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Microstructure of internally oxidized layer in Ag–Sn–Cu alloy
Corrosion Science 50 (2008) 3508–3518
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Microstructure of internally oxidized layer in Ag–Sn–Cu alloy C.P. Wu, D.Q. Yi *, C.H. Xu, J. Li, B. Wang, F. Zheng School of Materials Science and Engineering, Central South University, Changsha 410083, PR China
a r t i c l e
i n f o
Article history: Received 6 June 2008 Accepted 2 September 2008 Available online 19 September 2008 Keywords: A. Ag–Sn–Cu alloy C. Internal oxidation
a b s t r a c t Ag–7.49 wt%Sn–1.16 wt%Cu alloy was internally oxidized in air at 800 °C. The results showed that the size of dispersed particles and hardness in internally oxidized layer became larger with the distance from the surface of the specimens. The relationship between the depth of internally oxidized layer and oxidation time followed a parabolic law. The fracture stress of particles in internally oxidized layer increased with the particle radius increasing. Cold rolling was effective for internal oxidation because of the increased diffusivity of oxygen in internally oxidized layer. Internal oxidation of the alloy was found to be a process of reaction diffusion. Ó 2008 Elsevier Ltd. All rights reserved.
1. Introduction Ag–SnO2 materials prepared by internally oxidizing Ag–Sn alloy have been widely used in electrical and electronic industries due to their good electrical property and service performance [1–21]. However, the mechanism underlaying internal oxidation is still not clear. So investigation on internal oxidation of Ag–Sn alloy is rather necessary and meaningful for both industry production and academic research. Wang has studied the internal oxidation of Ag–Sn alloys [22]. He did a diffusion experiment on oxygen in the pure silver and Ag–Sn alloy. The experimental results showed that the oxidizable metal encapsulated by pure silver was oxidized absolutely, but that encapsulated by Ag–Sn alloy was not oxidized. So the oxygen can not diffuse into the Ag–Sn sosoloid. Lin and his coauthors studied the effect of Bi on internal oxidation of Ag–Sn alloys [23]. They found that internal oxidation is controlled by oxygen diffusion and Bi can change the oxidation mechanism. Wen and his colleague investigated the effect of the ammonium paratungstate (APT) on oxidation process of Ag–Sn alloy powders [24]. They found that the decomposition product of APT and new phase Ag2WO2 formed in the oxidation act as the sources of active oxygen atom, and enhance the oxidation of tin. Joo et al. studied the effect of tellurium addition on the internal oxidation of Ag–Sn alloys [25]. It may be possible that nucleation rate of (Sn, Te)O2 oxides is much faster than that of pure SnO2 oxides and tellurium oxides nucleate first but reacts with incoming Sn atoms to form (Sn, Te)O2 oxides. The nucleation of tellurium oxide is the fastest of all and as a result, addition of trace amount of Te can accelerate the oxidation rate of Ag–Sn alloys. Deng and his group investigated the effect of some metal elements (Cu, Bi, Sb and Zn) on the internal oxidation rate of * Corresponding author. Tel.: +86 731 8830263; fax: +86 731 8836320. E-mail address: [email protected] (D.Q. Yi). 0010-938X/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.corsci.2008.09.010
Ag–Sn alloy [26]. They found that these metal elements (Cu, Bi, Sb and Zn) can accelerate the internal oxidation rate of Ag–Sn but the effect of addition Bi is the best. Bi will capture the oxygen atom before Sn and form (2Bi + 3O) atomic group in the silver matrix due to the large affinity between Bi and O. Crystal lattice of Bi belongs to rhombus but that of Bi2O3 is body-centered cubic structure. The crystal cell volume of Bi2O3 is about seven times larger than that of Bi [27], so there is a volume effect around Bi atoms. An irregular high energy interface between the (2Bi + 3O) atomic group and silver matrix is produced because of the volume effect. This interface can act as the diffused channel of oxygen atoms and greatly reduce the diffused active energy of oxygen. The diffused oxygen atoms react with Sn around the Bi2O3 and form SnO2. SnO2 can not form dense film due to the existence of Bi2O3 and Sn also has no time to diffuse outside. So the addition of Bi obviously enhanced the oxidation rate of Ag–Sn alloy. Internal oxidation of Ag–Sn alloy is important to prepare Ag–SnO2 electrical contact materials using by internal oxidation method. Although the kinetics of internal oxidation of Ag–Sn has been explored many times, little information is available in literatures on microstructure, size and distribution of the dispersed particles and Vickers hardness of internal oxidation layer in Ag–Sn alloy. In addition, CuO can improve the wettability between SnO2 and Ag matrix and enhance the welding resistance of Ag–SnO2 electrical contact materials. Consequently, the purpose of this study is to investigate the microstructure of internal oxidation layer in Ag–Sn–Cu alloy. The size and distribution of the dispersed particles and Vickers hardness of the internally oxidized layer in Ag–Sn–Cu alloy have been studied. Furthermore, we will also discuss the formation process of oxide particles, the effect of cold rolling on internal oxidation and the oxidation mechanism of this alloy. The investigation will be helpful for industrial production and to better understand the oxidation process of Ag–Sn–Cu alloy for academic researchers.
C.P. Wu et al. / Corrosion Science 50 (2008) 3508–3518
2. Experimental procedure 2.1. Materials High purity metal silver (99.99%) and tin, copper (99.9%) were used to prepare Ag–7.49%Sn–1.16%Cu (chemical composition, wt%) alloy samples. 2.2. Melting, casting and internal oxidation Metals were melted in a medium-frequency induction furnace using a graphite crucible. The melt was poured into a preheated square steel mould in the temperature ranging from 1100 to 1200 °C and cooled in air. The ingot (9.4 2.25 2.1 cm3) was annealed in a vacuum (102 –103 Pa) for 24 h at 800 °C for homogenization and then cold rolled into 1.5 mm thick plates. Specimen was cut into 10 10 mm2 square plates. The plates were cleaned thoroughly in acetone, alcohol and water to remove surface contaminations and were annealed in a vacuum (102– 103 Pa) for 0.5 h at 450 °C for recrystallization before internal oxidation. Internal oxidation experiments were carried out in a muffle furnace at 800 °C under an oxygen pressure of 0.21 105 Pa (means oxidation in air). Process flow diagram of experiment was presented in Fig. 1. 2.3. Characterization Microstructure of internally oxidized layer was characterized by an optical microscope (POLYVAR-MET), a scanning electron micro-
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scope (Sirion200 or KYKY2800) and the composition was analyzed by an energy dispersion spectrometer (Gensis60). Dispersion parameter such as particle radius and inter-particle spacing was measured on the internally oxidized layer. The measurement of hardness was carried out on polished samples at room temperature using a Digital Microhardness Tester (HVS-1000), diamond penetrator and load of 0.245 N. 3. Results 3.1. Microstructure evolution of Ag–Sn–Cu alloy Microstructures of Ag–Sn–Cu alloy sample at different processing conditions are presented in Fig. 2. There is fir-tree crystal on the casting Ag–Sn–Cu alloy sample and the distance of inter dendritic is small (Fig. 2a). The twin crystal is observed on the casting alloy sample after homogenization but the fir-tree crystal has disappeared (Fig. 2b). The crystal grains have deformed on the cold rolling sample (Fig. 2c–d). The fine equiax crystal is observed on the cold rolling sample after annealing and the grain size is about 5–15 lm (Fig. 2e). There are many oxide bands in the internally oxidized layer of sample A (Fig. 2f). The oxidation front of sample A is nearly flat (Fig. 2g) but that of sample B is zigzagged (Fig. 2h), which means internal oxidation proceeds well into the plates sample A and passivation has already started to occur on the sample B. Solute atoms are known to diffuse to the oxidation front during internal oxidation. If the diffusion of oxygen is hindered by the precipitation of oxides, the formation of oxides will mainly occur by the diffusion of solute atoms to the oxidation front. This will result in the dense precipitation of oxides in the oxidation front. Further diffusion of oxygen will be limited and so further internal oxidation will cease to occur, which means passivation has already started to occur. On the other hand, if oxidation proceeds faster than the diffusion of solute atoms, internal oxidation occurs mainly by the diffusion of oxygen and dense precipitation of oxides can be avoid. Internal oxidation of alloys with a uniform distribution of oxides will be possible and internal oxidation proceeds well into the alloy. Sample A have very high dislocation density, the oxygen can successfully diffuse into the alloy by short circuit diffusion, so the oxidation front is almost flat. While oxygen diffusion in sample B is hindered by the precipitation of oxides due to the low dislocation density. So the oxidation front is not flat but zigzagged. 3.2. Microstructure of the internally oxidized Ag–Sn–Cu alloy
Fig. 1. Process flow diagram of experiment.
SEM images of sample A are presented in Fig. 3. Fig. 3a shows the microstructure of the internally oxidized layer of sample A. Fig. 3b–e are the high magnified images of regions (1–4) in Fig. 3a, respectively. The oxide particles are very fine near the surface of the specimen (Fig. 3b). Compared to Fig. 3b, the oxide particles are larger in Fig. 3c–e. So we can suggest that the size of dispersed oxide particles in the internally oxidized layer become larger with the distance from the surface of the specimen. There are many oxide bands in the Fig. 3d–e, where the dark particles are observed on the oxide bands. EDS results show that the dark particles are CuO and the gray ones are SnO2 and the composition of oxide bands is SnO2. SEM images of sample B are presented in Fig. 4. Fig. 4a shows the microstructure of the internally oxidized layer of sample B. Fig. 4b–g are the high magnified images of regions (1–6) in Fig. 4a, respectively. In Fig. 4b we can see the fine gray SnO2 particles and dark CuO particles uniformly dispersed on the silver matrix. Compared to Fig. 4b, the oxide particles are larger in
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Fig. 2. Microstructure of Ag–Sn alloy sample at different processing conditions. (a)-Casting alloy; (b)-Homogenizing; (c)-Cold rolling, rolling surface; (d)-Cold rolling, vertical section; (e)-Recrystallization annealing; (f), (g)-Sample A; (h)-Sample B.
Fig. 4c–g. So the size of dispersed oxide particles in the internally oxidized layer became larger with the distance from the surface of the specimen. In Fig. 4c the CuO particles are large rod-shaped and granular shape, but SnO2 particles are fine granular shape. In Fig. 4d the CuO particles mainly are rod-shaped. We can see the massive shape CuO in Fig. 4e and the banding shape SnO2 in Fig. 4f. In Fig. 4g we can observe the large blocky SnO2 and CuO in oxide band, but we can see the fine SnO2 particles inside the CuO particles. Consequently, we suggest that the shape of oxide particles is different with the distance from the surface of the specimens. Microstructures of the internally oxidized layer of sample A are clearly presented in Fig. 5, where we can see the granular shape (Fig. 5a) and rod-shaped CuO (Fig. 5b) on the oxide band and the size of CuO particles are larger than that of SnO2 particles. When the interface between silver and SnO2 is incoherent interface, oxide band interface will be high energy boundary like grain boundary. The incoherent interface will act as a nucleation site for CuO particles. So the CuO particles after nucleation will grow easily and then form the large granular CuO particles or rod shaped CuO particles.
The high magnified microstructures of the internally oxidized layer of sample A are presented in Fig. 6. The oxide band is not continuous and dense (Fig. 6a), which means diffusion of oxygen does not hindered. The fine gray SnO2 particles are formed inside the dark CuO particles and surface of the dark CuO particles cover gray SnO2 particles (Fig. 6b1). From the result of thermodynamics calculation as shown in Fig. 7, we can know that Sn element is more active than Cu element, which means Sn will first react with oxygen at the same oxidation condition and then the formed SnO2 particles act as the nucleation site of CuO particles. Sn element around CuO particles diffuse to the surface of CuO particles and react with oxygen. So we can see the SnO2 particles on the surface of CuO particles. Fig. 6b2 shows the model of the SnO2 and CuO particles formation. In addition, the fine gray SnO2 particles and the large dark CuO particles uniformly disperse on the matrix (Fig. 6c). 3.3. Inter-particle spacing and particle radius in internally oxidized layer As we know N/X2 = 1/l2 and 4pr3/3l3 100 = vol%, where N is the numbers of particles in a square area X2, X is the length of area
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a Oxidation-free zone Oxidation front
4 3
Oxidation zone 2
Oxide band
1
Surface
c
b
50µm
50µm
d
e
50µm
50µm
Fig. 3. Microstructure of the internally oxidized layer of sample A. (b)–(e) are high magnified image of regions (1–4) in (a).
with N particles, l is inter-particle spacing, r is particle radius and vol% is the volume percent of particles. From the above two equations, we can calculate the value of inter-particle spacing and particle radius in the internally oxidized layer. The results show that the inter-particle spacing and particle radius increase with the distance from surface increasing (Figs. 8 and 9). In addition, the interparticle spacing and particle radius of sample B are larger than those of sample A. The oxygen concentration is very high on the surface of specimens and it becomes lower with the distance from the surface of the specimens. The oxygen diffusion flux, at the oxidation front, which is also the fixation rate of oxygen atoms per unit surface area is [28]:
" #1=2 C 0O DO C 0O DO mC 0M J¼ t 1=2 ¼ n 2
ð1Þ
where DO and C 0O are, respectively, the oxygen coefficient and the solubility of oxygen in pure silver, imposed by the temperature and the oxygen pressure, t is the O/M ratio in the oxide MOv, and C 0M is the solute concentration, J is the oxygen diffusion flux, n is the distance from surface and t is oxidation time. So the diffusion flux of oxygen is decreased with the distance from surface increasing. The oxide particles near the surface of specimen will only nucleate but not grow or the growing time will be very short and the ones away from the surface will grow continuously. Conse-
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a
Oxidation-free zone
6
Oxidation front
5 4
Oxidation zone
3 2 1
Surface
b
c
d
Rod-shaped Granular shape 5µm
e
5µm
5µm
g
f
SnO2 Massive shape Banding shape
5µm
5µm
5µm
CuO
Fig. 4. Microstructure of the internally oxidized layer of sample B. (b)–(g) are high magnified image of regions (1–6) in (a).
quently, the particle radius is increase with the distance from surface increasing. 3.4. Diffusion coefficient of oxygen in the internally oxidized layer
" #1=2 ðSÞ 2N 0 D0 t
ð3Þ
mNðSÞ M ðSÞ
The experimental results show that the relationship between the penetration depth (E) that is the depth of the internally oxidized layer and the oxidation time (t) of specimens are well expressed by a parabolic law (Fig. 10). So we can get the following equation:
E2 ¼ kt
E¼
ð2Þ
where k is the velocity constant of the internally oxidized layer. From Fig. 10 we can calculate the value of k, k = dE2/dt. The calculation result shows that the velocity constant of the sample A is 5.69 109cm2/s and that of sample B is 2.39 109cm2/s. Because the diffusion rate of oxygen in the alloy is much higher than that of alloy element, the alloy element is thought to be motionless in the alloy. According to the Wagner’s equation [29]:
where N 0 is the oxygen concentration on surface, imposed by the temperature and the oxygen pressure, and we can calculate the vaðSÞ lue from Sievert’s law [30], NM is Sn concentration before internal oxidation, D0 is diffusion coefficient of oxygen in internally oxidized layer, m is the atomic ratio of O and Sn to form SnO2. According to the Eqs. (2) and (3), we can get the following equation:
D0 ¼
mNðSÞ M k
ð4Þ
ðSÞ
2N 0
ðSÞ
ðSÞ
where m is 2, NM is 6.9at%, N 0 is 5.87 103at%, kA is the velocity constant of the sample A that value is 5.69 109cm2/s, kB is the velocity constant of the sample B that value is 2.39 109cm2/s. Consequently, we can calculate the diffusion coefficient of oxygen ðAÞ in sample A and sample B. D0 is the diffusion coefficient of oxygen ðBÞ in sample A and D0 is the diffusion coefficient of oxygen in sample ðAÞ B. The calculation result show that D0 = 6.69 106cm2/s and
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a
a
Oxidation band Gap
Granular CuO
Oxide band
b
b
b2 SnO2 CuO
Rod-shaped CuO
CuO
SnO2
SnO2
Fig. 5. Microstructure of the internally oxidized layer of sample A.
ðBÞ
ðAÞ
c
ðBÞ
D0 = 2.81 106cm2/s. D0 2.4D0 , so oxygen diffusion in sample A is obviously faster than that in sample B. This is an effect of short circuit diffusion of oxygen. In this work, the short circuit diffusion is mainly dislocation diffusion. Like the grain boundary diffusion, the dislocation diffusion will accelerate the atom diffusion, such as the edge dislocation. The diffusion coefficient in dislocation center is larger than that in perfect crystal. The dislocation may be considered as the ‘‘pipeline”. The atoms can quickly diffuse along the ‘‘pipeline”. The dislocation density in sample A is higher than that in sample B, so the diffusion of oxygen in sample A is faster than that in sample B. The diffusion coefficient of oxygen in pure silver is 1.6 106cm2/s at 850 °C [31]. While in this work, the diffusion coefficient of oxygen in both samples A and B is larger than that of in pure silver. As a result, the oxygen diffusion coefficients are increased in this work. 3.5. Hardness of the internally oxidized Ag–Sn–Cu alloy The relationship between Vickers hardness (Hv) and distance from the surface of the internally oxidized Ag–Sn–Cu alloy show that the hardness of the alloy after internal oxidation is obviously increased (Fig. 11). This is a dispersion hardening effect by internally oxidized particles. Internally oxidized silver metal oxide contact materials are initially cast as homogeneous binary or ternary substitutional solid solutions. The silver is the solvent and the solute consists of the elements which are to be oxidized, such as tin. Prior to internal oxidation the materials, being solid solutions, are relatively free to un-
SnO2 CuO
Fig. 6. High magnified image of the internally oxidized layer of sample A.
dergo dislocation nucleation and slip when subjected to an applied load. It is the movement of these dislocations (line imperfection in the crystalline array of atoms) that enable metals to undergo plastic deformation [32]. During the internal oxidation process, the solute atoms (Sn and Cu) are selectively oxidized to form a second phase of discrete oxide particles dispersed throughout the silver matrix. This second phase, no longer a part of the silver crystalline structure, increases the difficulty of dislocation movement. The dislocations must move between them for plastic deformation. This result in a much higher level of stress needed to plastically deform the internally oxidized materials when compared to that needed for the solid solution that existed prior to oxidation. This is the reason for the increased hardness in these alloys after internal oxidation.
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200000 Ag2O
SnO2
1.8
CuO
1.4
E2 (×10—3cm2)
0 -100000
ΔG/J
Sample A Sample B
1.6
100000
-200000 -300000
1.2 1.0 0.8 0.6 0.4
-400000
0.2 -500000
0.0
200
400
600
800
1000
1200
0
1400
12
24
36
48
60
72
84
Oxidation time (t) / h
T/K Fig. 7. Curves of DG–T on oxidizing reaction of Ag, Sn and Cu elements.
Fig. 10. The relationship between square of penetration depth and oxidation time.
Sample A Sample B
0.29
100
Oxidation front
95
0.28
90
0.27
Sample A Sample B
85
Hardness / Hv
Inter-particle spacing (l) / μm
0.30
0.26 0.25 0.24 0.23 0.22
80 75 70
Oxidation-free zone
65 60 55
0.21
50 0
50
100
150
200
250
300
350
Distance from surface / μm
Oxidation zone
45 0
Fig. 8. The relationship between inter-particle spacing and distance from surface.
100
200
300
400
500
Distance from surface / μm Fig. 11. Relationship between hardness and distance from the surface.
Particle radius (r) / μm
0.100
Sample A Sample B
0.085
eratures on formation process of oxides and hardness analysis of the internally oxidized Ag–Sn–Cu alloy. So we will discuss the formation process of CuO and SnO2. In addition, we also discuss the hardness in the internally oxidized layer and the effect of cold rolling on internal oxidation rate of Ag–Sn–Cu alloy.
0.080
4.1. The formation process of CuO particles
0.075
From Fig. 5 we can find that the size of CuO particles are larger than that of SnO2 particles, which means nucleation of CuO particles is harder than that of SnO2 particles. The result of oxidation thermodynamics of Cu and Sn elements show that Sn will firstly be oxidized when the oxidation condition is same. So nucleation of CuO particles is harder than that of SnO2 particles. In addition, we can see that the CuO particles mainly formed on the SnO2 bands. About the phenomena we will discuss from interface between Ag matrix and SnO2 band. Firstly, we suppose that the interface between Ag matrix and SnO2 band is coherent (Fig. 12a), which means the SnO2 band will be low energy boundary. Hence nucleation of CuO particles is hard on the SnO2 band, which is opposite appearance to our experimental result. So the interface between Ag (structure: fcc) and SnO2 (structure: rutile) is incoherent (Fig. 12b). Because incoherent interface has high energy boundary and large lattice defect (Fig. 12c), CuO particles can nucleate on the SnO2 band and grow up. The incoherent interface acts as nucle-
0.095 0.090
0.070 0
50
100
150
200
250
300
350
Distance from surface / μm Fig. 9. The relationship between particle radius and distance from surface.
In addition, from Fig. 11 we can see that the hardness of the internally oxidized layer is increased with the distance from surface increasing and the hardness of sample B is higher than that of sample A. 4. Discussion We can get much information on the internal oxidation kinetics of Ag–Sn–Cu alloys. However, little information is available in lit-
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a
b
Incoherent interface Ag
Coherent interface Ag
SnO2
SnO2
c
CuO nucleation position
d
Cu
O
Ag
Ag
SnO2 SnO2
e
Cu
O
f
Ag
Ag
SnO2
SnO2 CuO nucleation
g
CuO particles
Cu
O
CuO growth
h
Ag
CuO particles SnO2 Fig. 12. Schematic plan of CuO particles formation process.
ation site for CuO particles. About the process of CuO particles forming, we have the following explanation. Firstly, the Cu and O atoms diffuse into the SnO2 lattice defect (Fig. 12d) and then Cu atoms react with O atoms and form CuO nucleus (Fig. 12e). Secondly, the O and Cu atoms continuously diffuse into the SnO2 lattice defect and make the CuO to grow (Fig. 12f). Finally, CuO particles form on the SnO2 band (Fig. 12g and h). Because CuO particles continuously grow, the size of CuO particles is larger than that of SnO2 particles. This means that nucleation of CuO is harder than that of SnO2. Fig. 12d–g shows the schematic plan of CuO particles formation process. 4.2. Hardness in the internally oxidized layer When the second-phase particles are harder and the interface between the second-phase particles and matrix is incoherent, it is difficult for the dislocations to cut it because the second-phase particles do not deform. So the dislocations movement is hindered, it must bend and round the particles and then form a dislocation
loop around the particles if the dislocations continue to move. This dislocation moved mechanism is called as the Orowan mechanism. From Fig. 11 we can find that the hardness of the internally oxidized layer is increased with the distance from surface increasing. However, Shoji and his coauthor [33–34] found that the hardness of the internally oxidized layer was decreased with the distance from the surface increasing, in which Orowan mechanism operated. As we know Hv/sor = Gb/l for Orowan mechanism, where sor is decreased with the inter-particle spacing increasing. As a result, the hardness of the internally oxidized layer is decreased with the distance from the surface increasing in Shoji case. But in this experiment, it was found that the hardness and inter-particle spacing increase with increasing the distance from the surface in internally oxidized layer. So cutting mechanism is thought to operate. Hv / s ¼ F P =lb ¼ sf pr2 =lb for cutting mechanism, so Hv / sf r2 =l, which means the hardness is increased with sf and r2/l increasing. Where Hv is Vickers hardness, s is applied stress, FP is strength of particle, l is inter-particle spacing, b is Burgers vector, sf is fracture stress of particle and r is particle radius.
C.P. Wu et al. / Corrosion Science 50 (2008) 3508–3518
0.033
Fracture stress of particle (tf ) / Pa
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Sample A Sample B
0.032
r2/l (μm)
0.031 0.030 0.029 0.028 0.027 0.026 0.025
Sample A Sample B
250000 240000 230000 220000 210000 200000
0.024 0
50
100
150
200
250
300
350
0.075
0.080
0.085
0.090
0.095
0.100
Particle radius (r) / μm
Distance from surface / μm Fig. 13. Relationship between r2/l and distance from the surface.
Fig. 14. Relationship between fracture stress of particle and particle radius.
According to the value of r and l, we can calculate the value of r2/l and then we can get the relationship between r2/l and distance from surface of specimens. The result shows that the value of r2/l in the internally oxidized layer became larger with the distance from the surface of the specimens (Fig. 13). Consequently, the hardness of the internally oxidized layer is increased with the distance from the surface increasing. In addition, the value of r2/l of sample B is larger than that of sample A. So the hardness of sample B is higher than that of sample A. For the cutting mechanism, there are two related equations as the following:
particles is low. However, when the radius of oxide particles is very large, the oxides are in accordance with the stoichiometric proportion (such as SnO2). The large oxides are thought to be hard. The dislocations can not cut them but round them to lead Orowan mechanism when the dislocations move between the large oxide particles. As a result, the fracture stress of particles increased with the particle radius increasing.
Hv ¼ 3r ¼ 3 3:06s
s¼
sf pr2 lb
ð5Þ ð6Þ
So we can get the following equation:
sf ¼
9:18pr 2 lbHv
ð7Þ
where sf is fracture stress, Hv is Vicker hardness, r is radius of particles and l is inter-particles spacing, b is Burgers vector of silver crystal (FCC). With regard to the face-centered cubic crystal:
b¼
a < 110 > 2
ð8Þ
where a is lattice constant of silver and the value is 0.40857 nm. So we can get the value of b according to the Eq. (8). According to the value of Hv, r, l and b, we can calculate the sf. The relationship between fracture stress of oxide particles and particle radius is presented in Fig. 14, where the fracture stress of particles increased with the particle radius increasing and the fracture stress of particle is higher in sample B than that in sample A even for same particle radius. Charrin and his coauthors [28] investigated the variation of the quantity (oxygen atoms)/(solute atoms) (O/M) as a function of time in silver alloy. They found that the O/M ratio starts to increase, reaches a maximum (O/M)max and then slowly decreases to a final value (O/M)min. It means that the oxide is non stoichiometric proportion at the beginning and then the oxide slowly accord with the stoichiometric proportion. So the fine oxides, formed on the initial stage of the oxidation, are thought to be non stoichiometric composition and the large oxides, formed in the evening of the oxidation, are in accordance with the stoichiometric proportion. Because the non stoichiometric compound has some defects such as vacancy, the small oxides are thought to be soft. The dislocations can cut them easily when the dislocations move between the small oxide particles. Consequently, the fracture stress of small oxide
4.3. Effect of cold rolling on internal oxidation rate The experimental results show that the oxidation front of sample A is flat (Fig. 2g), but that of sample B is not flat but zigzagged (Fig. 2h). It indicates that the internal oxidation proceeds well into the plates of sample A, while that of sample B is not well. As we know that the specimen after cold rolling will have many dislocations and sub-boundaries, which means the specimen has very high dislocation density. High dislocation density and sub-boundaries are helpful to internal oxidation of alloy because of high diffusivity due to short circuit diffusion. It can increase the internal oxidation rate and accelerate the internal oxidation process and make the internal oxidation proceeded well into the plates. So cold rolling can accelerate the internal oxidation of alloy. However, the specimen after annealing for recrystallization will decrease the dislocation density and form fine equiax crystal. Reduction of dislocation density is not benefited to internal oxidation. So the internal oxidation of sample B is not better than that of sample A. Compared the microstructure of internal oxidation layer of sample A with sample B, you can find that the thickness of internal oxidation layer of sample A is larger than that of sample B. But sample B is internally oxidized for 72 h while sample A is internally oxidized for 60 h at 800 °C. Consequently, the oxidation rate of sample A is faster than that of sample B. In addition, from the above analysis we can know that the diffusion coefficient of oxygen in the internally oxidized layer of sample A is bigger than that of Sample B(DA0 2.4DB0 ), which indicates that cold rolling can accelerate the internal oxidation rate. We suggest that the preparation process of sample A is more effective for commercial run. 4.4. Internal oxidation mechanism According to the microstructure of internal oxidation layer of sample A, we can imply that the internal oxidation process of Ag–Sn–Cu alloy is a process of reaction diffusion. Firstly, oxygen adsorbs on the alloy surface and then it decomposes into oxygen atoms. The oxygen atoms react with Sn and Cu atoms and form SnO2 and CuO on the surface. Secondly, the oxygen atoms diffuse
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O2
Oxygen adsorption
[O]
Oxygen decomposition
O2
CuO MeO
SnO2
Reaction on surface
[O] Diffusion and reaction
SnO2 Sn
Oxide band formation
[O] and Sn diffusion
CuO Oxidation reaction
[O] and Sn diffusion
Cu [O] and Cu diffusion
Oxide band formation
CuO Oxide band
Fig. 15. Schematic plan of oxidation process of Ag–Sn–Cu alloy.
to inside the alloy and react with alloying agents (Sn and Cu) and form the metal oxides. Because Sn elements react with oxygen atoms and form SnO2, the Sn content is lower in the internally oxidized front than that of inside the alloy. Hence the Sn elements diffuse outside while oxygen atoms diffuse inside, they react and form SnO2 band (means oxide band). Thirdly, the Cu elements diffuse to the oxide band and react with oxygen atoms and form CuO. Finally, recycle the above oxidation process. Consequently, we can get the microstructure of internal oxidation layer like Figs. 3 and 5. Through the above analysis on internal oxidation process, we can
plot the schematic plan of internal oxidation process of Ag–Sn– Cu alloy, which is presented in Fig. 15.
5. Conclusion In this article, Ag–7.49 wt%Sn–1.16 wt%Cu alloy was internally oxidized in air at 800 °C. The size and distribution of the dispersed particles and Vickers hardness of the internally oxidized layer have been determined. In addition, the formation process of CuO and the
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effect of cold rolling on internal oxidation rate of Ag–Sn–Cu alloy were discussed. According to the above analysis and discussion, we came to the following conclusions: The relationship between the depth of the internally oxidized layer and the oxidation time of specimens are well expressed by a parabolic law. The size of dispersed oxide particles in the internally oxidized layer became larger and the shape of oxide particles is different with the distance from the surface of the specimens. (1) Vickers hardness of the internally oxidized layer increased with the distance from the surface of the specimens increasing. The hardness can be analyzed by the cutting mechanism. This is different with Shoji research result, in which the Orowan mechanism operates. (2) The fracture stress of particles increased with the particle radius increasing and the fracture stress of particle is higher in sample B than that in sample A even for same particle radius. (3) Cold rolling is effective for internal oxidation and it can increase obviously the diffusion of oxygen in the internally oxidized layer and accelerate the oxidation process. (4) Internal oxidation of Ag–Sn–Cu alloy is a process of reaction diffusion. It mainly includes three processes: the first process is the adsorption and decomposition of oxygen at the surface; the second one is oxidizing reaction to form SnO2 particles and oxide band; the third one is diffusion of Cu and CuO formation.
Acknowledgements This work is supported by China Science and Technology program (2006BAE03B03) and by the Production and Study program (2006D90404015) of the Ministry of Education of Guangdong province of China. The authors wish to acknowledge the assistance and direction of Prof. Shoji Goto. References [1] P. Verm, O.P. Pandey, A. Verma, Influence of metal oxides on the arc erosion behaviour of silver metal oxides electrical contact materials, Materials Science and Technology 20 (2004) 49–54. [2] F.Y. Guo, G.Q. Wang, N. Dong, F.W. Leuschner, The arc erosive characteristics and crack formation mechanisms analysis of silver-based contact materials, Proceedings of the CSEE (Chinese Society for Electrical Engineering) 24 (2004) 209–217. [3] M. Xie, S.G. Cao, L. Chen, Ag–SnO2 Electrical Contact Materials and Fabrication Method [P], 200410079614.X, 2004. [4] Y.P. Wang, T. Tian, K. Lu, Fabrication Method of one Kind of Silver Metal Oxides Electrical Contact Material [P], 02132791.2, 2004.
[5] T.Z. Cai, Internally oxidized Ag–Sn alloy electrical contact material and fabrication method [P], 86103279, (1991). [6] D.F. Chen, Producing Method of One Kind of Silver Tin Oxide Material [P], 03114863.8, (2003). [7] N. Tetsuya, S. Osamu, K. Hiroyuki, Method for producing Ag-ZnO electric contact material and electric contact material produced thereby [P], TW517095B, (2003). [8] S. Akira, Internal oxidized Ag–Sn–In system alloy electrical contact composite [P], US4672008, (1987). [9] H. Koji, K. Toru, Ag-oxide electrical contact material excellent in arc resistance [P], JP7166267, 1995. [10] T. Yasukazu, I. Shoji, T. Yasufumi, Internally Oxidized Electrical Contact Material of Ag–Sn Alloy [P], JP6136473, 1994. [11] T. Seiichi, H. Teruo, M. Yida, Internally Oxidized Ag–Sn–In Alloy Electrical Contact Materials and Manufacturing Method [P], EP0508746, 1992. [12] T. Seiichi, H. Teruo, M. Yida, Ag–SnO2 Electrical Contact Materials [P], US5147728, 1992. [13] S. Akira, Internally Oxidized Ag–Sn–Bi Alloy Electrical Contact Materials [P], US4514238, 1985. [14] S. Akira, Composite Electrical Contact Point Material Made of Ag–SnO2 Alloy [P], JP62250135, 1987. [15] S. Akira, Electrical Contact Material Made of Internally Oxidized Ag Alloy [P], JP6013197, 1985. [16] S. Akira, Ag-metal Oxides Electrical Contact Material Containing Internally Oxidized Indium Oxides and/or Tin Oxides [P], KR8301156, 1983. [17] S. Akira, A Composite Electrical Contact Material of Ag–Sn Oxides Alloy [P], KR8301155, 1983. [18] S. Akira, Electrical Contact Materials to Internally Oxidized Ag–Sn–Bi Alloy [P], CA1190770, 1985. [19] S. Akira, Electrical Contact Materials of Internally Oxidized Ag–Sn–Bi Alloy [P], CA1133285, 1982. [20] S. Akira, Composite Electrical Contact Material Comprising Ag and Intermetallic Compounds [P], US3880777, 1975. [21] S. Koichi, Ag Oxide Electrical Contact Material [P], JP63286542, 1988. [22] S. Wang, Discussion on internal oxidation problem of Ag–Sn alloy, Electrical Alloy 1 (1996) 11–14. [23] W.S. Lin, N.X. Fang, B. Lin, Effect of Bi on the mechanism of internal oxidation of Ag–Sn alloys, Powder Metallurgy Technology 20 (2002) 200–204. [24] J.H. Wen, B. Lin, Effect of APT on oxidation process of Ag–Sn alloy powders, Electrical Engineering Material 4 (2002) 14–18. [25] W.L. Joo, H.C. Lee, Effects of tellurium addition on the internal oxidation of Ag– Sn alloys, Scripta Material 42 (2000) 169–173. [26] Z.M. Deng, X.Y. Lv, F.Q. Zheng, M. Xie, J.L. Liu, A. Shi, The effect of some metal elements on the internal oxidation rate of Ag–Sn alloy, Precious Metals 22 (2001) 8–11. [27] C.J. Smithells, Metals Reference Book, Butter Worth, 1976. [28] L. Charrin, A. Combe, J. Cabane, Oxide particles in Ag–Mg alloys formed by internal oxidation, Oxidation of Metals 37 (1992) 65–80. [29] C. Wanger, Theoretical analysis of the diffusion processed determining oxidation rates of alloys, Journal of Electrochemical Society 99 (1952) 369– 380. [30] Q.Z. Du, J.S. Yang, Physical Chemistry, Chongqing University Press, Chongqing, 1997. [31] K. Hauffe, Reactions in solids and on their surface, Russian Translation 2 (1963) 115–127. [32] R.E. Reed-Hill, Physical Metallurgy Principles, second ed., D. Van Nostrand Co., New York, 1973 (Chapter 5). [33] G.T. Shoji, K.D. Shigeyasu, Internal oxidation of dilute Ni–Al solid solution alloys, Journal of the Japan Institute of Metals 32 (1968) 334–339. [34] G.T. Shoji, K.D. Shigeyasu, Structure of internally oxidized dilute nickel alloys containing chromium, Journal of the Japan Institute of Metals 31 (1967) 875– 880.
Contents lists available at ScienceDirect
Corrosion Science journal homepage: www.elsevier.com/locate/corsci
Microstructure of internally oxidized layer in Ag–Sn–Cu alloy C.P. Wu, D.Q. Yi *, C.H. Xu, J. Li, B. Wang, F. Zheng School of Materials Science and Engineering, Central South University, Changsha 410083, PR China
a r t i c l e
i n f o
Article history: Received 6 June 2008 Accepted 2 September 2008 Available online 19 September 2008 Keywords: A. Ag–Sn–Cu alloy C. Internal oxidation
a b s t r a c t Ag–7.49 wt%Sn–1.16 wt%Cu alloy was internally oxidized in air at 800 °C. The results showed that the size of dispersed particles and hardness in internally oxidized layer became larger with the distance from the surface of the specimens. The relationship between the depth of internally oxidized layer and oxidation time followed a parabolic law. The fracture stress of particles in internally oxidized layer increased with the particle radius increasing. Cold rolling was effective for internal oxidation because of the increased diffusivity of oxygen in internally oxidized layer. Internal oxidation of the alloy was found to be a process of reaction diffusion. Ó 2008 Elsevier Ltd. All rights reserved.
1. Introduction Ag–SnO2 materials prepared by internally oxidizing Ag–Sn alloy have been widely used in electrical and electronic industries due to their good electrical property and service performance [1–21]. However, the mechanism underlaying internal oxidation is still not clear. So investigation on internal oxidation of Ag–Sn alloy is rather necessary and meaningful for both industry production and academic research. Wang has studied the internal oxidation of Ag–Sn alloys [22]. He did a diffusion experiment on oxygen in the pure silver and Ag–Sn alloy. The experimental results showed that the oxidizable metal encapsulated by pure silver was oxidized absolutely, but that encapsulated by Ag–Sn alloy was not oxidized. So the oxygen can not diffuse into the Ag–Sn sosoloid. Lin and his coauthors studied the effect of Bi on internal oxidation of Ag–Sn alloys [23]. They found that internal oxidation is controlled by oxygen diffusion and Bi can change the oxidation mechanism. Wen and his colleague investigated the effect of the ammonium paratungstate (APT) on oxidation process of Ag–Sn alloy powders [24]. They found that the decomposition product of APT and new phase Ag2WO2 formed in the oxidation act as the sources of active oxygen atom, and enhance the oxidation of tin. Joo et al. studied the effect of tellurium addition on the internal oxidation of Ag–Sn alloys [25]. It may be possible that nucleation rate of (Sn, Te)O2 oxides is much faster than that of pure SnO2 oxides and tellurium oxides nucleate first but reacts with incoming Sn atoms to form (Sn, Te)O2 oxides. The nucleation of tellurium oxide is the fastest of all and as a result, addition of trace amount of Te can accelerate the oxidation rate of Ag–Sn alloys. Deng and his group investigated the effect of some metal elements (Cu, Bi, Sb and Zn) on the internal oxidation rate of * Corresponding author. Tel.: +86 731 8830263; fax: +86 731 8836320. E-mail address: [email protected] (D.Q. Yi). 0010-938X/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.corsci.2008.09.010
Ag–Sn alloy [26]. They found that these metal elements (Cu, Bi, Sb and Zn) can accelerate the internal oxidation rate of Ag–Sn but the effect of addition Bi is the best. Bi will capture the oxygen atom before Sn and form (2Bi + 3O) atomic group in the silver matrix due to the large affinity between Bi and O. Crystal lattice of Bi belongs to rhombus but that of Bi2O3 is body-centered cubic structure. The crystal cell volume of Bi2O3 is about seven times larger than that of Bi [27], so there is a volume effect around Bi atoms. An irregular high energy interface between the (2Bi + 3O) atomic group and silver matrix is produced because of the volume effect. This interface can act as the diffused channel of oxygen atoms and greatly reduce the diffused active energy of oxygen. The diffused oxygen atoms react with Sn around the Bi2O3 and form SnO2. SnO2 can not form dense film due to the existence of Bi2O3 and Sn also has no time to diffuse outside. So the addition of Bi obviously enhanced the oxidation rate of Ag–Sn alloy. Internal oxidation of Ag–Sn alloy is important to prepare Ag–SnO2 electrical contact materials using by internal oxidation method. Although the kinetics of internal oxidation of Ag–Sn has been explored many times, little information is available in literatures on microstructure, size and distribution of the dispersed particles and Vickers hardness of internal oxidation layer in Ag–Sn alloy. In addition, CuO can improve the wettability between SnO2 and Ag matrix and enhance the welding resistance of Ag–SnO2 electrical contact materials. Consequently, the purpose of this study is to investigate the microstructure of internal oxidation layer in Ag–Sn–Cu alloy. The size and distribution of the dispersed particles and Vickers hardness of the internally oxidized layer in Ag–Sn–Cu alloy have been studied. Furthermore, we will also discuss the formation process of oxide particles, the effect of cold rolling on internal oxidation and the oxidation mechanism of this alloy. The investigation will be helpful for industrial production and to better understand the oxidation process of Ag–Sn–Cu alloy for academic researchers.
C.P. Wu et al. / Corrosion Science 50 (2008) 3508–3518
2. Experimental procedure 2.1. Materials High purity metal silver (99.99%) and tin, copper (99.9%) were used to prepare Ag–7.49%Sn–1.16%Cu (chemical composition, wt%) alloy samples. 2.2. Melting, casting and internal oxidation Metals were melted in a medium-frequency induction furnace using a graphite crucible. The melt was poured into a preheated square steel mould in the temperature ranging from 1100 to 1200 °C and cooled in air. The ingot (9.4 2.25 2.1 cm3) was annealed in a vacuum (102 –103 Pa) for 24 h at 800 °C for homogenization and then cold rolled into 1.5 mm thick plates. Specimen was cut into 10 10 mm2 square plates. The plates were cleaned thoroughly in acetone, alcohol and water to remove surface contaminations and were annealed in a vacuum (102– 103 Pa) for 0.5 h at 450 °C for recrystallization before internal oxidation. Internal oxidation experiments were carried out in a muffle furnace at 800 °C under an oxygen pressure of 0.21 105 Pa (means oxidation in air). Process flow diagram of experiment was presented in Fig. 1. 2.3. Characterization Microstructure of internally oxidized layer was characterized by an optical microscope (POLYVAR-MET), a scanning electron micro-
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scope (Sirion200 or KYKY2800) and the composition was analyzed by an energy dispersion spectrometer (Gensis60). Dispersion parameter such as particle radius and inter-particle spacing was measured on the internally oxidized layer. The measurement of hardness was carried out on polished samples at room temperature using a Digital Microhardness Tester (HVS-1000), diamond penetrator and load of 0.245 N. 3. Results 3.1. Microstructure evolution of Ag–Sn–Cu alloy Microstructures of Ag–Sn–Cu alloy sample at different processing conditions are presented in Fig. 2. There is fir-tree crystal on the casting Ag–Sn–Cu alloy sample and the distance of inter dendritic is small (Fig. 2a). The twin crystal is observed on the casting alloy sample after homogenization but the fir-tree crystal has disappeared (Fig. 2b). The crystal grains have deformed on the cold rolling sample (Fig. 2c–d). The fine equiax crystal is observed on the cold rolling sample after annealing and the grain size is about 5–15 lm (Fig. 2e). There are many oxide bands in the internally oxidized layer of sample A (Fig. 2f). The oxidation front of sample A is nearly flat (Fig. 2g) but that of sample B is zigzagged (Fig. 2h), which means internal oxidation proceeds well into the plates sample A and passivation has already started to occur on the sample B. Solute atoms are known to diffuse to the oxidation front during internal oxidation. If the diffusion of oxygen is hindered by the precipitation of oxides, the formation of oxides will mainly occur by the diffusion of solute atoms to the oxidation front. This will result in the dense precipitation of oxides in the oxidation front. Further diffusion of oxygen will be limited and so further internal oxidation will cease to occur, which means passivation has already started to occur. On the other hand, if oxidation proceeds faster than the diffusion of solute atoms, internal oxidation occurs mainly by the diffusion of oxygen and dense precipitation of oxides can be avoid. Internal oxidation of alloys with a uniform distribution of oxides will be possible and internal oxidation proceeds well into the alloy. Sample A have very high dislocation density, the oxygen can successfully diffuse into the alloy by short circuit diffusion, so the oxidation front is almost flat. While oxygen diffusion in sample B is hindered by the precipitation of oxides due to the low dislocation density. So the oxidation front is not flat but zigzagged. 3.2. Microstructure of the internally oxidized Ag–Sn–Cu alloy
Fig. 1. Process flow diagram of experiment.
SEM images of sample A are presented in Fig. 3. Fig. 3a shows the microstructure of the internally oxidized layer of sample A. Fig. 3b–e are the high magnified images of regions (1–4) in Fig. 3a, respectively. The oxide particles are very fine near the surface of the specimen (Fig. 3b). Compared to Fig. 3b, the oxide particles are larger in Fig. 3c–e. So we can suggest that the size of dispersed oxide particles in the internally oxidized layer become larger with the distance from the surface of the specimen. There are many oxide bands in the Fig. 3d–e, where the dark particles are observed on the oxide bands. EDS results show that the dark particles are CuO and the gray ones are SnO2 and the composition of oxide bands is SnO2. SEM images of sample B are presented in Fig. 4. Fig. 4a shows the microstructure of the internally oxidized layer of sample B. Fig. 4b–g are the high magnified images of regions (1–6) in Fig. 4a, respectively. In Fig. 4b we can see the fine gray SnO2 particles and dark CuO particles uniformly dispersed on the silver matrix. Compared to Fig. 4b, the oxide particles are larger in
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Fig. 2. Microstructure of Ag–Sn alloy sample at different processing conditions. (a)-Casting alloy; (b)-Homogenizing; (c)-Cold rolling, rolling surface; (d)-Cold rolling, vertical section; (e)-Recrystallization annealing; (f), (g)-Sample A; (h)-Sample B.
Fig. 4c–g. So the size of dispersed oxide particles in the internally oxidized layer became larger with the distance from the surface of the specimen. In Fig. 4c the CuO particles are large rod-shaped and granular shape, but SnO2 particles are fine granular shape. In Fig. 4d the CuO particles mainly are rod-shaped. We can see the massive shape CuO in Fig. 4e and the banding shape SnO2 in Fig. 4f. In Fig. 4g we can observe the large blocky SnO2 and CuO in oxide band, but we can see the fine SnO2 particles inside the CuO particles. Consequently, we suggest that the shape of oxide particles is different with the distance from the surface of the specimens. Microstructures of the internally oxidized layer of sample A are clearly presented in Fig. 5, where we can see the granular shape (Fig. 5a) and rod-shaped CuO (Fig. 5b) on the oxide band and the size of CuO particles are larger than that of SnO2 particles. When the interface between silver and SnO2 is incoherent interface, oxide band interface will be high energy boundary like grain boundary. The incoherent interface will act as a nucleation site for CuO particles. So the CuO particles after nucleation will grow easily and then form the large granular CuO particles or rod shaped CuO particles.
The high magnified microstructures of the internally oxidized layer of sample A are presented in Fig. 6. The oxide band is not continuous and dense (Fig. 6a), which means diffusion of oxygen does not hindered. The fine gray SnO2 particles are formed inside the dark CuO particles and surface of the dark CuO particles cover gray SnO2 particles (Fig. 6b1). From the result of thermodynamics calculation as shown in Fig. 7, we can know that Sn element is more active than Cu element, which means Sn will first react with oxygen at the same oxidation condition and then the formed SnO2 particles act as the nucleation site of CuO particles. Sn element around CuO particles diffuse to the surface of CuO particles and react with oxygen. So we can see the SnO2 particles on the surface of CuO particles. Fig. 6b2 shows the model of the SnO2 and CuO particles formation. In addition, the fine gray SnO2 particles and the large dark CuO particles uniformly disperse on the matrix (Fig. 6c). 3.3. Inter-particle spacing and particle radius in internally oxidized layer As we know N/X2 = 1/l2 and 4pr3/3l3 100 = vol%, where N is the numbers of particles in a square area X2, X is the length of area
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a Oxidation-free zone Oxidation front
4 3
Oxidation zone 2
Oxide band
1
Surface
c
b
50µm
50µm
d
e
50µm
50µm
Fig. 3. Microstructure of the internally oxidized layer of sample A. (b)–(e) are high magnified image of regions (1–4) in (a).
with N particles, l is inter-particle spacing, r is particle radius and vol% is the volume percent of particles. From the above two equations, we can calculate the value of inter-particle spacing and particle radius in the internally oxidized layer. The results show that the inter-particle spacing and particle radius increase with the distance from surface increasing (Figs. 8 and 9). In addition, the interparticle spacing and particle radius of sample B are larger than those of sample A. The oxygen concentration is very high on the surface of specimens and it becomes lower with the distance from the surface of the specimens. The oxygen diffusion flux, at the oxidation front, which is also the fixation rate of oxygen atoms per unit surface area is [28]:
" #1=2 C 0O DO C 0O DO mC 0M J¼ t 1=2 ¼ n 2
ð1Þ
where DO and C 0O are, respectively, the oxygen coefficient and the solubility of oxygen in pure silver, imposed by the temperature and the oxygen pressure, t is the O/M ratio in the oxide MOv, and C 0M is the solute concentration, J is the oxygen diffusion flux, n is the distance from surface and t is oxidation time. So the diffusion flux of oxygen is decreased with the distance from surface increasing. The oxide particles near the surface of specimen will only nucleate but not grow or the growing time will be very short and the ones away from the surface will grow continuously. Conse-
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a
Oxidation-free zone
6
Oxidation front
5 4
Oxidation zone
3 2 1
Surface
b
c
d
Rod-shaped Granular shape 5µm
e
5µm
5µm
g
f
SnO2 Massive shape Banding shape
5µm
5µm
5µm
CuO
Fig. 4. Microstructure of the internally oxidized layer of sample B. (b)–(g) are high magnified image of regions (1–6) in (a).
quently, the particle radius is increase with the distance from surface increasing. 3.4. Diffusion coefficient of oxygen in the internally oxidized layer
" #1=2 ðSÞ 2N 0 D0 t
ð3Þ
mNðSÞ M ðSÞ
The experimental results show that the relationship between the penetration depth (E) that is the depth of the internally oxidized layer and the oxidation time (t) of specimens are well expressed by a parabolic law (Fig. 10). So we can get the following equation:
E2 ¼ kt
E¼
ð2Þ
where k is the velocity constant of the internally oxidized layer. From Fig. 10 we can calculate the value of k, k = dE2/dt. The calculation result shows that the velocity constant of the sample A is 5.69 109cm2/s and that of sample B is 2.39 109cm2/s. Because the diffusion rate of oxygen in the alloy is much higher than that of alloy element, the alloy element is thought to be motionless in the alloy. According to the Wagner’s equation [29]:
where N 0 is the oxygen concentration on surface, imposed by the temperature and the oxygen pressure, and we can calculate the vaðSÞ lue from Sievert’s law [30], NM is Sn concentration before internal oxidation, D0 is diffusion coefficient of oxygen in internally oxidized layer, m is the atomic ratio of O and Sn to form SnO2. According to the Eqs. (2) and (3), we can get the following equation:
D0 ¼
mNðSÞ M k
ð4Þ
ðSÞ
2N 0
ðSÞ
ðSÞ
where m is 2, NM is 6.9at%, N 0 is 5.87 103at%, kA is the velocity constant of the sample A that value is 5.69 109cm2/s, kB is the velocity constant of the sample B that value is 2.39 109cm2/s. Consequently, we can calculate the diffusion coefficient of oxygen ðAÞ in sample A and sample B. D0 is the diffusion coefficient of oxygen ðBÞ in sample A and D0 is the diffusion coefficient of oxygen in sample ðAÞ B. The calculation result show that D0 = 6.69 106cm2/s and
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a
a
Oxidation band Gap
Granular CuO
Oxide band
b
b
b2 SnO2 CuO
Rod-shaped CuO
CuO
SnO2
SnO2
Fig. 5. Microstructure of the internally oxidized layer of sample A.
ðBÞ
ðAÞ
c
ðBÞ
D0 = 2.81 106cm2/s. D0 2.4D0 , so oxygen diffusion in sample A is obviously faster than that in sample B. This is an effect of short circuit diffusion of oxygen. In this work, the short circuit diffusion is mainly dislocation diffusion. Like the grain boundary diffusion, the dislocation diffusion will accelerate the atom diffusion, such as the edge dislocation. The diffusion coefficient in dislocation center is larger than that in perfect crystal. The dislocation may be considered as the ‘‘pipeline”. The atoms can quickly diffuse along the ‘‘pipeline”. The dislocation density in sample A is higher than that in sample B, so the diffusion of oxygen in sample A is faster than that in sample B. The diffusion coefficient of oxygen in pure silver is 1.6 106cm2/s at 850 °C [31]. While in this work, the diffusion coefficient of oxygen in both samples A and B is larger than that of in pure silver. As a result, the oxygen diffusion coefficients are increased in this work. 3.5. Hardness of the internally oxidized Ag–Sn–Cu alloy The relationship between Vickers hardness (Hv) and distance from the surface of the internally oxidized Ag–Sn–Cu alloy show that the hardness of the alloy after internal oxidation is obviously increased (Fig. 11). This is a dispersion hardening effect by internally oxidized particles. Internally oxidized silver metal oxide contact materials are initially cast as homogeneous binary or ternary substitutional solid solutions. The silver is the solvent and the solute consists of the elements which are to be oxidized, such as tin. Prior to internal oxidation the materials, being solid solutions, are relatively free to un-
SnO2 CuO
Fig. 6. High magnified image of the internally oxidized layer of sample A.
dergo dislocation nucleation and slip when subjected to an applied load. It is the movement of these dislocations (line imperfection in the crystalline array of atoms) that enable metals to undergo plastic deformation [32]. During the internal oxidation process, the solute atoms (Sn and Cu) are selectively oxidized to form a second phase of discrete oxide particles dispersed throughout the silver matrix. This second phase, no longer a part of the silver crystalline structure, increases the difficulty of dislocation movement. The dislocations must move between them for plastic deformation. This result in a much higher level of stress needed to plastically deform the internally oxidized materials when compared to that needed for the solid solution that existed prior to oxidation. This is the reason for the increased hardness in these alloys after internal oxidation.
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200000 Ag2O
SnO2
1.8
CuO
1.4
E2 (×10—3cm2)
0 -100000
ΔG/J
Sample A Sample B
1.6
100000
-200000 -300000
1.2 1.0 0.8 0.6 0.4
-400000
0.2 -500000
0.0
200
400
600
800
1000
1200
0
1400
12
24
36
48
60
72
84
Oxidation time (t) / h
T/K Fig. 7. Curves of DG–T on oxidizing reaction of Ag, Sn and Cu elements.
Fig. 10. The relationship between square of penetration depth and oxidation time.
Sample A Sample B
0.29
100
Oxidation front
95
0.28
90
0.27
Sample A Sample B
85
Hardness / Hv
Inter-particle spacing (l) / μm
0.30
0.26 0.25 0.24 0.23 0.22
80 75 70
Oxidation-free zone
65 60 55
0.21
50 0
50
100
150
200
250
300
350
Distance from surface / μm
Oxidation zone
45 0
Fig. 8. The relationship between inter-particle spacing and distance from surface.
100
200
300
400
500
Distance from surface / μm Fig. 11. Relationship between hardness and distance from the surface.
Particle radius (r) / μm
0.100
Sample A Sample B
0.085
eratures on formation process of oxides and hardness analysis of the internally oxidized Ag–Sn–Cu alloy. So we will discuss the formation process of CuO and SnO2. In addition, we also discuss the hardness in the internally oxidized layer and the effect of cold rolling on internal oxidation rate of Ag–Sn–Cu alloy.
0.080
4.1. The formation process of CuO particles
0.075
From Fig. 5 we can find that the size of CuO particles are larger than that of SnO2 particles, which means nucleation of CuO particles is harder than that of SnO2 particles. The result of oxidation thermodynamics of Cu and Sn elements show that Sn will firstly be oxidized when the oxidation condition is same. So nucleation of CuO particles is harder than that of SnO2 particles. In addition, we can see that the CuO particles mainly formed on the SnO2 bands. About the phenomena we will discuss from interface between Ag matrix and SnO2 band. Firstly, we suppose that the interface between Ag matrix and SnO2 band is coherent (Fig. 12a), which means the SnO2 band will be low energy boundary. Hence nucleation of CuO particles is hard on the SnO2 band, which is opposite appearance to our experimental result. So the interface between Ag (structure: fcc) and SnO2 (structure: rutile) is incoherent (Fig. 12b). Because incoherent interface has high energy boundary and large lattice defect (Fig. 12c), CuO particles can nucleate on the SnO2 band and grow up. The incoherent interface acts as nucle-
0.095 0.090
0.070 0
50
100
150
200
250
300
350
Distance from surface / μm Fig. 9. The relationship between particle radius and distance from surface.
In addition, from Fig. 11 we can see that the hardness of the internally oxidized layer is increased with the distance from surface increasing and the hardness of sample B is higher than that of sample A. 4. Discussion We can get much information on the internal oxidation kinetics of Ag–Sn–Cu alloys. However, little information is available in lit-
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a
b
Incoherent interface Ag
Coherent interface Ag
SnO2
SnO2
c
CuO nucleation position
d
Cu
O
Ag
Ag
SnO2 SnO2
e
Cu
O
f
Ag
Ag
SnO2
SnO2 CuO nucleation
g
CuO particles
Cu
O
CuO growth
h
Ag
CuO particles SnO2 Fig. 12. Schematic plan of CuO particles formation process.
ation site for CuO particles. About the process of CuO particles forming, we have the following explanation. Firstly, the Cu and O atoms diffuse into the SnO2 lattice defect (Fig. 12d) and then Cu atoms react with O atoms and form CuO nucleus (Fig. 12e). Secondly, the O and Cu atoms continuously diffuse into the SnO2 lattice defect and make the CuO to grow (Fig. 12f). Finally, CuO particles form on the SnO2 band (Fig. 12g and h). Because CuO particles continuously grow, the size of CuO particles is larger than that of SnO2 particles. This means that nucleation of CuO is harder than that of SnO2. Fig. 12d–g shows the schematic plan of CuO particles formation process. 4.2. Hardness in the internally oxidized layer When the second-phase particles are harder and the interface between the second-phase particles and matrix is incoherent, it is difficult for the dislocations to cut it because the second-phase particles do not deform. So the dislocations movement is hindered, it must bend and round the particles and then form a dislocation
loop around the particles if the dislocations continue to move. This dislocation moved mechanism is called as the Orowan mechanism. From Fig. 11 we can find that the hardness of the internally oxidized layer is increased with the distance from surface increasing. However, Shoji and his coauthor [33–34] found that the hardness of the internally oxidized layer was decreased with the distance from the surface increasing, in which Orowan mechanism operated. As we know Hv/sor = Gb/l for Orowan mechanism, where sor is decreased with the inter-particle spacing increasing. As a result, the hardness of the internally oxidized layer is decreased with the distance from the surface increasing in Shoji case. But in this experiment, it was found that the hardness and inter-particle spacing increase with increasing the distance from the surface in internally oxidized layer. So cutting mechanism is thought to operate. Hv / s ¼ F P =lb ¼ sf pr2 =lb for cutting mechanism, so Hv / sf r2 =l, which means the hardness is increased with sf and r2/l increasing. Where Hv is Vickers hardness, s is applied stress, FP is strength of particle, l is inter-particle spacing, b is Burgers vector, sf is fracture stress of particle and r is particle radius.
C.P. Wu et al. / Corrosion Science 50 (2008) 3508–3518
0.033
Fracture stress of particle (tf ) / Pa
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Sample A Sample B
0.032
r2/l (μm)
0.031 0.030 0.029 0.028 0.027 0.026 0.025
Sample A Sample B
250000 240000 230000 220000 210000 200000
0.024 0
50
100
150
200
250
300
350
0.075
0.080
0.085
0.090
0.095
0.100
Particle radius (r) / μm
Distance from surface / μm Fig. 13. Relationship between r2/l and distance from the surface.
Fig. 14. Relationship between fracture stress of particle and particle radius.
According to the value of r and l, we can calculate the value of r2/l and then we can get the relationship between r2/l and distance from surface of specimens. The result shows that the value of r2/l in the internally oxidized layer became larger with the distance from the surface of the specimens (Fig. 13). Consequently, the hardness of the internally oxidized layer is increased with the distance from the surface increasing. In addition, the value of r2/l of sample B is larger than that of sample A. So the hardness of sample B is higher than that of sample A. For the cutting mechanism, there are two related equations as the following:
particles is low. However, when the radius of oxide particles is very large, the oxides are in accordance with the stoichiometric proportion (such as SnO2). The large oxides are thought to be hard. The dislocations can not cut them but round them to lead Orowan mechanism when the dislocations move between the large oxide particles. As a result, the fracture stress of particles increased with the particle radius increasing.
Hv ¼ 3r ¼ 3 3:06s
s¼
sf pr2 lb
ð5Þ ð6Þ
So we can get the following equation:
sf ¼
9:18pr 2 lbHv
ð7Þ
where sf is fracture stress, Hv is Vicker hardness, r is radius of particles and l is inter-particles spacing, b is Burgers vector of silver crystal (FCC). With regard to the face-centered cubic crystal:
b¼
a < 110 > 2
ð8Þ
where a is lattice constant of silver and the value is 0.40857 nm. So we can get the value of b according to the Eq. (8). According to the value of Hv, r, l and b, we can calculate the sf. The relationship between fracture stress of oxide particles and particle radius is presented in Fig. 14, where the fracture stress of particles increased with the particle radius increasing and the fracture stress of particle is higher in sample B than that in sample A even for same particle radius. Charrin and his coauthors [28] investigated the variation of the quantity (oxygen atoms)/(solute atoms) (O/M) as a function of time in silver alloy. They found that the O/M ratio starts to increase, reaches a maximum (O/M)max and then slowly decreases to a final value (O/M)min. It means that the oxide is non stoichiometric proportion at the beginning and then the oxide slowly accord with the stoichiometric proportion. So the fine oxides, formed on the initial stage of the oxidation, are thought to be non stoichiometric composition and the large oxides, formed in the evening of the oxidation, are in accordance with the stoichiometric proportion. Because the non stoichiometric compound has some defects such as vacancy, the small oxides are thought to be soft. The dislocations can cut them easily when the dislocations move between the small oxide particles. Consequently, the fracture stress of small oxide
4.3. Effect of cold rolling on internal oxidation rate The experimental results show that the oxidation front of sample A is flat (Fig. 2g), but that of sample B is not flat but zigzagged (Fig. 2h). It indicates that the internal oxidation proceeds well into the plates of sample A, while that of sample B is not well. As we know that the specimen after cold rolling will have many dislocations and sub-boundaries, which means the specimen has very high dislocation density. High dislocation density and sub-boundaries are helpful to internal oxidation of alloy because of high diffusivity due to short circuit diffusion. It can increase the internal oxidation rate and accelerate the internal oxidation process and make the internal oxidation proceeded well into the plates. So cold rolling can accelerate the internal oxidation of alloy. However, the specimen after annealing for recrystallization will decrease the dislocation density and form fine equiax crystal. Reduction of dislocation density is not benefited to internal oxidation. So the internal oxidation of sample B is not better than that of sample A. Compared the microstructure of internal oxidation layer of sample A with sample B, you can find that the thickness of internal oxidation layer of sample A is larger than that of sample B. But sample B is internally oxidized for 72 h while sample A is internally oxidized for 60 h at 800 °C. Consequently, the oxidation rate of sample A is faster than that of sample B. In addition, from the above analysis we can know that the diffusion coefficient of oxygen in the internally oxidized layer of sample A is bigger than that of Sample B(DA0 2.4DB0 ), which indicates that cold rolling can accelerate the internal oxidation rate. We suggest that the preparation process of sample A is more effective for commercial run. 4.4. Internal oxidation mechanism According to the microstructure of internal oxidation layer of sample A, we can imply that the internal oxidation process of Ag–Sn–Cu alloy is a process of reaction diffusion. Firstly, oxygen adsorbs on the alloy surface and then it decomposes into oxygen atoms. The oxygen atoms react with Sn and Cu atoms and form SnO2 and CuO on the surface. Secondly, the oxygen atoms diffuse
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O2
Oxygen adsorption
[O]
Oxygen decomposition
O2
CuO MeO
SnO2
Reaction on surface
[O] Diffusion and reaction
SnO2 Sn
Oxide band formation
[O] and Sn diffusion
CuO Oxidation reaction
[O] and Sn diffusion
Cu [O] and Cu diffusion
Oxide band formation
CuO Oxide band
Fig. 15. Schematic plan of oxidation process of Ag–Sn–Cu alloy.
to inside the alloy and react with alloying agents (Sn and Cu) and form the metal oxides. Because Sn elements react with oxygen atoms and form SnO2, the Sn content is lower in the internally oxidized front than that of inside the alloy. Hence the Sn elements diffuse outside while oxygen atoms diffuse inside, they react and form SnO2 band (means oxide band). Thirdly, the Cu elements diffuse to the oxide band and react with oxygen atoms and form CuO. Finally, recycle the above oxidation process. Consequently, we can get the microstructure of internal oxidation layer like Figs. 3 and 5. Through the above analysis on internal oxidation process, we can
plot the schematic plan of internal oxidation process of Ag–Sn– Cu alloy, which is presented in Fig. 15.
5. Conclusion In this article, Ag–7.49 wt%Sn–1.16 wt%Cu alloy was internally oxidized in air at 800 °C. The size and distribution of the dispersed particles and Vickers hardness of the internally oxidized layer have been determined. In addition, the formation process of CuO and the
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effect of cold rolling on internal oxidation rate of Ag–Sn–Cu alloy were discussed. According to the above analysis and discussion, we came to the following conclusions: The relationship between the depth of the internally oxidized layer and the oxidation time of specimens are well expressed by a parabolic law. The size of dispersed oxide particles in the internally oxidized layer became larger and the shape of oxide particles is different with the distance from the surface of the specimens. (1) Vickers hardness of the internally oxidized layer increased with the distance from the surface of the specimens increasing. The hardness can be analyzed by the cutting mechanism. This is different with Shoji research result, in which the Orowan mechanism operates. (2) The fracture stress of particles increased with the particle radius increasing and the fracture stress of particle is higher in sample B than that in sample A even for same particle radius. (3) Cold rolling is effective for internal oxidation and it can increase obviously the diffusion of oxygen in the internally oxidized layer and accelerate the oxidation process. (4) Internal oxidation of Ag–Sn–Cu alloy is a process of reaction diffusion. It mainly includes three processes: the first process is the adsorption and decomposition of oxygen at the surface; the second one is oxidizing reaction to form SnO2 particles and oxide band; the third one is diffusion of Cu and CuO formation.
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