Effects of La content (4%, 6%, 8%, 10%) on microstructure, mechanical properties, electrical and thermal conductivity of die-cast Al-La alloys were studied. La has a very small solid solubility in aluminum matrix and is easily enriched at the front of α-Al solidification interface, causing component supercooling, reducing crystallization temperature of liquid phase around α-Al, and then inhibiting growth of α-Al to achieve refinement. Al11La3 second phase generated during solidification can not only strengthen alloy, but also serve as a heterogeneous nucleation point for α-Al to achieve refinement, thanks to its small mismatch of 5.96% with α-Al matrix. As La content increases from 4% to 10%, yield strength and tensile strength of alloy increase from 48.6 MPa and 113.9 MPa to 92.3 MPa and 186.7 MPa, respectively, but elongation decreases from 26.1% to 9.6%. In addition, thermal conductivity and electrical conductivity of Al-La alloy decrease linearly with increase of La content, from 207.8 W/(m·K) and 32.0 MS/m to 173.1 W/(m·K) and 26.1 MS/m, respectively.
Rapid development of automobile industry has brought about increasingly serious environmental and energy consumption problems, lightweight and integration of parts are effective ways to solve these problems. Among them, replacement of steel splicing parts with integrated aluminum alloy die castings has become mainstream. Aluminum alloy structural parts for lightweight automobiles are usually more complex and are often produced by high-pressure casting. Degree of supercooling in die-casting process is large, and it is easy to obtain fine structures. Process also has advantages of high dimensional accuracy, high production efficiency, and good economic benefits. Studies have shown that every 10% weight reduction in a car can improve fuel efficiency by 6% to 8% and reduce pollutant emissions by 5% to 6%. Density of aluminum alloy is about one-third of that of steel, making it an excellent lightweight material for automobiles. However, as parts continue to develop in direction of integration and miniaturization, comprehensive performance requirements for alloy materials are becoming increasingly higher. For example, motor rotors for new energy vehicles require aluminum alloy materials to have high electrical conductivity (conductivity ≥ 28 MS/m, conventional die-cast aluminum alloy conductivity is less than 20 MS/m) while taking into account a certain strength. Liquid cooling plates require aluminum alloy materials to have high thermal conductivity while taking into account a certain strength. Therefore, conventional aluminum alloy materials are difficult to meet needs of these parts for structural and functional integrated aluminum alloy materials.
In industry, metamorphic processes and refining processes are usually used to improve quality of aluminum melts and aluminum alloys, thereby improving comprehensive performance of alloys. Rare earth is an important strategic resource in 21st century. Adding rare earth elements to aluminum can play a positive role in degassing and slag removal, grain refinement, and strength improvement. This is due to its active chemical properties, easy formation of Al-RE phase with good strengthening effect with aluminum, and heterogeneous nucleation effect of Al-RE relative to α-Al. Studies have shown that compared with solid solution atomic form, alloying elements in the form of fine dispersed second phases have a more significant strengthening effect, have the least effect on thermal and electrical conductivity. Light rare earth element La has a very low solid solubility in aluminum matrix and often exists in the form of a second phase in aluminum alloys. Therefore, it has little impact on high thermal conductivity and high electrical conductivity properties of pure aluminum while being strengthened.
In recent years, research on La has become more and more in-depth. Salem et al. explored refinement effect of La on A390 aluminum alloy and found that adding La can significantly improve mechanical properties of A390 alloy; Zhu et al. found that La will cause supercooling of solid-liquid interface of Mg-Al-Zn alloy, thereby causing second phase to be discontinuously distributed. If La is added to pure aluminum as main element, it is very likely to react with aluminum to form fine dispersed Al11La3 phase (a=0.443 2 nm, b=1.315 7 nm, c=1.013 753 nm, space group is Immm), which will improve strength of alloy while minimizing impact on high thermal conductivity and high electrical conductivity of pure aluminum. In this study, 4%, 6%, 8%, and 10% La element (mass fraction) were added to pure aluminum to study evolution of microstructure, mechanical properties, thermal conductivity, and electrical conductivity of die-cast Al-La alloys with La content, providing a reference for development of high thermal conductivity and high conductivity die-cast aluminum alloys based on Al-La alloys.

Al-xLa (x=4%, 6%, 8%, 10%) aluminum alloys were prepared using industrial pure aluminum (99.97%, mass fraction, same below) and Al-20La master alloy as raw materials. Put pure aluminum ingot and Al-20La into a crucible, heat to 740 ℃ until completely melted, stir and let stand, add refining agent and slag after refining, and die-cast at 720 ℃. Figure 1 is a schematic diagram of die-casting. This experiment uses a TOYO-BD350V5 cold chamber die-casting machine, mold preheating temperature is 200 ℃, low speed is 0.25 m/s, high speed is 2 m/s, and casting pressure is 70 MPa. Chemical composition of alloy is determined by inductively coupled plasma atomic emission spectroscopy (ICP-AES). As shown in Table 1, actual composition is close to nominal composition.

 

Figure 1 Schematic diagram of die casting

Table 1 Chemical composition of experimental alloys wB/%
Samples were cut from plate A and polished for XRD test, thermal and electrical conductivity test. XRD test used Ultima Ⅳ diffractometer, with Cu target, voltage of 40 kV, current of 30 mA, scanning speed of 5°/min, and scanning range of 20°~90°; thermal conductivity test sample was a disc with a diameter of 12.7 mm and a thickness of 2.5 mm. Thermal diffusion coefficient of alloy was measured by Netzsch LFA 447 thermal conductivity meter (average of 3 times). Before measurement, a layer of carbon coating was sprayed on the surface of sample to improve absorption of light pulses; room temperature density of each alloy was determined by an electronic balance based on Archimedes method (Sartorius Quintix124-1CN) and density meter (YDK03P) (average of 5 times); specific heat capacity is calculated by Neumann-Kopp equation:

 

Where: Cp is specific heat capacity of alloy, Cp,i is specific heat capacity of each component, and is weight fraction of each component. Thermal conductivity λ can be calculated by equation 2:

 

Where: α is thermal diffusion coefficient, ρ is density, and Cp is specific heat capacity. Conductivity σ (average of 5 times) of each alloy was measured by Sigma 2008 Digital Eddy Current Metal Conductivity Instrument (Sigma 2008).
As shown in Figure 1, rod-shaped specimen B on die casting is a tensile specimen with a total length of 170 mm, a parallel section length of 50 mm, and a parallel section diameter of 6.5 mm. Tensile test was performed on a Zwick Z100 universal material testing machine at a tensile rate of 1 mm/min. Test results were average of 5 tests, where σ0.2 was taken as yield strength of alloy. As shown in Figure 1, sample was cut from plate A and used for microstructure observation after grinding, polishing and etching. Corrosive agent was a hydrofluoric acid solution with a volume fraction of 0.5%, and etching time was 10 s. Alloy microstructure and tensile fracture morphology were observed using an optical microscope (Zeiss Axio Observer A1) and a scanning electron microscope with energy spectrum (Phenom). Scanning voltage was 15 kV, characterization was performed in backscattering mode BSE and secondary electron mode SE. Image-Pro Plus software was used to count α-Al size of each alloy, and result was average of 10 metallographic photos.

As shown in Figure 2a, equilibrium phase diagram of Al-La alloy was calculated using Pandat thermodynamic software. On Al-rich side, Al and La will undergo eutectic reaction at about 637.8 ℃ to generate Al+Al11La3 eutectic structure. It is worth mentioning that limit solubility of La in Al is only 0.001 795%, so added La element is almost entirely in the form of Al11La3 phase, which has little effect on high thermal conductivity and high electrical conductivity of pure aluminum while improving strength of pure aluminum. Compared with equilibrium phase diagram, non-equilibrium solidification path calculated based on Scheil model will be closer to actual die-casting result. As shown in Figure 2b, during solidification of molten metal, α-Al is first generated from liquid phase. Thanks to extremely low solid solubility in Al matrix, La element will be enriched in liquid phase with generation of α-Al, and Al+Al11La3 eutectic structure will be generated when eutectic reaction composition is reached. As La content increases, solidification start temperature of alloy decreases, α-Al decreases and eutectic Al+Al11La3 increases.

 

Figure 2 Al-La alloy phase diagram and non-equilibrium solidification path
As shown in Figure 3, Al-xLa alloy consists of two phases, α-Al and Al11La3. When La content increases from 4% to 10%, phase composition does not change. As La content increases, diffraction peak of Al11La3 is significantly enhanced, indicating that volume fraction of Al11La3 phase is gradually increasing.

 

Figure 3 XRD results of Al-xLa alloy
Figures 4 and 5 show microstructures of Al-xLa alloys with different La contents. Combined with Al-La phase diagram and XRD diffraction results, it can be seen that when La content is 4%, alloy consists of two structures, primary α-Al and eutectic Al11La3. Primary α-Al matrix is relatively coarse, while eutectic Al+Al11La3 structure distributed on primary α-Al dendrite boundary is relatively small. As shown in Figure 5, when La content is low, Al11La3 phase mainly exists in eutectic structure in a fine lamellar form. When La content increases to 8%, lamellar Al11La3 phase becomes significantly coarser. As La content increases, volume fraction of eutectic structure gradually increases, while phase composition does not change, but primary α-Al is refined with increase of La content, as shown in Figures 4a-b and 5a-b. α-Al sizes of four alloys Al-4La, Al-6La, Al-8La and Al-10La were statistically analyzed, results were (21.6±3.5)μm, (18.3±3.1)μm, (15.2±2.7)μm and (14.5±2.1)μm, respectively. Refinement of α-Al is mainly due to two reasons: on the one hand, solid solubility of La in Al matrix is extremely small. During growth of primary α-Al, a large amount of La elements are discharged to solid-liquid interface and continuously enriched. According to phase diagram shown in Figure 2a, in hypoeutectic Al-La alloy, a higher La content will lead to a lower crystallization temperature, so La element enriched at the front of solidification interface will cause composition to be supercooled, making it difficult for surrounding liquid phase to crystallize and solidify, thereby inhibiting growth of primary α-Al. On the other hand, as shown in Figures 4c-d and 5c-d, when La content reaches 8%, a blocky Al11La3 primary phase begins to appear inside α-Al matrix, its distribution form indicates that blocky primary phase can serve as a heterogeneous nucleation site for α-Al during solidification. When La content reaches 10%, as shown by yellow arrows in Figures 4d and 5d, a U-shaped blocky Al11La3 primary phase appears in organization, indicating that during solidification, α-Al can also serve as a heterogeneous nucleation site for Al11La3. Due to uneven distribution of solutes in liquid phase, concentration of local areas is prone to fluctuations, which makes local solute concentration of hypoeutectic alloy exceed eutectic composition point. In addition, La is easy to enrich during non-equilibrium solidification, so Al11La3 primary phase hypereutectic structure will appear in hypoeutectic alloys such as Al-8La and Al-10La.

 

Figure 4 Metallographic structure of alloy

 

Figure 5 Scanning structure of alloy
Heterogeneous nucleation ability can be determined by lattice mismatch. Generally, when lattice mismatch is between 0~6%, it indicates good nucleation ability, when it is between 6%~15%, it has medium nucleation ability, and when it is greater than 15%, it indicates no nucleation ability. Misfit degree can be calculated using Bramfitt's misfit degree equation:

 

Where: (hkl)s is low-index crystal plane of nucleation substrate, (hkl)n is low-index crystal plane of nucleation phase, [uvw]s is crystal direction on (hkl)s plane of nucleation substrate, [uvw]n is crystal direction on (hkl)n plane of nucleation phase, d[uvw]s is atomic spacing on [uvw]s crystal direction of nucleation substrate, d[uvw]n is atomic spacing on [uvw]n crystal direction of nucleation phase, θ is angle between [uvw]s and [uvw]n. α-Al is a face-centered cubic structure with a lattice constant of 4.05 , while Al11La3 is a body-centered orthorhombic structure (a=0.443 2 nm, b=1.315 7 nm, c=1.013 753 nm, space group is Immm), as shown in Figure 6a. (011) crystal plane is selected as low-index plane of Al11La3, which is also close-packed plane of Al11La3; (111) crystal plane is selected as low-index plane of α-Al, which is also close-packed plane of α-Al. Atomic arrangements on two corresponding low-index planes are shown in Figures 6b-c. Since atomic spacing between two phases in selected directions is quite different, and six atomic spacing of α-Al in [110]-direction is approximately equal to atomic spacing of Al11La3 in [011]-direction, some approximate processing can be used to obtain atomic matching relationship shown in Figure 6d.

 

Figure 6 Schematic diagram of Al11La3 and α-Al as each other's nucleation substrates
Substituting into Bramfitt mismatch equation for calculation, result can be obtained: if α-Al uses Al11La3 as a heterogeneous nucleation site, mismatch of α-Al (111) // Al11La3 (011) is 5.96%, that is, theoretically Al11La3 can be used as a nucleation site for α-Al and has good heterogeneous nucleation ability; if Al11La3 uses α-Al as a nucleation site, mismatch of Al11La3 (011) // α-Al (111) is 6.47%, that is, theoretically α-Al can also be used as a nucleation site for Al11La3 and has good heterogeneous nucleation ability. In hypoeutectic Al-10La alloy close to eutectic composition point, composition fluctuation and solidification segregation make hypereutectic structure and hypoeutectic structure coexist in alloy, so primary Al11La3 and primary α-Al can be intertwined, and lattice mismatch between the two is low, so they can serve as nucleation base for each other, so U-shaped Al11La3 phase is easy to appear. That is, Al11La3 nucleates and grows using α-Al formed first as nucleation site, then becomes wrapped in α-Al matrix as nucleation site of α-Al.

Figure 7 shows engineering stress-strain curves and corresponding mechanical properties of four alloys. As La content increases from 4% to 10%, yield strength and tensile strength of alloy increase from 48.6 MPa and 113.9 MPa to 92.3 MPa and 186.7 MPa respectively. When La content is greater than 8%, improvement effect decreases; elongation decreases from 26.1% to 9.6%, and there is a sudden drop when La content increases from 6% to 8%. Combined with microstructure analysis, it is not difficult to find that as La content increases, Al11La3 phase content increases. As a hard and brittle second phase, Al11La3 is dispersed in fine forms in eutectic structure when La content is low, and has a significant strengthening effect. When alloy undergoes plastic deformation under stress, dislocation movement will be blocked by second phase particles. At this time, the more particles and the smaller distance between particles, the more significant strengthening effect. But correspondingly, dislocation motion bypassing or cutting through second phase particles will also encounter greater resistance, so plasticity will decrease accordingly. When La content increases to 8%, large Al11La3 second phases appear. Strengthening effect of such large second phases is limited, but it has a greater impact on plasticity. Large second phases themselves and phase interface are prone to cracking, which promotes crack formation, thereby greatly reducing plasticity of alloy. Therefore, when La content reaches 8%, due to presence of more La elements in the form of large Al11La3 primary phases, strength of alloy is limited, but elongation decreases significantly.
Figure 8 shows tensile fracture morphology of each Al-xLa alloy. It can be seen that there are many dimples in fracture morphology of four alloys, indicating that fracture forms of these alloys include ductile fracture. However, in fracture morphology of aluminum alloys with different La contents, size and depth of dimples are quite different, that is, there are great differences in plasticity of four alloys, among which large and deep dimples imply better plasticity. Size of dimples is related to distribution of second phase particles. Second phase particles in Al-4La alloy exist in eutectic structure, as shown in Figure 8a. At this time, primary α-Al dendrite is larger, and eutectic structure where second phase is located is less, so its tensile dimple is large and deep. In Al-6La alloy, hard second phase increases, corresponding eutectic structure spacing decreases, and obstruction to dislocation movement is enhanced. Compared with Al-4La alloy, dimples in its tensile fracture morphology are smaller and shallower, as shown in Figure 8b, which also indicates that its plasticity has decreased to a certain extent. When La content increased to 8%, primary phase of Al11La3 appeared in structure. This large hard and brittle phase is easy to become a crack source and quickly reduce plasticity. As shown in Figure 8c, large Al11La3 phase cracks and participates in fracture process, which is one of main reasons for sharp decline in plasticity. Compared with Al-8La alloy, tensile fracture morphology of Al-10La alloy has more broken bulk Al11La3 phases, which causes a further decrease in plasticity.

 

Figure 7 Engineering stress-strain curve and mechanical properties of Al-xLa alloy

 

Figure 8 Fracture morphology of Al-La alloy

As La content increases, thermal diffusion coefficient of Al-La alloy gradually decreases, as shown in Figure 9a, while density of alloy gradually increases, as shown in Figure 9b. This is because Al11La3 phase density (4.04 g/cm³) in microstructure is much larger than Al matrix. Calculate specific heat capacity of alloy according to equation (1), where specific heat capacities of La and Al can be calculated using equation (4) and equation (5) respectively:

 

Specific heat capacities of La and Al components at room temperature are 0.195 J/(g·K) and 0.846 J/(g·K) respectively. Thermal conductivity of each alloy is calculated according to equation (2). As shown in Figure 9c, as La content increases, thermal conductivity roughly shows a linear downward trend. It dropped from 207.8 W/(m·K) in Al-4La alloy to 173.1 W/(m·K) in Al-10La alloy. As La content increases from 4% to 10%, electrical conductivity of aluminum alloy decreases from 32.0 MS/m to 26.1 MS/m, as shown in Figure 9d. According to modified Weidmann-Franz law, thermal conductivity and electrical conductivity have following relationship:

 

In formula: A and B are parameters related to alloy, L0 is Lorentz constant, which is , T is temperature, and σ is conductivity. For aluminum alloys, A=0.909, B=10.5 W/(m·K). After calculation, it can be found that electrical conductivity converted from thermal conductivity is relatively close to measured value, as shown in Figure 9d. At room temperature, electrical and thermal conductivity of aluminum alloy materials mainly depends on directional movement of electrons. Al11La3 phase generated by adding La element will produce distortion at phase interface, reducing mean free path of electrons, thereby reducing thermal conductivity and electrical conductivity. Increase in La content has a more obvious refining effect on Al matrix, which in turn leads to an increase in number of grain boundaries. However, atomic arrangement at grain boundaries is relatively chaotic and distortion is large. Therefore, an increase in number of grain boundaries will increase scattering of electrons, causing electrons to scatter. Mean free path decreases, thereby reducing thermal conductivity and electrical conductivity. In addition, formation of Al11La3 intermetallic compounds binds more electrons, resulting in poor thermal and electrical conductivity. Therefore, increase in second phase of Al11La3 is also one of reasons for decrease in thermal conductivity and electrical conductivity.

 

Figure 9 Thermophysical parameters of Al-xLa alloy at 25 ℃

Effect of La content on microstructure, mechanical properties, thermal and electrical conductivity of binary Al-xLa alloy was studied, a high thermal conductivity and high conductivity die-cast aluminum alloy material with a certain strength was developed. Main conclusions are as follows.
(1) Enrichment of La element at the front edge of solidification interface will cause supercooling of composition. At the same time, mismatch degree between α-Al and Al11La3 is as low as 5.96%, allowing Al11La3 to serve as a heterogeneous nucleation site for α-Al. The two reasons jointly promote refinement of α-Al. As La content increases from 4% to 10%, size of α-Al decreases from 21.6 μm to 14.5 μm.
(2) As La content increases from 4% to 10%, yield strength and tensile strength of alloy increase from 48.6 MPa and 113.9 MPa to 92.3 MPa and 186.7 MPa respectively, and elongation decreases from 26.1% to 9.6%. When La content is less than 8%, strengthening effect of needle-like Al11La3 second phase is better; when La content is higher than 8%, strengthening effect is weakened due to formation of massive Al11La3 primary phase, and elongation is significantly deteriorated.
(3) In Al-La alloy, there is no big difference in the influence of second relative thermal conductivity and electrical conductivity of the two different forms of Al11La3. Thermal conductivity and electrical conductivity of alloy generally have a linear downward trend as La content increases. They dropped from 207.8 W/(m·K) and 32.0 MS/m to 173.1 W/(m·K) and 26.1 MS/m respectively.