The effect of alloying elements on thermal stability of five different near eutectic Al-Ce based alloys, i.e., Al-12Ce, Al-12Ce-4Si, Al-12Ce-0.4Mg, Al-12Ce-4Si-0.4Mg and Al-12Ce-4Si-0.4Mg-0.25Sr alloys were investigated. The alloys were heat treated at three different temperatures, i.e., 200 °C, 300 °C and 400 °C to establish the thermal stability. Binary Al-12Ce alloy consisting of α-Al and showed higher resistance to coarsening at all temperatures. Addition of Si decreases the thermal stability above 200 °C while combined addition of Si and Mg increases the thermal stability up to 300 °C. Addition of Sr to quaternary alloy was not beneficial to enhance the thermal stability.
Research Article
Effect of alloying elements on thermal stability of Aluminium-Cerium based alloys
https://doi.org/10.21203/rs.3.rs-67587/v1
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Aluminium alloys are desirable for automotive and aerospace industries due to their lightweight, outstanding castability and excellent mechanical properties. These alloys can be used as replacement in the automotive sector with currently used iron based alloys and titanium alloys wherever it is suitable. The high strength to weight ratio of aluminium alloys lead to better fuel economy and help reduce greenhouse gases. Besides, aluminium alloys tend to make a robust passivizing protective oxide layer that makes it corrosion resistant. One of the major drawbacks of aluminium alloys is that it does not retain their mechanical properties at higher temperatures. The addition of rare earth elements to aluminium alloys can fill this gap. In this context, recent studies on Al-Ce alloys show improved design efficiency for the transport industry and better mechanical properties for high temperature applications. At the hyper eutectic point, Al-12Ce alloy contains a eutectic mixture of Al and Al11Ce3 intermetallic. This Al11Ce3 present in aluminium matrix is metastable and does not respond to heat treatment. In lanthanide series, earlier lanthanides up to Sm form Al11RE3 (RE refers to rare earth) intermetallic and later lanthanides from Eu form Al3RE phase that is stable in aluminium matrix. In Al-RE system, thermodynamically stable Al3RE phase precipitates out during solidification and no further heat treatment is needed for Al3RE precipitation [1]. Al-Ce alloys are castable for a wide range of Ce contents and compatible with available casting infrastructure. Addition of Mg to Al-Ce binary alloy allows precipitation of intermetallic phase during heat treatment and ensures better properties than previously developed binary Al-Ce alloys [2]. The addition of Si in Al-Ce-Mg alloys has a negative impact on castability of Al-Ce alloys and creates a flaky type CeAlSi intermetallic that adversely affects the mechanical properties.
Zachary C. Sims et al. have found that a lower amount of Si addition to Al-Ce based alloys does not degrade the castability of alloys and provides better wear resistance at room temperature [2]. It was found that the addition of Ce up to 12 wt. % into Al increases the mechanical properties of binary system (Figure1) [3]. To develop reasonable room temperature and high temperature (400 °C) strength, other alloying elements (Si, Mg and Cu) were added to binary Al-Ce alloys. Figures 1 and 2 represent the summary of mechanical properties of Al-Ce based alloys available in literature. Previous studies on binary Al-Ce alloys show improved thermal stability at high temperatures for various applications. Therefore, the main objective of the present work is to understand the effect of alloying elements and correlate the microstructure-hardness-thermal stability of Al-Ce based alloys like Al-12Ce, Al-12Ce-4Si, Al-12Ce-0.4Mg, Al-12Ce-4Si-0.4Mg and Al-12Ce-4Si-0.4Mg-0.25Sr (wt. %) alloys.
Experimental details
2.1 Alloy preparation
Near-eutectic Al-Ce based alloys with composition of Al-12Ce, Al-12Ce-4Si, Al-12Ce-0.4Mg, Al-12Ce-4Si-0.4Mg and Al-12Ce-4Si-0.4Mg-0.25Sr (wt. %) were cast for the present study. Commercial pure aluminium (CPAl) ingot and the following master alloys having purity level 99.7% were used as feedstock: Al-50Si, Al-20Ce, Al-20Mg, and Al-10Sr (wt. %). All the alloys were prepared using resistance type furnace (KANTHAL, SN 1015) and clay bonded graphite crucible. Master alloys of Al-Ce, Al-Si and CPAl together were placed in a crucible and allowed to melt in furnace held at 785 °C. Al-Mg and Al-Sr master alloy were added to the melt at the end with intermittent stirring to ensure chemical homogeneity and minimize oxidation loss. After complete melting, the alloy was degassed with a commercially available degasser, C2Cl6. The top oxide layer of the melt was skimmed off before pouring the molten alloy into mild steel mould (1.5 cm diameter and 15 cm height), which was preheated at 300 °C in an air circulated oven. After casting, the cylindrical alloy bars were sectioned using the abrasive cutter to prepare a sample of 2-3 mm thickness for metallography, heat treatment and subsequent studies.
2.2 Metallography and microstructure characterization
For microstructural analysis, ASTM E3 standard metallographic practice was adopted, where final polishing was carried out using 0.01 μm colloidal silica. Keller's reagent was used for chemical etching, whenever required. The FESEM (Field Emission Scanning Electron Microscope) was equipped with a backscattered electron (BSE) and a secondary electron (SE) detector. The attached energy dispersive spectroscopy (EDS) was used for phase specific chemical analysis with 5 kV and 20 kV acceleration voltages. Phase specific EDS analysis was carried out on three random locations for each phase and average composition calculated. For heat treatment studies, samples were polished using alumina slurry up to 12-13 μm surface roughness. Image analysis was carried out using Image J software (version 1.53b) and the systematic manual point count method (ASTM E562) was used for calculation of volume fraction of phases. X-ray diffractometer was equipped with a solid-state detector and operated using Cu-Kα radiation. Quantification of phases and XRD pattern was analysed using Xpert high score (3.0.0) software.
3.1 Al-12Ce alloy
Binary Al-12Ce (Figure 3a) alloy shows the presence of Al (light grey in the matrix) and intermetallic Al11Ce3. Al11Ce3 is also referred to as Al4Ce by some authors [1].The alloy shows a very fine faceted eutectic mixture of Al and lathe like intermetallic Al11Ce3. Figure 3b shows the XRD pattern of as cast and selected heat treated alloys. The XRD pattern confirms the presence of Al and Al11Ce3 phases. From the heat treatment studies (Figure 4), few critical points (encircled) were selected based on high hardness variation compared to as cast alloy. The XRD pattern shows that no new phases form on heat treatment up to 100 hours. The calculated lattice parameters for Al (cubic) and Al11Ce3 (orthorhombic) was found to be a=4.07Å and a=4.37Å b=16.63Å c=8.24Å, respectively which are in agreement with reported a=4.39Å b=13.2Å c=10.09Å [5].
Figure 4a and b show heat-treated (300 °C for 10 hours) and as cast microstructure of Al-12Ce alloy respectively. The heat-treated alloy showed coarsening of phases. Heat treatment at 300°C for 10 hours results in a eutectic microstructure that has undergone minor morphological changes. TheAl11Ce3 phase seems to have spheroidized at many regions and become less intertwined as compared to small and entangled laths in as cast alloy. This suggests a localized minimization of micro constituent surface energy at the eutectic through surface diffusion within the intermetallic and accompanying spheroidization, rather than bulk diffusion through the matrix. In as cast condition, volume fraction of Al11Ce3 was found to be 15.5 ± 1.2 % as determined using the systematic manual point count method. Theoretical volume fraction of Al11Ce3 from equilibrium phase diagram was found to be 9 .45 % [6, 7]. This disparity in volume fraction is due to the non-uniform distribution of intermetallic particle into the Al matrix. The volume fraction of Al11Ce3 increased from 0.155 to 0.185 upon heat treatment at 300 °C for 10 hours as compared to as-cast alloy. This is in agreement with XRD pattern (Figure 3b). It shows that the ratio of area integrated intensity ( ~13.9 to 13.6) slightly changed for a fixed plane (( , ) and 2 interval with heat treatment [8]. This indicates the gradual change in phase fraction of intermetallic with time and temperature. Strong vacancy binding of Al to Ce atoms decreases the degradation of Al11Ce3 intermetallic and therefore reduces vacancy diffusion (the dominant transport mechanism for solute atoms within the matrix) [9, 10]. The intermetallic is trapped by the zero solubility of Ce in aluminium matrix. This trapping prevents the system from minimizing surface energy through diffusion which in turn prevents the alloys from coarsening. So, low solubility and large atomic size difference between Ce (1.81 Å) and Al (1.43 Å) result in a low diffusion coefficient when compared to other alloying elements. As an illustration, the diffusion data for Al, Ce and other solutes in Al were calculated using the data available in literature (Table1).
Table 1: Measured diffusion data for elements (solutes) in Al used in the present study [10-12].
|
Phase |
Pre-exponential D° |
Activation Energy, Q(KJ/mole) |
D at 200 °C ( |
D at 300 °C ( |
D at 400 °C ( |
|
Al |
1.37× |
124.0 |
2.77× |
6.79× |
3.25× |
|
Ce |
1.90× |
111.0 |
1.04× |
1.47× |
4.60× |
|
Mg |
1.49× |
120.5 |
7.33× |
1.54× |
6.61× |
|
Si |
1.38× |
117.6 |
1.42× |
2.62× |
1.02× |
Figure 4c shows the heat treatment of binary Al-Ce alloys at three different temperatures of 200 °C, 300 °C and 400 °C for up to 100 hours. Al-12Ce shows microhardness of 510 ± 36.29 MPa as compared to 200 MPa for pure Al [13]. The increased hardness from as-cast to heat treated condition for 300 °C up to 10 hours can be approximated by Orowan strengthening.
An attempt was made to understand the strengthening mechanism in Al-Ce based alloys. Solid solution strengthening, Hall-Petch hardening and precipitation hardening contribute to significant hardening in aluminium alloys. In the presence of precipitates, precipitation hardening can dominate all other hardening mechanisms. Orowan described that precipitation strengthening is a result of precipitate-dislocation interaction in the matrix leading to formation of dislocation loop around the precipitate and increase in yield strength of the material is given by [14]
Where f: precipitate volume fraction
The parameters for aluminium in Equation 1are as follows: M=3.06 [18], ν=0.345 [18], b=0.286 nm [19] and G=25.4 GPa [19].
By using equation 1, 2 and 3, increase in Orowan strength for as cast and heat treated condition (300 °C for 10 hours) was estimated to be MPa and 42.27 MPa that is significantly lower than the experimentally measured (ΔHV/3) values of 103.37 MPa and 112.52 MPa respectively (Table 2). Here ΔHV/3 is approximated as the increment in strength and defined as the difference in microhardness values of as-received alloy and pure Al [20]. Z. C Sims et al. [21] explained the disparity in strength. The neutron diffraction study showed that the load transfer mechanism played significant role in improving the strength. Orowan strengthening mechanism and load transfer mechanism are expected to be active at higher temperatures, although less efficient than at ambient temperature, as dislocation can climb to bypass Al11Ce3 precipitate and the fast creeping Al matrix transfers less load to Al11Ce3 precipitate. An increase in the mean diameter of Al11Ce3 from 142 ± 26 nm in as cast alloy to 175 ± 21 nm in heat treated alloy at 300 °C for 10 hours was observed (Figure 4a and b). However, Eric T. Stromme et al. [22] observed the ageless behaviour in Al-Ce alloys. Although there was coarsening after heat treatment at 300 °C for 10 hours (Figure 3d), Orowan strengthening still dominated in the heat-treated alloy due to increase in volume fraction of intermetallic [23]. The hardness values show a peak for all temperatures studied and then stabilized on prolonged heat treatment for up to 100 hours. This demonstrates that both the hardening mechanisms were active at room and elevated. This study shows the thermal stability of intermetallic Al11Ce3, after heat treatment. The thermal stability of Al11Ce3 can also be ascribed to its high melting point above 1200 °C [24]
Table 2: Calculated increase in strength by Orowan/load transfer mechanism
|
Alloy |
Total increment in strength (MPa) = (ΔHV/3) |
Orowan strengthening Contribution(MPa) |
Load transfer contribution (MPa) |
|
Al-12Ce (as cast) |
103.37 |
42.27 |
61.10 |
|
Al-12Ce (after 10 hours of heat treatment at 300 °C) |
112.52 |
53.07 |
59.45 |
3.2 Al-12Ce-4Si alloy
Cerium reacts favourably with Si and results in formation of a new ternary tetragonal intermetallic Ce(Si1-XAlX)2 with x = 0.1-0.9 (Figure 5a). The addition of Si suppresses the formation of intermetallic Al11Ce3(-0.349 eV formation energy per atom). It forms a new stable intermetallic phase Al2Ce (-0.458 eV formation energy per atom) and CeAlSi (-0.585 eV formation energy per atom) which was confirmed by the XRD pattern in Figure 5b [25-27]. As in Al-12Ce alloy, few critical points (encircled) were selected from the heat treatment study of Al-12Ce-4Si alloy, based on large hardness variation from the as cast condition. XRD pattern at those critical points do not show formation of any new phase in Al-12Ce-4Si alloy even after heat treatment for up to 100 hours.
The microstructure of as-cast Al-12Ce-4Si comprises of α-Al, Al2Ce and CeAlSi phase. The intermetallic laths could have formed through an invariant reaction between Al, Ce and Si. The needle like phases (white) persisted even after heat treatment at different temperatures (Figures 6a, b). This shows the high thermal stability of intermetallic phase in aluminium [28-30]. This result can be justified with the higher melting point (more than 1400 °C) of Al2Ce [31].
The effect of Si on yield strength appears to be inconsistent and affects the ultimate tensile strength and work hardening [22].Therefore, Si addition in Al-12Ce alloy results in marginal change in hardness as compared to binary Al-12Ce alloy. Figure 6c shows the heat treatment studies for Al-12Ce-4Si alloy. After heat treatment at 200 °C for 10 hours, there is a significant increase in hardness. This could be due to the combined effect of dispersion strengthening, solid solution strengthening and load transfer mechanism activated at this temperature. Diffusion data (Table 1) shows that diffusion coefficient of Si at 400 °C is 104 times higher than 200 °C. Thus, diffusion time for Si at high temperature is much lower and strengthening due to solid solution could be lowered. Though microstructural changes observed at 200 and 400 °C were not significant (Figure 6a and b) but decrease in hardness at 400 °C after 10 hours of aging time was significant suggesting poor thermal stability of the alloy.
3.3 Al-12Ce-0.4Mg alloy
Figure 7a shows the as cast microstructure of Al-12Ce-0.4Mg alloy containing eutectic mixture of Al and Al11Ce3in which Al11Ce3lathes are firmly interlinked. The XRD pattern also shows Al andAl11Ce3 in as cast alloy. However, upon heat treatment at higher temperatures two new phases, Al3Mg2 and Mg3Al, appear (Figure 7b).This could be due to precipitation of phases in the alloy assisted by higher diffusion coefficient of Mg in Al (Table 1).
The corresponding microstructures of the alloy are shown in Figure 8a and c. Lathe-like interconnected Al11Ce3transforms into discrete particles after heat treatment at 400 °C for 10 hours and becomes more globular as compared to heat treatment at 200 °C for 100 hours (Figure 8c).These alloys showed a 23% increase in hardness at 200 °C for 100 hours, while at 400 °C, there is a 10% decrease in hardness. The variation in the hardness can be inferred from solid solution strengthening, Orowan strengthening and load transfer mechanism. In order to find the contribution of Orowan strengthening microstructural study was conducted on the critical point (Table 3).
Table 3 shows that the aspect ratio of Al11Ce3decreases at 400 °C compared to 200 °C. The difference in diffusivity of 3-4 orders (Table 1) between 200 °C and 400 °C explains the extent of fragmentation at two temperatures. The increment in Orowan strength for heat-treated alloy at 200 °C for 100 hours and 400 °C for 10 hours was calculated to be 15 MPa and 18.8 MPa that is significantly lower than experimentally measured (ΔHV/3) values of 179.23 MPa and 132.80 MPa respectively (Table 3). This suggests that the strength increase was mostly due to solid solution strengthening and a mechanism of load transfer. The decrease in hardness at 400 °C after 10 hours of heat treatment can be inferred to loss of solid solution strengthening due to the release of strain energy, inactivation of load transfer mechanism due to fragmentation of Al111Ce3 lathes and softening of phases low melting point phases like Al3Mg2(~447 °C)[33]). It was difficult to quantify the individual contribution of solid solution strengthening and load transfer mechanisms due to the formation of new phases during heat treatment. Increasing the heat treatment time beyond 50 hours shows a rapid decrease in hardness value accompanied by softening of phases
Table 3: Data for area fraction, aspect ratio and increased strength by Orowan/ load transfer/solid solution strengthening mechanisms at critical points
|
Alloy |
Area fraction of Al11Ce3 (%) |
Aspect ratio (length/diameter) of Al11Ce3 |
Total increment in strength (MPa) = (ΔHV/3) |
Orowan strengthening Contribution (MPa) |
Load transfer + solid solution strengthening contribution (MPa) |
|
Al-12Ce-0.4Mg (after 100 hours of aging time at 200 °C) |
23.07± 1.23 |
9.66 ± 9.50 |
179.23 |
15.12 |
164.11 |
|
Al-12Ce-0.4Mg (after 10 hours of aging time at 400 °C) |
21.67± 2.63 |
4.52 ± 4.65 |
132.80 |
18.84 |
113.96 |
3.4 Al-12Ce-4Si-0.4Mg alloy
Addition of 4 wt. % Si and 0.4 wt. % Mg to binary Al-12Ce alloy leads to complete suppression of intermetallic Al11Ce3 due to formation of Ce(Si1-XAlX)2with x=0.1 to 0.9, which has lower formation energy (-0.585 eV) than Al11Ce3 (-0.349 eV) [25, 27]. Figure 9a show fine eutectic mixture of Al9Siand Al matrix with dispersed Ce(AlXSi1-X)2.CeAl1.2Si0.8 phase is tetragonal with lattice parameters of a = b = 4.24, c = 14.538 A° [34].XRD pattern shows that no phase transformation occurs on heat treatment as compared to as-cast condition (Figure 9b).After the heat treatment, based on high hardness variation, some critical points(encircled) were selected for microstructural study. Figure 10a and b show the micrograph of as cast Al-12Ce-4Si-0.4Mg alloy at 200 °C and 400 °C respectively after 10 hours of heat treatment. Figure 10c shows the heat treatment analysis of Al-12Ce-4Si-0.4Mg alloy. Heat treatment at high temperature (400 °C) results in fragmentation of Al9Si intermetallic lathe and transformation into particle-like morphology. Hardness improves by 33 % at 200 ° C for 10 hours of aging time compared to as-cast condition (Figure 10c). This increase in hardness was associated with the combined effect of Si and Mg. The decrease in hardness at 400 °C could be expected due to the loss in solid solution strengthening at high temperature and inactivation of load carrying capacity of the microstructure due to fragmentation (Figure 10b).
3.5 Al-12Ce-4Si-0.4Mg-0.25Sr alloy
Figure 11a demonstrates that further addition of Sr to quaternary alloy refines intermetallic Ce(Si1-XAlX)2.Based on hardness values, some critical points (encircled) were selected and XRD study was performed (Figure 11b).Al0.9Mg3.1 phase was observed in the as-cast condition and disappears after heat treatment possibly due to its lower melting point. Si intermetallic, as observed in the alloy without Sr, is not observed in the alloy with Sr. Figure 11c reported the heat treatment study up to 100 hours for quinary alloy. It shows that Sr addition to quaternary alloy enhances the quinary alloy's room temperature strength. This may be due to increment in solid solution strengthening by addition of Sr. After 10 hours of heat treatment, a significant reduction in hardness was observed at 200 °C. Quinary alloy is characterised by presence of multiple phases and thus making the analysis challenging. The decrease in the hardness can be correlated with less thermal stability and softening and possibly dissolution of Mg3Al phase which is confirmed by the XRD pattern (Figure 11b). For a prolonged heat treatment period, Mg3Al phase disappears.
Microstructure stability of five different Al-Ce based alloys were studied after heat treatment at three different temperatures, i.e., 200 °C, 300 °C and 400 °C. Microhardness measurements were used to ascertain microstructure stability and strengthening mechanisms were discussed. The major conclusions from the work are as follows:
- Al-Ce binary alloys show thermal stability at all the temperatures studied (200 – 400 ° C) due to higher stability of
- Al-12Ce-4Si is thermally stable at 200 °C but there is progressive decrease in stability at higher temperatures.
- Al-12Ce-0.4Mg and Al-12Ce-4Si-0.4Mg show thermal stability at 200 °C and 300 °C. The strengthening mechanisms involved are solid solution strengthening, load transfer mechanism and Orowan strengthening.
- The addition of Sr to Al-12Ce-4Si-0.4Mg alloy fragmented the AlCeSi intermetallic and enhance the room temperature strength. Heat treatment at higher temperatures results in a significant reduction in hardness.
Authorship contribution statement
Dheeraj Kumar Saini: Writing original draft. Rahul Gope: Data collection. Animesh Mandal: Conceptualization, supervision, writing- review and editing.
Acknowledgement
The authors would like to thank the institute for providing all the research facilities and MHRD for providing the financial assistance in the form of scholarship to the master student and research scholar.
Competing interests: The authors whose names are listed immediately below certify that they have NO affiliations with or involvement in any organization or entity with any financial interest (such as honoraria; educational grants; participation in speakers’ bureaus; membership, employment, consultancies, stock ownership, or other equity interest; and expert testimony or patent-licensing arrangements), or non-financial interest (such as personal or professional relationships, affiliations, knowledge or beliefs) in the subject matter or materials discussed in this manuscript. Author's Name Dheeraj Kumar Saini Rahul Gope Animesh Mandal
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