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Effect of scandium on structure and hardening of Al–Ca eutectic alloys
Journal of Alloys and Compounds 646 (2015) 741e747
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Effect of scandium on structure and hardening of AleCa eutectic alloys N.A. Belov a, *, E.A. Naumova b, A.N. Alabin a, c, I.A. Matveeva c a
National Research and Technological University “MISIS”, 4, Leninsky pr., Moscow 119049 Russia Bauman Moscow State Technical University, 5, 2 ul. Baumanskaya, Moscow, 105005 Russia c UC RUSAL, 13/1, Nikoloyamskaya st., Moscow, 109240, Russia b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 21 April 2015 Received in revised form 14 May 2015 Accepted 17 May 2015 Available online 19 June 2015
The phase composition, structure and hardening of alloys in the aluminium corner of the AleCaeSc system were studied in the range up to 10% Ca and up to 1% Sс. The experimental study (optical, scanning and transmission electron microscopy with electron-microprobe analysis, differential thermal analysis and hardness measurements) was combined with Thermo-Calc software simulation for the optimization of the alloy composition. It was shown that only phases of the binary systems (Al4Ca и Al3Sc) might be in equilibrium with the aluminium solid solution. It was shown that the (Al) þ Al4Ca eutectic had a much finer structure as compared with the AleSi eutectic, which suggests a possibility of reaching higher mechanical properties as compared to commercial alloys of the A356 type. The influence of the annealing temperature within the range up to 600 С on the structure and hardness of the AleCaeSc experimental alloys was studied. It was determined that the maximum hardening corresponded to the annealing at 300 С, which was due to the precipitation of Al3Sc nanoparticles with their further coarsening. With an example of an Al-7.6% Ca-0.3% Sc model experimental alloy, a principal possibility of manufacturing aluminium casting alloys based on the (Al) þ Al4Ca eutectic was demonstrated. Unlike commercial alloys of the A356 type, the model alloy does not require quenching, as hardening particles are formed in the course of annealing of casting. © 2015 Elsevier B.V. All rights reserved.
Keywords: AleCaeSc system AleCa eutectic Al3Sc nano-particles Phase composition Microstructure Heat treatment Hardening
1. Introduction Scandium is one of the most efficient hardening additives in aluminium alloys [1,2], due to the formation of coherent Al3Sc (L12) phase precipitates with the size less than 10 nm [3e5]. These nanoparticles form in the process of annealing from supersaturated aluminium solid solution (hereinafter referred to as (Al)). Such a specific feature of these alloys enables a significant increase in strength of aluminium alloys not exposed to the classic T6 hardening heat treatment (solution treatment þ quenching þ ageing). Among these alloys, wrought alloys based on the AleMg system [6,7] are used most widely. Despite its high cost, at the moment scandium is considered to be one of the most promising alloying elements in new-generation aluminium alloys [2,8]. As for commercial casting alloys, the addition of scandium does not have the same effect as in wrought ones. First of all, this concerns alloys based on the AleSi system that constitute the majority of the total production of castings of aluminium alloys [9,10]. The
* Corresponding author. E-mail address: [email protected] (N.A. Belov). http://dx.doi.org/10.1016/j.jallcom.2015.05.155 0925-8388/© 2015 Elsevier B.V. All rights reserved.
reason for this is that silicon reduces significantly solubility of scandium in (Al) and, as a consequence, makes it impossible to form a sufficient amount of hardening nanoparticles of the Al3Sc phase in annealing. In addition, Si forms a ternary phase with Al and Sc [1], further reducing the amount of free Sc. It should also be noted that castings of widely used alloys such as А356/357 containing 0.2e0.5%Mg are exposed to heat treatment as per T6 mode in order to achieve the maximum hardening [11]. The possibility to avoid this would be appropriate under industrial conditions, in particular, for large-sized castings. Assuming that casting alloys should contain a sufficient amount of eutectic [9], it seems reasonable to search for other eutecticforming elements that would not reduce the hardening effect due to addition of scandium. We believe that one of the most promising elements among them is calcium that, similar to silicon, forms with aluminium a diagram of eutectic type. According to the data [12], in the AleCa system the L/(Al) þ Al4Ca eutectic reaction takes place at 7.6%Ca and 617 C, which is rather close to the calculated values obtained in the Thermo-Calc software application (Fig. 1a). In terms of abundance in nature, calcium holds the 3rd place among all the metals (about 3.4 wt%), behind aluminium and iron [9], and its
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N.A. Belov et al. / Journal of Alloys and Compounds 646 (2015) 741e747 Table 2 Annealing regimes of experimental alloys.
Fig. 1. Calculated phase diagrams AleCa (a) and AleCaeSc (b):b)experimental alloys are marked, liquidus lines: bold ecalculated (equilibrium), dottede shift after unequilibrium solidification (experimental).
3
density is lower than that of silicon (1.542 versus 2.328 g/sm ). In recent years, many publications considering calcium-containing magnesium alloys have appeared [13e15]. At the same time, the use of calcium for alloying of aluminium alloys is very limited [16e18]. As a rule, this element is considered to be a harmful impurity [10e12]. Limited data regarding the AleCaeSc phase diagram can be found [12,19,20]. Based on the above and taking into consideration the scarcity of information regarding aluminium alloys with addition of calcium, the following main objectives of this work were formulated: 1) to study the phase composition and structure of alloys of the AleCaeSc system (in the area close to the (Al) þ Al4Ca eutectic) in as-cast and heat-treated states; 2) to compare the hardening effect in AleCaeSc alloys, reached without quenching, with hardening in commercial AleSieMg alloys of the А356 type after the Т6 heat treatment. The purpose of these studies is to demonstrate the possibility of using the (Al) þ Al4Ca eutectic alloys with addition of scandium, hardened without quenching, in manufacturing castings of complex form.
For the experimental study, 6 alloys containing Ca in the amount of 4e10 wt% (3 binary and 3 ternary with addition of scandium) and 3 reference alloys were prepared (Table 1). The smelting was conducted in an electric resistance furnace in clay-graphite crucibles. All alloys (with the exception of B2 and B3) were prepared based on high purity aluminium (99.99%). Calcium was added in pure metallic form and scandium was added as an Ale2% Sc master alloy. Commercial alloys of the A356 and A413 types were used to prepare the reference alloys B2 and B3. The melt was poured into a graphite mould at 740e750 С to produce flat castings Table 1 Chemical composition of experimental alloys.
A1 A2 A3 A4 A5 A6 B1 B2 B3
Annealing regime
S200 S250 S300 S350 S400 S450 S500 S550 S600
200 C, 3 h S200 þ 250 S250 þ 300 S300 þ 350 S350 þ 400 S400 þ 450 S450 þ 500 S500 þ 550 S550 þ 600
C, C, C, C, C, C, C, C,
3 3 3 3 3 3 3 3
h h h h h h h h
15х30х180 mm in size (the cooling rate during solidification was about 10 К/s). The chemical composition of alloys obtained by spectral analysis is given in Table 1. The hot tearing susceptibility was assessed using a ‘harp’ probe [21]. The heat treatment of the castings was carried out in a muffle electric furnace using step cycles in the range from 200 to 600 C (Table 2) with an accuracy of temperature maintenance ±2 C. The Brunell hardness was determined in a DuraVision-20/200/250 hardness testing machine with the following parameters: ball diameter e 2.5 mm, load e 612.9 N, dwell time e 30 s. The structure was examined by means of optical (OM, Axiovert 200 MMAT), transmission electron (TEM, JEM-2100), scanning electron (SEM, TESCAN VEGA 3) microscopes and by electron microprobe analysis (EMPA, OXFORD AZtec). The samples were cut from the castings, ground and polished using standard procedures. Mechanical polishing (Struers Labopol-5) with further etching using Keller's reagent was applied. Thin foils for transmission electron microscopy (TEM) were prepared by ion thinning with a PIPS (Precision Ion Polishing System, Gatan) machine and studied at 160 kV. For calculation of the phase composition, the Thermo-Calc software (TTAL5 database) was used [22].
3. Results and their discussion
2. Experimental methods
Alloy
Designation
Concentration, wt% Ca
Sc
Fe
Si
Mg
Al
4.21 7.48 10.21 7.53 6.85 10.23 <0.01 <0.01 <0.01
<0.01 <0.01 <0.01 0.28 0.96 0.31 0.33 <0.01 0.29
<0.03 0.04 0.05 0.06 0.04 0.03 0.02 0.12 0.29
0.01 0.02 0.01 0.03 0.02 0.01 <0.01 7.21 11.34
<0.01 <0.01 <0.01 <0.01 <0.01 <0.01 <0.01 0.31 0.09
Balance Balance Balance Balance Balance Balance Balance Balance Balance
3.1. AleCaeSc phase diagram The compositions of the experimental alloys (Table 1) were chosen according to their positions in the AleCa (Fig. 1a) and AleCaeSc (Fig. 1b) phase diagrams. The binary alloys had hypoeutectic (A1), eutectic (A2) and hypereutectic (A3) calcium concentrations. According to the calculation of the ternary phase diagram, calcium reduces solubility of scandium in (Al) down to 0.2%. However, under conditions of non-equilibrium solidification, there is a shift of phase boundaries (see the dotted line in Fig. 1b), therefore the 0.3 wt% Sc concentration was chosen for the A4 basic experimental alloy. The further microstructure analysis (including EMPA) confirmed that scandium was completely dissolved into (Al). The compositions of the A5 and A6 alloys were chosen so that the primary intermetallic compounds Al3Sc and Al4Ca, respectively, could be produced. The reference alloys were chosen based on the following considerations. The B1 alloy, containing only the scandium additive and studied thoroughly [1,2], served as a standard of precipitation hardening caused by the formation of Al3Sc nanoparticles. The composition of the B2 alloy corresponds to the average composition of commercial alloys of the A356 type hardened as per T6 mode [11]. And, finally, the third reference alloy B3 was prepared (based on a commercial alloy of the A413 type) in order to demonstrate the deficiency of adding scandium into AleSi eutectic alloys.
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Fig. 2. Microstructures of as-cast alloys A1 (a), A2 (b), A3 (c) and B3 (d), a, c) SEM, b, d) OM a) hypoeutectic; b, d) eutectic; c) hypereutectic.
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Fig. 4. Primary crystals of phase Al3Sc in alloy A5, SEM/EMPA: a) SEM; b) Al; c) Ca; d) Sc.
Fig. 3. Morphology of eutectic (Al) þ Al4Ca in as-cast alloy A4, SEM: a) 1,300, b) 13,300.
3.2. Microstructure and hot tearing As expected, the castings of the А1eА3 binary alloys have hypoeutectic (Fig. 2a), eutectic (Fig. 2b) and hypereutectic (Fig. 2c) structures, respectively. The calcium-containing eutectic has a finer morphology than the aluminium-silicon one (Fig. 2d). Moreover, the optical microscopy cannot resolve the internal structure of the former eutectic as the eutectic dendrites have submicron sizes. As Fig. 3 shows, the average thickness of eutectic dendritic branches in the A4 experimental alloy is about 0.5 mm. The scandium content of the (Al) þ Al4Ca eutectic colonies corresponds to its content of the alloy A4 (Table 1), and no primary crystals of the Al3Sc phase are detected. Such crystals are found in the A5 alloy in the form of polyhedrons (Fig. 4), which is also typical for the binary AleSc alloys [1,2]. According to the EMPA data, there is no calcium in these crystals. At the same time, in the A6 alloy there are large crystals of the Al4Ca phase that hardly contain scandium (Fig. 5). Based on the obtained data, it may be concluded that in the aluminium corner of AleCaeSc system there are no ternary compounds, which concur with the calculation (Fig. 1b). The structure of the B3 alloy reveals Sc-containing crystals 5e10 mm in size (Fig.6), in addition to the particles of the siliconand iron-containing phases. Thus concentration of scandium in (Al) is very small. According to EMPA, besides Sc and Al, these particles
Fig. 5. Primary crystals of phase Al4Ca in alloy A6, SEM/EMPA. a) SEM; b) Al; c) Ca; d) Sc.
contain Si also. So this ternary phase can be identified as compound AlSc2Si2 (V) [1]. The eutectic composition of the A4 experimental alloy suggests good casting properties as it has a very narrow solidification range (DT) [18,29]. According to the calculation using the ScheileGulliver model, the scandium additive should not have a significant influence on the nature of solidification as this element affects only slightly the value of DT. The differential thermal analysis confirms the results of the calculation (Fig. 7). The assessment of hot tearing of the A4 alloy using a ‘harp’ sample showed a total absence of cracks (Fig. 8a). Among commercial heat treatable aluminium casting alloys, only alloys of 3xx series show the same indicator for such a sample (our own results). For comparison, in Fig. 8b similar
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500 С, almost all Al4Ca particles take a globular form, but their sizes remain submicron (Fig. 9a). At the maximum tested temperature of annealing (600 С), the structure becomes much coarser (Fig. 9b), the size of some particles reaches 5 mm (Fig. 9с). The second essential structural change in annealing is the decomposition of (Al) in Sc-containing alloys (A4-A6). As it appears from numerous works regarding AleSc alloys [3e5], the complete decomposition occurs at the temperature of about 300 С. Additives of other element, particularly Zr [23e25], Er [26,27], Yb [28], can rise the peak temperature up to 400e450 C. So these elements may be considered for AleCaeSc alloys also. 3.3. TEM investigations
Fig. 6. Primary crystals of ternary phase (Al,Sc,Si) in alloy B3, SEM/EMPA: a) SEM, b) Al; c) Si; d) Fe, e, f) Sc (f, enlarged image).
Fig. 7. DTA curve of alloy A4.
casting of a heat treatable alloy A224 (4.80% Cu, 0.45%Mn, 0.08%Fe, 0.05%Si) shows cracks. This experimental result gives confidence in producing mould castings of a complex geometry using (Al) þ Al4Ca eutectic alloys. It is known that the fineness of the eutectic structure in the ascast state determines to a great extent its capability to change its form during heat treatment [10,20]. The study on the influence of an annealing temperature (with 3-h holding) on the morphology of the eutectic constituents detects the first symptoms of fragmentation at 450 С. With the increase in the temperature, the changes become detectible at the resolution of optical microscopy. At
The principal difference between the as-cast structures of the experimental A4 alloy and the reference B1 alloy is that the size of the dendritic branches of the aluminum solid solution in the (Al) þ Al4Ca eutectic of the A4 alloy is less than 1 mm (Fig.3b). Therefore, the aluminum solid solution dissolution and the formation of Al3Sc precipitates occur near the (Al)/Al4Ca interfaces. To study these processes, the method of target photography was used. First, the foil of the A4 alloy was heated in the chamber of the microscope to 300 C, then to 500 C. Similar to the previous experiments (see in Table 2), stage heating by 50 C with the exposure at each stage for 3 h was used. Photographs were obtained for each stage in both bright and dark fields. Fig.10 shows, that in the all studied states extra diffraction spots are identified in all the studied states. These spots of the precipitated phase L12 in the selected area diffraction pattern enable their identification by dark-field imaging using these superlattice spots [3,4], which are at the positions of prohibited reflections of the face-centered cubic aluminium matrix. In the state S300, which corresponds to the maximum hardening (Fig.11), the size of L12 precipitates inside (Al) dendrites is minimal (<5 nm). Interestingly, the Sc-containing precipitates at the interface with the Al4Ca eutectic particles are much larger (Fig. 10a, b). When the temperature is increasing up to 350 C, the size of the precipitates inside the (Al) dendrites is increasing. However, they are smaller than those located near the Al4Ca particle. Heating at 400 C leads to further enlargement of L12 precipitates, with the difference with interphase precipitates being almost eliminated (Fig. 10e,f). Coherent precipitates L12 can be visualized in bright field through a strain contrast caused by elastic distortions of the adjacent matrix. This produces “coffee bean” contrasts in two-beam imaging conditions as shown in many previous TEM investigations [3,4]. This contrast is detected particularly clearly after heating at 450 C (Fig.10g), when the size of the precipitates reaches 50 nm (Fig.10h) Apparently from Fig. 10, density of these particles is rather high as the fraction of (Al) in the eutectic makes about 70 wt. %. We can elucidate that the concentration of scandium in the eutectic branches of (Al) makes not 0.3%, but about 0.43 wt%. As a consequence, the number of Al3Sc particles increases in comparison with reference alloy B1. With the increase in the temperature, the particle coarsen considerably which makes it possible to detect them using scanning electron microscopy [25]. The first signs of Al3Sc particles in the experimental alloy can be seen after annealing at 550 С; and after annealing at 600 С they are seen quite distinctly (Fig.9c). 3.4. Hardness measurements The change in hardness depending on the annealing temperature is a consequence of the two above mentioned processes. The initial state of all the experimental and reference alloys (except for B2) was as-cast. The B2 alloy corresponding to an А356 commercial
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Fig. 8. Castings ‘harp’ (hot tearing test) of alloys A4 (a) and 224 (b).
alloy was studied after heat treatment to the maximum hardening e T6 (heating at 540 С, 6 h, quenching in cold water, ageing at 175 С, 6 h). It is evident from Fig. 11 that the heating of the binary Ca-containing alloy A2 up to 400 С does not have an effect on hardness, which can be explained from the absence of any structural changes. At higher temperatures, softening takes place, which can be explained by the change in the Al4Ca eutectic morphology (Fig. 9). Dependences HBeT for binary alloys A1 and A3 are similar (not shown in Fig.11). In alloys A4 and B1 the visible hardening appears at 250 С (Fig. 11), which is likely to be connected with precipitation of Al3Sc nanoparticles [4,5]. At 300 С, their hardening effect reaches the maximum level, which can be connected with complete precipitation of scandium from (Al) with retention of sizes at the level of nanoparticles (Fig. 10a,b). Softening of the AleCaeSc eutectic alloy A4 at higher temperatures of annealing is caused, firstly, by coarsening of Al3Sc second-phase precipitates (beginning from 350 С, see in Fig. 10ceh), and, secondly, by the morphology change of Al4Ca eutectic particles (Fig. 8a). It should be noted that the hardness increment in the А4 and B1 alloys, containing 0.3% Sc, is very similar (Fig.11), which enables us to make a conclusion about similar features of Al3Sc nanoparticles. In scandium-containing alloys A5 and A6 as well as in A4 alloy the maximum hardening is reached at 300 C, and the hardness gain (in comparison with a as-cast state) makes 31 and 25 HB respectively. 3.5. Discussion It should be noted that, in terms of the absolute value of hardness, the calcium-containing alloy A4 after annealing at 300 С is as good as the B2 reference alloy in the T6 state. The estimation of volume fraction (Qv) of hardening precipitates in the alloys under comparison shows close values: 0.74 vol.% Al3Sc and ~0.8 vol.% b00 (MgSi) respectively [29]. This also explains the close values of hardness of these alloys (after the maximum hardening). However, if the Al3Sc phase is thermally stable up to 300e350 С, for the Mgcontaining phases, heating above 200 С is critical as this
temperature corresponds to the stage of over aging [30e32]. The consequence is that hardness of the alloy A356 at this temperature decreases visibly, and at 250e350 С it becomes unacceptably low (Fig. 12). It is also follows from Fig.11 that introduction of scandium to alloys on the basis of an (Al) þ (Si) eutectic does not lead to the noticeable strengthening effect. It is caused by low concentration of this element in (Al) that is reflected at Fig.6. It is also necessary to take into account results of work [33] in which it is shown that the addition of 0.12%Sc in an Al-7% Si-0.6% Mg alloy does not influence its hardening after T6 heat treatment. Thus, the experimental alloy A4 based on the calciumcontaining eutectic with the addition of scandium has the following advantages compared with commercial alloys of the А356 type: 1) in using a simplified heat treatment (without quenching e heat treatment of T5 type), to obtain hardening that is reached in alloys of the А356 type after the T6 heat treatment; 2) to combine hardening heat treatment with thermal stabilisation (i.e. resistance to heating below the annealing temperature); 3) achieve higher operating temperatures of parts made of the experimental alloy than those made of alloys of the A356 type (300 С against 200 С). It should also be noted that the volume fraction of Al4Ca particles in the alloy of eutectic composition is about 33 vol.%, which is much higher than the amount of silicon particles in AleSi eutectic alloys (about 11 vol.%). This makes it possible to increase considerably the properties that are controlled by the mixture rule (e.g., thermal-expansion coefficient, modulus of elasticity). In this study, by the example of the model composition of Al7.6% Ca-0.3% Sc, the possibility to develop aluminium casting alloys based on the (Al) þ Al4Ca eutectic has been substantiated. Unlike commercial alloys of the A356 type, the model alloy does not require quenching, as hardening particles are formed in the course of annealing of castings.
Fig. 9. Morphology of eutectic (Al) þ Al4Ca in alloy A4 after annealing, OM (a,b) SEM (c): a) S500, b, c) S600.
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Fig. 11. Effect of annealing temperature on hardness of experimental alloys.
Fig. 12. Нardness of experimental alloys after annealing at 300 C.
Fig. 10. Al3Sc precipitates in alloy A4 after annealing S300 (a,b), S350 (c,d), S400 (e,f) and S450 (g,h), TEM (target photography): a, c, e, g) bright field; b, d, f, h) dark field.
4. Conclusions 1. The phase composition and structure of alloys of the AleCaeSc system in the aluminium corner (up to 10% Ca and up to 1% Sс) have been studied. It is shown that only phases of the binary systems (Al4Ca и Al3Sc) may be in equilibrium with the aluminium solid solution. The solubility of scandium in the Al4Ca phase and that of calcium in the Al3Sc phase are negligible. 2. It is shown that the (Al) þ Al4Ca eutectic has a much finer structure than the (Al) þ Si eutectic, which suggests the possibility to reach higher mechanical properties as compared with commercial alloys of the A356 type.
3. The influence of the annealing temperature within the range up to 600 С on the structure and hardness of the AleCaeSc experimental alloy has been studied. It has been determined that the maximum hardening is reached at 300 С, which is caused by precipitation of nanoparticles of the Al3Sc phase (with their further coarsening). 4. By the example of the Al-7.6% Ca-0.3% Sc model experimental alloy, the principal possibility to produce the complex castings (that are currently produced from AleSi alloys) has been substantiated. Moreover, after annealing at 300e350 С, the experimental alloy demonstrates hardening similar to the alloys of the А356 type after the T6 heat treatment. 5. It is shown that introduction of scandium to alloys on the basis of aluminium-silicon eutectic does not lead to the noticeable strengthening effect. It is caused by the formation of Al3Sc crystals during solidification, with subsequent low concentration of Sc in (Al). Acknowledgements The work has been supported by Russian Science Foundation's grant 14-19-00632. References [1] L.S. Toropova, D.G. Eskin, M.L. Kharakterova, T.V. Dobatkina, Advanced Aluminium Alloys Containing Scandium: Structure and Properties, Gordon and Breach Science Publishers, Amsterdam, 1998, p. 175. [2] R. Øyset, N. Ryum, Scandium in aluminium alloys, Int. Mater. Rev. 50 (2005) 19e44. [3] E.A. Marquis, D.N. Seidman, Nanoscale structural evolution of Al3Sc precipitates in Al (Sc) alloys, Acta Mater. 49 (2001) 1909e1919.
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[20] N.A. Belov, D.G. Eskin, A.A. Aksenov, Multicomponent Phase Diagrams: Applications for Commercial Aluminium Alloys, Elsevier, 2005, p. 414. [21] I.I. Novikov, Goryachelomkost’ Tsvetnyh Metallov I Splavov (Hot Tearing of Non-ferrous Metals and Alloys), Nauka, Moscow, 1966, p. 299 (in Russian). [22] Information on http://www.thermocalc.com. [23] C.B. Fuller, D.N. Seidman, Temporal evolution of the nanostructure of Al(Sc,Zr) alloys: part II-coarsening of Al3(Sc1xZrx) precipitates, Acta Mater. 53 (2005) 5415e5428. [24] W. Lefebvre, F. Danoix, H. Hallem, B. Forbord, A. Bostel, K. Marthinsen, Precipitation kinetic of Al3(Sc,Zr) dispersoids in aluminium, J. Alloys Compd. 470 (2009) 107e110. [25] N.A. Belov, A.N. Alabin, D.G. Eskin, V.V. Istomin-Kastrovskiy, Optimization of Hardening of AleZreSc Casting Alloys, J. Mater. Sci. 41 (2006) 5890e5899. [26] C. Booth-Morrison, D.N. Seidman, D.C. Dunand, Effect of Er additions on ambient and high-temperature strength of precipitation-strengthened AleZreSceSi alloys, Acta Mater. 60 (2012) 3643e3654. [27] C. Booth-Morrison, D.C. Dunand, D.N. Seidman, Coarsening resistance at 400 C of precipitation-strengthened AleZreSceEr alloys, Acta Mater. 59 (2011) 7029e7042. [28] M.E. van Dalen, T. Gyger, D.C. Dunand, D.N. Seidman, Effects of Yb and Zr microalloying additions on the microstructure and mechanical properties of dilute AleSc alloys, Acta Mater. 59 (2011) 7615e7626. [29] N.A. Belov, Fazoviy Sostav Promyshlennykh I Perspektivnykh Aluminievykh Splavov (Phase composition of commercial and advanced aluminium alloys), Izdatel’skiy dom MISIS, Moscow, 2010, p. 511 (in Russian). [30] Bao Li, Hongwei Wang, Jinchuan Jie, Zunjie Wei, Effects of yttrium and heat treatment on the microstructure and tensile properties of Al-7.5Si-0.5Mg alloy, Mater. Des. 32 (2011) 1617e1622. [31] M. Abdulwahab, I.A. Madugu, S.A. Yaro, S.B. Hassan, A.P.I. Popoola, Effects of multiple-step thermal ageing treatment on the hardness characteristics of A356.0-type AleSieMg alloy, Mater. Des. 32 (2011) 1159e1166. [32] Hengcheng Liao, Yuna Wu, Ke Ding, Hardening response and precipitation behavior of Al-7%Si-0.3%Mg alloy in a pre-aging process, Mater. Sci. Eng. A560 (2013) 811e816. [33] Yu-Chih Tzeng, Chih-Ting Wu, Hui-Yun Bor, Jain-Long Horng, Mu-Lin Tsai, Sheng-Long Lee, Effects of scandium addition on iron-bearing phases and tensile properties of Ale7Sie0.6Mg alloys, Mater. Sci. Eng. A593 (2014) 103e110.
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Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom
Effect of scandium on structure and hardening of AleCa eutectic alloys N.A. Belov a, *, E.A. Naumova b, A.N. Alabin a, c, I.A. Matveeva c a
National Research and Technological University “MISIS”, 4, Leninsky pr., Moscow 119049 Russia Bauman Moscow State Technical University, 5, 2 ul. Baumanskaya, Moscow, 105005 Russia c UC RUSAL, 13/1, Nikoloyamskaya st., Moscow, 109240, Russia b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 21 April 2015 Received in revised form 14 May 2015 Accepted 17 May 2015 Available online 19 June 2015
The phase composition, structure and hardening of alloys in the aluminium corner of the AleCaeSc system were studied in the range up to 10% Ca and up to 1% Sс. The experimental study (optical, scanning and transmission electron microscopy with electron-microprobe analysis, differential thermal analysis and hardness measurements) was combined with Thermo-Calc software simulation for the optimization of the alloy composition. It was shown that only phases of the binary systems (Al4Ca и Al3Sc) might be in equilibrium with the aluminium solid solution. It was shown that the (Al) þ Al4Ca eutectic had a much finer structure as compared with the AleSi eutectic, which suggests a possibility of reaching higher mechanical properties as compared to commercial alloys of the A356 type. The influence of the annealing temperature within the range up to 600 С on the structure and hardness of the AleCaeSc experimental alloys was studied. It was determined that the maximum hardening corresponded to the annealing at 300 С, which was due to the precipitation of Al3Sc nanoparticles with their further coarsening. With an example of an Al-7.6% Ca-0.3% Sc model experimental alloy, a principal possibility of manufacturing aluminium casting alloys based on the (Al) þ Al4Ca eutectic was demonstrated. Unlike commercial alloys of the A356 type, the model alloy does not require quenching, as hardening particles are formed in the course of annealing of casting. © 2015 Elsevier B.V. All rights reserved.
Keywords: AleCaeSc system AleCa eutectic Al3Sc nano-particles Phase composition Microstructure Heat treatment Hardening
1. Introduction Scandium is one of the most efficient hardening additives in aluminium alloys [1,2], due to the formation of coherent Al3Sc (L12) phase precipitates with the size less than 10 nm [3e5]. These nanoparticles form in the process of annealing from supersaturated aluminium solid solution (hereinafter referred to as (Al)). Such a specific feature of these alloys enables a significant increase in strength of aluminium alloys not exposed to the classic T6 hardening heat treatment (solution treatment þ quenching þ ageing). Among these alloys, wrought alloys based on the AleMg system [6,7] are used most widely. Despite its high cost, at the moment scandium is considered to be one of the most promising alloying elements in new-generation aluminium alloys [2,8]. As for commercial casting alloys, the addition of scandium does not have the same effect as in wrought ones. First of all, this concerns alloys based on the AleSi system that constitute the majority of the total production of castings of aluminium alloys [9,10]. The
* Corresponding author. E-mail address: [email protected] (N.A. Belov). http://dx.doi.org/10.1016/j.jallcom.2015.05.155 0925-8388/© 2015 Elsevier B.V. All rights reserved.
reason for this is that silicon reduces significantly solubility of scandium in (Al) and, as a consequence, makes it impossible to form a sufficient amount of hardening nanoparticles of the Al3Sc phase in annealing. In addition, Si forms a ternary phase with Al and Sc [1], further reducing the amount of free Sc. It should also be noted that castings of widely used alloys such as А356/357 containing 0.2e0.5%Mg are exposed to heat treatment as per T6 mode in order to achieve the maximum hardening [11]. The possibility to avoid this would be appropriate under industrial conditions, in particular, for large-sized castings. Assuming that casting alloys should contain a sufficient amount of eutectic [9], it seems reasonable to search for other eutecticforming elements that would not reduce the hardening effect due to addition of scandium. We believe that one of the most promising elements among them is calcium that, similar to silicon, forms with aluminium a diagram of eutectic type. According to the data [12], in the AleCa system the L/(Al) þ Al4Ca eutectic reaction takes place at 7.6%Ca and 617 C, which is rather close to the calculated values obtained in the Thermo-Calc software application (Fig. 1a). In terms of abundance in nature, calcium holds the 3rd place among all the metals (about 3.4 wt%), behind aluminium and iron [9], and its
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N.A. Belov et al. / Journal of Alloys and Compounds 646 (2015) 741e747 Table 2 Annealing regimes of experimental alloys.
Fig. 1. Calculated phase diagrams AleCa (a) and AleCaeSc (b):b)experimental alloys are marked, liquidus lines: bold ecalculated (equilibrium), dottede shift after unequilibrium solidification (experimental).
3
density is lower than that of silicon (1.542 versus 2.328 g/sm ). In recent years, many publications considering calcium-containing magnesium alloys have appeared [13e15]. At the same time, the use of calcium for alloying of aluminium alloys is very limited [16e18]. As a rule, this element is considered to be a harmful impurity [10e12]. Limited data regarding the AleCaeSc phase diagram can be found [12,19,20]. Based on the above and taking into consideration the scarcity of information regarding aluminium alloys with addition of calcium, the following main objectives of this work were formulated: 1) to study the phase composition and structure of alloys of the AleCaeSc system (in the area close to the (Al) þ Al4Ca eutectic) in as-cast and heat-treated states; 2) to compare the hardening effect in AleCaeSc alloys, reached without quenching, with hardening in commercial AleSieMg alloys of the А356 type after the Т6 heat treatment. The purpose of these studies is to demonstrate the possibility of using the (Al) þ Al4Ca eutectic alloys with addition of scandium, hardened without quenching, in manufacturing castings of complex form.
For the experimental study, 6 alloys containing Ca in the amount of 4e10 wt% (3 binary and 3 ternary with addition of scandium) and 3 reference alloys were prepared (Table 1). The smelting was conducted in an electric resistance furnace in clay-graphite crucibles. All alloys (with the exception of B2 and B3) were prepared based on high purity aluminium (99.99%). Calcium was added in pure metallic form and scandium was added as an Ale2% Sc master alloy. Commercial alloys of the A356 and A413 types were used to prepare the reference alloys B2 and B3. The melt was poured into a graphite mould at 740e750 С to produce flat castings Table 1 Chemical composition of experimental alloys.
A1 A2 A3 A4 A5 A6 B1 B2 B3
Annealing regime
S200 S250 S300 S350 S400 S450 S500 S550 S600
200 C, 3 h S200 þ 250 S250 þ 300 S300 þ 350 S350 þ 400 S400 þ 450 S450 þ 500 S500 þ 550 S550 þ 600
C, C, C, C, C, C, C, C,
3 3 3 3 3 3 3 3
h h h h h h h h
15х30х180 mm in size (the cooling rate during solidification was about 10 К/s). The chemical composition of alloys obtained by spectral analysis is given in Table 1. The hot tearing susceptibility was assessed using a ‘harp’ probe [21]. The heat treatment of the castings was carried out in a muffle electric furnace using step cycles in the range from 200 to 600 C (Table 2) with an accuracy of temperature maintenance ±2 C. The Brunell hardness was determined in a DuraVision-20/200/250 hardness testing machine with the following parameters: ball diameter e 2.5 mm, load e 612.9 N, dwell time e 30 s. The structure was examined by means of optical (OM, Axiovert 200 MMAT), transmission electron (TEM, JEM-2100), scanning electron (SEM, TESCAN VEGA 3) microscopes and by electron microprobe analysis (EMPA, OXFORD AZtec). The samples were cut from the castings, ground and polished using standard procedures. Mechanical polishing (Struers Labopol-5) with further etching using Keller's reagent was applied. Thin foils for transmission electron microscopy (TEM) were prepared by ion thinning with a PIPS (Precision Ion Polishing System, Gatan) machine and studied at 160 kV. For calculation of the phase composition, the Thermo-Calc software (TTAL5 database) was used [22].
3. Results and their discussion
2. Experimental methods
Alloy
Designation
Concentration, wt% Ca
Sc
Fe
Si
Mg
Al
4.21 7.48 10.21 7.53 6.85 10.23 <0.01 <0.01 <0.01
<0.01 <0.01 <0.01 0.28 0.96 0.31 0.33 <0.01 0.29
<0.03 0.04 0.05 0.06 0.04 0.03 0.02 0.12 0.29
0.01 0.02 0.01 0.03 0.02 0.01 <0.01 7.21 11.34
<0.01 <0.01 <0.01 <0.01 <0.01 <0.01 <0.01 0.31 0.09
Balance Balance Balance Balance Balance Balance Balance Balance Balance
3.1. AleCaeSc phase diagram The compositions of the experimental alloys (Table 1) were chosen according to their positions in the AleCa (Fig. 1a) and AleCaeSc (Fig. 1b) phase diagrams. The binary alloys had hypoeutectic (A1), eutectic (A2) and hypereutectic (A3) calcium concentrations. According to the calculation of the ternary phase diagram, calcium reduces solubility of scandium in (Al) down to 0.2%. However, under conditions of non-equilibrium solidification, there is a shift of phase boundaries (see the dotted line in Fig. 1b), therefore the 0.3 wt% Sc concentration was chosen for the A4 basic experimental alloy. The further microstructure analysis (including EMPA) confirmed that scandium was completely dissolved into (Al). The compositions of the A5 and A6 alloys were chosen so that the primary intermetallic compounds Al3Sc and Al4Ca, respectively, could be produced. The reference alloys were chosen based on the following considerations. The B1 alloy, containing only the scandium additive and studied thoroughly [1,2], served as a standard of precipitation hardening caused by the formation of Al3Sc nanoparticles. The composition of the B2 alloy corresponds to the average composition of commercial alloys of the A356 type hardened as per T6 mode [11]. And, finally, the third reference alloy B3 was prepared (based on a commercial alloy of the A413 type) in order to demonstrate the deficiency of adding scandium into AleSi eutectic alloys.
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Fig. 2. Microstructures of as-cast alloys A1 (a), A2 (b), A3 (c) and B3 (d), a, c) SEM, b, d) OM a) hypoeutectic; b, d) eutectic; c) hypereutectic.
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Fig. 4. Primary crystals of phase Al3Sc in alloy A5, SEM/EMPA: a) SEM; b) Al; c) Ca; d) Sc.
Fig. 3. Morphology of eutectic (Al) þ Al4Ca in as-cast alloy A4, SEM: a) 1,300, b) 13,300.
3.2. Microstructure and hot tearing As expected, the castings of the А1eА3 binary alloys have hypoeutectic (Fig. 2a), eutectic (Fig. 2b) and hypereutectic (Fig. 2c) structures, respectively. The calcium-containing eutectic has a finer morphology than the aluminium-silicon one (Fig. 2d). Moreover, the optical microscopy cannot resolve the internal structure of the former eutectic as the eutectic dendrites have submicron sizes. As Fig. 3 shows, the average thickness of eutectic dendritic branches in the A4 experimental alloy is about 0.5 mm. The scandium content of the (Al) þ Al4Ca eutectic colonies corresponds to its content of the alloy A4 (Table 1), and no primary crystals of the Al3Sc phase are detected. Such crystals are found in the A5 alloy in the form of polyhedrons (Fig. 4), which is also typical for the binary AleSc alloys [1,2]. According to the EMPA data, there is no calcium in these crystals. At the same time, in the A6 alloy there are large crystals of the Al4Ca phase that hardly contain scandium (Fig. 5). Based on the obtained data, it may be concluded that in the aluminium corner of AleCaeSc system there are no ternary compounds, which concur with the calculation (Fig. 1b). The structure of the B3 alloy reveals Sc-containing crystals 5e10 mm in size (Fig.6), in addition to the particles of the siliconand iron-containing phases. Thus concentration of scandium in (Al) is very small. According to EMPA, besides Sc and Al, these particles
Fig. 5. Primary crystals of phase Al4Ca in alloy A6, SEM/EMPA. a) SEM; b) Al; c) Ca; d) Sc.
contain Si also. So this ternary phase can be identified as compound AlSc2Si2 (V) [1]. The eutectic composition of the A4 experimental alloy suggests good casting properties as it has a very narrow solidification range (DT) [18,29]. According to the calculation using the ScheileGulliver model, the scandium additive should not have a significant influence on the nature of solidification as this element affects only slightly the value of DT. The differential thermal analysis confirms the results of the calculation (Fig. 7). The assessment of hot tearing of the A4 alloy using a ‘harp’ sample showed a total absence of cracks (Fig. 8a). Among commercial heat treatable aluminium casting alloys, only alloys of 3xx series show the same indicator for such a sample (our own results). For comparison, in Fig. 8b similar
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500 С, almost all Al4Ca particles take a globular form, but their sizes remain submicron (Fig. 9a). At the maximum tested temperature of annealing (600 С), the structure becomes much coarser (Fig. 9b), the size of some particles reaches 5 mm (Fig. 9с). The second essential structural change in annealing is the decomposition of (Al) in Sc-containing alloys (A4-A6). As it appears from numerous works regarding AleSc alloys [3e5], the complete decomposition occurs at the temperature of about 300 С. Additives of other element, particularly Zr [23e25], Er [26,27], Yb [28], can rise the peak temperature up to 400e450 C. So these elements may be considered for AleCaeSc alloys also. 3.3. TEM investigations
Fig. 6. Primary crystals of ternary phase (Al,Sc,Si) in alloy B3, SEM/EMPA: a) SEM, b) Al; c) Si; d) Fe, e, f) Sc (f, enlarged image).
Fig. 7. DTA curve of alloy A4.
casting of a heat treatable alloy A224 (4.80% Cu, 0.45%Mn, 0.08%Fe, 0.05%Si) shows cracks. This experimental result gives confidence in producing mould castings of a complex geometry using (Al) þ Al4Ca eutectic alloys. It is known that the fineness of the eutectic structure in the ascast state determines to a great extent its capability to change its form during heat treatment [10,20]. The study on the influence of an annealing temperature (with 3-h holding) on the morphology of the eutectic constituents detects the first symptoms of fragmentation at 450 С. With the increase in the temperature, the changes become detectible at the resolution of optical microscopy. At
The principal difference between the as-cast structures of the experimental A4 alloy and the reference B1 alloy is that the size of the dendritic branches of the aluminum solid solution in the (Al) þ Al4Ca eutectic of the A4 alloy is less than 1 mm (Fig.3b). Therefore, the aluminum solid solution dissolution and the formation of Al3Sc precipitates occur near the (Al)/Al4Ca interfaces. To study these processes, the method of target photography was used. First, the foil of the A4 alloy was heated in the chamber of the microscope to 300 C, then to 500 C. Similar to the previous experiments (see in Table 2), stage heating by 50 C with the exposure at each stage for 3 h was used. Photographs were obtained for each stage in both bright and dark fields. Fig.10 shows, that in the all studied states extra diffraction spots are identified in all the studied states. These spots of the precipitated phase L12 in the selected area diffraction pattern enable their identification by dark-field imaging using these superlattice spots [3,4], which are at the positions of prohibited reflections of the face-centered cubic aluminium matrix. In the state S300, which corresponds to the maximum hardening (Fig.11), the size of L12 precipitates inside (Al) dendrites is minimal (<5 nm). Interestingly, the Sc-containing precipitates at the interface with the Al4Ca eutectic particles are much larger (Fig. 10a, b). When the temperature is increasing up to 350 C, the size of the precipitates inside the (Al) dendrites is increasing. However, they are smaller than those located near the Al4Ca particle. Heating at 400 C leads to further enlargement of L12 precipitates, with the difference with interphase precipitates being almost eliminated (Fig. 10e,f). Coherent precipitates L12 can be visualized in bright field through a strain contrast caused by elastic distortions of the adjacent matrix. This produces “coffee bean” contrasts in two-beam imaging conditions as shown in many previous TEM investigations [3,4]. This contrast is detected particularly clearly after heating at 450 C (Fig.10g), when the size of the precipitates reaches 50 nm (Fig.10h) Apparently from Fig. 10, density of these particles is rather high as the fraction of (Al) in the eutectic makes about 70 wt. %. We can elucidate that the concentration of scandium in the eutectic branches of (Al) makes not 0.3%, but about 0.43 wt%. As a consequence, the number of Al3Sc particles increases in comparison with reference alloy B1. With the increase in the temperature, the particle coarsen considerably which makes it possible to detect them using scanning electron microscopy [25]. The first signs of Al3Sc particles in the experimental alloy can be seen after annealing at 550 С; and after annealing at 600 С they are seen quite distinctly (Fig.9c). 3.4. Hardness measurements The change in hardness depending on the annealing temperature is a consequence of the two above mentioned processes. The initial state of all the experimental and reference alloys (except for B2) was as-cast. The B2 alloy corresponding to an А356 commercial
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Fig. 8. Castings ‘harp’ (hot tearing test) of alloys A4 (a) and 224 (b).
alloy was studied after heat treatment to the maximum hardening e T6 (heating at 540 С, 6 h, quenching in cold water, ageing at 175 С, 6 h). It is evident from Fig. 11 that the heating of the binary Ca-containing alloy A2 up to 400 С does not have an effect on hardness, which can be explained from the absence of any structural changes. At higher temperatures, softening takes place, which can be explained by the change in the Al4Ca eutectic morphology (Fig. 9). Dependences HBeT for binary alloys A1 and A3 are similar (not shown in Fig.11). In alloys A4 and B1 the visible hardening appears at 250 С (Fig. 11), which is likely to be connected with precipitation of Al3Sc nanoparticles [4,5]. At 300 С, their hardening effect reaches the maximum level, which can be connected with complete precipitation of scandium from (Al) with retention of sizes at the level of nanoparticles (Fig. 10a,b). Softening of the AleCaeSc eutectic alloy A4 at higher temperatures of annealing is caused, firstly, by coarsening of Al3Sc second-phase precipitates (beginning from 350 С, see in Fig. 10ceh), and, secondly, by the morphology change of Al4Ca eutectic particles (Fig. 8a). It should be noted that the hardness increment in the А4 and B1 alloys, containing 0.3% Sc, is very similar (Fig.11), which enables us to make a conclusion about similar features of Al3Sc nanoparticles. In scandium-containing alloys A5 and A6 as well as in A4 alloy the maximum hardening is reached at 300 C, and the hardness gain (in comparison with a as-cast state) makes 31 and 25 HB respectively. 3.5. Discussion It should be noted that, in terms of the absolute value of hardness, the calcium-containing alloy A4 after annealing at 300 С is as good as the B2 reference alloy in the T6 state. The estimation of volume fraction (Qv) of hardening precipitates in the alloys under comparison shows close values: 0.74 vol.% Al3Sc and ~0.8 vol.% b00 (MgSi) respectively [29]. This also explains the close values of hardness of these alloys (after the maximum hardening). However, if the Al3Sc phase is thermally stable up to 300e350 С, for the Mgcontaining phases, heating above 200 С is critical as this
temperature corresponds to the stage of over aging [30e32]. The consequence is that hardness of the alloy A356 at this temperature decreases visibly, and at 250e350 С it becomes unacceptably low (Fig. 12). It is also follows from Fig.11 that introduction of scandium to alloys on the basis of an (Al) þ (Si) eutectic does not lead to the noticeable strengthening effect. It is caused by low concentration of this element in (Al) that is reflected at Fig.6. It is also necessary to take into account results of work [33] in which it is shown that the addition of 0.12%Sc in an Al-7% Si-0.6% Mg alloy does not influence its hardening after T6 heat treatment. Thus, the experimental alloy A4 based on the calciumcontaining eutectic with the addition of scandium has the following advantages compared with commercial alloys of the А356 type: 1) in using a simplified heat treatment (without quenching e heat treatment of T5 type), to obtain hardening that is reached in alloys of the А356 type after the T6 heat treatment; 2) to combine hardening heat treatment with thermal stabilisation (i.e. resistance to heating below the annealing temperature); 3) achieve higher operating temperatures of parts made of the experimental alloy than those made of alloys of the A356 type (300 С against 200 С). It should also be noted that the volume fraction of Al4Ca particles in the alloy of eutectic composition is about 33 vol.%, which is much higher than the amount of silicon particles in AleSi eutectic alloys (about 11 vol.%). This makes it possible to increase considerably the properties that are controlled by the mixture rule (e.g., thermal-expansion coefficient, modulus of elasticity). In this study, by the example of the model composition of Al7.6% Ca-0.3% Sc, the possibility to develop aluminium casting alloys based on the (Al) þ Al4Ca eutectic has been substantiated. Unlike commercial alloys of the A356 type, the model alloy does not require quenching, as hardening particles are formed in the course of annealing of castings.
Fig. 9. Morphology of eutectic (Al) þ Al4Ca in alloy A4 after annealing, OM (a,b) SEM (c): a) S500, b, c) S600.
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Fig. 11. Effect of annealing temperature on hardness of experimental alloys.
Fig. 12. Нardness of experimental alloys after annealing at 300 C.
Fig. 10. Al3Sc precipitates in alloy A4 after annealing S300 (a,b), S350 (c,d), S400 (e,f) and S450 (g,h), TEM (target photography): a, c, e, g) bright field; b, d, f, h) dark field.
4. Conclusions 1. The phase composition and structure of alloys of the AleCaeSc system in the aluminium corner (up to 10% Ca and up to 1% Sс) have been studied. It is shown that only phases of the binary systems (Al4Ca и Al3Sc) may be in equilibrium with the aluminium solid solution. The solubility of scandium in the Al4Ca phase and that of calcium in the Al3Sc phase are negligible. 2. It is shown that the (Al) þ Al4Ca eutectic has a much finer structure than the (Al) þ Si eutectic, which suggests the possibility to reach higher mechanical properties as compared with commercial alloys of the A356 type.
3. The influence of the annealing temperature within the range up to 600 С on the structure and hardness of the AleCaeSc experimental alloy has been studied. It has been determined that the maximum hardening is reached at 300 С, which is caused by precipitation of nanoparticles of the Al3Sc phase (with their further coarsening). 4. By the example of the Al-7.6% Ca-0.3% Sc model experimental alloy, the principal possibility to produce the complex castings (that are currently produced from AleSi alloys) has been substantiated. Moreover, after annealing at 300e350 С, the experimental alloy demonstrates hardening similar to the alloys of the А356 type after the T6 heat treatment. 5. It is shown that introduction of scandium to alloys on the basis of aluminium-silicon eutectic does not lead to the noticeable strengthening effect. It is caused by the formation of Al3Sc crystals during solidification, with subsequent low concentration of Sc in (Al). Acknowledgements The work has been supported by Russian Science Foundation's grant 14-19-00632. References [1] L.S. Toropova, D.G. Eskin, M.L. Kharakterova, T.V. Dobatkina, Advanced Aluminium Alloys Containing Scandium: Structure and Properties, Gordon and Breach Science Publishers, Amsterdam, 1998, p. 175. [2] R. Øyset, N. Ryum, Scandium in aluminium alloys, Int. Mater. Rev. 50 (2005) 19e44. [3] E.A. Marquis, D.N. Seidman, Nanoscale structural evolution of Al3Sc precipitates in Al (Sc) alloys, Acta Mater. 49 (2001) 1909e1919.
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