Structure development in rapidly solidified Al–Fe–V–Si ribbons

Materials Science and Engineering A 373 (2004) 90–98

Structure development in rapidly solidified Al–Fe–V–Si ribbons S. Yaneva a,∗ , A. Kalkanlı b , K. Petrov c , R. Petrov d , Ir. Yvan Houbaert d , S. Kassabov c a

Institute of Metal Science, BAS, 67 Shipchenski Prohod Street, 1574 Sofia, Bulgaria Department of Metallurgical and Materials Engineering, METU, Ankara, Turkey c Institute of General and Inorganic Chemistry, BAS, 1113 Sofia, Bulgaria Department of Metallurgy and Materials Science, Ghent University, Technologiepark 903, B-9052 Gent, Belgium b

d

Received 30 July 2003; received in revised form 9 December 2003; accepted 22 December 2003

Abstract Microcrystalline Al–8.5 wt.% Fe–1.06 wt.% V–2.75 wt.% Si, ribbon was produced by a planar flow casting (PFC) technique. Environmental scanning electron microscopy, X-ray diffraction and Mössbauer spectroscopy proved that the polycrystalline alloy consists of a supersaturated Al matrix and quaternary Al–S–Fe–V dispersoids. The observed single exothermal peak in the differential scanning calorimetry curves was ascribed to partial decomposition of the Al supersaturated solid solution. After prolonged heating at 873 K, the microcrystalline alloy still consists of supersaturated Al matrix with lattice parameter 0.40485(4) nm, reinforced by silicide particles with chemical formula Al13.18 (Fe,V)1.84 Si and lattice parameter 1.2578(8) nm. © 2004 Published by Elsevier B.V. Keywords: Microcrystalline Al–Si alloys; Supersaturated Al matrix; Quaternary silicide phases; Differential scanning calorimetry; X-ray diffraction; Mössbauer spectroscopy

1. Introduction The hardening mechanism of Al–Fe–V–Si alloys is based on the high volume fraction of finely distributed silicides, which are stable at high temperatures due to low diffusivity and limited solubility of the alloying elements [1,2]. The microstructure of the bulk alloy is usually coarse grained, which is a reason for bad mechanical properties. Different methods for rapid solidification are extremely effective for producing materials with a fine-grained microstructure. By means of such methods it is possible to produce materials with grain sizes in the range of micrometers or nanometers even in alloys, which are usually coarse grained, like some Cu-based alloys [3,4]. Final products are usually ribbons with thickness between 20 and 80 ␮m, or powders. These products could be compacted and extruded in order to pro∗ Corresponding author. Tel.: +359-2-7142-327; fax: +359-2-8703-207. E-mail addresses: s [email protected], [email protected] (S. Yaneva).

0921-5093/$ – see front matter © 2004 Published by Elsevier B.V. doi:10.1016/j.msea.2003.12.034

duce a bulk material keeping the ultra-fine microstructure of the initial ribbons or powders. Hot extrusion is one of the simplest methods to do this. There has been a considerable amount of articles devoted to the development of aluminum alloys with high temperature stability, which can replace titanium alloys. Al–Fe–V–Si alloys produced by compaction of rapidly solidified ribbons or powders represent a class of structural materials for applications in aerospace industry. Unfortunately, during the compaction, the fine-grained material is usually subjected to reheating and precipitation and coarsening can develop. To avoid undesirable coarsening of the microstructure during compaction or practical use, it is of great importance to have a better understanding of the microstructural development at high temperatures of the as-cast rapidly solidified ribbons. The knowledge of structural changes at high temperatures of microcrystalline ribbons could support the processing route of these alloys. The goal of this work is to study the structure development in microcrystalline Al–Fe–V–Si ribbons during reheating in the temperature range between room temperature and 873 K that is appropriate for hot extrusion.

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2. Experimental Microcrystalline ribbons of Al–Fe–V–Si alloy with a chemical composition, given in Table 1, were prepared by means of planar flow casting (PFC) using a wheel-surface linear velocity of 19 m/s. A detailed description of this equipment is given elsewhere [5]. Annealing of as-cast ribbon specimens was performed in a laboratory furnace with temperature control ±1.5 K. The ribbon microstructure in the as-cast and annealed states was studied after electrolytic polishing. The particle size distribution was monitored for both, the wheel-side and air-side (free) surfaces of the ribbons in a JEM-7A transmission electron microscope and in an optical microscope Reichert MeF-2. The size and chemical composition of the phases in as-cast and heat-treated ribbons were studied by means of an XL30 environmental scanning electron microscope (ESEM), energy-dispersive X-ray spectroscopy (EDS) with sapphire detector SUTW, with ultra-thin Be window at an acceleration voltage 15 kV. The microstructure studies were implemented in the normal plane or in the plane perpendicular to the transverse direction (TD) of the ribbon according to the scheme shown in Fig. 1. A Perkin-Elmer DSC-2 differential scanning calorimeter in transient heating mode (scanning rate 20 K/min) was used to study the heat release due to phase transformations and decomposition of supersaturated aluminum matrix. X-ray diffraction (XRD) measurements were carried out on a DRON automatic powder diffractometer using filtered Cu K␣ radiation and a scintillation counter. Diffraction pat-

Fig. 1. Schematic representation of the plains, in which the SEM microstructural study was carried out.

terns were recorded in step-scan mode at steps of 0.02◦ (2θ) and counting time of 3 s per step, in the angle interval from 15◦ to 120◦ (2θ). The samples represented rectangular pieces of compact ribbon with dimensions of 10 mm×15 mm. Standard programs (TREOR and ITO, CRYSFIRE sets [6]) were used for indexing and unit cell parameter determination. Coherent domain size was estimated from the half-width parameters of the experimental diffraction lines, corrected for instrumental broadening (programs POWDERCELL and WINFIT [7]). The room temperature 57 Fe Mössbauer transmission spectra were recorded on a conventional spectrometer operating

Table 1 Chemical analysis of as-cast and annealed ribbons Element as-cast ribbons

Composition (wt.%) Integral content (AA) ± ± ± ±

Fe Si V Al

8.5 2.8 1.1 87.7

0.1 0.1 0.1 0.1

Ribbon after isothermal annealing for 1 h at 200 ◦ C

Composition in point 1 in Fig. 5

Fe Si V Al

17.5 5.8 2.2 74.5

Ribbon after annealing with temperature arrests at 473, 573 and 673 K for 1 h each

Composition in point 1 in Fig. 6 wt.%

Fe Si V Al

16.2 5.9 2.4 75.5

wt.%

Point 1 in Fig. 4 10.7 3.0 1.3 85.0

± ± ± ±

0.1 0.1 0.1 0.1

0.1 0.1 0.1 0.1

9.4 6.3 1.3 83.0

wt.% ± ± ± ±

0.1 0.1 0.1 0.1

0.1 0.1 0.1 0.1

10.6 2.9 0.9 85.5

at.% ± ± ± ±

0.1 0.1 0.1 0.1

5.5 3.0 0.5 91.0

± ± ± ±

0.1 0.1 0.1 0.1

Composition in point 2 in Fig. 6 at.%

± ± ± ±

10.6 ± 0.1 3.0 ± 0.1 1.00 ± 0.1 85.4 ± 0.1 Composition in point 2 in Fig. 5

at.% ± ± ± ±

Point 2 in Fig. 4

8.7 6.3 1.4 83.6

wt.% ± ± ± ±

0.1 0.1 0.1 0.1

11.8 3.9 1.5 82.7

at.% ± ± ± ±

0.1 0.1 0.1 0.1

6. ± 0.12 4.1 ± 0.1 0.9 ± 0.1 88.9 ± 0.1

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in a constant-acceleration mode. A gamma-ray source of 57 Co/Rh was used. The spectrometer velocity scale was calibrated using a metal iron foil as a reference sample. All the isomer shift values presented are given against this standard. The experimental spectra were fitted to discrete Lorentzian lines by a least-squares method. A standard χ2 (chi-squared) test was utilized to check the quality of the fitting.

3. Results and discussion The ribbons have different microstructures at surfaces formed on the wheel-side and on that exposed to air, see Fig. 2. As can be seen in the cross-section of the ribbons, there is a large, nearly structureless part at the wheel-side, while on the free-side, the silicide particles are about 1 ␮m in diameter. Transmission electron microscopy images of both zones are shown in Fig. 3, and they clearly demonstrate the size difference of silicide particles. Fig. 4 shows the microstructure of the as-cast wheel-side surface, where two different zones can be seen. The size of the white spots corresponds approximately to the diameter of the electron beam. The compositions of the marked areas are given in wt.% in Table 1. The fine-grained microstructure on the wheel-side might be the reason of having nearly equal chemical compositions of the two zones, but a slightly increased V content is registered in the white zones. The results from the local analysis and that determined by atomic absorption (AA) for the entire ribbon did not differ substantially (Table 1). After annealing for 1 h at 473 K, the microstructure appears coarser, and spherical particles with sizes between 0.3 and 1 ␮m are clearly visible (Fig. 5). The particles are not uniformly distributed in the matrix. The larger particles are preferably situated at grain boundaries. The composition in two specific regions (points 1 and 2) in Fig. 5 is given in Table 1. In order to estimate a chemical formula of the intermetallic phase, the composition in point 1 was normalized by dividing the at.% for Al and (Fe + V), by the value for Si. The formula Al13.23 (Fe,V)1.7 Si was found. The estima-

Fig. 3. TEM analysis: (a) spherical particles on the wheel-side of the ribbon; (b) spherical particles at the free-side of the ribbon.

tion of this formula is based on the microstructure observed near the chill-side of the ribbons, where the particles’ mean size is of the order of several nanometers, see Fig. 3(a). The difference between the above formula for quaternary silicides and those reported in the literature may be attributed to the small size of the particles investigated and a possible instrument error (about 1 at.%). Some authors offered

Fig. 2. Microstructure of cross-section of as-cast ribbons (light microscopy).

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Fig. 4. Microstructure of as-cast Al–Fe–V–Si ribbon in ND-plane displaying two different zones: white zone (1) with 1.3% V and zone (2) with 1% V.

a stoichiometry of quaternary silicide as Al12 (Fe,V)3 Si [9–11], while others suggest Al13 (Fe,V)3 Si [12–15]. To study the temperature stability of the ribbons’ microstructure, a complicated thermal treatment was performed. After annealing with successive temperature arrests at 473, 573 and 673 K for 1 h each, the particles’ sizes slightly increased to 0.5–1.2 ␮m in diameter, as shown in Fig. 6. The chemical composition of the precipitates (white particles—point 1 in Fig. 6) also changes a little, see Table 1. It corresponds to a possible chemical formula Al13.25 (Fe,V)1.6 Si. In close vicinity of the particles (point 2 in Fig. 6), the chemical composition is similar to the composition of the matrix (see Table 1). The coarsening of the structure after successive heating up to 673 K is similar to that after annealing during 1 h at

473 K, and second-phase size and chemical composition are similar. After further annealing at temperatures up to 873 K, the microstructure was studied by XRD. Experimental powder diffraction patterns of the as-cast sample and that treated at 873 K for 2 h are shown in Fig. 7. The calculated values of unit cell parameters and mean coherent scattering domain are summarized in Table 2. The XRD patterns of the as-cast ribbons contain peaks of the Al matrix and the quaternary silicide phase, common for alloys with the composition studied here. The diffraction indices of the latter phase show systematic extinction of a body centered cubic (bcc) lattice, i.e. h + k + l = 2n. The crystal structure of this phase can be derived from that of the well known ␣-Mn12 (Al,Si)57 , cubic (Pm3), Pearson notation cP138 [8–10]. It represents pseudo-body-centered

Fig. 5. Microstructure of as-cast Al–Fe–V–Si ribbons in TD-plane after annealing for 1 h at 473 K.

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Fig. 6. Spherical particles (1) surrounded by gray phase (2) in the grain boundary zone after annealing with successive temperature arrests up to 673 K. Table 2 Lattice parameters a (nm) and mean crystallite sizes D (nm) of Al matrix and Al13.18 (Fe,V)1.84 Si phase in the Al–Fe–V–Si alloy in as-cast condition and after 2 h of ageing at 873 K Sample

a(Al) (nm)

D(Al) (nm)

a(Alx Fey Vz Sip ) (nm)

D(Alx Fey Vz Sip ) (nm)

As-cast As-cast + 2 h at 600 ◦ C

0.40443(3) 0.40485(4)

42(2) 53(3)

1.2559(9) 1.2578(8)

29(2) 30(2)

Fig. 7. XRD patterns of Al–Fe–V–Si ribbons: (a) as-cast; (b) heat-treated at 873 K. The lines of the ␣-Al13 phase are indexed. The lines of Al are marked with asterisk.

packing of three-shell Mackay-type atom clusters with icosahedral symmetry [10] and contains two independent Mn atoms. Mn(1) is bonded to 10 nearest Al/Si atoms placed at distances ranging from 2.37 to 2.63 Å. Mn(2) is less symmetrically surrounded by nine Al/Si atoms (eight at distances ranging from 2.50 to 2.60 Å and one very short bond of 2.30 Å). Such atomic arrangement gives rise to pseudo-extinction, i.e. a strong reduction of the intensities of diffraction lines having h + k + l = 2n + 1. Nevertheless, the true Bravais lattice is a primitive one. This lattice type was ascribed to the known Pm3 ␣12 -AlFeVSi phase [12]. A bcc Im3 ␣13 -AlFeVSi phase has also been reported [13]. Its structure is related to that of the Im3 ␣-AlFeSi phase [9]. The latter is composed of two interpenetrating primitive cubic lattices in which Fe atoms and only one half of the Al atoms are symmetry related by the lattice translation (0, 0, 0; 1/2, 1/2, 1/2)+ . The rest of the Al atoms have split positions with partial site occupancies. It should be emphasized that although the “average” structure belongs to the Im3 space group, its two symmetry-related Fe atoms have different local Al/Si coordination. For the samples studied, it is important that the thermal treatment does not affect significantly either the lattice parameter of the quaternary phase or the crystallite size. This supports the idea of thermal stability of Al–Fe–V–Si alloys. The investigated ribbons contain two phases: Al matrix and a quaternary ␣13 intermetallic compound. The Al matrix of the as-cast sample actually is a supersaturated solid solution, which transforms at elevated temperatures to a less supersaturated matrix. This transformation is ac-

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companied by an increase of unit cell parameter and of coherent scattering domain mean size of the Al matrix crystallites (see Table 2). In other Al microcrystalline alloys, a full decomposition was observed after annealing [16,17]. Even annealing at 873 K leads to a two-phase structure; aluminum matrix with lower supersaturation (lattice parameter 0.40485(4) nm compared with the value of pure Al 0.40492 nm) and quaternary silicides with parameter 1.2578(8) nm. Due to the difference of diffusivity and solubility of alloying elements [11] a redistribution of Fe and V at high temperatures probably leads to a change of lattice parameters, see Table 2. The XRD results are consistent with the Mössbauer spectroscopy (MS) data. The fitted MS spectra obtained from both as-cast and heated at 873 K alloys are plotted in Fig. 8. The experimental spectra show a paramagnetic doublet. The strong asymmetry of the spectral doublet indicates the presence of more than one nearest-neighbor configuration for iron atoms. Several fits to the experimental spectra were tested. It was found that the calculated χ2 values were not acceptable when only doublet(s) without a single line were fitted to the spectra. The best fits obtained were a superposition of a single line and two overlapping symmetric quadrupole-split doublets. The calculated hyperfine parameters are summarized in Table 3, where the singlet and doublets are denoted as Fe(s) and Fe(1,2), respectively. In Fig. 8, the singlet is marked as Fe(s). The two stronger symmetric Lorentzian lines in both spectra correspond to the Fe(1) doublet, and the two weaker ones are assigned to the Fe(2) doublet. A single line in the Mössbauer spectra of Al-based alloys is assigned to isolated iron atoms in solid solution [18,19]. The quadrupole doublets can be assigned to iron atoms occupying different Fe–Al polyhedra in the quaternary silicide phase, identified by XRD analysis. According to the non-interacting point-charge model of partial isomer shifts, the isomer shift (IS) of iron atoms is a linear function of the ligand numbers [20–22]. The larger coordination number yields the larger IS. Assuming this and taking into account the XRD results, the doublet Fe(1) with a larger IS should be assigned to the 10-coordinated

Table 3 Mössbauer

57 Fe

95

Fig. 8. Mössbauer spectra of as-cast ribbon (a) and annealed at 873 K ribbon (b).

Fe–Al/Si polyhedron, while the doublet Fe(2) with a lower IS corresponds to the 9-coordinated Fe–Al/Si polyhedron. The less symmetrically 9-coordinated Fe(2) yields a larger quadrupole splitting. The relative distribution of the iron atoms in the two sites is estimated directly from the absorption areas of the corresponding singlet and doublets, assuming the same recoil-free fractions. Thermal annealing of the as-cast alloy led to redistribution of Fe atoms in both registered phases; the solid solution and the quaternary silicide. It was estimated that the solid solution phase fraction slightly decreases from 24.9 to 21.6% after heating. Redistribution of iron atoms between

hyperfine parameters of as-cast and heated alloys

Sample

Site

ISa (mm/s)

QSb (mm/s)

FWHMc (mm/s)

Relative area (%)

As-cast

Fe(1)–10Al Fe(2)–9Al Fe(s)

0.228 ± 0.002 0.208 ± 0.002 0.269 ± 0.001

0.300 ± 0.003 0.492 ± 0.004 –

0.229 ± 0.004 0.229 ± 0.004 0.277 ± 0.015

47.76 ± 1.40 27.35 ± 1.00 24.89 ± 1.87

Heated

Fe(1)–10Al Fe(2)–9Al Fe(s)

0.233 ± 0.002 0.218 ± 0.001 0.272 ± 0.001

0.282 ± 0.004 0.460 ± 0.004 –

0.225 ± 0.004 0.225 ± 0.004 0.279 ± 0.019

44.33 ± 1.54 34.06 ± 1.37 21.61 ± 2.25

a b c

IS—isomer shift values given against metal iron at 293 K. QS—quadrupole splitting values. FWHM—full-width at half maximum of single Lorentzian peak.

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Fig. 9. Microstructure of the wheel-side surface of a ribbon annealed at 870 K for 2 h.

Table 4 Composition of particles in points 1 and 2 in Fig. 8, annealing temperature 873 K Elements

Fe Si V Al

Concentrations in point 1

Concentrations in point 2

wt.%

at.%

wt.%

at.%

2.5 ± 0.1 1.25 ± 0.1 0.4 ± 0.1 95.9 ± 0.1

4.9 1.3 0.7 93.1

19.7 ± 0.1 5.8 ± 0.1 1.25 ± 0.1 73.25 ± 0.1

10.7 6.2 0.7 82.3

± ± ± ±

0.1 0.1 0.1 0.1

± ± ± ±

0.1 0.1 0.1 0.1

both polyhedra in the quaternary silicide is also visible. The absence of drastic changes in Mössbauer parameters of both solid solution and quaternary silicide phases after heating is a strong evidence of alloy stability up to 873 K. The microstructure of a specimen annealed at 873 K is shown in Fig. 9, and the chemical composition in points 1 and 2 is given in Table 4. The chemical formula of the intermetallic phase is Al13.18 (Fe,V)1.84 Si. The decomposition of the matrix is was studied by DSC. The DSC curves presented in Fig. 10 display tran-

sient changes (with relative units) in three samples: as-cast ribbon, ribbon annealed at 473 K and ribbon annealed at 593 K. One thermal effect (exothermal peak) was observed in as-cast ribbons only. This assumed one stage of structural change—decomposition of supersaturated matrix that begins at nearly 600 K and it does not finish up to 750 K. By extrapolating the experimental curve, one can conclude that full decomposition can be reached at about 850 K. XRD data (Table 2) of the value of matrix lattice parameter at 873 K confirms that the matrix is still supersaturated. From Fig. 10 it is seen that previously annealed samples do not show any heat release. According to Table 5, the supersaturation of the matrix decreases steeply at 473 K and at higher temperatures remains nearly constant. In Table 5, the lattice parameters of microcrystalline Al–Fe–V–Si ribbons are given after heating at 473, 593 and 873 K. The lattice parameters are very close and lower than the value of pure Al (0.40492 nm), see Table 5. Slow coarsening of the structure similar to Ostwald ripening is possible during annealing at these temperatures. Due to slow diffusivity of the alloying elements, this process is very slow and does not cause any significant change of the microstructure. The thermal stability of Al–Fe–V–Si alloys is probably the result of the stability of the supersaturated matrix also. It is found that about 7 wt.% of alloying elements remained in the matrix Table 5 Lattice parameters of Al matrix in microcrystalline alloys after annealing

Fig. 10. DSC curve representing heat-release between 580 and 830 K.

Temperature (K)

Lattice parameters (nm)

As-cast (about 397) 473 593 873

0.40443(5) 0.40481(5) 0.40478(5) 0.40485(5)

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4. Conclusions

Fig. 11. The influence of annealing temperature on the microhardness of microcrystalline ribbons [17].

despite the long heating at 873 K, see Table 4, concentration in point 1. As was shown in [23] the microhardness of microcrystalline Al–Fe–V–Si ribbons depends on the annealing temperature. These results are shown in Fig. 11, which—compared with Fig. 10—allows the decrease in microhardness to be connected with the partial decomposition of the supersaturated matrix. It can be assumed that by decrease of extrusion temperature it is possible to increase the microhardness and the tensile strength of compacted alloys. Compaction of rapidly solidified ribbons into bulk material was done by hot extrusion at 753 K. A detailed procedure for Al ribbons’ extrusion is given elsewhere [24]. The microhardness of extruded compacts shows nearly the same values as the ribbons annealed at the same temperature (Fig. 12). It can be assumed that the microstructure and mechanical properties of the alloys depend strongly on the temperature, and that this is valid for hot extrusion process, too. Valuable data for parameters of hot-extrusion processing can thus be obtained by studying the development of the microstructure in rapidly solidified ribbons during the annealing, see Figs. 11 and 12. As can be seen from Fig. 11, a significant improvement of microhardness of the extruded alloy can be obtained by decreasing the extrusion temperature to temperature below 673 K.

Fig. 12. Distribution of microhardness through the cross-section of compacted sample.

As-cast and annealed Al–Fe–V–Si microcrystalline ribbons have a two-phase microstructure, supersaturated aluminum matrix and quaternary silicides. The supersaturation of the matrix is quite stable in the studied temperature interval according to the Mössbauer spectroscopy and XRD results. There is a strong relation between the microstructure and microhardness of the microcrystalline alloys. Changes in microhardness of the ribbons during annealing can be connected both with the growth of quaternary silicides and the decomposition of supersaturated solid solution. A significant microhardness improvement of the extruded alloy can be obtained by decreasing the extrusion temperature to values below 673 K.

Acknowledgements Financial support from Bulgarian National Fund via Contract TN 1002/00 is gratefully acknowledged.

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