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Al2O3–FeCrAl composites and functionally graded materials fabricated by reactive hot pressing
Composites: Part A 38 (2007) 615–620 www.elsevier.com/locate/compositesa
Al2O3–FeCrAl composites and functionally graded materials fabricated by reactive hot pressing J.Q. Li a
a,b,*
, W.A. Sun
a,b
, W.Q. Ao
a,b
, K.M. Gu
a,b
, P. Xiao
c
Department of Materials Science and Engineering, Shenzhen University, Shenzhen, Guangdong 518060, PR China b Shenzhen Key Laboratory of Special Functional Materials, Shenzhen 518060, PR China c Manchester Materials Science Centre, University of Manchester, Manchester M1 7HS, UK Received 20 February 2005; received in revised form 28 November 2005; accepted 14 February 2006
Abstract Al2O3–FeCrAl composites were fabricated by mixing Fe2O3, Al and Cr powders and then reactive hot pressing. The high temperature alloy FeCrAl was formed by the reaction of extra Al, Cr and the Fe reduced from Fe2O3. The Al2O3–FeCrAl composites with various Al2O3 fractions were successfully fabricated by the proper addition of extra Fe, Cr, Al or Al2O3 powders. A five-layer functionally graded material of YSZ–FeCrAl was fabricated using the Al2O3–FeCrAl composites with compositions of 25, 53.2 and 75 vol.% Al2O3 as interlayer. The results from XRD analysis, optical microscope observation and thermal cycling test show that the composites fabricated by this method consist of a-Al2O3 phase and (Fe, Cr, Al) solid solution. The a-Al2O3 grain formed by this in-situ reaction between Fe2O3 and Fe is ultrafine and uniform distribution. The three-point bending strength is 305.0 MPa for the composite with 53.2 vol.% Al2O3 prepared by the reactive hot pressing, about 20% higher than that of the composite with same composition prepared by ex situ hot pressing method (252.0 MPa). No cracking was found in the functionally graded materials after 10 thermal cycles up to 1000 C due to the better metal–ceramic bond, continuous in microstructure at interface of FGM and good oxidation resistance component FeCrAl alloy formed in the FGM. 2006 Elsevier Ltd. All rights reserved. Keywords: B. Fracture; B. Mechanical properties; B. Strength
1. Introduction The fast progress of modern high technology requires more and more new materials with various special properties or functions. Metal–ceramic composites and functionally graded materials (FGMs) have been developed for some critical structural applications, such as in the environment of super-high temperature, thermal cycling, great temperature gradient, wear and corrosion [1,2]. The ceramic side offers the resistances to heat, corrosion or wear, and the metal side provides mechanical strength and ther* Corresponding author. Address: Department of Materials Science and Engineering, Shenzhen University, Shenzhen, Guangdong 518060, PR China. Fax: +86 755 26536239. E-mail address: [email protected] (J.Q. Li).
1359-835X/$ - see front matter 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.compositesa.2006.02.012
mal conductivity. Metal–ceramic FGMs are introduced to eliminate and reduce the residual stresses which would cause cracking in the materials during cooling from processing temperature or during service because of the difference in the coefficient of thermal expansion (CTE) between the ceramics and metals. Many ceramic–metal FGMs have been fabricated and studied for various applications, such as Nb5Si3–Nb [3], SiC–Al 2124 [4], 3Y–TZP–SUS304 [5], ZrO2–NiAl [6], PSZ–Mo [7] and YSZ–NiCr [8]. For metal–ceramic composites and FGMs fabrication, in general, the ceramic particulates are prepared independently prior to composite fabrication. The particulates are introduced into the matrix via ingot casting or powder metallurgy (PM) processes, termed as ex situ metal–ceramic composites. It is generally known that agglomeration of ceramic particulates may occur during processing of
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ex situ metal–ceramic composites, which may weaken the composites. Moreover, possible interfacial chemical reactions between the reinforcement phases and the matrix may also degrade the mechanical properties of ex situ metal–ceramic composites [9]. In recent years, novel processing techniques based on the in situ production of reinforced ceramic particles have been developed. These techniques include exothermic dispersion (XD) [10], reactive hot pressing (RHP) [11], combustion-assisted cast [12] and direct reaction synthesis [13]. Among these, RHP process is attractive due to its simplicity and flexibility. Ultrafine ceramic particulates are formed in situ by the exothermic reaction between the element constituents under hot pressing conditions. The reduction in the particle size is generally known to exhibit a beneficial effect in improving the mechanical strength. In situ formed ceramic particulates exhibit thermodynamic compatibility at the metal–ceramic interface, resulting in a stronger metal–ceramic bond. Such unique properties make in situ metal–ceramic composites to possess excellent mechanical properties than their ex situ counterparts. Alumina and zirconia ceramics have high fracture toughness and strength and high resistance to wear and corrosion. They are important in engineering applications, such as gas turbines, engines, solid oxide fuel cell. FeCrAl alloys are being used in various applications up to temperature as high as 1200 C due to their excellent oxidation resistance [14]. Therefore Al2O3–FeCrAl composites should be good candidate for high temperature applications. Investigations of Al2O3–Fe, Al2O3–FeAl [15] and Al2O3–FeAlNi [16] composites fabricated by in-situ displacement reaction have been reported. But Al2O3–FeCrAl composites or FGMs fabricated by reactive hot pressing has not been studied. In this work, we investigated the Al2O3/FeCrAl composites with various Al2O3 fraction and YSZ–FeCrAl FGMs using the composites as interlayer with various Al2O3 fractions fabricated by reactive hot pressing. 2. Experiment Fe2O3 powder (99% purity, average size: 20 lm), Cr, Al powders (99.8% purity, average size: 35 lm), Fe powder (99.9% purity, average size: 20 lm), a-Al2O3 powder (99.9% purity, average size: 35 nm) and fully stabilized zirconia (YSZ) powder (8 mol.% yittria, 99.9% purity, particle size: <1 lm) were used as starting materials. The mixtures of Fe2O3, Cr, Al, Fe or a-Al2O3 for the Al2O3–FeCrAl (Fe75Cr20Al5 in wt.%) in situ composites were prepared by ball milling for 24 h using ethanol as a solvent. After being dried and ground, the powder mixture were stacked in the boron nitride coated graphite die for the composite preparation by reactive hot pressing. The YSZ–FeCrAl FGM sample were prepared by stacking the powders layer by layer with composition change from pure FeCrAl side to pure ceramic side through three interlayers with about 25 vol.% Al2O3 increment. To keep the correct compositional distribution, each layer filled in the
die was pre-compacted under a low pressure before stacking the next layer. The theoretical thickness of 0.2 mm for each interlayer calculated from the mixture of fully dense Al2O3 and FeCrAl phases was prepared. The reactive hot pressing was performed in a flowing high purity argon at 700 C, 900 C or 1300 C for 30 min with an applied pressure of 22 MPa. The heating and the cooling rate were both 20 C min1. The thermal behavior and oxidation resistance for the FGM sample were examined by thermal cycling experiment performed in air between room temperature and 1000 C, kept at 1000 C for 1 h, with an equal heating and cooling rate of 20 C min1. lDTA (Netzch STA PG/PC) was performed for the green compact of mixed powder to determine the reaction temperature. The hot pressed samples, including the composites and FGM samples were cross-sectioned, polished and observed using optical (Olympus Gx71) and scanning electron microscopy (SEM) (Jeol JXA-840) before and after thermal cycling. XRD analysis (Philip PW 1140/00) was used to identify the phases in the composite. The fracture strength for the composite was measured using a threepoint bending test by a Reger M-50 test machine.
3. Results and discussion 3.1. Al2O3–FeCrAl composite An objective of this study was to use the following net reaction to convert a porous Al/Fe2O3/Fe/Cr preform into a dense Al2O3–FeCrAl composite by hot pressing: 2:474Al þ Fe2 O3 þ 0:676Cr ) Al2 O3 þ 2FeCr0:338 Al0:237 ð1Þ The alloy FeCr0.338Al0.237 (in at.%) corresponding to the high temperature alloy Fe75Cr25Al5 (in wt.%) was formed by the reaction between extra Al, Cr and the nascent Fe reduced from Fe2O3. The net reaction (1) can be considered to be a combination of the following two reactions: 2AlðlÞ þ Fe2 O3 ) Al2 O3 þ 2Fe Fe þ 0:338Cr þ 0:237Al ) FeCr0:338 Al0:237
ð2Þ ð3Þ
Reaction Eq. (2) is the well known exothermic thermite reaction, for which the standard enthalpy of reaction ranges 881.4 to 879.8 kJ over the temperature range of 670 C to 1527 C [17]. The Cr in Eq. (3) can dissolve in the iron aluminide to form the (Fe, Cr, Al) solid solution at 1300 C [18]. The results of XRD analysis confirmed this ideal to design and fabricate the Al2O3/FeCrAl composite. Fig. 1(a), (b) and (c) show the XRD patterns of the powder mixture, reactive hot pressing at 900 C and 1300 C for the sample with composition of reaction (1) respectively. Experimental results show that the phases for the powder mixture are the Fe2O3 phase, the Al phase and a small
J.Q. Li et al. / Composites: Part A 38 (2007) 615–620 650
Fe2O3 Al Cr (Fe, Cr, Al) α-Al 2O3
600 550 500
(a) as-milled
450
cps
400 350 300
(b) 900 οC
250 200 150
(b) 1300οC
100 50 0 20
30
40
2θ
50
60
70
80
Fig. 1. XRD patterns of the powder mixture (a), reactive hot pressing at 900 C (b) and reactive hot pressing at 1300 C (c) for the sample with the composition of Eq. (1).
amount of the Cr phase (Fig. 1(a)). The phases in the sample become a-Al2O3 phase and (Fe, Cr, Al) solution after reactive hot pressing at 900 C. No unreacted Cr, Al or Fe2O3 was found (Fig. 1(b)). It is mean that the powder mixture has reacted to form Al2O3–FeCrAl composite completely in this experimental conditions. Increasing the hot pressing temperature to 1300 C, the existence of phases in the composite does not longer change
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(Fig. 1(c)) but the density of the composite is higher as compared with that of the sample fabricated at 900 C. Although the volume fraction of a-Al2O3 in the composite produced by reaction (1) is 53.2 calculated based on the densities of a-Al2O3 (3.98 g cm1) and FeCrAl (7.22 g cm1), its XRD pattern is quite low but broad, which indicates that the grains size of a-Al2O3 in the composite formed by the in situ reaction between Fe2O3 and Al under hot pressing conditions is very small. It is beneficial in improving the mechanical strength of the composite. In order to compare the fracture strength for the composite prepared by this reactive hot pressing (in-situ, RHP) with that for the composite prepared by hot pressing (ex situ, HP), we prepared the samples with same composition of 53.2 vol.% Al2O3 by both methods for a three-point bending test. The bending strength is the average value of three testing for each condition. Experimental results show the average bending strength is 305.0 MPa for the composite prepared by RHP, while 252.0 MPa for that prepared by HP. The bending strength for the composite prepared by RHP is 20% higher than that of the composite prepared by HP. It may be because that the Al2O3 particles are ultrafine, both Al2O3 and FeCrAl phases in the composite prepared by RHP form network due to their in situ formation (see Fig. 2(b)). Based on the composition of reaction Eq. (1), we fabricated the Al2O3–FeCrAl composites with various Al2O3
Fig. 2. Microstructures of the Al2O3–FeCrAl composites with (a) 25, (b) 53.2 and (c) 75 vol.% Al2O3 by reactive hot pressing at 1300 C.
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content by adding an extra Fe, Cr, Al or Al2O3 powders into the powder mixture. Fig. 2 shows the microstructures of the Al2O3–FeCrAl composites with 25, 53.2 and 75 vol.% respectively by reactive hot pressing at 1300 C. In the microstructures, EDX analysis indicates that the Al2O3 ceramic phase appears dark gray and FeCrAl phase appears light gray. The FeCrAl alloy (metal phase) serves as matrix phase in the composite with 25 vol.% Al2O3 content (Fig. 2(a)) while the Al2O3 phase becomes matrix phase in the composite with 75 vol.% Al2O3 content (Fig. 2(c)). From the microstructures, one can see that the composites prepared in this work are dense and homogenous. Both Al2O3 ceramic and FeCrAl alloy form network
due to their in situ formation. The Al2O3 particles are ultrafine which is in good agreement with the results from XRD analysis. 3.2. Functionally graded materials and thermal cycling behaviors Based on the conditions for fabrication of the Al2O3– FeCrAl composites with various Al2O3 content, we successfully fabricated a dense crack-free five-layer functionally graded material (FGM) from pure YSZ to pure FeCrAl alloy by reactive hot pressing at 1300 C using the Al2O3–FeCrAl composites with compositions of 25,
Fig. 3. Microstructures of a cross-section for the YSZ–FeCrAl functionally graded material using Al2O3–FeCrAl composites with 25, 53.2 and 75 vol.% Al2O3 as interlayers by hot pressing at 1300 C; general view (a) and different interfaces (b), (c), (d) and (e).
J.Q. Li et al. / Composites: Part A 38 (2007) 615–620
53.2 and 75 vol.% Al2O3 as interlayers. Fig. 3 shows the general view and different interfaces details of optical microstructures of a cross section for this FGM with about 220 lm thick of each interlayer. Two ends of the FGM are pure YSZ (left in Fig. 3(a)) and pure FeCrAl alloy (right in Fig. 3(a)). In the microstructures, dark gray is the YSZ or Al2O3 ceramic phases and light gray is FeCrAl phase. With the variation of composition, the microstructure of interlayer changes in a stepwise manner from FeCrAl alloy side to YSZ side. From the microstructures in details for the different interfaces of the FGM, one can see that both the ceramic and metal components are continuous in microstructures due to the immigration of these phases formed by the in-situ method at processing temperature even on the pre-stacked layer section where chemical composition stepwise jumped at these interfaces. Thus there is none of macroscopic interface in the FGM. This good continuity of microstructure can eliminate the disadvantage of traditional macroscopic interface in ceramic/metal joint and reflects the design ideal of FGM. Based on the coefficients of thermal expansion (CTEs), 8.0 · 106 K1 for a-Al2O3 and 14.7 · 106 K1 for FeCrAl alloy, the CTEs for the Al2O3–FeCrAl composites with compositions of 25, 53.2, 75 vol.% Al2O3 are 13.0, 11.1 and 9.7 · 106 K1 respectively calculated using the simplest model a = acVc + amVm, where subscripts m and c indicate metal and ceramic, and a and V are CTE and volume fraction. The CTE for YSZ (10.0 · 106 K1) is very closed to its adjacent composite interlayer (9.6 · 106 K1). The thermal stress between these two layers of FGM should be close to zero. The CTEs for this FGM changes linearly from 14.7 · 106 K1 in pure FeCrAl side to 10 · 106 K1 in pure YSZ side. To examine the thermal stability, oxidation resistance and thermal fatigue behaviors of the FGM, the samples were heated and cooled in air cyclically between room temperature and 1000 C with an equal heating and cooling rate of 20 C min1, and were kept at 1000 C for 1 h. The microstructure observation for the FGM sample after 10 thermal cycles shows that there is not obvious change in microstructure as comparing as-prepared one. No cracking can be found in the FGMs in this work (Fig. 4), indicating that the FGM has good strength, good thermal stability, and good resistance to oxidation and thermal shock due to the better metal–ceramic bond, continuous in microstructure at interface of FGM and good oxidation resistance component FeCrAl alloy formed in the FGM. The thermal stress should be existed due to the difference of the coefficients of thermal expansion (CTEs) between two components and two different compositional interlayers. During the thermal cycling, the alternate thermal stress takes place in the interlayer or at interfaces induced by the difference of the CTEs. No cracking in the FGM during the thermal cycling indicates the bonding between both ceramic and metal components, and the bonding between two different compositional interlayers of the FGM prepared in this work are stronger than the thermal stress.
619
Fig. 4. General view of a cross-section for the YSZ–FeCrAl functionally graded material after 10 thermal cycles up to 1000 C.
In our previous work [8], the cracks were found in the YSZ–NiCr FGM fabricated by powder metallurgy hot pressing even after one thermal cycle while there is crackfree in the as-prepared sample, while no cracking was found in YSZ–NiCrAl FGM prepared by reactive hot pressing [19]. A similar result may occur in this Al2O3– FeCrAl FGM, which is the purpose of this work. 4. Conclusion The Al2O3–FeCrAl composites with various Al2O3 fraction were fabricated by the proper Fe2O3, Al, Fe, Cr and Al2O3 powders mixture using reactive hot pressing. A five-layer functionally graded material of YSZ–FeCrAl was fabricated using the composites with compositions of 25, 53.2, 75 vol.% Al2O3 as interlayers. The composite consists of a-Al2O3 phase and (Fe, Cr, Al) solid solution. The a-Al2O3 grains formed by the in-situ reaction between Fe2O3 and Al were ultrafine and uniform distribution. No cracking in the functionally graded materials after 10 thermal cycles up to 1000 C indicates that the composites and the functional gradient material fabricated in this work have good strength, good resistance to oxidation and thermal shock due to the better metal–ceramic bond, continuous in microstructure at interface of FGM and good oxidation resistance component FeCrAl alloy formed in the FGM. Acknowledgements This work is supported by Shenzhen Research Grant (200322). The authors thank Mr. Z.G. Liang for his help in the fabrication experiments. References [1] Neubrand A, Roedel J. Gradient materials: An overview of a novel concept. Z Metallkd 1997;88(5):358. [2] Hirai T, Chen L. Recent and prospective development of functionally graded materials in Japan. Mater Sci Forum 1998–1999;308–311:509.
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[3] Heian EM, Gibeling JC, Munir Z. Synthesis and characterization of Nb5Si3/Nb functionally graded materials. Mater Sci Eng A 2004;A368:168. [4] Lin CY, Bathias C, Mcshane HB, Rawlings RD. Production of silicon carbide Al 2124 alloy functionally graded materials by mechanical powder metallurgy technique. Powder Metall 1999; 42(1):29. [5] Jung YG, Choi SC. Fabrication of 3Y-TZP/SUS304 functionally graded materials by slip casting, application of porous alumina molds. Mater Sci Lett 1998;37(6):312. [6] He YR, Subramanian V, Lannutti JJ. Phase instability in ZrO2–NiAl functionally graded materials. J Mater Res 1997;12(10):2589. [7] Li YK, Wang Y, Li SG, Han WB. PSZ/Mo functionally graded materials. Acta Mater Comp Sin 2003;20:42. [8] Li JQ, Zeng XR, Tang JN, Xiao P. Fabrication and thermal properties of YSZ–NiCr joint with YSZ–NiCr functionally graded materials interlayer. J Eur Ceram Soc 2003;23:1847. [9] Qin CD, Derby B. Diffusion bonding of a nickel (chromium) alloy to zirconia–mechanical-properties and interface microstructures. J Mater Sci Lett 1993;28:4366. [10] Kuruvilla AK, Prased KS, Mahajan YR. Microstructure-property correlation in Al/TiB2 composites. Scripta Metall Mater 1990;24(5):873. [11] Ma ZY, Bi J, Lu YX. In-situ ceramic particle-reinforced aluminum matrix composite fabricated by reaction pressing in the TiO2(Ti)–Al– B(B2O3) system. Metall Trans A 1997;28(7):1931.
[12] Ranganath S, Vijayakumar M, Subrahamanyam J. Combustionassisted synthesis of Ti–TiB–TiC composite via the casting route. Mater Sci Eng A 1992;A149(1–2):253. [13] Maity PC, Panigrahi SC, Chakraborty PN. Preparation of aluminum-alumina in situ particle composite by addition of titania to aluminum melt. Scripta Metall Mater 1993;28:549. [14] Wessel E, Kochubey V, Naumenko D, Niewolak L, Singheiser L, Quadakkers WJ. Effect of Zr addition on the microstructure of the alumina scales on FeCrAlY-alloys. Scripta Mater 2004;51: 987. [15] Subramanian R, McKamey CG, Buck LR, Schneibel JH. Synthesis of iron aluminide–Al2O3 composites by in-situ displacement reactions. Mater Sci Eng A 1997;239–240:640. [16] Travitzky N, Kumar P, Sandhage KH, Janssen R, Claussen N. Rapid synthesis of Al2O3 reinforced Fe–Cr–Ni composites. Mater Sci Eng A 2003;A344:245. [17] Pankratz LB, Stuve JM, Gokcen NA. Thermodynamic Data for Mineral Technology, vol. 677. USA: US Department of Interior/ Bureau of Mines; 1984, 262. [18] Subramanian R, McKamey CG, Schneibel JH, Buck LR, Menchhofer PA. Iron aluminide–Al2O3 composites by in situ displacement reactions: processing and mechanical properties. Mater Sci Eng A 1998;254:119. [19] Li JQ, Gu KM, Tang JN, Xie SH, Zhuang YH. Al2O3–NiCrAl composites and functional gradient materials fabricated by reactive hot pressing. Mater Chem Phy 2006;97:37.
Al2O3–FeCrAl composites and functionally graded materials fabricated by reactive hot pressing J.Q. Li a
a,b,*
, W.A. Sun
a,b
, W.Q. Ao
a,b
, K.M. Gu
a,b
, P. Xiao
c
Department of Materials Science and Engineering, Shenzhen University, Shenzhen, Guangdong 518060, PR China b Shenzhen Key Laboratory of Special Functional Materials, Shenzhen 518060, PR China c Manchester Materials Science Centre, University of Manchester, Manchester M1 7HS, UK Received 20 February 2005; received in revised form 28 November 2005; accepted 14 February 2006
Abstract Al2O3–FeCrAl composites were fabricated by mixing Fe2O3, Al and Cr powders and then reactive hot pressing. The high temperature alloy FeCrAl was formed by the reaction of extra Al, Cr and the Fe reduced from Fe2O3. The Al2O3–FeCrAl composites with various Al2O3 fractions were successfully fabricated by the proper addition of extra Fe, Cr, Al or Al2O3 powders. A five-layer functionally graded material of YSZ–FeCrAl was fabricated using the Al2O3–FeCrAl composites with compositions of 25, 53.2 and 75 vol.% Al2O3 as interlayer. The results from XRD analysis, optical microscope observation and thermal cycling test show that the composites fabricated by this method consist of a-Al2O3 phase and (Fe, Cr, Al) solid solution. The a-Al2O3 grain formed by this in-situ reaction between Fe2O3 and Fe is ultrafine and uniform distribution. The three-point bending strength is 305.0 MPa for the composite with 53.2 vol.% Al2O3 prepared by the reactive hot pressing, about 20% higher than that of the composite with same composition prepared by ex situ hot pressing method (252.0 MPa). No cracking was found in the functionally graded materials after 10 thermal cycles up to 1000 C due to the better metal–ceramic bond, continuous in microstructure at interface of FGM and good oxidation resistance component FeCrAl alloy formed in the FGM. 2006 Elsevier Ltd. All rights reserved. Keywords: B. Fracture; B. Mechanical properties; B. Strength
1. Introduction The fast progress of modern high technology requires more and more new materials with various special properties or functions. Metal–ceramic composites and functionally graded materials (FGMs) have been developed for some critical structural applications, such as in the environment of super-high temperature, thermal cycling, great temperature gradient, wear and corrosion [1,2]. The ceramic side offers the resistances to heat, corrosion or wear, and the metal side provides mechanical strength and ther* Corresponding author. Address: Department of Materials Science and Engineering, Shenzhen University, Shenzhen, Guangdong 518060, PR China. Fax: +86 755 26536239. E-mail address: [email protected] (J.Q. Li).
1359-835X/$ - see front matter 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.compositesa.2006.02.012
mal conductivity. Metal–ceramic FGMs are introduced to eliminate and reduce the residual stresses which would cause cracking in the materials during cooling from processing temperature or during service because of the difference in the coefficient of thermal expansion (CTE) between the ceramics and metals. Many ceramic–metal FGMs have been fabricated and studied for various applications, such as Nb5Si3–Nb [3], SiC–Al 2124 [4], 3Y–TZP–SUS304 [5], ZrO2–NiAl [6], PSZ–Mo [7] and YSZ–NiCr [8]. For metal–ceramic composites and FGMs fabrication, in general, the ceramic particulates are prepared independently prior to composite fabrication. The particulates are introduced into the matrix via ingot casting or powder metallurgy (PM) processes, termed as ex situ metal–ceramic composites. It is generally known that agglomeration of ceramic particulates may occur during processing of
616
J.Q. Li et al. / Composites: Part A 38 (2007) 615–620
ex situ metal–ceramic composites, which may weaken the composites. Moreover, possible interfacial chemical reactions between the reinforcement phases and the matrix may also degrade the mechanical properties of ex situ metal–ceramic composites [9]. In recent years, novel processing techniques based on the in situ production of reinforced ceramic particles have been developed. These techniques include exothermic dispersion (XD) [10], reactive hot pressing (RHP) [11], combustion-assisted cast [12] and direct reaction synthesis [13]. Among these, RHP process is attractive due to its simplicity and flexibility. Ultrafine ceramic particulates are formed in situ by the exothermic reaction between the element constituents under hot pressing conditions. The reduction in the particle size is generally known to exhibit a beneficial effect in improving the mechanical strength. In situ formed ceramic particulates exhibit thermodynamic compatibility at the metal–ceramic interface, resulting in a stronger metal–ceramic bond. Such unique properties make in situ metal–ceramic composites to possess excellent mechanical properties than their ex situ counterparts. Alumina and zirconia ceramics have high fracture toughness and strength and high resistance to wear and corrosion. They are important in engineering applications, such as gas turbines, engines, solid oxide fuel cell. FeCrAl alloys are being used in various applications up to temperature as high as 1200 C due to their excellent oxidation resistance [14]. Therefore Al2O3–FeCrAl composites should be good candidate for high temperature applications. Investigations of Al2O3–Fe, Al2O3–FeAl [15] and Al2O3–FeAlNi [16] composites fabricated by in-situ displacement reaction have been reported. But Al2O3–FeCrAl composites or FGMs fabricated by reactive hot pressing has not been studied. In this work, we investigated the Al2O3/FeCrAl composites with various Al2O3 fraction and YSZ–FeCrAl FGMs using the composites as interlayer with various Al2O3 fractions fabricated by reactive hot pressing. 2. Experiment Fe2O3 powder (99% purity, average size: 20 lm), Cr, Al powders (99.8% purity, average size: 35 lm), Fe powder (99.9% purity, average size: 20 lm), a-Al2O3 powder (99.9% purity, average size: 35 nm) and fully stabilized zirconia (YSZ) powder (8 mol.% yittria, 99.9% purity, particle size: <1 lm) were used as starting materials. The mixtures of Fe2O3, Cr, Al, Fe or a-Al2O3 for the Al2O3–FeCrAl (Fe75Cr20Al5 in wt.%) in situ composites were prepared by ball milling for 24 h using ethanol as a solvent. After being dried and ground, the powder mixture were stacked in the boron nitride coated graphite die for the composite preparation by reactive hot pressing. The YSZ–FeCrAl FGM sample were prepared by stacking the powders layer by layer with composition change from pure FeCrAl side to pure ceramic side through three interlayers with about 25 vol.% Al2O3 increment. To keep the correct compositional distribution, each layer filled in the
die was pre-compacted under a low pressure before stacking the next layer. The theoretical thickness of 0.2 mm for each interlayer calculated from the mixture of fully dense Al2O3 and FeCrAl phases was prepared. The reactive hot pressing was performed in a flowing high purity argon at 700 C, 900 C or 1300 C for 30 min with an applied pressure of 22 MPa. The heating and the cooling rate were both 20 C min1. The thermal behavior and oxidation resistance for the FGM sample were examined by thermal cycling experiment performed in air between room temperature and 1000 C, kept at 1000 C for 1 h, with an equal heating and cooling rate of 20 C min1. lDTA (Netzch STA PG/PC) was performed for the green compact of mixed powder to determine the reaction temperature. The hot pressed samples, including the composites and FGM samples were cross-sectioned, polished and observed using optical (Olympus Gx71) and scanning electron microscopy (SEM) (Jeol JXA-840) before and after thermal cycling. XRD analysis (Philip PW 1140/00) was used to identify the phases in the composite. The fracture strength for the composite was measured using a threepoint bending test by a Reger M-50 test machine.
3. Results and discussion 3.1. Al2O3–FeCrAl composite An objective of this study was to use the following net reaction to convert a porous Al/Fe2O3/Fe/Cr preform into a dense Al2O3–FeCrAl composite by hot pressing: 2:474Al þ Fe2 O3 þ 0:676Cr ) Al2 O3 þ 2FeCr0:338 Al0:237 ð1Þ The alloy FeCr0.338Al0.237 (in at.%) corresponding to the high temperature alloy Fe75Cr25Al5 (in wt.%) was formed by the reaction between extra Al, Cr and the nascent Fe reduced from Fe2O3. The net reaction (1) can be considered to be a combination of the following two reactions: 2AlðlÞ þ Fe2 O3 ) Al2 O3 þ 2Fe Fe þ 0:338Cr þ 0:237Al ) FeCr0:338 Al0:237
ð2Þ ð3Þ
Reaction Eq. (2) is the well known exothermic thermite reaction, for which the standard enthalpy of reaction ranges 881.4 to 879.8 kJ over the temperature range of 670 C to 1527 C [17]. The Cr in Eq. (3) can dissolve in the iron aluminide to form the (Fe, Cr, Al) solid solution at 1300 C [18]. The results of XRD analysis confirmed this ideal to design and fabricate the Al2O3/FeCrAl composite. Fig. 1(a), (b) and (c) show the XRD patterns of the powder mixture, reactive hot pressing at 900 C and 1300 C for the sample with composition of reaction (1) respectively. Experimental results show that the phases for the powder mixture are the Fe2O3 phase, the Al phase and a small
J.Q. Li et al. / Composites: Part A 38 (2007) 615–620 650
Fe2O3 Al Cr (Fe, Cr, Al) α-Al 2O3
600 550 500
(a) as-milled
450
cps
400 350 300
(b) 900 οC
250 200 150
(b) 1300οC
100 50 0 20
30
40
2θ
50
60
70
80
Fig. 1. XRD patterns of the powder mixture (a), reactive hot pressing at 900 C (b) and reactive hot pressing at 1300 C (c) for the sample with the composition of Eq. (1).
amount of the Cr phase (Fig. 1(a)). The phases in the sample become a-Al2O3 phase and (Fe, Cr, Al) solution after reactive hot pressing at 900 C. No unreacted Cr, Al or Fe2O3 was found (Fig. 1(b)). It is mean that the powder mixture has reacted to form Al2O3–FeCrAl composite completely in this experimental conditions. Increasing the hot pressing temperature to 1300 C, the existence of phases in the composite does not longer change
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(Fig. 1(c)) but the density of the composite is higher as compared with that of the sample fabricated at 900 C. Although the volume fraction of a-Al2O3 in the composite produced by reaction (1) is 53.2 calculated based on the densities of a-Al2O3 (3.98 g cm1) and FeCrAl (7.22 g cm1), its XRD pattern is quite low but broad, which indicates that the grains size of a-Al2O3 in the composite formed by the in situ reaction between Fe2O3 and Al under hot pressing conditions is very small. It is beneficial in improving the mechanical strength of the composite. In order to compare the fracture strength for the composite prepared by this reactive hot pressing (in-situ, RHP) with that for the composite prepared by hot pressing (ex situ, HP), we prepared the samples with same composition of 53.2 vol.% Al2O3 by both methods for a three-point bending test. The bending strength is the average value of three testing for each condition. Experimental results show the average bending strength is 305.0 MPa for the composite prepared by RHP, while 252.0 MPa for that prepared by HP. The bending strength for the composite prepared by RHP is 20% higher than that of the composite prepared by HP. It may be because that the Al2O3 particles are ultrafine, both Al2O3 and FeCrAl phases in the composite prepared by RHP form network due to their in situ formation (see Fig. 2(b)). Based on the composition of reaction Eq. (1), we fabricated the Al2O3–FeCrAl composites with various Al2O3
Fig. 2. Microstructures of the Al2O3–FeCrAl composites with (a) 25, (b) 53.2 and (c) 75 vol.% Al2O3 by reactive hot pressing at 1300 C.
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content by adding an extra Fe, Cr, Al or Al2O3 powders into the powder mixture. Fig. 2 shows the microstructures of the Al2O3–FeCrAl composites with 25, 53.2 and 75 vol.% respectively by reactive hot pressing at 1300 C. In the microstructures, EDX analysis indicates that the Al2O3 ceramic phase appears dark gray and FeCrAl phase appears light gray. The FeCrAl alloy (metal phase) serves as matrix phase in the composite with 25 vol.% Al2O3 content (Fig. 2(a)) while the Al2O3 phase becomes matrix phase in the composite with 75 vol.% Al2O3 content (Fig. 2(c)). From the microstructures, one can see that the composites prepared in this work are dense and homogenous. Both Al2O3 ceramic and FeCrAl alloy form network
due to their in situ formation. The Al2O3 particles are ultrafine which is in good agreement with the results from XRD analysis. 3.2. Functionally graded materials and thermal cycling behaviors Based on the conditions for fabrication of the Al2O3– FeCrAl composites with various Al2O3 content, we successfully fabricated a dense crack-free five-layer functionally graded material (FGM) from pure YSZ to pure FeCrAl alloy by reactive hot pressing at 1300 C using the Al2O3–FeCrAl composites with compositions of 25,
Fig. 3. Microstructures of a cross-section for the YSZ–FeCrAl functionally graded material using Al2O3–FeCrAl composites with 25, 53.2 and 75 vol.% Al2O3 as interlayers by hot pressing at 1300 C; general view (a) and different interfaces (b), (c), (d) and (e).
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53.2 and 75 vol.% Al2O3 as interlayers. Fig. 3 shows the general view and different interfaces details of optical microstructures of a cross section for this FGM with about 220 lm thick of each interlayer. Two ends of the FGM are pure YSZ (left in Fig. 3(a)) and pure FeCrAl alloy (right in Fig. 3(a)). In the microstructures, dark gray is the YSZ or Al2O3 ceramic phases and light gray is FeCrAl phase. With the variation of composition, the microstructure of interlayer changes in a stepwise manner from FeCrAl alloy side to YSZ side. From the microstructures in details for the different interfaces of the FGM, one can see that both the ceramic and metal components are continuous in microstructures due to the immigration of these phases formed by the in-situ method at processing temperature even on the pre-stacked layer section where chemical composition stepwise jumped at these interfaces. Thus there is none of macroscopic interface in the FGM. This good continuity of microstructure can eliminate the disadvantage of traditional macroscopic interface in ceramic/metal joint and reflects the design ideal of FGM. Based on the coefficients of thermal expansion (CTEs), 8.0 · 106 K1 for a-Al2O3 and 14.7 · 106 K1 for FeCrAl alloy, the CTEs for the Al2O3–FeCrAl composites with compositions of 25, 53.2, 75 vol.% Al2O3 are 13.0, 11.1 and 9.7 · 106 K1 respectively calculated using the simplest model a = acVc + amVm, where subscripts m and c indicate metal and ceramic, and a and V are CTE and volume fraction. The CTE for YSZ (10.0 · 106 K1) is very closed to its adjacent composite interlayer (9.6 · 106 K1). The thermal stress between these two layers of FGM should be close to zero. The CTEs for this FGM changes linearly from 14.7 · 106 K1 in pure FeCrAl side to 10 · 106 K1 in pure YSZ side. To examine the thermal stability, oxidation resistance and thermal fatigue behaviors of the FGM, the samples were heated and cooled in air cyclically between room temperature and 1000 C with an equal heating and cooling rate of 20 C min1, and were kept at 1000 C for 1 h. The microstructure observation for the FGM sample after 10 thermal cycles shows that there is not obvious change in microstructure as comparing as-prepared one. No cracking can be found in the FGMs in this work (Fig. 4), indicating that the FGM has good strength, good thermal stability, and good resistance to oxidation and thermal shock due to the better metal–ceramic bond, continuous in microstructure at interface of FGM and good oxidation resistance component FeCrAl alloy formed in the FGM. The thermal stress should be existed due to the difference of the coefficients of thermal expansion (CTEs) between two components and two different compositional interlayers. During the thermal cycling, the alternate thermal stress takes place in the interlayer or at interfaces induced by the difference of the CTEs. No cracking in the FGM during the thermal cycling indicates the bonding between both ceramic and metal components, and the bonding between two different compositional interlayers of the FGM prepared in this work are stronger than the thermal stress.
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Fig. 4. General view of a cross-section for the YSZ–FeCrAl functionally graded material after 10 thermal cycles up to 1000 C.
In our previous work [8], the cracks were found in the YSZ–NiCr FGM fabricated by powder metallurgy hot pressing even after one thermal cycle while there is crackfree in the as-prepared sample, while no cracking was found in YSZ–NiCrAl FGM prepared by reactive hot pressing [19]. A similar result may occur in this Al2O3– FeCrAl FGM, which is the purpose of this work. 4. Conclusion The Al2O3–FeCrAl composites with various Al2O3 fraction were fabricated by the proper Fe2O3, Al, Fe, Cr and Al2O3 powders mixture using reactive hot pressing. A five-layer functionally graded material of YSZ–FeCrAl was fabricated using the composites with compositions of 25, 53.2, 75 vol.% Al2O3 as interlayers. The composite consists of a-Al2O3 phase and (Fe, Cr, Al) solid solution. The a-Al2O3 grains formed by the in-situ reaction between Fe2O3 and Al were ultrafine and uniform distribution. No cracking in the functionally graded materials after 10 thermal cycles up to 1000 C indicates that the composites and the functional gradient material fabricated in this work have good strength, good resistance to oxidation and thermal shock due to the better metal–ceramic bond, continuous in microstructure at interface of FGM and good oxidation resistance component FeCrAl alloy formed in the FGM. Acknowledgements This work is supported by Shenzhen Research Grant (200322). The authors thank Mr. Z.G. Liang for his help in the fabrication experiments. References [1] Neubrand A, Roedel J. Gradient materials: An overview of a novel concept. Z Metallkd 1997;88(5):358. [2] Hirai T, Chen L. Recent and prospective development of functionally graded materials in Japan. Mater Sci Forum 1998–1999;308–311:509.
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