- Email: [email protected]
Ni coating on TC4 alloy
Accepted Manuscript Diffusion behavior and mechanical properties of Cu/Ni coating on TC4 alloy Yongnan Chen, Shuangshuang Liu, Yongqing Zhao, Qiang Liu, Lixia Zhu, Xuding Song, Yong Zhang, Jianmin Hao PII:
S0042-207X(17)30188-4
DOI:
10.1016/j.vacuum.2017.06.004
Reference:
VAC 7445
To appear in:
Vacuum
Received Date: 12 February 2017 Revised Date:
21 April 2017
Accepted Date: 4 June 2017
Please cite this article as: Chen Y, Liu S, Zhao Y, Liu Q, Zhu L, Song X, Zhang Y, Hao J, Diffusion behavior and mechanical properties of Cu/Ni coating on TC4 alloy, Vaccum (2017), doi: 10.1016/ j.vacuum.2017.06.004. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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Diffusion behavior and mechanical properties of Cu/Ni coating on TC4 alloy ∗
Yongnan Chen 1) ; Shuangshuang Liu 1); Yongqing Zhao 2); Qiang Liu 3); Lixia Zhu 3); Xuding Song 4);
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Yong Zhang1); Jianmin Hao1); 1) School of Material science and Engineering, Chang’an University, Xi’an 710064, China; 2) Northwest Institute for Nonferrous Metal Research, Xi’an 710016, China; 3) CNPC Tubular Goods Research Institute, Xi’an 710017, China;
(1) Yongnan Chen1)∗;
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4）School of construction machinery, Chang’an University, Xi’an, 710064, People’s Republic of China.
Corresponding authors: [email protected] Shuangshuang Liu1); [email protected] Yong Zhang1);
Jianmin Hao1);
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[email protected]
[email protected]
China;
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School of Material science and Engineering, Chang’an University, Xi’an 710064,
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(2) Yongqing Zhao2); [email protected]
Northwest Institute for Nonferrous Metal Research, Xi’an 710016, China;
(3) Qiang Liu3);
[email protected] Lixia Zhu3); [email protected] CNPC Tubular Goods Research Institute, Xi’an 710017, China; ∗
Corresponding author, Ph.D, E-mail: [email protected] ( Yongnan Chen)
ACCEPTED MANUSCRIPT (4) Xuding Song4); [email protected] School of construction machinery, Chang’an University, Xi’an, 710064, People’s
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Republic of China
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Diffusion behavior and mechanical properties of Cu/Ni coating on TC4 alloy ∗
Yongnan Chen 1) ; Shuangshuang Liu 1); Yongqing Zhao 2); Qiang Liu 3); Lixia Zhu 3); Xuding Song 4);
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Yong Zhang1); Jianmin Hao1); 1) School of Material science and Engineering, Chang’an University, Xi’an 710064, China; 2) Northwest Institute for Nonferrous Metal Research, Xi’an 710016, China; 3) CNPC Tubular Goods Research Institute, Xi’an 710017, China;
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4）Key Laboratory of road construction technology and equipment of Ministry of Education, Chang’an University, Xi’an, 710064, China.
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Abstract
Diffusion process during heat treatment can be successfully employed to connect dissimilar metals. In order to get interconnected plating structure with a good biological performance on TC4 alloy, the most widely used titanium alloy, the surface of TC4 alloys were modified by deposition of Ni and Cu layers, and heat treatment
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was performed to increase the diffusivity at the interface. In this paper, the elements diffusion behaviors of Cu/Ni and Ni/Ti at the interfaces of Cu/Ni/Ti were investigated. A diffusion model was developed to reveal the diffusion coefficients and diffusion
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activation energies based on the Fick’s law and Arrhenius-type equation. The adhesion strength and hardness of the coatings were also measured to evaluate the
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influence of diffusion behavior on mechanical properties. It was found that the diffusion behavior was significantly influenced by heat treatment temperature. The diffusion coefficient increased with the increase of heat treatment temperature, especially above 700°C. The difference in the diffusion activation energies can be attributed to the difference in contact area and defect density caused by different coating processing technologies. In addition, the improvement of adhesion strength and hardness were mainly due to the solution strengthening and intermetallic compounds (IMCs) in the coating. ∗
Corresponding author, Ph.D, E-mail: [email protected] ( Yongnan Chen) 1
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Keywords: Titanium alloy; Diffusion behavior; Heat treatment; Intermetallic compounds; Mechanical properties.
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1. Introduction
Titanium and its alloys are used as functional and structural materials in areas of aerospace, vessels, metallurgy, etc. for their excellent properties of high specific
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strengths, low densities, high melting points and corrosion resistances [1-3]. Among them, TC4 alloy is the most widely used titanium alloy especially in petrochemical,
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airplanes, navigations and automobiles manufacturing [4-6]. However, the applications of TC4 alloy are limited by the low thermal conductivities and poor wear resistances. To overcome these limitations, surface treatments such as electroplating, micro arc oxidation and nitridation were usually performed. For example, Yao et al. [7] indicated that Cu coating electroplated on TC4 alloy could greatly enhance the
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thermal conductivity and wear resistance. Other researches on the electroplating of TC4 alloy [8-10] showed that the use of suitable interlayer materials could enhance the adhesion strength between the substrate and coating.
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The recent researches [11,12] on the relationship of electrodeposit and mechanical properties indicated that internal stress of electrodeposit has significant effect on the adhesion strength. To overcome this problem, further researches [13,14]
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on the improvement of the adhesion strength suggested that the application of heat treatment could remove internal stress and improve the adhesion strength between the coating and substrate. Lee et al. [15] developed the manufacturing method with massive solid-state of diffusion combination of multi-sheets of titanium alloy under the specific process parameters such as diffusion temperature, time, pressure and vacuum environment. The effects of heat treatment parameters on plating adhesion for TC4 alloys were exactly obtained, and the metallurgical bonding could be easily formed to improve the adhesion [13]. The diffusion between Ti and Cu elements was achieved by heat treatment and the adhesion strength was improved after heat 2
ACCEPTED MANUSCRIPT treatment at 890°C [16]. Moreover, the research on the phase transitions during heat treatment of Ni and Ti elements revealed that the adhesion strength was significant associated with the formation of IMCs (TixNiy) in the coatings after heat treated at 850°C and 900°C [2]. The above facts indicate that formation of IMCs via metal
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plating followed by diffusion treatment can improve mechanical properties of titanium alloys.
However, the detail diffusion behavior and its effects of phase transitions on mechanical properties of Cu/Ni coating on TC4 alloy are still unknown, which are
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crucial for the wide engineering implementation. The present research focuses on the diffusion behavior and phase transitions of the Cu/Ti using Ni as an intermediate
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material in the heat treatment temperature range from 600°C to 800°C. The diffusion coefficients and activation energies of Cu/Ni and Ni/Ti at the interface of Cu/Ni/Ti were calculated according to the Fick’s law, respectively. The formation of IMCs during diffusion was analyzed and its effects on the mechanical properties of coating
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were also discussed.
2. Materials and Methods
2.1. Materials and heat treatment
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In this work, Cu/Ni coating was prepared on TC4 alloy by using conventional electroplate method. Rectangular samples of commercial TC4 alloys (10×10×5mm)
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were used as the substrates, and its chemical composition is listed in Table 1. Table 1 Chemical composition of TC4 alloy (wt.%) Elements
Al
V
Fe
Mn
Si
Zn
Ti
Nominal
5.0-6.5
3.3-4.5
0.3-0.9
0.5
0.4
0.3
Bal
The substrates were mechanically ground and then ultrasonically cleaned in distilled water. Using hydrofluoric acid and formamide as activating solution, pretreatment process was carried out. It is necessary to notice the method of preparing nickel and copper electroplating solution. For the electroplating bath of the Ni layer, 3
ACCEPTED MANUSCRIPT the 180g/L NiSO4·6H2O should be dissolved completely in distilled water at room temperature. Then 70g/L Na2SO4, 30g/L MgSO4, 10g/L NaCl and 30g/L H3BO3 were put into the solution in turns. For the electroplating bath of the Cu layer, the 220g/L CuSO4·5H2O should be dissolved completely in distilled water at room temperature.
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Then 20mg/L NaCl and 70g/L H2SO4 were put into the solution in turns. Simultaneously, the solution was stirred continually until they were dissolved thoroughly. Then, Ni-pre electroplating process was performed on rinsed and dried samples with an applied voltage of 3V for 8min at room temperature. The plating of
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Cu was performed on Ni-plating samples with an applied voltage of 0.65V for 20min at room temperature. After electroplating, samples were sealed into vacuum furnace
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(OTF-1200X) with the vacuum level of 1×10-2Pa to avoid oxidization during heat treatment. The plating samples were heat treated at 600°C, 700°C and 800°C for 3h, respectively, and then cooled down to room temperature in vacuum furnace. The surface morphology and the corresponding cross-sectional microstructure of TC4
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alloy electroplated Cu/Ni are shown in Figure 1(a) and (b), respectively.
Figure 1 The surface morphology (a) and the corresponding cross-sectional microstructure (b) of TC4 alloy electroplated Cu/Ni. A bright, flat and compact Cu/Ni coating was formed on TC4 alloys. The Cu coating grows as cell-shaped structure and the microstructure was compact.
2.2. Analysis of micro-morphology The surface morphology and cross-sectional microstructure of TC4 alloy electroplated Cu/Ni were observed using scanning electron microscope (SEM, Hitachi-S4800) at 20kV with an energy-dispersive spectroscopy (EDS, Hitachi-S4800) to determine the 4
ACCEPTED MANUSCRIPT composition of the diffusion layer. To interpret the diffusion process during heat treatment, the element distributions of Cu, Ni and Ti at the Cu/Ni and Ni/Ti interfaces were determined to calculate diffusion coefficients and diffusion activation energies. The Ni atoms diffused to the Cu layer and Ti matrix were found with EDS to follow
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linear Arrhenius dependencies with the pre-exponential factors, and the diffusion activation energies Q of Cu/Ni and Ni/Ti were calculated by Fick’s law and Arrhenius-type equation, the results were averaged over three tests and calculations. To analyze the thermal reaction procedure of coatings, the differential scanning
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calorimeter (DSC, Netzsch STA 449C) was performed at the heating rates of 10°C/min in argon atmosphere. The phase structures of the samples were
20~80° with a step of 0.02° (=2θ)/s.
2.3. Mechanical properties
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characterized via X-ray diffraction (XRD, XRD D/M2500) by scanning span of
The cross-cut tests and adhesion tests were employed to evaluate the adhesion strength between the coating and the substrate. In the cross-cut test, two sets of six
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cuts with a spacing of 1mm were made perpendicular to each other to make a lattice of 25 small blocks [17]. A standardized tape was then stuck on the lattice and pulled off with a constant force and average value calculated on five tested data. The
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adhesion score of coating was calculated and obtained according to ISO 4624 [18]: S=
Nu N
(1)
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Where S is adhesion score, N u is number of blocks unshed, and N is number of
all blocks. The adhesion strength of the heat treated samples at different temperatures was evaluated using a microcomputer-controlled electron universal testing machine, as shown in Figure 2.
The hardness variation along the depth was obtained at various intervals of samples being heat treated at different temperatures by a MH-5 Digimatic Vickers hardness tester using a 50gf load for 15s. The distance between any two neighboring indentations was more than 2µm. The results are averaged over five hardness tests. 5
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Figure 2 The schematic diagrams for adhesion tests. At least five samples for each condition were tested to obtain an average value.
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3. Results and discussion
3.1 Cu/Ni/Ti elements diffusion during heat treatment
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The morphology and composition after heat treatment were analyzed by cross-sectional observation with EDS examination, as shown in Figure 3. The generalized element contents of Cu, Ni and Ti in different layers (Figure 3) are listed in Table 2. Almost no element diffusion of Ti, Ni and Cu occurred without heat treatment according to the result from Figure 3(a) and Table 2. With the increase of
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heat treatment temperature, the content of element decreased from 94.15wt.% to 68.19wt.% for Cu in zone A, and from 95.14wt.% to 67.93wt.% for Ti in zone C, respectively. The tendency of elements diffusion from the Ni to the Cu sides and the
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Ti to the Ni sides was higher than that in the reverse direction, which implied that the elements diffusion of Ti becomes faster than that of Ni at the Ni/Ti interface and the
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Ni elements diffused along Cu grain boundaries through fast pipe diffusion path at the same heat treatment temperature [19,20].
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Figure 3 Cross-sectional microstructures and elements distribution of TC4 alloy electroplated Cu/Ni composite coating under different heat treatment temperatures: (a) unheated, (b) 600°C, (c)
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respectively.
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700°C and (d) 800°C. A, B and C represented Cu-rich layer, Ni-rich layer and Ti-rich layer,
Table 2 EDS analysis results for the samples after heat treatment Heat treatment temperature/°C
Unheated
600
Elements content /wt.% Region Cu
Ni
Ti
A
99.83
0.16
0.01
B
1.12
98.12
0.76
C
0.01
1.11
98.88
A
94.15
5.66
0.19
B
2.41
94.35
3.24
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800
0.59
4.27
95.14
A
84.47
15.14
0.39
B
3.03
63.89
33.08
C
1.67
16.97
81.36
A
68.19
20.89
10.92
B
5.89
C
3.50
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700
C
50.77
43.34
28.57
67.93
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Note: Each datum in the table is an average value of five tested data in each region.
To study the diffusion behaviors of Ti, Ni and Cu elements on an atomic scale,
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the diffusion models for Cu/Ni and Ni/Ti system were established based on the general solution of Fick’s law, respectively, and experimental model system is given by following equation [21]:
x C ( x, t ) = B + Aerf 2 Dt
(2)
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By considering the boundary conditions ( t = 0 : x > 0 , C Ni = 1 ; x < 0 , C Ni = 0 ; t ≥ 0 : x = ∞ , C Ni = 1 ; x = −∞ , C Ni = 0 ) the analytical solution is described as
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follows:
x C ( x, t ) = 0.5erf 2 Dt
(3)
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Using the following formula, the distribution of Ni elements was achieved by
point fitting as shown in Figure 4, the fitting parameters a and b could be obtained.
C( x, t ) = 0.5(1 - erf (ax - b ))
(4)
Where the a and b could be expressed as follows:
(
a = 1 2 Dt
)
(
b = x0 2 Dt
(5)
)
(6)
The diffusion coefficients were calculated through the following equation:
(
D = 1 4a 2t 8
)
(7)
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Figure 4 The Ni element contents penetrated across the interface and then achieved point fitting at different heat treatment temperatures (a) 600°C and (b) 800°C for Cu/Ni interface, (c) 600°C and
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(d) 800°C for Ni/Ti interface.
Ni The diffusion coefficients of Cu/Ni ( DCu ) and Ni/Ti ( DTiNi ) were calculated under
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Ni different heat treatment temperatures, respectively, as shown in Table 3. The DCu
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and DTiNi increased with increase of heat treatment temperature in the range from 600°C to 800°C, respectively. The relationship between diffusion coefficient and temperature as well as the diffusion activation energy can be expressed by following equation [22]:
Q D = D0 exp RT
(8)
Where D is diffusion coefficient, D0 is pre-exponential factor, and Q is diffusion activation energy. The logarithmic formula is shown as the following.
ln D = A - B T 9
(9)
ACCEPTED MANUSCRIPT Where the A and B can be express as follows: A = ln D0
(10)
B=Q R
(11)
The pre-exponential factor D0 and the diffusion activation energy Q were
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obtained from the fit of a straight line to the data in Arrhenius plot and compared with the literature values listed in Table 4. In previous researches, Yan et al. [23] investigated the diffusion upon annealing Cu/Ni multilayers structures by ion
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sputtering from 325°C to 375°C for 30min and obtained the diffusion activation energy Q (101.40kJ/mol) of Cu/Ni multilayers. Abdul-Lettif [24] studied the diffusion
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behavior of vacuum-deposited Cu/Ni bilayer thin films and calculated the diffusion activation energy Q (96.40kJ/mol) for nickel in copper at constant temperatures (300-500°C) for an annealing time between 5min and 190min. The above results indicated that the diffusion process for Cu/Ni at low temperature was controlled by grain boundary diffusion, where the grain boundaries and a high defect density played
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a dominant role in the diffusion process. It is known that different coating processing technologies could affect the microstructure and result in different diffusion activation energies. For example, the use of ion sputtering could involve changes in both the creation of vacancies and energy of removed vacancies in the grain boundary which
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act as paths of easy diffusion. In the present work, less grain boundaries and vacancies in the Cu/Ni coating was obtained by electroplating and heat treatment compared with
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that by ion sputtering, which is the main reason for the high diffusion activation energy Q (111.95kJ/mol).
Ni
Ni
Table 3 The calculated values of DCu and DTi Heat treat temperature/°C
600
700
800
Ni DCu (cm2/s)
3.68×10-16
1.95×10-15
7.09×10-15
DTiNi (cm2/s)
1.53×10-14
3.03×10-14
6.08×10-14
Ni
Ni
Note: The DCu and DTi
represent the diffusion coefficients of nickel diffused to copper 10
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As for the diffusion activation energy of Ti/Ni interface, Panigrahi et al. [25] calculated the activation energy Q (46.20kJ/mol) of Ti-50Ni alloy by powder
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metallurgy in the higher temperature range (above 780°C), which is lower than that in this work (52.03kJ/mol). This is due to the fact that surface diffusion is considered to be controlled by diffusion contact area, compared with Ni electroplating on the Ti alloy in this work, larger contact area and less insufficient binding between particles
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were obtained by hot-pressing powder sintering processing, which supplied more paths for diffusion of Ni/Ti and required less energy to diffusion. In addition, Zhou et
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al. [26] fabricated Ti-Ni couples by upset butt welding with a current of 10A and obtained the diffusion activation energy Q (42.11kJ/mol) of the whole interfacial layer in Ti-Ni couple by isothermal diffusion treatment in the temperature range from 500°C to 700°C. For a Ti-Ni couple, the current, flowing in both directions, could increase the concentration and the mobility of defects, which plays a main role in
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promoting the behavior of the Ni and Ti atoms.
The above results of diffusion activation energy of Ni atoms in Cu coating and Ti matrix were lower than that in this work, which is due to the difference in larger
technologies.
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contact area and high defect density caused by different coating processing
Based on above calculated results, the diffusion coefficients of the Cu/Ni and
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Ni/Ti interface after heat treatment in the temperature range from 600°C to 800°C is shown as the following formulas, respectively:
DCuNi = 1.99 × 10− 9 exp(−
111953 )
（12）
)
（13）
RT DTiNi = 2.00 × 10−11 exp(−
52031
RT Table 4 The pre-exponential factor (D0) and diffusion activation energy (Q) of nickel in copper and nickel in TC4 alloy obtained in the present work in comparison with the results of other works
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Source
D0(cm2/s)
Q(kJ/mol)
Present work
1.99×10-9
111.95
Ref.[23]
6.20×10-9
101.40
Ref.[24]
2.00×10-7
96.40
Present work
2.00×10-11
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(at Cu/Ni interface)
52.03
(at Ni/Ti interface) /
Ref.[26]
/
49.83 42.11
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Ref.[25]
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3.2 The phase transitions during heat treatment
The diffusion of Cu/Ni/Ti during heat treatment could form new phase in the range from 600°C to 800°C [27]. In order to reveal the phase transitions during heat treatment, DSC test was performed (Figure 5a) and the results showed that a broad exothermic peak positioned at 700°C for the Cu/Ni/Ti compact. The broad exothermic
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peak for the Cu/Ni/Ti compact corresponds to the formation of IMCs via solid-state exothermic reactions in the middle of elemental Cu, Ni and Ti [28], the CuTi, TiNi and Ti2Ni were obtained after heat treatment at 700°C, as shown in Figure 5(b). This
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result was also confirmed by Simoes [27], where several phases formed in the TC4 alloy and Ni/Ti multilayer interface after heat treated at 750°C, 800°C and 900°C.
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Besides that, Ti3Ni4 was detected at higher heat treatment (800°C) (as summarized in Table 5).
Figure 5 DSC curves (a) of Ti/Ni/Cu compacts with a heating rate of 10°C/min. Surface XRD 12
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Table 5 The phases formed at different heat treatment temperatures Phases
600
/
700
TiNi, Ti2Ni, CuTi
800
TiNi, Ti2Ni, CuTi, Ti3Ni4
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Heat treatment temperature/°C
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In addition, no CuxNiy IMCs was found at the Cu/Ni interface for the reason that a complete solid solution was formed in the temperature range from 600°C to 800°C
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[29]. For the Ni/Ti interface, the diffusion coefficient of Ni/Ti at 700°C was greater than that of at 600°C (Table 3). The TixNiy IMCs were not detected at the Ni/Ti interface after heat treated at 600°C, which was due to the lower diffusion coefficient of Ni/Ti. With the increase of temperature, the elements diffusion of Ni and Ti increased to form TiNi and Ti2Ni phases at the Ni/Ti interface after heat treated at
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700°C. Based on the Ti-Ni binary system, the TixNiy IMCs mainly consist of TiNi3, TiNi and Ti2Ni from the Ni side to the Ti side and the free energy of TixNiy IMCs can be express as following [30]:
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G (Ti2 Ni ) = −49120 + 17.208T ( J / mol )
(14)
G (TiNi3 ) = −55585 + 15.962T ( J / mol )
(15)
G (TiNi ) = −54600 + 18.133T ( J / mol )
(16)
According to Eqs. from (14) to (16), the free energy for TixNiy IMCs at different
heat treatment temperatures were calculated, as shown in Table 6. The TiNi3 phase can be easy to nucleate and grow at the heat treatment temperatures from 600°C to 800°C. Based on the formation energy in this temperature range, TiNi3 phase was primarily formed at Ni/Ti interface, and Ti2Ni was afterward formed. However, the TiNi3 phase was not found at the Ni/Ti interface in the experiments, this may due to the occurrence of eutectoid from Ni3Ti to NiTi phases [31]. In addition, the varieties 13
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that the Ti3Ni4 phases formed in a short holding time.
Table 6 Calculated free energy of formation of TixNiy IMCs at different heat treatment temperatures (kJ/mol) 600°C
700°C
Ti2Ni
−34.09
−32.37
TiNi
−38.77
−36.95
−35.14
TiNi3
−41.65
−40.05
−38.46
800°C
−30.65
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Phase
Owing to the presence of Ni interlayer, the CuxTiy IMCs were not found after heat treatment at 600°C. But the CuTi phase was detected after heat treatment at 700°C, which implies that the Ni layer delays the elements diffusion between Ti and
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Cu (Figure 3c). The phase stability of CuxTiy IMCs during heat treatment has significant effect on the mechanical properties of coating [33]. To estimate the stability of CuxTiy IMCs, the formation enthalpy (∆H) of each CuxTiy IMCs can be calculated by [34]:
(
)
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bulk ∆H = [ Etotal - xECu + yETibulk ] ( x + y )
(17)
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Where Etotal is the total cell energy of a CuxTiy primitive cell including x Cu bulk atoms and y Ti atoms, ECu and ETibulk are the energy of a Cu atom and a Ti atom in
the bulk state, respectively. Limited Ti atoms diffused into the Cu coating for the existence of Ni layer. In addition, based on the calculated results, the CuTi phase was the most stable phase due to the minimal formation enthalpy. As a result, only CuTi phases existed in this study above 700°C. No ternary intermediate phase formed in the diffusion processes, which may due to low heat treated temperature and short holding time [10,35].
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3.3. Effect of diffusion behavior on mechanical properties Previous research [36] on the interface of titanium and different metals showed that the presence of IMCs at the interfacial regions strongly affected the adhesion strength between the substrate and coating. To study the effect of diffusion behavior on
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mechanical properties, the relationships between adhesion strength and heat treatment temperature is shown in Figure 6. It was found that the diffusion could improve the quality and adhesion property of the coating, which was due to the formation of metallurgical bonding at the interface. The adhesion strength of heat treated samples
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was higher than that of the unheated one, and the highest adhesion strength of the sample (about 120MPa) was obtained after heat treated at 700°C. Similar results on
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improvement of mechanical properties of Ni/Ti [9] and Cu/Ti [16] coatings revealed that the adhesion strength was mainly controlled by IMCs. However, the drop of adhesion strength at 800°C in this work was considered to be the excessive growth of
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the TiNi and Ti2Ni IMCs at the Ni/Ti interface [35].
Figure 6 The adhesion score and adhesion strength of the electroplated samples treated at different heat treatment temperatures. The results were averaged over five tension tests.
Based on the results of hardness tests, the hardness curve can be divided into three regions as shown in Figure 7. It seems that the first area of hardness curve is Cu coating, while the third area is Ti substrate. The hardness measurements were mainly made in the second area, which also marks the properties of new phases. Furthermore, 15
ACCEPTED MANUSCRIPT the Cu side of the second area exhibits lower hardness (from 100HV to 200HV), while the Ti side exhibits higher hardness values (from 400HV to 700HV). The hardness measured from the Cu side of the heat treated samples start to increase at the interface. The hardness values decrease again in the Ti area after the second area is
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crossed. It is also seen that the hardness values obtained from the second area are higher when compared to the other areas. The above results confirmed that the presence of IMCs in the coating could enhance the hardness values, which are consistent with the previous researches on surface strengthen effects of the TC4 alloy
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[35-38]. In addition, the width of the second area increases with the increase of heat treatment temperature, which is probably due to a new phase formation induced by
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the element diffusion at the interface. The highest hardness value is seen for the specimens at the heat treatment temperature of 700°C. This is due to the solution strengthening and the increase of IMCs with the increase of heat treatment temperature [31]. However, with a further rise in heat treatment temperature to 800°C, the hardness of coating decreases. This may due to larger grain size at high
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temperature.
Figure 7 The hardness variation along the depth was obtained on the cross-sectional samples (a) and the dependence of the surface hardness on the depth from the surface for the heat treated specimens at different temperatures (b).
4. Conclusions The diffusion behavior and mechanical properties of Cu/Ni coating on TC4 alloy were investigated under different heat treatment temperatures, the following conclusions 16
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The diffusion coefficient increased with the increase of heat treatment temperature. The elements diffusions of Cu/Ni/Ti during heat treatment caused the formation of new phases. The IMCs including TiNi, Ti2Ni and Ti3Ni4 were observed at the Ni/Ti interface, which was attributed to the change of the Ni contents and the possibility of
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nucleation. Besides that, the Ni interlayer could not restrict the diffusion of Ti elements to the Cu side and CuTi phase formed for temperatures up to 700°C. The
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maximum adhesion strength (about 120MPa) between the substrate and coating was obtained after heat treatment at 700°C. However, the adhesion strength dropped with an increase in the temperature to 800°C, which was due to the excessive growth of TiNi and Ti2Ni at the Ni/Ti interface. Hardness analysis confirmed that the presence of IMCs in the coating could enhance the hardness values and the highest hardness
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value is obtained at the heat treatment temperature of 700°C.
Acknowledgments
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Y. Chen, S. Liu，Q. Liu and L. Zhu planned and conducted all experiments. Y. Chen, S. Liu, Y. Zhao and Y. Zhang examined the experimental results, performed data
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analyses and wrote the article draft together. X. Song and J. Hao were involved in the discussion and writing.
Formatting of funding sources The authors acknowledge financial supported by National Natural Science Foundation of China (grant no. 51471136, and 51401032), International Cooperation Foundation of Shaanxi Province (2016KW-051), China Postdoctoral Science Foundation (2016M590909), and the Special Fund for Basic Scientific Research of Central Colleges (310831163401). 17
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Conflicts of Interest The authors declare no conflict of interest.
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Chem. Phys. 155 (2015) 113-121. [32] P. Lang, E. Povoden-Karadeniz, C.D. Cirstea, E. Kozeschnik, T. Wojcik, Crystal structure and free energy of Ti2Ni3 precipitates in Ti-Ni alloys from first principles, Comp. Mater. Sci.
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[33] G.Q. Chen, B.G. Zhang, L. Wei, J.C. Feng, Influence of electron-beam superposition welding on intermetallic layer of Cu/Ti joint, T. Nonferr. Metal. Soc. 22(10) (2012) 2416-2420. [34] S. Chen, Y.H. Duan, B. Huang, W.C. Hu, Structural properties, phase stability, elastic properties and electronic structures of Cu-Ti intermetallics, Philos. Mag. 95(32) (2015) 1-19. [35] S. Semboshi, A. Iwase, T. Takasugi, Surface hardening of age-hardenable Cu-Ti alloy by plasma carburization, Surf. Coat. Tech. 283 (2015) 262-267. 20
ACCEPTED MANUSCRIPT [36] A. Elrefaey, W. Tillmann, Brazing of titanium to steel with different filler metals: analysis and comparison, J. Mater. Sci. 45(16) (2010) 4332-4338. [37] K.P. Gupta, The Cu-Ni-Ti (Copper-Nickel-Titanium) System, Cheminform, 23(6) (2002) 541-547.
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bronze diffusion-bonded joint, T. Nonferr. Metal. Soc. 19(2) (2009) 414-417.
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Figure Captions Figure 1 The surface morphology (a) and the corresponding cross-sectional microstructure (b) of TC4 alloy electroplated Cu/Ni. A bright, flat and compact Cu/Ni coating was formed on TC4
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alloys. The Cu coating grows as cell-shaped structure and the microstructure was compact.
Figure 2 The schematic diagrams for adhesion tests. At least five samples for each condition were
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tested to obtain an average value.
Figure 3 Cross-sectional microstructures and elements distribution of TC4 alloy electroplated
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Cu/Ni composite coating under different heat treatment temperatures: (a) unheated, (b) 600°C, (c) 700°C and (d) 800°C. A, B and C represented Cu-rich layer, Ni-rich layer and Ti-rich layer, respectively.
Figure 4 The Ni element contents penetrated across the interface and then achieved point fitting at
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different heat treatment temperatures (a) 600°C and (b) 800°C for Cu/Ni interface, (c) 600°C and (d) 800°C for Ni/Ti interface.
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Figure 5 DSC curves (a) of Ti/Ni/Cu compacts with a heating rate of 10°C/min. Surface XRD
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images (b) of the electroplated samples heat treated at different temperatures for 3h.
Figure 6 The adhesion score and adhesion strength of the electroplated samples treated at different heat treatment temperatures. The results were averaged over five tension tests.
Figure 7 The hardness variation along the depth was obtained on the cross-sectional samples (a) and the dependence of the surface hardness on the depth from the surface for the heat treated specimens at different temperatures (b).
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Tables with captions Table 1 Chemical composition of TC4 alloy (wt.%) Al
V
Fe
Mn
Si
Zn
Ti
Nominal
5.0-6.5
3.3-4.5
0.3-0.9
0.5
0.4
0.3
Bal
Table 2 EDS analysis results for the samples after heat treatment
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Elements
Elements content /wt.% Heat treatment temperature/°C
Region Cu
700
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800
0.16
0.01
B
1.12
98.12
0.76
C
0.01
1.11
98.88
A
94.15
5.66
0.19
B
2.41
94.35
3.24
C
0.59
4.27
95.14
A
84.47
15.14
0.39
B
3.03
63.89
33.08
C
1.67
16.97
81.36
A
68.19
20.89
10.92
B
5.89
50.77
43.34
C
3.50
28.57
67.93
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99.83
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600
Ti
A
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Unheated
Ni
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Note: Each datum in the table is an average value of five tested data in each region.
Ni
Ni
Table 3 The calculated values of DCu and DTi Heat treat temperature/°C
600
700
800
Ni DCu (cm2/s)
3.68×10-16
1.95×10-15
7.09×10-15
DTiNi (cm2/s)
1.53×10-14
3.03×10-14
6.08×10-14
Ni
Ni
Note: The DCu and DTi
represent the diffusion coefficients of nickel diffused to copper
coating and nickel diffused to the titanium matrix, respectively.
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Source
D0(cm2/s)
Q(kJ/mol)
Present work
1.99×10-9
111.95
Ref.[23]
6.20×10-9
Ref.[24]
2.00×10-7
Present work
2.00×10-11
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and nickel in TC4 alloy obtained in the present work in comparison with the results of other works
(at Cu/Ni interface) 101.40 96.40
52.03
/
Ref.[26]
/
49.83 42.11
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Ref.[25]
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(at Ni/Ti interface)
Table 5 The phases formed at different heat treatment temperatures Heat treatment temperature/°C 600
800
/
TiNi, Ti2Ni, CuTi
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700
Phases
TiNi, Ti2Ni, CuTi, Ti3Ni4
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Table 6 Calculated free energy of formation of TixNiy IMCs at different heat treatment temperatures (kJ/mol)
600°C
700°C
800°C
Ti2Ni
−34.09
−32.37
−30.65
TiNi
−38.77
−36.95
−35.14
TiNi3
−41.65
−40.05
−38.46
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Phase
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Highlights
►The diffusion model of the Ni/Ti and Cu/Ni interface was established. ►The formation mechanism of IMCs relies on elements diffusion behavior.
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►The IMCs have a significantly effect on their mechanical properties.
S0042-207X(17)30188-4
DOI:
10.1016/j.vacuum.2017.06.004
Reference:
VAC 7445
To appear in:
Vacuum
Received Date: 12 February 2017 Revised Date:
21 April 2017
Accepted Date: 4 June 2017
Please cite this article as: Chen Y, Liu S, Zhao Y, Liu Q, Zhu L, Song X, Zhang Y, Hao J, Diffusion behavior and mechanical properties of Cu/Ni coating on TC4 alloy, Vaccum (2017), doi: 10.1016/ j.vacuum.2017.06.004. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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Diffusion behavior and mechanical properties of Cu/Ni coating on TC4 alloy ∗
Yongnan Chen 1) ; Shuangshuang Liu 1); Yongqing Zhao 2); Qiang Liu 3); Lixia Zhu 3); Xuding Song 4);
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Yong Zhang1); Jianmin Hao1); 1) School of Material science and Engineering, Chang’an University, Xi’an 710064, China; 2) Northwest Institute for Nonferrous Metal Research, Xi’an 710016, China; 3) CNPC Tubular Goods Research Institute, Xi’an 710017, China;
(1) Yongnan Chen1)∗;
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4）School of construction machinery, Chang’an University, Xi’an, 710064, People’s Republic of China.
Corresponding authors: [email protected] Shuangshuang Liu1); [email protected] Yong Zhang1);
Jianmin Hao1);
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[email protected]
[email protected]
China;
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School of Material science and Engineering, Chang’an University, Xi’an 710064,
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(2) Yongqing Zhao2); [email protected]
Northwest Institute for Nonferrous Metal Research, Xi’an 710016, China;
(3) Qiang Liu3);
[email protected] Lixia Zhu3); [email protected] CNPC Tubular Goods Research Institute, Xi’an 710017, China; ∗
Corresponding author, Ph.D, E-mail: [email protected] ( Yongnan Chen)
ACCEPTED MANUSCRIPT (4) Xuding Song4); [email protected] School of construction machinery, Chang’an University, Xi’an, 710064, People’s
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Republic of China
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Diffusion behavior and mechanical properties of Cu/Ni coating on TC4 alloy ∗
Yongnan Chen 1) ; Shuangshuang Liu 1); Yongqing Zhao 2); Qiang Liu 3); Lixia Zhu 3); Xuding Song 4);
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Yong Zhang1); Jianmin Hao1); 1) School of Material science and Engineering, Chang’an University, Xi’an 710064, China; 2) Northwest Institute for Nonferrous Metal Research, Xi’an 710016, China; 3) CNPC Tubular Goods Research Institute, Xi’an 710017, China;
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4）Key Laboratory of road construction technology and equipment of Ministry of Education, Chang’an University, Xi’an, 710064, China.
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Abstract
Diffusion process during heat treatment can be successfully employed to connect dissimilar metals. In order to get interconnected plating structure with a good biological performance on TC4 alloy, the most widely used titanium alloy, the surface of TC4 alloys were modified by deposition of Ni and Cu layers, and heat treatment
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was performed to increase the diffusivity at the interface. In this paper, the elements diffusion behaviors of Cu/Ni and Ni/Ti at the interfaces of Cu/Ni/Ti were investigated. A diffusion model was developed to reveal the diffusion coefficients and diffusion
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activation energies based on the Fick’s law and Arrhenius-type equation. The adhesion strength and hardness of the coatings were also measured to evaluate the
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influence of diffusion behavior on mechanical properties. It was found that the diffusion behavior was significantly influenced by heat treatment temperature. The diffusion coefficient increased with the increase of heat treatment temperature, especially above 700°C. The difference in the diffusion activation energies can be attributed to the difference in contact area and defect density caused by different coating processing technologies. In addition, the improvement of adhesion strength and hardness were mainly due to the solution strengthening and intermetallic compounds (IMCs) in the coating. ∗
Corresponding author, Ph.D, E-mail: [email protected] ( Yongnan Chen) 1
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Keywords: Titanium alloy; Diffusion behavior; Heat treatment; Intermetallic compounds; Mechanical properties.
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1. Introduction
Titanium and its alloys are used as functional and structural materials in areas of aerospace, vessels, metallurgy, etc. for their excellent properties of high specific
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strengths, low densities, high melting points and corrosion resistances [1-3]. Among them, TC4 alloy is the most widely used titanium alloy especially in petrochemical,
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airplanes, navigations and automobiles manufacturing [4-6]. However, the applications of TC4 alloy are limited by the low thermal conductivities and poor wear resistances. To overcome these limitations, surface treatments such as electroplating, micro arc oxidation and nitridation were usually performed. For example, Yao et al. [7] indicated that Cu coating electroplated on TC4 alloy could greatly enhance the
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thermal conductivity and wear resistance. Other researches on the electroplating of TC4 alloy [8-10] showed that the use of suitable interlayer materials could enhance the adhesion strength between the substrate and coating.
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The recent researches [11,12] on the relationship of electrodeposit and mechanical properties indicated that internal stress of electrodeposit has significant effect on the adhesion strength. To overcome this problem, further researches [13,14]
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on the improvement of the adhesion strength suggested that the application of heat treatment could remove internal stress and improve the adhesion strength between the coating and substrate. Lee et al. [15] developed the manufacturing method with massive solid-state of diffusion combination of multi-sheets of titanium alloy under the specific process parameters such as diffusion temperature, time, pressure and vacuum environment. The effects of heat treatment parameters on plating adhesion for TC4 alloys were exactly obtained, and the metallurgical bonding could be easily formed to improve the adhesion [13]. The diffusion between Ti and Cu elements was achieved by heat treatment and the adhesion strength was improved after heat 2
ACCEPTED MANUSCRIPT treatment at 890°C [16]. Moreover, the research on the phase transitions during heat treatment of Ni and Ti elements revealed that the adhesion strength was significant associated with the formation of IMCs (TixNiy) in the coatings after heat treated at 850°C and 900°C [2]. The above facts indicate that formation of IMCs via metal
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plating followed by diffusion treatment can improve mechanical properties of titanium alloys.
However, the detail diffusion behavior and its effects of phase transitions on mechanical properties of Cu/Ni coating on TC4 alloy are still unknown, which are
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crucial for the wide engineering implementation. The present research focuses on the diffusion behavior and phase transitions of the Cu/Ti using Ni as an intermediate
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material in the heat treatment temperature range from 600°C to 800°C. The diffusion coefficients and activation energies of Cu/Ni and Ni/Ti at the interface of Cu/Ni/Ti were calculated according to the Fick’s law, respectively. The formation of IMCs during diffusion was analyzed and its effects on the mechanical properties of coating
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were also discussed.
2. Materials and Methods
2.1. Materials and heat treatment
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In this work, Cu/Ni coating was prepared on TC4 alloy by using conventional electroplate method. Rectangular samples of commercial TC4 alloys (10×10×5mm)
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were used as the substrates, and its chemical composition is listed in Table 1. Table 1 Chemical composition of TC4 alloy (wt.%) Elements
Al
V
Fe
Mn
Si
Zn
Ti
Nominal
5.0-6.5
3.3-4.5
0.3-0.9
0.5
0.4
0.3
Bal
The substrates were mechanically ground and then ultrasonically cleaned in distilled water. Using hydrofluoric acid and formamide as activating solution, pretreatment process was carried out. It is necessary to notice the method of preparing nickel and copper electroplating solution. For the electroplating bath of the Ni layer, 3
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Then 20mg/L NaCl and 70g/L H2SO4 were put into the solution in turns. Simultaneously, the solution was stirred continually until they were dissolved thoroughly. Then, Ni-pre electroplating process was performed on rinsed and dried samples with an applied voltage of 3V for 8min at room temperature. The plating of
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Cu was performed on Ni-plating samples with an applied voltage of 0.65V for 20min at room temperature. After electroplating, samples were sealed into vacuum furnace
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(OTF-1200X) with the vacuum level of 1×10-2Pa to avoid oxidization during heat treatment. The plating samples were heat treated at 600°C, 700°C and 800°C for 3h, respectively, and then cooled down to room temperature in vacuum furnace. The surface morphology and the corresponding cross-sectional microstructure of TC4
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alloy electroplated Cu/Ni are shown in Figure 1(a) and (b), respectively.
Figure 1 The surface morphology (a) and the corresponding cross-sectional microstructure (b) of TC4 alloy electroplated Cu/Ni. A bright, flat and compact Cu/Ni coating was formed on TC4 alloys. The Cu coating grows as cell-shaped structure and the microstructure was compact.
2.2. Analysis of micro-morphology The surface morphology and cross-sectional microstructure of TC4 alloy electroplated Cu/Ni were observed using scanning electron microscope (SEM, Hitachi-S4800) at 20kV with an energy-dispersive spectroscopy (EDS, Hitachi-S4800) to determine the 4
ACCEPTED MANUSCRIPT composition of the diffusion layer. To interpret the diffusion process during heat treatment, the element distributions of Cu, Ni and Ti at the Cu/Ni and Ni/Ti interfaces were determined to calculate diffusion coefficients and diffusion activation energies. The Ni atoms diffused to the Cu layer and Ti matrix were found with EDS to follow
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linear Arrhenius dependencies with the pre-exponential factors, and the diffusion activation energies Q of Cu/Ni and Ni/Ti were calculated by Fick’s law and Arrhenius-type equation, the results were averaged over three tests and calculations. To analyze the thermal reaction procedure of coatings, the differential scanning
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calorimeter (DSC, Netzsch STA 449C) was performed at the heating rates of 10°C/min in argon atmosphere. The phase structures of the samples were
20~80° with a step of 0.02° (=2θ)/s.
2.3. Mechanical properties
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characterized via X-ray diffraction (XRD, XRD D/M2500) by scanning span of
The cross-cut tests and adhesion tests were employed to evaluate the adhesion strength between the coating and the substrate. In the cross-cut test, two sets of six
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cuts with a spacing of 1mm were made perpendicular to each other to make a lattice of 25 small blocks [17]. A standardized tape was then stuck on the lattice and pulled off with a constant force and average value calculated on five tested data. The
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adhesion score of coating was calculated and obtained according to ISO 4624 [18]: S=
Nu N
(1)
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Where S is adhesion score, N u is number of blocks unshed, and N is number of
all blocks. The adhesion strength of the heat treated samples at different temperatures was evaluated using a microcomputer-controlled electron universal testing machine, as shown in Figure 2.
The hardness variation along the depth was obtained at various intervals of samples being heat treated at different temperatures by a MH-5 Digimatic Vickers hardness tester using a 50gf load for 15s. The distance between any two neighboring indentations was more than 2µm. The results are averaged over five hardness tests. 5
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Figure 2 The schematic diagrams for adhesion tests. At least five samples for each condition were tested to obtain an average value.
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3. Results and discussion
3.1 Cu/Ni/Ti elements diffusion during heat treatment
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The morphology and composition after heat treatment were analyzed by cross-sectional observation with EDS examination, as shown in Figure 3. The generalized element contents of Cu, Ni and Ti in different layers (Figure 3) are listed in Table 2. Almost no element diffusion of Ti, Ni and Cu occurred without heat treatment according to the result from Figure 3(a) and Table 2. With the increase of
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heat treatment temperature, the content of element decreased from 94.15wt.% to 68.19wt.% for Cu in zone A, and from 95.14wt.% to 67.93wt.% for Ti in zone C, respectively. The tendency of elements diffusion from the Ni to the Cu sides and the
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Ti to the Ni sides was higher than that in the reverse direction, which implied that the elements diffusion of Ti becomes faster than that of Ni at the Ni/Ti interface and the
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Ni elements diffused along Cu grain boundaries through fast pipe diffusion path at the same heat treatment temperature [19,20].
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Figure 3 Cross-sectional microstructures and elements distribution of TC4 alloy electroplated Cu/Ni composite coating under different heat treatment temperatures: (a) unheated, (b) 600°C, (c)
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respectively.
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700°C and (d) 800°C. A, B and C represented Cu-rich layer, Ni-rich layer and Ti-rich layer,
Table 2 EDS analysis results for the samples after heat treatment Heat treatment temperature/°C
Unheated
600
Elements content /wt.% Region Cu
Ni
Ti
A
99.83
0.16
0.01
B
1.12
98.12
0.76
C
0.01
1.11
98.88
A
94.15
5.66
0.19
B
2.41
94.35
3.24
7
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800
0.59
4.27
95.14
A
84.47
15.14
0.39
B
3.03
63.89
33.08
C
1.67
16.97
81.36
A
68.19
20.89
10.92
B
5.89
C
3.50
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700
C
50.77
43.34
28.57
67.93
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Note: Each datum in the table is an average value of five tested data in each region.
To study the diffusion behaviors of Ti, Ni and Cu elements on an atomic scale,
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the diffusion models for Cu/Ni and Ni/Ti system were established based on the general solution of Fick’s law, respectively, and experimental model system is given by following equation [21]:
x C ( x, t ) = B + Aerf 2 Dt
(2)
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By considering the boundary conditions ( t = 0 : x > 0 , C Ni = 1 ; x < 0 , C Ni = 0 ; t ≥ 0 : x = ∞ , C Ni = 1 ; x = −∞ , C Ni = 0 ) the analytical solution is described as
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follows:
x C ( x, t ) = 0.5erf 2 Dt
(3)
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Using the following formula, the distribution of Ni elements was achieved by
point fitting as shown in Figure 4, the fitting parameters a and b could be obtained.
C( x, t ) = 0.5(1 - erf (ax - b ))
(4)
Where the a and b could be expressed as follows:
(
a = 1 2 Dt
)
(
b = x0 2 Dt
(5)
)
(6)
The diffusion coefficients were calculated through the following equation:
(
D = 1 4a 2t 8
)
(7)
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Figure 4 The Ni element contents penetrated across the interface and then achieved point fitting at different heat treatment temperatures (a) 600°C and (b) 800°C for Cu/Ni interface, (c) 600°C and
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(d) 800°C for Ni/Ti interface.
Ni The diffusion coefficients of Cu/Ni ( DCu ) and Ni/Ti ( DTiNi ) were calculated under
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Ni different heat treatment temperatures, respectively, as shown in Table 3. The DCu
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and DTiNi increased with increase of heat treatment temperature in the range from 600°C to 800°C, respectively. The relationship between diffusion coefficient and temperature as well as the diffusion activation energy can be expressed by following equation [22]:
Q D = D0 exp RT
(8)
Where D is diffusion coefficient, D0 is pre-exponential factor, and Q is diffusion activation energy. The logarithmic formula is shown as the following.
ln D = A - B T 9
(9)
ACCEPTED MANUSCRIPT Where the A and B can be express as follows: A = ln D0
(10)
B=Q R
(11)
The pre-exponential factor D0 and the diffusion activation energy Q were
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obtained from the fit of a straight line to the data in Arrhenius plot and compared with the literature values listed in Table 4. In previous researches, Yan et al. [23] investigated the diffusion upon annealing Cu/Ni multilayers structures by ion
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sputtering from 325°C to 375°C for 30min and obtained the diffusion activation energy Q (101.40kJ/mol) of Cu/Ni multilayers. Abdul-Lettif [24] studied the diffusion
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behavior of vacuum-deposited Cu/Ni bilayer thin films and calculated the diffusion activation energy Q (96.40kJ/mol) for nickel in copper at constant temperatures (300-500°C) for an annealing time between 5min and 190min. The above results indicated that the diffusion process for Cu/Ni at low temperature was controlled by grain boundary diffusion, where the grain boundaries and a high defect density played
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a dominant role in the diffusion process. It is known that different coating processing technologies could affect the microstructure and result in different diffusion activation energies. For example, the use of ion sputtering could involve changes in both the creation of vacancies and energy of removed vacancies in the grain boundary which
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act as paths of easy diffusion. In the present work, less grain boundaries and vacancies in the Cu/Ni coating was obtained by electroplating and heat treatment compared with
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that by ion sputtering, which is the main reason for the high diffusion activation energy Q (111.95kJ/mol).
Ni
Ni
Table 3 The calculated values of DCu and DTi Heat treat temperature/°C
600
700
800
Ni DCu (cm2/s)
3.68×10-16
1.95×10-15
7.09×10-15
DTiNi (cm2/s)
1.53×10-14
3.03×10-14
6.08×10-14
Ni
Ni
Note: The DCu and DTi
represent the diffusion coefficients of nickel diffused to copper 10
ACCEPTED MANUSCRIPT coating and nickel diffused to the titanium matrix, respectively.
As for the diffusion activation energy of Ti/Ni interface, Panigrahi et al. [25] calculated the activation energy Q (46.20kJ/mol) of Ti-50Ni alloy by powder
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metallurgy in the higher temperature range (above 780°C), which is lower than that in this work (52.03kJ/mol). This is due to the fact that surface diffusion is considered to be controlled by diffusion contact area, compared with Ni electroplating on the Ti alloy in this work, larger contact area and less insufficient binding between particles
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were obtained by hot-pressing powder sintering processing, which supplied more paths for diffusion of Ni/Ti and required less energy to diffusion. In addition, Zhou et
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al. [26] fabricated Ti-Ni couples by upset butt welding with a current of 10A and obtained the diffusion activation energy Q (42.11kJ/mol) of the whole interfacial layer in Ti-Ni couple by isothermal diffusion treatment in the temperature range from 500°C to 700°C. For a Ti-Ni couple, the current, flowing in both directions, could increase the concentration and the mobility of defects, which plays a main role in
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promoting the behavior of the Ni and Ti atoms.
The above results of diffusion activation energy of Ni atoms in Cu coating and Ti matrix were lower than that in this work, which is due to the difference in larger
technologies.
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contact area and high defect density caused by different coating processing
Based on above calculated results, the diffusion coefficients of the Cu/Ni and
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Ni/Ti interface after heat treatment in the temperature range from 600°C to 800°C is shown as the following formulas, respectively:
DCuNi = 1.99 × 10− 9 exp(−
111953 )
（12）
)
（13）
RT DTiNi = 2.00 × 10−11 exp(−
52031
RT Table 4 The pre-exponential factor (D0) and diffusion activation energy (Q) of nickel in copper and nickel in TC4 alloy obtained in the present work in comparison with the results of other works
11
Source
D0(cm2/s)
Q(kJ/mol)
Present work
1.99×10-9
111.95
Ref.[23]
6.20×10-9
101.40
Ref.[24]
2.00×10-7
96.40
Present work
2.00×10-11
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(at Cu/Ni interface)
52.03
(at Ni/Ti interface) /
Ref.[26]
/
49.83 42.11
SC
Ref.[25]
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3.2 The phase transitions during heat treatment
The diffusion of Cu/Ni/Ti during heat treatment could form new phase in the range from 600°C to 800°C [27]. In order to reveal the phase transitions during heat treatment, DSC test was performed (Figure 5a) and the results showed that a broad exothermic peak positioned at 700°C for the Cu/Ni/Ti compact. The broad exothermic
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peak for the Cu/Ni/Ti compact corresponds to the formation of IMCs via solid-state exothermic reactions in the middle of elemental Cu, Ni and Ti [28], the CuTi, TiNi and Ti2Ni were obtained after heat treatment at 700°C, as shown in Figure 5(b). This
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result was also confirmed by Simoes [27], where several phases formed in the TC4 alloy and Ni/Ti multilayer interface after heat treated at 750°C, 800°C and 900°C.
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Besides that, Ti3Ni4 was detected at higher heat treatment (800°C) (as summarized in Table 5).
Figure 5 DSC curves (a) of Ti/Ni/Cu compacts with a heating rate of 10°C/min. Surface XRD 12
ACCEPTED MANUSCRIPT images (b) of the electroplated samples heat treated at different temperatures for 3h.
Table 5 The phases formed at different heat treatment temperatures Phases
600
/
700
TiNi, Ti2Ni, CuTi
800
TiNi, Ti2Ni, CuTi, Ti3Ni4
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Heat treatment temperature/°C
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In addition, no CuxNiy IMCs was found at the Cu/Ni interface for the reason that a complete solid solution was formed in the temperature range from 600°C to 800°C
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[29]. For the Ni/Ti interface, the diffusion coefficient of Ni/Ti at 700°C was greater than that of at 600°C (Table 3). The TixNiy IMCs were not detected at the Ni/Ti interface after heat treated at 600°C, which was due to the lower diffusion coefficient of Ni/Ti. With the increase of temperature, the elements diffusion of Ni and Ti increased to form TiNi and Ti2Ni phases at the Ni/Ti interface after heat treated at
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700°C. Based on the Ti-Ni binary system, the TixNiy IMCs mainly consist of TiNi3, TiNi and Ti2Ni from the Ni side to the Ti side and the free energy of TixNiy IMCs can be express as following [30]:
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G (Ti2 Ni ) = −49120 + 17.208T ( J / mol )
(14)
G (TiNi3 ) = −55585 + 15.962T ( J / mol )
(15)
G (TiNi ) = −54600 + 18.133T ( J / mol )
(16)
According to Eqs. from (14) to (16), the free energy for TixNiy IMCs at different
heat treatment temperatures were calculated, as shown in Table 6. The TiNi3 phase can be easy to nucleate and grow at the heat treatment temperatures from 600°C to 800°C. Based on the formation energy in this temperature range, TiNi3 phase was primarily formed at Ni/Ti interface, and Ti2Ni was afterward formed. However, the TiNi3 phase was not found at the Ni/Ti interface in the experiments, this may due to the occurrence of eutectoid from Ni3Ti to NiTi phases [31]. In addition, the varieties 13
ACCEPTED MANUSCRIPT of the Ni contents and the formation of Ti2Ni and TiNi phases induced the formation of metastable Ti3Ni4 phases at 800°C. This result was consistent with the research on the stable and metastable IMCs of TiNi alloys by first principles [32], which indicated
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that the Ti3Ni4 phases formed in a short holding time.
Table 6 Calculated free energy of formation of TixNiy IMCs at different heat treatment temperatures (kJ/mol) 600°C
700°C
Ti2Ni
−34.09
−32.37
TiNi
−38.77
−36.95
−35.14
TiNi3
−41.65
−40.05
−38.46
800°C
−30.65
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Phase
Owing to the presence of Ni interlayer, the CuxTiy IMCs were not found after heat treatment at 600°C. But the CuTi phase was detected after heat treatment at 700°C, which implies that the Ni layer delays the elements diffusion between Ti and
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Cu (Figure 3c). The phase stability of CuxTiy IMCs during heat treatment has significant effect on the mechanical properties of coating [33]. To estimate the stability of CuxTiy IMCs, the formation enthalpy (∆H) of each CuxTiy IMCs can be calculated by [34]:
(
)
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bulk ∆H = [ Etotal - xECu + yETibulk ] ( x + y )
(17)
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Where Etotal is the total cell energy of a CuxTiy primitive cell including x Cu bulk atoms and y Ti atoms, ECu and ETibulk are the energy of a Cu atom and a Ti atom in
the bulk state, respectively. Limited Ti atoms diffused into the Cu coating for the existence of Ni layer. In addition, based on the calculated results, the CuTi phase was the most stable phase due to the minimal formation enthalpy. As a result, only CuTi phases existed in this study above 700°C. No ternary intermediate phase formed in the diffusion processes, which may due to low heat treated temperature and short holding time [10,35].
14
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3.3. Effect of diffusion behavior on mechanical properties Previous research [36] on the interface of titanium and different metals showed that the presence of IMCs at the interfacial regions strongly affected the adhesion strength between the substrate and coating. To study the effect of diffusion behavior on
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mechanical properties, the relationships between adhesion strength and heat treatment temperature is shown in Figure 6. It was found that the diffusion could improve the quality and adhesion property of the coating, which was due to the formation of metallurgical bonding at the interface. The adhesion strength of heat treated samples
SC
was higher than that of the unheated one, and the highest adhesion strength of the sample (about 120MPa) was obtained after heat treated at 700°C. Similar results on
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improvement of mechanical properties of Ni/Ti [9] and Cu/Ti [16] coatings revealed that the adhesion strength was mainly controlled by IMCs. However, the drop of adhesion strength at 800°C in this work was considered to be the excessive growth of
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the TiNi and Ti2Ni IMCs at the Ni/Ti interface [35].
Figure 6 The adhesion score and adhesion strength of the electroplated samples treated at different heat treatment temperatures. The results were averaged over five tension tests.
Based on the results of hardness tests, the hardness curve can be divided into three regions as shown in Figure 7. It seems that the first area of hardness curve is Cu coating, while the third area is Ti substrate. The hardness measurements were mainly made in the second area, which also marks the properties of new phases. Furthermore, 15
ACCEPTED MANUSCRIPT the Cu side of the second area exhibits lower hardness (from 100HV to 200HV), while the Ti side exhibits higher hardness values (from 400HV to 700HV). The hardness measured from the Cu side of the heat treated samples start to increase at the interface. The hardness values decrease again in the Ti area after the second area is
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crossed. It is also seen that the hardness values obtained from the second area are higher when compared to the other areas. The above results confirmed that the presence of IMCs in the coating could enhance the hardness values, which are consistent with the previous researches on surface strengthen effects of the TC4 alloy
SC
[35-38]. In addition, the width of the second area increases with the increase of heat treatment temperature, which is probably due to a new phase formation induced by
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the element diffusion at the interface. The highest hardness value is seen for the specimens at the heat treatment temperature of 700°C. This is due to the solution strengthening and the increase of IMCs with the increase of heat treatment temperature [31]. However, with a further rise in heat treatment temperature to 800°C, the hardness of coating decreases. This may due to larger grain size at high
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EP
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temperature.
Figure 7 The hardness variation along the depth was obtained on the cross-sectional samples (a) and the dependence of the surface hardness on the depth from the surface for the heat treated specimens at different temperatures (b).
4. Conclusions The diffusion behavior and mechanical properties of Cu/Ni coating on TC4 alloy were investigated under different heat treatment temperatures, the following conclusions 16
ACCEPTED MANUSCRIPT can be derived from the achieved results: The diffusion behaviors of the Ti, Ni and Cu elements occurred and the diffusion coefficient increased with the increase of heat treatment temperature. It was found that the diffusion behavior was significantly influenced by heat treatment temperature.
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The diffusion coefficient increased with the increase of heat treatment temperature. The elements diffusions of Cu/Ni/Ti during heat treatment caused the formation of new phases. The IMCs including TiNi, Ti2Ni and Ti3Ni4 were observed at the Ni/Ti interface, which was attributed to the change of the Ni contents and the possibility of
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nucleation. Besides that, the Ni interlayer could not restrict the diffusion of Ti elements to the Cu side and CuTi phase formed for temperatures up to 700°C. The
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maximum adhesion strength (about 120MPa) between the substrate and coating was obtained after heat treatment at 700°C. However, the adhesion strength dropped with an increase in the temperature to 800°C, which was due to the excessive growth of TiNi and Ti2Ni at the Ni/Ti interface. Hardness analysis confirmed that the presence of IMCs in the coating could enhance the hardness values and the highest hardness
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value is obtained at the heat treatment temperature of 700°C.
Acknowledgments
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Y. Chen, S. Liu，Q. Liu and L. Zhu planned and conducted all experiments. Y. Chen, S. Liu, Y. Zhao and Y. Zhang examined the experimental results, performed data
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analyses and wrote the article draft together. X. Song and J. Hao were involved in the discussion and writing.
Formatting of funding sources The authors acknowledge financial supported by National Natural Science Foundation of China (grant no. 51471136, and 51401032), International Cooperation Foundation of Shaanxi Province (2016KW-051), China Postdoctoral Science Foundation (2016M590909), and the Special Fund for Basic Scientific Research of Central Colleges (310831163401). 17
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Conflicts of Interest The authors declare no conflict of interest.
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under high-cycle fatigue, Rare. Metal. Mat. Eng. 44(5) (2015) 1185-1190. [5] Y.G. Zhou, W.D. Zeng, H.Q. Yu, An investigation of a new near-beta forging process for titanium alloys and its application in aviation components, Mater. Sci. Eng. A. 393(1-2)
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[15] H.S. Lee, J.H. Yoon, Y.M. Yi, Fabrication of titanium parts by massive diffusion bonding, J. Mater. Process. Tech. 201(1) (2008) 280-284.
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with Cu on the surface of Mg-Li alloy, Surf. Coat. Tech. 225(225) (2013) 119-125. [18] ISO 4624, Paints and varnishes - pull-off test for adhesion, International Organisation for Standardisation. Geneva, Switzerland, (2002).
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ACCEPTED MANUSCRIPT 364(3) (2015) 434-40. [24] A.M. Abdul-Lettif, Investigation of interdiffusion in copper-nickel bilayer thin films, Physica. B. 388(388) (2007) 107-111. [25] B.B. Panigrahi, M.M. Godkhindi, Dilatometric sintering study of Ti-50Ni elemental powders,
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Intermetallics. 14(2) (2006) 130-135. [26] Y. Zhou, Q. Wang, D.L Sun, X.L. Han, Co-effect of heat and direct current on growth of intermetallic layers at the interface of Ti-Ni diffusion couples, J. Alloy. Compd. 509(4) (2011) 1201-1205.
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[27] S. Simões, F. Viana, A.S. Ramos, M.T. Vieira, M.F. Vieira, Reaction zone formed during diffusion bonding of TiNi to Ti6Al4V using Ni/Ti nanolayers, J. Mater. Sci. 48(21) (2013)
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[30] P. He, D. Liu, Mechanism of forming interfacial intermetallic compounds at interface for solid state diffusion bonding of dissimilar materials, Mater. Sci. Eng. A. 437(2) (2006) 430-435.
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[31] P. Novák, P. Pokorný, V. Vojtěch, A. Knaislová, A. Školáková, J. Čapek, M. Karlíkb, J. Kopečekc, Formation of Ni-Ti intermetallics during reactive sintering at 500-650°C, Mater.
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Chem. Phys. 155 (2015) 113-121. [32] P. Lang, E. Povoden-Karadeniz, C.D. Cirstea, E. Kozeschnik, T. Wojcik, Crystal structure and free energy of Ti2Ni3 precipitates in Ti-Ni alloys from first principles, Comp. Mater. Sci.
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[38] H. Zhao, J. Cao, J.C. Feng, Microstructure and mechanical properties of Ti6Al4V/Cu-10Sn
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bronze diffusion-bonded joint, T. Nonferr. Metal. Soc. 19(2) (2009) 414-417.
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Figure Captions Figure 1 The surface morphology (a) and the corresponding cross-sectional microstructure (b) of TC4 alloy electroplated Cu/Ni. A bright, flat and compact Cu/Ni coating was formed on TC4
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alloys. The Cu coating grows as cell-shaped structure and the microstructure was compact.
Figure 2 The schematic diagrams for adhesion tests. At least five samples for each condition were
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tested to obtain an average value.
Figure 3 Cross-sectional microstructures and elements distribution of TC4 alloy electroplated
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Cu/Ni composite coating under different heat treatment temperatures: (a) unheated, (b) 600°C, (c) 700°C and (d) 800°C. A, B and C represented Cu-rich layer, Ni-rich layer and Ti-rich layer, respectively.
Figure 4 The Ni element contents penetrated across the interface and then achieved point fitting at
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different heat treatment temperatures (a) 600°C and (b) 800°C for Cu/Ni interface, (c) 600°C and (d) 800°C for Ni/Ti interface.
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Figure 5 DSC curves (a) of Ti/Ni/Cu compacts with a heating rate of 10°C/min. Surface XRD
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images (b) of the electroplated samples heat treated at different temperatures for 3h.
Figure 6 The adhesion score and adhesion strength of the electroplated samples treated at different heat treatment temperatures. The results were averaged over five tension tests.
Figure 7 The hardness variation along the depth was obtained on the cross-sectional samples (a) and the dependence of the surface hardness on the depth from the surface for the heat treated specimens at different temperatures (b).
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Tables with captions Table 1 Chemical composition of TC4 alloy (wt.%) Al
V
Fe
Mn
Si
Zn
Ti
Nominal
5.0-6.5
3.3-4.5
0.3-0.9
0.5
0.4
0.3
Bal
Table 2 EDS analysis results for the samples after heat treatment
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Elements
Elements content /wt.% Heat treatment temperature/°C
Region Cu
700
EP
800
0.16
0.01
B
1.12
98.12
0.76
C
0.01
1.11
98.88
A
94.15
5.66
0.19
B
2.41
94.35
3.24
C
0.59
4.27
95.14
A
84.47
15.14
0.39
B
3.03
63.89
33.08
C
1.67
16.97
81.36
A
68.19
20.89
10.92
B
5.89
50.77
43.34
C
3.50
28.57
67.93
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99.83
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600
Ti
A
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Unheated
Ni
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Note: Each datum in the table is an average value of five tested data in each region.
Ni
Ni
Table 3 The calculated values of DCu and DTi Heat treat temperature/°C
600
700
800
Ni DCu (cm2/s)
3.68×10-16
1.95×10-15
7.09×10-15
DTiNi (cm2/s)
1.53×10-14
3.03×10-14
6.08×10-14
Ni
Ni
Note: The DCu and DTi
represent the diffusion coefficients of nickel diffused to copper
coating and nickel diffused to the titanium matrix, respectively.
ACCEPTED MANUSCRIPT Table 4 The pre-exponential factor (D0) and diffusion activation energy (Q) of nickel in copper
Source
D0(cm2/s)
Q(kJ/mol)
Present work
1.99×10-9
111.95
Ref.[23]
6.20×10-9
Ref.[24]
2.00×10-7
Present work
2.00×10-11
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and nickel in TC4 alloy obtained in the present work in comparison with the results of other works
(at Cu/Ni interface) 101.40 96.40
52.03
/
Ref.[26]
/
49.83 42.11
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Ref.[25]
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(at Ni/Ti interface)
Table 5 The phases formed at different heat treatment temperatures Heat treatment temperature/°C 600
800
/
TiNi, Ti2Ni, CuTi
TE D
700
Phases
TiNi, Ti2Ni, CuTi, Ti3Ni4
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Table 6 Calculated free energy of formation of TixNiy IMCs at different heat treatment temperatures (kJ/mol)
600°C
700°C
800°C
Ti2Ni
−34.09
−32.37
−30.65
TiNi
−38.77
−36.95
−35.14
TiNi3
−41.65
−40.05
−38.46
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Phase
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Highlights
►The diffusion model of the Ni/Ti and Cu/Ni interface was established. ►The formation mechanism of IMCs relies on elements diffusion behavior.
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►The IMCs have a significantly effect on their mechanical properties.