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Interfacial reaction between Co–Cr–Mo alloy and liquid Al
Corrosion Science 75 (2013) 262–268
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Corrosion Science journal homepage: www.elsevier.com/locate/corsci
Interfacial reaction between Co–Cr–Mo alloy and liquid Al Ning Tang, Yunping Li ⇑, Yuichiro Koizumi, Shingo Kurosu, Akihiko Chiba ⇑ Institute for Materials Research, Tohoku University, Sendai 980-8577, Japan
a r t i c l e
i n f o
Article history: Received 6 December 2012 Accepted 4 June 2013 Available online 15 June 2013 Keywords: A. Aluminium A. Cobalt A. Intermetallics B. TEM C. Interfaces
a b s t r a c t The interfacial reaction between Co–Cr–Mo alloy and liquid Al was investigated using immersion tests. Microstructure characterization indicated that the Co–Cr–Mo alloy was corroded by liquid Al homogeneously, with the formation of a (Co,Cr,Mo)2Al9 layer close to alloy matrix and ‘‘(Cr,Mo)7Al45 + Al’’ layer close to Al. Kinetics analysis showed that the corrosion of the Co–Cr–Mo alloy followed a linear relationship with the immersion duration. Compared with pure Co–liquid Al reaction system, the alloying of Cr and Mo changed the solid–liquid interface structure, but the corrosion of the solid metal was still dominated by the dissolution of an intermetallic layer. Crown Copyright Ó 2013 Published by Elsevier Ltd. All rights reserved.
1. Introduction Co-based alloys have been well developed as wear resistant alloys, heat resistant alloys, and corrosion resistant alloys by virtue of their superior properties under severe environments [1]. Recently, because of their relatively higher resistance to both thermo-mechanical fatigue (TMF) cracking [2–4] and molten Al corrosion/erosion [5–7], Co-based alloys are expected to be better candidates for Al die casting tools than the currently used hotwork tool steels. In relation to the TMF properties, Birol carried out a series of works to compare Co-based alloys with Ni-based alloys and Fe-based alloys, and ascribed the relatively higher TMF resistance of Co-based alloys to their higher resistance to oxidation and temper softening [2–5]. However, for the corrosion behavior of Co-based alloys in molten Al, the reports were very limited. Mihelich and Kecker [7] just mentioned that Co-based alloys are more resistant to molten Al than both Fe-based alloys and Ni-based alloys, but did not report further details concerning their corrosion behavior and mechanisms. We previously studied the effects of oxidation treatment on the corrosion resistance of Co–Cr–Mo alloy (CCM alloy) in liquid Al at 1013 K [8]. The results indicated that the thickness loss of mirror-polished CCM alloy in liquid Al roughly followed a linear relationship with the immersion time, and two intermediate layers were observed between the CCM alloy matrix and solidified Al after the immersion tests. However, the intermetallic compounds (IMCs) that formed were not identified. Thus, the corrosion mechanism and effects of the alloying elements on the
⇑ Corresponding authors. Tel.: +81 22 215 2118 (Y. Li). E-mail addresses: [email protected] (Y. Li), [email protected] (A. Chiba).
corrosion were unclear. Therefore, to further improve the resistance of Co-based alloys to corrosion of liquid Al, their interfacial reactions with liquid Al should be well clarified. In our previous research, to exclude the effects of alloying elements, the interfacial reaction between solid Co and liquid Al was investigated by performing immersion tests, microstructure characterization, kinetics analysis, and theoretical modeling [9,10]. The results indicated that a Co2Al5 layer was formed at the solid–liquid interface during the immersion tests, and the reaction rate was controlled by both the formation and dissolution of this Co2Al5 layer. In this study, CCM alloy was selected to reveal the general regulation of the interfacial reactions between Cobased alloys and liquid Al. Based on our previous results [8–10], the interfacial reaction was studied through interface structure observation and reaction kinetics analysis, and the effects of Cr and Mo on the interfacial reaction were also investigated.
2. Experimental procedures 2.1. Materials The composition of the CCM alloy in this work was 28.85 Cr, 5.97 Mo, 0.46 Si, and 0.40 Mn (wt%), with the remainder being Co. The ingot was fabricated by vacuum melting and subjected to homogenization treatment at 1200 °C for 12 h, resulting in a biphase structure consisting of the c phase (FCC structure) and e phase (HCP structure) (Fig. 1). For immersion tests in liquid Al, the ingots were cut into small samples (20 5 2 mm3) and ground with SiC abrasive paper, followed by polishing with 1- and 0.3-lm alpha alumina suspensions (AP-A suspension, Struers) in sequence. Finally, after being cleaned in ethanol using
0010-938X/$ - see front matter Crown Copyright Ó 2013 Published by Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.corsci.2013.06.009
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(a)
(b)
100 µm µ
100 µ µm
HCP phase
FCC phase
HCP phase
FCC phase
Fig. 1. Initial structure of CCM alloy. (a) Inverse pole figure map of EBSD. (b) Phase map of EBSD.
an ultrasonic cleaner and dried with a blower, the mirror-polished samples were ready for the immersion tests. 2.2. Immersion test Owing to the Al oxide that naturally forms on the surface of liquid Al prevents the further oxidation of liquid Al, the immersion tests were conducted in air with the apparatus illustrated schematically in Fig. 2 [9]. For each immersion test, about 300 g of pure Al (99.99 wt%) was melted (about 125 cm3 [11]) in the alumina crucible, and a K-type thermocouple covered with a closed-ended alumina tube was immersed into the liquid Al to monitor the temperature. The immersion tests were conducted at the temperature ranging from 953 to 1043 K. After the temperature of liquid Al reached the target level and stabilized, the Al oxide layer on liquid Al was removed with a refractory tool, and then the prepared CCM alloy sample was immersed immediately. After being im-
mersed statically for a given duration ranging from 10 to 3600 s, the sample was taken out (with some attached Al) and cooled in air. The Co, Cr, and Mo concentrations in the liquid Al after the immersion tests were roughly estimated by the consumption of the solid samples, indicating that the liquid Al was far away from saturation even after immersion for 3600 s.
2.3. Structure characterization The initial structure of the CCM alloy was observed by electron backscatter diffraction (EBSD) using a field-emission scanning electron microscope (FEI XL30S-FEG) equipped with a TSL OIM system. The sample was ground with SiC abrasive paper (up to 1500 grade), then polished with 1- and 0.3-lm alpha alumina suspensions (APA suspension, Struers) in sequence, and finally polished with colloidal silica suspensions (OP-U suspension, Struers).
Thermocouple Sample holder
Specimen
Alumina tube with a closed end
Heating wire Thermal shields
Alumina crucible
Fig. 2. Schematic illustration of apparatus for immersion tests [9].
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After the immersion tests, the samples were cut perpendicular to their longest dimension and mounted in resin, together with the attached Al. The cross section was ground and polished using the above-mentioned routine. Without additional etching, the polished samples were observed under a scanning electron microscope (SEM, Hitachi S-3400N) and an electron probe micro-analyzer (EPMA, JEOL JXA-8530F). Samples for transmission electron microscopy (TEM) observation were prepared using an ion slicer (JEOL EM-09100IS) from the section perpendicular to the reaction interface. The detailed interface structure was observed using the TEM (Topcon EM002B) operated at 200 kV. Selected area diffraction patterns (SADP) were obtained for different phases and analyzed with the Crystallography 3.1 software. 2.4. Reaction kinetics To quantitatively analyze the interfacial reactions, the thicknesses of the sample matrix and intermediate layers formed by the interfacial reaction were measured in the cross sections. Then, the thickness loss of the metal matrix (DX) was calculated using the following equation:
DX ¼ ðX 0 XÞ=2
ð1Þ
where DX is the thickness loss of the solid metal matrix (mm), X0 is the initial thickness of the solid metal matrix (mm), and X is the thickness of the solid metal after immersion for a certain duration (mm). The difference between X0 and X was divided by 2 because both sides of the specimens were exposed to the liquid Al. 3. Results and analysis 3.1. Typical interface morphology between CCM alloy and solidified Al The typical interface structure between the CCM alloy and solidified Al is shown in Fig. 3. Two layers were observed between the CCM alloy and solidified Al: an intermetallic layer stuck closely to the CCM alloy matrix and a multiphase layer between the intermetallic layer and Al [8]. The intermetallic layer possessed a clear and smooth interface with the CCM alloy matrix, but an uneven interface with the multiphase layer. The multiphase layer was composed of Al solution (dark) and isolated phases (gray) with irregular shapes and various particle sizes from 1 lm to 5 lm.
Multiphase layer y + + 1
2
+ 3
+ 4
+ 6
Al +
+
5
7
3.2. Results from EPMA observation EPMA analysis was applied to the marked points in Fig. 3, and the results are tabulated in Table 1. Point 1 was the matrix of the CCM alloy, and only a little Al was detected by EPMA. The results of points 2 and 3 represent the composition of the intermetallic layer. At these two points, high concentrations of Al was observed (about 84 at%). Apart from Al, the concentrations of cobalt were observed to be about 10 at%, while the Cr and Mo contents were relatively low. Points 4 and 5 are the particles in the multiphase layer. Compared with the intermetallic layer, the Al contents in those particles in the multiphase layer increased to about 89 at%, particularly, the Co contents decreased to nearly zero, and the Cr and Mo contents showed obvious increases. Point 6 is the Al solution in the multiphase layer. Other than the Al (about 98 at%), the Cr content was the highest among the Co, Cr, and Mo. Point 7 is solidified Al, with very low contents of the other elements. Considering the compositions and phase diagrams, the Al content of the intermetallic layer (about 84 at%) is close to that of the Co2Al9 phase in the Al–Co binary phase diagram [12]. Similarly, the Al content of the particles in the multiphase layer (about 89 at%) is close to that of the Cr7Al45 phase in the Al–Cr binary phase diagram [13]. 3.3. Results from TEM observation The interface structure between the CCM alloy and solidified Al (after the immersion test at 1013 K for 600 s) was observed under TEM. Fig. 4 shows the typical structure of the intermetallic layer. In the bright field image (Fig. 4a), numerous fine columnar grains (100–200 nm in width, 0.5–2 lm in length) with heavy strain contrast were observed. These columnar grains were repeatedly analyzed using the SADPs for different grains, and the typical results are shown in Figs. 4b and c, which were taken in the dashed circle in Fig. 4a by tilting the sample with an angle of 17.5°. These two and patterns were consistent with the beam directions of ½012 34 in Co2Al9, respectively. Moreover, the angle between the ½1 and ½1 34 directions in Co2Al9 (17.8°) was close to the tilt an½012 gle of the sample (17.5°). Therefore, by combining the composition results and crystal structure analysis results, it can be concluded that the intermetallic layer was composed of a derivative phase of Co2Al9. In this research, it was named as (Co,Cr,Mo)2Al9. Similarly, the typical structure of the multiphase layer is shown in Fig. 5. In the bright field image (Fig. 5a), particles with different morphologies are clearly observed distributing in the Al. SADPs were repeatedly obtained for the particles distributed in different areas of the multiphase layer. Every pair of SADPs that were taken at the same position by tilting the sample can be indexed using a pair of crystallographic directions in Cr7Al45 (the typical results are shown in Fig. 5b and c), indicating that all of the particles in the multiphase layer were the same phase. Similar to the phase of (Co,Cr,Mo)2Al9 in the intermetallic layer, the particles in the multiphase layer were a derivative phase of Cr7Al45, which was called (Cr,Mo)7Al45 based on its composition and crystal structure. Table 1 EPMA results of points in Fig. 5 (at%).
A
B
C
D
E
5 µm Fig. 3. Typical interface structure between CCM alloy and solidified Al. After immersion in liquid Al at 1013 K for 600 s, two intermediate layers formed between CCM alloy and Al: an intermetallic layer next to CCM alloy and a multiphase layer next to Al.
Point
Co
Cr
Mo
Al
1 2 3 4 5 6 7
62.73 10.44 10.17 0.51 0.74 0.12 0.06
33.46 5.73 3.47 8.83 6.88 1.71 0.30
3.55 0.61 0.98 1.99 1.62 0.43 0.01
0.25 83.22 85.39 88.67 90.77 97.74 99.62
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(a)
265
(b)
(c)
1 µm Fig. 4. TEM observation of intermetallic layer (after immersion test at 1013 K for 600 s). (a) Bright field image. (b) and (c) SADPs (the sample was tilted by 17.5°, and the angle and ½1 34 axes was 17.8° in Co2Al9. between the ½012
(b)
(a)
Al
(c) Particles
1 µm Fig. 5. TEM observation of particles in multiphase layer (after immersion test at 1013 K for 600 s). (a) Bright field image. (b) and (c) SADPs (the sample was tilted by 16.4°, and 5 and ½1 30 axes was 16.9° in Cr7Al45. 2 the angle between the ½3
Therefore, the multiphase layer was composed of Al and the (Cr,Mo)7Al45 phase. 3.4. Kinetics of interfacial reaction For the test temperature of 1013 K, the thickness of the intermediate layers between the CCM alloy and Al is shown in Fig. 6 as a function of the immersion time. The intermetallic layer grew rapidly at the initial stage (10 s). Then, the growth rate decreased gradually and finally maintained a stable value (about 10 lm) after immersion for 600 s. Similar to the intermetallic layer, the multiphase layer grew rapidly at the beginning and then grew more and more slowly. However, the growth rate never decreased to
zero, and the thickness constantly increased during the entire test time. The thickness losses of the CCM alloy matrix during the immersion tests in liquid Al at different temperatures are shown in Fig. 7 versus the immersion time. The thickness losses were observed to approximately follow a linear law with the immersion time, and the rate of thickness loss increased greatly with the increasing temperature. To estimate the temperature dependence quantitatively, the thickness loss of the CCM alloy at each temperature was fitted using the linear law shown in the following equation
DX CCM ¼ aCCM þ K CCM t
ð2Þ
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Fig. 6. Thickness evolution of intermetallic layer and multiphase layer at 1013 K.
Fig. 7. Thickness losses of CCM alloy matrix after immersion tests at different temperatures and different durations.
where DX CCM is the thickness loss at time t, aCCM is a constant, and K CCM is the reaction coefficient. Then, the effect of the temperature on the reaction coefficient was evaluated using the Arrhenius equation,
Q K CCM ¼ K CCM0 exp CCM RT
dðln K CCM Þ R dð1=TÞ
Co-based alloys in liquid Al. The results indicated that, during the immersion test, the IMC layer between the solid metal and liquid metal would be simultaneously formed and dissolved at the solid metal–IMC interface and IMC–liquid metal interface, respectively. In addition, the corrosion rate of the solid metal in the liquid metal was controlled by both the formation and dissolution of the IMC layer. In Co–Al system, the formation and dissolution of the Co2Al5 layer achieved the equilibrium in a very short time, and the corrosion of the Co matrix was mainly controlled by the dissolution of the Co2Al5 layer. Based on these results, the reaction mechanism of the CCM alloy with liquid Al was clarified by considering its interface structure and reaction kinetics. The interface structure between the CCM alloy and Al was further divided into three sub-interfaces (A, C, and E) and two zones (B and D), as shown in Fig. 3. During the immersion test, reactions occurred in the subinterfaces simultaneously and the elements diffused through the intermediate layers along the direction opposite to the concentration gradient. The details of the reaction process can be explained as follows. (1) In interface A, Al atoms that diffused from liquid Al reacted with the CCM alloy and formed more (Co,Cr,Mo)2Al9 in the following reaction:
CCM alloy
ð3Þ
þ Al ðdiffused through the ðCo; Cr; MoÞ2 Al9 layerÞ
where K CCM0 is the pre-exponential factor, Q CCM is the apparent activation energy, R is the universal gas constant, and T is the test temperature. From Eq. (3), the apparent activation energy can be derived as
Q CCM ¼
Fig. 8. Logarithm of linear coefficient KCCM and KCo [9] versus reciprocal temperature. The apparent activation energies were 168.26 and 136.01 kJ/mol for the thickness losses of CCM alloy and pure Co, respectively.
ð4Þ
By plotting ln K CCM versus the reciprocal temperature and fitting the plot linearly (Fig. 8), the apparent activation energy of the thickness loss of the CCM alloy in liquid Al was calculated to be Q CCM = 168.26 kJ/mol. 4. Discussion 4.1. Reaction mechanism and controlling mechanism The interfacial reaction between solid Co and liquid Al was first investigated in our previous studies [9,10], which laid a foundation for understanding the corrosion behavior and mechanisms of
! ðCo; Cr; MoÞ2 Al9 : (2) In zone B, Al, Co, Cr, and Mo interdiffused in the intermetallic layer. (3) In interface C, the (Co,Cr,Mo)2Al9 phase further dissolved into the liquid Al. Possibly, as a result of the different solubilities of the elements in liquid Al (Table 2) [14–16], the Co in the intermetallic layer readily dissolved into the liquid Al and then diffused away, while the Cr and Mo concentrated to form the isolated (Cr,Mo)7Al45 phases in the multiphase layer in the following reaction:
Table 2 Solubilities of Co, Cr, and Mo in liquid Al at 1013 K (at%). Element
Solubility
Refs.
Co Cr Mo
1.53 0.58 0.17
[14] [15] [16]
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ðCo; Cr; MoÞ2 Al9 þ Al ðliquidÞ ! Co ðin liquid AlÞ þ ðCr; MoÞ7 Al45 : (4) In zone D, Co, Cr, and Mo diffused in the liquid Al. (5) In interface E, the (Cr,Mo)7Al45 phase gradually disappeared during the slow dissolution process in the following reaction:
ðCr; MoÞ7 Al45 þ Al ðliquidÞ ! Cr ðin liquid AlÞ þ Mo ðin liquid AlÞ þ liquid Al: In the solid Co–liquid Al reaction system [9,10], a thin layer of Co2Al5 was formed at the solid–liquid interface. The formation and dissolution of the Co2Al5 layer reached an equilibrium state in a very short time at the beginning of the immersion test, and the thickness loss of the Co matrix roughly followed a linear relationship with the immersion time. The theoretical model indicated that the thickness of the Co was mainly dominated by the dissolution of the Co2Al5 layer, and the corresponding activation energy (136.01 kJ/mol) was the activation energy of the dissolution process of Co2Al5, which was composed of the heat of solution of the Co2Al9 in the liquid Al (74.6 kJ/mol) and the activation energy for the dissolution rate constant (61.5 kJ/mol). Similarly, the stable thickness of the (Co,Cr,Mo)2Al9 layer after the initial period indicated the equilibrium between its formation and dissolution. For this complex IMC ((Co,Cr,Mo)2Al9), the parameters for the quantitative analysis [10] are not available. However, it can still be concluded that, after the thickness of the intermetallic layer stabilized, the thickness loss of the CCM alloy in the liquid Al was controlled by the dissolution of the (Co,Cr,Mo)2Al9 layer, and the corresponding apparent activation energy (168.26 kJ/mol) was mainly the activation energy of the dissolution process of (Co,Cr,Mo)2Al9. 4.2. Effects of Cr and Mo on interfacial reactions Fig. 9 show the interface structure of the Co–Al system. By comparing Figs. 4a and 9, it was obvious that the alloying of Cr and Mo into Co matrix changed the interface structure greatly. Although
Co
Co2Al5
Fig. 10. Thickness losses of Co [9] and CCM alloy matrix at 1013 and 1043 K versus immersion time.
the IMC layer formed in both reaction systems, the (Co,Cr,Mo)2Al9 phase (about 10 lm at 1013 K (Fig. 3)) formed next to the CCM alloy in the CCM–Al system, instead of the Co2Al5 phase (about 2 lm at 1013 K (Fig. 9)) forming next to the pure Co in the Co–Al system. In addition, besides the IMC layer, there was a multiphase layer, which was composed of isolated (Cr,Mo)7Al45 particles and liquid Al, formed between the IMC layer and liquid Al. The formation of the IMC layer in the diffusion couple depends on many factors, including the chemical reaction, nucleation conditions, interdiffusion coefficient of IMC, and composition range of IMC [17–20]. In the Co–Al system, Co2Al5 is prone to form rather than Co2Al9. However, the alloying of Cr and Mo obviously changed the relative factors and made (Co,Cr,Mo)2Al9 form during the interfacial reaction instead of (Co,Cr,Mo)2Al5. And according to our previous study [10], due to the different properties (both formation and dissolution) of the corresponding IMC layers, the steady state thicknesses of the IMC layers would be different in different reaction system. Therefore, the different steady state thicknesses of (Co,Cr,Mo)2Al9 in the CCM alloy–Al system and Co2Al5 in the Co– Al system were attributed to their different properties of formation and dissolution. As for the formation of the multiphase layer in the CCM–Al reaction system, it can be attributed to the relatively lower solubility of Cr and Mo in liquid Al, as analyzed in Section 4.1. The thickness loss rates (Fig. 10) and corresponding apparent activation energies (Fig. 8) were compared between the Co–Al and CCM–Al systems. Though the solubilities of Cr and Mo in liquid Al are lower than that of Co (Table 2), the Cr and Mo alloying increased the thickness loss of the solid metal matrix compared with pure Co (Fig. 10). Moreover, the apparent activation energy of the thickness loss of solid matrix in the CCM–Al system (168.26 kJ/mol) was also a little higher than that in the Co–Al system (136.01 kJ/mol). Because the interfacial reactions in both the Co–Al and CCM–Al systems were considered to be mainly controlled by the dissolution process of the IMC layer, the different dissolution behavior of Co2Al5 and (Co,Cr,Mo)2Al9 were responsible for the difference in the thickness loss rates and corresponding apparent activation energies between the Co–Al and CCM–Al systems. 5. Conclusions
1 µm Fig. 9. Thin layer of Co2Al5 formed next to Co matrix after immersing pure Co in liquid Al at 1013 K for 600 s [10].
(1) The Co–Cr–Mo alloy is homogeneously corroded by liquid Al and two intermediate layers form between the alloy matrix and liquid Al: (Co,Cr,Mo)2Al9 layer close to the alloy matrix and ‘‘(Cr,Mo)7Al45 + Al’’ layer close to the Al.
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(2) The thickness of the (Co,Cr,Mo)2Al9 layer is almost stable after the beginning stage of the immersion test. The thickness loss of the Co–Cr–Mo alloy follows a linear relationship with the immersion time, and the apparent activation energy of the thickness loss was calculated to be 168.26 kJ/ mol. (3) The mechanisms of the interface reactions are as follows:
Co—Cr—Mo alloy þ Al ðdiffused through the ðCo; Cr; MoÞ2 Al9 layerÞ ! ðCo; Cr; MoÞ2 Al9 ; ðCo; Cr; MoÞ2 Al9 þ Al ðliquidÞ ! Co ðin liquid AlÞ þ ðCr; MoÞ7 Al45 ; ðCr; MoÞ7 Al45 þ Al ðliquidÞ ! Cr ðin liquid AlÞ þ Mo ðin liquid AlÞ þ liquid Al: (4) Compared with pure Co, the alloying of Cr and Mo changes the solid–liquid interface structure, but the thickness loss of the solid metal is still controlled by the dissolution of an intermetallic layer.
Acknowledgements This study was mainly supported by a Regional Innovation Cluster Program 2010 from the Ministry of Education, Culture, Sports, Science and Technology of Japan. The authors would like to express their appreciation to Mr. Issei Narita for his assistance with EPMA observation, and Mr. Shun Ito for his assistance with TEM work. References [1] J.R. Davis, ASM Specialty Handbook: Nickel, Cobalt, and Their Alloys, International ASM, 2000. p. 362.
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Corrosion Science journal homepage: www.elsevier.com/locate/corsci
Interfacial reaction between Co–Cr–Mo alloy and liquid Al Ning Tang, Yunping Li ⇑, Yuichiro Koizumi, Shingo Kurosu, Akihiko Chiba ⇑ Institute for Materials Research, Tohoku University, Sendai 980-8577, Japan
a r t i c l e
i n f o
Article history: Received 6 December 2012 Accepted 4 June 2013 Available online 15 June 2013 Keywords: A. Aluminium A. Cobalt A. Intermetallics B. TEM C. Interfaces
a b s t r a c t The interfacial reaction between Co–Cr–Mo alloy and liquid Al was investigated using immersion tests. Microstructure characterization indicated that the Co–Cr–Mo alloy was corroded by liquid Al homogeneously, with the formation of a (Co,Cr,Mo)2Al9 layer close to alloy matrix and ‘‘(Cr,Mo)7Al45 + Al’’ layer close to Al. Kinetics analysis showed that the corrosion of the Co–Cr–Mo alloy followed a linear relationship with the immersion duration. Compared with pure Co–liquid Al reaction system, the alloying of Cr and Mo changed the solid–liquid interface structure, but the corrosion of the solid metal was still dominated by the dissolution of an intermetallic layer. Crown Copyright Ó 2013 Published by Elsevier Ltd. All rights reserved.
1. Introduction Co-based alloys have been well developed as wear resistant alloys, heat resistant alloys, and corrosion resistant alloys by virtue of their superior properties under severe environments [1]. Recently, because of their relatively higher resistance to both thermo-mechanical fatigue (TMF) cracking [2–4] and molten Al corrosion/erosion [5–7], Co-based alloys are expected to be better candidates for Al die casting tools than the currently used hotwork tool steels. In relation to the TMF properties, Birol carried out a series of works to compare Co-based alloys with Ni-based alloys and Fe-based alloys, and ascribed the relatively higher TMF resistance of Co-based alloys to their higher resistance to oxidation and temper softening [2–5]. However, for the corrosion behavior of Co-based alloys in molten Al, the reports were very limited. Mihelich and Kecker [7] just mentioned that Co-based alloys are more resistant to molten Al than both Fe-based alloys and Ni-based alloys, but did not report further details concerning their corrosion behavior and mechanisms. We previously studied the effects of oxidation treatment on the corrosion resistance of Co–Cr–Mo alloy (CCM alloy) in liquid Al at 1013 K [8]. The results indicated that the thickness loss of mirror-polished CCM alloy in liquid Al roughly followed a linear relationship with the immersion time, and two intermediate layers were observed between the CCM alloy matrix and solidified Al after the immersion tests. However, the intermetallic compounds (IMCs) that formed were not identified. Thus, the corrosion mechanism and effects of the alloying elements on the
⇑ Corresponding authors. Tel.: +81 22 215 2118 (Y. Li). E-mail addresses: [email protected] (Y. Li), [email protected] (A. Chiba).
corrosion were unclear. Therefore, to further improve the resistance of Co-based alloys to corrosion of liquid Al, their interfacial reactions with liquid Al should be well clarified. In our previous research, to exclude the effects of alloying elements, the interfacial reaction between solid Co and liquid Al was investigated by performing immersion tests, microstructure characterization, kinetics analysis, and theoretical modeling [9,10]. The results indicated that a Co2Al5 layer was formed at the solid–liquid interface during the immersion tests, and the reaction rate was controlled by both the formation and dissolution of this Co2Al5 layer. In this study, CCM alloy was selected to reveal the general regulation of the interfacial reactions between Cobased alloys and liquid Al. Based on our previous results [8–10], the interfacial reaction was studied through interface structure observation and reaction kinetics analysis, and the effects of Cr and Mo on the interfacial reaction were also investigated.
2. Experimental procedures 2.1. Materials The composition of the CCM alloy in this work was 28.85 Cr, 5.97 Mo, 0.46 Si, and 0.40 Mn (wt%), with the remainder being Co. The ingot was fabricated by vacuum melting and subjected to homogenization treatment at 1200 °C for 12 h, resulting in a biphase structure consisting of the c phase (FCC structure) and e phase (HCP structure) (Fig. 1). For immersion tests in liquid Al, the ingots were cut into small samples (20 5 2 mm3) and ground with SiC abrasive paper, followed by polishing with 1- and 0.3-lm alpha alumina suspensions (AP-A suspension, Struers) in sequence. Finally, after being cleaned in ethanol using
0010-938X/$ - see front matter Crown Copyright Ó 2013 Published by Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.corsci.2013.06.009
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(a)
(b)
100 µm µ
100 µ µm
HCP phase
FCC phase
HCP phase
FCC phase
Fig. 1. Initial structure of CCM alloy. (a) Inverse pole figure map of EBSD. (b) Phase map of EBSD.
an ultrasonic cleaner and dried with a blower, the mirror-polished samples were ready for the immersion tests. 2.2. Immersion test Owing to the Al oxide that naturally forms on the surface of liquid Al prevents the further oxidation of liquid Al, the immersion tests were conducted in air with the apparatus illustrated schematically in Fig. 2 [9]. For each immersion test, about 300 g of pure Al (99.99 wt%) was melted (about 125 cm3 [11]) in the alumina crucible, and a K-type thermocouple covered with a closed-ended alumina tube was immersed into the liquid Al to monitor the temperature. The immersion tests were conducted at the temperature ranging from 953 to 1043 K. After the temperature of liquid Al reached the target level and stabilized, the Al oxide layer on liquid Al was removed with a refractory tool, and then the prepared CCM alloy sample was immersed immediately. After being im-
mersed statically for a given duration ranging from 10 to 3600 s, the sample was taken out (with some attached Al) and cooled in air. The Co, Cr, and Mo concentrations in the liquid Al after the immersion tests were roughly estimated by the consumption of the solid samples, indicating that the liquid Al was far away from saturation even after immersion for 3600 s.
2.3. Structure characterization The initial structure of the CCM alloy was observed by electron backscatter diffraction (EBSD) using a field-emission scanning electron microscope (FEI XL30S-FEG) equipped with a TSL OIM system. The sample was ground with SiC abrasive paper (up to 1500 grade), then polished with 1- and 0.3-lm alpha alumina suspensions (APA suspension, Struers) in sequence, and finally polished with colloidal silica suspensions (OP-U suspension, Struers).
Thermocouple Sample holder
Specimen
Alumina tube with a closed end
Heating wire Thermal shields
Alumina crucible
Fig. 2. Schematic illustration of apparatus for immersion tests [9].
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After the immersion tests, the samples were cut perpendicular to their longest dimension and mounted in resin, together with the attached Al. The cross section was ground and polished using the above-mentioned routine. Without additional etching, the polished samples were observed under a scanning electron microscope (SEM, Hitachi S-3400N) and an electron probe micro-analyzer (EPMA, JEOL JXA-8530F). Samples for transmission electron microscopy (TEM) observation were prepared using an ion slicer (JEOL EM-09100IS) from the section perpendicular to the reaction interface. The detailed interface structure was observed using the TEM (Topcon EM002B) operated at 200 kV. Selected area diffraction patterns (SADP) were obtained for different phases and analyzed with the Crystallography 3.1 software. 2.4. Reaction kinetics To quantitatively analyze the interfacial reactions, the thicknesses of the sample matrix and intermediate layers formed by the interfacial reaction were measured in the cross sections. Then, the thickness loss of the metal matrix (DX) was calculated using the following equation:
DX ¼ ðX 0 XÞ=2
ð1Þ
where DX is the thickness loss of the solid metal matrix (mm), X0 is the initial thickness of the solid metal matrix (mm), and X is the thickness of the solid metal after immersion for a certain duration (mm). The difference between X0 and X was divided by 2 because both sides of the specimens were exposed to the liquid Al. 3. Results and analysis 3.1. Typical interface morphology between CCM alloy and solidified Al The typical interface structure between the CCM alloy and solidified Al is shown in Fig. 3. Two layers were observed between the CCM alloy and solidified Al: an intermetallic layer stuck closely to the CCM alloy matrix and a multiphase layer between the intermetallic layer and Al [8]. The intermetallic layer possessed a clear and smooth interface with the CCM alloy matrix, but an uneven interface with the multiphase layer. The multiphase layer was composed of Al solution (dark) and isolated phases (gray) with irregular shapes and various particle sizes from 1 lm to 5 lm.
Multiphase layer y + + 1
2
+ 3
+ 4
+ 6
Al +
+
5
7
3.2. Results from EPMA observation EPMA analysis was applied to the marked points in Fig. 3, and the results are tabulated in Table 1. Point 1 was the matrix of the CCM alloy, and only a little Al was detected by EPMA. The results of points 2 and 3 represent the composition of the intermetallic layer. At these two points, high concentrations of Al was observed (about 84 at%). Apart from Al, the concentrations of cobalt were observed to be about 10 at%, while the Cr and Mo contents were relatively low. Points 4 and 5 are the particles in the multiphase layer. Compared with the intermetallic layer, the Al contents in those particles in the multiphase layer increased to about 89 at%, particularly, the Co contents decreased to nearly zero, and the Cr and Mo contents showed obvious increases. Point 6 is the Al solution in the multiphase layer. Other than the Al (about 98 at%), the Cr content was the highest among the Co, Cr, and Mo. Point 7 is solidified Al, with very low contents of the other elements. Considering the compositions and phase diagrams, the Al content of the intermetallic layer (about 84 at%) is close to that of the Co2Al9 phase in the Al–Co binary phase diagram [12]. Similarly, the Al content of the particles in the multiphase layer (about 89 at%) is close to that of the Cr7Al45 phase in the Al–Cr binary phase diagram [13]. 3.3. Results from TEM observation The interface structure between the CCM alloy and solidified Al (after the immersion test at 1013 K for 600 s) was observed under TEM. Fig. 4 shows the typical structure of the intermetallic layer. In the bright field image (Fig. 4a), numerous fine columnar grains (100–200 nm in width, 0.5–2 lm in length) with heavy strain contrast were observed. These columnar grains were repeatedly analyzed using the SADPs for different grains, and the typical results are shown in Figs. 4b and c, which were taken in the dashed circle in Fig. 4a by tilting the sample with an angle of 17.5°. These two and patterns were consistent with the beam directions of ½012 34 in Co2Al9, respectively. Moreover, the angle between the ½1 and ½1 34 directions in Co2Al9 (17.8°) was close to the tilt an½012 gle of the sample (17.5°). Therefore, by combining the composition results and crystal structure analysis results, it can be concluded that the intermetallic layer was composed of a derivative phase of Co2Al9. In this research, it was named as (Co,Cr,Mo)2Al9. Similarly, the typical structure of the multiphase layer is shown in Fig. 5. In the bright field image (Fig. 5a), particles with different morphologies are clearly observed distributing in the Al. SADPs were repeatedly obtained for the particles distributed in different areas of the multiphase layer. Every pair of SADPs that were taken at the same position by tilting the sample can be indexed using a pair of crystallographic directions in Cr7Al45 (the typical results are shown in Fig. 5b and c), indicating that all of the particles in the multiphase layer were the same phase. Similar to the phase of (Co,Cr,Mo)2Al9 in the intermetallic layer, the particles in the multiphase layer were a derivative phase of Cr7Al45, which was called (Cr,Mo)7Al45 based on its composition and crystal structure. Table 1 EPMA results of points in Fig. 5 (at%).
A
B
C
D
E
5 µm Fig. 3. Typical interface structure between CCM alloy and solidified Al. After immersion in liquid Al at 1013 K for 600 s, two intermediate layers formed between CCM alloy and Al: an intermetallic layer next to CCM alloy and a multiphase layer next to Al.
Point
Co
Cr
Mo
Al
1 2 3 4 5 6 7
62.73 10.44 10.17 0.51 0.74 0.12 0.06
33.46 5.73 3.47 8.83 6.88 1.71 0.30
3.55 0.61 0.98 1.99 1.62 0.43 0.01
0.25 83.22 85.39 88.67 90.77 97.74 99.62
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(a)
265
(b)
(c)
1 µm Fig. 4. TEM observation of intermetallic layer (after immersion test at 1013 K for 600 s). (a) Bright field image. (b) and (c) SADPs (the sample was tilted by 17.5°, and the angle and ½1 34 axes was 17.8° in Co2Al9. between the ½012
(b)
(a)
Al
(c) Particles
1 µm Fig. 5. TEM observation of particles in multiphase layer (after immersion test at 1013 K for 600 s). (a) Bright field image. (b) and (c) SADPs (the sample was tilted by 16.4°, and 5 and ½1 30 axes was 16.9° in Cr7Al45. 2 the angle between the ½3
Therefore, the multiphase layer was composed of Al and the (Cr,Mo)7Al45 phase. 3.4. Kinetics of interfacial reaction For the test temperature of 1013 K, the thickness of the intermediate layers between the CCM alloy and Al is shown in Fig. 6 as a function of the immersion time. The intermetallic layer grew rapidly at the initial stage (10 s). Then, the growth rate decreased gradually and finally maintained a stable value (about 10 lm) after immersion for 600 s. Similar to the intermetallic layer, the multiphase layer grew rapidly at the beginning and then grew more and more slowly. However, the growth rate never decreased to
zero, and the thickness constantly increased during the entire test time. The thickness losses of the CCM alloy matrix during the immersion tests in liquid Al at different temperatures are shown in Fig. 7 versus the immersion time. The thickness losses were observed to approximately follow a linear law with the immersion time, and the rate of thickness loss increased greatly with the increasing temperature. To estimate the temperature dependence quantitatively, the thickness loss of the CCM alloy at each temperature was fitted using the linear law shown in the following equation
DX CCM ¼ aCCM þ K CCM t
ð2Þ
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Fig. 6. Thickness evolution of intermetallic layer and multiphase layer at 1013 K.
Fig. 7. Thickness losses of CCM alloy matrix after immersion tests at different temperatures and different durations.
where DX CCM is the thickness loss at time t, aCCM is a constant, and K CCM is the reaction coefficient. Then, the effect of the temperature on the reaction coefficient was evaluated using the Arrhenius equation,
Q K CCM ¼ K CCM0 exp CCM RT
dðln K CCM Þ R dð1=TÞ
Co-based alloys in liquid Al. The results indicated that, during the immersion test, the IMC layer between the solid metal and liquid metal would be simultaneously formed and dissolved at the solid metal–IMC interface and IMC–liquid metal interface, respectively. In addition, the corrosion rate of the solid metal in the liquid metal was controlled by both the formation and dissolution of the IMC layer. In Co–Al system, the formation and dissolution of the Co2Al5 layer achieved the equilibrium in a very short time, and the corrosion of the Co matrix was mainly controlled by the dissolution of the Co2Al5 layer. Based on these results, the reaction mechanism of the CCM alloy with liquid Al was clarified by considering its interface structure and reaction kinetics. The interface structure between the CCM alloy and Al was further divided into three sub-interfaces (A, C, and E) and two zones (B and D), as shown in Fig. 3. During the immersion test, reactions occurred in the subinterfaces simultaneously and the elements diffused through the intermediate layers along the direction opposite to the concentration gradient. The details of the reaction process can be explained as follows. (1) In interface A, Al atoms that diffused from liquid Al reacted with the CCM alloy and formed more (Co,Cr,Mo)2Al9 in the following reaction:
CCM alloy
ð3Þ
þ Al ðdiffused through the ðCo; Cr; MoÞ2 Al9 layerÞ
where K CCM0 is the pre-exponential factor, Q CCM is the apparent activation energy, R is the universal gas constant, and T is the test temperature. From Eq. (3), the apparent activation energy can be derived as
Q CCM ¼
Fig. 8. Logarithm of linear coefficient KCCM and KCo [9] versus reciprocal temperature. The apparent activation energies were 168.26 and 136.01 kJ/mol for the thickness losses of CCM alloy and pure Co, respectively.
ð4Þ
By plotting ln K CCM versus the reciprocal temperature and fitting the plot linearly (Fig. 8), the apparent activation energy of the thickness loss of the CCM alloy in liquid Al was calculated to be Q CCM = 168.26 kJ/mol. 4. Discussion 4.1. Reaction mechanism and controlling mechanism The interfacial reaction between solid Co and liquid Al was first investigated in our previous studies [9,10], which laid a foundation for understanding the corrosion behavior and mechanisms of
! ðCo; Cr; MoÞ2 Al9 : (2) In zone B, Al, Co, Cr, and Mo interdiffused in the intermetallic layer. (3) In interface C, the (Co,Cr,Mo)2Al9 phase further dissolved into the liquid Al. Possibly, as a result of the different solubilities of the elements in liquid Al (Table 2) [14–16], the Co in the intermetallic layer readily dissolved into the liquid Al and then diffused away, while the Cr and Mo concentrated to form the isolated (Cr,Mo)7Al45 phases in the multiphase layer in the following reaction:
Table 2 Solubilities of Co, Cr, and Mo in liquid Al at 1013 K (at%). Element
Solubility
Refs.
Co Cr Mo
1.53 0.58 0.17
[14] [15] [16]
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ðCo; Cr; MoÞ2 Al9 þ Al ðliquidÞ ! Co ðin liquid AlÞ þ ðCr; MoÞ7 Al45 : (4) In zone D, Co, Cr, and Mo diffused in the liquid Al. (5) In interface E, the (Cr,Mo)7Al45 phase gradually disappeared during the slow dissolution process in the following reaction:
ðCr; MoÞ7 Al45 þ Al ðliquidÞ ! Cr ðin liquid AlÞ þ Mo ðin liquid AlÞ þ liquid Al: In the solid Co–liquid Al reaction system [9,10], a thin layer of Co2Al5 was formed at the solid–liquid interface. The formation and dissolution of the Co2Al5 layer reached an equilibrium state in a very short time at the beginning of the immersion test, and the thickness loss of the Co matrix roughly followed a linear relationship with the immersion time. The theoretical model indicated that the thickness of the Co was mainly dominated by the dissolution of the Co2Al5 layer, and the corresponding activation energy (136.01 kJ/mol) was the activation energy of the dissolution process of Co2Al5, which was composed of the heat of solution of the Co2Al9 in the liquid Al (74.6 kJ/mol) and the activation energy for the dissolution rate constant (61.5 kJ/mol). Similarly, the stable thickness of the (Co,Cr,Mo)2Al9 layer after the initial period indicated the equilibrium between its formation and dissolution. For this complex IMC ((Co,Cr,Mo)2Al9), the parameters for the quantitative analysis [10] are not available. However, it can still be concluded that, after the thickness of the intermetallic layer stabilized, the thickness loss of the CCM alloy in the liquid Al was controlled by the dissolution of the (Co,Cr,Mo)2Al9 layer, and the corresponding apparent activation energy (168.26 kJ/mol) was mainly the activation energy of the dissolution process of (Co,Cr,Mo)2Al9. 4.2. Effects of Cr and Mo on interfacial reactions Fig. 9 show the interface structure of the Co–Al system. By comparing Figs. 4a and 9, it was obvious that the alloying of Cr and Mo into Co matrix changed the interface structure greatly. Although
Co
Co2Al5
Fig. 10. Thickness losses of Co [9] and CCM alloy matrix at 1013 and 1043 K versus immersion time.
the IMC layer formed in both reaction systems, the (Co,Cr,Mo)2Al9 phase (about 10 lm at 1013 K (Fig. 3)) formed next to the CCM alloy in the CCM–Al system, instead of the Co2Al5 phase (about 2 lm at 1013 K (Fig. 9)) forming next to the pure Co in the Co–Al system. In addition, besides the IMC layer, there was a multiphase layer, which was composed of isolated (Cr,Mo)7Al45 particles and liquid Al, formed between the IMC layer and liquid Al. The formation of the IMC layer in the diffusion couple depends on many factors, including the chemical reaction, nucleation conditions, interdiffusion coefficient of IMC, and composition range of IMC [17–20]. In the Co–Al system, Co2Al5 is prone to form rather than Co2Al9. However, the alloying of Cr and Mo obviously changed the relative factors and made (Co,Cr,Mo)2Al9 form during the interfacial reaction instead of (Co,Cr,Mo)2Al5. And according to our previous study [10], due to the different properties (both formation and dissolution) of the corresponding IMC layers, the steady state thicknesses of the IMC layers would be different in different reaction system. Therefore, the different steady state thicknesses of (Co,Cr,Mo)2Al9 in the CCM alloy–Al system and Co2Al5 in the Co– Al system were attributed to their different properties of formation and dissolution. As for the formation of the multiphase layer in the CCM–Al reaction system, it can be attributed to the relatively lower solubility of Cr and Mo in liquid Al, as analyzed in Section 4.1. The thickness loss rates (Fig. 10) and corresponding apparent activation energies (Fig. 8) were compared between the Co–Al and CCM–Al systems. Though the solubilities of Cr and Mo in liquid Al are lower than that of Co (Table 2), the Cr and Mo alloying increased the thickness loss of the solid metal matrix compared with pure Co (Fig. 10). Moreover, the apparent activation energy of the thickness loss of solid matrix in the CCM–Al system (168.26 kJ/mol) was also a little higher than that in the Co–Al system (136.01 kJ/mol). Because the interfacial reactions in both the Co–Al and CCM–Al systems were considered to be mainly controlled by the dissolution process of the IMC layer, the different dissolution behavior of Co2Al5 and (Co,Cr,Mo)2Al9 were responsible for the difference in the thickness loss rates and corresponding apparent activation energies between the Co–Al and CCM–Al systems. 5. Conclusions
1 µm Fig. 9. Thin layer of Co2Al5 formed next to Co matrix after immersing pure Co in liquid Al at 1013 K for 600 s [10].
(1) The Co–Cr–Mo alloy is homogeneously corroded by liquid Al and two intermediate layers form between the alloy matrix and liquid Al: (Co,Cr,Mo)2Al9 layer close to the alloy matrix and ‘‘(Cr,Mo)7Al45 + Al’’ layer close to the Al.
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(2) The thickness of the (Co,Cr,Mo)2Al9 layer is almost stable after the beginning stage of the immersion test. The thickness loss of the Co–Cr–Mo alloy follows a linear relationship with the immersion time, and the apparent activation energy of the thickness loss was calculated to be 168.26 kJ/ mol. (3) The mechanisms of the interface reactions are as follows:
Co—Cr—Mo alloy þ Al ðdiffused through the ðCo; Cr; MoÞ2 Al9 layerÞ ! ðCo; Cr; MoÞ2 Al9 ; ðCo; Cr; MoÞ2 Al9 þ Al ðliquidÞ ! Co ðin liquid AlÞ þ ðCr; MoÞ7 Al45 ; ðCr; MoÞ7 Al45 þ Al ðliquidÞ ! Cr ðin liquid AlÞ þ Mo ðin liquid AlÞ þ liquid Al: (4) Compared with pure Co, the alloying of Cr and Mo changes the solid–liquid interface structure, but the thickness loss of the solid metal is still controlled by the dissolution of an intermetallic layer.
Acknowledgements This study was mainly supported by a Regional Innovation Cluster Program 2010 from the Ministry of Education, Culture, Sports, Science and Technology of Japan. The authors would like to express their appreciation to Mr. Issei Narita for his assistance with EPMA observation, and Mr. Shun Ito for his assistance with TEM work. References [1] J.R. Davis, ASM Specialty Handbook: Nickel, Cobalt, and Their Alloys, International ASM, 2000. p. 362.
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