. High temperature resistance copper alloys find their main applications as electrical contacts, components for X-Ray tubes, welding electrodes and electrical solar cells with high conversion efficiency. A common way of increasing the low intrinsic strength of copper while remaining its high electrical 456

2 Fabrication and Characterization of Nanostructured Cu-15wt.% Mo Compound by Mechanical Alloying 457 conductivity is alloying with B.C.C elements such as Cr and Mo, with or without a dispersion of reinforcing particles 2. Cu-Mo system is known to be mutually immiscible in both solid and liquid state and do not form any compound or alloy using the conventional material processing 3. In Cubased binary alloys, the supersaturation state of solute atom in many alloys can be achieved by non equilibrium processing such as Rapid Solidification Processing (RSP), Mechanical Alloying (MA) of solid powder elements, vapor deposition and laser processing. Table 1 shown the maximum deviation from equilibrium achieved for various non-equilibrium processes that indicate the ability of different processing teqnique to synthesis metastable structures. In all of these non equilibrium processing methods, RSP and MA are often used to obtain bulk supersaturated solid solution for practical engineering applications. Since the maximum deviation from the equilibrium achievable by MA is higher than that by RSP for a given alloy system, it is expected that MA can achieve a larger solid solubility extension than RSP. Table1. Maximum deviation from equilibrium in various non-equilibrium processes 4 Processes Maximum deviation from equilibrium (kj/mol) Solid state quench 16 Quench from liquid (Rapid solidification) 24 Condensation from vapor 160 Irradiation/ion implantation 30 Mechanical cold work 1 Mechanical alloying 30 In the recent years, a variety of supersaturated Cu-based solid solutions e.g. Cu- Cr,Cu-Fe,Cu-Nb,Cu-Ag and etc. have been prepared and characterized by mechanical alloying. While Cu-Mo system less than others have been studied 4. In the present work nanostructured Cu-Mo Compound with 15wt.% Mo was prepared by mechanical alloying of pure Cu and Mo powders. Then the powder particles were characterized by X-Ray diffraction, SEM observation and microhardness measurements and the effect of milling time on the formation mechanism of nanostructured compound was investigated. 2. Materials and Methods A mixture of pure Copper (99.99% purity and particle size of 20 µm) and molybdenum (99.9% purity and particle size of 4 µm) were milled in order to produce nanostructured Cu- 15wt.% Mo compound. The MA was performed in a planetary ball mill at room temperature under argon atmosphere. The experiments were carried out in hardened chromium steel container with five steel balls. The ball to powder weight ratio and rotational speed were 10:1 and 500 rpm, respectively. The structural and crystallite

3 458 S. Sabooni, T. Mousavi & F. Karimzadeh size evolution during milling were determined by X-Ray diffraction (XRD) with Cu k α radiation (λ=1.5405nm). The crystallite size and internal strain of milled powders were determined by Williamson-Hall formula 5. The microstructure and morphology of mechanically alloyed powders were examined with scanning electron microscopy (SEM) in a Philips XL 30.Vickers microhardness measurements were carried out in a mounted and polished powders under a load of 50 g and dwell time of 10s for several indentation and average value of hardness were reported. 3. Result and discussion: The XRD patterns of Cu-15wt.% Mo powder mixture in different milling times are shown in Fig 1. It is apparent that the intensity of XRD peaks decreases and width at the half height of every peak increases by increasing the milling time. These trends are due to refinement of crystallite size of Cu and Mo powders and increase of internal strain during milling. The average crystallite size and internal strain of powders can be determined from Broadening of XRD peaks by Williamson-Hall formula: βcos θ = (0.9 λ/d) + 2εSinθ (1) Where β is the full width at half maximum intensity, λ is the wavelength of X-ray used, D is the average crystallite size, θ the Bragg angle and ε, the average internal strain. Fig. 1. XRD patterns of Cu-15wt.% Mo powder mixture in different milling times. The crystallite size and internal strain of Cu matrix in Cu-15Wt.% Mo are shown in Fig 2. Due to the mechanical deformation introduced into powder, Crystallite refinement occured and internal strain increased. At the early stage of MA, the crystallite size of Cu decreases rapidly and further refinement of the crystallite size occurs gradually to 35 nm

4 Fabrication and Characterization of Nanostructured Cu-15wt.% Mo Compound by Mechanical Alloying 459 after 60 h of MA and then seems to reach an equilibrium value. The internal strain also increases rapidly and finally reaches to about a steady-state value of 0.34%. Fig. 2. Crystallite size and internal strain of Cu-15wt.% Mo powder mixture as a function of milling time. The displacement of Cu(111) XRD peak during ball milling process is presented in Fig 3. It is revealed that Cu peaks shifted slightly to lower angels with increasing the milling time. This suggests that the lattice parameter of Cu increased as a result of dissolution of larger Mo atom size into Cu lattice. It should be noted that after 60h of milling time, several peaks of Mo phase are still visible at the XRD pattern indicating that only partial amount of Mo can be dissolved during MA process. According to Vegard formula 6, the solid solubility of Mo in Cu after 60h of milling was achieved to about 9.5Wt.% that is higher than that presented by Shengqi Xi et al 7. Since Cu-Mo system is known to be mutually immiscible in both solid and liquid state, the MA product is a supersaturated solid solution matrix with reinforcing of Mo particles. Fig. 3. Displacement of Cu(111) peak at different milling time.

5 460 S. Sabooni, T. Mousavi & F. Karimzadeh The formation of supersaturated Cu(Mo) matrix in Cu-15Wt.% Mo can be explained by means of the dislocation solute-pumping mechanism introduced by Schwarz 8. According to this model the dislocation core acts as a pump for the solutes. Due to the larger size of Mo atoms, the lattice near the Mo atoms is compressed and the dislocation situated in an expended lattice. Thus, the interaction between Mo atoms and dislocations were take place. The solute atoms diffuse along the dislocation line faster than that of the inner grain regions without any lattice defect. Thus, Mo atoms can arrange in the dislocation line within the Cu matrix. The stress in powder particles, due to collision of balls, exceeds the flow stress of Cu and so dislocation starts to glides. As the Mo atoms can not move easily, they will be left behind the dislocation lines. The Mo atoms remain a few nanometers within the Cu grain whereby the matrix is locally supersaturated. In fact the driving force for this mechanism is the interaction between dislocation and dissolved atoms. In order to achieve the supersaturated state in the whole volume, the grain size has to be reduced to nanometric scale. This means that formation of supersaturated solid solution is closely related to achievement a nanocrystalline grain. Fig 4 shows the change of Microhardness of Cu-15wt.% Mo powder mixture during the milling process. The microhardness of powders increases strongly to 285HV in the early stage of milling. By increasing the milling time the gradient of microhardness changes, decrease and after 60 h the powders have a microhardness of about 350 HV. In fact the increasing of hardness during milling is due to crystallite refinement, solid solution hardening and dispersion hardening of Mo reinforcements. Microhardness (HV) Milling time (h) Fig. 4. Microhardness of Cu-15wt.% Mo mechanically alloyed Compound as a function of milling time Fig 5 shows the Hall-petch plot for mechanically alloyed Cu-15wt.% Mo compound. At the larger crystallite size, the slope is high while by decreasing grain size, the slope

6 Fabrication and Characterization of Nanostructured Cu-15wt.% Mo Compound by Mechanical Alloying 461 will decrease. The similar trends have been reported in other materials. Many models of deformation in nanocrystalline metals have been proposed to describe this phenomenon 9. Meyers and Ashworth proposed a model of strengthening by grain refinement that is based on the generation of dislocation at grain boundaries 10. In this model, as material deforms, elastic anisotropy results in stress concentration in the boundary region that give rise to the generation of dislocation that effectively form hardened reinforcing second phase network. In order to ensure a d -(1/2) dependence of σ y at large grain size, Meyers and Ashworth set the thickness of the hardened layer proportional to d (1/2). The resulting expression for σ y contains not only the usual d -(1/2) term but also a second term, proportional to d -1, that becomes important at small value of d, and acts to lower slope of H-P plot in this grain size region. So the Hall-petch plot obtained in this work was in agreement with previous trends and also confirmed the proposed model. Fig. 5. Hall-Petch plot for mechanically alloyed Cu-15Wt.% Mo Compound The cross- sectional SEM micrographs of the powder particles after 4 and 60h MA are presented in Fig 6(a,b).The light phase is Mo and the dark phase is Cu(Mo) solid solution. In the early stage of milling (4hr), the Mo particles were homogenously distributed in Cu matrix due to high elastic module of Mo( E Mo = 330 Gpa) 11. In fact Cu is a ductile metal and Mo is brittle and because of this, Mo particles fractured by colliding balls and Cu were surround it. This fine homogeny distribution of Mo in Cu matrix, is a very important reason for extension the solid solubility of Mo in Cu. In fact by increasing the contact surface between Mo and Cu the solid solubility will be increased. Finally after 60h of milling a very fine distribution of Mo in Cu(Mo) matrix was observed. This means that after 60h milling of powder mixture (Cu-15wt.% Mo), a nanocomposite was formed. In this nanocomposite, matrix is supersaturated solid solution and reinforcement is Mo particles with size of about ( ) nm.

7 462 S. Sabooni, T. Mousavi & F. Karimzadeh Fig. 6. Cross-sectional SEM micrographs of Cu-15Wt.% Mo powders at :a) 4h b)60h milling time The morphology of powders at different milling times are shown in Fig 7(a-d). As received powder particles include denderitic Cu and polygonal Mo particles. After 15h of milling (Fig 7b.), most particles were equiaxed and have a larger particle size than early stage of milling. By increasing the milling time up to 30h (Fig 7.c), the powder particles become finer while their shape remained unchanged. In fact in the initial stage of milling the rate of agglomeration is higher than fragmentation due to high rate of cold welding. Work hardening of Cu powders occurred by increasing the milling time and causes to increase the rate of fragmentation and reduces particle size. Finally the powders after 60h milling have very fine particles of about 2µm. Fig. 7. Morphology of milled powders at different milling times : a)0h b) 15h c)30h d)60h

8 Fabrication and Characterization of Nanostructured Cu-15wt.% Mo Compound by Mechanical Alloying Conclusions By mechanical alloying of Cu-15wt.% Mo powder mixture, a nonocomposite was formed that contains supersaturated Cu(Mo) solid solution matrix and reinforcing of Mo particles. The formation of homogenous supersaturated Cu(Mo) solid solution matrix is favored by microstructural refinement experienced during milling. In fact a nanocrystalline grain structure possesses a large number of defects that can enhance atomic diffusion. Solid solubility of Mo in Cu after 60h milling was increased to 9.5wt.%.The microhardness of formed nanocomposite increased to 350HV due to grain refinement, formation of solid solution and dispersion hardening that is very higher than unmilled powders and indicates that mechanical alloying is a suitable process for producing materials with high strength compared to conventional coarse-grain materials. References 1. JR.Davis, editor. Copper and Copper alloys, ASM international handbooks, (ASM international materials park, 2001). 2. E.Botcharova, M.Heilmaier, J.Freudenberger, G.Drew, D.Kudashow, U.Martin, L.schultz, J. Alloys Compd., 351, 119, (2003). 3. T.B.Masalski, H.Okamoto, P.R.Subramanian, L.Kacprzak,(Eds.), Binary alloy Phase diagram, 2 nd ed, (ASM international materials park,1990). 4. C.Suryanarayana, prog. Mater Sci., 46, 1, (2001). 5. K.Williamson, W.H.Hall, Acta Metall., 1, 22, (1953). 6. W.B.Pearson, A Handbook of lattice spacing and structures of metals and alloys, (Oxford, pergamon, 1967 ). 7. Shenggqi Xi, Kesheng Zuo, Xiaogang Li, Guang ran, Jingen Zhou, Acta Mater., 56, 6050, (2008). 8. R.B.Schwarz, Mater. Sci. Forum, , 665, (1998). 9. J.R.Weertman, Mechanical behavior of nanocrystalline metals, in: Koch CC, "Nanostructured materials: processing, properties andpotential applications", (noyes publication, 2002). 10. M.A.Meyers, E.Ashworth, Philos.Mag.A, 46, 737, (1982). 11. E.Botcharova, J.freudenberger, L.Schultz, J. Mater. Sci., 39, 5287, (2004).