Effect of yttrium content on microstructure and tensile properties of as-cast and as-solutionized Al-Zn-Mg-Cu alloys

Four kinds of 7xxx series aluminum alloys with different Y elementcontents were obtained by ordinary gravity casting. The effect of Y content on the microstructure and mechanical properties of as-cast and as-solutionized Al-7.9Zn-3Mg-2.4Cu-0.13Zr (wt%) alloys were investigated by means of x-ray diffraction (XRD), electron backscattering diffraction (EBSD), scanning electron microscopy (SEM), energy dispersive spectrometer (EDS) and room-temperature tensile tests. The results shows that, in as-cast condition, Y element can refine the grain and reduce the content of Mg(Zn, Cu, Al)2 lamellar phases at the interdendritic. (Al, Zn)8Cu4Y block-shaped phases form in interdendritic regions. After solution treatment, the undissolved Mg(Zn, Cu, Al)2 phases evolved from lamellar to bulk-like which distribution in interdendritic, but no obvious change in (Al, Zn)8Cu4Y phase. The tensile testing results shows that the optimal yttrium content is 0.45 wt%. At 0.45 wt% Y, the ultimate tensile strength and elongation are 267 MPa and 2.4% in as-cast condition and 420 MPa and 3.6% in as-solutionized condition.


Introduction
In recent years, with the rapid development of aerospace and high-speed railway trains, the comprehensive properties of Al-Zn-Mg-Cu alloys have been continuously improved in the existing researches [1][2][3]. Among them, adding rare Earth elements such as Sc, Er and Y to the alloy have been validated previously as an excellent means for the increase in performance of the alloy. For example, adding trace amounts of Er and Y elements can not only refine the grains of cast aluminum alloy [4][5][6], but also improve the recrystallization resistance of the alloy during hot working, and form fine and uniform Al 3 Er and Al 8 Cu 4 Y particles, which helps the alloy to form a stable and fine sub-crystal structure during deformation processing [7], thereout, improve the strength of the alloy.
Some studies have shown that adding Y element to Al-Cu alloy forms the Al 8 Cu 4 Y phase with good thermal stability at the grain boundary, and this phase will strengthen the grain boundary at high temperature [8]. Moreover, Li et al [9]. designed and optimized the composition of Al, Zn, Mg, Cu, Ti, Y and Ce elements through machine learning, obtaining aluminum alloy containing trace yttrium with tensile strength of more than 900 MPa, and discovered Al 8 Cu 4 Y phase. However, a large number of precipitates in artificial aging is one of the main reasons for improving the strength of the aluminum alloy, but Y element and Y-containing phase also greatly affect the microstructure and properties of the alloy. In order to study the effect of Y element and Y-containing phase on the microstructure and mechanical properties of Al-Zn-Mg-Cu alloy before precipitation, this article studies the grain size, mechanical properties and fracture mechanism of four Al-Zn-Mg-Cu alloys with yttrium content gradient, and discusses the influence of yttrium content on the microstructure and mechanical properties of as-cast and as-solutionized Al-7.9Zn-3Mg-2.4Cu-0.13Zr (wt%) alloys. Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI.

Materials and methods
This paper designed four kinds of Al-7.9Zn-3Mg-2.4Cu-0.13Zr-(x) Y (x = 0; 0.15; 0.45; 0.75) alloys. They were prepared using high-purity Al, pure Mg and pure Zn ingots and Al-50 wt% Cu, Al-20 wt% Zr and Al-10 wt% Y master alloys under normal gravity casting conditions. The burning loss rate of each element verified by a large number of experiments has been considered. All raw materials were melted in a resistance furnace and poured into an iron mold (with an inner dimension of Φ35 mm × 100 mm) at ∼720°C. Solid solution samples are obtained by room temperature water quenching after holding at 465°C for 24 h in an air furnace. Square samples (6 mm × 6 mm × 6 mm) for characterization were derived from the bottom of the cast billet. The sheet tensile sample was sampled in the middle of the ingot parallel to the bottom. The width at the gauge distance of the sheet tensile sample is 2.3 mm and the thickness is 2 mm.
The chemical compositions of the alloys were examined by an inductively coupled plasm-optical emission spectrometer (ICP-OES). Four alloys with chemical compositions are shown in table 1. Microstructural examination was conducted on a field-emission scanning electron microscope (SEM, JSM-7800F) coupled with an energy dispersive x-ray spectroscopy (EDS) detector. Electron backscatter diffraction (EBSD) samples were mechanically polished with 50 nm silica polishing solution and cleaned by ultrasonic oscillation. The step size and acceleration voltage used for EBSD measurements were 2.3 μm and 20 kV, respectively. The successful rates of the EBSD specimens were above 80%. The EBSD data were analyzed by HKL Channel 5 software. The grain size and dendrite arm distance were measured by Image-Pro-Plus software.

Microstructure
As-cast microstructure pictures of four alloys are shown in figure 1. The average grain size of 0Y, 0.15Y, 0.45Y, and 0.75Y alloys was evaluated using EBSD analyses in figure 1(a)-(d) respectively. It can be noticed from figure 1 that adding Y element can refine the grain size. When the content of Y reach to 0.45wt%, the grain size decreased to a minimum. This may because the effective Y particles act as nucleation sites are reach saturation when the yttrium content is meeting 0.45 wt%, so the grain refinement effect of 0.15Y and 0.75Y alloys is not as strong as that of 0.45Y alloy [10]. The grain size of 0.45Y alloy is the most uniform among the four alloys. The uniform size grains have better coordinated during deformation, which is beneficial to the properties of the alloy [11].
The as-cast four alloys XRD spectrums are shown in figure 2. The main phase of the four as-cast alloys is MgZn 2 . Al 8 Cu 4 Y phases are detected in 0.15Y, 0.45Y and 0.75Y alloys. The scanning electron micrographs of ascast and as-solutionized 0Y, 0.15Y, 0.45Y and 0.75Y alloys are shown in figure 3, and the corresponding EDS analysis results are listed in table 2. The EDS results show that the shiny white particles at the interdendrites of the alloy with Y element added in figure 3 are (Al, Zn) 8 Cu 4 Y and the gray phase is Mg(Zn, Cu, Al) 2 . The microstructure of the four as-cast alloys is dendrite structure. According to the as-cast structure pictures of the four alloys in figure 3, when the content of Y increases from 0 wt% to 0.45 wt%, the second phases of as-cast alloy become more uniform, but 0.75Y alloy somewhat anomalously. This may be the increase of yttrium content shortens the solidification temperature range of aluminum alloy [12]. The intergranular phases of as-cast 0Y alloy is almost Mg(Zn, Cu, Al) 2 lamellar phases, the increased Y element forms bulk-like (Al, Zn) 8 Cu 4 Y phases at interdendrites. Adding Y element to Al-7.9Zn-3Mg-2.4Cu-0.13Zr alloy can refine the grain, but some coarse (Al, Zn) 8 Cu 4 Y phases are also introduced, which will affect the mechanical properties of the alloy [13]. The second phase particles are smaller, or even reaches the nanometer level, and more disperse distribute in matrix, have better effect of hindering dislocation movement, this is beneficial to improving the final properties of the alloy [14].
According to the microstructure pictures of as-solutionized alloys in figure 3 and EDS results, the undissolved spheroidized Mg(Zn, Cu, Al) 2 phases are distribute in the matrix of as-solutionized 0Y alloy. After solid solution, for comparison, the second phase content of the four alloys is decrease after solid solution. With

Tensile properties
The ultimate tensile strength (UTS) and elongation (EL) of the four alloys in as-cast and as-solutionized are given in figure 4. With increased yttrium concentration, the UTS of the as-cast alloys are first increases and then decreases, and the EL demonstrate the same trend. The peak values of UTS achieve 267 MPa. After the addition of yttrium, the UTS and EL become higher than the original state, which is mainly attributed to the second phase strengthening. Otherwise, the smaller dendrite grain size also controls the mechanical properties. After solid solution, the UTS and EL of the alloys increase firstly and then decrease with the increase of yttrium content, reach up to a maximum when yttrium content is 0.45 wt% (UTS = 420 MPa, EL = 3.6%). The  0.75Y alloy shows UTS is superior to the 0.15Y alloy, but with lower elongation properties. This may be due to the Y element which has a larger atomic size than other elements, and its high amount of addition in 0.75Y alloy may lead to larger lattice distortion. For this large distortion energies, the 0.75Y alloy has a large barrier which will lead the hindrance of the dislocation movement. However, the coarse and agglomerated (Al, Zn) 8 Cu 4 Y phases in the 0.75Y alloy decrease the EL. The as-cast samples after solution treated, the intergranular secondary phases are dissolved into the matrix, so the alloys elongation increases, but the influence of casting defects [15], the improvement is limited. Figure 5 shows the SEM images of tensile fracture surfaces of 0Y, 0.15Y, 0.45Y and 0.75Y alloys in as-cast and as-solutionized. The EDS inspection ( figure 5(c) point G) showed that the (Al, Zn) 8 Cu 4 Y phase particles are inside the dimple, and the Mg(Zn, Cu, Al) 2 phases are distribute on the shear platforms near the dimples. By comparing the fracture surface of as-cast four alloys, it can be seen that there are no dimples and less steps on the cleavage of 0Y alloy. However, some dimples were found in the fracture structure of the Y-containing alloys, which indicates that Y element can improve the plasticity of the alloy and this result is consistent with the mechanical properties [16]. The microstructure and mechanical behavior of the Al-Zn-Mg-Cu alloys are largely affected by the second phase particles [17,18]. It is known from the SEM observation in figure 3(b), (d), (f) and (h) that (Al, Zn) 8 Cu 4 Y phase particles become coarser with the increase of yttrium content. The second phase particles are more coarser, the fracture or debonding will take place before the macro cracks appear, which is one of the reasons for the decline of the properties of 0.75Y alloys.   Comparing the fracture surfaces of the four as-solutionized alloys in figure 5 and combining with EDS results, the second phase particles in the fracture dimples of 0Y alloy are Mg(Zn, Cu, Al) 2 phases. Most of the second phases in the fracture dimples of 0.15Y, 0.45Y and 0.75Y alloys are (Al, Zn) 8 Cu 4 Y phases, and the size of dimples can properly reflect the toughness of the alloy [19]. The dimples of 0.45Y alloy are more obvious than those of 0.15Y and 0.75Y alloys, and the size is more uniform.

Future outlook
The microstructure examination shows that the base alloy has lamellar Mg(Zn, Cu, Al) 2 phases in as-cast condition ( figure 3(b), (d), (f) and (h)) and second phases distributed along the grain boundary in assolutionized condition (figure 3(j), (i), (n) and (p)). Previous studies have shown that the distribution of coarse brittle particles along the grain boundary will have a negative impact on the properties [20]. Therefore, subsequent plastic deformation of the alloy, such as extrusion and rolling, will break the second phases and make it uniform distribute in the matrix. In this process we will analyze the effect of Y element on the grains and second phases of base alloy after plastic deformation and we will calculate and simulate the bonding strength between (Al, Zn) 8 Cu 4 Y phases and the matrix, such as this article [21]. As Al-Zn-Mg-Cu alloy is a solution strengthening alloy, we will also study the process of solution and artificial aging after plastic deformation.

Conclusions
The effect of yttrium content on microstructure and tensile properties of Al-7.9Zn-3Mg-2.4Cu-0.13Zr alloy (the base alloy) was investigated systematically.
1. Adding Y element to the base alloy will introduce a new second phase ((Al, Zn) 8 Cu 4 Y) in the intergranular and reduce the Mg(Zn, Cu, Al) 2 lamellar phases at interdendrite. With the increase of Y element, the as-cast grain size of the base alloy increases first then subsequently decreases, and reaches the minimum when Y content is 0.45 wt%.
2. In as-cast condition, with the increase of yttrium content, the content of (Al, Zn) 8 Cu 4 Y phases in interdendrite increases. In as-solutionized condition, the Mg(Zn, Cu, Al) 2 lamellar phases at the interdendrite partially are dissolved into the matrix, and the remaining phases existed in the interdendrite as granule-like. Except that the volume fraction of (Al, Zn) 8 Cu 4 Y phases changes with the adjustment of Y element content, no significant change was recorded.
3. Whether in as-cast or as-solutionized condition, the UTS and EL of the base alloy increase first and then decrease slightly with the increase of yttrium content, and both reach the highest when yttrium content was 0.45 wt%. The 0.45Y alloy as-cast UTS = 267 MPa; EL = 2.4%, and as-solutionized UTS = 420 MPa; EL = 3.6%.
4. Adding Y element to the base alloy to introduce a new second phase brings about the second phase strengthening. As a rare Earth element, yttrium has a small solubility and a large atomic radius in Al matrix, which provides more nucleation points during solidification, thus refining dendrite grains. Part of Y element dissolves into the matrix and causes lattice distortion, which hinders dislocation movement, and ultimately improves mechanical properties.