Effects of atomics growing orientation to mechanical properties of Cu/Ta bilayer using molecular dynamics simulation

ISSN 2354-0575
 EFFECTS OF ATOMICS GROWING ORIENTATION
 TO MECHANICAL PROPERTIES OF Cu/Ta BILAYER
 USING MOLECULAR DYNAMICS SIMULATION
 Anh-Tuan Do, Anh-Son Tran
 Hung Yen University of Technology and Education
 Received: 10/01/2019
 Revised: 15/01/2019
 Accepted for publication: 26/02/2019
Abstract:
 In this article, the effects of different atomics growing orientations to mechanical properties of 
Cu/Ta nanofilms with a circle void defect under tension process are studied using molecular dynamics 
simulation. The stress-strain relationship, structural phase transformations, dislocation mechanism, and 
local stress concentration are examined. The results show that the Cu[100]/Ta[111] nanofilm exhibited the 
most excellent mechanical properties. The FCC structures are mainly transformed into HCP structures, and 
, dislocations occurred in Cu sections. The local stress concentrations are focused around 
the intersection regions between void defect and Cu/Ta interface.
Keywords: Molecular dynamics; Cu/Ta nanofilms; Hirth dislocations; Thompson tetrahedron.
1. Introduction nanofilms commonly used in aerospace, electronic, 
 Recently, metallic nanofilms are widely optical, and magnetic industries. Therefore, studying 
studied because of their excellent mechanical the mechanical properties of Cu/Ta is a very urgent 
properties as follows: good toughness, high requirement. However, it is very difficult to establish 
strength, and great hardness, etc [1]. Besides, a Cu/Ta nanofilm with different atomics growing 
metallic nanofilms are very easy in fabrication orientations by the experiment. With the strong 
and adjustment corresponding to different specific development of computer technology, molecular 
conditions [2]. There are many factors that affect dynamics (MD) simulation is a very accurate and 
the mechanical properties of metallic nanofilms, reasonable choice to investigate the properties of 
however, atomics growing orientation directly materials at nanoscale [3-6].
influence on the deformation mechanisms and In this study, the stress-strain relationship, 
mechanical responses of metallic nanofilms under structural phase transformations, dislocation 
testing processes. Therefore, in order to analyze mechanism, and local stress concentration of 
the changes in the structures of metallic nanofilms, Cu/Ta nanofilms are studied by MD simulation 
a careful and systematic study of the deformation under tension process. The Cu/Ta specimens 
mechanisms and mechanical properties of nanofilms are constructed with different atomics growing 
in different atomics growing orientations conditions orientations. The results obtained provide a 
is required. deeper insight on the deformation and mechanical 
 Cu/Ta nanofilms is one of the most metallic properties of Cu/Ta nanofilms.
 .
 Fig. 1. The simulation Cu/Ta nanofilm specimen under tension process
20 Khoa học & Công nghệ - Số 21/Tháng 3 - 2019 Journal of Science and Technology
 ISSN 2354-0575
2. Methodology 0.012 – 0.020 range. Then, the plasticity stage is 
 Fig. 1 shows the simulation models of Cu/ going on, which can be presented by the vibration 
Ta nanofilms under tension process. The sizes of of the stress-strain curves. The plasticity stage 
the simulation specimen are 40 nm (length) × 15 lasts due to the effects of circle void defect in the 
nm (height) × 4 nm (thickness) corresponding specimens. The plasticity deformation is dominated 
to the x-, y-, and z-axes. Many previous studies by nucleation of dislocation at the surface of the 
have performed simulations with samples of void defect. The stress concentration is distributed 
approximately sizes [7,8]. The thickness is enough and transferred around the void defect causes the 
to out the size effect. The essential difference is at prolongation of the plasticity stage. In the cases of 
the boundaries: the bulk Cu and Ta systems with Cu/Ta nanofilms with [111][100], [111][110], and 
periodic boundaries have no locations for dislocation [111][111] growing orientations, the stress values 
initiation. The diameter (D) of void defect is 3 nm. in plasticity stages are higher than these values 
The different atomics growing orientations along x in the cases of Cu/Ta nanofilms with growing 
direction of the Cu/Ta nanofilms are [100]/[100], orientations of [100][100], [100][110], and [100]
[100]/[110], [100]/[111], [111]/[100], [111]/[110], [111]. However, the opposite situation occurs with 
[111]/[111]. Periodic boundary conditions are the tensile strength values. The tensile strength 
determined in y- and z-axes, and the axial tension is values are 2.51, 2.64, 2.83, 2.44, 2.47, and 2.05 
specified along the x-axis. Gpa corresponding to [100][100], [100][110], 
 The testing conditions of the simulations are [100][111], [111][100], [111][110], and [111][111] 
at room temperature, a strain rate of 108 s-1, and a growing orientations, respectively. The specimen 
time step of 2 fs. 105 time steps have been run in with growing orientation of [100][111] exhibits the 
each simulation. best tensile strength.
 The MD simulation package Large-scale 
Atomic/Molecular Massively Parallel Simulator 
(LAMMPS) [6] is applied to simulate the tension 
processes. The interaction potential energies 
between the atoms in Cu/Ta nanofilms are evaluated 
by the embedded atom method (EAM) [9]. The 
OVITO software [10] is used to indicate the tension 
processing. In OVITO software, the common 
neighbor analysis (CNA) is used to classify atoms 
that are connected with particular phases and defects 
in crystalline systems. The dislocation extraction 
algorithm (DXA) [10] is applied to determine 
the dislocations in the Cu/Ta nanofilms structures 
during the process, and exports the representational 
line for each dislocation defect and also useful for 
identifying the Burgers vectors.
3. Results and discussion
 In this section, the mechanical properties 
and deformation behaviors of Cu/Ta nanofilms are 
assessed in terms of tensile stress-strain responses, 
dislocations, distributions of stress, and structural 
transformations of Cu, Ta atomics.
3.1. Tensile stress-strain responses Fig. 2. The stress-strain relationship of Cu/Ta 
 The stress-strain response is the most nanofilms with different growing orientations under 
characteristic element to evaluate the mechanical tension process at a temperature of 300 K, a strain 
properties of materials. rate of 108 s-1
 Fig. 2 shows the stress-strain relationship of 
Cu/Ta nanofilms with different growing orientations 3.2. Structural transformations and local stress 
under tension process at room temperature and a distributions
strain rate of 108 s-1. The maximum stress values The structural transformations and local 
achieved is at about the corresponding strain in stress distributions are very important factors to 
Khoa học & Công nghệ - Số 21/Tháng 3 - 2019 Journal of Science and Technology 21
ISSN 2354-0575
investigate the deformation mechanism of Cu/Ta oblique dislocations occur together with growing 
nanofilms under tension process. Fig. 3 presents the orientations of Cu/Ta are [111][100], [111][110], 
dynamic response of Cu/Ta nanofilms with different and [111][111] in Fig. 3(b). Almost of these oblique 
growing orientations under tension precess over the dislocations are originated from the interface 
peak stress value at room temperature and strain between Cu and Ta elements. Compare the dynamic 
rate of 108 s-1. As we can see, the FCC structures are responses of the Cu/Ta nanofilms under the tension 
transformed into HCP structures in Cu element, while process over the peak point, the Cu/Ta with growing 
the BCC structures are changed into amorphous orientations [100][111] exhibits the most stable in 
structures in Ta element. The dynamic responses structural materials. That means the Cu/Ta [100]
of Cu/Ta specimens with growing orientations are [111] exposes the best mechanical properties than 
[100][100], [100][110], and [100][111] are shown other cases, which is consistent with the result in 
in Fig. 3(a). The oblique dislocations are obviously tensile strength of this specimen in section 3.1.
observed in Cu elements. However, the vertical and 
Fig. 3. Dynamic response of Cu/Ta nanofilms with different growing orientations under tension process 
over the peak stress value at room temperature and strain rate of 108 s-1. (a) The growing orientation of Cu 
 is [100] and (b) The growing orientation of Cu is [111]
 Fig. 4 illustrates the local stress distributions intersections between the circle voids and the 
of Cu/Ta nanofilms under the tension process over interfaces of Cu/Ta elements, where the most severe 
the peak stress at 300 K and strain rate of 108 s-1. The deformations are observed. Atomic bonds in these 
larger stresses mainly concentrated on Cu elements areas are the weakest. All distortions originate from 
due to the more intense deformation happened in these locations, then spread to the surrounding areas 
Cu elements. The maximum stress concentrations causes the appearance of cracks.areas.
focus around the circle voids, in particular, at the 
Fig. 4. The local stress distributions of Cu/Ta nanofilms under the tension process over the peak stress at 
room temperature and strain rate of 108 s-1. (a) The growing orientation of Cu is [100] and (b) The growing 
 orientation of Cu is [111]
22 Khoa học & Công nghệ - Số 21/Tháng 3 - 2019 Journal of Science and Technology
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3.3. Dislocations and fracture structures tension process, the Thompson tetrahedron used 
 In order to investigate the dislocations for indexing Burgers vectors and slip planes of Cu 
and fracture structures of Cu/Ta nanofilms under crystal (FCC) is presented in Fig. 5.
 Fig. 5. The Thompson tetrahedron used for indexing Burgers vectors and slip planes of FCC structure
 The dislocations and fracture structures of the intrinsic stacking faults. The intersection of the 
Cu/Ta nanofilms under the tension process over two leading partials on different [111] planes causes 
the peak stress at room temperature and a strain their movements being blocked and the stair-rod 
rate of 108 s-1 is shown in Fig. 6. The dislocations dislocation is generated, which is described as:
intensely appeared in the Cu elements. The γα = γD + Dα or 1/6a = 1/6a + 1/6a<2
generality dislocations are and in the 1 1 > (in vector form) (2)
Cu elements, and in the Ta elements. This As a kind of sessile dislocation which is 
result is consistent with Lu et al. [4] and Zhang et incapable of moving, the stair-rod dislocation is 
al. [5] in previous studies. able to be dissociated into two trailing partials so 
 The α and γ plane control all thplane control that plastic deformation can proceed. The perfect 
all the dislocation slip systems in Cu crystal. The dislocation is also formed according to the Shockley 
Shockley partial dislocations [11] play a role as the partial as follows:
leading partials. The glide of a leading partial in BD = Dα + Bα or 1/2a = 1/6a + 1/6a<12
the swept areas under the tension process formed 1 > (in vector form) (3)
Fig. 6. The dislocations and fracture structures of Cu/Ta nanofilms over the peak stress at room temperature and 
strain rate of 108 s-1. (a) The growing orientation of Cu is [100] and (b) The growing orientation of Cu is [111]
 In addition to dissociating the stair-rod formed, which is called Hirth dislocation [12]. 
dislocations, the trailing partials are able to eliminate This dislocation is the reaction product of a leading 
the stacking faults which result from the leading partial and a trailing partial and can be expressed as:
ones as well. Not only the stair-rod dislocation, γα/BD = γB + αD or 1/3a = 1/6a + 
but also another kind of sessile dislocation is 1/6a (in vector form) (4)
Khoa học & Công nghệ - Số 21/Tháng 3 - 2019 Journal of Science and Technology 23
ISSN 2354-0575
4. Conclusion (3) The FCC structures are mainly transferred 
 The mechanical properties and deformation into HCP structures.
mechanisms of Cu/Ta nanofilms with different (4) The and dislocations 
growing orientations under tension process are mainly occur in Cu elements, dislocations 
studied using MD simulation. The conclusions are are found in Ta elements.
listed as follows:
 (1) The Cu/Ta[100]/[111] nanofilms exhibits ACKNOWLEDGMENT
the most excellent mechanical properties. This study is conducted by the research 
 (2) The local stress concentration is intensely team UTEHY.T016 and funded by the Center for 
distributed at intersections between circle voids and Applied Science and Technology Research, Hung 
interfaces of Cu/Ta specimens. Yen University of Technology and Education.
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 ẢNH HƯỞNG CỦA SỰ ĐỊNH HƯỚNG NGUYÊN TỬ
 ĐẾN TÍNH CHẤT CƠ HỌC CỦA TẤM NANO Cu/Ta
 SỬ DỤNG MÔ PHỎNG ĐỘNG LỰC HỌC PHÂN TỬ 
Tóm tắt:
 Trong bài báo này, ảnh hưởng của sự định hướng nguyên tử khác nhau đến tính chất cơ học của tấm 
nano Cu/Ta với một lỗ trống hình tròn dưới tác động của quá trình kéo được nghiên cứu bằng cách sử dụng 
phương pháp mô phỏng động lực học phân tử. Mối quan hệ giữa sức căng-ứng suất, sự biến đổi cấu trúc 
nguyên tử, cơ chế xô lệch mạng tinh thể và ứng suất tập trung cục bộ được đánh giá. Kết quả cho thấy rằng 
tấm nano Cu[100]/Ta[111] thể hiện các tính chất cơ học tốt nhất. Các cấu trúc FCC chủ yếu bị chuyển biến 
thành cấu trúc HCP, và các hướng lệch mạng tinh thể , xuất hiện ở phần Cu. Ứng suất tập 
trung cục bộ được phân bố xung quanh các vùng giao nhau giữa lỗ trống của mẫu và giao diện của Cu/Ta.
Từ khóa: Động lực học phân tử; tấm nano Cu/Ta; sự lệch mạng Hirth; tứ diện Thompson.
24 Khoa học & Công nghệ - Số 21/Tháng 3 - 2019 Journal of Science and Technology