Abstract
Four alloy rods of Al-Zn-Mg-Cu-Zr-xEr (x=0, 0.1, 0.2, 0.5, wt%) were prepared by directional solidification. The microstructure and second phase morphology of alloys were investigated by optical microscopy (OM), electron probe micro-analysis (EPMA), energy dispersive spectrometer (EDS) and other methods. The results indicate that Er element can increase the number of dendrites in the directional solidification structure of Al-Zn-Mg-Cu-Zr alloy, and reduce the primary dendrite arm spacing and secondary dendrite arm spacing of the alloy. A proper amount of Er element can reduce the content of second phase in the alloy and increase the tendency to form round second phase. The formation of Al8Cu4Er phase can reduce the Cu content in the T phase (AlZnMgCu phase), which plays a role of rounding the boundary of the T phase, and can change the morphology and internal structure of the T phase. The Al8Cu4Er phase serves as the nucleation position of the T phase, and part of the T phase grows around this phase. The addition of Er can improve the microhardness of the alloy.
Science Press
Ultra-high-strength Al-Zn-Mg-Cu series alloy (7xxx series aluminum alloy) has become a very popular aluminum alloy series due to its advantages of high strength, high hardness, light mass, easy processing and excellent thermal conduc-tivity. It is widely used in aerospace industry and military fields. Therefore, Al-Zn-Mg-Cu series aluminum alloy occupies an important position in the fields of national defense, military, and people’s daily lif
In order to improve the comprehensive performance of the Al-Zn-Mg-Cu series alloy, many material workers have added different trace elements to the 7xxx series aluminum alloy. Among them, Er element has gradually entered the material workers’ vision due to its relatively cheap price and good micro-alloying effect. Wang et a
For Al-Zn-Mg-Cu series alloys, the use of products should go through the processes of ingot casting, homogenization heat treatment, heat deformation processing, solid solution and aging heat treatment. In different process stages, the microstructure of alloy and morphology of the second phase are different, which may have different effects on the next process and the performance of the final product. Mehdi Malekan et a
The second phases precipitated in Al-Zn-Mg-Cu alloys are generally the products of eutectic reaction, such as T phase (AlZnMgCu phase), θ phase (Al2Cu phase
Four Al-Zn-Mg-Cu-Zr alloys containing Er elements with different mass fractions were used in the experiment. The nominal chemical compositions of alloys were Al-9.0Zn-2.1Mg-2.1Cu-0.13Zr-xEr (x=0, 0.1, 0.2, 0.5, wt%), named as alloy 1, alloy 2, alloy 3, and alloy 4, respectively. The raw materials were aluminum blocks, zinc sections, magnesium blocks with a purity of 99.99%, Al-50Cu intermediate alloys, Al-10Zr intermediate alloys and Al-20Er intermediate alloys, which were repeatedly smelted in a vacuum arc furnace for 4 times and then cast into a master alloy ingot with a mass of about 700 g. Each alloy was cut by wire cutting to obtain two sample rods with a diameter of 12 mm and a length of 100 mm. After removing the surface oxide scale, they were ultrasonically cleaned with acetone, then dried, and filled into an Al2O3 ceramic tube with an inner diameter of 12 mm and a length of 210 mm. The Bridgman type liquid metal cooling directional solidification furnace was evacuated to 6.0×1
The directional solidification rod was split in the middle along the drawing direction, one half was used to observe the cross-sectional structure, and the other half was used to observe the longitudinal cross-sectional structure of different solidification positions. Carl Zeiss metallographic microscope, JXA-8100 type electron probe micro-analysis (EPMA) and attached INCA energy spectrometer (EDS) were used to investigate the microstructure and phase composition of the alloys, and the hardness of the alloys was measured by FM800 micro-Vickers hardness tester. Image Pro Plus 6.0 professional image analysis software was was to count the size and area fraction of microstructure. The corrosive agent was Keller reagent (95 mL H2O+2.5 mL HNO3+1.5 mL HCl+1.0 mL HF) and Weck reagent (100 mL H2O+5 g KMnO4+1 g NaOH).
Fig.1 Cross section (a~d) and longitudinal section (e~h) microstructures and color metallographys (i~l) of alloy 1 (a, e, i), alloy 2 (b, f, j), alloy 3 (c, g, k) and alloy4 (d, h, l)
Fig.3 BSE images of longitudinal section of alloys: (a) alloy 1, (b) alloy 2, (c) alloy 3, and (d) alloy 4
After adding Er, the morphology of second phase changes obviously. Among them, the boundary of second phase in alloy 2 and alloy 3 is gradually rounded, and the content of second phase in the circular shape is greatly increased compared to alloy 1; the boundary of second phase in alloy 4 is sharper, and there are mainly irregular stripe-shaped second phases. The ratios of round-shape second phase in the alloys are shown in Fig.4. The ratio of the round-shape second phase in alloy 3 is the highest, 68.79%, which is 59.79% higher than 43.05% of alloy 1; alloy 4 has the lowest round-shape second phase ratio of 21.67%, which is 49.66% lower than that of alloy 1.
The hardness of alloys with different Er contents was measured, and the hardness values of the cross section and the longitudinal section of the alloy were also measured. The measurement results are shown in Fig.5. As the Er content increases, the hardness value of the alloy gradually increases; the hardness value of the cross section increases from 1180 MPa to 1420 MPa, and that of the longitudinal section increases from 1380 MPa to 1540 MPa. The primary dendrite arm spacing and secondary dendrite arm spacing of the alloy decrease with the increase of the addition amount of Er elemen
Al8Cu4Er phase is precipitated in the alloy with adding Er element. The formation law of the alloy phase depends on the interaction between the atoms in the alloy, and all the interactions between the atoms can be attributed to the interaction between the electrons. Among them, the theory of the interaction strength between the alloy elements can be used to judge the formation of the alloy phase la
The theoretical formula of the interaction strength between two alloying elements is:
| (1) |
where rA and rB are the atomic radius of solvent A and solute B, respectively; NA and NB are the electronegativity of solvent A and solute B, respectively.
According to the theory of interaction strength between alloy elements, the interaction strength of Er with Cu, Al, Zn, Mg, and Zr is 8.23, 5.39, 4.73, 2.30, and 1.63, respectively, which gradually decreases. The interaction strength of Er-Cu is the highest, indicating that the addition of Er will inhibit the solid solution of Cu, reduce the solid solubility of Cu in the α-Al matrix, and increase the tendency of forming compounds. Therefore, many Er- and Cu-rich second phases are found in the second phase of the alloy with Er addition. It is observed that the interaction strength of Er-Mg and Er-Zr is small, indicating that it is not easy to form a compound between Er and these two elements, which will promote the solid solution of these two elements.
In addition, the interaction strength of Er-Zn is 4.73, indicating that Er-Zn compound may form. However, due to the small amount of Er addition, this process is inhibited. Al8Cu4Er contains a small amount of Zn. Therefore, EDS results of phase B1 in Section 2.1 are explained.
The two main second phases generated in all alloys are Al8Cu4Er phase and T phase, with melting points of about 575 and 473 ℃, respectively. During the solidification process, as the temperature decreases, the Al8Cu4Er phase and the T phase are sequentially formed. In alloy 1, the main second phase is the T phase. In alloy 2, alloy 3, and alloy 4, due to the addition of the Er, the Al8Cu4Er phase is preferentially formed in the alloys, and more Cu participates in the reaction process with Er element, so the Cu contents of T phase in these alloys are reduced. According to the analysis result of Fig.4, the second phase morphology in alloy 1 is mainly round or irregular strips, and the content of round-shape second phase is 43.05%. After the addition of Er element, the ratio of round-shape second phase in the alloy changes significantly, and the ratio of round-shape second phase in alloy 3 is the largest, reaching 68.79%. The BSE images of the irregular strip-like second phase in alloy 1 and the round-shape second phase in alloy 3 are shown in Fig.6a and 6b, respectively and the Cu content at each position is shown in Fig.6c. It is found that the Cu content at different positions of the T phase in alloy 1 changes greatly, and the Cu content in the round shape of alloy 3 does not change much, indicating that there is segregation of Cu element in the bright white of the irregular second phase in alloy 1. At the second phase boundary, a sharper phase boundary is formed. When the homogenization process cannot completely eliminate the second phase, the remaining sharp phase boundary often causes inconvenience in the subsequent processing of the alloy and affects the final performance of the product. By adding an appropriate amount of Er, the Al8Cu4Er phase formed in the alloy will reduce the Cu element content in the T phase, which plays a role of rounding the T phase boundary, then forming a large number of round T phases.
The morphology of the second phase of alloys with different compositions changes significantly with the addition of Er, and the morphology of the generated round T phases is also different. The round T phases of the four alloys with different compositions are shown in
Fig.7 T phase morphologies of alloys with different compositions: (a) alloy 1, (b) alloy 2, (c) alloy 3, and (d) alloy 4
The Al8Cu4Er phase (bright white part in the second phase) precipitated in alloys with different Er element contents is shown in
As the Er element is continuously added, the decrease in the degree of supercooling caused by the continuous decrease in the Cu content dominates, so the interlamellar spacing of the round second phase in alloy 3 and alloy 4 increases.
Fig.9 Al8Cu4Er phase morphologies in alloys with different Er contents: (a, d, g) alloy 2, (b, e, h) alloy 3, and (c, f, i) alloy 4
The formation of the T phase (AlZnMgCu) is as follows, according to the non-equilibrium eutectic reaction formula:
| L→α-Al+T(AlZnMgCu) | (2) |
The two products produced by the reaction are the α-Al phase and the AlZnMgCu phase. As the reaction progresses, the two phases nucleate and grow alternately. During the participating of four elements in the reaction, the diffusion rate of Cu is the slowest and has the greatest influence on the whole reaction. When the α-Al phase nucleates, excess Cu is discharged from the crystal since the Cu content in the α-Al phase is less than that in the liquid phase, and the Cu content near the reaction interface increases, which promotes the AlZnMgCu phase to grow. The α-Al and AlZnMgCu phases alternately nucleate and grow, showing a network layered structure macroscopically. Because the diffusion rate of Cu element is slow, it is easy to segregate at the T phase boundary and easier to form a sharp phase boundary. When Er element is added, Cu element participates in the reaction of Er element. Therefore, in the reaction of forming T phase, the content of Cu element decreases accordingly. After the α-Al phase is formed, the Cu element discharged from the crystal to the liquid phase in reaction interface decreases. The ability to promote the formation of AlZnMgCu phase is reduced, and the Cu element is not easy to segregate. In order to reduce the free energy of the system, the macroscopic appearance is a round second phase, and there is a certain network layer structure inside.
After adding 0.5wt% Er element, the content of the second phase in the round shape is the smallest among the four alloys. This is because as the Er content increases, the number of Al8Cu4Er phases generated increases accordingly, which act as nuclei. The site makes the AlZnMgCu phase nucleate and grow around the Al8Cu4Er phase, and the macroscopic appearance is an irregular second phase shape.
1) In Al-Zn-Mg-Cu-Zr-xEr (x=0, 0.1, 0.2, 0.5, wt%) alloys, with the increase of Er content, the dendrite spacing becomes smaller while the number of dendrites increases. After adding 0.5wt% Er element, continuous dendrites are formed in the alloy.
2) Adding an appropriate amount of Er element can effectively reduce the precipitation degree of the second phase in the alloy, and can change the morphology of the second phase, so that the content of the second phase with round shape increases. However, when the addition amount is too large, the degree of precipitation of the second phase in the alloy is greatly increased, and the content of the round second phase is greatly reduced. The addition of Er element can improve the hardness of the alloy.
3) The Al8Cu4Er phase is formed in the alloy added with Er element, which reduces the Cu content in the T phase (AlZnMgCu), and changes the morphology of the T phase; and the internal structure changes accordingly.
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