Intermetallic Phases Examination in Cast AlSi5Cu1Mg and AlCu4Ni2Mg2 Aluminium Alloys in As-Cast and T6 Condition

Cast Al-Si-Cu-Mg and Al-Cu-Ni-Mg alloys have a widespread application, especially in the marine structures, automotive and aircraft industry due to their excellent properties. The main alloying elements – Si, Cu, Mg and Ni, partly dissolve in the primary α-Al matrix, and to some extent present in the form of intermetallic phases. A range of different intermetallic phases may form during solidification, depending upon the overall alloy composition and crystallization condition. Their relative volume fraction, chemical composition and morphology exert significant influence on a technological properties of the alloys (MrowkaNowotnik G., at al., 2005; Zajac S., at al., 2002; Warmuzek M., at al. 2003). Therefore the examination of microstructure of aluminium and its alloys is one of the principal means to evaluate the evolution of phases in the materials and final products in order to determine the effect of chemical composition, fabrication, heat treatments and deformation process on the final mechanical properties, and last but not least, to evaluate the effects of new procedures of their fabrication and analyze the cause of failures (Christian, 1995; Hatch, 1984; Karabay et al., 2004). Development of morphological structures that become apparent with the examination of aluminium alloys microstructure arise simultaneously with the freezing, homogenization, preheat, hot or cold reduction, anneling, solution and precipitation heat treatment of the aluminium alloys. Therefore, the identification of intermetallic phases in aluminium alloys is very important part of complex investigation. These phases are the consequence of equilibrium and nonequilibrium reactions occurred during casting af aluminium alloy. It worth to mention that good interpretation of microstructure relies on heaving a complete history of the samples for analysis. Commercial aluminium alloys contains a number of second-phase particles, some of which are present because of deliberate alloying additions and others arising from common impurity elements and their interactions. Coarse intermetallic particles are formed during solidification in the interdendric regions, or whilst the alloy is at a relatively high temperature in the solid state, for example, during homogenization, solution treatment or recrystallization (Cabibbo at al., 2003; Gupta at al., 2001; Gustafsson at al., 1998; Griger at al., 1996; Polmear, 1995; Zhen at al., 1998). They usually contain Fe and other alloying elements


Material and methodology
The investigation was carried out on the AlCu4Ni2Mg2 and AlSi5Cu1Mg casting aluminium alloys.The chemical composition of the alloys is indicated in Table 1.Microstructure analysis was carried out on the as-cast and in T6 condition aluminium alloys.The alloys were subjected to T6 heat treatment: solution heat treated at 520°C for 5 h followed by water cooling and aging at 250°C for 5 h followed by air cooling.The Intermetallic Phases Examination in Cast AlSi5Cu1Mg and AlCu4Ni2Mg2 Aluminium Alloys in As-Cast and T6 Condition 21 microstructure of examined alloy was observed using an optical microscope on the polished sections etched in Keller solution (0.5 % HF in 50ml H 2 O).The observation of specimens morphology was performed on a scanning electron microscope (SEM), operating at 6-10 kV in a conventional back-scattered electron mode and a transmission electron microscopes (TEM) operated at 120, 180 and 200kV.The thin foils were prepared by the electrochemical polishing in: 260 ml CH 3 OH + 35 ml glycerol + 5 ml HClO 4 .The chemical composition of the intermetallics was made by energy dispersive spectroscopy (EDS) attached to the SEM.The intermetallic particles from investigated AlCu4Ni2Mg2 and AlSi5Cu1Mg alloys in T6 condition were extracted chemically in phenol.The samples in the form of disc were cut out from the rods of ∅12 mm diameter.Then ~0.8 mm thick discs were prepared by two-sided grinding to a final thickness of approximately 0.35 mm.The isolation of phases was performed according to following procedure: 1.625 g of the sample to be dissolved was placed in a 300 ml flask containing 120 mm of boiling phenol (182°C).The process continued until the complete dissolution of the sample occurred ~10 min.The phenolic solution containing the residue was treated with 100 ml benzyl alcohol and cooled to the room temperature.The residue was separated by centrifuging a couple of times in benzyle alcohol and then twice more in the methanol.The dried residue was refined in the mortar.After sieving of residue ~0.2 g isolate was obtained.The intermetallic particles from the powder extract were identified by using X-ray diffraction analysis.The X-ray diffraction analysis of the powder was performed using a diffractometer -Cu Kα radiation at 40kV.DSC measurements were performed using a calorimeter with a sample weight of approximately 80-90 mg.Temperature scans were made from room temperature ~25°C to 800°C with constants heating rates of 5°C in a dynamic argon atmosphere.The heat effects associated with the transformation (dissolution/precipitation) reactions were obtained by subtracting a super purity Al baseline run and recorded.

Results and discussion
DSC curves obtained by heating (Fig. 1a) and cooling (Fig. 1b) as-cast specimens of the examined AlSi5Cu1Mg alloy are shown in Fig. 1.DSC curves demonstrate precisely each reactions during heating and solidification process of as-cast AlSi5Cu1Mg alloy.One can see from the figures that during cooling the reactions occurred at lower temperatures (Fig. 1b) compared to the values recorded during heating of the same alloy (Fig. 1a).Solidification process of this alloy is quite complex (Fig. 1) and starts from formation of aluminum reach (α-Al) dendrites.Additional alloying elements such as: Mg, Cu, as well as impurities: Mn, Fe, leads to more complex solidification reaction.Therefore, as-cast microstructure of AlSi5Cu1Mg alloy presents a mixture of intermetallic phases (Fig. 2).The solidification reactions (the exact value of temperature) obtained during DSC investigation were compared with the literature data (Bäckerud at al., 1992;Li, et al., 2004) and presented in Table 2. Results obtained in this work very well corresponding to the (Bäckerud at al., 1992;Li, et al., 2004;Dobrzański at al., 2007).Fig. 2 shows as-cast microstructure of AlSi5Cu1Mg alloy.The analyzed microstructure contains of primary aluminium dendrites and substantial amount of different intermetallic phases constituents varied in shape, (i.e.: needle, plate-like, block or "Chinese script"), size and distribution.They are located at the grain boundaries of α-Al and form dendritic network structure (Fig. 2).In order to identify the intermetallic phases in the examined alloy, series of elemental maps were performed for the elements line Al-K, Mg-K, Fe-K, Si-K, Cu-K and Mn-K (Fig. 3 and 4).
The maximum pixel spectrum clearly shows the presence of Al, Mg, Fe, Si, Cu and Mn in the scanned microstructure.In order to identify the presence of the elements in the observed phases, characteristic regions of the mapped phase with high Mg, Fe, Si, Cu and Mn concentration were marked and their spectra evaluated (Fig. 5).).In addition the Cu-containing intermetallics nucleating as dark grey rod, primary eutectic Si particles with "Chinese script" morphology were also observed.Fe has a very low solid solubility in Al alloy (maximum 0.05% at equilibrium) (Mondolfo, 1976), and most of Fe in aluminium alloys form a wide variety of Fe-containing intermetallics depending on the alloy composition and its solidification conditions (Ji et al., 2008).In the investigated as-cast AlSi5Cu1Mg alloy Fe-containing intermetallics such as light grey needle like β-Al 5 FeSi (Fig. 5a) and blockly phase consisting of Al, Si, Mn and Fe (Fig. 5a) were observed.On the basic of literature date (Liu Y.L. et al., 1999;Mrówka-Nowotnik et al., a,b, 2007;Wierzbińska et al., 2008) and EDS results (Fig. 5 and Tab. 3) this particles were identified as α-Al(FeMn)Si phase.The following phases were identified in the as-cast AlSi5Cu1Mg alloy based on DSC results and microstructure -LM and SEM observations (Tab. 2 and 3, Fig. 1-5): Si, β-Al 5 FeSi, Al 5 Cu 2 Mg 8 Si 6 , Al 2 Cu, α-Al(FeMn)Si.These results can suggest, that in this alloys occur five solidification reactions (Tab.4).The data presented in Tab. 4 shows that the solidification sequence of AlSi5Cu1Mg alloy differ only slightly from this obtained by Backerud and Li (Tab.2).Microstructure of AlSi5Cu1Mg alloy in T6 condition is presented in Fig. 6.Analyzing the micrographs of the alloy after heat treatment at 520°C for 5h it had been found that during solution heat treatment the morphology of primary eutectic Si changes from relatively large needle like structure to the more refined "Chinese script" and spherical in shape particles.
Most of the needle like particles of β-Al 5 FeSi phase transform into spherical-like α-Al(FeMn)Si (Kuijpers at al, 2002;Liu at al., 1999;Christian, 1995) as shown in Figure 6  silicon ones, whereas the rod-like and "Chinese script" shaped, are inclusions of the phase consisting of Al, Si, Mn and Fe (Fig. 2,7 and Tab.3).Since it is rather difficult to produce detailed identification of intermetallic using only one method (e.g.microscopic examination) therefore XRD and TEM techniques was utilized to provide confidence in the results of phase classification based on metallographic study.The microstructure of the examined alloy AlSi5Cu1Mg in T6 state consists of the primary precipitates of intermetallic phases combined with the highly dispersed particles of hardening phases.The TEM micrographs and the selected area electron diffraction patterns analysis proved that the dispersed precipitates shown in Figure 8 and 9 were the precipitates of hardening phase β-Mg 2 Si (Fig. 8) and θ′-Al 2 Cu (Fig. 9).The results of XRD investigation are shown in Fig. 8. X-ray diffraction analysis of AlSi1MgMn alloy confirmed metalograffic observation.Additionaly the presented above results were compared to the analysis of particles extracted from the AlSi5Cu1Mg alloy using phenolic dissolution technique (Fig. 11).The EDS spectra revealed the presence of Al, Mg, Mn, Si, Fe and Cu -bearing particles in the extracted powder (Fig. 11).The EDS analysis results proof that analyzed particles extracted from the AlSi5Cu1Mg alloy were: Si, AlMnFeSi, Al 5 FeSi, Al 5 Mg 8 Cu 2 Si 6 phases.DSC curves obtained by heating (Fig. 12a) and cooling (Fig. 12b) of as-cast specimens of AlCu 4 Ni 2 Mg 2 alloy are shown in Fig. 12. DSC curves demonstrate reactions which occurred during heating and solidification process of the alloy.The obtained results were similar to the peaks observed during cooling of the samples of AlSi5Cu1Mg alloy -the recorded peaks were shifted to the lower values (Fig. 12b).
The solidification sequence of this alloy can be quite complex and dependent upon the cooling rate (Fig. 12).Possible reactions which occurred during solidification of AlCu4Ni2Mg2 alloy are presented in Tab. 5. Aluminum reach (α-Al) dendrites are formed at the beginning of solidification process.Additional alloying elements into the alloys (Ni, Cu, Mg) as well as impurities (eg.Fe) change the solidification path and reaction products.Therefore, as-cast microstructure of the tested alloy exhibit the appearance of mixture of intermetallic phases (Fig. 13a).The solidification reactions (the exact value of temperature) obtained during DSC investigation presented in Tab. 5.The analyzed microstructure in as-cast state (Fig. 13a) contains of primary aluminium dendrites and substantial amount of different intermetallic phases constituents varied in shape, size and distribution.They are located at the grain boundaries of α-Al and form dendrites network structure (Fig. 13a).
The analyzed microstructure of investigated AlCu4Ni2Mg2 alloy in T6 condition (Fig. 13b) consists different precipitates varied in shape, i.e.: fine sphere-like, complex rod-like and ellipse-like distributed within interdendritic areas of the α-Al alloy.Large number of fine sphere-like strengthening phase are located in the boundary zone.However, small volume of this phase is also present homogenously throughout the sample (Fig. 13b).In order to identify the intermetallic phases in the examined alloy, series of distribution maps were performed for the elements line Mg-K, Al-K, Fe-K, Ni-K, Cu-K (Fig. 14).The maximum pixel spectrum clearly shows the presence of Ni and Cu in the scanned microstructure.In order to identify the presence of the elements in the observed phases, two regions of the mapped phase with high nickel and copper concentration were marked and their spectra evaluated.The microstructure of the examined alloy AlCu4Ni2Mg2 in T6 state consists of the primary precipitates of intermetallic phases combined with the highly dispersed particles of hardening phases.The TEM micrographs and the selected area electron diffraction patterns analysis proved that the dispersed precipitates shown in Fig. 13b are the intermetallic phases S-Al 2 CuMg (Fig. 16) and Al 6 Fe (Fig. 17) besides the precipitates of hardening phase θ′-Al 2 Cu were present in AlCu4Ni2Mg2 alloy (Fig. 18).The approximate size of the S phase was 0,5 μm.The results of the SEM/EDS analysis of the particles extracted with boiling phenol from AlCu4Ni2Mg2 alloy (Fig. 19) were compared with X-ray diffraction pattern (Fig. 20).The observed peaks confirmed SEM and TEM results.The majority of the peaks were from Al 7 Cu 4 Ni, Al 6 Fe, S-Al 2 CuMg, and Al 3 (CuFeNi) 2 .On the other hand, it is nearly impossible to make unambiguous identification of the all intermetallics present in an aluminium alloy which are rather complex, even applying all well-known experimental techniques.X-ray diffraction analysis is one of the most powerful and appropriate technique giving the possibility to determine most of verified intermetallics based on their crystallographic parameters.Our analysis shows that the difficulties of having reliable results of all the possible existing phases in a microstructure of the alloy is related to the procedure of phase isolation.The residue is separated by centrifuging and since some of the particles are very fine and available sieves are having too big outlet holes there is no chance prevents them from being flowing out from a solution.

Conclusion
Currently, efforts are being directed towards the development of analytical techniques which rapidly achieve an accurate determination of phase components in an alloy.
According to the obtained results, the applicability of the proposed methods provides a practical alternative to other techniques.The phenol extraction procedure was also successfully applied to the examined aluminium alloys.The main advantages of dissolution techniques are its reliability -when used properly you will always get pure residue -and its low price.Fine intermetallic particles (<lμm) are formed during artificial aging of heat-treatable alloys and are more uniformly distributed than constituent particles or dispersoids.Dimensions, shape and distribution of these particles may have important effects on the ductility of alloys and more needs to be known regarding their formation, structure and composition.For example, the coarse particles can influence the recrystallization, fracture, surface, and corrosion behavior, while the dispersoids control grain size and provide stability to the metallurgical structure.The dispersoids can also affect the fracture performance and may limit strain localization during deformation.The formation of particles drains solute from the matrix and, consequently, changes the strength properties of the material.This is specially relevant in the heat-treatable alloys, where depletion in Cu, Mg, and Si can significantly change the metastable precipitation processes and age hardenability of a material.Therefore, particle characterization is essential not only for choosing the best processing routes, but also for designing optimized alloy composition.Thus, particle characterization is important not only to decide what sort of processing courses should be applied, but also for designing optimized chemical composition of a material.A variety of microscopic techniques are well appropriate to characterize intermetallics but only from a small section of an analyzed sample.From commercial point of view it is extremely advantageous to provide use quick, reliable and economical examination technique capable of providing data of particles from different locations of a full scale-sized ingot.One of these methods is dissolving the matrix of an aluminium alloy chemically or electrochemically.

Acknowledgment
This work was carried out with the financial support of the Ministry of Science and Higher Education under grant No. N N507 247940

Fig. 3 .
Fig. 3. SEM image of the AlSi5Cu1Mg alloy and corresponding elemental maps of: Al, Mg, Fe, Si and Cu Fig.5shows the SEM micrographs with corresponding EDS-spectra of intermetallics observed in the as-cast AlSi5Cu1Mg alloy.The EDS analysis indicate that the oval particles are Al 2 Cu (Fig.5a).Besides Al 2 Cu phase, another Cu containing phase Al 5 Mg 8 Cu 2 Si 6 was observed (Fig.4,5).In addition the Cu-containing intermetallics nucleating as dark grey rod, primary eutectic Si particles with "Chinese script" morphology were also observed.Fe has a very low solid solubility in Al alloy (maximum 0.05% at equilibrium)(Mondolfo, 1976), and most of Fe in aluminium alloys form a wide variety of Fe-containing intermetallics depending on the alloy composition and its solidification conditions(Ji et al., 2008).In the investigated as-cast AlSi5Cu1Mg alloy Fe-containing intermetallics such as light grey needle like β-Al 5 FeSi (Fig.5a) and blockly phase consisting of Al, Si, Mn and Fe (Fig.5a) were observed.On the basic of literature date(Liu Y.L. et al., 1999; Mrówka-Nowotnik et al., a,b,  2007;Wierzbińska et al., 2008) and EDS results (Fig.5and Tab. 3) this particles were identified as α-Al(FeMn)Si phase.Fig.5shows SEM micrographs with corresponding EDS-spectra of intermetallics observed in as-cast AlSi5Cu1Mg alloy.The EDS spectra indicate that the oval particles are Al 2 Cu (Fig.5a).Besides Al 2 Cu phase, another Cu containing phase AlCuMgSi is observed (Fig5b).The results of EDS analysis are summarized in Tab. 3 versus the results obtained by earlier investigators.

Fig. 5
Fig.5shows the SEM micrographs with corresponding EDS-spectra of intermetallics observed in the as-cast AlSi5Cu1Mg alloy.The EDS analysis indicate that the oval particles are Al 2 Cu (Fig.5a).Besides Al 2 Cu phase, another Cu containing phase Al 5 Mg 8 Cu 2 Si 6 was observed (Fig.4,5).In addition the Cu-containing intermetallics nucleating as dark grey rod, primary eutectic Si particles with "Chinese script" morphology were also observed.Fe has a very low solid solubility in Al alloy (maximum 0.05% at equilibrium)(Mondolfo, 1976), and most of Fe in aluminium alloys form a wide variety of Fe-containing intermetallics depending on the alloy composition and its solidification conditions(Ji et al., 2008).In the investigated as-cast AlSi5Cu1Mg alloy Fe-containing intermetallics such as light grey needle like β-Al 5 FeSi (Fig.5a) and blockly phase consisting of Al, Si, Mn and Fe (Fig.5a) were observed.On the basic of literature date(Liu Y.L. et al., 1999; Mrówka-Nowotnik et al., a,b,  2007;Wierzbińska et al., 2008) and EDS results (Fig.5and Tab. 3) this particles were identified as α-Al(FeMn)Si phase.Fig.5shows SEM micrographs with corresponding EDS-spectra of intermetallics observed in as-cast AlSi5Cu1Mg alloy.The EDS spectra indicate that the oval particles are Al 2 Cu (Fig.5a).Besides Al 2 Cu phase, another Cu containing phase AlCuMgSi is observed (Fig5b).The results of EDS analysis are summarized in Tab. 3 versus the results obtained by earlier investigators.

Fig. 5
Fig. 5. a) SEM micrographs of the AlSi5Cu1Mg alloy in the as-cast state

and 7 .Fig. 8 .
Fig. 8. TEM micrograph of AlSi5Cu1Mg alloy in T6 conditions showing the precipitate of the β-Mg 2 Si phase (a,b), and corresponding electron diffraction pattern (c)

Fig. 13 .
Fig. 13.The microstructure of AlCu4Ni2Mg2 alloy in as-cast state (a) and the T6 condition (b)

Fig. 14 .Chemical
Fig. 14.SEM image of the AlCu4Ni2Mg2 alloy and corresponding elemental maps of: Al, Mg, Fe, Ni and CuAs seen in the elemental maps in Fig.14, the regions enriched in Ni and Cu correspond to the formation of type precipitates (complex rod-like) and ellipse-like precipitates observed in Fig.13.Fig.15shows the scanning electron micrographs and EDS analysis of particles in the AlCu4Ni2Mg2 alloy.The EDS analysis performed on the phases present in microstructure of the alloy revealed, that complex rod-like phase is the Al 7 Cu 4 Ni one, whereas the ellipse-like is Al 3 (CuFeNi) 2(Fig.15 and Tab. 6)

Fig. 17 .
Fig. 17.TEM micrograph of AlCu4Ni2Mg alloy in T6 condition showing the precipitate of the Al 6 Fe phase (a), and corresponding electron diffraction pattern (b) Fig. 19.SEM micrographs of the particles Al 7 Cu 4 Ni (a,c) and Al 3 (CuFeNi) 2 (b,d) extracted from the AlCu4Ni2Mg2 alloy along with EDS spectra (e,f)

Table 3
. The chemical composition of the intermetallic phases in AlSi5Cu1Mg alloy in the as-cast state The major disadvantageous of phenol extraction method are the possible contamination of the residue and the time needed.The examined alloys AlSi5Cu1Mg and AlCu4Ni2Mg2 possessed a complex microstructure.By using various instruments and techniques (LM, SEM-EDS, TEM and XRD) a wide range of intermetallics phases were identified.The microstructure of investigated AlSi5Cu1Mg alloy included: β-Al 5 FeSi, α-Al 12 (FeMn) 3 Si, Al 2 Cu, Q-Al 5 Cu 2 Mg 8 Si 6 , Si and Mg 2 Si phases.The microstructure of AlCu4Ni2Mg2 alloy included five phases, namely: Al 7 Cu 4 Ni, θ′-Al 2 Cu, Al 6 Fe, S-Al 2 CuMg, and Al 3 (CuFeNi) 2 .A size and distribution of these various dispersoids depend on the time and temperature of the homogenization and/or annealing processes.