Molecular dynamics study of pressure effect on structure and mechanical properties of CuNi alloy

82 HNUE JOURNAL OF SCIENCE DOI: 10.18173/2354-1059.2018-0074 Natural Sciences 2018, Volume 63, Issue 11, pp. 82-86 This paper is available online at MOLECULAR DYNAMICS STUDY OF PRESSURE EFFECT ON STRUCTURE AND MECHANICAL PROPERTIES OF CuNi ALLOY Nguyen Thi Thao 1 and Le Van Vinh 2 1 Department of Theoretical Physics, Hanoi National University of Education 2 Department of Computational Physics, Hanoi University of Science and Technology Abstract. The structure and mechanical properties of CuNi alloy have been investigated by means of molecular dynamic (MD) simulation. The interactions between atoms of the system were calculated by Sutton-Chen type of embedded atom method. The results show that when the sample was cooled down from 2000K to 300K at the cooling rate of 0.01 K/ps, both Ni and Cu atoms are crystallized into face centered cubic (fcc) and the hexagonal close packed (hcp) phases. The transformation to crystalline phase is analyzed through the Common Neighbor Analysis (CNA) methods. Furthermore, we focus on the pressure dependence of mechanical properties of CuNi alloy. Keywords: Structure, molecular dynamics, CuNi alloy, CNA method, mechanical properties. 1. Introduction CuNi alloys have gained a variety of interesting applications because of their specific characteristics. Compared to pure Cu, CuNi alloys are better in terms of electrical resistance, durability, hardness, thermal properties and corrosion resistance [1-3]. Their properties depend heavily on manufacturing techniques such as casting, welding, or 3D printing. During the cooling process, different types of CuNi alloys can be obtained depending on the cooling rate. A detailed understanding of the microstructure of CuNi alloys can provide information on the properties of the material. Many studies have shown the process of glass phase transition, the crystallization of CuNi amorphous alloys during heat treatment, liquid CuNi alloys during cooling and the dependence on the pressure of the processes [4-7]. In the work of molecular dynamics simulations of pressure-induced structural and mechanical property changes in amorphous Al 2O3 [8]. The result show that the Young's modulus increases with increasing pressure due to the increasing fraction of high coordination number and decrease of the volume of each AlOx type. However, there is no work to clarify the structure of CuNi alloys with different atomic concentrations at different temperatures and pressures as well as the effect of microstructure on the mechanical properties of this material. Therefore, the research focuses on the mechanical properties of CuNi alloy. Experimental research has been difficult and unlimited in changing the conditions of the sample, such as atomic concentration, temperature, pressure, etc. This limitation is overcome by the use of simulation, the method has been successful in studying disorder systems. Received November 7, 2018. Revised November 22, 2018. Accepted November 29, 2018. Contact Nguyen Thi Thao, e-mail address: ntthao.hnue@gmail.com Molecular dynamics study of pressure effect on structure and mechanical properties of CuNi alloy 83 2. Content 2.1. Computational procedures A molecular dynamics (MD) simulation was conducted to study CuNi alloys. The Sutton- Chen type of embedded atom method was used to describe the inter-atomic potential between atoms [5]. This version of EAM was widely used for investigating the metallic systems and their alloys. EMA based potentials have also been used in the investigations of liquid and amorphous phases [9]. The MD simulation is performed for sample of CuNi containing 4000 particles under periodic boundary conditions. Samples of CuNi alloy are heated from 300 K to 2000 K. These samples then were cooled to 300 K with a cooling rate of 0.01 K/ps to study their crystallization. By this way, five CuNi alloy samples which contain 3200 Cu and 800 B atoms have been constructed at five different pressure from 0GPa to 45GPa. The transformation to crystalline phase is analyzed through the Common Neighbor Analysis (CNA) methods [10]. 2.2. Results and discussion 2 4 6 8 0 2 4 6 8 10 12 14 16 r(Å) P=0GPa P=10GPa P=20GPa P=30GPa P=45GPa g (r ) 2 4 6 8 0 2 4 6 8 10 12 14 16 18 P=0GPa P=10GPa P=20GPa P=30GPa P=45GPa G C u -N i( r) r(Å) a) b) 2 4 6 8 0 2 4 6 8 10 12 14 16 18 P=0GPa P=10GPa P=20GPa P=30GPa P=45GPa G C u -C u (r ) r(Å) 2 4 6 8 0 5 10 15 20 25 30 G N i- N i( r) r(Å) P=0GPa P=10GPa P=20GPa P=30GPa P=45GPa c) d) Figure 1. The RDF of CuNi samples at 300K:a) The total radial distribution function G(r) of CuNi samples; b) The pair RDF GCu-Ni(r) for Cu-Ni pair ; c) The pair RDF GCu-Cu(r) for Cu-Cu pair; d) The pair RDF GNi-Ni(r) for Ni-Ni pair Nguyen Thi Thao and Le Van Vinh 84 Figure 1 displays the total RDFs for G(r) and pair RDFs of CuNi samples at 300K. For the total RDFs G(r) exhibiting the short-order structure of amorphours phase at 0GPa and 10 GPa (see Figure 1a). We can see that the agreement of the position of the first two peaks, which are located at 2.57 Ǻ and 4.23 Ǻ. With increasing pressure, the total RDFs of CuNi samples shows crystalline structure (faced centered cubic - fcc or hexagonal closed packed - hcp). For the GCu-Ni(r) exhibiting the Cu-Ni bond distance (see Figure 1b), the position of the first peak is unchanged with increasing pressure, the height of the first peak increases with increasing pressure. When pressure is larger than 20GPa, RDFs of these samples also shows crystalline structure. This phenomenon is similar to GCu-Cu(r) (see Figure 1c). For the GNi-Ni(r) exhibiting the Ni-Ni bond distance (see Figure 1d), the shape is almost unchanged except the height and position of the first peak with increasing pressure. It shows crystalline structure. For a detailed explanation of the structure of these samples, we used a common neighbor analysis (CNA) method. With the CNA method, we found samples containing both faced centered cubic (fcc) or hexagonal closed packed (hcp) structures at 300K. The pressure dependence of the number of Cu-Ni alloy samples are listed in Table 1. When the pressure increasing, the total number of crystal atoms increases from 2298 atoms to 3882 atoms. At pressure of 45 GPa, the crystallizatiton of Cu-Ni alloy sample is almost complete with 97% number of crystal atoms. Table 1. Pressure dependence of the number of crystal atoms of CuNi alloy P(GPa) Number of Ni crystal atoms Number of Cu crystal atoms Total number of Crystal atoms 0 555 1743 2298 10 680 1551 2231 20 757 2571 3328 30 767 2860 3627 45 785 3097 3882 Figure 2. The snapshot atoms of CuNi samples under compression Molecular dynamics study of pressure effect on structure and mechanical properties of CuNi alloy 85 Figure 3. Stress-strain curves for CuNi samples upon compression The crystallization of CuNi alloy samples can be seen from the snapshot of spatial arrangement of atoms. As shown in Figure 2, a crystal structure forms inside the samples and then grows with increasing pressure. As the number of crystal-atoms increases from 2298 atoms at pressure of 0GPa to 3882 atoms at pressure of 45GPa. Here we also investigate the mechanical behavior of CuNi alloy samples upon compression. During deformation of the samples, the stress was calculated as a function of uniaxial strain. The stress-strain curves in samples at different pressure obtained from the MD simulation are presented in Figure 3. The elastic modulus is given by the slope of the stress-strain curve in the linear region. From the stress-strain curves, we intimated the elastic modulus of the samples as presented in Table 2. The elastic modulus increase with increasing pressure (from 102.94 GPa at pressure of 0 GPa to 260.05 GPa at pressure of 45 GPa), and this result is good agreement with one in Ref. [8]. Table 2. Pressure dependence of the elastic modulus of CuNi alloy samples P(GPa) 0 10 20 30 45 E(GPa) 102.94 108.2138 128.1077 204.6369 260.0567 3. Conclusions In this paper, crystallization of CuNi alloys has been investigated by means of MD simulation. The structural transformation to crystalline phase is analyzed through the radial distribution function (RDF). 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