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Mechanical behavior of coated T91 steel in contact with lead–bismuth liquid alloy at 300 °C
Surface & Coatings Technology 205 (2011) 4521–4527
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Surface & Coatings Technology j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / s u r f c o a t
Mechanical behavior of coated T91 steel in contact with lead–bismuth liquid alloy at 300 °C I. Proriol Serre a,⁎, I. Diop b, N. David b, M. Vilasi b, J.-B. Vogt a a b
Unité Matériaux et Transformations, UMR CNRS/ENSCL/USTL 8207, Métallurgie Physique et Génie des Matériaux, Université de Lille 1, Bat C6, F-59655 Villeneuve d'Ascq, France Institut Jean Lamour, UMR 7198, Faculté des Sciences, BP 70239, 54506 Vandoeuvre-Les-Nancy Cedex, France
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Article history: Received 29 October 2010 Accepted in revised form 24 March 2011 Available online 29 March 2011 Keywords: Pack cementation process Small Punch Test Fracture surface analysis Liquid metal accelerated damage
a b s t r a c t Different coatings, deposited by pack cementation process, were developed to protect the T91 steel against dissolution by Lead Bismuth Eutectic (LBE): iron aluminide, iron boride and an iron solid solution enriched in chromium and covered by carbide. The mechanical behavior of the T91 steel with the different coatings is studied in air and in LBE at 300 °C by the use of the Small Punch Test (SPT). Though the coatings are brittle, they do not affect the mechanical strength of the substrate in air, except the iron boride one. In LBE, the most critical situation is found when fracture of the interface between the T91 steel and the coating occurs. Then, the coating provides the set of conditions to trigger liquid metal accelerated damage. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Heavy Liquid Metals (HLM) such as the Pb–Bi eutectic (LBE) alloy are envisioned to be used for heat transfer applications or spallation target, in Accelerator Driven Systems (ADS) or in Generation IV nuclear concepts. Therefore the compatibility of structural materials with liquid metals must be investigated: corrosion resistance, liquid metal embrittlement and liquid metal accelerated damage prevention. Liquid Metal Embrittlement refers to a loss of ductility of an otherwise ductile material when stressed in contact with a liquid metal [1,2]. In this case, the liquid metal induces a ductile to brittle transition in the mechanical behavior of the material. In some cases, the liquid metal provokes an evolution of the mechanical behavior without any change in the characteristic of the fracture (ductile or brittle): decreasing of the fatigue life in liquid metal in comparison with that observed in air [3], and localization of the plastic strain by liquid metal [4]. These last cases refer to as Liquid Metal Accelerated Damage. These last years, the corrosive and mechanical behavior of the T91 steel (Z10CrMoNbV9-1) which is a material of prime interest for the nuclear systems using HLM, was studied extensively [5,6]. The T91 steel is a modified 9Cr1Mo steel developed to increase the mechanical strength by addition of Nb and V. This martensitic steel was chosen for its good resistance to creep and to swelling under irradiation. But, it was observed that the behavior of the T91 steel is strongly affected by the presence of the LBE [3,6–10], particularly according to the
⁎ Corresponding author. Tel.: +33 3 20 43 66 06; fax: +33 3 20 43 40 40. E-mail address: [email protected] (I. Proriol Serre). 0257-8972/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2011.03.089
chemical properties of LBE. Indeed, oxygen concentration and temperature of LBE control the mechanism of degradation, namely: i) oxidation when the oxygen concentration is high enough to permit the magnetite formation according to thermodynamic equilibrium and ii) dissolution of the metal substrate when the oxygen concentration is too low [11]. Dissolution as a corrosion phenomenon is very critical because first it corresponds to a loss of matter. Secondly, the dissolution of the T91 steel occurs preferentially at grain boundaries resulting in surface roughness and in nucleation of surface defects which act localizing the mechanical damage of the T91 steel in the presence of LBE [12]. At higher oxygen concentrations, oxide layer is formed [11] and decreases the fatal impact of LBE on the mechanical properties of the T91 steel [12]. Generally, the effect of liquid metal on the mechanical properties of materials is indeed connected to a direct contact (without oxide layer) between the material and the liquid metal [1,2,4,7,13]. So the formation of an oxide layer on the surface of the T91 steel in contact with LBE would appear as a means to limit not only the corrosion of the steel by the metal liquid but also the decrease of mechanical properties more particularly the fatigue resistance [12]. But, the idea to limit the interactions between the liquid metal and the surface of the materials of nuclear reactor components by natural oxidation thanks to dissolved oxygen in LBE cannot be foreseen because of the heterogeneities in temperature and oxygen concentration in LBE bath. These heterogeneities generate instabilities in oxide layer formation and properties and so do not allow consider oxide layer as a protective layer. The protection of T91 steel against corrosion by means of coatings has therefore been a subject of research. Aluminized coatings have been considered for protection of T91 steel against corrosion in static and flowing LBE. They appeared efficient in static LBE up to 500 °C even for low oxygen concentration
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and long duration [14]. Performances can be increased if the GESA process that uses pulsed electron beams is employed [15]. The performances of the aluminized coatings to protect the T91 steel from the corrosion by LBE have been demonstrated; then, in this paper, the mechanical behavior of aluminized coatings obtained by pack cementation is studied. Furthermore, another way was investigated: the synthesis of coatings for which the superficial layer is constituted by a ceramic material such as carbide or boride. The goal of the present work is to study the mechanical behavior in LBE of the T91 steel protected by the different coatings. Small Punch Tests (SPT) [16] were performed at 300 °C in air and in LBE to evaluate the behavior of the different coatings and the influence of LBE on the mechanical damage of the materials. SPT is used because it appeared drastic to evidence the effects of liquid metal on the mechanical properties of materials: liquid metal embrittlement or accelerated damage by liquid metal [16]. The fracture surfaces of the T91 steel and of the coatings were observed after test by scanning electron microscopy to study the fracture mode. 2. Materials and experimental procedure
phase composition [17,18], chromizing–aluminizing of Ni and Fe-base alloys, or chromizing–siliconizing of steels are successful [19]. 2.2. Materials In the present study, four different coatings have been synthesized on T91 steel samples. Most of them are constituted by a stacking of intermetallic layers consisting respectively from the surface to the substrate by the following succession (Fig. 1): – a superficial M23C6 (M = Cr + Fe) compound covering a thick layer of Cr1 − yFey (0 b yb weight %) solid solution (Fig. 1a); – a superficial layer of (Fe,Cr)2B compound over a (Fe,Cr,B) one (Fig. 1b); – a superficial FeAl3 thin layer covering a thick Fe2Al5 one under which a very thin FeAl layer grows (Fig. 1c). The last one is a single layer of FeAl (Fig. 1d). The main features of the process and the coating composition are reported in the Table 1. The results of thermodynamic calculations lead to the use of:
2.1. The pack cementation process Among the CVD (Chemical Vapor Deposition) coating techniques, pack cementation is probably the most competitive. In the cementation process, the part to be coated is heated in a powder mixture comprising an inert material (usually Al2O3), the powder of the element to be deposited (Al, Cr or Si for example), and a halide salt activator. Before heating, the silica or metallic crucible containing all the previous powders (the pack) is evacuated. At the process temperature and under low pressure (Ptotal = 10− 1 Pa), a volatile metal halide forms and diffuses in the gaseous phase and reacts with the substrate to form a coating, which growth is governed by solid diffusion. The pack cementation process allows the coating of small parts uniformly. With thermodynamic calculations of the gaseous
– CrCl3 rather than the classical NH4 Cl for chromizing and aluminizing in order to avoid the formation of H2 and HCl, – KBF3 for forming selectively boron halides during boriding; – 950 °C during chromizing in order to optimize the Cr coating reaction rather than the loss of iron from the T91 substrate through the volatile iron halide formation. The superficial layer of the two first coatings is a ceramic material which is very stable in temperature, but sensitive to oxidation in presence of oxygen. Indeed, during isothermal annealing in air at 815 °C, the iron solid solution enriched in chromium and covered by a M23C6 carbide coating forms a protective Cr2O3 layer with a slow growing rate during 50 h. For iron boride coating, a mix (Fe,Cr)2O3
Fig. 1. SEM micrographs (SE) of cross sections of the T91 steel covered by the different studied coatings.
I. Proriol Serre et al. / Surface & Coatings Technology 205 (2011) 4521–4527 Table 1 Main features of the coating process parameters. Coating-type
Master alloy powder
Halide salt activator
Coating temperature (°C)
Duration (hour)
M23C6 + Cr1 − xFex FeAl3 + Fe2Al5 + FeAl FeAl (Fe,Cr)B + (Fe,Cr)2B
Fe0.3Cr0.7 Pure Al Fe2Al5 Pure B
CrCl3 CrCl3 CrCl3 KBF4
980 650 650 950
4 1 1 2
oxide appears after 4 h. For the last two coatings, the presence of aluminum allows the spontaneous formation of an alumina protective layer in low oxygen environment. The surface of the T91 specimen which constitutes the substrate was mechanical and electrochemical polished before the coatings formation. Before coating the T91 steel, the standard heat treatment normalizing at 1050 °C and air quenching followed by a tempering at 750 °C for 1 h is applied. The microstructure consists of a tempered martensite with an average prior austenite grain size of 20 μm. The formation temperature of the aluminum based coating (650 °C) is lower than the transformation temperature of the tempered T91 steel. In this case, no heat treatment was performed after pack cementation deposition. For the other two coatings, because pack cementation involves high temperature (950 °C and 980 °C), the T91 substrate undergoes phase transformation. Therefore, the coated T91 was subjected to the standard heat treatment after pack cementation process.
2.3. Small Punch Test setup and the test conditions In SPT, specimens in the form of disk are punched in their middle by a ball. The SPT setup used in the study is a non standard one specially designed for testing materials in liquid metals at high temperature [16]. It consists of a disk specimen holder, a pushing rod and a ball. The specimen holder includes a lower die and an upper die which is also used as the tank for the liquid metal. The load is transferred onto the specimen by the means of pushing a 2.5 mm diameter tungsten carbide ball in contact with the lower surface of the disk specimen. In this way, the puncher is under the specimen. The upper surface of the latter, so the coating deposited on the T91 steel, is in contact with the liquid metal and is submitted to tensile loading. SPT were performed using an electro mechanical machine with a controlled cross-head speed of 0.5 mm/min. Load–cross-head dis-
placement curves were recorded during the tests. Additional details can be found in [16]. The tests were performed at 300 °C in air or in LBE without any control of the oxygen. The specimens are in the form of disk 8.9 mm in diameter and 480–490 μm in thickness. The coatings were deposited by pack cementation process on both surfaces of the disks. After coating (and the heat treatment for the coatings with a ceramic superficial layer), the disks were directly tested by SPT. SPT on T91 steel without coating were also performed in air and in LBE, to evaluate the effect of the coatings on the mechanical properties of the material. 3. Mechanical characterization and discussion 3.1. Mechanical behavior of the coatings with a ceramic superficial layer The load–displacement curves (Fig. 2), corresponding to the T91 steel coated by iron boride or by an iron solid solution enriched in chromium and covered by a M23C6 carbide deformed in air and in LBE exhibit a typical shape of ductile materials. They suggest that these materials are not sensitive to the presence of LBE. But the mechanical resistance of the iron boride coated T91 steel is reduced in comparison with the one of the T91 steel, in air and in LBE. The fracture mode of the materials (substrate and coatings) is not affected by the presence of LBE. So, in air as in LBE, the main cracks are circular with sometimes radial cracks (Figs. 3a and 4a); the fracture surface of the substrate is ductile (Figs. 3c and 4c). In the case of the iron boride coating, the fractographic analyses exhibited a brittle fracture for the superficial layer composed of (Fe,Cr)2B compound, and a ductile fracture surface for the (Fe,Cr,B) layer (Fig. 3b and d). For the second coating, the superficial M23C6 (M = Cr + Fe) compound is brittle (Fig. 4b and d) while ductile fracture was observed for layer of Cr1 − yFey solid solution (Fig. 4b). The intrinsic behavior of the superficial layer of the coatings is brittle and the internal layer is ductile whatever the environment, air or LBE. The differences in behavior in air between the different layers of the coatings agree with the ductility of the compounds. But the propagation in LBE of a brittle crack in the superficial surface does not induce a modification of the fracture mode of the material. 3.2. The mechanical behavior of the aluminum based coatings The load–displacement curves (Fig. 5) have shown that the T91 steel coated by Al-based coatings is sensitive to the presence of LBE. 2000
2000
Coating : iron boride
Coating : iron solid solution enriched in chromium and covered by a carbide
1500
Load (N)
Load (N)
1500
1000
1000
500
500
0
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coated T91 - air coated T91 - LBE T91 - air T91 - LBE
coated T91 - air coated T91 - LBE T91 - air T91 - LBE 0
0.5
1
Displacement (mm)
1.5
2
0
0
0.5
1
Displacement (mm)
Fig. 2. Load–displacement curves of the coated T91 steel tested either in air or in LBE.
1.5
2
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Fig. 3. Fracture surfaces of the iron boride coated T91 steel: top view of the sample fractured in LBE with the main circular crack and radial cracks (a), ductile fracture of the internal layer and brittle fracture of the external layer of the coating after fracture in air (b) and in LBE (d), ductile fracture of the T91 steel after test in LBE (c).
Fig. 4. Fracture surfaces of T91 steel coated with an iron solid solution enriched in chromium and covered by a M23C6 carbide: top view of the sample fractured in LBE with the main circular crack (a), ductile fracture of the internal layer and brittle fracture of the superficial layer of the coating after fracture in air (b) and in LBE (d), ductile fracture of the T91 steel after test in LBE (c).
I. Proriol Serre et al. / Surface & Coatings Technology 205 (2011) 4521–4527
2000
2000 Fe2Al5 coating
a FeAl layer
1500
Load (N)
Load (N)
1500
1000
500
0 0
4525
500
coated T91 - air coated T91 -- LBE T91 - air T91 - LBE 0.5
1
1.5
1000
2
Displacement (mm)
0 0
coated T91 - air coated T91 - LBE T91 - air T91 - LBE 0.5
1
1.5
2
Displacement (mm)
Fig. 5. Load–displacement curves of the T91 steel with Al-based coatings.
The curves of the tests performed in air exhibit the typical aspect observed for ductile materials. In LBE, the curves have the same shape as in air. However, an earlier fracture of coated samples tested in LBE occurred with a lower load and a lower displacement at fracture smaller in LBE than in air. The observations of the fracture surfaces for both Al-based coatings are similar. The main crack of the fractured specimen, in air (Fig. 6a and c) as in LBE (Fig. 6b and d), is circular. However, in LBE, the circular crack is accompanied with radial cracks, especially fine in the case of the FeAl coating. The SEM analysis of the fracture surfaces shows that both coatings are brittle in air (Fig. 7a and c) and in LBE (Fig. 7b and d).
In air as in LBE, cracking occurred at the interface between the coatings (Fe2Al5 and FeAl) and the T91 substrate (Fig. 7). This crack may result from a microstructural and chemical discontinuities at the interface coating/substrate. In the case of Fe2Al5 coating, the phenomena could be emphasized by the large difference in microhardness between the coating (900 Hv50 with the presence of cracks at the corner of the indentation) and the T91 steel (between 255 and 295 Hv50) and so the difference in elastic and plastic deformation capacities. During SPT, this difference in the mechanical behavior induces shear stresses at the interface coating/substrate. The FeAl (B2 cubic structural-type) coating was developed because it is more ductile than the Fe2Al5 (orthorhombic) coating in order to reduce the
Fig. 6. Top views of the T91 steel samples with the Al-based coatings: with Fe2Al5 coating after test in air (a) and in LBE (b), with FeAl coating after test in air (c) and in LBE (d).
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Fig. 7. Fracture surfaces of the Al-based coatings: Fe2Al5 coating after test in air (a) and in LBE (b), FeAl coating after test in air (c) and in LBE (d).
difference in behavior between the substrate and the coating. But in spite of a smaller hardness and a smaller thickness of the coating, a fracture between the T91 steel and the FeAl coating is still observed. In air as well as in LBE, the T91 steel fracture surface is ductile (Fig. 8). In LBE, no brittle fracture surface of the substrate was observed. The fracture surface of the circular cracks is ductile (Fig. 8a) in air and in LBE. But, after test in LBE, the fracture surface of the radial cracks presents shear zones (Fig. 8b). To understand the evolution of the macroscopic behavior of the Albased coated T91 steel in air and in LBE, the results are compared with those of the T91 steel without coating (Fig. 5). In air, the macroscopic behavior of the T91 specimen with and without coating is similar in terms of maximal load and displacement at fracture. So, the brittle behavior of the coatings does not involve a large evolution of the mechanical properties and fracture mode of the T91 steel. Because of the brittleness of the Al-based coatings, they do not seem participat-
ing in the mechanical resistance of the coated T91 steel. To verify this point, the fracture energy (derived from the area under the load versus displacement curve, noted Jf) per thickness of the T91 substrate was calculated. The fracture energy does not vary from one specimen to another one (Table 2) in air. So the mechanical resistance of the T91 steel covered by Al-based coating in air during the Small Punch Test is only due to the resistance of the substrate. The uncoated T91 steel, in its standard heat treatment, exhibits the same behavior in air and in LBE [16,20]. To find if the decrease in the mechanical resistance of the Al-based coated T91 in LBE results or not from the property of the T91 substrate, some Al-based coated T91 specimens were polished to remove the coating, and, then tested in LBE. The fracture energy per thickness (Table 2) after test in LBE is similar for the as-received T91 steel and the de-coated one. Therefore, the decrease in mechanical resistance of Al-based coated T91 results from a combined effect of the presence of LBE and of the coating.
Fig. 8. Fracture surfaces of the T91 steel coated with Fe2Al5 and tested in LBE: circular crack (a) and radial cracks (b).
I. Proriol Serre et al. / Surface & Coatings Technology 205 (2011) 4521–4527 Table 2 Fracture energy per thickness (J/mm) of the T91 steel, the T91 steel covered by Al-based coating and the T91 substrate of the FeAl coated T91. Materials
In air
In LBE
T91 steel Fe2Al5 FeAl T91 substrate of the FeAl coating
3.29 ± 0.22 3.30 ± 0.42 3.70 ± 0.09 –
3.53 ± 0.31 1.93 ± 0.16 2.58 ± 0.05 3.44 ± 0.21
3.3. Discussion The coatings considering in this study have little influence on the mechanical behavior of the coated T91 steel in air, although for the greater part, they are brittle. The fracture cracking of the coatings does not affect the mechanical resistance of the material. The iron boride coating was the only one which provokes a decrease of the plastic response and of the mechanical resistance of the T91 steel. Electron probe micro analyses performed in the vicinity of the interface suggested diffusion of boron and formation of boron precipitates in the T91 steel substrate during the processing of the coating. The changes occurring on the T91 steel substrate promote an evolution of the mechanical resistance of the T91 steel in air, and so of the coated T91 steel. The influence of liquid metal on mechanical properties of materials depends on different parameters: the liquid metal/solid metal couple, the yield strength of the material, stress concentrations, a direct contact between the material and the liquid metal [1,2,4,7,13,20]. Thus, the harder materials are generally more severely influenced by liquid metal [7,20]. During small punch tests, for ductile materials, the first stages of the curves are related with an elastic bending of the specimen followed by a plastic bending. With further displacement of the ball, a stretching of the membrane occurs: the deformation of the sample is not caused by bending but by stretching around the contact area between the ball and the sample. Then, a roughness at the surface of the sample and microvoids in the bulk of the material due to plastic deformation are generated. Finally, the main circular crack forms by coalescence of the microvoids and propagates. Because the upper surface is the most stressed, brittle fracture of the coatings occurs very early in the first moment of the test. However, the cracks stop at the interface and do not propagate in the substrate. During SPT in liquid metal, plasticity-induced roughness at the surface of the sample contributes to form fresh surfaces (without oxide) of the material in contact of liquid metal, giving rise to a possible liquid metal damage. The behavior of the sample results then from a competition between the bulk damage due to plastic deformation and the surface damage due to the interactions with the liquid metal. In air, the studied coatings have little influence on the mechanical behavior of the material. In the presence of LBE, though opened cracks would permit direct contact of LBE and steel, neither a brittle behavior of the substrate nor a decrease in the mechanical resistance of the steel is observed. Because the overcoming the interface coating/ substrate by a crack requires a stress intensity factor at crack tip close to the critical value, one would expect that the longer is the crack, i.e. the coating thickness, in addition with LBE at the very tip, the most detrimental is the response. This is however not observed as suggests the response of the T91 steel coated by a brittle FeAl layer of 7 μm and that of T91 steel coated by a 40 μm thickness iron boride coating. More important is the cohesion between the coating and the substrate which seems to be a key factor for an effect of LBE on the mechanical properties of the coated T91 steel. In the case of Al-based coatings, the fracture between the substrate and the coating promotes a direct contact (without native oxide of the T91 steel) between the liquid metal and the steel and therefore a possible effect of LBE. This direct contact combined with stress concentrations near the surface could
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explain an evolution in the plastic deformation and the observation of shear fractures. Conversely, the two other coatings seem efficient for a resistance of T91 steel in LBE protecting, since the coating avoids the contact of the T91 substrate with LBE, the coated T91 steel behaves in LBE as in air and damages by pure nucleation and coalescence of microvoids in bulk. 4. Conclusion The mechanical behavior in air and in LBE of T91 steel coated by pack cementation process, were studied: iron aluminide, iron boride and an iron solid solution enriched in chromium and covered by a chromium carbide. In air, the coatings have little influence on the mechanical behavior of the coated T91 steel, although the greater part of them is brittle. The mechanical resistance of the iron-boride coated T91 steel is reduced in comparison with that of the T91 steel without coating. This is due to the diffusion process, which induces a modification of the T91 substrate during the pack cementation. The fracture of the interface between the steel and the Al-based coatings promotes an effect of LBE on the plasticity deformation and on the damage of the T91 substrate. Once the crack propagates through the coating layer and spreads along the steel/coating interface, LBE comes into contact with a large part of the T91 substrate. This condition of direct contact between the liquid metal and the material is necessary to observe a LBE effect. In the case of the iron boride coating and the iron–chromium solid solution covered by a M23C6 carbide coating, the mechanical damage is not influenced by the LBE. Indeed, in the first steps of the test, cracks propagate only through the external part of the coating and stop because of the ductility of the internal part of the coatings. The damage of this part of coating and of the T91 substrate is also due to large plastic deformation which promotes formation of voids in the bulk of the specimen as for tests performed in air. Acknowledgement This work has been supported by the French CNRS GdR GEDEPEON as well as by the European project DEMETRA part of the 6th FP EUROTRANS program (contract no. FI6 W-CT-2004-516520). References [1] [2] [3] [4] [5]
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Contents lists available at ScienceDirect
Surface & Coatings Technology j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / s u r f c o a t
Mechanical behavior of coated T91 steel in contact with lead–bismuth liquid alloy at 300 °C I. Proriol Serre a,⁎, I. Diop b, N. David b, M. Vilasi b, J.-B. Vogt a a b
Unité Matériaux et Transformations, UMR CNRS/ENSCL/USTL 8207, Métallurgie Physique et Génie des Matériaux, Université de Lille 1, Bat C6, F-59655 Villeneuve d'Ascq, France Institut Jean Lamour, UMR 7198, Faculté des Sciences, BP 70239, 54506 Vandoeuvre-Les-Nancy Cedex, France
a r t i c l e
i n f o
Article history: Received 29 October 2010 Accepted in revised form 24 March 2011 Available online 29 March 2011 Keywords: Pack cementation process Small Punch Test Fracture surface analysis Liquid metal accelerated damage
a b s t r a c t Different coatings, deposited by pack cementation process, were developed to protect the T91 steel against dissolution by Lead Bismuth Eutectic (LBE): iron aluminide, iron boride and an iron solid solution enriched in chromium and covered by carbide. The mechanical behavior of the T91 steel with the different coatings is studied in air and in LBE at 300 °C by the use of the Small Punch Test (SPT). Though the coatings are brittle, they do not affect the mechanical strength of the substrate in air, except the iron boride one. In LBE, the most critical situation is found when fracture of the interface between the T91 steel and the coating occurs. Then, the coating provides the set of conditions to trigger liquid metal accelerated damage. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Heavy Liquid Metals (HLM) such as the Pb–Bi eutectic (LBE) alloy are envisioned to be used for heat transfer applications or spallation target, in Accelerator Driven Systems (ADS) or in Generation IV nuclear concepts. Therefore the compatibility of structural materials with liquid metals must be investigated: corrosion resistance, liquid metal embrittlement and liquid metal accelerated damage prevention. Liquid Metal Embrittlement refers to a loss of ductility of an otherwise ductile material when stressed in contact with a liquid metal [1,2]. In this case, the liquid metal induces a ductile to brittle transition in the mechanical behavior of the material. In some cases, the liquid metal provokes an evolution of the mechanical behavior without any change in the characteristic of the fracture (ductile or brittle): decreasing of the fatigue life in liquid metal in comparison with that observed in air [3], and localization of the plastic strain by liquid metal [4]. These last cases refer to as Liquid Metal Accelerated Damage. These last years, the corrosive and mechanical behavior of the T91 steel (Z10CrMoNbV9-1) which is a material of prime interest for the nuclear systems using HLM, was studied extensively [5,6]. The T91 steel is a modified 9Cr1Mo steel developed to increase the mechanical strength by addition of Nb and V. This martensitic steel was chosen for its good resistance to creep and to swelling under irradiation. But, it was observed that the behavior of the T91 steel is strongly affected by the presence of the LBE [3,6–10], particularly according to the
⁎ Corresponding author. Tel.: +33 3 20 43 66 06; fax: +33 3 20 43 40 40. E-mail address: [email protected] (I. Proriol Serre). 0257-8972/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2011.03.089
chemical properties of LBE. Indeed, oxygen concentration and temperature of LBE control the mechanism of degradation, namely: i) oxidation when the oxygen concentration is high enough to permit the magnetite formation according to thermodynamic equilibrium and ii) dissolution of the metal substrate when the oxygen concentration is too low [11]. Dissolution as a corrosion phenomenon is very critical because first it corresponds to a loss of matter. Secondly, the dissolution of the T91 steel occurs preferentially at grain boundaries resulting in surface roughness and in nucleation of surface defects which act localizing the mechanical damage of the T91 steel in the presence of LBE [12]. At higher oxygen concentrations, oxide layer is formed [11] and decreases the fatal impact of LBE on the mechanical properties of the T91 steel [12]. Generally, the effect of liquid metal on the mechanical properties of materials is indeed connected to a direct contact (without oxide layer) between the material and the liquid metal [1,2,4,7,13]. So the formation of an oxide layer on the surface of the T91 steel in contact with LBE would appear as a means to limit not only the corrosion of the steel by the metal liquid but also the decrease of mechanical properties more particularly the fatigue resistance [12]. But, the idea to limit the interactions between the liquid metal and the surface of the materials of nuclear reactor components by natural oxidation thanks to dissolved oxygen in LBE cannot be foreseen because of the heterogeneities in temperature and oxygen concentration in LBE bath. These heterogeneities generate instabilities in oxide layer formation and properties and so do not allow consider oxide layer as a protective layer. The protection of T91 steel against corrosion by means of coatings has therefore been a subject of research. Aluminized coatings have been considered for protection of T91 steel against corrosion in static and flowing LBE. They appeared efficient in static LBE up to 500 °C even for low oxygen concentration
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and long duration [14]. Performances can be increased if the GESA process that uses pulsed electron beams is employed [15]. The performances of the aluminized coatings to protect the T91 steel from the corrosion by LBE have been demonstrated; then, in this paper, the mechanical behavior of aluminized coatings obtained by pack cementation is studied. Furthermore, another way was investigated: the synthesis of coatings for which the superficial layer is constituted by a ceramic material such as carbide or boride. The goal of the present work is to study the mechanical behavior in LBE of the T91 steel protected by the different coatings. Small Punch Tests (SPT) [16] were performed at 300 °C in air and in LBE to evaluate the behavior of the different coatings and the influence of LBE on the mechanical damage of the materials. SPT is used because it appeared drastic to evidence the effects of liquid metal on the mechanical properties of materials: liquid metal embrittlement or accelerated damage by liquid metal [16]. The fracture surfaces of the T91 steel and of the coatings were observed after test by scanning electron microscopy to study the fracture mode. 2. Materials and experimental procedure
phase composition [17,18], chromizing–aluminizing of Ni and Fe-base alloys, or chromizing–siliconizing of steels are successful [19]. 2.2. Materials In the present study, four different coatings have been synthesized on T91 steel samples. Most of them are constituted by a stacking of intermetallic layers consisting respectively from the surface to the substrate by the following succession (Fig. 1): – a superficial M23C6 (M = Cr + Fe) compound covering a thick layer of Cr1 − yFey (0 b yb weight %) solid solution (Fig. 1a); – a superficial layer of (Fe,Cr)2B compound over a (Fe,Cr,B) one (Fig. 1b); – a superficial FeAl3 thin layer covering a thick Fe2Al5 one under which a very thin FeAl layer grows (Fig. 1c). The last one is a single layer of FeAl (Fig. 1d). The main features of the process and the coating composition are reported in the Table 1. The results of thermodynamic calculations lead to the use of:
2.1. The pack cementation process Among the CVD (Chemical Vapor Deposition) coating techniques, pack cementation is probably the most competitive. In the cementation process, the part to be coated is heated in a powder mixture comprising an inert material (usually Al2O3), the powder of the element to be deposited (Al, Cr or Si for example), and a halide salt activator. Before heating, the silica or metallic crucible containing all the previous powders (the pack) is evacuated. At the process temperature and under low pressure (Ptotal = 10− 1 Pa), a volatile metal halide forms and diffuses in the gaseous phase and reacts with the substrate to form a coating, which growth is governed by solid diffusion. The pack cementation process allows the coating of small parts uniformly. With thermodynamic calculations of the gaseous
– CrCl3 rather than the classical NH4 Cl for chromizing and aluminizing in order to avoid the formation of H2 and HCl, – KBF3 for forming selectively boron halides during boriding; – 950 °C during chromizing in order to optimize the Cr coating reaction rather than the loss of iron from the T91 substrate through the volatile iron halide formation. The superficial layer of the two first coatings is a ceramic material which is very stable in temperature, but sensitive to oxidation in presence of oxygen. Indeed, during isothermal annealing in air at 815 °C, the iron solid solution enriched in chromium and covered by a M23C6 carbide coating forms a protective Cr2O3 layer with a slow growing rate during 50 h. For iron boride coating, a mix (Fe,Cr)2O3
Fig. 1. SEM micrographs (SE) of cross sections of the T91 steel covered by the different studied coatings.
I. Proriol Serre et al. / Surface & Coatings Technology 205 (2011) 4521–4527 Table 1 Main features of the coating process parameters. Coating-type
Master alloy powder
Halide salt activator
Coating temperature (°C)
Duration (hour)
M23C6 + Cr1 − xFex FeAl3 + Fe2Al5 + FeAl FeAl (Fe,Cr)B + (Fe,Cr)2B
Fe0.3Cr0.7 Pure Al Fe2Al5 Pure B
CrCl3 CrCl3 CrCl3 KBF4
980 650 650 950
4 1 1 2
oxide appears after 4 h. For the last two coatings, the presence of aluminum allows the spontaneous formation of an alumina protective layer in low oxygen environment. The surface of the T91 specimen which constitutes the substrate was mechanical and electrochemical polished before the coatings formation. Before coating the T91 steel, the standard heat treatment normalizing at 1050 °C and air quenching followed by a tempering at 750 °C for 1 h is applied. The microstructure consists of a tempered martensite with an average prior austenite grain size of 20 μm. The formation temperature of the aluminum based coating (650 °C) is lower than the transformation temperature of the tempered T91 steel. In this case, no heat treatment was performed after pack cementation deposition. For the other two coatings, because pack cementation involves high temperature (950 °C and 980 °C), the T91 substrate undergoes phase transformation. Therefore, the coated T91 was subjected to the standard heat treatment after pack cementation process.
2.3. Small Punch Test setup and the test conditions In SPT, specimens in the form of disk are punched in their middle by a ball. The SPT setup used in the study is a non standard one specially designed for testing materials in liquid metals at high temperature [16]. It consists of a disk specimen holder, a pushing rod and a ball. The specimen holder includes a lower die and an upper die which is also used as the tank for the liquid metal. The load is transferred onto the specimen by the means of pushing a 2.5 mm diameter tungsten carbide ball in contact with the lower surface of the disk specimen. In this way, the puncher is under the specimen. The upper surface of the latter, so the coating deposited on the T91 steel, is in contact with the liquid metal and is submitted to tensile loading. SPT were performed using an electro mechanical machine with a controlled cross-head speed of 0.5 mm/min. Load–cross-head dis-
placement curves were recorded during the tests. Additional details can be found in [16]. The tests were performed at 300 °C in air or in LBE without any control of the oxygen. The specimens are in the form of disk 8.9 mm in diameter and 480–490 μm in thickness. The coatings were deposited by pack cementation process on both surfaces of the disks. After coating (and the heat treatment for the coatings with a ceramic superficial layer), the disks were directly tested by SPT. SPT on T91 steel without coating were also performed in air and in LBE, to evaluate the effect of the coatings on the mechanical properties of the material. 3. Mechanical characterization and discussion 3.1. Mechanical behavior of the coatings with a ceramic superficial layer The load–displacement curves (Fig. 2), corresponding to the T91 steel coated by iron boride or by an iron solid solution enriched in chromium and covered by a M23C6 carbide deformed in air and in LBE exhibit a typical shape of ductile materials. They suggest that these materials are not sensitive to the presence of LBE. But the mechanical resistance of the iron boride coated T91 steel is reduced in comparison with the one of the T91 steel, in air and in LBE. The fracture mode of the materials (substrate and coatings) is not affected by the presence of LBE. So, in air as in LBE, the main cracks are circular with sometimes radial cracks (Figs. 3a and 4a); the fracture surface of the substrate is ductile (Figs. 3c and 4c). In the case of the iron boride coating, the fractographic analyses exhibited a brittle fracture for the superficial layer composed of (Fe,Cr)2B compound, and a ductile fracture surface for the (Fe,Cr,B) layer (Fig. 3b and d). For the second coating, the superficial M23C6 (M = Cr + Fe) compound is brittle (Fig. 4b and d) while ductile fracture was observed for layer of Cr1 − yFey solid solution (Fig. 4b). The intrinsic behavior of the superficial layer of the coatings is brittle and the internal layer is ductile whatever the environment, air or LBE. The differences in behavior in air between the different layers of the coatings agree with the ductility of the compounds. But the propagation in LBE of a brittle crack in the superficial surface does not induce a modification of the fracture mode of the material. 3.2. The mechanical behavior of the aluminum based coatings The load–displacement curves (Fig. 5) have shown that the T91 steel coated by Al-based coatings is sensitive to the presence of LBE. 2000
2000
Coating : iron boride
Coating : iron solid solution enriched in chromium and covered by a carbide
1500
Load (N)
Load (N)
1500
1000
1000
500
500
0
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coated T91 - air coated T91 - LBE T91 - air T91 - LBE
coated T91 - air coated T91 - LBE T91 - air T91 - LBE 0
0.5
1
Displacement (mm)
1.5
2
0
0
0.5
1
Displacement (mm)
Fig. 2. Load–displacement curves of the coated T91 steel tested either in air or in LBE.
1.5
2
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Fig. 3. Fracture surfaces of the iron boride coated T91 steel: top view of the sample fractured in LBE with the main circular crack and radial cracks (a), ductile fracture of the internal layer and brittle fracture of the external layer of the coating after fracture in air (b) and in LBE (d), ductile fracture of the T91 steel after test in LBE (c).
Fig. 4. Fracture surfaces of T91 steel coated with an iron solid solution enriched in chromium and covered by a M23C6 carbide: top view of the sample fractured in LBE with the main circular crack (a), ductile fracture of the internal layer and brittle fracture of the superficial layer of the coating after fracture in air (b) and in LBE (d), ductile fracture of the T91 steel after test in LBE (c).
I. Proriol Serre et al. / Surface & Coatings Technology 205 (2011) 4521–4527
2000
2000 Fe2Al5 coating
a FeAl layer
1500
Load (N)
Load (N)
1500
1000
500
0 0
4525
500
coated T91 - air coated T91 -- LBE T91 - air T91 - LBE 0.5
1
1.5
1000
2
Displacement (mm)
0 0
coated T91 - air coated T91 - LBE T91 - air T91 - LBE 0.5
1
1.5
2
Displacement (mm)
Fig. 5. Load–displacement curves of the T91 steel with Al-based coatings.
The curves of the tests performed in air exhibit the typical aspect observed for ductile materials. In LBE, the curves have the same shape as in air. However, an earlier fracture of coated samples tested in LBE occurred with a lower load and a lower displacement at fracture smaller in LBE than in air. The observations of the fracture surfaces for both Al-based coatings are similar. The main crack of the fractured specimen, in air (Fig. 6a and c) as in LBE (Fig. 6b and d), is circular. However, in LBE, the circular crack is accompanied with radial cracks, especially fine in the case of the FeAl coating. The SEM analysis of the fracture surfaces shows that both coatings are brittle in air (Fig. 7a and c) and in LBE (Fig. 7b and d).
In air as in LBE, cracking occurred at the interface between the coatings (Fe2Al5 and FeAl) and the T91 substrate (Fig. 7). This crack may result from a microstructural and chemical discontinuities at the interface coating/substrate. In the case of Fe2Al5 coating, the phenomena could be emphasized by the large difference in microhardness between the coating (900 Hv50 with the presence of cracks at the corner of the indentation) and the T91 steel (between 255 and 295 Hv50) and so the difference in elastic and plastic deformation capacities. During SPT, this difference in the mechanical behavior induces shear stresses at the interface coating/substrate. The FeAl (B2 cubic structural-type) coating was developed because it is more ductile than the Fe2Al5 (orthorhombic) coating in order to reduce the
Fig. 6. Top views of the T91 steel samples with the Al-based coatings: with Fe2Al5 coating after test in air (a) and in LBE (b), with FeAl coating after test in air (c) and in LBE (d).
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Fig. 7. Fracture surfaces of the Al-based coatings: Fe2Al5 coating after test in air (a) and in LBE (b), FeAl coating after test in air (c) and in LBE (d).
difference in behavior between the substrate and the coating. But in spite of a smaller hardness and a smaller thickness of the coating, a fracture between the T91 steel and the FeAl coating is still observed. In air as well as in LBE, the T91 steel fracture surface is ductile (Fig. 8). In LBE, no brittle fracture surface of the substrate was observed. The fracture surface of the circular cracks is ductile (Fig. 8a) in air and in LBE. But, after test in LBE, the fracture surface of the radial cracks presents shear zones (Fig. 8b). To understand the evolution of the macroscopic behavior of the Albased coated T91 steel in air and in LBE, the results are compared with those of the T91 steel without coating (Fig. 5). In air, the macroscopic behavior of the T91 specimen with and without coating is similar in terms of maximal load and displacement at fracture. So, the brittle behavior of the coatings does not involve a large evolution of the mechanical properties and fracture mode of the T91 steel. Because of the brittleness of the Al-based coatings, they do not seem participat-
ing in the mechanical resistance of the coated T91 steel. To verify this point, the fracture energy (derived from the area under the load versus displacement curve, noted Jf) per thickness of the T91 substrate was calculated. The fracture energy does not vary from one specimen to another one (Table 2) in air. So the mechanical resistance of the T91 steel covered by Al-based coating in air during the Small Punch Test is only due to the resistance of the substrate. The uncoated T91 steel, in its standard heat treatment, exhibits the same behavior in air and in LBE [16,20]. To find if the decrease in the mechanical resistance of the Al-based coated T91 in LBE results or not from the property of the T91 substrate, some Al-based coated T91 specimens were polished to remove the coating, and, then tested in LBE. The fracture energy per thickness (Table 2) after test in LBE is similar for the as-received T91 steel and the de-coated one. Therefore, the decrease in mechanical resistance of Al-based coated T91 results from a combined effect of the presence of LBE and of the coating.
Fig. 8. Fracture surfaces of the T91 steel coated with Fe2Al5 and tested in LBE: circular crack (a) and radial cracks (b).
I. Proriol Serre et al. / Surface & Coatings Technology 205 (2011) 4521–4527 Table 2 Fracture energy per thickness (J/mm) of the T91 steel, the T91 steel covered by Al-based coating and the T91 substrate of the FeAl coated T91. Materials
In air
In LBE
T91 steel Fe2Al5 FeAl T91 substrate of the FeAl coating
3.29 ± 0.22 3.30 ± 0.42 3.70 ± 0.09 –
3.53 ± 0.31 1.93 ± 0.16 2.58 ± 0.05 3.44 ± 0.21
3.3. Discussion The coatings considering in this study have little influence on the mechanical behavior of the coated T91 steel in air, although for the greater part, they are brittle. The fracture cracking of the coatings does not affect the mechanical resistance of the material. The iron boride coating was the only one which provokes a decrease of the plastic response and of the mechanical resistance of the T91 steel. Electron probe micro analyses performed in the vicinity of the interface suggested diffusion of boron and formation of boron precipitates in the T91 steel substrate during the processing of the coating. The changes occurring on the T91 steel substrate promote an evolution of the mechanical resistance of the T91 steel in air, and so of the coated T91 steel. The influence of liquid metal on mechanical properties of materials depends on different parameters: the liquid metal/solid metal couple, the yield strength of the material, stress concentrations, a direct contact between the material and the liquid metal [1,2,4,7,13,20]. Thus, the harder materials are generally more severely influenced by liquid metal [7,20]. During small punch tests, for ductile materials, the first stages of the curves are related with an elastic bending of the specimen followed by a plastic bending. With further displacement of the ball, a stretching of the membrane occurs: the deformation of the sample is not caused by bending but by stretching around the contact area between the ball and the sample. Then, a roughness at the surface of the sample and microvoids in the bulk of the material due to plastic deformation are generated. Finally, the main circular crack forms by coalescence of the microvoids and propagates. Because the upper surface is the most stressed, brittle fracture of the coatings occurs very early in the first moment of the test. However, the cracks stop at the interface and do not propagate in the substrate. During SPT in liquid metal, plasticity-induced roughness at the surface of the sample contributes to form fresh surfaces (without oxide) of the material in contact of liquid metal, giving rise to a possible liquid metal damage. The behavior of the sample results then from a competition between the bulk damage due to plastic deformation and the surface damage due to the interactions with the liquid metal. In air, the studied coatings have little influence on the mechanical behavior of the material. In the presence of LBE, though opened cracks would permit direct contact of LBE and steel, neither a brittle behavior of the substrate nor a decrease in the mechanical resistance of the steel is observed. Because the overcoming the interface coating/ substrate by a crack requires a stress intensity factor at crack tip close to the critical value, one would expect that the longer is the crack, i.e. the coating thickness, in addition with LBE at the very tip, the most detrimental is the response. This is however not observed as suggests the response of the T91 steel coated by a brittle FeAl layer of 7 μm and that of T91 steel coated by a 40 μm thickness iron boride coating. More important is the cohesion between the coating and the substrate which seems to be a key factor for an effect of LBE on the mechanical properties of the coated T91 steel. In the case of Al-based coatings, the fracture between the substrate and the coating promotes a direct contact (without native oxide of the T91 steel) between the liquid metal and the steel and therefore a possible effect of LBE. This direct contact combined with stress concentrations near the surface could
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explain an evolution in the plastic deformation and the observation of shear fractures. Conversely, the two other coatings seem efficient for a resistance of T91 steel in LBE protecting, since the coating avoids the contact of the T91 substrate with LBE, the coated T91 steel behaves in LBE as in air and damages by pure nucleation and coalescence of microvoids in bulk. 4. Conclusion The mechanical behavior in air and in LBE of T91 steel coated by pack cementation process, were studied: iron aluminide, iron boride and an iron solid solution enriched in chromium and covered by a chromium carbide. In air, the coatings have little influence on the mechanical behavior of the coated T91 steel, although the greater part of them is brittle. The mechanical resistance of the iron-boride coated T91 steel is reduced in comparison with that of the T91 steel without coating. This is due to the diffusion process, which induces a modification of the T91 substrate during the pack cementation. The fracture of the interface between the steel and the Al-based coatings promotes an effect of LBE on the plasticity deformation and on the damage of the T91 substrate. Once the crack propagates through the coating layer and spreads along the steel/coating interface, LBE comes into contact with a large part of the T91 substrate. This condition of direct contact between the liquid metal and the material is necessary to observe a LBE effect. In the case of the iron boride coating and the iron–chromium solid solution covered by a M23C6 carbide coating, the mechanical damage is not influenced by the LBE. Indeed, in the first steps of the test, cracks propagate only through the external part of the coating and stop because of the ductility of the internal part of the coatings. The damage of this part of coating and of the T91 substrate is also due to large plastic deformation which promotes formation of voids in the bulk of the specimen as for tests performed in air. Acknowledgement This work has been supported by the French CNRS GdR GEDEPEON as well as by the European project DEMETRA part of the 6th FP EUROTRANS program (contract no. FI6 W-CT-2004-516520). References [1] [2] [3] [4] [5]
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