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Inhibition of Al grain coarsening by quasicrystalline icosahedral phase in the rapidly solidified powder metallurgy Al–Fe–Ti–Cr alloy
Scripta Materialia 56 (2007) 785–788 www.actamat-journals.com
Inhibition of Al grain coarsening by quasicrystalline icosahedral phase in the rapidly solidified powder metallurgy Al–Fe–Ti–Cr alloy Michiaki Yamasaki,* Yusuke Nagaishi and Yoshihito Kawamura Department of Materials Science, Kumamoto University, Kumamoto 860-8555, Japan Received 10 July 2006; revised 10 December 2006; accepted 4 January 2007 Available online 6 February 2007
The quasicrystalline icosahedral phase prevents Al matrix grain coarsening in the rapidly solidified powder metallurgy Al92.5Fe2.5Ti2.5Cr2.5 alloy due to a thermally stable crystallographic orientation relationship between the icosahedral and Al phases. The 2-fold axis of the icosahedral phase is along the h1 1 2i axis of face centered cubic Al; this orientation relationship was observed in alloys annealed at 573 K for 1000 h. A highly dispersed intergranular icosahedral phase located on the Al matrix significantly enhances the heat resistance of the alloys. 2007 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. Keywords: Quasicrystal; Orientation relationship; Aluminum; Al–Fe–Ti–Cr; Rapid solidification
Rapidly solidified powder metallurgy (RS P/M) technology is suited to the design of Al alloys for use at elevated temperatures because of the finely dispersed intermetallic strengthening phases that form during rapid solidification. Much of the development in the field of RS P/M Al alloys has sought to disperse lowdiffusion-coefficient transition metals as intermetallic phases in the alloys [1]. Typical examples include Al– Fe–V–Si [2,3], Al–Fe–Mo [4], Al–Fe–Mo–Si [5] and Al–Fe–Ti [6] RS P/M alloys. Recently, Al–Fe–Ti–Cr [7–10] alloys have also attracted a great deal of attention because of their high heat resistance and the formation of nano-scale quasicrystalline phases. However, the role of the quasicrystalline phase in improving heat resistance is still unknown. Therefore, in this study, we investigated the change in the mechanical properties and microstructure of the RS P/M Al92.5Fe2.5Ti2.5Cr2.5 alloy as a result of annealing at a high-temperature of 573 K for 1000 h (more than 40 days). Particular attention has been paid to the annealing-time evolution of the quasicrystalline particle distribution and the crystallographic orientation relationship (OR) between the quasicrystalline and Al matrix phases.
* Corresponding author. Tel.: +81 96 342 3705; fax: +81 96 342 3710; e-mail: [email protected]
Al92.5Fe2.5Ti2.5Cr2.5 (at.%) ingots were prepared by arc melting a mixture of pure metals in an argon atmosphere. The RS P/M alloy was prepared by a closed P/M processing system [11]. RS powder was produced by high-pressure argon-gas atomization at a dynamic gas pressure of 9.8 MPa. The powder was sieved to less than 38 lm and pressed into a copper billet with an inner diameter of 20 mm and an outer diameter of 23 mm. In order to prevent the oxidation of these powders, all the processes (i.e. collecting and sieving of atomized powders, and packing into a copper billet) were performed in a closed chamber and glove box, in which oxygen and moisture contents in the argon atmosphere were maintained at less than 0.5 ppm by a gas purifier. Powders in copper billets were degassed at 623–773 K for 900 s at 102 Pa [12,13]. Extrusion was performed at an extrusion ratio of 5, at 623–773 K and a ram speed of 2.5 mm s1. Tensile strength and elongation were investigated at a strain rate of 5 · 104 s1 using an Instron-type tensile testing machine. The structure of the extruded bulk alloys was investigated by X-ray diffractometry (XRD) with a JDX-8030 using Cu Ka radiation and transmission electron microscopy (TEM) with a JEM-2000FX operating at 200 kV. The composition of the intermetallic compounds was estimated by energy-dispersive X-ray spectroscopy (EDS) with a JEM-3000F operating at 300 kV.
1359-6462/$ - see front matter 2007 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.scriptamat.2007.01.012
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Figure 1. Room temperature tensile yield strength and elongation of the Al92.5Ti2.5Fe2.5Cr2.5 RS P/M alloys. Extrusion ratio is 5:1.
Figure 1 shows the tensile yield strength and elongation of the Al92.5Ti2.5Fe2.5Cr2.5 RS P/M alloys consolidated at various extrusion temperatures. Although tensile yield strength decreased with an increase in the extrusion temperature, the alloy exhibited comparatively high strength at room and elevated temperatures despite the low extrusion ratio of 5:1. This is because preparation of the alloys by powder consolidation in a high-purity argon atmosphere results in sound metalto-metal bonding between powder particles during extrusion [10–13]. Figure 2 compares the elevated temperature tensile yield strength of the as-consolidated specimens with those annealed at the high-temperature of 573 K for 100 h and 1000 h. It is noteworthy that the elevated temperature strength of the alloy annealed at 573 K for 100 h increased with increasing extrusion temperature. After annealing at 573 K for 1000 h, the yield strength and ultimate strength of the as-consolidated specimens and those annealed at the higher temperature remained at the high level of 213 and 230 MPa, respectively. Microstructure characterization in the RS P/M alloy consolidated at 773 K, which shows excellent mechani-
Figure 2. Elevated temperature tensile yield strength of the Al92.5Ti2.5Fe2.5Cr2.5 RS P/M alloys consolidated and then annealed at 573 K. Extrusion ratio is 5:1.
Figure 3. X-ray diffraction patterns of the Al92.5Ti2.5Fe2.5Cr2.5 RS P/M alloys. As-consolidated, and annealed at 573 K for 1000 h.
cal properties at elevated temperature, has been performed by XRD, TEM observation and EDS analysis. Figure 3 shows XRD patterns of the Al92.5Ti2.5Fe2.5Cr2.5 RS P/M alloys that consolidated at 773 K and were then annealed at 573 K for 1000 h. These RS P/M alloys mainly consist of face centered cubic (fcc) Al, D022 Al3Ti, L12 Al3Ti and quasicrystalline icosahedral phases (I-phase). No phase change occurred during lengthy annealing. Figure 4 shows low-magnification TEM micrographs of as-consolidated and annealed RS P/M Al92.5Fe2.5Ti2.5Cr2.5 alloys. It is noteworthy that the Al grain sizes remain nearly unchanged even after 1000 h of annealing. The RS P/M Al92.5Fe2.5Ti2.5Cr2.5 alloy consists mainly of equiaxed grains with a grain size of 50–400 nm. Small amounts of square-shaped particles and many spherically shaped particles were observed. The selected area electron diffraction (SAED) patterns indicate that the square and spherically shaped particles constitute L12 Al3Ti and the I-phase, respectively, as shown in Figure 5. A chemical analysis by EDS showed the averaged compositions of Al matrix grains and Al3Ti type compound are (at.%) Al–0.7 ± 0.6 Fe– 0.8 ± 0.1 Ti–1.5 ± 0.7 Cr and Al–22.3 ± 1.7 Ti– 1.0 ± 0.4 Fe–10.9 ± 0.6 Cr, respectively. The average composition for the I-phase grain in RS P/M Al92.5Fe2.5Ti2.5Cr2.5 alloy was found to be approximately (at.%) Al–12.7 ± 2.9 Fe–11.3 ± 1.6 Cr–1.2 ± 0.6 Ti; the I-phase significantly differs in composition from other icosahedral quasicrystals that have so far been
Figure 4. Low-magnification TEM micrographs of the Al92.5Ti2.5Fe2.5Cr2.5 RS P/M alloys. (a) As-consolidated. Annealed at 573 K for (b) 100 h and (c) 1000 h.
M. Yamasaki et al. / Scripta Materialia 56 (2007) 785–788
Figure 5. TEM micrograph of (a) L12 Al3Ti phase and (c) icosahedral phase in the as-consolidated Al92.5Ti2.5Fe2.5Cr2.5 RS P/M alloy. SAED patterns taken from (b) L12 Al3Ti phase and icosahedral phase; (d) 2fold axis //EB, (e) 3-fold axis //EB, (f) 5-fold axis //EB.
reported, such as Al84.6Cr15.4 [14], Al82Fe18 [15], Al94Fe4Cr [16] and other quarternary quasicrystals [17] in RS Al–Fe–Ti–Cr alloys. Moreover, the Al13Cr2 compound was not detected by EDS analysis and TEM observation in this study, although the formation of the Al13Cr2 compound has often been reported in RS Al–Fe–Ti–Cr alloys. The volume fraction of the I-phase is estimated to be approximately 8% in the as-consolidated RS P/M alloy. Almost all the I-phase particles exist next to Al matrix grains as intergranular particles, and seem to cover the corners of the Al particles. Furthermore, it was noted that there is a crystallographic OR between the I-phase and the Al particles. Figure 6 shows TEM micrographs of the RS P/M Al92.5Fe2.5Ti2.5Cr2.5 alloy annealed at 573 K for 1000 h. Nanoscale Al matrix grains enclosed by I-phase particles were
Figure 6. (a) TEM micrographs of the Al92.5Ti2.5Fe2.5Cr2.5 RS P/M alloy annealed at 573 K for 1000 h. SAED patterns taken from (b) icosahedral, (c) both icosahedral and a-Al, and (d) a-Al phases. Diffraction spots are indexed by an fcc Al crystal.
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often observed. SAED patterns revealed that the twofold axis of the I-phase is along the h1 1 2i axis of fcc Al. The OR was retained after annealing for a long time at 573 K. In the P/M Al92.5Fe2.5Ti2.5Cr2.5 alloy consolidated by extrusion at 773 K, the I-phase and Al3Ti phase have been compositionally identified from the results of XRD, EDS analysis and TEM observation. Although these phases may contribute to the improvement of mechanical properties of the alloys, the difference between the role of the Al3Ti phase and the I-phase needs be considered. Finely dispersed L12 Al3Ti compounds in equiaxed grain microstructure may effectively enhance mechanical properties, especially at room temperature. In contrast, the I-phase particle dispersion in the alloy microstructure results in improved heat resistance of the alloys [18]. The I-phase is effective in increasing the heat resistance of the alloys because of the crystallographic OR. The OR between the quasicrystalline I-phase and fcc Al matrix in, for example, Al–Mn [19] and Al–Li–Cu– Mg [20,21] is well known. The intergranular quasicrystalline I-phases exhibit several definite ORs with the fcc Al matrix. The first orientation relationship (OR1) in an ideal case is denoted as follows: three 2-fold axes (i2) which are perpendicular to each other are parallel to [1 1 1], ½1 1 0 and ½1 1 2 of the fcc Al. This OR1 can be expressed as: i2//[1 1 1]fcc, i2//½1 1 0fcc and i2// ½1 1 2fcc . The second orientation relationship (OR2) is i2//[0 0 1]fcc, i2//[1 1 0]fcc and i2//½1 1 0fcc . The third orientation relationship (OR3) is i5//[1 1 0]fcc, i2//½1 1 0fcc and i2//½1 1 1fcc [19]. In the case of equiaxed nano-scale grain Al–Fe–Ti–Cr RS P/M alloy, almost all the OR observed in this study was i2//½1 1 2fcc , and this OR can be classified into OR1, which is the most common (about 80% of cases) [21]. The thermal stability of the I-phases in Al– Fe–Ti–Cr alloys has also been investigated. Prima et al. reported that I-phases in Al93Fe3Ti2Cr2 RS ribbons are stable up to 673 K and that the decomposition process starts at 693 K [23]. Sahoo and Stone reported that the decomposition temperature of I-phases in Al93Fe3Ti2Cr2 RS ribbons is approximately 753 K [22]. With Al92.5Fe2.5Ti2.5Cr2.5 RS P/M alloys, however, icosahedral Al75Fe13Cr11Ti1 quasicrystal remained intact after extrusion at 773 K followed by high-temperature degassing, as shown in Figures 5 and 6. Moreover, the I-phase was thermally stable at 573 K, maintained the OR between the I-phase and Al matrix, and showed no grain growth during long-term annealing. The I-phase with an OR to fcc-Al suppresses Al grain coarsening, resulting in enhancement of heat resistance of the alloys. In conclusion, we have investigated the microstructure and mechanical properties of Al92.5Fe2.5Ti2.5Cr2.5 RS P/M alloys prepared by the closed powder metallurgy processing technique. The following conclusions have been drawn: (1) The RS P/M Al92.5Fe2.5Ti2.5Cr2.5 alloy consolidated by extrusion at 773 K shows excellent mechanical properties at elevated temperature together with high heat resistance. The yield strength and ultimate strength of the alloys after annealing for 1000 h at 573 K are 215 and 230 MPa, respectively.
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(2) RS P/M Al92.5Fe2.5Ti2.5Cr2.5 alloy contains fcc Al, L12 Al3Ti, D022 Al3Ti and Al75Fe13Cr11Ti1 icosahedral quasicrystal. The Al75Fe13Cr11Ti1 quasicrystalline phase is thermally stable up to 773 K. (3) There is a crystallographic OR between the icosahedral and Al phases. The 2-fold axis of the icosahedral is along the h1 1 2i axis of the fcc Al, resulting in suppression of Al grain coarsening. Highly dispersed intergranular I-phases with a strong OR in equiaxed microstructure significantly enhance the heat resistance of the alloy. The microstructure observations are supported in part by the ‘‘Nanotechnology Support Project’’ of the Ministry of Education, Science, Sport and Culture, Japan (MEXT). We are indebted to and thank Dr. Nishijima, Institute for Materials Research, Tohoku University, for his support with the EDS analysis. This work was financially supported by the Grant-in-aid from MEXT, and also by the Japan Aluminium Association. [1] J.R. Davis (Ed.), ASM Specialty Handbook, Aluminum and Aluminum Alloys, ASM International, Materials Park, OH, 1993, p. 154. [2] A. Seibold, Z. Metall. 72 (1981) 712. [3] D.J. Skinner, R.L. Bye, D. Raybould, A.M. Brown, Scripta Metall. 20 (1986) 867. [4] I.G. Palmer, M.P. Thomas, G.J. Marshall, in: Y.W. Kim, W.M. Griffith (Eds.), Dispersion Strengthened Aluminum Alloys, The Minerals, Metals & Materials Society, Warrendale, PA, 1988, p. 213. [5] P.Y. Li, H.J. Yu, S.C. Chai, Y.R. Li, Scripta Mater. 49 (2003) 819.
[6] J.Q. Guo, N.S. Kazama, Mater. Sci. Eng. A 232 (1997) 177. [7] Y. Kawamura, A. Inoue, M. Takagi, T. Imura, Mater. Trans. JIM 40 (1999) 392. [8] Y. Kawamura, A. Inoue, M. Takagi, H. Ohta, T. Imura, T. Masumoto, Scripta Mater. 40 (1999) 1131. [9] A. Inoue, H.M. Kimura, Mater. Sci. Eng. A 286 (2000) 1. [10] Y. Nagaishi, M. Yamasaki, Y. Kawamura, Mater. Sci. Eng. A, doi:10.1016/j.msea.2006.02.383. [11] Y. Kawamura, A. Inoue, T. Masumoto, Scripta Metall. 29 (1993) 25. [12] M. Yamasaki, Y. Kawamura, Mater. Trans. 45 (2004) 1335. [13] M. Yamasaki, Y. Kawamura, Mater. Trans. 47 (2006) 1902. [14] A.P. Tsai, A. Inoue, T. Masumoto, J. Mater. Sci. Lett. 7 (1988) 322. [15] E. Gopal, S. Baranidharan, J. Sekhar, Mater. Sci. Eng. 99 (1988) 413. [16] R. Manaila, V. Florescu, A. Jianu, A. Badescu, Phys. Status Solidi A 109 (1988) 61. [17] H.M. Kimura, K. Sasamori, A. Inoue, Mater. Sci. Eng. A 294–296 (2000) 168. [18] M. Yamasaki, Y. Nagaishi, Y. Kawamura, Collected Abstracts of the 110th Meeting of the Japan Inst. Light Metals, The Japan Institute of Light Metals, Tokyo, 2006, p. 203; M. Yamasaki, Y. Kawamura, Powder Metallurgy, in preparation. [19] C. Beeli, T. Ishimasa, H.U. Nissen, Philos. Mag. B 57 (1988) 599. [20] A. Loiseau, G. Lapasset, Philos. Mag. Lett. 56 (1987) 165. [21] A. Loiseau, G. Lapasset, J. Phys. Paris 47 (1986) C3-331. [22] F. Prima, M. Tomut, I. Stone, B. Cantor, D. Janickovic, G. Vlasak, P. Svec, Mater. Sci. Eng. A375–377 (2004) 772. [23] K.L. Sahoo, I.C. Stone, Philo. Mag. Lett. 85 (2005) 231.
Inhibition of Al grain coarsening by quasicrystalline icosahedral phase in the rapidly solidified powder metallurgy Al–Fe–Ti–Cr alloy Michiaki Yamasaki,* Yusuke Nagaishi and Yoshihito Kawamura Department of Materials Science, Kumamoto University, Kumamoto 860-8555, Japan Received 10 July 2006; revised 10 December 2006; accepted 4 January 2007 Available online 6 February 2007
The quasicrystalline icosahedral phase prevents Al matrix grain coarsening in the rapidly solidified powder metallurgy Al92.5Fe2.5Ti2.5Cr2.5 alloy due to a thermally stable crystallographic orientation relationship between the icosahedral and Al phases. The 2-fold axis of the icosahedral phase is along the h1 1 2i axis of face centered cubic Al; this orientation relationship was observed in alloys annealed at 573 K for 1000 h. A highly dispersed intergranular icosahedral phase located on the Al matrix significantly enhances the heat resistance of the alloys. 2007 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. Keywords: Quasicrystal; Orientation relationship; Aluminum; Al–Fe–Ti–Cr; Rapid solidification
Rapidly solidified powder metallurgy (RS P/M) technology is suited to the design of Al alloys for use at elevated temperatures because of the finely dispersed intermetallic strengthening phases that form during rapid solidification. Much of the development in the field of RS P/M Al alloys has sought to disperse lowdiffusion-coefficient transition metals as intermetallic phases in the alloys [1]. Typical examples include Al– Fe–V–Si [2,3], Al–Fe–Mo [4], Al–Fe–Mo–Si [5] and Al–Fe–Ti [6] RS P/M alloys. Recently, Al–Fe–Ti–Cr [7–10] alloys have also attracted a great deal of attention because of their high heat resistance and the formation of nano-scale quasicrystalline phases. However, the role of the quasicrystalline phase in improving heat resistance is still unknown. Therefore, in this study, we investigated the change in the mechanical properties and microstructure of the RS P/M Al92.5Fe2.5Ti2.5Cr2.5 alloy as a result of annealing at a high-temperature of 573 K for 1000 h (more than 40 days). Particular attention has been paid to the annealing-time evolution of the quasicrystalline particle distribution and the crystallographic orientation relationship (OR) between the quasicrystalline and Al matrix phases.
* Corresponding author. Tel.: +81 96 342 3705; fax: +81 96 342 3710; e-mail: [email protected]
Al92.5Fe2.5Ti2.5Cr2.5 (at.%) ingots were prepared by arc melting a mixture of pure metals in an argon atmosphere. The RS P/M alloy was prepared by a closed P/M processing system [11]. RS powder was produced by high-pressure argon-gas atomization at a dynamic gas pressure of 9.8 MPa. The powder was sieved to less than 38 lm and pressed into a copper billet with an inner diameter of 20 mm and an outer diameter of 23 mm. In order to prevent the oxidation of these powders, all the processes (i.e. collecting and sieving of atomized powders, and packing into a copper billet) were performed in a closed chamber and glove box, in which oxygen and moisture contents in the argon atmosphere were maintained at less than 0.5 ppm by a gas purifier. Powders in copper billets were degassed at 623–773 K for 900 s at 102 Pa [12,13]. Extrusion was performed at an extrusion ratio of 5, at 623–773 K and a ram speed of 2.5 mm s1. Tensile strength and elongation were investigated at a strain rate of 5 · 104 s1 using an Instron-type tensile testing machine. The structure of the extruded bulk alloys was investigated by X-ray diffractometry (XRD) with a JDX-8030 using Cu Ka radiation and transmission electron microscopy (TEM) with a JEM-2000FX operating at 200 kV. The composition of the intermetallic compounds was estimated by energy-dispersive X-ray spectroscopy (EDS) with a JEM-3000F operating at 300 kV.
1359-6462/$ - see front matter 2007 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.scriptamat.2007.01.012
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Figure 1. Room temperature tensile yield strength and elongation of the Al92.5Ti2.5Fe2.5Cr2.5 RS P/M alloys. Extrusion ratio is 5:1.
Figure 1 shows the tensile yield strength and elongation of the Al92.5Ti2.5Fe2.5Cr2.5 RS P/M alloys consolidated at various extrusion temperatures. Although tensile yield strength decreased with an increase in the extrusion temperature, the alloy exhibited comparatively high strength at room and elevated temperatures despite the low extrusion ratio of 5:1. This is because preparation of the alloys by powder consolidation in a high-purity argon atmosphere results in sound metalto-metal bonding between powder particles during extrusion [10–13]. Figure 2 compares the elevated temperature tensile yield strength of the as-consolidated specimens with those annealed at the high-temperature of 573 K for 100 h and 1000 h. It is noteworthy that the elevated temperature strength of the alloy annealed at 573 K for 100 h increased with increasing extrusion temperature. After annealing at 573 K for 1000 h, the yield strength and ultimate strength of the as-consolidated specimens and those annealed at the higher temperature remained at the high level of 213 and 230 MPa, respectively. Microstructure characterization in the RS P/M alloy consolidated at 773 K, which shows excellent mechani-
Figure 2. Elevated temperature tensile yield strength of the Al92.5Ti2.5Fe2.5Cr2.5 RS P/M alloys consolidated and then annealed at 573 K. Extrusion ratio is 5:1.
Figure 3. X-ray diffraction patterns of the Al92.5Ti2.5Fe2.5Cr2.5 RS P/M alloys. As-consolidated, and annealed at 573 K for 1000 h.
cal properties at elevated temperature, has been performed by XRD, TEM observation and EDS analysis. Figure 3 shows XRD patterns of the Al92.5Ti2.5Fe2.5Cr2.5 RS P/M alloys that consolidated at 773 K and were then annealed at 573 K for 1000 h. These RS P/M alloys mainly consist of face centered cubic (fcc) Al, D022 Al3Ti, L12 Al3Ti and quasicrystalline icosahedral phases (I-phase). No phase change occurred during lengthy annealing. Figure 4 shows low-magnification TEM micrographs of as-consolidated and annealed RS P/M Al92.5Fe2.5Ti2.5Cr2.5 alloys. It is noteworthy that the Al grain sizes remain nearly unchanged even after 1000 h of annealing. The RS P/M Al92.5Fe2.5Ti2.5Cr2.5 alloy consists mainly of equiaxed grains with a grain size of 50–400 nm. Small amounts of square-shaped particles and many spherically shaped particles were observed. The selected area electron diffraction (SAED) patterns indicate that the square and spherically shaped particles constitute L12 Al3Ti and the I-phase, respectively, as shown in Figure 5. A chemical analysis by EDS showed the averaged compositions of Al matrix grains and Al3Ti type compound are (at.%) Al–0.7 ± 0.6 Fe– 0.8 ± 0.1 Ti–1.5 ± 0.7 Cr and Al–22.3 ± 1.7 Ti– 1.0 ± 0.4 Fe–10.9 ± 0.6 Cr, respectively. The average composition for the I-phase grain in RS P/M Al92.5Fe2.5Ti2.5Cr2.5 alloy was found to be approximately (at.%) Al–12.7 ± 2.9 Fe–11.3 ± 1.6 Cr–1.2 ± 0.6 Ti; the I-phase significantly differs in composition from other icosahedral quasicrystals that have so far been
Figure 4. Low-magnification TEM micrographs of the Al92.5Ti2.5Fe2.5Cr2.5 RS P/M alloys. (a) As-consolidated. Annealed at 573 K for (b) 100 h and (c) 1000 h.
M. Yamasaki et al. / Scripta Materialia 56 (2007) 785–788
Figure 5. TEM micrograph of (a) L12 Al3Ti phase and (c) icosahedral phase in the as-consolidated Al92.5Ti2.5Fe2.5Cr2.5 RS P/M alloy. SAED patterns taken from (b) L12 Al3Ti phase and icosahedral phase; (d) 2fold axis //EB, (e) 3-fold axis //EB, (f) 5-fold axis //EB.
reported, such as Al84.6Cr15.4 [14], Al82Fe18 [15], Al94Fe4Cr [16] and other quarternary quasicrystals [17] in RS Al–Fe–Ti–Cr alloys. Moreover, the Al13Cr2 compound was not detected by EDS analysis and TEM observation in this study, although the formation of the Al13Cr2 compound has often been reported in RS Al–Fe–Ti–Cr alloys. The volume fraction of the I-phase is estimated to be approximately 8% in the as-consolidated RS P/M alloy. Almost all the I-phase particles exist next to Al matrix grains as intergranular particles, and seem to cover the corners of the Al particles. Furthermore, it was noted that there is a crystallographic OR between the I-phase and the Al particles. Figure 6 shows TEM micrographs of the RS P/M Al92.5Fe2.5Ti2.5Cr2.5 alloy annealed at 573 K for 1000 h. Nanoscale Al matrix grains enclosed by I-phase particles were
Figure 6. (a) TEM micrographs of the Al92.5Ti2.5Fe2.5Cr2.5 RS P/M alloy annealed at 573 K for 1000 h. SAED patterns taken from (b) icosahedral, (c) both icosahedral and a-Al, and (d) a-Al phases. Diffraction spots are indexed by an fcc Al crystal.
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often observed. SAED patterns revealed that the twofold axis of the I-phase is along the h1 1 2i axis of fcc Al. The OR was retained after annealing for a long time at 573 K. In the P/M Al92.5Fe2.5Ti2.5Cr2.5 alloy consolidated by extrusion at 773 K, the I-phase and Al3Ti phase have been compositionally identified from the results of XRD, EDS analysis and TEM observation. Although these phases may contribute to the improvement of mechanical properties of the alloys, the difference between the role of the Al3Ti phase and the I-phase needs be considered. Finely dispersed L12 Al3Ti compounds in equiaxed grain microstructure may effectively enhance mechanical properties, especially at room temperature. In contrast, the I-phase particle dispersion in the alloy microstructure results in improved heat resistance of the alloys [18]. The I-phase is effective in increasing the heat resistance of the alloys because of the crystallographic OR. The OR between the quasicrystalline I-phase and fcc Al matrix in, for example, Al–Mn [19] and Al–Li–Cu– Mg [20,21] is well known. The intergranular quasicrystalline I-phases exhibit several definite ORs with the fcc Al matrix. The first orientation relationship (OR1) in an ideal case is denoted as follows: three 2-fold axes (i2) which are perpendicular to each other are parallel to [1 1 1], ½1 1 0 and ½1 1 2 of the fcc Al. This OR1 can be expressed as: i2//[1 1 1]fcc, i2//½1 1 0fcc and i2// ½1 1 2fcc . The second orientation relationship (OR2) is i2//[0 0 1]fcc, i2//[1 1 0]fcc and i2//½1 1 0fcc . The third orientation relationship (OR3) is i5//[1 1 0]fcc, i2//½1 1 0fcc and i2//½1 1 1fcc [19]. In the case of equiaxed nano-scale grain Al–Fe–Ti–Cr RS P/M alloy, almost all the OR observed in this study was i2//½1 1 2fcc , and this OR can be classified into OR1, which is the most common (about 80% of cases) [21]. The thermal stability of the I-phases in Al– Fe–Ti–Cr alloys has also been investigated. Prima et al. reported that I-phases in Al93Fe3Ti2Cr2 RS ribbons are stable up to 673 K and that the decomposition process starts at 693 K [23]. Sahoo and Stone reported that the decomposition temperature of I-phases in Al93Fe3Ti2Cr2 RS ribbons is approximately 753 K [22]. With Al92.5Fe2.5Ti2.5Cr2.5 RS P/M alloys, however, icosahedral Al75Fe13Cr11Ti1 quasicrystal remained intact after extrusion at 773 K followed by high-temperature degassing, as shown in Figures 5 and 6. Moreover, the I-phase was thermally stable at 573 K, maintained the OR between the I-phase and Al matrix, and showed no grain growth during long-term annealing. The I-phase with an OR to fcc-Al suppresses Al grain coarsening, resulting in enhancement of heat resistance of the alloys. In conclusion, we have investigated the microstructure and mechanical properties of Al92.5Fe2.5Ti2.5Cr2.5 RS P/M alloys prepared by the closed powder metallurgy processing technique. The following conclusions have been drawn: (1) The RS P/M Al92.5Fe2.5Ti2.5Cr2.5 alloy consolidated by extrusion at 773 K shows excellent mechanical properties at elevated temperature together with high heat resistance. The yield strength and ultimate strength of the alloys after annealing for 1000 h at 573 K are 215 and 230 MPa, respectively.
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