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Interface characteristics in diffusion bonding of Fe3Al with Cr18-Ni8 stainless steel
Journal of Colloid and Interface Science 285 (2005) 201–205 www.elsevier.com/locate/jcis
Interface characteristics in diffusion bonding of Fe3Al with Cr18-Ni8 stainless steel Juan Wang a,∗ , Yajiang Li a,b , Yansheng Yin a a Key Lab of Liquid Structure and Heredity of Materials, Ministry of Education, School of Materials Science and Engineering, Shandong University,
Jinan 250061, People’s Republic of China b National Key Lab of Advanced Welding Production Technology, Harbin Institute of Technology, Harbin 150001, People’s Republic of China
Received 16 September 2004; accepted 27 October 2004 Available online 8 January 2005
Abstract Fe3 Al and Cr18-Ni8 stainless steel were diffusion-bonded in vacuum and a Fe3 Al/Cr18-Ni8 interface with reaction layer was formed. Microstructure in the reaction layer at Fe3 Al/Cr18-Ni8 interface was analyzed by means of scanning electron microscope (SEM) and electron probe micro-analyzer (EPMA). The growth of reaction layer with heating temperature (T ) and holding time (t) was researched. The results indicate that FeAl, Fe3 Al, Ni3 Al, and α-Fe (Al) solid solution are formed in the reaction layer. These phases are favorable to promote the element diffusion and to accelerate the formation of the reaction layer at Fe3 Al/Cr18-Ni8 interface. The growth of reaction layer obeys the parabolic law and its thickness (X) is expressed by X 2 = 7.5 × 10−4 exp(−83.59/RT )(t − t0 ). 2004 Elsevier Inc. All rights reserved. Keywords: Fe3 Al/Cr18-Ni8 interface; Diffusion bonding; Reaction layer; Characteristics
1. Introduction Fe3 Al intermetallics have high hardness and excellent resistance to abrasion, oxidization, and corrosion because of its special DO3 ordered superlattice structure [1–3]. It is expected that Fe3 Al intermetallics can be applied to petrochemical industry, pressure vessel, electric power, and so on. In recent years, the plasticity and toughness of Fe3 Al intermetallics have been promoted greatly and the percentage elongation at room temperature has been up to 8–10% by controlling alloy composition and improving heat machining technology [4,5]. If the joining of Fe3 Al intermetallics and commonly used Cr18-Ni8 austenitic stainless steel is realized and a composite structure with above materials is manufactured successfully, advantages in economy and properties will be attained fully and mutually.
* Corresponding author.
E-mail address: [email protected] (J. Wang). 0021-9797/$ – see front matter 2004 Elsevier Inc. All rights reserved. doi:10.1016/j.jcis.2004.10.071
The vacuum diffusion bonding technique has progressed increasingly with the development of computer and vacuum techniques, and it is used in the joining of brittle materials and dissimilar materials [6,7]. Fe3 Al intermetallics and Cr18-Ni8 austenitic stainless steel are bonded by diffusionbonding technology, and cracks due to welding could be avoided. This will be the key to promoting the application of Fe3 Al intermetallics in aviation, petrochemistry, and electric power. During diffusion bonding, an interface with a reaction layer can be formed between substrates. The reacted phase in the interface and the thickness of the reaction layer are the key to determine the performance in the interface [8,9]. In this paper, Fe3 Al and Cr18-Ni8 stainless steel were diffusion-bonded in vacuum. Microstructure and concentration in the reaction layer at Fe3 Al/Cr18-Ni8 interface were analyzed by means of a scanning electron microscope (SEM) and an electron probe micro-analyzer (EPMA). The relation between the thickness of reaction layer and technological parameters during bonding and the growth of reaction layer were studied. The results will provide experi-
202
J. Wang et al. / Journal of Colloid and Interface Science 285 (2005) 201–205
Table 1 Chemical compositions and thermophysical properties of Fe3 Al intermetallics and Cr18-Ni8 steel Chemical compositions (wt%) Base metals Al
Cr
Nb
Ni
Zr
B
Ce
S
P
Fe
– 9.43
0.1 –
0.04 –
0.15 –
– 0.03
– 0.03
Bal. Bal.
Fe3 Al 16.5 2.5 1.0 Cr18-Ni8 – 18.21 – Thermophysical properties Base Structure Order critical Young’s metals temperature (K) modulus (GPa)
Melting point (K)
Coefficient of heat expansion (10−6 K−1 )
Density (g cm−3 )
Tensile strength (MPa)
Elongation (%)a
Hardness
Fe3 Al Cr18-Ni8
1813 –
11.5 16.7
6.72 8.03
455 520
3 40
30 HRC 70 HRB
DO3 –
813 –
140 –
a Elongation refers to the ratio of the increase in length (subtracting original length from the total length of broken specimen) to the original length.
joint for microstructure observation and the research on growth behavior. 2.2. Characterization
Fig. 1. Assemble and position of samples in the vacuum chamber.
mental and theoretical basis on joining of Fe3 Al with other materials.
The interface samples were ground by a series of types of sandpaper and then polished and finally etched with a solution consisting of 70% HCl and 30% HNO3 . The microstructure in the reaction layer at the Fe3 Al/Cr18-Ni8 interface was observed by means of JXA-840 scanning electron microscopy (SEM). Element concentration from Fe3 Al intermetallics to Cr18-Ni8 steel across the interface was measured with JXA-8800R electron probe micro-analyzer (EPMA). The growth of reaction layer was researched by parabolic equation.
2. Experimental
3. Results and discussion
2.1. Materials and interface preparation
3.1. Microstructure in the interface
Materials used in the test are Fe3 Al intermetallics and Cr18-Ni8 austenitic stainless steel. Fe3 Al intermetallics was melted by the vacuum induction furnace and then fabricated into plate used for bonding with Cr18-Ni8 steel. The chemical compositions and thermophysical properties of Fe3 Al intermetallics and Cr18-Ni8 steel are listed in Table 1. Fe3 Al intermetallics sample was machined into the dimension of 100 × 30 × 20 mm3 and Cr18-Ni8 steel sample was 100 × 30 × 10 mm3 . The oxide film on the sample surface should be eliminated by a series of treatments including grounding by sand paper, acetone dipping, alcohol cleaning, then water flushing and dry. The samples were put into the vacuum chamber and the position in the vacuum chamber is shown in Fig. 1. The diffusion bonding was immediately carried out in a 4.5 × 10−4 Pa vacuum at heating temperatures (T ) ranging from 1000 to 1080 ◦ C for a holding time (t) from 15 to 80 min. The heating rate is 12 ◦ C/min in each run. The chamber temperature wan cooled 100 ◦ C by circulating water, followed by furnace cooling. After the diffusion bonding, the interface sample was cut from a diffusion-bonded
Atoms from substrates diffuse continuously toward the Fe3 Al/Cr18-Ni8 interface during bonding under the action of heating and concentration grads. When elements concentration is to certain extent, the elements will react mutually to produce new phase forming reaction layer, in which the structure is different from that in substrates. Fig. 2 shows features of the reaction layer at Fe3 Al/Cr18-Ni8 interface. With the enhancing of heating temperature and holding time, the thickness of reaction layer increases and the microstructure in the layer is becoming coarse. In order to analyze the reacted phase, element concentration from Fe3 Al intermetallics to Cr18-Ni8 steel across the Fe3 Al/Cr18-Ni8 interface was measured by means of EPMA. The measured location and results are shown in Fig. 3. Al concentration increases abruptly due to the effect of Cr and Ni in the reaction layer near the side of Fe3 Al. Al concentrates in the small region of Fe3 Al side and its content is more than that of Fe3 Al. Thus, FeAl is produced according to iron–aluminum phase diagram. From the reaction layer to Cr18-Ni8 steel, Al, Fe concentrations decrease and Ni, Cr concentrations increase gradually to form Ni3 Al and α-Fe (Al) solid solution. Ni3 Al is the reacted phase of Ni from
J. Wang et al. / Journal of Colloid and Interface Science 285 (2005) 201–205
203
Fig. 2. Feature of the reaction layer at the Fe3 Al/Cr18-Ni8 interface: (a) 1020 ◦ C × 60 min, (b) 1040 ◦ C × 60 min.
Cr18-Ni8 steel and Al from Fe3 Al. The diffusion coefficient of Al is small, so that Al concentration in the region of Cr18Ni8 steel side is quiet low. Finally, α-Fe (Al) solid solution can be formed after cooling. Fig. 3. EPMA for the reaction layer at Fe3 Al/Cr18-Ni8 interface: (a) measured location (1060 ◦ C × 60 min), (b) element concentration.
3.2. Growth of diffusion reaction layer Atom diffusion in the interface is a nonstabilized dynamic process during diffusion bonding [10,11]. The diffusion distance of Al, Fe, Ni, and Cr in Fe3 Al/Cr18-Ni8 interface with different heating temperature and holding time was measured with EPMA. Fig. 4 shows the relation between diffusion distance and square root of holding time at temperature ranging from 1000 to 1060 ◦ C. The diffusion distance and square root of holding time at Fe3 Al/Cr18-Ni8 interface are approximately linear relationship. The good linear relationship indicated the growth of reaction layer obeys the parabolic law x 2 = Kp (t − t0 ),
(1)
in which x is diffusion distance (µm), Kp is parabolic constant (µm2 /s), t is holding time (s), t0 is latent period (s). Parabolic constant Kp increases from 1000 to 1060 ◦ C. It is well known that parabolic constant Kp is related to the element diffusion coefficient at certain temperature. Diffusion distance is also expressed by the diffusion coefficient D according to the empirical formulas put forward by Arrhenius [12], 2 CD(t − t0 ), (2) Ci in which x is diffusion distance (µm), C is concentration difference on the sides of interface (%), Ci is element
x2 =
Table 2 Diffusion coefficients of Al, Fe, Ni, and Cr in the reaction layer at Fe3 Al/ Cr18-Ni8 interface Temperature (◦ C)
1000
1020
1040
1060
0.49 0.5 0.49 0.40
0.8 0.7 0.5 0.47
1.6 1.2 0.8 0.49
2.1 1.5 1.4 0.7
−0.71 −0.69 −0.71 −0.92
−0.22 −0.36 −0.69 −0.76
0.47 0.18 −0.22 −0.71
D (µm2 /s) Al Fe Ni Cr ln D (µm2 /s) Al Fe Ni Cr
0.74 0.41 0.34 −0.36
concentration (%), D is diffusion coefficient (µm2 /s), t is holding time (s), and t0 is latent period (s). On the basis of Eqs. (1) and (2), diffusion coefficients of Al, Fe, Ni, and Cr in the reaction layer at Fe3 Al/Cr18-Ni8 interface are shown in Table 2. The relation between diffusion coefficient and heating temperature is shown in Fig. 5. According to the slope and intercept of linear relationship between diffusion coefficient (natural logarithm value) and heating temperature (reciprocal value), the diffusion ac-
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J. Wang et al. / Journal of Colloid and Interface Science 285 (2005) 201–205
Fig. 4. Relation between diffusion distance and holding time in the reaction layer at Fe3 Al/Cr18-Ni8 interface: (a) Al, (b) Fe, (c) Ni, and (d) Cr.
the difference is least when the thickness of reaction layer is represented by diffusion distance of Cr. Thus based on the diffusion factor and activation energy of Cr, the thickness of reaction layer X is calculated to be 83.59 2 −4 (t − t0 ). X = 7.5 × 10 exp − (3) RT
Fig. 5. Relation between diffusion coefficient (ln D) and temperature (1/T ) in the reaction layer.
tivation energy (Q) and diffusion factor (D0 ) of Al, Fe, Ni, Cr are calculated and the result is shown in Table 3. It can be seen in Table 3 that diffusion activation energy at the Fe3 Al/Cr18-Ni8 interface is less than that in substrates including Fe3 Al intermetallics and Cr18-Ni8 steel. This indicates that microstructure in the reaction layer at Fe3 Al/Cr18-Ni8 interface is more favorable to the element diffusion and the growth of the reaction layer. Since diffusion distance of every element at the interface is different due to its distinct diffusion factor and activation energy, the thicknesses of the reaction layer are represented by the diffusion distances of Al, Fe, Ni, and Cr, respectively. By comparisons of the calculated and EPMAmeasured thickness of reaction layer, the result indicates that
The thickness of the reaction layer X increases gradually with the enhancement of heating temperature T and holding time t. According to formula (3), the calculated thickness of reaction layer is compared with the EPMA measured value in Fig. 6. The result shows that the calculated thickness is little larger than the measured value and the difference between them is only less than 5% when the holding time is not longer than 60 min. Bonding parameters can be determined by the growth rule of the thickness of reaction layer with heating temperature and holding time. In addition, there exists a latent period for the formation of reaction layer. When the thickness of reaction layer is certain, the latent period becomes short with the increasing of heating temperature. Therefore, holding time may be decreased properly to raise bonding efficiency when heating temperature increases.
4. Summary FeAl, Fe3 Al, Ni3 Al, and α-Fe (Al) solid solution were formed in the reaction layer at the Fe3 Al/Cr18-Ni8 interface by controlling diffusion-bonding technological parameters.
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Table 3 Diffusion activation energy and factor of elements at Fe3 Al/Cr18-Ni8 interface Parameters D0 (106 µm2 /s) Q (kJ/mol)
Fe3 Al intermetallicsa
Cr18-Ni8 steelb
Fe3 Al/Cr18-Ni8 interface
Al
Fe
Cr
Al
Fe
Ni
Cr
Fe
Ni
Cr
1.7 211.09
4 166.36
20 308.6
26,362 158.72
2650 49.90
25.87 56.35
7.34 83.59
500 163.02
1.8 108.68
20 216.11
a Data of Al, Fe, and Cr in Fe Al intermetallics is from Ref. [13]. 3 b Data of Fe, Cr, and Ni in Cr18-Ni8 steel is from Ref. [14].
(t − t0 ). Thus technological parameters during bonding can be determined by the relation between thickness of reaction layer and heating temperature and holding time.
Acknowledgments This project was supported by the National Natural Science Foundation of China (Grant 50375088) and the Shandong Province Natural Science Foundation (Y2003F05). The authors express their heartfelt thanks for this support.
References
Fig. 6. Thickness of reaction layer at Fe3 Al/Cr18-Ni8 interface: (a) effect of heating temperature, (b) effect of holding time.
These phases can decrease diffusion activation energy to promote element diffusion and even to accelerate diffusion reaction layer formation. The formation of diffusion reaction layer requires a latent period t0 and the growth of reaction layer obeys parabolic law. The thickness of reaction layer X is expressed to be X 2 = 7.5 × 10−4 exp(−83.59/RT ) ×
[1] C.G. McKamey, J.H. Devan, P.E. Tortorelli, J. Mater. Res. 6 (1991) 1779. [2] G.H. Fair, J.V. Wood, J. Mater. Sci. 29 (1994) 1935. [3] S. Zuqing, G. Dechun, Y. Wangyue, H. Xiaxu, H. Jihua, Chin. J. Mater. Res. 15 (2001) 69. [4] B.K. Kad, S.E. Schoenfeld, C.G. Mckamey, Acta Mater. 45 (1997) 1333. [5] G. Frommeyer, C. Derder, J.A. Jimenez, Mater. Sci. Technol. 18 (2001) 981. [6] S.A. David, M.L. Santella, Adv. Weld. Metall. 31 (1990) 65. [7] N.F. Kazakov, Diffusion Bonding of Materials, Mir, Moscow, 1990. [8] S.P. Garg, G.B. Kale, R.V. Patil, T. Kundu, Intermetallics 7 (1999) 901. [9] Y.M. Sung, J. Mater. Sci. Lett. 20 (2001) 1433. [10] M.A. Dayananda, Metall. Mater. Trans. A 27 (1996) 2505. [11] S.K. Kailasam, M.E. Glicksman, Acta Mater. 47 (1999) 905. [12] H. Jihua, Diffusion in Metal and Alloys, Metallurgical Industry, Beijing, 1996. [13] Y.H. Sohn, M.A. Dayananda, Metall. Mater. Trans. A 33 (2002) 3375. [14] H. Kangsheng, C. Xiongfu, Welding of Dissimilar Metal, Mechanical Industry, Beijing, 1996.
Interface characteristics in diffusion bonding of Fe3Al with Cr18-Ni8 stainless steel Juan Wang a,∗ , Yajiang Li a,b , Yansheng Yin a a Key Lab of Liquid Structure and Heredity of Materials, Ministry of Education, School of Materials Science and Engineering, Shandong University,
Jinan 250061, People’s Republic of China b National Key Lab of Advanced Welding Production Technology, Harbin Institute of Technology, Harbin 150001, People’s Republic of China
Received 16 September 2004; accepted 27 October 2004 Available online 8 January 2005
Abstract Fe3 Al and Cr18-Ni8 stainless steel were diffusion-bonded in vacuum and a Fe3 Al/Cr18-Ni8 interface with reaction layer was formed. Microstructure in the reaction layer at Fe3 Al/Cr18-Ni8 interface was analyzed by means of scanning electron microscope (SEM) and electron probe micro-analyzer (EPMA). The growth of reaction layer with heating temperature (T ) and holding time (t) was researched. The results indicate that FeAl, Fe3 Al, Ni3 Al, and α-Fe (Al) solid solution are formed in the reaction layer. These phases are favorable to promote the element diffusion and to accelerate the formation of the reaction layer at Fe3 Al/Cr18-Ni8 interface. The growth of reaction layer obeys the parabolic law and its thickness (X) is expressed by X 2 = 7.5 × 10−4 exp(−83.59/RT )(t − t0 ). 2004 Elsevier Inc. All rights reserved. Keywords: Fe3 Al/Cr18-Ni8 interface; Diffusion bonding; Reaction layer; Characteristics
1. Introduction Fe3 Al intermetallics have high hardness and excellent resistance to abrasion, oxidization, and corrosion because of its special DO3 ordered superlattice structure [1–3]. It is expected that Fe3 Al intermetallics can be applied to petrochemical industry, pressure vessel, electric power, and so on. In recent years, the plasticity and toughness of Fe3 Al intermetallics have been promoted greatly and the percentage elongation at room temperature has been up to 8–10% by controlling alloy composition and improving heat machining technology [4,5]. If the joining of Fe3 Al intermetallics and commonly used Cr18-Ni8 austenitic stainless steel is realized and a composite structure with above materials is manufactured successfully, advantages in economy and properties will be attained fully and mutually.
* Corresponding author.
E-mail address: [email protected] (J. Wang). 0021-9797/$ – see front matter 2004 Elsevier Inc. All rights reserved. doi:10.1016/j.jcis.2004.10.071
The vacuum diffusion bonding technique has progressed increasingly with the development of computer and vacuum techniques, and it is used in the joining of brittle materials and dissimilar materials [6,7]. Fe3 Al intermetallics and Cr18-Ni8 austenitic stainless steel are bonded by diffusionbonding technology, and cracks due to welding could be avoided. This will be the key to promoting the application of Fe3 Al intermetallics in aviation, petrochemistry, and electric power. During diffusion bonding, an interface with a reaction layer can be formed between substrates. The reacted phase in the interface and the thickness of the reaction layer are the key to determine the performance in the interface [8,9]. In this paper, Fe3 Al and Cr18-Ni8 stainless steel were diffusion-bonded in vacuum. Microstructure and concentration in the reaction layer at Fe3 Al/Cr18-Ni8 interface were analyzed by means of a scanning electron microscope (SEM) and an electron probe micro-analyzer (EPMA). The relation between the thickness of reaction layer and technological parameters during bonding and the growth of reaction layer were studied. The results will provide experi-
202
J. Wang et al. / Journal of Colloid and Interface Science 285 (2005) 201–205
Table 1 Chemical compositions and thermophysical properties of Fe3 Al intermetallics and Cr18-Ni8 steel Chemical compositions (wt%) Base metals Al
Cr
Nb
Ni
Zr
B
Ce
S
P
Fe
– 9.43
0.1 –
0.04 –
0.15 –
– 0.03
– 0.03
Bal. Bal.
Fe3 Al 16.5 2.5 1.0 Cr18-Ni8 – 18.21 – Thermophysical properties Base Structure Order critical Young’s metals temperature (K) modulus (GPa)
Melting point (K)
Coefficient of heat expansion (10−6 K−1 )
Density (g cm−3 )
Tensile strength (MPa)
Elongation (%)a
Hardness
Fe3 Al Cr18-Ni8
1813 –
11.5 16.7
6.72 8.03
455 520
3 40
30 HRC 70 HRB
DO3 –
813 –
140 –
a Elongation refers to the ratio of the increase in length (subtracting original length from the total length of broken specimen) to the original length.
joint for microstructure observation and the research on growth behavior. 2.2. Characterization
Fig. 1. Assemble and position of samples in the vacuum chamber.
mental and theoretical basis on joining of Fe3 Al with other materials.
The interface samples were ground by a series of types of sandpaper and then polished and finally etched with a solution consisting of 70% HCl and 30% HNO3 . The microstructure in the reaction layer at the Fe3 Al/Cr18-Ni8 interface was observed by means of JXA-840 scanning electron microscopy (SEM). Element concentration from Fe3 Al intermetallics to Cr18-Ni8 steel across the interface was measured with JXA-8800R electron probe micro-analyzer (EPMA). The growth of reaction layer was researched by parabolic equation.
2. Experimental
3. Results and discussion
2.1. Materials and interface preparation
3.1. Microstructure in the interface
Materials used in the test are Fe3 Al intermetallics and Cr18-Ni8 austenitic stainless steel. Fe3 Al intermetallics was melted by the vacuum induction furnace and then fabricated into plate used for bonding with Cr18-Ni8 steel. The chemical compositions and thermophysical properties of Fe3 Al intermetallics and Cr18-Ni8 steel are listed in Table 1. Fe3 Al intermetallics sample was machined into the dimension of 100 × 30 × 20 mm3 and Cr18-Ni8 steel sample was 100 × 30 × 10 mm3 . The oxide film on the sample surface should be eliminated by a series of treatments including grounding by sand paper, acetone dipping, alcohol cleaning, then water flushing and dry. The samples were put into the vacuum chamber and the position in the vacuum chamber is shown in Fig. 1. The diffusion bonding was immediately carried out in a 4.5 × 10−4 Pa vacuum at heating temperatures (T ) ranging from 1000 to 1080 ◦ C for a holding time (t) from 15 to 80 min. The heating rate is 12 ◦ C/min in each run. The chamber temperature wan cooled 100 ◦ C by circulating water, followed by furnace cooling. After the diffusion bonding, the interface sample was cut from a diffusion-bonded
Atoms from substrates diffuse continuously toward the Fe3 Al/Cr18-Ni8 interface during bonding under the action of heating and concentration grads. When elements concentration is to certain extent, the elements will react mutually to produce new phase forming reaction layer, in which the structure is different from that in substrates. Fig. 2 shows features of the reaction layer at Fe3 Al/Cr18-Ni8 interface. With the enhancing of heating temperature and holding time, the thickness of reaction layer increases and the microstructure in the layer is becoming coarse. In order to analyze the reacted phase, element concentration from Fe3 Al intermetallics to Cr18-Ni8 steel across the Fe3 Al/Cr18-Ni8 interface was measured by means of EPMA. The measured location and results are shown in Fig. 3. Al concentration increases abruptly due to the effect of Cr and Ni in the reaction layer near the side of Fe3 Al. Al concentrates in the small region of Fe3 Al side and its content is more than that of Fe3 Al. Thus, FeAl is produced according to iron–aluminum phase diagram. From the reaction layer to Cr18-Ni8 steel, Al, Fe concentrations decrease and Ni, Cr concentrations increase gradually to form Ni3 Al and α-Fe (Al) solid solution. Ni3 Al is the reacted phase of Ni from
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203
Fig. 2. Feature of the reaction layer at the Fe3 Al/Cr18-Ni8 interface: (a) 1020 ◦ C × 60 min, (b) 1040 ◦ C × 60 min.
Cr18-Ni8 steel and Al from Fe3 Al. The diffusion coefficient of Al is small, so that Al concentration in the region of Cr18Ni8 steel side is quiet low. Finally, α-Fe (Al) solid solution can be formed after cooling. Fig. 3. EPMA for the reaction layer at Fe3 Al/Cr18-Ni8 interface: (a) measured location (1060 ◦ C × 60 min), (b) element concentration.
3.2. Growth of diffusion reaction layer Atom diffusion in the interface is a nonstabilized dynamic process during diffusion bonding [10,11]. The diffusion distance of Al, Fe, Ni, and Cr in Fe3 Al/Cr18-Ni8 interface with different heating temperature and holding time was measured with EPMA. Fig. 4 shows the relation between diffusion distance and square root of holding time at temperature ranging from 1000 to 1060 ◦ C. The diffusion distance and square root of holding time at Fe3 Al/Cr18-Ni8 interface are approximately linear relationship. The good linear relationship indicated the growth of reaction layer obeys the parabolic law x 2 = Kp (t − t0 ),
(1)
in which x is diffusion distance (µm), Kp is parabolic constant (µm2 /s), t is holding time (s), t0 is latent period (s). Parabolic constant Kp increases from 1000 to 1060 ◦ C. It is well known that parabolic constant Kp is related to the element diffusion coefficient at certain temperature. Diffusion distance is also expressed by the diffusion coefficient D according to the empirical formulas put forward by Arrhenius [12], 2 CD(t − t0 ), (2) Ci in which x is diffusion distance (µm), C is concentration difference on the sides of interface (%), Ci is element
x2 =
Table 2 Diffusion coefficients of Al, Fe, Ni, and Cr in the reaction layer at Fe3 Al/ Cr18-Ni8 interface Temperature (◦ C)
1000
1020
1040
1060
0.49 0.5 0.49 0.40
0.8 0.7 0.5 0.47
1.6 1.2 0.8 0.49
2.1 1.5 1.4 0.7
−0.71 −0.69 −0.71 −0.92
−0.22 −0.36 −0.69 −0.76
0.47 0.18 −0.22 −0.71
D (µm2 /s) Al Fe Ni Cr ln D (µm2 /s) Al Fe Ni Cr
0.74 0.41 0.34 −0.36
concentration (%), D is diffusion coefficient (µm2 /s), t is holding time (s), and t0 is latent period (s). On the basis of Eqs. (1) and (2), diffusion coefficients of Al, Fe, Ni, and Cr in the reaction layer at Fe3 Al/Cr18-Ni8 interface are shown in Table 2. The relation between diffusion coefficient and heating temperature is shown in Fig. 5. According to the slope and intercept of linear relationship between diffusion coefficient (natural logarithm value) and heating temperature (reciprocal value), the diffusion ac-
204
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Fig. 4. Relation between diffusion distance and holding time in the reaction layer at Fe3 Al/Cr18-Ni8 interface: (a) Al, (b) Fe, (c) Ni, and (d) Cr.
the difference is least when the thickness of reaction layer is represented by diffusion distance of Cr. Thus based on the diffusion factor and activation energy of Cr, the thickness of reaction layer X is calculated to be 83.59 2 −4 (t − t0 ). X = 7.5 × 10 exp − (3) RT
Fig. 5. Relation between diffusion coefficient (ln D) and temperature (1/T ) in the reaction layer.
tivation energy (Q) and diffusion factor (D0 ) of Al, Fe, Ni, Cr are calculated and the result is shown in Table 3. It can be seen in Table 3 that diffusion activation energy at the Fe3 Al/Cr18-Ni8 interface is less than that in substrates including Fe3 Al intermetallics and Cr18-Ni8 steel. This indicates that microstructure in the reaction layer at Fe3 Al/Cr18-Ni8 interface is more favorable to the element diffusion and the growth of the reaction layer. Since diffusion distance of every element at the interface is different due to its distinct diffusion factor and activation energy, the thicknesses of the reaction layer are represented by the diffusion distances of Al, Fe, Ni, and Cr, respectively. By comparisons of the calculated and EPMAmeasured thickness of reaction layer, the result indicates that
The thickness of the reaction layer X increases gradually with the enhancement of heating temperature T and holding time t. According to formula (3), the calculated thickness of reaction layer is compared with the EPMA measured value in Fig. 6. The result shows that the calculated thickness is little larger than the measured value and the difference between them is only less than 5% when the holding time is not longer than 60 min. Bonding parameters can be determined by the growth rule of the thickness of reaction layer with heating temperature and holding time. In addition, there exists a latent period for the formation of reaction layer. When the thickness of reaction layer is certain, the latent period becomes short with the increasing of heating temperature. Therefore, holding time may be decreased properly to raise bonding efficiency when heating temperature increases.
4. Summary FeAl, Fe3 Al, Ni3 Al, and α-Fe (Al) solid solution were formed in the reaction layer at the Fe3 Al/Cr18-Ni8 interface by controlling diffusion-bonding technological parameters.
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Table 3 Diffusion activation energy and factor of elements at Fe3 Al/Cr18-Ni8 interface Parameters D0 (106 µm2 /s) Q (kJ/mol)
Fe3 Al intermetallicsa
Cr18-Ni8 steelb
Fe3 Al/Cr18-Ni8 interface
Al
Fe
Cr
Al
Fe
Ni
Cr
Fe
Ni
Cr
1.7 211.09
4 166.36
20 308.6
26,362 158.72
2650 49.90
25.87 56.35
7.34 83.59
500 163.02
1.8 108.68
20 216.11
a Data of Al, Fe, and Cr in Fe Al intermetallics is from Ref. [13]. 3 b Data of Fe, Cr, and Ni in Cr18-Ni8 steel is from Ref. [14].
(t − t0 ). Thus technological parameters during bonding can be determined by the relation between thickness of reaction layer and heating temperature and holding time.
Acknowledgments This project was supported by the National Natural Science Foundation of China (Grant 50375088) and the Shandong Province Natural Science Foundation (Y2003F05). The authors express their heartfelt thanks for this support.
References
Fig. 6. Thickness of reaction layer at Fe3 Al/Cr18-Ni8 interface: (a) effect of heating temperature, (b) effect of holding time.
These phases can decrease diffusion activation energy to promote element diffusion and even to accelerate diffusion reaction layer formation. The formation of diffusion reaction layer requires a latent period t0 and the growth of reaction layer obeys parabolic law. The thickness of reaction layer X is expressed to be X 2 = 7.5 × 10−4 exp(−83.59/RT ) ×
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