Some Factors Affected on Structure, Mechanical of Ni Bulk

The article examines the effect of atomic number, temperature and tempering time on microstructure and mechanical of Ni bulk by molecular dynamics simulation and deformation z-axis. Samples Ni with N = 4000, 5324, 6912, and 8788 atoms at 300 K, 6912 atoms at T = 1100, 900, 700, 500, 300 K and 6912 atoms at 900 K after different annealing time. The samples were incubated with the same heating rate 12 4 10 K s T t ∆ = × ∆ . Combined with common neighborhood analysis method shown in sample is always existing four types structure: FCC, HCP, BCC, and Amor. In particular, structural units FCC, HCP and Amor always prevail and BCC are very small and appear only at 300, 500 K with 6912 atoms. When increasing atomic number, lowering temperature or increasing tempering time will facilitate crystallization process leading to increased FCC and HCP units number. The increasing FCC, HCP units number and additional appearance BCC structure led to change microstructure and mechanical of material: When increasing atom, lowering temperature and increasing incubation time lead to an increase in density of atoms that increase mechanical properties of the material.

The results do not always give the desired results. Phase transitions, heteroge-Advances in Materials Physics and Chemistry neous dynamics, surface shape, size, and crystallization lead to changes structures: BCC, FCC, HCP, [17]- [24]. Cause when size material limited led to appearance quantum effects (Size effects, surface effects), materials appear much different natures [25] [26] [27] [28].
To get a better understanding of the factors that influence on microstructure and mechanical of material. Molecular dynamics simulation is considered an effective tool for empirical research to study the microstructure of metals [29]- [38], deformation of z-axis to determine mechanical properties of metal have structured FCC (Al, Cu và Ni) and BCC (Fe, Cr, W). Macmillan, Kelley [39] and Parrinello, Rahman found [40] that: With Ni bulk, deformation of material depends greatly on the direction of strain and the intensity of applied pressure. Komandari et al. [41] [42], Park et al. [43] [44] stretched nanowires in different directions and Wu [45] [46], Golovnev [47] studied the mechanical properties of Cu nanowires and determined relationship between temperatures.
Dimensions are always proportional to deformation, and Lin Yuan and colleagues [48] examined monoclinic mechanics at different temperatures. The results show, we cannot predict the deformation of materials at high temperatures and high heating rates [49] [50] [51] [52]. Therefore, study a number of factors affecting the microstructure, and mechanics of nano-sized nanomaterials, will contribute to the fabrication of new materials [53]. In this paper, we focus on the influence of factors such as atomic number, temperature, microstructure and mechanical properties and termination; and the relationship among size, stress and number of structural units FCC, HCP, BCC, and Amor.
With: r ij is distance between two atoms i, j; a is parameter with dimension of length; ρ i is atomic density i; E tot is total energy of the system; Φ(r ij ) is energy between two atoms i, j; F(ρ i ) is interaction force on atom i; r c is radius disconnect, ε is energy; C, m, n, N is constant. With ε = 7.3767meV, C = 84.745, n = 10, m = 5 và a = 3.52 Å selected for accurate results on microstructure of materials at different temperatures.
Previously, interactive embedding Sutton-Chen has been used extensively to study phase transition in metals [63] [64] [65] [66] [67]. In addition, study microstructure of materials we use Common Neighbor Analysis methods (CNA) [68] [69] [70]. To study mechanical properties of Ni, we used z-axis deformation method (2) to determine the relationship between stress and strain.
Here, m i is mass of atom i, i v α is velocity of atom i along axis α and F ij is interaction force between atom i and atom j, r ij is distance between atom i, j, and r ij,α is vector in α axis from atom i to atom j, V i is volume of atom i. Where E is stress, G is slipping and µ is deformation coefficient, α is factor deformation, ε zz is deformation z-axis; σ xx , σ yy , σ zz is stress according to x, y, z, and σ αβ is stress model. Deformation is determined by the work [71] [72].

The Influence of the Atomic Number
The microstructure of Ni 4000 , Ni 5324 , Ni 6912 , Ni 8788 at temperature 300 K, determined by radial distribution function (RDF), resulting in Figure 1.
The Ni 4000 has the first peak of RDF prevail with valuable is 2.45 Å that shows sample Ni bulk do not far order exist that always exist near order (Figure 1(a)) and have average coordination number is 12 (Figure 1(b)). When increasing atoms number then first peak height of RDF decreased from 7.35 (Ni 4000 ) down 5.76 (Ni 6912 ) and increased 7.61 (Ni 8788 ) leading to increased atomic density. The first peak of RDF increases and decreases do not follow specific rules. The minimum value at Ni 6912 , due to in material has microstructural units existence, results show in the second peak of RDF. The second position peak varies of RDF from 3.45 to 3.55Å reaching and a maximum value at 3.55Å. The first peak height RDF Ni bulk sample smaller nanoparticles, second peak position Ni bulk larger nanoparticles. These results are entirely consistent with Ni nanoparticles [73]. Confirm this, we use simulation method, resulting in Figure 2 and Table 1 The results show that Ni 4000 at 300 K has three types of structures: FCC, HCP,   Figure 3 and Table 2 (Figure 3(b)). This result is consistent with influence atoms number on microstructure and number structural units.

Effect of Temperature
The microstructural, mechanical properties of Ni 6912 bulk at 1100, 900, 700, 500 and 300 K are shown in Figure 4.
The results show that at 1100 K on sample there existed only 13 atoms has HCP structure (Figure 4(a1)). The first peak of radial distribution function g(r) is 3.4 Å (Figure 4(a2)) and stress E = 0 and slope G = 0 (Figure 4(a3)). When temperature lowered 900, 700, 500 and 300 K then structure units number FCC, This result is consistent with Ni 4000 bulk has crystalline temperature is 800 K [32], Ni 5324 nanoparticles have T m = 800 K [73] and when N increases then T m increasing [30]. In addition, phase transition temperature depends not only on number of atoms but also on shape and size [35] such as: Wen et al. assume that melting temperature T m of Ni nanowires is inversely proportional with size D [31] and Trong Dung Nguyen assumes that for Ni nanoparticles then D is proportional with N −1/3 , E is inverse with N and confirm that transition temperature of sample is not applicable to nanoparticles [73]. The other side, when temperature decreases then E, G increase [87]- [91] as by deformation of z-axis, E = 171 GPa [92]. This confirms when temperature decreases then E increases and determination at 900K is phase transition temperature of Ni 6912 bulk.

The Effect of Crystallization
The energy, size, number of structural units, radial distribution function, E and G of Ni 6912 bulk after incubation time are shown in Table 3, Figure 5. Advances in Materials Physics and Chemistry   Figure 5(d3)). These results simulation are consistent with results of the experiment [83]. This confirms that there is a great influence of atoms number, temperature and thermal time on microstructure and mechanical properties of Ni bulk.

Conclusion
The  Figure 5. Structural unit number, radial distribution function, mechanical of Ni 6912 bulk after different heating time.
from 4000 to 8788 atoms then g(r) decreases, structure units number FCC and HCP increase as α, E, G increase. At Ni 6912 add structure BCC lead to E reach maximum value, this suggests that addition of BCC structure increases entropy. This is an indispensable basis for the balance of Ni bulk also when decreasing temperature and increasing incubation time lead to stress increases, is due to in structure units number FCC and HCP increase. The results are based on theoretical foundations of structural units number FCC, HCP, and BCC. Amors are unclear and should be encouraged in further studies.