Rare Earth Metal-Based Intermetallics Formation in Al–Cu–Mg and Al–Si–Cu–Mg Alloys: A Metallographic Study

.is study was conducted on Al–Cu–Mg and Al–Si–Cu–Mg alloys containing either 5%La or 5%Ce. Two levels of Ti addition were examined, i.e., 0.05% and 0.15%. .ermal analysis was the only technique used to obtain castings, from which samples were then sectioned for metallographic examination. Based on the results obtained, the following points may be highlighted. Addition of a fairly large amount of RE metals (La or Ce) leads to the appearance of several peaks in the solidification curve between the precipitation of the primary α-Al phase and the (Al–Al2Cu) eutectic reaction. Although a significant drop in the eutectic temperature is caused by the addition of 5%La or Ce, the correspondingmodification of the eutectic Si is marginal. Twomain types of intermetallics were documented: a gray phase in the form of sludge with a fixed composition and a white phase in the shape of thin platelets. Due to the high affinity of RE to react with Si, Fe, and Cu, several compositions were obtained explaining the observed multiple peaks in the solidification curve. Judging by the morphology of the gray phase, it is assumed that this phase is precipitated in the liquid state and acts as a nucleation site for the white phase. Lanthanum and Ce can substitute each other.


Introduction
e effect of trace Ce additions on the microstructure and mechanical properties of A356 (Al-7Si-0.35Mg)aluminum alloys was analyzed by Tsai et al. [1].
eir results show that two kinds of intermetallic compounds are formed, namely, Ce-23%Al-22%Si and Al-17%Ce-12%Ti-2%Si-2%Mg phases (percentages here and elsewhere are in wt.%).e thermal analysis data reveal that there is no direct relationship between the eutectic growth temperature and the silicon morphology/modification rating [2,3].In another study on the effect of rare earth elements' addition on microstructures and mechanical properties of A356 alloy, Tsai et al. [4] observed the precipitation of AlTiLa(Ce)Mg and AlSiLa(Ce) phases.
Intermetallic phases in nonmodified and Sr-modified Al-Si cast alloys containing mischmetal (MM) were investigated by Elsebaie et al. [5] who reported the formation of two distinct intermetallic phases at high solidification rate with the addition of 6 wt.% MM to A356.2 alloy: (i) a grey sludge phase containing Ti with a high Ce/La ratio (3 : 4 : 1) and low Mg content (0.26 wt.%), and (ii) a white spherical phase with a low Ce/La ratio (1 : 32 : 1) with Sr content of 1.5 wt.% and 0.4 wt.%Mg. is white spherical phase was also observed at low solidification rate, having the same chemical composition but exhibiting larger sized particles.At low solidification rate, a white Chinese script phase, Al 2 MMSi 2 , is formed with an addition of 6 wt.% of MM with a low Ce/La ratio (1 : 5 : 1) associated with 0.26 wt.%Mg content.
Effect of Ce addition on the microstructure and mechanical properties of Al-20%Si alloy was investigated by Joy Yii et al. [6].e results show that addition of 0.46 to 2.24 wt.% of Ce led to the formation of fine cells consisting of a mixture of eutectic Si particles and intermetallic Al 3 Ce and CeAl 1.2 Si 0.8 phases in the Al matrix.In Al-17%Si alloys, La begins to form intermetallics when its concentration exceeds 1 wt.%. e La-rich phase could be represented as AlSi 2 La 2 [7].Differential thermal analysis (DTA) was carried out by Hosseinifar [8] on two alloys with the compositions Al-20.1 wt.%La-19.9wt.%Mg and Al-15.07 wt.% La-14.93 wt.%Mg; he reported on the precipitation of Al 11 La 3 at 458.5 °C, Al 3 La at 554.5 °C, and AlLa at 521 °C.
According to Bäckerud et al. [9], the onset of di erent solidi cation reactions can be determined with the aid of thermal analyses.e crudest interpretation of the phenomena taking place can be obtained by direct observation of the temperature-time curve, as most reactions are exothermic and result in the reduction in the cooling rate or, in some cases, can result in the increase in temperature due to recalescence [9][10][11].ese analyses can be conducted by inserting either one or more thermocouples in the mold containing the solidifying metal.Di erences in temperature can be detected between di erent thermocouples, or the readings from one of these devices can be derived as a function of time to evaluate the time and temperature at which the reactions take place [12][13][14].
e present study was undertaken to investigate precipitation of intermetallics in Al-2%Cu-0.5%Mgand Al-8% Si-2%Cu-0.5%Mgalloys containing 5%La or Ce using the thermal analysis technique as the main tool (solidi cation rate was approximately 0.8 °C/s).Samples sectioned from the thermal analysis castings were used for examining the microstructures.Phases were identi ed using an electron probe microanalyzer equipped with energy dispersive X-ray spectroscopic (EDS) and wavelength dispersive spectroscopic (WDS) facilities.

Experimental Procedure
e alloys used in this study were supplied in the form of 12.5 kg ingots.e chemical composition of the base alloy  used for this research is listed in Table 1.ermal analysis was used to obtain the solidi cation curves and to identify the main reactions and corresponding temperatures occurring during solidi cation of the di erent melt compositions prepared.Melting was carried out in a cylindrical graphite crucible of 2 kg capacity, using an electrical resistance furnace; the melting temperature was maintained at 750 °C while alloying elements were added.Rare earth metals (5%La or 5%Ce) were added in the form of Al-15 wt.%RE master alloys, whereas Ti in the amount of 0.15% was added in the form of Al-5 wt.%Ti-1 wt.%B master alloy.
A high sensitivity type-K (chromel-alumel) thermocouple, which is insulated using a double-holed ceramic tube, is attached to the centre of the graphite mold.
e temperature-time data are collected using a highspeed data acquisition system linked to a computer to record the temperature-time data every 0.1 second.Figure 1 shows a schematic representation of the graphite mold (preheated at 600 °C), thermocouple, and thermal analysis setup.From the data obtained, the solidi cation curves and the corresponding rst derivative curves for a number of selected alloys were plotted to identify the main reactions occurring during solidi cation and their corresponding temperatures.In order to support the data obtained from thermal analysis, DSC runs were carried out in the temperature range 400-700 °C at the rate of 10 °C/min.
Samples for microstructural characterization were sectioned from the central portion of the casting containing the thermocouple tip as explained elsewhere [3].e prepared samples were examined by means of a Leica DM LM optical microscope.e grain-size measurements were carried out using a Clemex image analyzer in conjunction with the optical microscope.Phase identi cation was carried out using an electron probe microanalyzer (EPMA) in conjunction with energy dispersive X-ray spectroscopic (EDS)

DSC Runs.
e DSC heating curves of the B0-based alloys are shown in Figure 1(a).Generally, all the alloys displayed two common endothermic peaks (B and D) at temperatures ranging from 621 to 627 °C and 635 to 637 °C, respectively.ese peaks can be attributed to the melting of white intermetallic phases and α-Al, respectively.ree of the B0-based alloys displayed a small endothermic peak (A) at temperature 576-580 °C which may also be related to the melting of white phases.
e two Ce-containing B0 alloys exhibited an additional endothermic peak (C) at 625 °C in the low-Ti alloy and at 630 °C in the high-Ti alloy. is peak corresponds to the melting of the graycolored Al-Ti-Ce phase.
e height of this peak in the latter alloy is noticeably higher than that in the former alloy, which reveals that increasing the level of Ti signi cantly      1(c).e endothermic peaks H, K, and L represent the melting of the Al 2 Cu phase, eutectic Si, and α-Al, respectively.e temperatures of these peaks varied with the alloy composition to range from 505 to 507 °C, 569 to 573 °C, and 586 to 600 °C, respectively.e endothermic peaks I and J which occurred at 540 °C and 552 °C, respectively, could be attributed to the melting of the white-colored Ce-/La-rich intermetallic phases.Taking into consideration the increase in its size with increasing the Ti level whether for Ce-or Lacontaining alloys, the last endothermic peak (M) which occurred at 599-607 °C could be related to the melting of the gray Al-Ti-Ce/La phase.Figure 1(d e other exothermic peaks, namely O, P, and R, could be due to the solidi cation of whitecolored Ce-/La-rich intermetallic phases.No distinct exothermic peak was displayed by the DSC curve for the formation of the gray Al-Ti-Ce/La phase, which implies that this phase may be cosolidi ed with α-Al.Eutectic modi cation of Al-Si alloys with rare earth metals was studied by Nogita et al. [16].e authors suggested that all of the rare earth elements caused a depression of the eutectic growth temperature.At best, the RE elements resulted in only a small degree of re nement of the plate-like silicon.Also, many of the rare earth additions signi cantly altered the eutectic solidi cation mode from that of the unmodi ed alloy. Figures 3 and 4 demonstrate the e ect of adding a relatively large amount of RE metals (without or with 0.15%Ti) to both base alloys.Based on these gures, it is clear that these additions resulted in (1) appearance of several new peaks in the zone between α-Al and (Al-Al 2 Cu) eutectic, (2) depression in the (Al-Si) eutectic temperature in D0 alloys (approximately 16 °C), (3) increase in the solidi cation zone by about 18 °C.As reported previously by the present authors [3,17], the observed depression in the eutectic temperature due    It should be mentioned that only La revealed partial modi cation but was not as e ective a modi er as Sr in the Sr-treated alloys.For example, the Si particle average area is initially 17.85 µm 2 compared to 15.64 µm 2 and 10.67 µm 2 with the addition of 5% of Ce and La, respectively, and 3.2 µm 2 in the Sr-treated alloy.Figure 6 exhibits the importance of adding Ti in rening the alloy grain size.However, due to the high a nity of Ti to react with the RE metals, this leads to precipitation of a fairly large amount of intermetallics as displayed in Figure 5(d).e nature of these intermetallics will be discussed in the next section.e observed increase in the freezing zone in RE-treated alloys may lead to formation of shrinkage porosity.Since the molten metal was not degassed prior to casting, the volume fraction of porosity may not be reliable.

Microstructural Characterization
In this section, a series of electron micrographs will be presented to illustrate the e ect of the added RE metals without (<0.05%)and with (0.15%) Ti on the morphology and density of the precipitated intermetallics.Figure 7(a) exhibits the microstructure of the base B0 alloy showing the three main phases as described in the previous section.12 Advances in Materials Science and Engineering Addition of RE (La or Ce) resulted in the precipitation of an intensive amount of thin, long particles of a white phase.e high-magni cation micrograph in Figure 7(c) reveals the platelet-like morphology of this white phase.Addition of Ti + RE caused the precipitation of a gray phase in the form of "sludge" as shown in Figure 7(e)-note the precipitation of the white phase on the edges of the sludge.
Based on the chemical composition listed in Table 1, the Fe-based sludge should precipitate at about 650 °C, which exceeds the melting temperatures of the two alloys.Similarly, the gray-phase particles (judging by their morphology) could as well have precipitated in the liquid state prior to solidi cation and acted as nucleation sites for the white phase.
From the present results, both La-and Ce-rich precipitates are found to have more or less the same shape, as illustrated in Figures 7(f) and 7(g).In order to distinguish between the di erent types of particles, WDS and EDS techniques were employed.It should be mentioned as well that similar observations were made when D0 alloy was used; that is, the amount of Si seems to have no bearing on the precipitated phases.illustrate the distribution of Ce and La in the gray-phase particles compared to Ti and Si.From these figures, it is evident that Ti is the main element in the gray phase, with traces of RE and Si. Figure 11 shows the distribution of La and Ti in such particles observed in the D0 + La + Ti alloy sample.
Another interesting observation made is the interaction between the RE and transition metals, in particular  12 shows the interaction between La and Cu in the D0 alloy containing 0.15%Ti.Lanthanum platelets seem to attract Cu at their edges as shown in Figure 12(b) which is an enlarged portion of Figure 12(a).On the other hand, it is evident that neither Cu nor Si exhibits an affinity to react with the gray phase.Considering the white phase, observed in Figures 13 and 14, Cu has a relatively higher affinity to react with La compared to Fe as inferred from their relative intensities and the size of their corresponding peaks in Figure 13(b).It should be borne in mind that the white platelets are very thin (less than 1.5 nm thick) which would explain the variation in their composition (the diameter of the area examined by the electron beam is ∼3 µm).
Based on these observations, it would be reasonable to say that the gray phase (due to its larger size) would exhibit a definite composition overall, whereas the composition of the white phase would vary from one particle to another.Table 2 summarizes the WDS analysis carried out on the phases observed in the present alloys.e composition of the gray phase could be written as Al 21 Ti 2 RE (RE � La or Ce). e white phase has several compositions caused by its reactivity with the other elements in the matrix, particularly Si, Cu, and Fe. e phases formed in the alloys investigated are as follows:

Concluding Remarks
Based on the results obtained in the present study, the following remarks may be highlighted.Addition of a fairly large amount of RE metals (La or Ce) leads to the appearance of several peaks in the solidification curve between the primary α-Al and (Al-Al 2 Cu) eutectic phases.Although a significant drop in the eutectic temperature is caused by the addition of 5%La or Ce, the corresponding modification of the eutectic Si is marginal.Two main categories of intermetallics were documented: a gray phase in the form of sludge with a fixed composition and a white phase in the shape of thin platelets.Due to the high affinity of RE to react with Si, Fe, and Cu, several compositions were obtained explaining the observed multiple peaks in the solidification curve.Judging by the morphology of the gray phase, it is assumed that this phase was precipitated in the liquid state and acted as a nucleation site for the white phase.Both La and Ce are substitutable.

Figure 1 :Figure 3 :Figure 2 :
Figure 1: DSC heating and cooling curves of B0-based (a and b) and D0-based (c and d) alloys.

Figure 1 (Figure 8 :
Figure 8: (a) Backscattered electron image and distribution of Al, Ce, and Ti in D0 + Ce + Ti alloy and (b) EDS spectrum corresponding to the gray phase marked X in (a).(Note the weak peak of Si compared to the Ti peak.) ) shows the DSC cooling curves of the D0-based alloys.e two major exothermic peaks (N and Q) correspond to the solidi cation reaction of α-Al and the eutectic Si, respectively.

Figure 9 :
Figure 9: (a) Backscattered electron image and distribution of Ti, La, and Al in D0 + La + Ti alloy and (b) EDS spectrum corresponding to the gray phase marked X in (a).

Figure 10 :
Figure 10: (a) Backscattered electron image and distribution of Ti and Ce in D0 + Ce + Ti alloy and (b) EDS spectrum corresponding to the gray phase marked X in (a).White arrows indicate high Ce concentration in the white-phase particles.

Figure 11 :
Figure 11: (a) Backscattered electron image and (b) elemental distribution in D0 + La + Ti alloy, (c) X-ray images of Ti, La, and Si in (a), showing the presence of La in the RE-rich plate, EDS spectrum corresponding to the white phase marked X in (a).

10
Advances in Materials Science and Engineering to addition of RE metals is not necessarily related to modi cation of the eutectic Si particles as shown in Figure5.

Figure 12 :
Figure 12: (a) Backscattered electron image and distribution of Cu, Ti, and La in D0 + La + Ti alloy, (b) enlarged micrograph of circled area in (a), and (c) EDS spectrum corresponding to the gray phase marked X in (b).(Note the weak Cu and Si peaks in the gray phase.)

Figure 13 :
Figure 13: La-transition metal interactions in D0 + La alloy: (a) backscattered electron image and corresponding Cu, Fe, and La images and (b) EDS spectrum of white phase circled in (a).
Figure 8 reveals the possibility of the precipitation of RE-based intermetallics on the existing Ti-rich particles (possibly Al 3 Ti particles).Figures 9 and 10

Figure 14 :
Figure 14: La-Cu interactions in D0 + La alloy: (a) backscattered image, (b) corresponding X-ray images of Si, La, Al, and Cu, and (c) EDS spectrum of white phase circled in (a).

Table 1 :
Chemical composition of the two base alloys used in the present work.

Table 2 :
WDS analysis of the RE-based phases observed in the alloys studied.