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Vol. 8. Issue 5.
Pages 4130-4140 (September - October 2019)
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Vol. 8. Issue 5.
Pages 4130-4140 (September - October 2019)
Original Article
DOI: 10.1016/j.jmrt.2019.07.022
Open Access
Effect of Zr and Sc on microstructure and properties of 7136 aluminum alloy
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Shaokun Tiana, Jingyuan Lia,
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lijy@ustb.edu.cn

Corresponding author.
, Junlong Zhanga, Zhumabieke Wulabiekea, Dan Lvb
a School of Materials Science and Engineering, University of Science and Technology Beijing, Beijing 100083, China
b North-east Light Alloy Co. Ltd., Harbin, 150060, China
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Tables (3)
Table 1. Chemical composition of the two alloys (mass fraction/%).
Table 2. Valence electron structure for Ll2–Al3Zr and DO23–Al3Zr.
Table 3. Mechanical properties of alloys under solution and aging treatment.
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Abstract

We have linked experimental results to theoretical calculations and discussed the precipitation behavior of Zr and Sc in 7136 alloy and the influence of precipitation on the microstructure and mechanical properties of the alloy. The experimental results show that the added Sc atom can replace the Zr atom in the cubic Al3Zr phase with a lattice constant of a = 0.4417 nm, and form Al3(ScxZr1-x), which is also cubic and has a lattice constant of a = 0.4212 nm. A heterogeneous nucleation core promotes crystal-grain refinement, and the average size of the as-cast crystal grains is reduced from 82.4 μm to 51.7 μm. The valence electron structure and strongest bond energy of the Al3Zr phase with different structures was analyzed by the empirical electron theory. For Ll2-Al3Zr, the covalent electron number and bond energy of the strongest bonds are 0.3226 and 52.8 kJ/mol, respectively, and for DO23-Al3Zr, the numerical values are 0.4471 and 71.0 kJ/mol, respectively. Combined with the ab initio calculation, Ll2-Al3Zr has a good stability. Under suitable conditions, Ll2-Al3Zr will transform into the more stable DO23-Al3Zr. Quantitative analysis of the effect of strengthening mechanism such as fine grain strengthening and dispersion strengthening caused by the addition of Sc element on the yield strength of the alloy. The calculation results show that 0.2% elemental Sc addition can increase the alloy strength by 36.63 MPa, which is very similar to the measured result of 38.61 MPa.

Keywords:
7136 Aluminum alloy
Scandium
Valence electronic structure
Microstructure
Mechanical property
Full Text
1Introduction

The aluminum content in the earth's crust is ˜8%, which is the highest content of metal elements in the earth's crust. Aluminum and its alloys are used widely because of their wide distribution and excellent properties. The main areas of focus of research into aluminum alloys are as follows. One area of focus is the high-temperature performance of aluminum alloys. Su et al. [1] investigated the behavior of aluminum-alloy (6061-T6 and 6063-T5) beams at elevated temperatures by using finite-element analyses, and reliability analysis has also been conducted to evaluate the reliability level of the aforementioned design methods for aluminum-alloy beams at elevated temperatures. Bian et al. [2] conducted an in-depth study on the thermal stability of Al–Fe–Ni eutectic alloys that were produced by a gravity cast. The second area of focus was an aluminum matrix composite. Krishnan et al. [3] studied the feasibility of using scrap aluminum alloy car wheels as the matrix material and spent alumina catalyst from oil refineries as reinforcement material and obtained a combination of composites – scrap aluminum alloy + alumina. Bach et al. [4] developed a new aluminum alloy using Mg and AlN composite and hot extrusion processing, and the corrosion resistance of new materials was studied in depth. Imran et al. [5] reported the mechanical properties, tribological properties and corrosion behavior of Al-7075 metal matrix composites with particulate SiC, Al2O3, Gr, TiO2, bagasse ash and other reinforcements. Another popular area of focus is a new type of aluminum alloy, such as the Al–Li alloy. Rino et al. [6] studied the thermal stability of an ultrafine-grained AA8090 Al–Li alloy with an average grain size of 2 μm in detail. In addition, the effect of rare earth metals on the properties of the aluminum alloy have been studied. Souza et al. [7] explored the effect of solute content (hipoperitectic Al–0.22 wt.%Zr and hiperperitectic Al–0.32 wt.%Zr) on the precipitation hardening and microstructural evolution of dilute Al–Zr alloys that had been aged isothermally. The thermal stability, microstructure and properties of Al–Mg–Sc alloys after severe plastic deformation (such as repetitive corrugation and straightening and high-pressure torsion) was analyzed by Pereira [8] and Bhovi et al. [9] in depth. The effects of rare-earth elements on the microstructure and properties of Al–Zn–Mg–Cu ultra-high-strength aluminum alloys were investigated.

The Al–Zn–Mg–Cu alloy has been developed rapidly in the fields of automobiles, military, energy and power, construction and aerospace by virtue of its excellent processing performance, high fracture toughness, good corrosion resistance and fatigue performance and excellent electrical and thermal conductivity [10–13]. The 7136 aluminum alloy was developed by the Universal Alloy corporation in 2004 on the basis of 7055 aluminum alloy. It is characterized by a significant increase in the content of elemental Zn to improve the Zn/Mg ratio of the alloy, and optimize the proportion of other alloy elements to improve the alloy microstructure [14,15]. Because of its excellent comprehensive performance, 7136 alloy extrusion plates have been used in some aircraft [16,17].

Elemental Sc is in twenty-first place in the periodic table, belongs to the 3d transitional elements, and is also a kind of rare-earth element. When elemental Sc is added to aluminum alloy, Al3Sc second-phase particle can be generated in the solidification process, which can improve the performance of the alloy by refining grains, precipitation strengthening and inhibiting deformed-metal recrystallization. However, extensive elemental Sc is needed to form this phase, and elemental Sc is expensive, which limits its application. The addition of trace Zr into the alloy can form an Al3Zr phase, which can refine the grain, have a strong nailing effect, hinder the slip and climbing of the dislocation, inhibit grain-boundary movement, increase the recrystallization temperature and improve the comprehensive performance of the alloy [18–21]. Although some studies have shown that the combination of elemental Sc and Zr can form the Al3(Sc,Zr) phase, which can enhance the alloy properties significantly [22–26], many controversies exist related to the Al3Zr and Al3(Sc, Zr) phases. Some studies have shown [27,28] that the Al3Zr phase is a tetragonal phase, which does not refine the grain of aluminum alloy effectively. In the aluminum alloy, Sc and Zr addition can form an Al3Sc phase preferentially that is similar to the α-Al lattice constant. The Zr atom replaces part of the Sc atoms in the Al3Sc particles by diffusion to form Al3(Sc, Zr) with a core–shell structure. The Al3(Sc, Zr) phase with a core–shell structure can strengthen the alloy and reduce the tendency of the Al3Sc particles to coarsen. It has been shown [29] that the Al3(Sc,Zr) particles that are formed by the combined addition of Sc and Zr in the aluminum alloy are multi-layer composite structures. In the 7136 alloy, it is not known what structure of the second phase particles formed by adding Zr and Sc.7136 aluminum alloy itself contains much Zr elements. It is not known whether the microstructure of 7136 aluminum alloy can be optimized and the alloy performance improved by the addition of a small amount of elemental Sc.

We studied the effect of a small amount of Sc on the microstructure and mechanical properties of 7136 aluminum alloy by adding 0.2% elemental Sc to 7136 aluminum alloy. The effects of fine grain strengthening, precipitation strengthening and solid–solution strengthening on the alloy strength were analyzed quantitatively.

2Experimental

High-purity aluminum (99.99%); high-purity zinc (99.99%); high-purity magnesium (99.99%) and Al–50.80% Cu, Al–5.11% Zr, Al–10.13% Mn, Al–10.02% Ti and Al–2.02% Sc were used as raw materials. The alloy was smelted in a ZGJL 0.01-50-4 K vacuum induction melting furnace and cast into a 100-mm-diameter × 210-mm-long ingots in a graphite mold. The composition of the two sets of prepared alloys is shown in Table 1.

Table 1.

Chemical composition of the two alloys (mass fraction/%).

Sample  Zn  Mg  Cu  Zr  Mn  Ti  Sc  Al 
9.4  2.16  2.09  0.15  0.03  0.07  –  Bal 
9.4  2.19  2.08  0.15  0.03  0.11  0.2  Bal 

The alloy was processed according to the following procedure:as-cast alloy → homogenization treatment (460 °C/24 h + 465 °C/24 h) → hot extrusion (extrusion ratio: 12.96) → solution treatment (475 °C/2 h) → aging treatment (120 °C/24 h). The alloy microstructure was observed by optical microscopy (OM), transmission electron microscope (TEM) and electron backscattered diffraction (EBSD). The alloy hardness after solution treatment and aging treatment was tested. Each sample was measured 10 times, and the average value was calculated after removing the maximum and minimum values. The heat-treated sample was subjected to a tensile test to obtain the yield strength, tensile strength and elongation.

3Results3.1Effect of Sc on 7136 aluminum alloy microstructure

Fig. 1 shows the 7136 aluminum alloy microstructure in the casting, solution and aging state without and with 0.2% elemental Sc addition. Fig. 2 shows the average grain size of the two alloys in different states as calculated by ImageJ software. Fig. 1(a) shows that the as-cast grains of the 7136 aluminum alloy that contained no Sc are almost equiaxed, and the grain size was ˜40–120 μm. Statistics indicate that the average grain size is ˜82.4 μm. After magnifying the grain-boundary position, it can be seen that many continuous second phases exist at the grain boundary. An analysis of the phase diagram of the Al-rich phase of the Al–Zn–Mg alloy and a consideration of the Cu addition shows that the grain boundary is composed mainly of a T(Al2Zn3Mg3) phase, a small amount of S(Al2CuMg) phase, a small amount of malleable (Al2Cu) phase and an Al6(FeCu) impurity phase. These non-equilibrium eutectic structures have a width of up to 10 μm, which is detrimental to the alloy forming properties and therefore requires homogenization to eliminate the structure. Near the grain boundary, many short rod- and needle-like precipitate phases are visible, which is an η (MgZn2) phase according to the morphology and literature [30]. The as-cast microstructure of the 7136 aluminum alloy with 0.2% elemental Sc is shown in Fig. 1(b). When 0.2% elemental Sc is added, the alloy grain size in the as-cast state decreases from 40 to 120 μm to ˜30 to 80 μm, and a significant refinement occurs. According to statistics, after the addition of 0.2% elemental Sc, the average size of the as-cast crystal grains is ˜51.7 μm, and the crystal grains appear to be equiaxed in a uniform size. The enlargement shows that only a small amount of non-equilibrium eutectic phase exists at the grain boundary. In summary, the 7136 aluminum alloy grain without Sc is relatively coarse, and many non-equilibrium eutectic phases exist. After 0.2% elemental Sc addition, the as-cast grains are refined and the non-equilibrium eutectic phase is reduced significantly.

Fig. 1.

Microstructure of two alloys in different states (OM + EBSD). a, c, e: 7136 alloy; b, d, f: 7136 + 0.2% Sc; a, b: as-cast; c, d: solid–solution state; e, f: aging state.

(1.64MB).
Fig. 2.

Average grain size of two alloys in different states.

(0.31MB).

The alloy microstructure after solution and aging treatment is shown in Fig. 1(c)–(f). After 0.2% elemental Sc addition, the alloy grain size after solution treatment was reduced from 6 µm to 60 µm to ˜2 µm to 40 µm. The average grain size was reduced from 28.2 μm to 15.2 μm as calculated by ImageJ software. After aging treatment, the alloy grain size was reduced from 2 µm to 30 µm to ˜1 µm to 15 µm. The average grain size decreased from 8.6 μm to 2.9 μm as calculated by ImageJ software. The grains that contained the Sc alloy in the solid–solution state and the aging state were finer than those without elemental Sc addition. This behavior is analyzed in detail in Section 3.1.

Fig. 3 shows the TEM morphology of two particles of Al3Zr and Al3(ScxZr1-x). The Al3Zr particles are spherical and ˜25 nm in diameter. The Al3(ScxZr1-x) particles are "bean-like" and ˜20 nm to 60 nm. Fig. 4 is an FFT(Fast Fourier Transformation) diagram of selected regions of Fig. 3 (i.e., Al3Zr and Al3(ScxZr1-x) particles), (a) is along the [1(—)11] ribbon axis, and (b) is along the [112] ribbon axis.

Fig. 3.

TEM images of Al3Zr and Al3(ScxZr1-x).

(0.39MB).
Fig. 4.

FFT of Al3Zr and Al3(ScxZr1-x).

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3.2Valence electron structure analysis of Al3Zr with different crystal structures

Some controversy exists regarding the structure of the second phase that is formed by Zr and Sc in the aluminum alloy. The controversy regarding the Al3Zr particle structure is related mainly to the following two cases, cubic Ll2 and tetragnonal DO23, as shown in Fig. 5[31]. The valence electron structure and the strongest bond energy of Al3Zr with different crystal structures was analyzed by empirical electron theory (EET) in solid and molecules to explain the controversial causes.

Fig. 5.

Crystal structures for Al3Zr: (a) cubic Ll2, (b) tetragnonal DO23.

(0.21MB).

According to the EET [32,33], all atoms in a solid are hybridized by two states of the (near) ground state; the hybrid states are discontinuous; in general, covalent electron pairs always exist between two atoms in close proximity; for some elements, the electrons of their s-layer have functions that are similar to those of the d-layer electrons, which are termed equivalent electrons. The atomic state can be represented by the total number of valence electrons per unit molecule(nT), the number of covalent electrons(nc), the number of lattice electrons(nl), the number of magnetoelectronics(nm), the number of dummy pairs(nd), and the half-length of single bonds(R(l)). The commonly used calculation method is the bond-length difference method, and is expressed as:

where Dnαu-v is a covalent bond distance; u and v are two atoms that form a bond; nα is any number greater than zero, and the largest is represented by nαM; α represents any covalent bond; β is a parameter, and its selection condition is as shown in Eq. 2:
where, 0 ≤ ε < 0.05。

We assume that the covalent bond that is formed by the two nearest atoms is A(α = A), and the remaining covalent bonds are B, C, D… (α = B, C, D…). According to Eq. 1, we obtain:

By subtracting Eq. 4 from Eq. 3 yields Eq. 5:

We stipulate:

Eq. 5 is converted to Eq. 7:

When the system under study has been determined, rα' can be obtained from Eq. 7.

In the crystal, the total number of equivalent bonds with the same bond length that is formed by the same atoms is represented by Iα, which is calculated as:

where IM is the total number of atoms in the unit crystal, IS is the total number of equivalent bonds formed with a certain reference atom, IK is a parameter, and when the two atoms that forming a bond are the same atom, the value is 1, otherwise it is 2.

According to the principle that the basic structural unit in the system maintains electrical neutrality, a structural unit that is composed of j atoms with a total number of covalent electrons ∑jncj can be expressed by Eq. 9:

By combining Eq. 7 with Eq. 9, several solutions for nα can be obtained. By introducing nα into Eq. 1, the theoretical bond distance of each covalent bond can be obtained. When the theoretical bond distance D¯nαu-v and the experimental bond distance satisfy Eq. 10, then the set atom state can be considered to agree with reality.

According to the above calculation method, the valence electron structures of the Al3Zr phase of Ll2- and DO23-type were calculated, as shown in Table 2.

Table 2.

Valence electron structure for Ll2–Al3Zr and DO23–Al3Zr.

Crystal structure  Bond  Iα  D  nα 
Ll2DnAAl-Zr  24  22a  0.3226 
DnBAl-Al  24  22a  0.1613 
DnCZr-Zr  0.0068 
DnDAl-Al  18  0.0017 
DnEAl-Zr  48  62a  0.0001 
DO23DnAAl3-Zr  16  22a  0.4471 
DnBAl1-Al1  22a  0.1729 
DnCAl2-Al2  22a  0.1729 
DnDAl1-Zr  16  a2+c22  0.2888 
DnEAl2-Zr  16  a2+c22  0.2888 
DnFAl1-Al3  16  a2+c22  0.1116 
DnGAl2-Al3  16  a2+c22  0.1116 
DnHZr-Zr  0.0127 
DnIAl1-Al1  0.0019 
DnJAl2-Al2  0.0019 
DnKAl3-Al3  0.0019 
3.3Effect of Sc on mechanical properties of 7136 aluminum alloy

Microhardness is a comprehensive index of mechanical properties, such as the material strength and plastic deformability. Hardness testing is performed on the 7136 aluminum alloy with and without added Sc element. The hardness-versus-time curve of the two alloys at 120 °C is shown in Fig. 6. In the early stage of aging, the hardness of the 7136 aluminum alloy without elemental Sc addition increases rapidly, and then the upward trend slows down. At an aging time of 16 h, the hardness approaches the peak. When the aging time exceeds 24 h, the hardness begins to decrease slowly. With 0.2% elemental Sc addition to the alloy, the hardness change shows the same trend during aging. The alloy hardness is close to the peak at 16 h. After 24 h, the hardness decreases slightly, but the decrease in trend is not obvious. The peak hardness of the alloy without added elemental Sc was 220.7 V, and the peak hardness was 221.4 HV after 0.2% Sc addition. After 0.2% elemental Sc addition, the alloy hardness was not improved significantly.

Fig. 6.

Hardness curves of 7136 aluminum alloys aged at 120 °C.

(0.11MB).

The tensile properties of the two alloys at normal temperature are shown in Table 3. In the solid–solution state, when 0.2% elemental Sc is added, the yield strength of the alloy increases from 524.69 MPa to 570.95 MPa, which represents an increase of 8.82%. The tensile strength increased from 673.57 MPa to 714.19 MPa, which represents an increase of 6.03%. In the aging state, compared with the alloy without added elemental Sc, the yield strength increased from 697.99 MPa to 736.60 MPa with added Sc, which is an increase of 5.53%. The tensile strength increased from 717.80 MPa to 753.44 MPa, which is an increase of 4.97%. When elemental Sc is added, the elongation at break of the alloy is also improved to some extent. In the solid–solution state, the addition of elemental Sc increases the elongation at break of the alloy from 10.50% to 11.52%, which is an increase of 9.71%. After peak aging treatment, the addition of 0.2% elemental Sc increased the alloy elongation from 7.12% to 9.64%, which is an increase of 35.39%. This result is linked to the grain refinement that is caused by the addition of elemental Sc. After 0.2% elemental Sc addition to 7136 aluminum alloy, the strength and plasticity of the alloy improved to some extent.

Table 3.

Mechanical properties of alloys under solution and aging treatment.

Sample  Status  Yield strength/MPa  Tensile strength/Mpa  Elongation/% 
1Solid solution  524.69  673.57  10.50 
Aging  697.99  717.80  7.12 
2Solid solution  570.95  714.19  11.52 
Aging  736.60  753.44  9.64 
4Discussion4.1Effect of Sc on 7136 aluminum alloy microstructure

Many studies have shown that during solidification, a peritectic reaction of L + Al3Zr →α-Al exists. The Zr content in the alloy studied reached 0.15%, which exceeded 0.11%, and the nascent Al3Zr particles with a tetragonal DO23 structure could be formed. The lattice structure of the phase has a lattice constant a = 0.4013 nm and c = 1.732 nm, which is different from the crystal structure of the Al matrix; and some lattice parameters of Al3Zr do not match the Al matrix. Therefore, although much elemental Zr exists in the 7136 aluminum alloy, it does not function well to refine the grains. When elemental Sc is added, primary Al3Sc particles of a face-centered cubic Ll2-type (AuCu3 type) structure can be formed. The lattice constants of the particles and Al are 0.4106 ± 0.007 nm and 0.4049 nm; and the lattice mismatch between the two particles is small. Therefore, the Al3Sc particles can serve as a heterogeneous nucleation core during solidification. The effect is to refine the alloy grain. However, only 0.2% of the elemental Sc was added to the alloy, and the Al3Sc second-phase particles could not be formed directly. Li et al. [34] have shown that under certain conditions, the melt does not form primary Al3Zr particles with a DO23 structure during solidification, but forms Ll2-type Al3Zr particles with a crystal structure and size that are similar to Al3Sc. Subsequently, the Sc atom replaces part of the Zr atoms gradually in the Al3Zr particles by diffusion to form Al3(ScxZr1-x) Ll2-type particles. The particles act as a heterogeneous nucleation core during solidification, which promotes as-cast alloy grain refinement. Therefore, the as-cast microstructure of the 7136 aluminum alloy to which 0.2% elemental Sc is added exhibits a relatively fine uniform equiaxed crystal.

Fig. 4 shows an FFT diagram of selected regions of Fig. 3 (i.e., Al3Zr and Al3(ScxZr1-x) particles): (a) is along the [1(—)11] ribbon axis and (b) is along the [112] ribbon axis. In Fig. 4(a), the actual lengths of the measurements d1, d2 and d3 and the scale are denoted as l1, l2, l3 and l0. By using Eq. 11, the true lengths of d1, d2 and d3 can be obtained:

The calculation indicates that d1 = 0.3200 nm, d2 = 0.3110 nm and d3 = 0.3061 nm, and α = 62.29° and β = 56.01° are obtained after measurement. The pdf card indicates that d1, d2 and d3 are interplanar spacings of the (1(—)01(—)), (110) and (011(—)) faces of the Al3Zr phase, respectively. Many studies have shown that Al3Zr particles have a tetragonal structure, and the lattice constant of Al3Zr particles can be obtained from Eq. 12:

The calculated lattice constants of the Al3Zr phase were a = 0.4525 nm and c = 0.4248 nm, which differs from the lattice constant a = 0.4013 nm and c = 1.7321 nm of the tetragonal phase Al3Zr. We assume that the Al3Zr phase has a cubic structure. The lattice constant of the cubic phase Al3Zr calculated by Eq. 13 is a = 0.4417 nm, and the calculation result is similar to the actual result. Experiments have shown that in the 7136 alloy, the Zr atom does not form a tetragonal Al3Zr phase with Al but forms an Al3Zr phase with a cubic structure. The lattice constant of the aluminum matrix is a = 0.4049 nm, and the difference between them is only 8.33%. Therefore, the Al3Zr particles can act as a heterogeneous nucleation core during solidification, which can explain that when no elemental Sc is added, the as-cast grains of the 7136 alloy are equiaxed rather than coarse dendrites. Similarly, in Fig. 4(b), by using Eq. 11, d1 = 0.242442 nm, d2 = 0.2909 nm and d3 = 0.11920 nm. α = 41.57° and β = 50.70° are measured. Eq. 13 shows that the lattice constant of the Al3(ScxZr1-x) phase is a = 0.4212 nm, which is 3.87% different from the lattice constant of the aluminum matrix (a = 0.4049 nm). Therefore, when 0.2% Sc is added, although the Al3Sc phase is not formed directly, the Al3Zr phase that is also a tetragonal structure and has a lattice constant that is similar to Al3Sc is formed. The atomic radii of Zr and Sc are 2.16 Å and 2.09 Å, therefore, Sc can replace the Zr atom by diffusion without causing a change in the crystal structure of the Al3Zr phase. Because the Al3(ScxZr1-x) particles are similar to the lattice constant of the aluminum matrix, the grain-refining effect is better. The crystal structure and lattice constant of the aluminum matrix, Al3Zr and Al3(ScxZr1-x) are shown in Fig. 7.

Fig. 7.

Crystal structure and lattice constant of aluminum matrix, Al3Zr and Al3(ScxZr1-x).

(0.31MB).

During the alloy solidification, the Al3Zr phase is formed preferentially instead of the Al3Sc phase. Zhimin et al. [35] explored the phase diagrams of Al–8Zn–(0–8)Mg–2Cu–0.3Zr–0.3Sc alloy with different magnesium contents. The results show that during solidification, the Al3Zr phase is formed first in the melt, and then Al3Sc is formed. For the alloy that was studied, the content of elemental Sc was only 0.2%. According to the Al–Sc binary phase diagram (as shown in Fig. 8) when the Sc content is only 0.2%, the Al3Sc phase is not formed directly in the melt, and all Sc is solid-dissolved in the matrix. As the temperature decreases, the solid solubility of Sc in the matrix decreases, and Sc combines with Al3Zr particles to form Al3(Sc,Zr) particles. Some studies in our laboratory have shown that in similar alloys, Al3(ScxZr1−x) particles also exhibit a tendency to coarsen and can grow up to hundreds of nanometers, which results in a significantly reduced alloy strengthening. Therefore, the author does not believe that the Al3(ScxZr1−x) particles have a core–shell structure.

Fig. 8.

Al–Sc binary phase diagram.

(0.08MB).

After elemental Sc addition, the grain refinement is inseparable from the Sc purification and metamorphism. Sc can form a stable compound with impurity elements, such as Si and Fe, which changes its original state of existence and suppresses the formation of a coarse iron-rich phase. Al is a face-centered cubic structure, and Sc is a close-packed hexagonal structure [19]. Because of the difference in crystal structure, the solid solubility of Sc in the Al matrix is small. Therefore, the Sc that was enriched at the front edge of the solid–liquid interface increases the composition of the supercooling, which reduces the secondary dendrite spacing, and refines the grains.

4.2Valence electron structure analysis of Al3Zr with different crystal structures

Table 2 shows that for the Al3Zr phase with the cubic Ll2, the strongest bond is the Al–Zr bond (bond A), the covalent electron is 0.3226. The Al atom is in the fifth stage of the hybrid, and the Zr atom is in the thirteenth stage of the A hybrid [31]. For the Al3Zr phase with tetragonal DO23, the strongest bond is the Al3–Zr bond (bond A), and the covalent electron number is 0.4471. Al1 and Al2 atoms are at the fifth stage of the hybrid, Al3 is at the sixth stage and Zr is at the ninth stage. The determination of the stage of the hybrid in this paper follows the principle of a minimum bond length difference. During the calculation of covalent electron number (nA) of the strongest bond, the shielding effect of the nuclear charge and the influence of the d-layer electron were not considered. Therefore, although the calculation can characterize the stability of phases of different structures to a certain extent, it is not as accurate as the energy of the strongest bond (EA) [31]. The calculation method of the covalent bond energy(Eα) is as follows [33]:

Because the strongest bond in the Al3Zr with cubic Ll2 and tetragnonal DO23 is the Al–Zr bond, we only need to consider the second case. In Eq. 14, B¯ is the average shielding coefficient, and F¯ represents the bonding ability of the covalent electrons that is contributed to by the two atoms of the bond to the bond, and B¯ and F¯ can be obtained from:

where bu and bv are the shielding coefficients of the bonding two atoms to the nuclear charge, respectively, and fu and fv are the bonding energies of the bonding orbitals of the two atoms that are bound to each other. bu and bv can be obtained by look-up in a table, and fu and fv can also be obtained by a hybrid orbit type and the hybridization coefficient [36].

The above calculation indicates that the strongest bond energies (EA) of Al3Zr with cubic Ll2 and tetragnonal DO23 are 52.8 kJ/mol and 71.0 kJ/mol, respectively. The DO23–Al3Zr has a stronger stability regardless of the number of covalent electrons on the strongest bond or the bond energy of the strongest bond. According to the ab initio calculation [37], the formation energies of Ll2–Al3Zr and DO23–Al3Zr are –45.5 kJ/mol and –46.9 kJ/mol, respectively. Ll2–Al3Zr will undergo a phase transition and be converted into a more stable DO23–Al3Zr. We only observed the existence of Ll2–Al3Zr, and DO23–Al3Zr particles were not observed because Ll2–Al3Zr has a good stability, as shown by the strongest bond energy.

4.3Effect of Sc on mechanical properties of 7136 aluminum alloy

Because the addition of Sc produces a refinement effect on the as-cast grains of the alloy, the grains of the alloy added with Sc in the solid–solution and aged states are finer than the alloy without elemental Sc addition. Grain refinement improves the alloy plasticity and increases the alloy hardness and strength. Based on the mechanism of fine grain strengthening and second-phase strengthening, the contribution of various strengthening mechanisms to the strength of the 7136 alloy after Sc addition was analyzed quantitatively. For the 7136 aluminum alloy, the factor that causes the change in alloy strength (yield strength) is expressed by [38]:

where σ0.2 is the yield strength of the alloy, σ0 is the material inherent frictional resistance (Peierls–Nabarro forces), σOR is the strength that is provided by dispersion strengthening, σHP is the strength that is provided by grain-boundary strengthening, σWH is the strength increase because of work hardening and σSO is the strength that is provided by solid–solution strengthening. After solution treatment, the work hardening of the alloy has been eliminated and σWH can be ignored.

Jmatpro was used to plot the content of the main strengthening phase (i.e., the MgZn2 phase) of the two alloys with temperature and the TEM of the MgZn2 phase as shown in Fig. 9. Under the same conditions, the amount of precipitated MgZn2 phase is almost the same, so the content of Mg and Zn in the solid solution in the aluminum matrix is almost the same, and σSO can be ignored. Therefore, Eq. 16 can be simplified to Eq. 17:

Fig. 9.

MgZn2 phase content-temperature curve of two alloys and the TEM of MgZn2 phase.

(0.42MB).

Fig. 1 shows that in the peak aging state, the grain size of the alloy without added Sc is ˜2 μm to 30 μm, and after 0.2% Sc addition, the grain size is reduced to 0.5 μm–10 μm. By including the statistical results of Fig. 2 in Eq. 18, which is the fine grain-strengthening formula, the intensity that is contributed by the refined grains can be obtained:

where k is a constant and d is the grain diameter. With dWith Sc = 2.9 μm, dWithout Sc = 8.6 μm and k = 0.04 MPa × m 1/2, the intensity value from grain refinement by Sc addition after peak aging was 9.85 MPa.

Fig. 9 shows that there is almost no difference in content of the main strengthening phase, that is, the MgZn2 phase in the two alloys, and therefore the intensity increase from the dispersion strengthening by the phase is almost the same. The second-phase particles that cause the main changes in strength are Al3Zr and Al3(ScxZr1−x). Fig. 3 shows that the Al3Zr particles in the 7136 aluminum alloy have a diameter of ˜25 nm, and the Al3(ScxZr1−x) particles that are formed after elemental Sc addition have an average diameter of ˜30 nm. Both diameters have exceeded the critical particle diameter of 4–6 nm from the mode of dislocation shearing phase to the Orowan looping mode, so both strengthen the alloy through the Orovan mechanism. The strength increase of the two particles that is caused by the Orovan strengthening mechanism can be calculated from Eq.s 19,20, and 21:

where M is the Taylor factor, ν is the Poisson ratio, G is the shear modulus, b is the Burgers vector of the aluminum matrix, K4 is a constant, ds- is the nominal particle diameter, λ is the effective particle spacing and fV is the volume percentage. We assume that the thickness of the observed transmission sample is 80 nm (the thickness condition of the microstructure can be observed under transmission in the bright field). According to the observation area and the number of particles observed in Fig. 3, the calculated volume fraction of Al3Zr particles and Al3(ScxZr1-x) particles is 4.60 × 10−3 and 9.63 × 10−3. By including the diameters of two particles and K4 = 0.127, M = 3.06, ν = 0.331, G = 27.8 GPa and b = 0.286 nm in Eq. 19, the calculated contribution of Al3Zr particles to the alloy strength is ˜84.18 MPa, and the contribution of Al3(ScxZr1-x) particles to the alloy strength is ˜110.96 MPa. In summary, compared with the alloy without added elemental Sc, the contribution of fine grain strengthening and dispersion strengthening to the alloy yield strength with 0.2% elemental Sc addition increased by 9.85 MPa and 26.78 MPa. The sum of the two values is 36.63 MPa, which is similar to the measured 38.61 MPa, and the difference between the two values is only 5.13%.

Because of the addition of Sc, the alloy grains are refined, so the alloy plasticity is improved to some extent. Li et al. [34] showed that Al3(ScxZr1-x) particles cannot be dislocated by dislocation during deformation. Therefore, a reduction in the number of dislocations slipped to the grain boundary reduces the stress concentration at the grain boundary, which avoids premature sample fracture and improves the alloy plasticity.

5Conclusions

  • (1)

    For Ll2-Al3Zr, the covalent electron number and bond energy of the strongest bond are 0.3226 and 52.8 KJ/mol, respectively, and for DO23-Al3Zr, the covalent electron number and bond energy of the strongest bond are 0.4471 and 71.0 KJ/mol, respectively. Ll2-Al3Zr itself has good stability. After elemental Sc addition, Al3 (ScxZr1−x) is formed based on Ll2-Al3Zr.

  • (2)

    After 0.2% elemental Sc addition to the 7136 aluminum alloy, the alloy grain is refined remarkably. The average grain size of the alloy in the as-cast, solid–solution and aged states decreased from 82.4 μm, 28.2 μm and 8.6 μm to 51.7 μm, 15.2 μm and 2.9 μm, respectively.

  • (3)

    In the peak aging state, the yield strength increment caused by the grain refinement through the addition of elemental Sc is 9.85 MPa. The yield strength increase that is caused by the dispersion strengthening of Al3(ScxZr1−x) particles is 110.96 MPa, which is larger than the 84.18 MPa of the Al3Zr particles. The addition of 0.2% elemental Sc to the 7136 alloy improves the alloy plasticity, and increases the alloy strength to some extent.

Conflicts of interest

The authors declare no conflicts of interest.

Acknowledgments

This work was financially supported by the National Key Research and Development Program of China (2016YFB0700300), the National Key R&D Program of China (2016YFB0300901) and academician work station of aluminum and magnesium alloys in Province Harbin.

References
[1]
Mei-Ni Su, Yu Zhang, Ben Young.
Design of aluminium alloy beams at elevated temperatures.
Thin-walled Struct, 140 (2019), pp. 506-515
[2]
Zeyu Bian, Shihan Dai, Liang Wu, Zhe Chen, Mingliang Wang, Dong Chen, et al.
Thermal stability of Al–Fe–Ni alloy at high temperatures.
J Mater Res Technol, 8 (2019), pp. 2538-2548
[3]
Pradeep Kumar Krishnan, John Victor Christy, Ramanathan Arunachalam, Abdel-Hamid I Mourad, Rajaraman Muraliraja, Majid Al-Maharbi, et al.
Production of aluminum alloy-based metal matrix composites using scrap aluminum alloy and waste materials: influence on microstructure and mechanical properties.
J Alloys Compd, 784 (2019), pp. 1047-1061
[4]
L.X. Bach, D.L. Son, M.T. Phong, L.V. Thang, M.Z. Bian, N.D. Nam.
A study on Mg and AlN composite in microstructural and electrochemical characterizations of extruded aluminum alloy.
Compos Part B Eng, 156 (2019), pp. 332-343
[5]
Mohammed Imran, A.R. Anwar Khan.
Characterization of Al-7075 metal matrix composites: a review.
J Mater Res Technol, 8 (2019), pp. 3347-3356
[6]
Jenix Rino, John Xavier Raj, Balasivanandha Prabu Shanmugavel.
Thermal stability of ultrafine grained AA8090 Al–Li alloy processed by repetitive corrugation and straightening.
J Mater Res Technol, 8 (2019), pp. 3251-3260
[7]
Pedro Henrique Lamarão Souza, Carlos Augusto Silva de Oliveira, JoséMaria do Vale Quaresma.
Precipitation hardening in dilute Al–Zr alloys.
J Mater Res Technol, 7 (2018), pp. 66-72
[8]
Pedro Henrique R. Pereira, Yi Huang, Terence G. Langdon.
Examining the microhardness evolution and thermal stability of an Al–Mg–Sc alloy processed by high-pressure torsion at a high temperature.
J Mater Res Technol, 6 (2017), pp. 348-354
[9]
Prabhakar M. Bhovi, Deepak C. Patil, S.A. Kori.
A comparison of repetitive corrugation and straightening and high-pressure torsion using an Al-Mg-Sc alloy.
J Mater Res Technol, 5 (2016), pp. 353-359
[10]
Kai Wen, Yunqiang Fan, Guojun Wang, Longbin Jin, Xiwu Li, Zhihui Li, et al.
Aging behavior and precipitate characterization of a high Zn-containing Al-Zn-Mg-Cu alloy with various tempers.
Mater Des, 101 (2016), pp. 16-23
[11]
A. Heinz, A. Haszler, C. Keidel, S. Moldenhauer, R. Benedictus, W.S. Miller.
Recent development in aluminum alloys for aerospace applications.
Mater Sci Eng A, 280 (2000), pp. 101-107
[12]
D. Dumont, A. Deschamps, Y. Brechet.
On the relationship between microstructure, strength and tonghness in AA7050 aluminum alloy.
Mater Sci Eng A, A356 (2003), pp. 326-336
[13]
Yifang Liu, Wei Tian, Yue Sun.
Research progress of microalloying of Al-Zn-Mg-Cu aluminum alloy.
Nonferrous Metal Mater Eng, 39 (2018), pp. 38-41
[14]
Sun Wenhui, Zhang Yongan, Li Xiwu, Li Zhihui, Wang Feng, Liu Hongwei, et al.
Effect of solution treatment on microstructures and mechanical properties of 7136 aluminum alloy.
J Aeronaut Mater, 34 (2014), pp. 35-41
[15]
Fang Hongjie, Sun Jie, Wang Hongbo, Yin Dengfeng.
Influence of alloy added trace cerium on microstructure and properties of 7136 aluminum.
J Chin Soc Rare Earth, 34 (2016), pp. 313-319
[16]
Zhu RanRan, Zhang Yongan, Xiong Baiqing, Li Zhihui, Li Xiwu, Liu Hongwei, et al.
Effect of solution treatment on microstructures and mechanical properties of 7136 aluminum alloy.
J Aeronaut Mater, 32 (2012), pp. 37-42
[17]
Fang Hongjie, Liu Hui, Yin Hongxia, Sun Jie.
Effect of trace element Cr on microstructure and performance of 7136 aluminum alloy.
Hot Working Technol, 46 (2017), pp. 74-77
[18]
Liu Huana, Yang Guangyu, Qi Yuanhao, Liu Shaojun, Jie Wanqi.
Effects of Sc on the microstructures and mechanical properties of Al-Zn-Mg-Cu casting aluminum alloy.
Foundry, 62 (2013), pp. 4-9
[19]
Kai Yang.
The effect of Sc on the microstructure and properties of 7075 aluminum alloy.
(2016),
[20]
Zhao Bin, Li Xiangbo, Wang Jiulin, Xue Kemin.
Influence of Micro-Sc on casting microstructures and mechanical properties of high strength aluminum alloy.
J Netshape Forming Eng, 7 (2015), pp. 70-75
[21]
Zhou Min, Gan Peiyuan, Deng Honghua, Zhan Haihong, Liu Chen, Zeng Jianmin.
Research status and prospect of Sc microalloying aluminum alloys.
Mater Chin, 37 (2018), pp. 154-160
[22]
He Yongdong, Zhang Xinming, You Jianghai.
Effect of minor Sc and Zr on microstructure and mechanical properties of Al-Zn-Mg-Cu alloy.
Trans Nonferrous Met Soc China, (2006), pp. 1228-1235
[23]
Yin Zhimin, Yang Lei, Pan Qinglin, Jiang Feng.
Effect of minor Sc and Zr on microstructures and mechanical properties of Al-Zn-Mg based alloys.
Trans Nonferrous Met Soc China, 11 (2001), pp. 822-825
[24]
Li Wenbin, Pan Qinglin, Zou Liang, Liang Wenjie, He Yunbin, Liu Junsheng.
Effects of minor Sc on the microstructure and mechanical properties of Al-Zn-Mg-Cu-Zr based alloys.
Rare Metals, 28 (2009), pp. 102-106
[25]
He Zhenbo, Yin Zhimin, Lin Sen, Deng Ying, Shang Baochuan, Zhou Xiang.
Preparation, microstructure and properties of Al-Zn-Mg-Sc alloy tubes.
J Rare Earths, 28 (2010), pp. 641-646
[26]
Xiaoyuan Dai, Changqing Xia, Xiaomin Peng, Ke Ma.
Structure and properties of an ultra-high strength 7xxx aluminum alloy contained Sc and Zr.
Materials, 15 (2008), pp. 276-279
[27]
P.W. Voorhees.
Scandium overtakes zirconium.
Nat Mater, 5 (2006), pp. 435-436
[28]
K.E. Knipling, R.A. Karnesky, C.P. Lee, David C. Dunand, David N. Seidman.
Precipitation evolution in Al-0.1Sc, Al-0.1Zr and Al-0.1Sc-0.1Zr(at. %) alloys during isochronal aging.
Acta Mater, 58 (2010), pp. 5184-5195
[29]
Dai Xiaoyuan, Xia Changqing, Long Chunguang, Kou Lili.
Morphology of primary A13(Sc, Zr) of As-Cast A1-Zn-Mg-Cu-Zr-Sc alloys.
Rare MRTA1 Mater Eng, 40 (2011), pp. 265-268
[30]
Jianhui Jiang.
Study on heat treatment process and microstructure of 7056 aluminum alloy.
(2012),
[31]
R.L. Zhang.
The empirical Electron theory of solids and molecules.
(1993),
[32]
Zhilin Liu, Zhilin Li, Weidong Liu.
Interface electronic structure and interface performance.
(2002),
[33]
Dongsheng Zhang.
Comparative analysis of the refinement mechanism of pure Al by Ti, Zr and Sc by valence electron structure.
(2011),
[34]
Li Hai, Yang Yingxin, Zheng Ziqiao, Wang Xiuzhi.
Effect of minor addition of scandium on microstructures and mechanical properties of 7055 aluminum alloy.
Mater Sci Technol, 14 (2006), pp. 46-49
[35]
Yin Zhimin, Pan Qinglin, Jiang Feng, Li Hanguang.
Scandium and its alloys.
(2007),
[36]
R..L. Zhang.
The ζ-Fe2N valence electron structure and the relations between the ε→ζ phase transformation and the valence electron structures in Fe-N system.
J Nat Sci Jilin Univ, 3 (1984), pp. 74-82
[37]
C. Colinet, A. Pasturel.
Phase stability and electronic structure in ZrAl3 compound.
J Alloys Compd, 319 (2001), pp. 154-161
[38]
Huiqin Cao.
Preparation and properties of oxide dispersion strengthened aluminum alloys.
(2015),
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