Special Issue on Aluminium and Its Alloys for Zero Carbon Society, ICAA 18
Effects of Sc and Zr Addition on the Mechanical Properties of 7000 Series Aluminum Alloys
2023 Volume 64 Issue 2 Pages 443-447
Details
2023 Volume 64 Issue 2 Pages 443-447
Scandium addition to aluminum alloys has been evaluated at various research institutions, and it is known that the Al3Sc precipitates effectively increase their strengths. In this study, the effect of Sc addition on the strengths of two types of 7000 series aluminum alloys was investigated. As a result, the strength increased by the Sc addition to both types of alloys, but the increased amounts were limited to 10–40 MPa. It was considered that because the strengthening effect by the η′ phase was sufficiently high, the precipitation strengthening by the dispersion of Al3(Sc1−xZrx) particles was relatively low in these alloys.
In the 7000 series aluminum alloys, because the strengthening effect by the η′ phase is sufficiently high, the precipitation strengthening by the dispersion of Al3(Sc1−xZrx) particles was relatively low.
There are two types of 7000 series aluminum alloys, i.e., the Al–Zn–Mg alloys, and the Al–Zn–Mg–Cu alloys. The Al–Zn–Mg–Cu alloys have a higher strength than the Al–Zn–Mg alloys and are used for aircraft, sports equipment, etc. The Al–Zn–Mg alloys contain little Cu, so they have improved weldability, extrusion workability, and corrosion resistance. The Al–Zn–Mg alloys are used as structural materials for railway vehicles, motorcycle frames, etc.
Transition elements, such as Mn, Cr, and Zr, are often added to the 7000 series aluminum alloys to control the grain size and prevent recrystallization. In addition, the quench sensitivity of Zr added alloys is lower than it of Mn or Cr added alloys. When Zr is added to the 7000 series aluminum alloys, Al3Zr particles form and prevent recrystallization during hot working.1)
Scandium also has the same effect of the preventing recrystallization. Fine Al3Sc particles are precipitated, and have the effect of preventing recrystallization.2) When both Zr and Sc are added to pure aluminum, Al3Sc particles preferentially form because the Sc diffusion rate in Al is higher than the Zr diffusion rate. Al3(Sc1−xZrx) phase form on the surface of the Al3Sc particles and a core-shell structure form.3)
Al3(Sc1−xZrx) particles are less likely to coarsen during heat treatment, and it is considered that the combined addition of Sc and Zr has a greater effect on suppressing recrystallization than the case of only adding Zr. Furthermore, the combined addition of Sc and Zr increases the precipitation number density, and the dispersion strengthening enhances the strength of the alloy.4) In one example of the Sc addition to pure aluminum, the 0.2% yield strength was improved by about 200 MPa.5)
For transportation equipment, such as aircraft and motorcycles, the strength increase of the aluminum alloys is required. Since Sc is one of the most effective precipitation hardening elements in aluminum alloys, the strength increase of the 7000 series aluminum alloy by adding Sc is expected. In this study, the effect of Sc addition on the strength of the 7000 series aluminum alloys was investigated.
The base alloys were alloy A (Al–10%Zn–2.6%Mg–1.5%Cu–0.12%Zr, mass%) and alloy B (Al–4.7%Zn–1.1%Mg–0.10%Cu–0.15%Zr, mass%), and the chemical compositions are shown in Table 1. Alloy A was developed for aircraft structural components6) and alloy B is widely used for motorcycle structures. Scandium of 0.1 mass% was added to both alloys (alloys A+Sc and B+Sc). These alloys were prepared by direct chill casting to give a billet of 90 mm diameter. The billets were homogenized under several conditions. After homogenization, the billets were extruded into rectangular bars that had a 2 mm thickness and 35 mm width. The manufacturing processes are shown in Table 2 and the details are listed below.
For alloys A and A+Sc, the condition of homogenization treatment was decided to be 400°C for 10 h according to the results of previous experiments. After extrusion, solution heat treatments were carried out. Because the Al3(Sc1−xZrx) precipitates might become coarser at the higher temperatures, the four conditions of solution heat treatments in Table 2 were completed to find the best condition. Following the solution heat treatments, artificial aging was conducted at 120°C for 24 h.
For alloys B and B+Sc, a press-quench process was adopted instead of a solution heat treatment. Because solution heat treatment is not carried out, the condition of the precipitates after the homogenization treatment affects the mechanical properties of the extruded samples. Then, the three conditions of homogenization treatments in Table 2 were carried out to determine the optimum conditions. After air cooling at the exit of the extrusion press, artificial aging at 120°C for 24 h was carried out.
After the artificial aging, tensile tests and microstructure observations were conducted. The direction of the tensile test was parallel to the extrusion direction. The tensile test specimens were formed into the JIS No. 5 shape whose width was 25 mm in the parallel portion and the gauge length was 50 mm. Microstructure observations by an optical microscope of the L-ST section and by a transmission electron microscope (TEM) of the L-LT section were carried out. Microstructure observations by an optical microscope was conducted after electrolytic etching.
In Fig. 1, the optical microscope images of alloy A and alloy A+Sc are shown. By adding Zr, the microstructures of alloy A showed a fibrous structure regardless of the solution heat treatment conditions. Alloy A+Sc also showed fibrous structures.
Microstructures of alloy A and A+Sc.
Figure 2 shows the optical microscope images of alloy B and alloy B+Sc. Usually, in the polarized microstructure of the fibrous structure, the color contrast between the crystal grains is strong and the boundaries can be clearly seen. However, when recovery occurs, the contrast is weak and the boundaries are obscured. Alloy B showed almost fibrous structures by adding Zr, but signs of partial recovery or recrystallization appeared. On the other hand, alloy B+Sc, which had both Sc and Zr added, had no signs of recovery or recrystallization, and the fibrous structure was maintained under all conditions. The specimen subjected to the homogenization treatment at 400°C had a particularly stable fibrous structure.
Microstructures of alloy B and B+Sc.
Figure 3 shows TEM images after the artificial aging. Alloy A+Sc of the solution treatment at 470°C for 1 h and alloy B+Sc of the homogenization treatment at 450°C for 8 h were observed because the ultimate tensile strengths were the highest as mentioned below. The Al3(Sc1−xZrx) particles have been observed in both alloys, and their sizes are 15–30 nm for alloy A+Sc and 5–20 nm for alloy B+Sc. It was confirmed that these particles contained Sc and Zr by using EDS. The EDS images of alloy B+Sc are shown in Fig. 4. The very fine black particles of 2 nm or less observed in Fig. 3 are suppose to be the η′ phase,7) and they are more finely and densely dispersed than the Al3(Sc1−xZrx).
Bright field TEM images of samples A+Sc and B+Sc. Arrows show Al3(Sc1−xZrx) particles.
EDS images of alloy B+Sc. Arrows show Al3(Sc1−xZrx) particles.
Figure 5 shows the mechanical properties of alloy A and alloy A+Sc. The ultimate tensile strengths and the 0.2% yield strengths became higher when the solution treatment temperature was higher and the solution heat treatment time was longer. The highest strength was then obtained by the solution heat treatment at 470°C for 1 h. By adding Sc, the ultimate tensile strengths were improved by 10–20 MPa and the 0.2% yield strengths were improved by 18–40 MPa.
Mechanical properties of alloy A and alloy A+Sc.
As shown in the TEM image of Fig. 3, the size of the Al3(Sc1−xZrx) particles was 15–30 nm after the solution heat treatment and the artificial aging. Because the Al3(Sc1−xZrx) particles were fine enough, the ultimate tensile strengths and the 0.2% yield strengths of the samples subjected to the solution treatment at 470°C were improved by adding Sc.
In the sample of which Sc was added, the improved amount of the 0.2% yield strength was low even if the solution treatment time was longer. Sc may have an effect on the precipitation behavior of the η′ phase, but the mechanism is needed to be addressed in future studies.
The mechanical properties of alloy B and alloy B+Sc are shown in Fig. 6. Regardless of the homogenization conditions, by adding Sc, the ultimate tensile strengths were improved by 20–28 MPa and the 0.2% yield strengths were improved by 34–38 MPa.
Mechanical properties of alloy B and alloy B+Sc.
The increasing strength of the alloy B+Sc was greater than that of the alloy A+Sc. It is considered that because alloy B+Sc has a lower amount of Zn, Mg, and Cu than the alloy A+Sc, the alloy has fewer precipitates, such as the η′ phase, after artificial aging. Therefore, precipitation strengthening by dispersion of the Al3(Sc1−xZrx) particles is considered to be more effective.
Under the conditions in which the highest strength was obtained after extrusion and final tempering, the amounts of the strength increase were 10 MPa in the ultimate tensile strength and 18 MPa in the 0.2% yield stress for alloy A, and they were 20 MPa in the ultimate tensile strength and 34 MPa in the 0.2% yield stress for alloy B. The effect of increasing the strength by adding Sc was not high for these 7000 series aluminum alloys. As described in the introduction, it was reported that the strength increased by 200 MPa when Sc was added to pure aluminum. Therefore, the results of these 7000 series aluminum alloys were extremely different from the result for pure aluminum.
The relationship between the number density of the precipitates and the increasing amount of the 0.2% yield strength is schematically shown in Fig. 7. For simplification, it was assumed that the strengthening mechanism is the orowan mechanism and the strengthening effect of the η′ phase is equal to the effect of the Al3(Sc1−xZrx) precipitates. Strictly speaking, the size and the number density of the η′ phase and the Al3(Sc1−xZrx) particles must be taken into consideration, but it is difficult to analyze the image from the current TEM observation results. Therefore, in this paper, the estimation was performed very simply by the number of the η′ phase and the Al3(Sc1−xZrx) particles.
Schematic diagram of relationship between number density of the precipitates and increase in the yield strength.
The orowan stress is proportional to $\sqrt{\text{F}_{\text{v}}} $ (number density of precipitates). Therefore, when the number density of the Al3(Sc1−xZrx) precipitates increases in a similar way, the effect of strengthening by the Al3(Sc1−xZrx) precipitates is high if there is no other precipitation phase that contributes to the strength. However, the degree of the increasing strength due to the Al3(Sc1−xZrx) precipitate is relatively low in the presence of the η′ phase. According to Fig. 3, the η′ phases precipitation was very fine with a high density. Therefore, it is considered that the contribution of the η′ phase to the strength was much higher than that of the Al3(Sc1−xZrx) phase, and the degree of the increasing strength due to the addition of Sc became low. Moreover, since the number density of the η′ phase of alloy B is less than that of alloy A, it is presumed that the increase in the strength due to the addition of Sc is higher in the case of alloy B.
The effect of strengthening by the Sc addition is low for age hardened alloys. On the other hand, in solid solution strengthening alloys, the effect of strengthening by the Sc addition is expected to be higher than for the age hardening alloys because of no strengthening precipitation phase.
The effect of Sc addition on the strength of two types of 7000 series aluminum alloys was investigated, and the following results were obtained.
