Characterization of Phases in an As-cast Copper-Manganese- Aluminum Alloy

J. Mater. Sci. Technol., Vol.22 No.6, 2006 779 Characterization of Phases in an As-cast Copper-Manganese- Aluminum Alloy J.Iqbal, F.Hasan and F.Ahmad Department of Metallurgical and Materials Engineering, University of Engineering and Technology, Lahore, Pakistan [Manuscript received February 20, 2006, in revised form May 14, 2006] Copper-manganese-aluminum (CMA) alloys, containing small additions of Fe, Ni, and Si, exhibit good strength and remarkable corrosion resistance against sea water. The alloys are used in as-cast condition, and their microstructure can show wide variations. The morphology, crystallography and composition of the phases presented in an as-cast (CMA) alloy of nominal composition Cu-14%Mn-8%Al-3%Fe-2%Ni were investigated using optical, electron optical, and microprobe analytical techniques. The as-cast microstructure consisted of the grains of fcc α and bcc β-phases alongwith intermetallic precipitates of various morphologies. The room temperature microstructure exhibited four different types of precipitates inside the α-grains: the large dendritic-shaped particles and the cuboid-shaped precipitates, which were rich in Fe and Mn and had an fcc structure, while the small dendritic-shaped particles and the globular precipitates were based on Fe 3 Al and had DO 3 structure. These four different morphologies of intermetallic precipitates exhibited discrete orientationrelationships with the α-matrix. The β-grains only contained very small cuboid shaped precipitates, which could only be resolved through transmission electron microscopy. These precipitates were found to be based on Fe 3 Al and had the DO 3 structure. KEY WORDS: Orientation relationship; DO 3 structure; Intermetallic phases 1. Introduction The requirement of high strength Cu-base alloys for use in marine environments has lead to the development of high-tensile brasses, the aluminumbronzes, nickel-aluminum-bronzes (NAB) (AB1 and AB2 types), and the copper-manganese-aluminum alloys (CMA, also known as superstones). NAB and CMA alloys are widely used for applications where high strength and corrosion resistance are the principal requirements [1]. The microstructure of CMA alloys is almost similar to that of high-tensile-brass and aluminumbronze [2] ; it consists of α, β and a dispersion of intermetallic precipitates. Although increasing interest has been shown in the microstructural study of NAB [2,3], the mechanical properties, castability and weldability of CMA have also been extensively studied [4 9]. But the published literature so far contains very limited information on the microstructure of CMA alloys. An early paper by Langham and Webb [10] had reported that CMA alloys consisted essentially of α, β and some Fe-rich precipitates, although some Ni-Al intermetallics were also believed to be present near α/β boundaries. Brezina [11] and Bradley [12] have suggested that the microstructure of CMA alloy consisted of, in addition to α and β, some Fe-rich and Ni-rich intermetallic precipitates, which Brezina [11] preferred to designate as κ-phase, in line with the nomenclature adopted to describe the microstructure of nickel-aluminum-bronzes [13]. West and Thomas [14] have investigated the Curich end of Cu-Mn-Al ternary diagram using X- ray diffraction. According to their results, in the composition range of 13% 14%Mn, 6% 8%Al, Prof., Ph.D., to whom correspondence should be addressed, E-mail: drfaiz@uet.edu.pk. Fig.1 Pseudo-binary diagram of Cu-Al-12Mn (+Fe+Ni) alloy after Knotek [15] the microstructure consists of α and β. A pseudobinary diagram [15] of a Cu-Al alloy containing 12%Mn-2.8%Fe-2%Ni, given in Fig.1, shows that the presence of Mn and small concentrations of Fe and Ni has a very limited influence on the position of phaseboundaries, and the effect is only to cause the precipitation of intermetallic κ-phase in both α and β in the temperature range of 750 850 C. 2. Experimental The composition of the alloy examined is given in Table 1. Specimens for optical microscopy were prepared by using conventional metallographic techniques. Thin foil specimens for transmission electron

780 J. Mater. Sci. Technol., Vol.22 No.6, 2006 Table 1 Chemical composition of the Alloy Element wt pct at. pct Cu 72.43 64.0 Mn 13.95 14.3 Al 7.78 16.2 Fe 3.24 3.24 Ni 2.17 3.30 Si 0.06 0.01 Fig.2 Optical micrographs of an as-cast CMA alloy. The α and β phases, and the various intermetallic precipitates are indicated by arrowhead as: (a) α phase, (b) β phase, (c) large dendritic particles, (d) smaller dendritic particles, (e) globular precipitates, (f) cuboid precipitates Ion Tech Super Micro-lap apparatus. Electron microscopy of the thin foil specimens was carried out on EM-400T transmission electron microscope operating at 100 kv. The microanalysis of various phases/precipitates was carried out with either bulk or thin specimens. The bulk microprobe analyses were carried out on a Philip-505 scanning electron microscope and the thin specimen microprobe analyses were carried on Philips EM-400T analytical electron microscope. These instruments were fitted with EDX (energy dispersive X-ray) detectors and ancillary electronics. Quantitative chemical analyses were obtained from the bulk specimens using standard ZAF correction procedure, and from the thin specimens using the ratio technique [16 17] and experimentally determined k factors. Lattice parameter measurements were obtained from electron diffraction patterns taken from samples which had been sputter-coated with a thin layer of gold. The gold rings provided a standard on each diffraction pattern and enabled lattice parameter data to be obtained, which was accurate to within about 1%. 3. Results and Discussion The typical microstructure of the as-cast CMA alloy is shown in Fig 2. It consists of light-etching areas of α-phase, dark-etching β-phase, and intermetallic precipitates of various morphologies. 3.1 α-phase The light-etching areas in Fig.2 constitute the α- phase, which is fcc Cu-rich solid solution. The chemical analysis of bulk specimens, as determined from the microprobe analysis given in Table 2, showed that α- phase contained about 12%Mn, 6%Al, 2.5%Fe, and 1.5%Ni. The lattice parameter of α-phase measured by electron diffraction was 0.371±0.004 nm. 3.2 β-phase The dark-etching areas in Fig.2 are the β-phase. The electron diffraction patterns taken from β-phase were indexed as bcc structure, with a lattice parameter of 0.297±0.003 nm. The chemical composition of the β as determined from the bulk microprobe analysis was Cu-13.5%Mn-12.5%Al-1.0%Fe-2.0%Ni (see Table 2). The orientation relationship between the α and β phases, illustrated by the diffraction pattern of Fig.3, was very close to the Nishiyama-Wasserman orientation relationship: [001]β [011]α, and [110]β [111]α Fig.3 Electron diffraction pattern showing a Nishiyama-Wasserman orientation-relationship between the α-phase and the β-phase: hkl α, hkl β [001]β(bcc) [011]α(fcc), and [110]β(bcc) [111]α(fcc) microscopic examination were prepared either by electro polishing in 25 percent nitric acid in methanol cooled to 40 C using a potential of 10 12 V, or by ion beam thinning with 5.5 kv argon ions in an 3.3 Intermetallic precipitates The as-cast CMA alloy contained intermetallic precipitates of various morphologies. The α-phase contained four different types of precipitates: the large dendritic-shaped particles, the small dendriticshaped particles, the globular precipitates near α/β grain boundaries, and the cuboid precipitates. These different intermetallic precipitates are indicated by arrowheads in Fig.2. Inside the β-phase, only one type of precipitates were present; these precipitates which could only be

J. Mater. Sci. Technol., Vol.22 No.6, 2006 781 Table 2 Chemical analyses of phases in as-cast CMA alloy, in wt pct and (at. pct) Phases Al Si Mn Fe Ni Cu Alpha (α) 05.9±0.2 0.2±0.1 12.1±0.2 02.4±0.5 1.4±0.1 78.0±1.0 (12.6) (0.4) (12.6) (2.5) (1.3) (70.6) Beta (β) 12.5±1.1 0.3±0.1 13.5±0.5 01.0±0.2 2.2±0.3 70.5±1.0 (24.6) (0.6) (13.0) (1.0) (2.0) (58.8) Large dendrite particles 03.6±1.0 1.8±0.3 29.5±0.7 56.4±0.8 1.3±0.5 07.4±0.9 (7.2) (3.5) (27.0) (54.8) (1.2) (6.3) Small dendrite particles 15.9±0.8 3.0±0.8 25.0±1.0 47.0±3.5 1.2±0.4 07.8±2.5 (27.6) (5.0) (21.3) (39.4) (0.9) (5.7) Globular precipitates 12.2±1.0 0.7±0.5 29.6±2.5 32.6±4.0 4.4±1.4 20.3±7.4 (22.8) (1.3) (26.6) (29.5) (3.8) (18.4) Cuboid precipitates in α 08.2±1.7 0.6±0.4 28.9±3.0 36.7±7.0 2.9±1.3 22.6±7.3 (15.9) (1.1) (27.5) (34.5) (2.6) (18.4) Small cuboid precipitates in β 12.7±1.2 0.2±0.2 28.1±4.2 36.1±2.8 1.4±0.1 21.4±3.7 (23.6) (0.4) (25.5) (32.4) (1.2) (16.9) Fig.4 Transmission electron micrograph showing a large dendritic-shaped particle in the α-grain Fig.5 Electron diffraction pattern (taken along <110>α <110>ppt) showing a nearly cubecube orientation-relationship between α-matrix and a large dendritic-shaped precipitate resolved by transmission electron microscopy, were very small cuboid shaped ones. 3.3.1 Large dendritic-shaped particles Large dendritic-shaped particles are shown in the optical micrographs of Fig.2, and its transmission electron micrograph is shown in Fig.4. These particles were invariably located at the centers of α-grains, (Fig.2), and were typically 20 40 µm across. Bulk microprobe analyses of these particles, indicated that they were rich in Fe and Mn, and contained more than 25 at. pct Mn (Table 2). The large dendritic-shaped particles had an fcc structure with a lattice parameter of 0.369±0.003 nm, and exhibited an orientation relationship with the α- matrix which was close to a cube-cube orientationrelationship (Fig.5). 3.3.2 Small dendritic-shaped particles The optical micrographs of small dendritic-shaped particles are shown in Fig.2, and the transmission electron micrograph is shown in Fig.6. These particles were distinguishable from the large dendritic-shaped particles by their size and location in the microstructure. They were typically 5 10 µm in size and were generally located near α/β boundaries (Fig.2). An electron diffraction study of small dendriticshaped particles revealed that they had DO 3 structure of the Fe 3 Al type with a lattice parameter of 0.294±0.004 nm. Additionally, from the thin specimen microprobe analysis of individual particles (Table 2), it appeared to have a composition based on Fe 3 Al, in which Fe was partially substituted by Mn, Cu and Ni. Whereas Al was substituted by Si. The small dendritic-shaped particles had, (as shown by the diffraction pattern of Fig.7), a Kurdjumov- Sachs orientation relationship with the α-phase, i.e., [111]ppt [011]α, and [101]ppt [111]α 3.3.3 Globular precipitates The globular precipitates, shown in Fig.8, are located in the α-grains near the α/β boundaries. Electron diffraction pattern and thin specimen microprobe analyses from the individual globular precipitates, showed that they had a composition and crystal structure similar to the small dendritic-shaped particles, i.e., they were based on Fe 3 Al and had the DO 3 structure with a lattice parameter of 0.295±0.004 nm. The orientation relation-

782 J. Mater. Sci. Technol., Vol.22 No.6, 2006 Fig.6 Transmission electron micrograph of a small dendritic-shaped particle Fig.9 Electron diffraction pattern showing orientationrelationship between the α-matrix and a globular precipitates Fig.7 Electron diffraction pattern showing orientationrelationship between α-matrix and a small dendritic-shaped precipitate Fig.10 Transmission electron of cuboid-shaped particles in the α-phase Fig.8 Transmission electron micrograph showing globular precipitates in α-phase ship between the globular precipitates and the α- matrix as illustrated by Fig.9 was: [011]ppt [011]α, and [001]ppt [111]α 3.3.4 Cuboid-shaped precipitates in α-grains The cuboid-shaped precipitates were distributed throughout the α-grains (Fig.10). These precipitates were typically about 0.1 0.2 µm in size. Convergent beam diffraction patterns taken from individual cuboid precipitates showed that these precipitates had fcc structure with a lattice parameter of 0.37±0.004 nm. Thin specimen microprobe analyses of these precipitates indicated that they were Fe-rich and contained about 25% 30% Mn (Table 2). The cuboid-shaped precipitates exhibited a cube-cube orientation relationship with the α-matrix. 3.3.5 Cuboid precipitates in β-grains Transmission electron microscopy of thin-foil specimens revealed the presence of very small cuboid precipitates in the β-phase (Fig.11). The chemical composition, given in Table 2, and the electron diffraction results obtained from the β region containing the cuboids (Fig.12), indicated that these precipitates were based on Fe 3 Al and had the DO 3 structure. The electron diffraction pattern of Fig.12 also shows that these cuboids had a cube-cube orientation relationship with the β matrix.

J. Mater. Sci. Technol., Vol.22 No.6, 2006 783 Fig.11 Transmission electron micrograph showing the cuboid precipitates in the β-phase Fig.12 Electron diffraction pattern along <110> taken from a region inside the β-phase that contained a number of small cuboid precipitates. The pattern contains the spots from the bcc matrix as well as the precipitates. The precipitate diffraction spots could be indexed as a DO 3 structure. It is also revealed that there is a cube-cube orientationrelationship between the matrix and the cuboid precipitates 4. Discussion The results of the microstructure of CMA alloy obtained during the present work are consistent with those of the previous work [10 12] in the composition of around Cu-14%Mn-8%Al (+Fe+Ni), the microstructure of which consists of α, β and intermetallic precipitates. However the identification of intermetallic precipitates carried out during the present work by electron diffraction and microanalysis, has shown that in the present alloy, there were structurally and chemically two different intermetallic phases present: one based on γ-fe (large dendritic-shaped particles and the cuboid precipitates in α-phase), and the other on Fe 3 Al (the small dendritic-shaped and the globular precipitates in α-phase, as well as the small cuboids in the β-phase). The nickel-rich precipitates reported by Langham and Webb [10], Brezina [11] and Bradley [12] were not observed during the present research. The compositions of α and β-phases as determined during the present work (see Table 2) indicate that the positions of the phase boundaries are essentially similar to those indicated in Fig.1 by Knotek [15]. The partitioning of Al between α and β-phases (given in Table 2) is very similar to that indicated in Fig.1. Additionally, it is also indicated from Table 2 that Fe appears to concentrate into the α-phases, whereas Ni tends to concentrate into β. However, it appears to be that the various intermetallic precipitates observed during the present work were all identified as a single κ-phase (having the DO 3 structure) by Knotek [15]. It also appears to be that since the Knotek s work was carried out by X-ray diffraction, the Fe and Mn-rich fcc precipitates reported in the present work, were not detected as a separate phase because the diffraction peaks from the two fcc phases with extremely similar lattice parameters (the Cu-rich α-phase and the Fe and Mn-rich γ-fe) would have been difficult to resolve in the photographed X-ray diffraction patterns. The present work carried out with analytical and transmission electron microscopy has, however, clearly distinguished all the different morphologies of intermetallic precipitates. The large dendritic precipitates seen in Fig.2 are very similar to those observed in nickel-aluminum bronze by Hasan et al. [2], whereas the dendritic morphology was attributed to the fact that these particles were the first to form during solidification. The large dendritic precipitates in the present alloy are also believed to be the first to form during solidification, as the work of Iqbal and Hasan [18] has shown that these particles do not dissolve even when the alloy is held for a long time at a temperature only a few degrees below the solidus. As the NAB and CMA alloys are used for similar applications, it is of the interest to draw a comparison between the microstructures of the two alloys. An extremely low volume fraction of β was reported [2] in a NAB alloy, of similar Al-content as that used in the present CMA alloy, while the CMA showed about 50 vol. pct β-phase. Additionally, in NAB alloys the retained β has been reported [19] to consist of martensite (although, very small cuboid shaped intermetallic precipitates were also observed in the martensite). In contrast, in the present CMA alloy, the β-phase retained its bcc lattice at room temperature. It appears to be that a high concentration of Mn has a stabilizing effect on β. The work of Bouchard and Thomas [20] on the Cu-Mn-Al alloys has shown that the addition of Mn decreases the M s temperature of the Cu-Al alloys. Thus, the present work provides support to the research carried out by Bouchard and Thomas [20]. The intermetallic precipitates in NAB alloy are based on Fe 3 Al and NiAl, while in CMA alloy the intermetallic precipitates are γ-fe and Fe 3 Al. The formation of γ-fe precipitates is due to the presence of a high concentration of Mn, which stabilizes the fcc form of iron. As the lattice parameter of γ-fe and cu are very similar (0.371 nm and 0.369 nm), the

784 J. Mater. Sci. Technol., Vol.22 No.6, 2006 large dendrites-shaped particles and the cuboid precipitates (both fcc) exhibit a cube-cube orientation relationship with the fcc Cu-rich matrix. The orientation relationship of small dendritic-shaped particles of Fe 3 Al with matrix as determined during the present work, is the same as that reported between the dendritic-shaped particles of κ (Fe 3 Al) and the α-matrix in NAB [2]. However the globular particles of Fe 3 Al in the present alloy show a different orientation relationship with the matrix. This may either be due to the difference in the proportion of Mn and Al present in these two types of precipitates or that the globular precipitates actually nucleate at the α/β boundaries during the growth of the α-matrix which may have had an effect on the orientation relationship of these precipitates with the α-phase. 5. Conclusions (1) The as-cast CMA alloy consists of α and β phases as well as intermetallic precipitates of various morphologies. (2) The α-phase is an fcc Cu-rich solid solution, while the β-phase has a bcc structure. (3) The large dendritic-shaped particles and the cuboid-shaped precipitates located in α, are based on γ-iron with an fcc structure and have a cube-cube orientation relationship with the α-matrix. (4) The small dendritic-shaped particles and the globular precipitates are based on Fe 3 Al which has the DO 3 structure. The small dendritic-shaped particles have a Kurdjumov-Sachs orientation relationship with the α-matrix, while the globular precipitates also exhibit a discrete orientation relationship with the α- matrix. (5) The small Fe-rich cuboid precipitates in the β-phase have a DO 3 structure, and a cube-cube orientation relationship with the matrix. REFERENCES [1 ] P.J.Macken and A.A.Smith: The Aluminum Bronzes, 2nd ed., CDA, London, 1966, 224. [2 ] F.Hasan, A.Jahanfrooz, G.W.Lorimer and N.Ridley: Metall. Trans., 1982, 13A, 1337. [3 ] F.Hasan: Ph.D. Thesis, Univ. of Manchester, 1984. [4 ] J.M.Langam and A.W.O.Webb: Modern Casting, 1962, 42, 62. [5 ] J.O.Edwards and D.A.Whittaker: Trans. AFS, 1961, 69, 862. [6 ] K.Rutkowski: Mod. Cast., 1962, 41, 99. [7 ] N.Bailey and A.W.O.Webb: Trans. AFS, 1964, 72, 321. [8 ] L.Raymond: Trans. AFS, 1979, 87, 537. [9 ] G.Bradshaw, M.M.Kennedy and S.H.Dorn: Mod. Cast., 1960, 39, 67. [10] J.M.Langham and A.W.O.Webb: British Foundryman, 1962, 55, 686. [11] P.Brezina: Inst. Met. Rev., 1982, 27, 77. [12] J.N.Bradley: Inst. Met. Rev., 1972, 17, 88. [13] M.Cook, W.P.Fentiman and E.Davis: J. Inst. Met., 1952, 80, 419. [14] D.R.F.West and D.O.L.Thomas: J. Inst. Met., 1956-57, 85, 505. [15] O.Knotek: Tech. Rundsch. Bern., 1968, 60(16), 25. [16] G.Cliff and G.W.Lorimer: J. Microscopy, 1975, 103, 203. [17] J.I.Goldstein, J.L.Costley, G.W.Lorimer and S.R.J.Reed: Proc. SEM 1977, ed. O.Johari, 11RT1 Chicago, 315. [18] J.Iqbal and F.Hasan: Proc. 2nd Int. Conf. on Structure, Processing and Properties of Materials, eds. E.Haque, A.S.M.A.Haseeb, A.K.M.B.Rashid, M.F.Islam and M.Moniruzzaman, Dhaka, 2004, 129. [19] F.Hasan, G.W.Lorimer and N.Ridley: J. Phys., Coloque C4, 1982, 43, 653. [20] M.Bouchard and G.Thomas: Acta Metall. Mater., 1975, 23, 1485.