Relationship of particle stimulated nucleation, recrystallization and mechanical properties responding to Fe and Si contents in hot-extruded 7055 aluminum alloys

1. Introduction

7055 aluminum alloy is widely used in critical aircraft parts owing to its superior combination of high strength, fracture toughness and stress corrosion cracking resistance. The mechanical properties are remarkably affected by recrystallization, which is considered importantly during industrial processing for 7055 aluminum alloy [[1], [2], [3]]. Insoluble primary Fe-rich phases and Si-rich phases, and undissolved η [Mg(Al, Cu, Zn)₂]/S (Al₂CuMg) phases are usually coarse intermetallic particles remain in the final product of 7055 aluminum alloys [[4], [5], [6]]. In contrast to the sub-micrometer size of Cr-, Zr- or Mn-bearing particles to retard recrystallization, coarse particles (>1 μm) generally promote recrystallization during or after deformation via particle stimulated nucleation (PSN) [[7], [8], [9], [10], [11]]. PSN is one of the main mechanisms of recrystallization in particle-containing aluminum alloys, which has a significant influence on the grain structure and texture, and then plays a role in the mechanical properties [3,[12], [13], [14]].

Fe and Si are the most common impurity elements in Al-Zn-Mg-Cu alloys, which generally give rise to the formation of coarse intermetallic particles such as Al₇Cu₂Fe, Al₃Fe, α-AlFeSi, and Mg₂Si with sizes larger than 1 μm [15,16]. The volume fraction and size of Fe-rich phases and Si-rich phases are directly proportional to the Fe and Si contents. At the same time, the volume fraction and size of the other primary phases such as η and S in Al-Zn-Mg-Cu alloys decrease with Fe and Si contents increasing because the prior formation of Fe- rich phases and Si-rich phases consumes some amounts of Mg and Cu atoms simultaneously required in the η and S phases [17,18]. It has been not well clarified that the evolution of recrystallization and texture of Al-Zn-Mg-Cu alloys results from the inverse variation between Fe-rich/Si-rich phases and η/S phases responding to Fe and Si contents, which is important for regulating Fe and Si contents to control recrystallization and texture. To find this out, the PSN behavior of each kind of coarse particles in Al-Zn-Mg-Cu alloys need to be illustrated first.

PSN from the coarse intermetallic particles and its influence on grain structure, texture and mechanical properties in Al-Zn-Mg-Cu aluminum alloy have been reported in some researches [1,3,[19], [20], [21]]. But less attention has been paid on the influence of intermetallic particles with different chemical compositions, sizes, morphologies, distributions and so on. It is well known that the recrystallization nucleus of PSN originates from the particle deformation zone (PDZ) in the vicinity of non-deforming particles. The PDZ with a high dislocation density and a large orientation gradient is created during deformation due to the incompatible deformation between the non-deforming particles and the surrounding matrix. The high dislocation density and large orientation gradient in the PDZ provide the force for recrystallization nucleation and growth [22,23].

The influence of particle size on PSN recrystallization has been clearly illustrated. The size of PDZ has been proved to be proportional to the particle size [24]. The maximum misorientation in PDZ is close to the particle-matrix interface and decreases with particle size [24,25]. Therefore, the particle size should be larger than a critical value in order to achieve the maximum misorientation to generate high-angle grain boundary (HAGB) for PSN [26,27]. Furthermore, the number of PSN nuclei increases with increasing the size of PDZ as well as the particle size [27,28].

A few researchers reported the impact of particle number density and shape on the PDZ and PSN in aluminum alloys. A finite element modeling on a multi-particle case exhibits an overlap of strain fields around the individual particles, resulting in a higher localized strain in the PDZ compared to a single particle case [29]. The experiment results verified that with increasing the number density of particles, the number density of PSN grains increases and the grain structure transforms from plates to small equiaxed PSN grains [30]. The correlation between the particle shape and PSN is mainly investigated by simulation [22,29]. The particles with small aspect ratio account for minor strain gradient compared to the particles with larger aspect ratio [29]. Moreover, different shapes of the PDZ and strain states at each position in the PDZ depending on particle shape have a significant effect on the orientation of PSN grains [22]. An early experiment work by transmission electron microscopy (TEM) found that the plate-like θ′-Al₂Cu particles give rise to more pronounced lattice rotations than the globular Al₆Mn particles, because very high dislocation densities are accumulated at the tips of the plate-like θ′-Al₂Cu particles [25]. The experiment works about the effect of the particle number density and shape on PSN are still very few, which are needed to be enriched further. In particular, how the chemical composition of particles affects PSN is still unclarified.

In this paper, the PDZ size, the number and orientation of PSN grains upon several kinds of particles with different chemical compositions, sizes, shapes and distributions in hot-extruded 7055 aluminum alloy were investigated by electron back scattered diffraction (EBSD). And then the evolution of different coarse particles with Fe and Si contents impacting on recrystallization and mechanical properties was discussed.

2. Experimental

Two kinds of 7055 aluminum alloys with the same nominal composition of Al-8.3 wt% Zn-2.2 wt% Mg-2.1 wt% Cu-0.15 wt% Zr, but different Fe and Si contents were cast. The actual chemical compositions of the two ingots were determined by inductively coupled plasma (ICP) spectrometry as shown in Table 1. After homogenized at 450 °C for 24 h and then slowly heating to 465 °C for 24 h, the ingots with a diameter of 120 mm were extruded at 400 °C to the bars with a cross-section size of 18 mm × 34 mm. The extruded bars were solution treated at 475 °C for 1 h and quenched in water. Finally, a retrogression and re-aging heat treatment (RRA) at 120 °C for 24 h, 190 °C for 1 h and again 120 °C for 24 h was conducted.

The chemical compositions and morphology of coarse second phases in the two alloys were analyzed by scanning electron microscopy (SEM) and energy-dispersive X-ray spectrometry (EDS). The sizes, number densities and area fractions of coarse intermetallic particles in aged alloys were quantitatively measured over a number of SEM images acquired in several different regions using an image analysis software. An area of 0.7 mm (normal direction) × 12 mm (extrusion direction) was characterized by electron back scattered diffraction (EBSD) for each aged alloy. The specimens for EBSD were subjected to mechanical polish and then vibratory polish for 2 h. EBSD was performed on a FEI NOVA NanoSEM equipped with a field emission gun (FEG) at an operation voltage of 20 kV. The longitudinal section microstructures of two aged alloy and the surrounding microstructures of each kind of coarse intermetallic particles with different sizes in the two alloys were detected. The obtained patterns were post processed by CHANNEL 5-Oxford Instruments software. The grains structure and texture of two aged alloys, the PDZ size, the number and orientation of PSN grains upon each particle were quantitatively analyzed from EBSD data. PSN recrystallized grains are identified as being partly or fully surrounded by a high-angle boundary (>15°). The tensile properties paralleled to the extrusion direction were tested by a Zwick/Roell testing machine with the rod tensile specimens.

3. Results and discussion

3.1. Evolution of coarse intermetallic particles responding to Fe and Si

The microstructures and second phases of as-cast, homogenized and aged alloys were examined via backscattered electron (BSE) and EDS equipped in SEM as shown in Fig. 1. Several grayscales of coarse second phases can be observed in the two alloys. Combined with EDS, the bright white phases could be proved to be η [Mg(Al, Cu, Zn)₂] or S (Al₂CuMg), the light gray phases are Al₇Cu₂Fe, the dark gray phases are Al₃Fe and the dark phases are Mg₂Si in two alloys. The identification of the second phases with different gray scales in 7055 alloys has been analyzed in more detail in our previous work [31]. It is indicated that the coarse particles in aged alloys originate from the coarse primary phases of the solidification microstructures, which were insoluble or not completely dissolved under homogenization and solution treatments because of their high melting points, but crushed to smaller sizes during hot extrusion [15,31]. It is found that the S phases occur and the Fe-rich phases in alloy 2 transform from Al₃Fe to the dominant Al₇Cu₂Fe after homogenization. This is due to the lower diffusion velocity of Cu than Mg and Zn when η phases dissolve into the matrix during homogenization [17,32]. In the aged alloys, the grayscale and morphology of η and S are very similar, and in some cases, they are interlaced because of the transformation from η to S during homogenization. It is difficult to divide the η phase and S phase alone, so the two phases are studied as one kind of particle in this work. A part of Al₇Cu₂Fe is around Al₃Fe phase in alloy 2, which are also regarded as one particle of Al₇Cu₂Fe in the following study. The composition and shape of coarse intermetallic particles in the two aged 7055 alloys are summarized in Table 2.

The size and number density of coarse η/S, Al₇Cu₂Fe and Mg₂Si particles (>1 μm) are quantitatively measured over a number of BSE images using image analysis software. As shown in Fig. 2(a), with increasing Fe and Si contents, the average length (l) of η/S particles is almost unchanged which is about 2 μm, and the average length of Mg₂Si particles slightly increases from 2 μm to 3 μm, while the average length of Al₇Cu₂Fe particles largely decreases from 11 μm to 7 μm. The aspect ratios of the three kinds of particles all reduce as the Fe and Si contents increase shown in Fig. 2(b). The Al₇Cu₂Fe particles in alloy 1 have the biggest aspect ratio among all the particles, but the aspect ratios of Al₇Cu₂Fe particles dramatically decrease to be close to those of the η/S and Mg₂Si particles in alloy 2. The irregular η/S, Mg₂Si and Al₇Cu₂Fe particles have small aspect ratios between 3 to 4, while the aspect ratio of rod-like Al₇Cu₂Fe particles in alloy 1 is larger than 6.

The number densities with lengths of 1-5 μm, 5-10 μm, 10-20 μm and larger than 20 μm (>20 μm) of coarse η/S, Al₇Cu₂Fe and Mg₂Si particles are respectively counted as shown in Fig. 2(c) and (d). It can be observed that the number density of η/S decreases with the length increasing in each alloy. The number density of η/S is much higher in 1-5 μm than >5 μm, which results in the average length is smaller than 5 μm. The number density of η/S with length larger than 5 μm decreases with increasing Fe and Si contents. The number densities of η/S with lengths of 5-10 μm, 10-20 μm and >20 μm are 1.23 × 10² mm^-2, 33.1 mm^-2 and 3.9 mm^-2 respectively in alloy 1, which correspondingly decrease to 37 mm^-2, 3.5 mm^-2 and zero in alloy 2. In contrary, with Fe and Si contents increasing, the number densities of Al₇Cu₂Fe with lengths of 1-5 μm, 5-10 μm, 10-20 μm and >20 μm increase from 13.7 mm^-2, 19.6 mm^-2, 13.7 mm^-2 and 7.2 mm^-2 to 1.24 × 10² mm^-2, 1.24 × 10² mm^-2, 50.9 mm^-2 and 9.1 mm^-2 respectively, and the number densities of Mg₂Si with lengths of 1-5 μm, 5-10 μm, 10-20 μm and >20 μm increase from 67.1 mm^-2, 2.8 mm^-2, zero and zero to 1.25 × 10³ mm^-2, 78.6 mm^-2, 18.9 mm^-2 and 31.5 mm^-2 respectively.

3.2. Effects of different coarse particles on PSN in 7055 alloys

The numbers of coarse Al₇Cu₂Fe and Mg₂Si particles grow with the Fe and Si contents increasing, but the number of coarse η/S particles decreases and the morphology of Al₇Cu₂Fe particles transforms from rod-like to irregular. The PSN behavior of each kind of particles was clearly illustrated by EBSD, in order to exactly understand the evolution of recrystallization and texture related to PSN in hot-extruded 7055 aluminum alloys with the change of Fe and Si contents.

There are four kinds of coarse particles in the two aged 7055 alloys, namely the irregular η/S and Mg₂Si in alloys 1 and 2, the rod-like Al₇Cu₂Fe in alloy 1 and the irregular Al₇Cu₂Fe in alloy 2. The lengths of η/S particles in alloy 1 and Mg₂Si particles in alloy 2 are respectively distributed in 1-70 μm and 1-60 μm, which contain the length range of 1-5 μm mainly for the η/S in alloy 2 and Mg₂Si in alloy 1. In the following analyses, the PSN recrystallized behavior of η/S in alloy 1 and Mg₂Si in alloy 2 with the larger range of length will be presented.

3.2.1. PDZ and the degree of local non-uniform deformation induced by different coarse particles

The microstructures of longitudinal sections of the two aged 7055 alloys were surveyed by EBSD. The EBSD patterns colored with inverse pole figures (IPF) in extrusion direction (ED) are presented. In all EBSD micrographs shown in this article, low angle grain boundaries (LAGBs) with misorientations of 1.5°-15° are displayed with the white lines and high angle grain boundaries (HAGBs) with misorientations larger than 15° are drawn with the black lines. The local deformation zones (outlined with yellow dashed lines in Fig. 3) of each kind of particles extending from the particle-matrix interface to the neighboring matrix are evidently observed through the EBSD micrographs.

Each of the PDZs is filled with the fine near-equiaxed recrystallized grains and subgrains. Mostly, recrystallized grains in the PDZ occur close to the particle-matrix interface. Two main orientation for the matrix microstructures are <111 > //ED (extrusion direction) and <100 > //ED according to the IPF (inverse pole figure) coloring, which are commonly formed in extruded aluminum alloys because of the preferred orientation during extrusion [33,34]. The PSN recrystallized grains are often associated with the orientations largely deviated from <111 > //ED and <001 > //ED, but subgrains are close to the initial extrusion orientation of the matrix.

For the rod-like Al₇Cu₂Fe particles in alloy 1, the PSN recrystallized grains occur more frequently at the tips of the particles, while the subgrains are primarily present at the sides as shown in Fig. 3(b). However, a number of recrystallized grains could be also observed at the sides, when the length of rod-like Al₇Cu₂Fe particle is larger than 70 μm in Fig. 3(c). The PSN recrystallized grains usually exist in the vicinity of the particle surface for the irregular Al₇Cu₂Fe and Mg₂Si particles in alloy 2 (Fig. 3(d)) as well as the irregular η/S (Fig. 3(a)).

The area of every PSN recrystallized grain and subgrain in a PDZ was quantitively measured by CHANNEL 5-Oxford Instruments software, and the sum of that was given as the area of PDZ in this work. Fig. 4 shows the area of PDZ around the coarse intermetallic particles at different lengths in the two 7055 alloys. The area of PDZ is almost zero for a particle with the length less than 10 μm since no clear deformation zone, fine PSN recrystallized grains or subgrains could be observed through the EBSD micrographs. The area of PDZ for all particles rises after the particle length is larger than 10 μm. However, at the same length, the PDZ areas of the irregular Al₇Cu₂Fe and Mg₂Si particles with small aspect ratios in alloy 2 are similar and greater than that of the rod-like Al₇Cu₂Fe particles in alloy 1, and the PDZ area of the irregular η/S particle is the smallest.

The formation of PDZ and PSN depends on the strain and the particle size for a kind of particle. PSN recrystallization originates at a pre-existing subgrain within the PDZ. A necessary criterion for the formation of a nucleus is that the maximum misorientation (θ_max) within the PDZ must be sufficient to form a high angle grain boundary (HAGB) with the surrounding matrix substructure, i.e. θ_max >10°-15° [26]. The θ_max for a given strain decreases with decreasing particle size [34]. Therefore, a critical particle size (l_c) is required for PSN [13,26]. In terms of the statistical analysis from the EBSD micrographs shown in Fig. 4, l_c for the irregular η/S, rod-like Al₇Cu₂Fe, irregular Al₇Cu₂Fe and Mg₂Si particles in 7055 alloys in this work could be inferred as about 10 μm.

It has been identified that the orientation gradient exists in a PDZ that the misorientation decreases with increasing the distance from the matrix-particle interface [24,25]. It also turned out that at the tips of the plate-like θ′-Al₂Cu particles in aluminum alloys give rise to stronger lattice rotations resulting in the maximum misorientation [12,25]. Therefore, PSN recrystallized grains preferentially form at the tips of the rod-like Al₇Cu₂Fe, but at the particle surface for the other three types of particles with the small aspect ratios of irregular shape. Meanwhile, subgrains still remain in the outer regions of the PDZs as shown in Fig. 3.

According to the relationship between the orientation gradient and particle size, the size of PDZ (L, including the particle in the center) for a given kind of particle could be definitely established [24,27,35]:

L = f_zone⋅l (1)

where the particle size l ≥ l_c, f_zone is a factor related to the degree of deformation. The area of PDZ around each kind of particles increases with the particle length in Fig. 4, which is in accordance with Eq. (1). Whereas, the different PDZ areas for different particles at the same length and applied strain in our results imply that f_zone may be different for each kind of particles attributed to different chemical compositions or shapes of particles, which substantially influence the degree of local non-uniform deformation. The area of each PDZ counted in Fig. 4 could be obtained by the area from Eq. (1) subtracting the area of relevant particle. From Fig. 3, the outer contour of each PDZ could be approximately regarded as ellipse, the cross section of rod-like Al₇Cu₂Fe is rectangle and the cross section of the irregular η/S, Al₇Cu₂Fe and Mg₂Si can be simply supposed to be ellipse. Based on the above assumptions and Eq. (1), for the rod-like Al₇Cu₂Fe, the area of PDZ (A_PDZ) can be given as:

A_PDZ = $\frac{πf_{zone}^2 - 1}{4λ}·l^2$ (2)

and for the irregular η/S, Al₇Cu₂Fe and Mg₂Si, the area of PDZ (A_PDZ) is:

A_PDZ=$\frac{π(f_{zone}^2- 1)}{4λ}·l^2$ (3)

The data in Fig. 4 were respectively fitted by Eqs. (2) and (3) and the fitted curves are solid lines in Fig. 4. Thus, f_zone for each kind of particles could be obtained as shown in Table 3. The values of f_zone for the rod-like Al₇Cu₂Fe, irregular Al₇Cu₂Fe and Mg₂Si approach to 2, which agrees with the experimental results and simulation by Humphreys [24,25] that the size of the PDZ is approximately 2 times of the particle size. The variation of f_zone for the four kinds of particles indicates that the rod-like Al₇Cu₂Fe induces the most non-uniform local deformation, followed by the irregular Al₇Cu₂Fe and Mg₂Si, and the η/S is the least. The distinction of aspect ratios among the different particles contributes to the variation of f_zone [25,29]. Moreover, the f_zone of η/S particles is significantly smaller than that of the irregular Al₇Cu₂Fe and Mg₂Si in spite of the similar aspect ratios, which manifests that the difference of the chemical compositions between η/S and Al₇Cu₂Fe or Mg₂Si particles has great impact on the degree of local non-uniform deformation.

Although the rod-like Al₇Cu₂Fe has the largest f_zone, its PDZ area is smaller than that of the irregular Al₇Cu₂Fe and Mg₂Si. This is because the narrow width restricts the PDZ size. Consequently, the shape or aspect ratio of particles not only influences the degree of local non-uniform deformation, but also play a role on the PDZ area as shown in Eqs. (2) and (3).

For the same particle length, the larger number density of particles in alloy 2 is also a reason for the larger PDZ area around the irregular Al₇Cu₂Fe and Mg₂Si than the rod-like Al₇Cu₂Fe. The number densities of coarse Al₇Cu₂Fe and Mg₂Si particles in alloy 2 both increase resulting from the increasing of Fe and Si contents, which reduces the spacing between the coarse particles. The adjacent PDZs interact with each other due to the narrow spacing leads to the increase of the PDZ area (Fig. 3(d)). The degree of local non-uniform deformation and the PDZ area are crucial to determine the number and orientations of PSN recrystallized grains, which will be clarified in Sections 3.2.2 and 3.2.3.

3.2.2. Number and size of PSN grains stimulated by different coarse particles

The number and average diameter of recrystallized grains and subgrains in the PDZs were quantitively assessed for the η/S, rod-like Al₇Cu₂Fe, irregular Al₇Cu₂Fe and Mg₂Si particles with different lengths as shown in Fig. 5. The number of recrystallized grains for the irregular Al₇Cu₂Fe in alloy 2 is the largest and the second is that for the irregular Mg₂Si in alloy 2, while that for the irregular η/S and rod-like Al₇Cu₂Fe particles in alloy 1 is smaller. With increasing particle length, the number of recrystallized grains goes up for the rod-like Al₇Cu₂Fe, irregular Al₇Cu₂Fe and Mg₂Si particles, but changes not much for the irregular η/S particles. A number of recrystallized grains occur at the side of the bigger rod-like Al₇Cu₂Fe particle besides the tip as shown in Fig. 3(c), which causes the remarkedly increased number of recrystallized grains.

The average diameter of recrystallized grains decreases at first, and then tends to be constant after the particle length is larger than 20 μm for the rod-like Al₇Cu₂Fe, irregular Al₇Cu₂Fe and Mg₂Si particles as seen in Fig. 5(b). The average diameter of recrystallized grains for the irregular η/S particles is similar and about 4-5 μm when the particle length is in the range of 10-45 μm, but it obviously increases and becomes larger than that for the other three particles when the particle length is over 45 μm. The stable value of the average diameter of recrystallized grains is the smallest for the Mg₂Si particles, and they are similar for the rod-like Al₇Cu₂Fe particles and the irregular Al₇Cu₂Fe particles.

In Fig. 5(c) and (d), compared with the other three particles, the number of subgrains is the smallest and the average diameter is relatively larger in the PDZ of the η/S particles. In contrast to a smaller number of recrystallized grains for the rod-like Al₇Cu₂Fe particles than the irregular Al₇Cu₂Fe particles, the number of subgrains for the rod-like Al₇Cu₂Fe particles is larger. Moreover, the number of subgrains is greater than that of recrystallized grains in the PDZ of the rod-like Al₇Cu₂Fe particles.

PSN recrystallization is driven by the strain stored energy of high dislocation density and misorientation in PDZ when the particle size is larger than the critical size [36]. The number of PSN nuclei for one particle (N_PSN) can be given as [27]:

N_PSN = H0⋅A_PDZ (4)

where H₀ is the probability of nucleation per unit area of the PDZ. For a certain kind of particle, the number of PSN grains increases with the particle length in Fig. 5(a), which could be interpreted by Eq. (4) because of the increase of A_PDZ. But the N_PSN among the different particles change inconsistently with the A_PDZ, which indicates that the chemical compositions and shape of the particles affect the H₀. Substituting Eqs. (2) and (3) into Eq. (4) respectively, the N_PSN for the rod-like Al₇Cu₂Fe is:

N_PSN =$ H_0\frac{πf_{zone}^2 - 1}{4λ}·l^2$ (5)

and the N_PSN for the irregular η/S, Al₇Cu₂Fe and Mg₂Si is:

N_PSN = $H_0\frac{π(f_{zone}^2- 1)}{4λ}·l^2$ (6)

The data in Fig. 5(a) were fitted by Eqs. (5) and (6) respectively and presented with the solid curves. In hence, the H₀ for each particle could be calculated as listed in Table 3. The H₀ of the irregular Al₇Cu₂Fe in alloy 2 is maximum, the next is the irregular Mg₂Si, and the minimum is for the irregular η/S.

The degree of the local non-uniform deformation in PDZ which is the driving force for PSN, has a great impact on the H₀. It has been identified in Section 3.2.1 that the chemical compositions and shape of the particles have significant effects on the degree of local non-uniform deformation. The degree of the local non-uniform deformation in the PDZ is greater with the larger f_zone. The f_zone of the rod-like Al₇Cu₂Fe is the largest, but its shape restricts the H₀ which is lower than that of the irregular Al₇Cu₂Fe and Mg₂Si. The reason is that the maximum strain caused by a cuboidal particle concentrates at the tips, whereas it is more widely distributed close to the surface of a spherical particle resulting in a higher recrystallization fraction of PSN [22,37]. Therefore, it is observed in Fig. 3 that PSN grains preferentially nucleate at the tips of the rod-like Al₇Cu₂Fe, but generally nucleate simultaneously at all of the surface of the irregular η/S, Al₇Cu₂Fe and Mg₂Si. Nevertheless, the evolution of the H₀ for the irregular η/S, Al₇Cu₂Fe and Mg₂Si with the similar shape is well positively correlated with the f_zone, which is dominated by the particle chemistry and shape as discussed in Section 3.2.1. Therefore, the number of PSN recrystallized grains for the irregular Al₇Cu₂Fe is the largest, followed by the irregular Mg₂Si and it is the smallest for the η/S, according to the H₀ and the A_PDZ.

Because of the low H₀, the number of PSN recrystallized grains for η/S does not increase as same as the PDZ area with increasing particle length. However, the increasing average diameter of PSN recrystallized grains with the η/S length in Fig. 5(c) suggests that the growth of grains prevail in the competition between nucleation and growth during recrystallization. The PSN recrystallized grains of η/S grow fast by consuming the strain stored energy in the PDZ, which is also an underlying reason for the low H₀. For the rod-like Al₇Cu₂Fe, irregular Al₇Cu₂Fe and Mg₂Si, when the particle length increases from 10 μm to 20 μm, the number of PSN recrystallized grains increases faster than the growth resulting in the decreased average diameter. But after then, the average diameter of PSN recrystallized grains keeps a constant owing to the balance between the number increasing and the growth.

A high angle misorientation and a critical size are two necessary criterions for a subgrain successfully becoming a new grain [23]. The subgrains which cannot meet the two criterions must grow by recovery or vanish during the solution. Subgrains still exist in the PDZs of the irregular η/S, rod-like Al₇Cu₂Fe, irregular Al₇Cu₂Fe and Mg₂Si. The strain distribution in the PDZ of the rod-like Al₇Cu₂Fe described above, gives rise to more subgrains than grains as shown in Fig. 5(a) and (b). The number of PSN grains and subgrains in the PDZ increase with the particle length due to the size of PDZ enlarging.

3.2.3. Orientations of PSN recrystallized grains stimulated by different coarse particles

The orientations of PSN recrystallized grains in particle-containing Al-alloys after cold or hot rolling have been confirmed to comprise a certain of random orientations, close to the initial matrix orientations and significant volume fractions of the rotated cube orientation {001}<310> and the P orientation {011}<122> [12,28,37,38]. The formed orientations different from the rolling-texture orientations would weaken the deformation textures, which affects the mechanical properties [13,37]. The influence of chemical compositions, size and shape of the particles on the orientation distribution of PSN had not been well established. In this part, the number of PSN recrystallized grains within different orientations for the irregular η/S, rod-like Al₇Cu₂Fe, irregular Al₇Cu₂Fe and Mg₂Si particles of different lengths in 7055 alloys were quantitively analyzed as illustrated in Fig. 6. The two main orientations of <111> and <001> fiber textures for the matrix microstructures are respectively mass in 10° of <111> and 15° of <100>. According to the angle deviated from <111 > //ED and <100 > //ED, the orientations of PSN recrystallized grains are divided into five groups, namely less than 10° from <111> (<111>_10°), less than 15° from <001> (<001>_15°), in 10°-25° from <111> (<111>_10°-25°), in 15°-30° from <001> (<001>_15°-30°) and the others as random (R).

It can be seen from Fig. 6 that the orientations of PSN recrystallized grains for the η/S particles are mainly <001>_15°, which is the same with the initial matrix orientations of <001> fiber texture. A small number of grains with <111>_10°-25° and <001>_15°-30° appear when the length of the η/S particles is larger than 30 μm, but no grains with R orientation could be found in the PDZs surrounding the η/S particles. However, the grains with <111>_10°-25°, <001>_15°-30° and R orientations form for the rod-like Al₇Cu₂Fe, irregular Al₇Cu₂Fe and Mg₂Si particles when the particle length is larger than 10 μm. Additionally, <111>_10°-25° and <001>_15°-30° orientations are dominant in the orientation distribution of PSN grains, and the number of all five orientations of PSN recrystallized grains increase with the particle length. The number of all five orientations of PSN recrystallized grains for the irregular Al₇Cu₂Fe and Mg₂Si particles in alloy 2 are greater than that for the η/S and the rod-like Al₇Cu₂Fe particles in alloy 1. The variation of the number of PSN recrystallized grains in each orientation among the four particles is consistent with the number of PSN recrystallized grains in Fig. 5(a).

In particle-containing Al-alloys after cold or hot rolling, the PSN recrystallized grains with the orientations close to the initial matrix orientations are mainly at regions very close to the particle-matrix interface where the deformation is effectively blocked on all possible slip systems, or in lightly deformed alloys. In heavily deformed alloys, the nuclei orientations are more randomly [37]. The orientations of <111>_10° and <001>_15° are two main orientations of the PSN recrystallized grains for the irregular η/S particles, which generate weak local non-uniform deformation (small f_zone). A few numbers of PSN recrystallized grains with <111>_10°-25° and <001>_15°-30° orientations largely deviated from the extruded fiber textures occur with the η/S particle length larger than 30 μm due to the increased local strains. The rod-like Al₇Cu₂Fe, irregular Al₇Cu₂Fe and Mg₂Si particles possess the ability to cause stronger local non-uniform deformation (larger f_zone), which provides higher misorientation for nucleation of the PSN grains with <111>_10°-25°, <001>_15°-30° and R orientations even at a small particle length of 10 μm. Because the heavy deformation gives rise to more random orientations of PSN grains, the number of PSN grains in each orientation for the above three types of particles increases with the particle length and larger than that for the η/S particles.

3.3. Recrystallization and texture of hot-extruded 7055 alloys influenced by PSN responding to Fe and Si contents

The EBSD microstructures of longitudinal sections of the two aged 7055 alloys in low magnification (Fig. 7(a) and (b)) show that the elongated grains along with ED are primary, inside which are divided by amounts of LAGBs. The fine and equiaxed recrystallized grains become more in alloy 2 and they mostly surround the coarse particles of Al₇Cu₂Fe and Mg₂Si as exhibited in Fig. 3. The grain boundaries misorientation distributions of alloys 1 and 2 are respectively diagramed in Fig. 8(a) and (b). The number fractions of grain boundaries misorientation in 1.5°-4°, 4°-10°, 10°-15°and larger than 15° are respectively calculated in Fig. 8(c). Although the LAGBs are dominant in the two alloys, the 1.5°-4° grain boundaries are obviously reduced and the 4°-10°, 10°-15°and larger than 15° grain boundaries are all increased in alloy 2 with higher Fe and Si contents. The grain boundaries patterns of alloys 1 and 2 (the 1.5°-4°, 4°-10°, 10°-15°and larger than 15° grain boundaries are respectively drawn with red, blue, yellow and black lines) in Fig. 7(c) and (d) express that the 4°-10°, 10°-15°and larger than 15° grain boundaries are mostly around the coarse particles.

The scattered IPFs in ED of the two alloys are shown in Fig. 9(a) and (b). The extruded fiber textures of <111 > //ED and <001 > //ED are still predominant. Whereas the proportion of grain orientation deviated from <111 > //ED and <001 > //ED and the deviated angle increase with the Fe and Si contents. The fractions of the <111>_10°, <001>_15°, <111>_10°-25°, <001>_15°-30° and R orientations are respectively counted in Fig. 9(c) and (d). The area fractions of <111>_10° and <001>_15° are respectively reduced by 5.3% and 20.5% with increasing Fe and Si contents from 0.041% and 0.024% to 0.272% and 0.134%. The area fractions of <111>_10°-25°, <001>_15°-30° and R orientations are raised with Fe and Si contents.

The variation of grain boundaries misorientation distributions in the two alloys are closely related to the PSN recrystallization stimulated by the coarse η/S, Al₇Cu₂Fe and Mg₂Si particles [39]. In this study, the critical size of PSN for η/S, Al₇Cu₂Fe and Mg₂Si particles is about 10 μm, and the PSN grains caused by the irregular Al₇Cu₂Fe and Mg₂Si are more than the irregular η/S and rod-like Al₇Cu₂Fe. Additionally, the numbers of PSN grains grow with the sizes of Al₇Cu₂Fe and Mg₂Si particles. Despite the coarse η/S larger than 10 μm is decreased in alloy 2 compared to alloy 1, the numbers of Al₇Cu₂Fe and Mg₂Si particles larger than 10 μm increase and the morphology of Al₇Cu₂Fe transforms to irregular that generate more PSN grains. In hence, the number fractions of 4°-10°, 10°-15°and larger than 15° grain boundaries are increased.

It can be observed in Fig. 7 that the PSN grains deviated from the extruded fiber textures frequently occur at the vicinity of coarse particles. It is evident that PSN from the coarse particles changes the proportion of deformation textures [40]. The orientation distributions of PSN grains from the irregular η/S, rod-like Al₇Cu₂Fe, irregular Al₇Cu₂Fe and Mg₂Si discussed in Section 3.2.3 indicate that the orientations of PSN grains from η/S are substantially the same with the extruded fiber textures, while the PSN grains stimulated by the rod-like Al₇Cu₂Fe, irregular Al₇Cu₂Fe and Mg₂Si mostly deviate from the extruded fiber textures. The influence of irregular η/S, rod-like Al₇Cu₂Fe, irregular Al₇Cu₂Fe and Mg₂Si on the texture are summarized in Fig. 10. At the same length, the areas of PSN grains deviated from <111>_10° and <001>_15° of the irregular Al₇Cu₂Fe and Mg₂Si are larger than the rod-like Al₇Cu₂Fe, and that of η/S is the smallest. The small area of PSN grains with <111>_10°-25° and <001>_15°-30° orientations and few PSN grains with R orientation are caused by η/S. Besides, the areas of PSN grains deviated from extruded fiber textures become large with the length of coarse particles. Therefore, the increase of large and irregular Al₇Cu₂Fe and Mg₂Si particles in alloy 2 contributes to raising the proportion of orientations deviated from the extruded fiber textures.

3.4. Mechanical properties of 7055 alloys with different Fe and Si contents

The tensile stress-strain curves of aged alloy 1 and alloy 2 loaded along extrusion direction are shown in Fig. 11(a). The average yield strength (YS) and the average ultimate tensile strength (UTS) are 575 MPa and 597 MPa respectively in alloy 1, which slightly decrease to 572 MPa and 593 MPa respectively in alloy 2 with Fe and Si contents increasing. The average elongation (El) to fracture obviously decreases from 9.8% in alloy 1 to 7.3% in alloy 2. The tensile fracture morphologies of alloys 1 and 2 were respectively observed by SEM in Fig. 11(b) and (c). The tensile fracture surfaces of alloys 1 and 2 both consist of coarse voids and fine dimples. The coarse voids usually initiate at the coarse particles and become more and larger with Fe and Si contents increasing [41].

The mechanical properties of aluminum alloys are intensely relevant to crystallographic texture. The strength in extrusion direction could be reinforced by stronger fiber texture of <111 > //ED [42,43]. The fraction of grains with <111 > //ED orientation decreases from 56.8% to 53.8% (Fig. 9(c)) due to the enhanced recrystallization caused by more PSN formation, which contributes to the slight reduction of strength with Fe and Si contents increasing from 0.041% and 0.024% to 0.272% and 0.134% respectively.

Although the number of coarse η/S is decreased with Fe and Si contents increasing, the numbers of coarse Al₇Cu₂Fe and Mg₂Si are both increased resulting in a larger number of total coarse particles (Fig. 2(c) and (d)). The coarse particles are easy to crack or separate from the matrix to form voids at a low stress, which are usually the source and propagation path of crack [44]. Therefore, the elongation of alloy 2 with higher Fe and Si contents is lower with more and larger voids in the fracture surface. The higher proportion of HAGBs in alloy 2 (Fig. 8) related to enhanced PSN recrystallization also lessens the elongation, because the intergranular fracture is promoted by HAGBs, which hinder the deformation more seriously than LAGBs [45].

4. Conclusions

The evolution of coarse intermetallic particles, recrystallization, texture and mechanical properties response to Fe and Si contents in hot-extruded 7055 aluminum alloys were investigated. The PSN behavior of each kind of coarse particles occurred in 7055 alloys was detailly recognized to understand the variation of recrystallization and texture responding to Fe and Si contents. The main conclusions could be summarized as follows:

(1) Four kinds of coarse particles of rod-like Al₇Cu₂Fe, irregular η/S, Al₇Cu₂Fe and Mg₂Si were found in the two experimental 7055 aged alloys. The average length and number density of coarse η/S particles are reduced with 0.041 wt% Fe and 0.024 wt% Si increasing to 0.272 wt% Fe and 0.134 wt% Si. The coarse Al₇Cu₂Fe particles transform from rod-like to irregular, which results in the large decrease of average length and aspect ratio. The number densities of coarse Al₇Cu₂Fe and Mg₂Si particles are both increased with a rise of Fe and Si contents.

(2) The rod-like Al₇Cu₂Fe particles with the largest aspect ratio generate the greatest degree of local non-uniform deformation, but smaller PDZ area than the irregular Al₇Cu₂Fe and Mg₂Si particles due to the restriction of shape. The irregular η/S particles cause the smallest degree of local non-uniform deformation, in spite of the similar aspect ratio with the irregular Al₇Cu₂Fe and Mg₂Si particles. The number of PSN recrystallized grains in PDZ follows the order of irregular Al₇Cu₂Fe, irregular Mg₂Si, rod-like Al₇Cu₂Fe and irregular η/S from most to least. It is consistent with the degree of local non-uniform deformation caused by different particles, except for the rod-like Al₇Cu₂Fe because of the smaller PDZ area. The orientations of PSN recrystallized grains stimulated by rod-like Al₇Cu₂Fe, irregular Al₇Cu₂Fe and Mg₂Si are dominantly deviated from the extruded fiber textures, owing to the larger degree of local non-uniform deformation. The irregular η/S particles mainly produce PSN recrystallized grains with orientations close to the extruded fiber textures.

(3) The coarse η/S particles are reduced with Fe and Si contents, but the increased Al₇Cu₂Fe and Mg₂Si particles are more liable to induce PSN recrystallization and produce PSN recrystallized grains with orientations largely deviated from the extruded fiber textures, which contribute to the more HAGBs and a lower percentage of extruded fiber textures. With Fe and Si contents increasing, the slight decline of strength is consistent with the weakened <111 > //ED fiber texture, while the larger number of coarse particles and more HAGBs contribute to reducing the elongation.

Acknowledgements

This research work was supported financially by the National Natural Science Foundation of China (No.51821001).

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