UDK 669.018:669.715:620.18 ISSN 1580-2949 Pregledni znanstveni članek MTAEC9, 38(6)289(2004) THE CHARACTERISATION OF INTERMETALLIC-COMPOUND PARTICLES IN AN ANNEALED Al-Mg-Cr-Fe ALLOY KARAKTERIZACIJA DELCEV INTERMETALNIH SPOJIN V ŽARJENI ZLITINI Al-Mg-Cr-Fe Milan Svoboda1, Jozef Janovec2, Monika Jenko2, Ante Vrankovi}3 1 Academy of Sciences of the Czech Republic, Institute of Physics of Materials, Žižkova 22, 616 62 Brno, Czech Republic 2 Institute of Metals and Technology, Lepi pot 11, 1000 Ljubljana, Slovenia 3 TLM, d. d., Narodnega preporoda 12, 22000 Šibenik, Croatia svobmŽipm.cz Prejem rokopisa – received: 2004-09-20; sprejem za objavo - accepted for publication: 2004-11-11 Three types of particles were identified in an annealed 5xxx-series Al-Mg-Cr-Fe alloy. The large particles (500-1000 nm), which contain aluminium and iron as the dominant elements, are isostructural with Al19Fe4MnSi2 (crystal group Im3). These particles were probably formed during the solidification of the alloy. The mostly twinned middle-size particles (100-600 nm) were found to be isostructural with Al18Cr2Mg3 (crystal group Fd3m). These particles are free of iron, but contain traces of titanium. In the small particles (5-50 nm), chromium, magnesium, iron and silicon were found next to aluminium. These small particles probably precipitated from the supersaturated solid solution of a-Al during annealing of the alloy. Key words: Al-Mg-Cr-Fe alloy, intermetallic compounds, twins, transmission electron microscopy V žarjeni zlitini Al-Mg-Cr-Fe, seriji 5xxx, so bile identificirane tri vrste delcev. Največji delci (500-1000 nm), bogati z aluminijem in železom, imajo kristalno strukturo grupe Im3, ki je ustrezna za Al19Fe4MnSi2. Tvorili so se verjetno med kristalizacijo zlitine. Srednji delci z dvojčki (100-600 nm) so bili identificirani kot Al18Cr2Mg3 (kristalna grupa Fd3m). Ti delci ne vsebujejo železa, titan je v njih samo v majhnih količinah. V majhnih delcih (5-50 nm) so bili poleg aluminija odkriti krom, magnezij, železo in silicij. Ti delci so se verjetno izločali iz nasičene trdne raztopine a-Al med žarjenjem zlitine. Ključne besede: zlitina Al-Mg-Cr-Fe, intermetalne spojine, dvojčki, transmisijska elektronska mikroskopija 1 INTRODUCTION Aluminium alloys are structural materials that start to play an important role in the automotive industry because of their low density and excellent corrosion resistance, as well as the ease with which they can be welded and recycled 13. The higher strength-to-weight ratio of these alloys compared to ferrite steels helps to enhance the fuel efficiency of automobiles 4,5. In order to enhance the formability of aluminium alloys, attention is paid to the controlled evolution of strain-induced micro-structures 6,7 and/or to the production of ultra-fine-grain materials 8,9. In both cases, the microstructure development is connected with the formation of intermetallic compounds. The intermetallic particles in aluminium alloys are mostly formed on solidification, during thermo-mecha-nical treatment, and/or in the course of annealing. On solidification, larger particles (hundreds or thousands of nanometres) crystallize from the liquid in eutectic (e.g., Fe4Al13, FeAl6, FeAlm, FeAlx) or pseudo-peritectic (e.g., a-AlFeSi, ß-AlFeSi, aT, q2, Al18Cr2Mg3, Al6Mn,) reactions 51011. The type of reaction depends on the alloy’s chemical composition and the solidification conditions (equilibrium or metastable) 512. The large intermetallic particles are present either at the grain boundaries or in the grain interior. The mechanical properties of age-hardenable alloys (e.g., the 2xxx and 7xxx series) can be improved by the formation of coherent or semi-coherent GP zones from the supersaturated solid solution of a-Al. When the annealing is prolonged, GP zones transform into non-coherent particles of different types, e.g., Al2Cu 13, Mg2Si 14, or MgZn215. Dispersive particles are also present in rapidly solidified or recrystallized microstructures of some non-hardenable alloys. For instance, Maeng et al. 16 reported the occurrence of small (50-200 nm) spherical precipitates in the microstructure of an Al-7Mg-0.8Mn alloy (the 5xxx series). Dispersive Mg2Si particles were also identified by Banovic and Foecke 7 in an Al-2.3Mg-0.2Cr-0.3Fe alloy (AA5052-H32). In this work, an Al-Mg-Cr-Fe alloy of the 5xxx series was investigated. The study was focused on characterising the intermetallic compounds formed in the alloy on solidification, during thermo-mechanical treatment, and/or in the course of annealing. The identification of twins in f.c.c. compounds susceptible to twinning was also performed. 2 EXPERIMENTAL The Al-Mg-Cr-Fe alloy (for the chemical composition see Table 1) was cast, homogenised, cold rolled and finally annealed at 250 °C for 1.5 h. From the MATERIALI IN TEHNOLOGIJE 38 (2004) 6 289 M. SVOBODA ET AL.: THE CHARACTERISATION OFINTERMETALLIC-COMPOUND PARTICLES ... Table 1: Chemical composition of the Al-Mg-Cr-Fe alloy. Contents of the elements are given in mass %. Tabela 1: Kemična sestava zlitine Al-Mg-Cr-Fe. Masni delež elementov w/% Element Mg Fe Cr Si Mn Cu Ti B Al Content 2.23 0.23 0.25 0.16 0.02 <0.01 0.03 <0.01 bal. Table 2: Particles of intermetallic compounds identified in the Al-Mg-Cr-Fe alloy Tabela 2: Delci intermetalnih spojin identificirani v zlitini Al-Mg-Cr-Fe Type Formula of intermetallic compound Size d/nm Results of EDX analysis Crystal system Space group Lattice parameter a/nm A Al19Fe4MnSi2 500-1000 76Al, 17Fe, 3Si, 2Mg, 1Mn, 1Cr b.c.c. Im3 1.256 B Al18Cr2Mg3 100-600 79Al, 10Mg, 10Cr, 1Ti f.c.c. Fd3m 1.453 C - 5-50 80Al, 7Cr, 5Mg, 5Fe, 3Si - - - H «11* ÖÖ0 Hî« ¦ . • - B = Š032] Figure 1: TEM micrographs of the annealed Al-Mg-Cr-Fe alloy: a) intermetallic particles of the A and B types; typical sizes and shapes, b) typical size and shape of the C-type intermetallic particles, c) dislocations in the aluminium matrix Slika 1: TEM-posnetki žarjene zlitine Al-Mg-Cr-Fe: a) intermetalni delci vrst A in B; tipična velikost in oblika, b) tipična velikost in oblika intermetalnih delcev vrste C, c) dislokacije v aluminijski matrici Figure 2: Characterisation of the A-type particle (Al19Fe4MnSi2): a) TEM micrograph showing the analysed particle, b) SAD pattern of the particle, zone axis Š100], c) SAD pattern of the particle, zone axis Š032]; reflections of the aluminium matrix are also indexed Slika 2: Karakterizacija delcev vrste A (Al19Fe4MnSi2): a) TEM-posnetek analiziranega delca, b) uklonska slika delca, conska os Š100], c) uklonska slika delca, conska os Š032]; indeksirani so tudi ukloni aluminijske matrice 290 MATERIALI IN TEHNOLOGIJE 38 (2004) 6 M. SVOBODA ET AL.: THE CHARACTERISATION OFINTERMETALLIC-COMPOUND PARTICLES ... Figure 3: TEM micrographs showing morphology of the B-type particle (Al18Cr2Mg3): a) zone axis Š211], b) zone axis close to Š211], to see twin boundaries Slika 3: TEM-posnetek, ki prikazuje obliko delcev vrste B (Al18Cr2Mg3): a) conska os Š211], b) conska os blizu Š211], če bi bilo videti meje dvojčkov annealed samples, thin foils were prepared for a TEM (transmission electron microscopy) investigation. Both electrochemical polishing in a TENUPOL and precision ion polishing in a GATAN 656 were used in the production of the foils. The intermetallic particles present in the solid solution of a-Al were characterised with selected-area electron diffraction (SAD) and energy-dispersive X-ray spectroscopy (EDX) in a Philips CM12 microscope operating at 120 kV. The chemical compositions of the identified phases were determined from EDX spectra. 3 RESULTS Typical particles present in the alloy are illustrated in the set of TEM micrographs in Figure 1. In Figure 1a are typical representatives of the large (type A, sizes 500-1000 nm) and the middle-sized (type B, sizes 100-600 nm) particles. The small particles (type C, sizes 5-50 nm) are shown in Figure 1b. A detailed view of the dislocations appearing in the aluminium matrix is illustrated in Figure 1c. The different types of particles identified are characterised briefly in Table 2. The typical size and morphology of the A-type particle is documented in Figure 2a. The corresponding SAD patterns (Figures 2b Figure 4: Characterisation of the twinned B-type particle (Al18Cr2Mg3): a) TEM micrograph showing the analysed particle with twin in the central part, b) SAD pattern obtained from area a (see Figure 4a), zone axis Š211], c) SAD pattern obtained from area a (see Figure 4a), zone axis Š110], d) SAD pattern obtained from area b (see Figure 4a), zone axis Š110] Slika 4: Karakterizacija delcev vrste B (Al18Cr2Mg3) z dvojčki: a) TEM-posnetek, ki prikazuje analiziran delec z dvojčkom v njegovem srednjem delu, b) uklonska slika delca v mestu a (gl. sliko 4a), conska os Š211], c) uklonska slika delca v mestu a (gl. sliko 4a), conska os Š110], d) uklonska slika delca v mestu b (gl. sl. 4a), conska os Š110] MATERIALI IN TEHNOLOGIJE 38 (2004) 6 291 M. SVOBODA ET AL.: THE CHARACTERISATION OFINTERMETALLIC-COMPOUND PARTICLES ... Figure 5: TEM micrographs showing B-type particle (Al18Cr2Mg3) in aluminium sheet with contamination spots (black dots) after EDX measurements in two different foil tilts: a) tilt 0°, b) tilt 45°; r represents distance between the spots (see Eq. 1) Slika 5: TEM-posnetek, ki prikazuje delec vrste B (Al18Cr2Mg3) v aluminijskem traku; ta delec obsega dve temni točki zaradi EDX-meritev izdelanih v dveh različnih nagibih: a) nagib 0°, b) nagib 45°; r je razdalja med dvema točkama (gl. enačbo 1) and 2c) revealed that the particle is isostructural with the Al19Fe4MnSi2 phase. In Figure 3, the twin-containing B-type particle is illustrated. To highlight the twin boundaries, the particle was taken in two different orientations. In the next step, electron diffraction was used for the crystallographic identification of the particle and for the characterisation of the miss-orientation between the twin and parent lattices. The diffraction patterns taken from area a (Figures 4a-4c) revealed that the particle is isostructural with the Al18Cr2Mg3 phase. The b area contains both the boundary between the twin and parent lattices (Figure 4a) that enabled the twin characterisation. The corresponding diffraction pattern (Figure 4d) is indexed in the small window situated in the upper-left corner of the figure. The twin reflections are underlined. In all the diffraction patterns the symbol B was used to characterize the zone axis. 4 DISCUSSION 4.1 Characterisation of the phases As mentioned above, the EDX/TEM technique was used to characterise the intermetallic particles in the thin foils. A quantitative EDX analysis of Al-Mg alloys is a * a Hafflď m plana i 110] ,;:. _ imi r 222F» 220P * * D Î13T» . Î2ZP-K2T D02T ĎÎSP ,,-:„ 1Ď1P - 1-ÎÎT V13P« 9 220T 002 P i 22DT ¦111P--111T \ " 113P «113P * i «HIP » Ht 0C2T* 2Z2P -22ZT ZMP 222P 11ĎT * Figure 6: Characterisation of twins in f.c.c. lattice of the Al18Cr2Mg3 phase: a) arrangement of atoms in (110) plane of the twinned crystal; orientations corresponding to the parent and twin lattices are labelled with subscripts P and T, respectively; angle between Š002]P and Š002]t orientations is 70.52°, symbol a in the upper scheme represents the lattice parameter, b) diffraction pattern of the twinned crystal modelled with the use of the Ca.R.Ine. program; reflections corresponding to the parent and twin lattices are labelled with symbols P and T, respectively; angle between (002)P and (002)T planes is 70.52°, c) the angle 70.52° between (002)P and (002)T planes illustrated in the experimentally obtained diffraction pattern (identical with Figure 4d) Slika 6: Karakterizacija dvojčkov v fazi Al18Cr2Mg3 z mrežo p.c.k.: a) razvrstitev atomov v ploskvi (110) kristala z dvojčkom; orientacije matičnega območja so označene P, orientacije dvojčka so označene T; kot med orientacijama Š002]P in Š002]T je 70.52°, simbol a v gornji shemi simbolizira parameter mreže, b) uklonska slika kristala z dvojčkom, modelirana s programom Ca.R.Ine.; ukloni od matičnega območja so označeni s P, ukloni od dvojčka so označeni s T; kot med ploskvami (002)P in (002)T je 70.52°, c) kot 70.52° med ploskvami (002)P and (002)T ponazorjen z eksperimentalno uklonsko sliko, ki je identična s tisto na sliki 4d 292 MATERIALI IN TEHNOLOGIJE 38 (2004) 6 M. SVOBODA ET AL.: THE CHARACTERISATION OFINTERMETALLIC-COMPOUND PARTICLES ... rather complicated task because the correction for absorption cannot be neglected. To consider the correction, appropriate values of the k-factor as well as the thickness and the density of the analysed foil are expected to be available. The values of the k-factor for Al and Mg were determined experimentally using the intermetallic Mg17Al12 phase as a standard. The k-factors for the other elements were taken from the literature 17. The used densities of particular phases ranged between 2700 kg m–3 and 2900 kg m–3. The thicknesses of the foils were determined by means of contamination spots. The spots consist of organic compounds (hydrocarbons from the vacuum-oil vapor, O-ring grease etc.) that are polymerized upon contact with the electron beam and condense on the specimen, creating the familiar cone shaped deposits. The size (diameter) of the spot is higher than the beam size and the region of the specimen from which the x-rays are emitted in the EDX technique. During the analysis the spot is produced on the top and the bottom of the thin foil, and when the foil is tilted through an appropriate angle two contamination spots, instead of one, can be observed, Figure 5. A measurement of the distance between two spots is made, and by simple geometry and a knowledge of the image magnification, the thickness (t) of the foil can be calculated: (1) M sinp where § is the tilt angle (45°), r is the measured spot distance (see Figure 5b) and M is the image magnification 13. The analysed A-type particles, isostructural with Al19Fe4MnSi2 (Table 2, Figures 2b and 2c), contain Al and Fe as major elements, and Si, Mn, Cr and Mg as minor elements. This is in good agreement with the stoichiometric formula. With respect to its lower bulk content, manganese was partially substituted with chromium. Traces of magnesium are acceptable because of its higher bulk content. The robust particles (about 1 µm, see Figure 2a) with mostly indented contours indicate that the formation of Al19Fe4MnSi2 takes place on solidification. The above suggestion is encouraged by the presence of iron in the particles 10. The interfaces between the particles and the aluminium matrix probably served as places convenient for the precipitation of small intermetallic particles during annealing. The evidence for it follows from Figure 2a, where many small precipitates are illustrated close to the large A-type particle. In dilute Al-Fe alloys, the primary iron aluminides crystallise mostly in structures showing lower symmetries (monoclinic, orthorhombic or tetragonal lattices) 10. However, the complex aluminides in Al-Mg alloys can crystallise in high-symmetry lattices (e.g., cubic), as was shown in this work. Compared to the A-type particles, the B-type particles are clearly smaller and have a blocky morphology. Besides the basic elements (Al, Mg and Cr) and traces of titanium they did not contain any other element, or iron (Table 2). Both the above facts indicate that the B-type particles, isostructural with the Al18Cr2Mg3 phase, appear in the microstructure later than the A-type particles. It was reported earlier that the Al18Cr2Mg3 phase forms dispersoids in Al-Mg alloys with the addition of chromium 18. If manganese is present in a sufficient amount, complex phases containing iron can be formed. The twins in the B-type particles (Figures 3-5) can arise from the plastic deformation on rolling. The small particles, labelled as C-type, were not characterised in detail. The results of the quantitative EDX analysis revealed that the particles contain Cr, Mg, Fe and Si next to aluminium (Table 2). With respect to the small size of the particles (5-50 nm, Table 2), their precipitation can be expected from the supersaturated solid solution of a-Al on annealing. The driving force for the precipitation is probably the chromium bulk content, because magnesium does not exceed the solubility limit in aluminium at 250 °C. If the earlier formation of Al18Cr2Mg3 does not reduce the chromium content in the aluminium matrix under its solubility limit, the precipitation of a non-stoichiometric (Al,Mg)x(Cr,Fe)y phase is possible. 4.2. Twins in the Al18Cr2Mg3 particles Twinning is known as a mode of plastic deformation in crystalline solids at temperatures below those at which individual atoms are mobile 19. Deformation twins have been observed in various materials, including intermetallic compounds 20. They arise from a homogeneous simple shear of the parent lattice that results in a highly coordinated displacement of individual atoms. One of the four crystallographic parameters characterising the twinning process is a twinning plane separating the areas of the twin and the parent lattices. If the twinning and projection planes are perpendicular to one another, the electron diffraction pattern of the area adjacent to the twinning plane contains two sets of reflections originating from both the twin and parent lattices 21. In Figure 4d, the diffraction pattern taken from the area b of the B-type particle (it contains both the twin and parent lattices) is documented for the zone axis Š110]. It is the convenient orientation because the twinning plane (111) is perpendicular to the projection plane (110), and the parent and twin lattices show mirror reflections across the twinning plane. In other words, the twinning in f.c.c. crystals (e.g., Al18Cr2Mg3) is associated with the 180° rotation of the parent lattice around the (111) plane (see Figure 6a). As a consequence, the stacking sequence across the twin plane is changed from ABCABC to BACBACBA 21. For instance, the angle