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Solid solution in Al–4.5 wt% Cu produced by mechanical alloying
Materials Science and Engineering A 433 (2006) 45–49
Solid solution in Al–4.5 wt% Cu produced by mechanical alloying J.B. Fogagnolo a,b,∗ , D. Amador a , E.M. Ruiz-Navas a , J.M. Torralba a a
Materials Science and Engineering Department, Universidad Carlos III de Madrid, Avda. Universidad 30, 28911 Legan´es, Spain b Universidade S˜ ao Francisco, Rua Alexandre Rodrigues Barbosa, 45, 13251-900 Itatiba-SP, Brazil Received 19 March 2006; received in revised form 4 June 2006; accepted 5 July 2006
Abstract Mechanical alloying has been used to produce oxide dispersion strengthened alloys, intermetallic compounds, aluminium alloys and to obtain nanostructured and amorphous materials, as well as to extend the solid solution limit. In this work, Al and Cu elemental powders were subjected to high-energy milling to produce Al–4.5 wt% Cu powder alloy. The powders obtained were characterized by scanning electron microscopy, X-ray diffraction (XRD) and differential scanning calorimetry (DSC), aiming to explore if the copper is present in solid solution or as small particles after high-energy milling. Related to the formation of a supersaturated solid solution, the results of scanning electron microscopy and X-ray diffraction are non-conclusive: the copper could be dispersed with a very small size, undetectable to both techniques. The Al2 Cu precipitation at temperatures between 160 and 230 ◦ C, verified by DSC and XRD analyses, substantiated that mechanical alloying had produced a supersaturated solid solution of copper in aluminium. The crystallite size as a function of milling time and annealing temperature was also determined by X-ray techniques. © 2006 Elsevier B.V. All rights reserved. Keywords: Mechanical alloying; Powder characterization; X-ray diffraction; Scanning electron microscopy; Thermal analysis
1. Introduction Mechanical alloying is a powder processing technique that allows the production of homogeneous materials, starting from blended elemental powder mixtures [1–4]. This process, first developed by Benjamin [5] to produce nickel super-alloys hardened by oxide dispersion, consists in repeated welding–fracturing–welding of a mixture of powder particles in a high-energy ball mill. The powder particles are trapped between the colliding balls during the milling and undergo deformation, welding or fracture, depending on the mechanical behaviour of the powder components [6–8]. Mechanical alloying breaks the oxide layer at the aluminium powder surface and thus produces homogenous oxide dispersion in aluminium alloys. Furthermore, aluminium carbide is formed due to the reaction between aluminium and carbon introduced as process control agent (PCA); this compound, undesired in casting aluminium alloys, is dispersed throughout the metal. Both, Al2 O3 and Al4 C can achieve submicrometer or even nanometer sizes, thus increasing the mechanical strength significantly ∗ Corresponding author at: Universidade S˜ ao Francisco, Rua Alexandre Rodrigues Barbosa, 45, 13251-900 Itatiba-SP, Brazil. Tel.: +55 11 4534 8034; fax: +55 11 4524 1933. E-mail address: [email protected] (J.B. Fogagnolo).
0921-5093/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2006.07.005
and stabilizing the high refinement of the aluminium matrix at elevated temperatures [9–11]. Two of the distinguishing attributes of mechanical alloying are the formation of a solid solution from milling elemental powders and the extension of the solid solubility limits, which can be determined and quantified by X-ray diffraction (XRD), generally from changes in the lattice parameter values calculated from shifts in peak positions or even the absence of second phase peaks. Suryanarayana [3] discusses the difficulties associated with the determination of the increase in the solid solubility limit promoted by mechanical alloying, by the absence of peaks of the second phase in the XRD pattern. Several factors influence the detectability of a second phase by XRD, such as the broadening effect due to the crystalline refinement and the lattice strain, the very small size of the second phase, the defects introduced by cold deformation, which are always present in mechanically milled materials, among others. Mechanical alloying also introduces impurities into the milled powder, from the materials of the container and the balls, and nitrogen, oxygen or carbon from the atmosphere or the PCA. This contamination can dilate or contract the lattice parameter, depending on the kind of impurity atom introduced, which can obscure the determination of the solid solution. Instead of forming a solid solution, a fully decomposed twophase mixture can be obtained after milling [12]. Depending
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J.B. Fogagnolo et al. / Materials Science and Engineering A 433 (2006) 45–49
on the size and the volume fraction of one-phase, XRD is unable to distinguish a (supersaturated) solid solution from a two-phase mixture. Thus, it is desirable to use more than one technique to unambiguously distinguish a solid solution from a nanosized, dispersed second phase. Transmission electron microscopy (TEM) is a powerful technique that has been used to do this [13], however there is no work in the literature that makes use of thermal analysis. This work describes the obtainment of Al–4.5 wt% Cu by mechanical alloying. The main objective is to explore if the copper is in solid solution or as small particles after the milling process. To do this, we used XRD, scanning electron microscopy (SEM), and differential scanning calorimetry (DSC), as characterization techniques. 2. Experimental procedure Elemental aluminium (gas atomised, purity 99.5%) and copper powders (electrolytic, purity 99.9%) were placed in a highenergy ball mill (ZOZ attritor with container and grinding
medium of stainless steel) to produce Al–4.5 wt% Cu powder alloy. The following parameters were used: balls/load ratio, 10/1 (wt./wt.); ball diameter, 5 mm; speed, 1000 rpm; atmosphere, argon. One percent (in weight) of microwax was added as PCA. Samples of the milled powders were collected at different milling times: 0, 2, 4, 6, 8 and 10 h. To investigate internal sections of the milled powder, the samples were impregnated in resin, polished and observed by scanning electron microscopy (SEM). Back scattering electron (BSE) mode and energy dispersive spectrum analyses (EDS) were used. The samples were also analysed by XRD. The crystallite size and lattice strain were estimated by measuring the broadening of the X-ray peaks (Hall–Williamson plot [2]). The powder milled for 10 h was analysed by differential scanning calorimeter (DSC), at a heating and cooling rate of 20 K min−1 . This analysis was performed successively, twice, to distinguish a history-dependent thermal event from a material intrinsic one. The same powder was heated up to 200, 325 and 450 ◦ C in the DSC device, using the same heating rate, to investigate the exothermic events founded on the DSC curves, by means of
Fig. 1. Microstructure of the mechanically alloyed Al–4.5 wt% Cu powders after different milling times: 2 h (a), 4 h (b), 6 h (c), 8 h (d) and 10 h (e).
J.B. Fogagnolo et al. / Materials Science and Engineering A 433 (2006) 45–49
47
XRD. Once having reached the desired temperatures, the samples were cooled at 100 K min−1 . 3. Results Fig. 1 shows the microstructure of the mechanically alloyed Al–4.5 wt% Cu powders after different milling times. After 2 h of milling, the particles had undergone deformation and welding was the dominating process. Several laminar particles of aluminium and copper are seen welded, copper being the brighter laminate, as confirmed by EDS. After 4 h of milling, the particle morphology had changed to an equiaxed one, with randomly oriented welding boundaries. After 6 h of milling, the copper phase did not appear as distinct as before; intermediate tones appeared, indicating a diffusion process of copper into aluminium. The copper contrast had almost disappeared after 8 h and, finally, after 10 h of milling, the copper contrast could not be detected by BSE. Iron contamination from the balls and grinding media, was revealed by EDS as indicated in Fig. 1e.
Fig. 2. XRD patterns of Al–4.5 wt% Cu after several milling times.
Fig. 3. Crystallite size and lattice strain as functions of milling time.
Fig. 2 shows XRD patterns of the powders after different milling times. At the bottom of the figure, the pattern of 10h-milled powder after heating up to 200 ◦ C is shown. The aluminium peaks tend to broaden as the milling time increases. The copper peaks, clearly present after 2 h of milling, disappeared entirely after 10 h of milling, which is in agreement with the SEM observations: the reduction and disappearance of the copper peaks in the XRD patterns is concurrent with the disappearance of the copper contrast observed in the SEM analysis. The concept of X-ray peak broadening related to the crystallite size is widely accepted in the study of mechanically alloyed powder particles [2]. Fig. 3 shows the crystallite size and lattice strain as functions of milling time. Due to the mechanical deformation introduced into the powder, crystallite refinement occurs and the lattice strain increases. The unmilled powder already presented a very refined crystallite, due to the high cooling rate imposed by the atomisation process, and the milling process decreased its size to about 45 nm; the lattice strain increased to about 0.8%. These results confirm the effectiveness of the highenergy milling process to produce nanostructured materials. Fig. 4 shows the DSC curves of the powder previously submitted to 10 h of milling. The two DSC curves differ signifi-
Fig. 4. DSC curves of the powder previously submitted to 10 h of milling.
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J.B. Fogagnolo et al. / Materials Science and Engineering A 433 (2006) 45–49
Fig. 5. Crystallite size and lattice strain of the powder previously submitted to 10 h of milling, as a function of the thermal treatment temperature.
cantly. The one related to the first heating shows two exothermic peaks: one between 160 and 230 ◦ C (peak 1), and another between 330 and 450 ◦ C (peak 2). The curve related to the second heating shows only a low intensity, broad endothermic peak between 330 and 525 ◦ C (peak 3). Fig. 5 shows the crystallite size and lattice strain of the powder previously submitted to 10 h of milling, estimated by the broadening of the X-ray peaks, as a function of the thermal treatment temperature. The lattice strain decreased continuously; while the crystallite size increased with increasing temperature, save between 200 and 325 ◦ C. 4. Discussion The diffusion of copper into the aluminium structure and the formation of a supersaturated solid solution cannot be undoubtedly postulated only from the SEM and XRD analysis. The copper could have been dispersed into the aluminium remaining as a distinct phase, of a very small size, so as to be undetectable by XRD and SEM. Another possibility is that the copper can be present in both forms: part solubilized and part as a nanosized, dispersed second phase. The identification of the phenomenon that caused the DSC peaks, mainly the first exothermic peak during the first heating, can help to determine, without a doubt, if a true solid solution has been formed by MA. The absence of exothermic peaks during the second heating confirms that the events, which produced exothermic peaks during the first heating are history-dependent. The absence of Al2 Cu in the as-milled powder and its presence in the same powder heated up to 200 ◦ C (see the XRD pattern at the bottom of Fig. 2) demonstrates that the exothermic peak observed between 160 and 230 ◦ C during the first heating was due to the precipitation of this intermetallic compound. The second exothermic peak observed during the first heating was possibly caused by the recovery that should have occurred, mainly between 330 and 450 ◦ C, as the mechanically alloyed powder presents a very deformed structure after milling. The increase in crystalline size and largely, the decrease of lattice strain, strongly corroborate this interpretation and advise that
the recovery has started at a lower temperature, but its DSC signal should have been hided by the stronger first endothermic peak. The absence of crystallite growth between 200 and 325 ◦ C can be explained by the anchor effect caused by precipitation of Al2 Cu particles that occurred between 160 and 230 ◦ C, as already shown. The endothermic peak observed between 330 and 525 ◦ C during the second heating was probably related to the solubilization of Al2 Cu into the aluminium, as predicted by the aluminium–copper phase diagram [14]. If the copper were dispersed in the aluminium as a distinct phase, it would have had to firstly be solubilized and subsequently be precipitated. Solubilization is an endothermic process and its occurrence should produce a signal in the DSC analysis, in spite of the nanosized scale of the supposed copper dispersoids. The precipitation of Al2 Cu between 160 and 230 ◦ C, substantiated by DSC and XRD analysis, clearly demonstrates that a supersaturated solid solution of 4.5 wt% of copper in aluminium could be obtained by mechanical alloying: to precipitate the Al2 Cu compound in the Al-based alloy, the Cu should previously be in solid solution. The non-occurrence of a similar exothermic peak in this range of temperatures during the second heating is due to the fact that Al2 Cu precipitated during the cooling of the first cycle: the cooling rate was low enough to permit this. It has to be pointed out that it was the feature of the Al–Cu system (solid solubility increasing with temperature), which allowed us to deduce the solid solution formation from the precipitation of an intermetallic phase. In systems, which do not present this feature, the use of thermal analysis to distinguish a solid solution from a nanosized, dispersed second phase should have different usefulness and would produce distinct inference. 5. Conclusions SEM and XRD results indicate the formation of a supersaturated solid solution of copper in aluminium by mechanical alloying. DSC and XRD analysis confirm that Al2 Cu precipitated while heating the mechanically alloyed powder at temperatures between 160 and 230 ◦ C, which substantiates that mechanical alloying produced a supersaturated solid solution of copper in aluminium. Therefore, thermal analysis could be used to explore if the copper is in solid solution or as small particles, after Al and Cu have been subjected to high-energy milling. Mechanical alloying decreases the crystallite size to about 45 nm and produces a cell lattice strain of about 0.8% in Al–4.5 wt% Cu powder alloy starting from elemental powders. References [1] [2] [3] [4]
J.S. Benjamin, Mater. Sci. Forum 88–90 (1992) 1–18. L. Lu, M.O. Lai, Mech. Alloying, Kluwer Academic Publishers, 1998. C. Suryanarayana, Prog. Mater. Sci. 46 (2001) 1–184. C. Suryanarayana, E. Ivanov, V.V. Boldyrev, Mater. Sci. Eng. A 304–306 (2001) 151–158. [5] J.S. Benjamin, Metall. Trans. 1 (1970) 2943–2951. [6] J.S. Benjamin, T.E. Volin, Metall. Trans. 5 (1974) 1929–1934.
J.B. Fogagnolo et al. / Materials Science and Engineering A 433 (2006) 45–49 [7] B.J.M. Aikin, T.H. Courtney, Metall. Trans. A 24 (1993) 647–657. [8] J.B. Fogagnolo, F. Velasco, M.H. Robert, J.M. Torralba, Mater. Sci. Eng. A 342 (2003) 131–143. [9] J.H. Weber, R.D. Shelleng, in: Y.-W. Kim, W.M. Griffith (Eds.), Dispersion Strengthening of Alluminum Alloys, TMS, 1988, pp. 467– 481. [10] M. Besterci, M. Sles´ar, G. Jangg, Powder Metall. Int. 24 (1992) 27–32.
49
[11] H. Arik, Mater. Des. 25 (2004) 31–40. [12] H.-S. Kim, D.-S. Suhr, G.-H. Kim, D.-W. Kum, Metall. Mater. 2 (1996) 15–21. [13] A.M. Harris, G.B. Schaffer, N.W. Page, J. Mater. Sci. Lett. 12 (1993) 160–161. [14] American Society for Metals, Alloy Phase Diagrams, vol. 3, 10th ed., ASM Handbook, 1985.
Solid solution in Al–4.5 wt% Cu produced by mechanical alloying J.B. Fogagnolo a,b,∗ , D. Amador a , E.M. Ruiz-Navas a , J.M. Torralba a a
Materials Science and Engineering Department, Universidad Carlos III de Madrid, Avda. Universidad 30, 28911 Legan´es, Spain b Universidade S˜ ao Francisco, Rua Alexandre Rodrigues Barbosa, 45, 13251-900 Itatiba-SP, Brazil Received 19 March 2006; received in revised form 4 June 2006; accepted 5 July 2006
Abstract Mechanical alloying has been used to produce oxide dispersion strengthened alloys, intermetallic compounds, aluminium alloys and to obtain nanostructured and amorphous materials, as well as to extend the solid solution limit. In this work, Al and Cu elemental powders were subjected to high-energy milling to produce Al–4.5 wt% Cu powder alloy. The powders obtained were characterized by scanning electron microscopy, X-ray diffraction (XRD) and differential scanning calorimetry (DSC), aiming to explore if the copper is present in solid solution or as small particles after high-energy milling. Related to the formation of a supersaturated solid solution, the results of scanning electron microscopy and X-ray diffraction are non-conclusive: the copper could be dispersed with a very small size, undetectable to both techniques. The Al2 Cu precipitation at temperatures between 160 and 230 ◦ C, verified by DSC and XRD analyses, substantiated that mechanical alloying had produced a supersaturated solid solution of copper in aluminium. The crystallite size as a function of milling time and annealing temperature was also determined by X-ray techniques. © 2006 Elsevier B.V. All rights reserved. Keywords: Mechanical alloying; Powder characterization; X-ray diffraction; Scanning electron microscopy; Thermal analysis
1. Introduction Mechanical alloying is a powder processing technique that allows the production of homogeneous materials, starting from blended elemental powder mixtures [1–4]. This process, first developed by Benjamin [5] to produce nickel super-alloys hardened by oxide dispersion, consists in repeated welding–fracturing–welding of a mixture of powder particles in a high-energy ball mill. The powder particles are trapped between the colliding balls during the milling and undergo deformation, welding or fracture, depending on the mechanical behaviour of the powder components [6–8]. Mechanical alloying breaks the oxide layer at the aluminium powder surface and thus produces homogenous oxide dispersion in aluminium alloys. Furthermore, aluminium carbide is formed due to the reaction between aluminium and carbon introduced as process control agent (PCA); this compound, undesired in casting aluminium alloys, is dispersed throughout the metal. Both, Al2 O3 and Al4 C can achieve submicrometer or even nanometer sizes, thus increasing the mechanical strength significantly ∗ Corresponding author at: Universidade S˜ ao Francisco, Rua Alexandre Rodrigues Barbosa, 45, 13251-900 Itatiba-SP, Brazil. Tel.: +55 11 4534 8034; fax: +55 11 4524 1933. E-mail address: [email protected] (J.B. Fogagnolo).
0921-5093/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2006.07.005
and stabilizing the high refinement of the aluminium matrix at elevated temperatures [9–11]. Two of the distinguishing attributes of mechanical alloying are the formation of a solid solution from milling elemental powders and the extension of the solid solubility limits, which can be determined and quantified by X-ray diffraction (XRD), generally from changes in the lattice parameter values calculated from shifts in peak positions or even the absence of second phase peaks. Suryanarayana [3] discusses the difficulties associated with the determination of the increase in the solid solubility limit promoted by mechanical alloying, by the absence of peaks of the second phase in the XRD pattern. Several factors influence the detectability of a second phase by XRD, such as the broadening effect due to the crystalline refinement and the lattice strain, the very small size of the second phase, the defects introduced by cold deformation, which are always present in mechanically milled materials, among others. Mechanical alloying also introduces impurities into the milled powder, from the materials of the container and the balls, and nitrogen, oxygen or carbon from the atmosphere or the PCA. This contamination can dilate or contract the lattice parameter, depending on the kind of impurity atom introduced, which can obscure the determination of the solid solution. Instead of forming a solid solution, a fully decomposed twophase mixture can be obtained after milling [12]. Depending
46
J.B. Fogagnolo et al. / Materials Science and Engineering A 433 (2006) 45–49
on the size and the volume fraction of one-phase, XRD is unable to distinguish a (supersaturated) solid solution from a two-phase mixture. Thus, it is desirable to use more than one technique to unambiguously distinguish a solid solution from a nanosized, dispersed second phase. Transmission electron microscopy (TEM) is a powerful technique that has been used to do this [13], however there is no work in the literature that makes use of thermal analysis. This work describes the obtainment of Al–4.5 wt% Cu by mechanical alloying. The main objective is to explore if the copper is in solid solution or as small particles after the milling process. To do this, we used XRD, scanning electron microscopy (SEM), and differential scanning calorimetry (DSC), as characterization techniques. 2. Experimental procedure Elemental aluminium (gas atomised, purity 99.5%) and copper powders (electrolytic, purity 99.9%) were placed in a highenergy ball mill (ZOZ attritor with container and grinding
medium of stainless steel) to produce Al–4.5 wt% Cu powder alloy. The following parameters were used: balls/load ratio, 10/1 (wt./wt.); ball diameter, 5 mm; speed, 1000 rpm; atmosphere, argon. One percent (in weight) of microwax was added as PCA. Samples of the milled powders were collected at different milling times: 0, 2, 4, 6, 8 and 10 h. To investigate internal sections of the milled powder, the samples were impregnated in resin, polished and observed by scanning electron microscopy (SEM). Back scattering electron (BSE) mode and energy dispersive spectrum analyses (EDS) were used. The samples were also analysed by XRD. The crystallite size and lattice strain were estimated by measuring the broadening of the X-ray peaks (Hall–Williamson plot [2]). The powder milled for 10 h was analysed by differential scanning calorimeter (DSC), at a heating and cooling rate of 20 K min−1 . This analysis was performed successively, twice, to distinguish a history-dependent thermal event from a material intrinsic one. The same powder was heated up to 200, 325 and 450 ◦ C in the DSC device, using the same heating rate, to investigate the exothermic events founded on the DSC curves, by means of
Fig. 1. Microstructure of the mechanically alloyed Al–4.5 wt% Cu powders after different milling times: 2 h (a), 4 h (b), 6 h (c), 8 h (d) and 10 h (e).
J.B. Fogagnolo et al. / Materials Science and Engineering A 433 (2006) 45–49
47
XRD. Once having reached the desired temperatures, the samples were cooled at 100 K min−1 . 3. Results Fig. 1 shows the microstructure of the mechanically alloyed Al–4.5 wt% Cu powders after different milling times. After 2 h of milling, the particles had undergone deformation and welding was the dominating process. Several laminar particles of aluminium and copper are seen welded, copper being the brighter laminate, as confirmed by EDS. After 4 h of milling, the particle morphology had changed to an equiaxed one, with randomly oriented welding boundaries. After 6 h of milling, the copper phase did not appear as distinct as before; intermediate tones appeared, indicating a diffusion process of copper into aluminium. The copper contrast had almost disappeared after 8 h and, finally, after 10 h of milling, the copper contrast could not be detected by BSE. Iron contamination from the balls and grinding media, was revealed by EDS as indicated in Fig. 1e.
Fig. 2. XRD patterns of Al–4.5 wt% Cu after several milling times.
Fig. 3. Crystallite size and lattice strain as functions of milling time.
Fig. 2 shows XRD patterns of the powders after different milling times. At the bottom of the figure, the pattern of 10h-milled powder after heating up to 200 ◦ C is shown. The aluminium peaks tend to broaden as the milling time increases. The copper peaks, clearly present after 2 h of milling, disappeared entirely after 10 h of milling, which is in agreement with the SEM observations: the reduction and disappearance of the copper peaks in the XRD patterns is concurrent with the disappearance of the copper contrast observed in the SEM analysis. The concept of X-ray peak broadening related to the crystallite size is widely accepted in the study of mechanically alloyed powder particles [2]. Fig. 3 shows the crystallite size and lattice strain as functions of milling time. Due to the mechanical deformation introduced into the powder, crystallite refinement occurs and the lattice strain increases. The unmilled powder already presented a very refined crystallite, due to the high cooling rate imposed by the atomisation process, and the milling process decreased its size to about 45 nm; the lattice strain increased to about 0.8%. These results confirm the effectiveness of the highenergy milling process to produce nanostructured materials. Fig. 4 shows the DSC curves of the powder previously submitted to 10 h of milling. The two DSC curves differ signifi-
Fig. 4. DSC curves of the powder previously submitted to 10 h of milling.
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J.B. Fogagnolo et al. / Materials Science and Engineering A 433 (2006) 45–49
Fig. 5. Crystallite size and lattice strain of the powder previously submitted to 10 h of milling, as a function of the thermal treatment temperature.
cantly. The one related to the first heating shows two exothermic peaks: one between 160 and 230 ◦ C (peak 1), and another between 330 and 450 ◦ C (peak 2). The curve related to the second heating shows only a low intensity, broad endothermic peak between 330 and 525 ◦ C (peak 3). Fig. 5 shows the crystallite size and lattice strain of the powder previously submitted to 10 h of milling, estimated by the broadening of the X-ray peaks, as a function of the thermal treatment temperature. The lattice strain decreased continuously; while the crystallite size increased with increasing temperature, save between 200 and 325 ◦ C. 4. Discussion The diffusion of copper into the aluminium structure and the formation of a supersaturated solid solution cannot be undoubtedly postulated only from the SEM and XRD analysis. The copper could have been dispersed into the aluminium remaining as a distinct phase, of a very small size, so as to be undetectable by XRD and SEM. Another possibility is that the copper can be present in both forms: part solubilized and part as a nanosized, dispersed second phase. The identification of the phenomenon that caused the DSC peaks, mainly the first exothermic peak during the first heating, can help to determine, without a doubt, if a true solid solution has been formed by MA. The absence of exothermic peaks during the second heating confirms that the events, which produced exothermic peaks during the first heating are history-dependent. The absence of Al2 Cu in the as-milled powder and its presence in the same powder heated up to 200 ◦ C (see the XRD pattern at the bottom of Fig. 2) demonstrates that the exothermic peak observed between 160 and 230 ◦ C during the first heating was due to the precipitation of this intermetallic compound. The second exothermic peak observed during the first heating was possibly caused by the recovery that should have occurred, mainly between 330 and 450 ◦ C, as the mechanically alloyed powder presents a very deformed structure after milling. The increase in crystalline size and largely, the decrease of lattice strain, strongly corroborate this interpretation and advise that
the recovery has started at a lower temperature, but its DSC signal should have been hided by the stronger first endothermic peak. The absence of crystallite growth between 200 and 325 ◦ C can be explained by the anchor effect caused by precipitation of Al2 Cu particles that occurred between 160 and 230 ◦ C, as already shown. The endothermic peak observed between 330 and 525 ◦ C during the second heating was probably related to the solubilization of Al2 Cu into the aluminium, as predicted by the aluminium–copper phase diagram [14]. If the copper were dispersed in the aluminium as a distinct phase, it would have had to firstly be solubilized and subsequently be precipitated. Solubilization is an endothermic process and its occurrence should produce a signal in the DSC analysis, in spite of the nanosized scale of the supposed copper dispersoids. The precipitation of Al2 Cu between 160 and 230 ◦ C, substantiated by DSC and XRD analysis, clearly demonstrates that a supersaturated solid solution of 4.5 wt% of copper in aluminium could be obtained by mechanical alloying: to precipitate the Al2 Cu compound in the Al-based alloy, the Cu should previously be in solid solution. The non-occurrence of a similar exothermic peak in this range of temperatures during the second heating is due to the fact that Al2 Cu precipitated during the cooling of the first cycle: the cooling rate was low enough to permit this. It has to be pointed out that it was the feature of the Al–Cu system (solid solubility increasing with temperature), which allowed us to deduce the solid solution formation from the precipitation of an intermetallic phase. In systems, which do not present this feature, the use of thermal analysis to distinguish a solid solution from a nanosized, dispersed second phase should have different usefulness and would produce distinct inference. 5. Conclusions SEM and XRD results indicate the formation of a supersaturated solid solution of copper in aluminium by mechanical alloying. DSC and XRD analysis confirm that Al2 Cu precipitated while heating the mechanically alloyed powder at temperatures between 160 and 230 ◦ C, which substantiates that mechanical alloying produced a supersaturated solid solution of copper in aluminium. Therefore, thermal analysis could be used to explore if the copper is in solid solution or as small particles, after Al and Cu have been subjected to high-energy milling. Mechanical alloying decreases the crystallite size to about 45 nm and produces a cell lattice strain of about 0.8% in Al–4.5 wt% Cu powder alloy starting from elemental powders. References [1] [2] [3] [4]
J.S. Benjamin, Mater. Sci. Forum 88–90 (1992) 1–18. L. Lu, M.O. Lai, Mech. Alloying, Kluwer Academic Publishers, 1998. C. Suryanarayana, Prog. Mater. Sci. 46 (2001) 1–184. C. Suryanarayana, E. Ivanov, V.V. Boldyrev, Mater. Sci. Eng. A 304–306 (2001) 151–158. [5] J.S. Benjamin, Metall. Trans. 1 (1970) 2943–2951. [6] J.S. Benjamin, T.E. Volin, Metall. Trans. 5 (1974) 1929–1934.
J.B. Fogagnolo et al. / Materials Science and Engineering A 433 (2006) 45–49 [7] B.J.M. Aikin, T.H. Courtney, Metall. Trans. A 24 (1993) 647–657. [8] J.B. Fogagnolo, F. Velasco, M.H. Robert, J.M. Torralba, Mater. Sci. Eng. A 342 (2003) 131–143. [9] J.H. Weber, R.D. Shelleng, in: Y.-W. Kim, W.M. Griffith (Eds.), Dispersion Strengthening of Alluminum Alloys, TMS, 1988, pp. 467– 481. [10] M. Besterci, M. Sles´ar, G. Jangg, Powder Metall. Int. 24 (1992) 27–32.
49
[11] H. Arik, Mater. Des. 25 (2004) 31–40. [12] H.-S. Kim, D.-S. Suhr, G.-H. Kim, D.-W. Kum, Metall. Mater. 2 (1996) 15–21. [13] A.M. Harris, G.B. Schaffer, N.W. Page, J. Mater. Sci. Lett. 12 (1993) 160–161. [14] American Society for Metals, Alloy Phase Diagrams, vol. 3, 10th ed., ASM Handbook, 1985.