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Growth and morphological characterization of Al–Cr–Nb eutectic alloys
Journal of Alloys and Compounds 402 (2005) 156–161
Growth and morphological characterization of Al–Cr–Nb eutectic alloys S.A. Souza a , C.T. Rios a , A.A. Coelho b , P.L. Ferrandini c , S. Gama b , R. Caram a,∗ a
DEMA/FEM, Universidade Estadual de Campinas, C.P. 6122, CEP 13083-970, SP, Brazil b IFGW, Universidade Estadual de Campinas, C.P. 6165, CEP 13083-970, SP, Brazil c Centro Universit´ ario da FEI, C.P. 85, CEP 09850-901, SP, Brazil Received 8 March 2005; received in revised form 14 April 2005; accepted 19 April 2005 Available online 13 June 2005
Abstract Directional solidification of eutectic alloys attracts considerable attention, when in situ composites are concerned. The eutectic alloys are regarded as presenting regular morphology (lamellar and fibrous structures). Besides, when directionally solidified they show high microstructure stability at high temperatures. This work reports a morphological study of an Al–Cr–Nb eutectic alloy. The solidification morphology of the alloys was studied both in the as-cast and in the directionally solidified conditions. The samples were first obtained in an arc furnace and then directionally solidified using Bridgman equipment. During the directional solidification process, the growth rates utilized varied from 5.0 to 30.0 mm/h. Optical (OM) and scanning electron microscopy (SEM) was used in order to determine the influence of the solidification conditions on the microstructure. The results obtained indicated that the eutectic transformation temperature is near 1347.9 ◦ C with formation of Al3 (Nb,Cr) and Cr(Al,Nb) phases. Also, it was noted that the Cr(Al,Nb) phase undergoes a polymorphic transformation (∼892.3 ◦ C) forming the Al(Nb)Cr2 compound, followed by eutectoid decomposition Cr(Al,Nb) → Al(Nb)Cr2 + Al8 Cr5. © 2005 Elsevier B.V. All rights reserved. Keywords: Metals; Intermetallics; Casting; Scanning electron microscopy; X-ray diffraction
1. Introduction Composite materials have been considered as highly advanced structural materials for no more than 50 years, which allows one to classify them as relatively recent materials. They are generated by the combination of two or more different materials, which gives them improved properties. Usually, composites are formed by a matrix and non-soluble second phase, where they are separated by a well-defined boundary. In the early 60s, according to McLean [1], Kraft proposed that a new class of composite materials could be obtained from the directional solidification of polyphase alloys, particularly the eutectic alloys. In the eutectic transformation, the solidification results in two or more phases aligned in the growth direction. When the solid/liquid transformation occurs in a well-defined direction (directional solidification) the solid phases grow in the heat extraction ∗
Corresponding author. Tel.: +55 19 37883314; fax: +55 19 37883314. E-mail address: [email protected] (R. Caram).
0925-8388/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2005.04.137
direction [2]. Hence, both matrix and strengthening phase are generated simultaneously and the resulting alloy is called in situ composite. The microstructure array of a eutectic alloy depends on the growth mode of the solid phases and distribution of the constituents in the interfacial liquid. The distribution of the constituents also affects the solid/liquid interface undercooling. Lamplough and Scott [3] observed that these are the causes for the wide variety of eutectic morphologies. Jackson [4] studied thermodynamic aspects of the solid growth from the melt and proposed that a solidifying liquid may present two types of interfacial morphology: faceted and non-faceted. The faceted morphology is typical of substances exhibiting complex crystal structures and directional bonding that leads to the formation of planar faces and hence, the growth occurs under preferential directions. When the solid/liquid interface is microscopically smooth, the atomic attachment kinetics does not depend on the crystal plane involved and no facets occur and consequently, it presents no preferential direction. The tendency to a specific morphology can be predicted by
S.A. Souza et al. / Journal of Alloys and Compounds 402 (2005) 156–161
the α factor (α = Sf ξ/R). Jackson and Hunt [5] classified the binary eutectic alloys into three groups based on the concept of α: (a) faceted/faceted (f/f), (b) faceted/non-faceted (f/nf) and (c) non-faceted/non-faceted (nf/nf). In terms of possible applications, the most important alloys are the nf/nf because they originate regular structures. The regular structures can be either fibrous or lamellar, and the kind of structure formed during solidification depends on the volume fraction of the phases involved. A volume fraction between 0 and 0.28 determines a fibrous eutectic structure and a volume fraction between 0.28 and 0.50 determines a lamellar structure. Several metallic systems present eutectic reactions that provide potential alloys to be used at high temperatures and the system Al–Cr–Nb is one of them. Despite being important for understanding the behaviour of several industrial alloys [6], literature information on this system is contradictory and not plentiful. The solidification of eutectic alloys in this system leads to the formation of the intermetallic compounds found in the binary systems Nb–Al, Cr–Al and Nb–Cr that are remarkably brittle. Therefore, specific processing techniques should be attempted in order to solidify these alloys and overcome the pronounced brittleness. At 1000 ◦ C, the maximum solubility of Cr in Al3 Nb is 10.8%, when Cr occupies the Nb sites. In the binary system, Al–Cr [7], the solubility of Al in Cr is higher than 40% but solubility of Nb is considerably low (∼3.0–4.0%). The (Cr,Al)2 Nb (C14), co-exists with the phases (Cr,Al)2 Nb (C15), Al3 (Nb,Cr), Nb2 (Al,Cr) and Cr(Al,Nb) over a large compositional range [8]. According to Kaufman and Nesor [9], the Al–Nb–Cr presents a eutectic transformation at the composition 54%Al–34%Cr–12%Nb and 1317 ◦ C, which would produce an in situ composite, where the matrix would be 50%Al–45%Cr–5%Nb strengthened by Al3 Nb fibres. Thomas [10] suggested that such a eutectic transformation occurs between 1346 and 1366 ◦ C, on a monovariant eutectic curve, which descends towards a pseudo binary eutectic between Cr2 Al3 and NbAl3 , at the composition 54%Al–34%Cr–12%Nb. Besides the work of Kaufman and Thomas, Costa Neto [11], by applying differential thermal analysis, found that the eutectic transformation occurs at 1374 ◦ C. However, according to Costa Neto, the eutectic transformation leads to the Al8 Cr5 , Al3 Nb and a Cr-rich phase. Based on the above information, the purpose of this work is to investigate the obtaining and characterization of the eutectic alloy in the Al-rich region of the Al–Cr–Nb system, both in the as-cast and in the directionally solidified conditions.
2. Experimental procedures All compositions in this work are given in atomic percentage. Ingots of 20 g were prepared using commercial purity elements. Al, Cr and Nb were arc-melted using a nonconsumable tungsten electrode furnace with a water-cooled
157
Table 1 Chemical compositions of the samples studied Alloy (at.%)
Al
Cr
Nb
1 2 3
49.9 49.3 45.8
37.6 39.7 43.5
12.5 11.0 10.7
copper hearth under an argon atmosphere. The purpose was to investigate the compositions near 54%Al–34%Cr–12%Nb, which was proposed by Thomas [10]. Among all alloys studied, three compositions presented interesting results, in terms of the eutectic transformation and thus, were analysed in more detail. They are seen in Table 1. Afterwards, the samples were directionally solidified using a Bridgman equipment with induction heating and Al2 O3 crucibles. Samples 6.0 cm long and 0.7 cm in diameter were processed by slowly shifting the crucible from the upper hot region to the lower cold region under an argon atmosphere. The temperature of the hot region was set at 1500 ± 10 ◦ C, and the temperature gradient was near 100 ◦ C/cm. The growth rates utilized were 5.0, 10.0, 20.0 and 30.0 mm/h. The samples, both in the as-cast and in the directionally solidified conditions were prepared for optical and scanning electron microscopy and the etchant solution used was 6% HNO3 , 3% HF, 91% H2 O (vol.%). Energy dispersive spectrometry (EDS) and computerized image analysis were also used. Phase presence was confirmed by X-ray diffraction and differential thermal analysis was utilized in order to determine the transformation temperatures, under heating and cooling rates of 10 o C/min.
3. Results and discussion All samples in the as-cast condition showed very thin eutectic microstructure with lamellar spacing between 0.5 and 1.0 m, as seen in Fig. 1. Their compositions are located slightly out of the Cr(Al,Nb) + Al3 (Nb,Cr) field in the 1000 ◦ C isotherm, suggested by Mahdouk and Gachon [8]. The microstructure analysis of sample 1, which is in the as-cast condition, reveals that it is formed by Al3 (Nb,Cr) and Al(Nb)Cr2 , that were identified by X-ray diffraction (Fig. 2) and EDS-obtained composition values are seen in Table 2. According to the 1000 ◦ C isotherm suggested by Mahdouk and Gachon [8], one is supposed to find Cr(Al,Nb). However, according to the Al–Cr binary phase diagram, one sees that the Cr-rich phase undergoes a polymorphic transformation, Table 2 EDS-obtained compositions of the phases found in sample 1 Element (at.%) Al Cr Nb Total
Al3 (Nb,Cr)
Al(Nb)Cr2
64.6 5.3 30.1
24.0 72.1 3.9
100.0
100.0
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Fig. 1. SEM images of the as-cast condition microstructures: (a) (back scattered electrons, BSE) hypereutectic (sample 1); (b) (BSE) eutectic (sample 2); (c) (BSE) hypoeutectic (sample 3); (d) (secondary electrons, SE) hypoeutectic (sample 3).
forming the AlCr2 compound and, the eutectoid decomposition (Crss → AlCr2 + Al8 Cr5 ). Therefore, at room temperature, a Cr-rich alloy presents AlCr2 , which comes from the Cr solid solution with an Al content and eventually, when considering the ternary phase diagram, a small Nb content either. Also, the analysis of the sample 1 shows a typically hypereutectic microstructure. Al3 (Nb,Cr) appears light colored as a faceted primary phase in Fig. 1a, while the dark film, which envelops the primary phase particles is Al(Nb)Cr2 . Fig. 1b shows the sample 2, which contains 39.7% Cr and presents a completely eutectic microstructure. Differential
thermal analysis was used in order to determine the eutectic transformation temperature (Fig. 3). The result show an exothermic peek at 1347.9 ◦ C, which is related to the eutectic formation of Al3 (Nb,Cr) and Cr(Al,Nb). The eutectic temperature found by DTA agrees with data found in literature [10] and the deviation most probably comes from the different heating/cooling rates and different equipment response. At 892.3 ◦ C, another exothermic peak was observed, which is associated with the formation of Al(Nb)Cr2 , from the Cr solid solution. This transformation is followed by the eutectoid decomposition, which forms Al8 Cr5 and Al(Nb)Cr2 also from the Cr solid solution. Since both transformations occur considerably near each other, it is quite difficult to identify
Fig. 2. X-ray diffraction pattern of sample 1. As-cast condition.
Fig. 3. DTA curve of the sample 2 (49.3%Al–39.7%Cr–11%Nb).
S.A. Souza et al. / Journal of Alloys and Compounds 402 (2005) 156–161
159
Fig. 6. Complex regular microstructure presented by the eutectic alloy 49.3%Al–39.7%Cr–11%Nb (sample 2) (OM).
Fig. 4. X-ray diffraction pattern of sample 2, heat-treated at 1000 ◦ C for 1 h: (a) water quenched and (b) furnace-cooled.
each one. The eutectoid transformation occurs near 60.5% Cr and at 870 ◦ C [7]. This result is in good agreement with the X-ray diffraction patterns that are seen in Fig. 4. The patterns refer to two samples of same composition and different cooling conditions. Both were heated at 1000 ◦ C for 1 h and afterwards, one of them was water quenched (Fig. 4a) and the other furnace-cooled, taking 18 h to reach the room temperature (Fig. 4b). The results allow one to verify the formation of Al3 (Nb,Cr) and Cr(Al,Nb) as well as the polymorphic transformation of Cr(Al,Nb) into Al(Nb)Cr2 . Al8 Cr5 was not found using X-ray diffraction. However, it does not allow one to say that this phase is not present, since X-ray diffraction demands a certain amount of the phase aimed at, and the volume of Al8 Cr5 is considerably low.
A further increase in the Cr content led the alloy to reach the hypoeutectic composition, which results in the precipitation of Cr solid solution. An interesting feature seen in the microstructure of all alloys studied, mostly sample 3, is the formation of the Widmanst¨atten morphology [12] within the hypoeutectic Cr(Al,Nb) primary phase, which is shown in Fig. 1c and d. The magnified view of the microstructure (Fig. 1d) allows one to notice that the phase previously identified by X-ray diffraction as Al(Nb)Cr2 shows acicular morphology. This kind of morphology comes from a solid/solid transformation. This solid/solid transformation of the mother phase, Cr(Al,Nb), occurs by nucleation on specific planes along the grain boundaries, showing a particular growth habit. These specific planes are selected as a way of minimizing the energy difference between the mother phase and the nuclei of the new phase. As a result of this phenomenon, the new phase will grow in some particular directions, giving rise to an acicular structure. The theory of regular eutectic alloys [13] says that the growth mode of the constituent phases in the eutectic transformation determines the resulting structure. The microstructure
Fig. 5. Optical microscopy (OM) of hypereutectic microstructures of the sample 1, which presents the faceted phase Al3 (Nb,Cr).
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analysis of samples 1 and 3 allows one to conclude that a near eutectic composition alloy presents both faceted and non-faceted primary phase morphologies. Fig. 5 shows the non-equilibrium microstructure of sample 1, as the solidification in an arc furnace completely hinders heat extraction control. Sample 1 presents Al3 (Nb,Cr) as primary phase, of faceted morphology, which is in agreement with the theory of Jackson [4], since Al3 (Nb,Cr) is an intermetallic compound. Faceted phases present high melting entropy (α > 2) and grow under preferential directions. It was also observed
that Al3 (Nb,Cr) nucleates within the co-operative zone, since the dendritic growth is rapidly hindered and absorbed by the eutectic structure. The nucleation of a second phase on the first one will determine the manner the primary phase will grow. Usually, a second phase nucleates as a film over the primary phase, because of the higher solute concentration in the solid/liquid interface, since the primary crystals are unable to nucleate the eutectic second phase. This is probably due to the existence of an asymmetric co-operative growth zone [14]. Fig. 6
Fig. 7. SEM images of directionally solidified microstructures under the growth rates (BSE): (a and b) 5.0 mm/h; (c and d) 10.0 mm/h; (e and f) 20.0 mm/h; (g and h) 30.0 mm/h.
S.A. Souza et al. / Journal of Alloys and Compounds 402 (2005) 156–161 Table 3 Lamellar spacing as a function of growth rate of sample 2, directionally solidified Growth rate,V (mm/h)
Lamellar spacing, λ (m)
Standard deviation (m)
5 10 20 30
1.6 1.4 1.1 1.0
0.3 0.3 0.5 0.2
allows one to observe that the Al3 (Nb,Cr) crystal is rapidly absorbed by the eutectic structure and presents a propensity to form morphology like fish spine cell. According to Elliot [15], the cells present faceted solid/liquid interface during the growth, which are due to the constitutional undercooling that occurs at the solid/liquid interface because of growth restrictions of the faceted phase. Besides, the cells are elongated, present plate secondary branches and are highly stable. On the other hand, the primary phase Cr(Al,Nb) dendrites (Fig. 1c) present completely isotropic growth, showing no preferential direction. This growth mode occurs, when the growth is totally diffusion controlled and no barrier to the atom transfer between solid and liquid exists. This growth mode is typically presented by non-faceted phases. The microstructure analysis of the eutectic sample 2 revealed that the microstructure shows low regularity morphology. This is probably because of the presence of a growth transition from faceted to non-faceted morphology. The complex regular structures seen in Fig. 6 are typical of faceted/non-faceted eutectic growth. One should note that more regular regions are seen, presenting well-defined eutectic structures that are constituted by short-range fine lamellae. As it was already mentioned, the samples were directionally solidified and it was observed that the interlamellar spacing is strongly dependent on the growth rate, since a growth rate increase causes microstructure refinement, which is shown in Table 3. The highest interlamellar spacing was obtained for 5.0 mm/h (Fig. 7a and b). A low growth rate allows more intense atomic diffusion. Thus, the high interlamellar spacing is attributed to the more intense cooperative growth and to the higher diffusion length. On the other hand, when a 30.0 mm/h growth rate was used, it was obtained the smallest interlamellar spacing (Fig. 7g and h) and the microstructure presented dendritic eutectic cells. This may be attributed to the presence of constitutional undercooling and hence, to the non-equilibrium growth condition. It was also observed that the growth rate increase raises the amount of defects, which leads to an irregular structure.
161
4. Conclusions The eutectic alloy was solidified under controlled and uncontrolled conditions and the results permit one to conclude that the eutectic composition is near 49.3%Al– 39.7%Cr–11%Nb, the melting point is 1347.9 ◦ C and the phases formed are Cr(Al,Nb) and Al3 (Nb,Cr). However, at 892.3 ◦ C, Cr(Al,Nb) undergoes a polymorphic transformation at which the compound Al(Nb)Cr2 is formed. Afterwards, Cr(Al,Nb) undergoes a eutectoid decomposition forming the phases Al(Nb)Cr2 and Al8 Cr5 . The growth morphology of this alloy tends to be faceted/non-faceted with the presence of complex regular structures and regions exhibiting short-range lamellae arranged regularly. We found evidence of the existence of an asymmetric co-operative growth zone. When the alloy is directionally solidified, the structures grown under lower growth rates present higher interlamellar spacing, since diffusion in the interfacial liquid is more intense.
Acknowledgments The authors are grateful to FAPESP, CNPq and CAPES for financial support.
References [1] M. McLean, Directionally Solidified Materials for High Temperature Service, The Metals Society, London, 1983. [2] W. Kurz, D.J. Fisher, Fundamentals of Solidification, third ed., Trans. Tech. Publications, Switzerland, 1992. [3] F.E.E. Lamplough, J.T. Scott, Proc. R. Soc. 90A (1914) 600. [4] K.A. Jackson, Mechanism of Growth Liquid Metals and Solidification, American Society for Metals, Cleveland, OH, 1958, p. 174. [5] K.A. Jackson, J.D. Hunt, Trans. AIME 236 (1966) 1129. [6] J.C. Zhao, M.R. Jackson, L.A. Peluso, J. Phase Equilib. Diffus. 25 (2) (2004) 152–159. [7] K. Mahdouk, J.-C. Gachon, J. Phase Equilib. 21 (2) (2000) 157–166. [8] K. Mahdouk, J.-C. Gachon, J. Alloys Compd. 321 (2001) 232–236. [9] L. Kaufman, H. Nesor, Annu. Rev. Mater. Sci. 3 (1973) 1. [10] M.K. Thomas, in: M.R. Jackson, J.L. Walter, R.W. Hertzberg (Eds.), Conference on In Situ Composites-II, Lake George, NY, 1976, pp. 37–47. [11] J.G. Costa Neto, Al–Cr and Cr–Nb phase diagrams and Al–Cr–Nb 1000 ◦ C section, M.Sc. Dissertation, UNICAMP, 1991, 133 p. (in Portuguese). [12] R.E. Reed-Hill, R. Abbaschian, Physical Metallurgy Principles, PWS Publishing Co., 1994. [13] J.D. Hunt, K.A. Jackson, Trans. Met. Soc. AIME 236 (1966) 843. [14] M.F.X. Gigliotti, G.A. Colligan, G.F. Powell, Met. Trans. AIME 1 (1970), pp. 891/2046. [15] R. Elliott, Int. Met. Rev. 219 (1977) 161.
Growth and morphological characterization of Al–Cr–Nb eutectic alloys S.A. Souza a , C.T. Rios a , A.A. Coelho b , P.L. Ferrandini c , S. Gama b , R. Caram a,∗ a
DEMA/FEM, Universidade Estadual de Campinas, C.P. 6122, CEP 13083-970, SP, Brazil b IFGW, Universidade Estadual de Campinas, C.P. 6165, CEP 13083-970, SP, Brazil c Centro Universit´ ario da FEI, C.P. 85, CEP 09850-901, SP, Brazil Received 8 March 2005; received in revised form 14 April 2005; accepted 19 April 2005 Available online 13 June 2005
Abstract Directional solidification of eutectic alloys attracts considerable attention, when in situ composites are concerned. The eutectic alloys are regarded as presenting regular morphology (lamellar and fibrous structures). Besides, when directionally solidified they show high microstructure stability at high temperatures. This work reports a morphological study of an Al–Cr–Nb eutectic alloy. The solidification morphology of the alloys was studied both in the as-cast and in the directionally solidified conditions. The samples were first obtained in an arc furnace and then directionally solidified using Bridgman equipment. During the directional solidification process, the growth rates utilized varied from 5.0 to 30.0 mm/h. Optical (OM) and scanning electron microscopy (SEM) was used in order to determine the influence of the solidification conditions on the microstructure. The results obtained indicated that the eutectic transformation temperature is near 1347.9 ◦ C with formation of Al3 (Nb,Cr) and Cr(Al,Nb) phases. Also, it was noted that the Cr(Al,Nb) phase undergoes a polymorphic transformation (∼892.3 ◦ C) forming the Al(Nb)Cr2 compound, followed by eutectoid decomposition Cr(Al,Nb) → Al(Nb)Cr2 + Al8 Cr5. © 2005 Elsevier B.V. All rights reserved. Keywords: Metals; Intermetallics; Casting; Scanning electron microscopy; X-ray diffraction
1. Introduction Composite materials have been considered as highly advanced structural materials for no more than 50 years, which allows one to classify them as relatively recent materials. They are generated by the combination of two or more different materials, which gives them improved properties. Usually, composites are formed by a matrix and non-soluble second phase, where they are separated by a well-defined boundary. In the early 60s, according to McLean [1], Kraft proposed that a new class of composite materials could be obtained from the directional solidification of polyphase alloys, particularly the eutectic alloys. In the eutectic transformation, the solidification results in two or more phases aligned in the growth direction. When the solid/liquid transformation occurs in a well-defined direction (directional solidification) the solid phases grow in the heat extraction ∗
Corresponding author. Tel.: +55 19 37883314; fax: +55 19 37883314. E-mail address: [email protected] (R. Caram).
0925-8388/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2005.04.137
direction [2]. Hence, both matrix and strengthening phase are generated simultaneously and the resulting alloy is called in situ composite. The microstructure array of a eutectic alloy depends on the growth mode of the solid phases and distribution of the constituents in the interfacial liquid. The distribution of the constituents also affects the solid/liquid interface undercooling. Lamplough and Scott [3] observed that these are the causes for the wide variety of eutectic morphologies. Jackson [4] studied thermodynamic aspects of the solid growth from the melt and proposed that a solidifying liquid may present two types of interfacial morphology: faceted and non-faceted. The faceted morphology is typical of substances exhibiting complex crystal structures and directional bonding that leads to the formation of planar faces and hence, the growth occurs under preferential directions. When the solid/liquid interface is microscopically smooth, the atomic attachment kinetics does not depend on the crystal plane involved and no facets occur and consequently, it presents no preferential direction. The tendency to a specific morphology can be predicted by
S.A. Souza et al. / Journal of Alloys and Compounds 402 (2005) 156–161
the α factor (α = Sf ξ/R). Jackson and Hunt [5] classified the binary eutectic alloys into three groups based on the concept of α: (a) faceted/faceted (f/f), (b) faceted/non-faceted (f/nf) and (c) non-faceted/non-faceted (nf/nf). In terms of possible applications, the most important alloys are the nf/nf because they originate regular structures. The regular structures can be either fibrous or lamellar, and the kind of structure formed during solidification depends on the volume fraction of the phases involved. A volume fraction between 0 and 0.28 determines a fibrous eutectic structure and a volume fraction between 0.28 and 0.50 determines a lamellar structure. Several metallic systems present eutectic reactions that provide potential alloys to be used at high temperatures and the system Al–Cr–Nb is one of them. Despite being important for understanding the behaviour of several industrial alloys [6], literature information on this system is contradictory and not plentiful. The solidification of eutectic alloys in this system leads to the formation of the intermetallic compounds found in the binary systems Nb–Al, Cr–Al and Nb–Cr that are remarkably brittle. Therefore, specific processing techniques should be attempted in order to solidify these alloys and overcome the pronounced brittleness. At 1000 ◦ C, the maximum solubility of Cr in Al3 Nb is 10.8%, when Cr occupies the Nb sites. In the binary system, Al–Cr [7], the solubility of Al in Cr is higher than 40% but solubility of Nb is considerably low (∼3.0–4.0%). The (Cr,Al)2 Nb (C14), co-exists with the phases (Cr,Al)2 Nb (C15), Al3 (Nb,Cr), Nb2 (Al,Cr) and Cr(Al,Nb) over a large compositional range [8]. According to Kaufman and Nesor [9], the Al–Nb–Cr presents a eutectic transformation at the composition 54%Al–34%Cr–12%Nb and 1317 ◦ C, which would produce an in situ composite, where the matrix would be 50%Al–45%Cr–5%Nb strengthened by Al3 Nb fibres. Thomas [10] suggested that such a eutectic transformation occurs between 1346 and 1366 ◦ C, on a monovariant eutectic curve, which descends towards a pseudo binary eutectic between Cr2 Al3 and NbAl3 , at the composition 54%Al–34%Cr–12%Nb. Besides the work of Kaufman and Thomas, Costa Neto [11], by applying differential thermal analysis, found that the eutectic transformation occurs at 1374 ◦ C. However, according to Costa Neto, the eutectic transformation leads to the Al8 Cr5 , Al3 Nb and a Cr-rich phase. Based on the above information, the purpose of this work is to investigate the obtaining and characterization of the eutectic alloy in the Al-rich region of the Al–Cr–Nb system, both in the as-cast and in the directionally solidified conditions.
2. Experimental procedures All compositions in this work are given in atomic percentage. Ingots of 20 g were prepared using commercial purity elements. Al, Cr and Nb were arc-melted using a nonconsumable tungsten electrode furnace with a water-cooled
157
Table 1 Chemical compositions of the samples studied Alloy (at.%)
Al
Cr
Nb
1 2 3
49.9 49.3 45.8
37.6 39.7 43.5
12.5 11.0 10.7
copper hearth under an argon atmosphere. The purpose was to investigate the compositions near 54%Al–34%Cr–12%Nb, which was proposed by Thomas [10]. Among all alloys studied, three compositions presented interesting results, in terms of the eutectic transformation and thus, were analysed in more detail. They are seen in Table 1. Afterwards, the samples were directionally solidified using a Bridgman equipment with induction heating and Al2 O3 crucibles. Samples 6.0 cm long and 0.7 cm in diameter were processed by slowly shifting the crucible from the upper hot region to the lower cold region under an argon atmosphere. The temperature of the hot region was set at 1500 ± 10 ◦ C, and the temperature gradient was near 100 ◦ C/cm. The growth rates utilized were 5.0, 10.0, 20.0 and 30.0 mm/h. The samples, both in the as-cast and in the directionally solidified conditions were prepared for optical and scanning electron microscopy and the etchant solution used was 6% HNO3 , 3% HF, 91% H2 O (vol.%). Energy dispersive spectrometry (EDS) and computerized image analysis were also used. Phase presence was confirmed by X-ray diffraction and differential thermal analysis was utilized in order to determine the transformation temperatures, under heating and cooling rates of 10 o C/min.
3. Results and discussion All samples in the as-cast condition showed very thin eutectic microstructure with lamellar spacing between 0.5 and 1.0 m, as seen in Fig. 1. Their compositions are located slightly out of the Cr(Al,Nb) + Al3 (Nb,Cr) field in the 1000 ◦ C isotherm, suggested by Mahdouk and Gachon [8]. The microstructure analysis of sample 1, which is in the as-cast condition, reveals that it is formed by Al3 (Nb,Cr) and Al(Nb)Cr2 , that were identified by X-ray diffraction (Fig. 2) and EDS-obtained composition values are seen in Table 2. According to the 1000 ◦ C isotherm suggested by Mahdouk and Gachon [8], one is supposed to find Cr(Al,Nb). However, according to the Al–Cr binary phase diagram, one sees that the Cr-rich phase undergoes a polymorphic transformation, Table 2 EDS-obtained compositions of the phases found in sample 1 Element (at.%) Al Cr Nb Total
Al3 (Nb,Cr)
Al(Nb)Cr2
64.6 5.3 30.1
24.0 72.1 3.9
100.0
100.0
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S.A. Souza et al. / Journal of Alloys and Compounds 402 (2005) 156–161
Fig. 1. SEM images of the as-cast condition microstructures: (a) (back scattered electrons, BSE) hypereutectic (sample 1); (b) (BSE) eutectic (sample 2); (c) (BSE) hypoeutectic (sample 3); (d) (secondary electrons, SE) hypoeutectic (sample 3).
forming the AlCr2 compound and, the eutectoid decomposition (Crss → AlCr2 + Al8 Cr5 ). Therefore, at room temperature, a Cr-rich alloy presents AlCr2 , which comes from the Cr solid solution with an Al content and eventually, when considering the ternary phase diagram, a small Nb content either. Also, the analysis of the sample 1 shows a typically hypereutectic microstructure. Al3 (Nb,Cr) appears light colored as a faceted primary phase in Fig. 1a, while the dark film, which envelops the primary phase particles is Al(Nb)Cr2 . Fig. 1b shows the sample 2, which contains 39.7% Cr and presents a completely eutectic microstructure. Differential
thermal analysis was used in order to determine the eutectic transformation temperature (Fig. 3). The result show an exothermic peek at 1347.9 ◦ C, which is related to the eutectic formation of Al3 (Nb,Cr) and Cr(Al,Nb). The eutectic temperature found by DTA agrees with data found in literature [10] and the deviation most probably comes from the different heating/cooling rates and different equipment response. At 892.3 ◦ C, another exothermic peak was observed, which is associated with the formation of Al(Nb)Cr2 , from the Cr solid solution. This transformation is followed by the eutectoid decomposition, which forms Al8 Cr5 and Al(Nb)Cr2 also from the Cr solid solution. Since both transformations occur considerably near each other, it is quite difficult to identify
Fig. 2. X-ray diffraction pattern of sample 1. As-cast condition.
Fig. 3. DTA curve of the sample 2 (49.3%Al–39.7%Cr–11%Nb).
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Fig. 6. Complex regular microstructure presented by the eutectic alloy 49.3%Al–39.7%Cr–11%Nb (sample 2) (OM).
Fig. 4. X-ray diffraction pattern of sample 2, heat-treated at 1000 ◦ C for 1 h: (a) water quenched and (b) furnace-cooled.
each one. The eutectoid transformation occurs near 60.5% Cr and at 870 ◦ C [7]. This result is in good agreement with the X-ray diffraction patterns that are seen in Fig. 4. The patterns refer to two samples of same composition and different cooling conditions. Both were heated at 1000 ◦ C for 1 h and afterwards, one of them was water quenched (Fig. 4a) and the other furnace-cooled, taking 18 h to reach the room temperature (Fig. 4b). The results allow one to verify the formation of Al3 (Nb,Cr) and Cr(Al,Nb) as well as the polymorphic transformation of Cr(Al,Nb) into Al(Nb)Cr2 . Al8 Cr5 was not found using X-ray diffraction. However, it does not allow one to say that this phase is not present, since X-ray diffraction demands a certain amount of the phase aimed at, and the volume of Al8 Cr5 is considerably low.
A further increase in the Cr content led the alloy to reach the hypoeutectic composition, which results in the precipitation of Cr solid solution. An interesting feature seen in the microstructure of all alloys studied, mostly sample 3, is the formation of the Widmanst¨atten morphology [12] within the hypoeutectic Cr(Al,Nb) primary phase, which is shown in Fig. 1c and d. The magnified view of the microstructure (Fig. 1d) allows one to notice that the phase previously identified by X-ray diffraction as Al(Nb)Cr2 shows acicular morphology. This kind of morphology comes from a solid/solid transformation. This solid/solid transformation of the mother phase, Cr(Al,Nb), occurs by nucleation on specific planes along the grain boundaries, showing a particular growth habit. These specific planes are selected as a way of minimizing the energy difference between the mother phase and the nuclei of the new phase. As a result of this phenomenon, the new phase will grow in some particular directions, giving rise to an acicular structure. The theory of regular eutectic alloys [13] says that the growth mode of the constituent phases in the eutectic transformation determines the resulting structure. The microstructure
Fig. 5. Optical microscopy (OM) of hypereutectic microstructures of the sample 1, which presents the faceted phase Al3 (Nb,Cr).
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analysis of samples 1 and 3 allows one to conclude that a near eutectic composition alloy presents both faceted and non-faceted primary phase morphologies. Fig. 5 shows the non-equilibrium microstructure of sample 1, as the solidification in an arc furnace completely hinders heat extraction control. Sample 1 presents Al3 (Nb,Cr) as primary phase, of faceted morphology, which is in agreement with the theory of Jackson [4], since Al3 (Nb,Cr) is an intermetallic compound. Faceted phases present high melting entropy (α > 2) and grow under preferential directions. It was also observed
that Al3 (Nb,Cr) nucleates within the co-operative zone, since the dendritic growth is rapidly hindered and absorbed by the eutectic structure. The nucleation of a second phase on the first one will determine the manner the primary phase will grow. Usually, a second phase nucleates as a film over the primary phase, because of the higher solute concentration in the solid/liquid interface, since the primary crystals are unable to nucleate the eutectic second phase. This is probably due to the existence of an asymmetric co-operative growth zone [14]. Fig. 6
Fig. 7. SEM images of directionally solidified microstructures under the growth rates (BSE): (a and b) 5.0 mm/h; (c and d) 10.0 mm/h; (e and f) 20.0 mm/h; (g and h) 30.0 mm/h.
S.A. Souza et al. / Journal of Alloys and Compounds 402 (2005) 156–161 Table 3 Lamellar spacing as a function of growth rate of sample 2, directionally solidified Growth rate,V (mm/h)
Lamellar spacing, λ (m)
Standard deviation (m)
5 10 20 30
1.6 1.4 1.1 1.0
0.3 0.3 0.5 0.2
allows one to observe that the Al3 (Nb,Cr) crystal is rapidly absorbed by the eutectic structure and presents a propensity to form morphology like fish spine cell. According to Elliot [15], the cells present faceted solid/liquid interface during the growth, which are due to the constitutional undercooling that occurs at the solid/liquid interface because of growth restrictions of the faceted phase. Besides, the cells are elongated, present plate secondary branches and are highly stable. On the other hand, the primary phase Cr(Al,Nb) dendrites (Fig. 1c) present completely isotropic growth, showing no preferential direction. This growth mode occurs, when the growth is totally diffusion controlled and no barrier to the atom transfer between solid and liquid exists. This growth mode is typically presented by non-faceted phases. The microstructure analysis of the eutectic sample 2 revealed that the microstructure shows low regularity morphology. This is probably because of the presence of a growth transition from faceted to non-faceted morphology. The complex regular structures seen in Fig. 6 are typical of faceted/non-faceted eutectic growth. One should note that more regular regions are seen, presenting well-defined eutectic structures that are constituted by short-range fine lamellae. As it was already mentioned, the samples were directionally solidified and it was observed that the interlamellar spacing is strongly dependent on the growth rate, since a growth rate increase causes microstructure refinement, which is shown in Table 3. The highest interlamellar spacing was obtained for 5.0 mm/h (Fig. 7a and b). A low growth rate allows more intense atomic diffusion. Thus, the high interlamellar spacing is attributed to the more intense cooperative growth and to the higher diffusion length. On the other hand, when a 30.0 mm/h growth rate was used, it was obtained the smallest interlamellar spacing (Fig. 7g and h) and the microstructure presented dendritic eutectic cells. This may be attributed to the presence of constitutional undercooling and hence, to the non-equilibrium growth condition. It was also observed that the growth rate increase raises the amount of defects, which leads to an irregular structure.
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4. Conclusions The eutectic alloy was solidified under controlled and uncontrolled conditions and the results permit one to conclude that the eutectic composition is near 49.3%Al– 39.7%Cr–11%Nb, the melting point is 1347.9 ◦ C and the phases formed are Cr(Al,Nb) and Al3 (Nb,Cr). However, at 892.3 ◦ C, Cr(Al,Nb) undergoes a polymorphic transformation at which the compound Al(Nb)Cr2 is formed. Afterwards, Cr(Al,Nb) undergoes a eutectoid decomposition forming the phases Al(Nb)Cr2 and Al8 Cr5 . The growth morphology of this alloy tends to be faceted/non-faceted with the presence of complex regular structures and regions exhibiting short-range lamellae arranged regularly. We found evidence of the existence of an asymmetric co-operative growth zone. When the alloy is directionally solidified, the structures grown under lower growth rates present higher interlamellar spacing, since diffusion in the interfacial liquid is more intense.
Acknowledgments The authors are grateful to FAPESP, CNPq and CAPES for financial support.
References [1] M. McLean, Directionally Solidified Materials for High Temperature Service, The Metals Society, London, 1983. [2] W. Kurz, D.J. Fisher, Fundamentals of Solidification, third ed., Trans. Tech. Publications, Switzerland, 1992. [3] F.E.E. Lamplough, J.T. Scott, Proc. R. Soc. 90A (1914) 600. [4] K.A. Jackson, Mechanism of Growth Liquid Metals and Solidification, American Society for Metals, Cleveland, OH, 1958, p. 174. [5] K.A. Jackson, J.D. Hunt, Trans. AIME 236 (1966) 1129. [6] J.C. Zhao, M.R. Jackson, L.A. Peluso, J. Phase Equilib. Diffus. 25 (2) (2004) 152–159. [7] K. Mahdouk, J.-C. Gachon, J. Phase Equilib. 21 (2) (2000) 157–166. [8] K. Mahdouk, J.-C. Gachon, J. Alloys Compd. 321 (2001) 232–236. [9] L. Kaufman, H. Nesor, Annu. Rev. Mater. Sci. 3 (1973) 1. [10] M.K. Thomas, in: M.R. Jackson, J.L. Walter, R.W. Hertzberg (Eds.), Conference on In Situ Composites-II, Lake George, NY, 1976, pp. 37–47. [11] J.G. Costa Neto, Al–Cr and Cr–Nb phase diagrams and Al–Cr–Nb 1000 ◦ C section, M.Sc. Dissertation, UNICAMP, 1991, 133 p. (in Portuguese). [12] R.E. Reed-Hill, R. Abbaschian, Physical Metallurgy Principles, PWS Publishing Co., 1994. [13] J.D. Hunt, K.A. Jackson, Trans. Met. Soc. AIME 236 (1966) 843. [14] M.F.X. Gigliotti, G.A. Colligan, G.F. Powell, Met. Trans. AIME 1 (1970), pp. 891/2046. [15] R. Elliott, Int. Met. Rev. 219 (1977) 161.