预处理对喷射沉积Al-Cu-Li合金中Al3Zr粒子析出及再结晶行为的影响

1 Introduction

Al-Cu-Li alloys, which have excellent mechanical properties, have been rapidly developed in recent years and are considered as ideal lightweight structural materials [1-3]. However, Al-Cu-Li alloys are prone to recrystallize during the hot deformation process to form coarse grains and reduce their mechanical properties [4]. Zirconium (Zr) is added to aluminum alloys to form Al3Zr dispersoids, which can inhibit the degree of recrystallization and improve mechanical properties because the grain boundaries, subgrain boundaries, and dislocation motion can be effectively pinned by fine Al3Zr particles [5-7]. However, the low solubility, high lattice misfit, and slow diffusion rate of Zr atoms in the Al matrix result in severe microsegregation of the Zr solute. The dendrite core is enriched in Zr, while a much lower concentration is present toward the dendrite edge [8-9]. Compared with the traditional casting process, spray-deposited technology with rapid cooling and solidification avoids severe segregation during casting. The spray-deposited alloy ingots are composed of equiaxial grains, with a lower degree of microsegregation and a more uniform microstructure [10-12].

In the past decade, relevant research in this field has focused on optimizing the homogenization process to regulate the precipitation of Al3Zr dispersoids and affect the microstructure and properties of the spray-deposited Al-Cu-Li alloys. LIU et al [13] showed two-step homogenization to facilitate the homogeneous nucleation of Al3Zr and the effect of the Al3Zr precipitate on the recrystallization behavior and on the strength, and the ductility of 2195 alloys. WANG et al [14] showed that two-stage and ramp heating homogenization processes can promote the precipitation of Al3Zr dispersoids in spray deposited Al-Li alloys. The Al3Zr dispersoids tend to precipitate when the T1 plates dissolve, which causes a nonuniform distribution and decreases the recrystallization resistance of the alloy. DUAN et al [15] studied the effects of cooling rates on precipitates in homogenized Al-Cu-Li alloys and found that regardless of the cooling rate, the β′ phase was formed at high temperatures. However, the precipitation behavior of Al3Zr in spray-deposited Al-Cu-Li alloy during homogenization and its precipitation mechanism have not been explained. Therefore, it is necessary to investigate the precipitation behavior of Al3Zr dispersoids during homogenization spray-deposited Al-Cu-Li alloys and its effect on recrystallization.

In this study, the effects of different pretreatments before homogenization on the precipitation behavior of Al3Zr particles in spray-deposited Al-Cu-Li alloy and recrystallization of the alloy were studied and analyzed.

2 Materials and methods

The materials used in this study were spray deposited Al-Cu-Li alloys and its chemical composition is shown in Table 1. The alloy composition was detected by inductively coupled plasma-atomic emission spectroscopy (ICP-AES). The ingots were pretreated in three different ways: annealing at 573 K for 12 h, compressing by 10% at 673 K, and compressing by 30% at 673 K. After pretreatment, the three groups of samples were treated with the same ramp heating homogenization: beginning at room temperature, heating for 10 h to 783 K, maintaining an isothermal plateau for 12 h. The three groups of homogenized samples were named samples PAS, PF1S and PF2S. The homogenized samples were hot compressed and then treated in solution at 783 K/h. Hot compression was carried out from 673 K with a total strain of 0.3. The microstructures were obtained with optical metallographe (OM), electron backscatter diffractmeter (EBSD, ZEISS EVO M10), and transmission electron microscope (TEM, Titan F20 G2). A statistical analysis of the number density and particle size of Al3Zr was carried out by Image J software. OM samples were observed after grinding and polishing. The EBSD samples were first polished by mechanical grinding followed by fabric polishing and were then electrically polished using a solution of 10% perchloric acid and 90% ethyl alcohol at a voltage of 20 V. EBSD patterns were obtained using a ZEISS EVO M10 scanning electron microscope with an OXFORD EBSD detector. Samples for TEM imaging analysis were prepared by mechanical grinding to a thickness of 80 mm and cut into 3 mm radius disks. Then, electropolishing was performed using a Tenupol 5 machine (Struers) with a solution of 30% nitric acid and 70% methanol at 243 to 253 K and 20 V. The process diagram of the whole heat treatment and test conditions are shown in Figure 1(a). Figure 1(b) shows an OM micrograph of the spray-deposited alloy, which reveals typical equiaxed grains with an average size of 28.5 μm. Some small pores can be found on the grain boundaries.

Table 1  Chemical composition of Al-Cu-Li alloy( wt% )

Si Fe Cu Mn Mg

0.03 0.04 4.1 0.04 0.28

Zr Ag Li Other Al

0.13 0.26 0.9 0.07 94.15

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Figure 1  (a) Schematic showing the heat treatment and test conditions; (b) OM micrograph of the spray deposited alloy

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3 Results

3.1 Precipitation behavior of Al3Zr precipitate

Figure 2 shows the STEM image of the three homogenized samples. In the PAS sample, the distribution of Al3Zr dispersoids was relatively heterogeneous, and there were more local precipitation-free zones (PFZs), as shown by the red enclosed dashed line in Figure 2(a). Compared with the PAS sample, it can be observed that the distribution uniformity of Al3Zr dispersoids in the PF1S sample was improved, while the local PFZs decreased, as shown in Figure 2(b). In the PF2S sample, the Al3Zr dispersoids presented a uniformly dense distribution as shown in Figure 2(c). Compared with the PAS and PF1S samples, the distribution uniformity of the dispersoids further increased, and the local PFZs disappeared. The number density and size distribution of the Al3Zr dispersoids were statistically analyzed in each homogenized specimen, as shown in Figure 3. Among the three homogenized samples, the Al3Zr dispersoids had the highest number density and the smallest size in the PF2S sample, and the Al3Zr dispersoids had the lowest number density and the largest size in the PAS sample. The average radii of the Al3Zr dispersoids in the three homogenized samples are 14.9, 13.6 and 11.0 nm, respectively, and the dispersoid radius gradually decreases.

Figure 2  STEM images of Al3Zr distributions taken near the [100]Al zones axis in three homogenized samples: (a) PAS; (b) PF1S; (c) PF2S

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Figure 3  Histogram of Al3Zr particles size distribution and number density obtained from STEM image in three homogenized samples: (a) PAS; (b) PF1S; (c) PF2S; (d) Number density

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Al3Zr dispersoids were randomly distributed near the grain boundaries, and there were clusters with certain shapes and occasionally several coarse particles, as shown in Figure 4. The number of Al3Zr dispersoids was significantly reduced compared to that of the intragranular dispersoids, as shown in Figure 4(d). Linear clusters consisted of several particle arrangements, as shown in Figure 4(a) by the red arrows. Compared with that of PAS, the number of Al3Zr dispersoids increased near the grain boundaries of the PF1S sample, as shown Figures 4(b) and (d). In contrast to the PAS and PF1S, the number density of Al3Zr dispersoids in the PF2S sample near the grain boundaries further increased (Figure 4(d)), and the distribution uniformity was significantly improved near the grain boundaries, as shown in Figure 4(c). In combination with the diffraction spots in the lower right corner of Figures 4(a) and (b), the linear cluster is parallel to <100>Al. These linear cluster phenomena of Al3Zr dispersoids have also been observed in many studies [16-17]. The precipitation and distribution of Al3Zr dispersoids are affected by different pretreatment processes. The Al3Zr dispersoids in the intragranular and near grain boundaries of the PAS sample are heterogeneously distributed and have a local PFZs, with the lowest number density, the largest size, and the highest nonuniformity. Compared with the PAS, the local PFZs in the intragranular and near grain boundaries of the PF1S samples were reduced, the number density of Al3Zr dispersoids increased, the size was decreased, and the nonuniformity was improved. The Al3Zr dispersoids of the PF2S sample were uniformly distributed in the intragranular and near grain boundaries, and the local PFZs were inhibited, showing the highest number density, the smallest size, and the highest uniformity. Combined with the distribution characteristics of Al3Zr particles in the intragranular and near grain boundaries, the precipitation and distribution of Al3Zr dispersoids of three different homogenized states are summarized, as shown in Figure 5.

Figure 4  STEM images of Al3Zr distributions near the grain boundary and number density of three homogenized samples: (a) PAS; (b) PF1S; (c) PF2S; (d) Number density

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Figure 5  Schematic of precipitation and distribution of Al3Zr dispersoids of three different pretreatment samples: (a) PAS; (b) PF1S; (c) PF2S

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3.2 Recrystallization behavior

To clarify the effect of the three different pretreatments on the precipitation of Al3Zr dispersoids and the recrystallization behavior, the hot compression samples were annealed at 783 K for 1 h, followed by water quenching to ambient temperature. The EBSD map of the samples after solution treatment by hot compression is shown in Figure 6. Boundaries with misorientation between 2° and 15° were defined as low-angle grain boundaries (LABGs) associated with subgrains; misorientations >15° were defined as high-angle grain boundaries (HABGs) associated with recrystallized grains. The white and black lines represent the LAGBs and HAGBs, respectively. After hot compression solution treatment of the samples, many subgrains developed intragranularly, while many fine recrystallized grains formed at the triangular boundaries. Some recrystallized grains at the triangular boundary of the PAS sample coarsened, as shown by the black arrow in Figure 6(a). The grain size and misorientation in the EBSD map of Figure 6 were calculated to obtain the average grain size and distribution misorientation (Table 2). The grain sizes of the three hot compressed specimens are very similar. In the three hot compressed samples, the percentage of HAGBs decreased from 30.3% to 17.3%. The gradual decrease in the percentage of HAGBs indicates that recrystallization resistance is gradually improved. Therefore, the number of recrystallized grains at the triangle grain boundary is as follows: PAS>PF1S>PF2S. From PAS to PF1S and PF2S, the recrystallization degree decreased gradually, and the deformed grains were better retained, as shown in Figures 6(a)-(c) and Figure 7.

Figure 6  EBSD maps of three hot compression samples: (a) PAS; (b) PF1S; (c) PF2S

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Table 2  Distribution misorientation (>2°) of three hot compression samples

Different hot compression sample LABGs/% HABGs% Average grain size/μm

PAS 32.8 30.3 26.1

PF1S 42.1 21.6 26.0

PF2S 55.5 17.3 26.4

LABS: low-angle grain boundries; HABS: high-angle grain boundries.

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Figure 7  Histogram of recrystallized fraction of three hot compression samples

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4 Discussion

SRINIVASAN et al [18] found that Al3Zr has small values of interatomic spacing misfit and interplanar spacing mismatch with respect to Al. The solubility limit of Zr in Al(fcc) is very low, and the maximum solubility is 0.23 wt% during the peritectic reaction [19]. The diffusion rate of Zr solutes at 470 ℃ (7.07×10-19 m2/s) [20] causes strong microsegregation of the Zr solute within a dendrite during casting. The formation of the Al3Zr dispersoids tends to produce a homogeneous distribution in the dendrite center where Zr supersaturation is high, leading to a heterogeneous distribution in interdendritic areas where supersaturation is significantly lower [21-22]. However, the characteristics of the low degree of microsegregation of the spray-deposited alloy with equiaxial grains will have a significant influence on the precipitation behavior of Al3Zr particles during homogenization and recrystallization after pretreatment.

Usually, dislocations are favorable locations for precipitation and growth of precipitates. During the homogenization process, Zr atoms diffused to dislocations to moderate their large atomic misfit with the Al matrix [23]. When the solubility of the Zr atoms near the dislocation reached the precipitation condition, the Al3Zr dispersoids began to precipitate. Meanwhile, the dislocation climbed under thermal activation. The dislocations climb drags Zr atoms out of the solid solution and Al3Zr forms by fast pipe diffusion. The interaction between the Zr solute and dislocation increases the precipitation of Al3Zr, which is called “repeated precipitation on dislocations” [24]. TEM images of Al3Zr particles by repeated precipitation on dislocations from linear clusters are shown in Figure 8.

Figure 8  Al3Zr dispersoids by repeated precipitation on dislocations

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PAS samples were annealed for a long pretreatment time, and the number density of dislocations was reduced. Therefore, Al3Zr dispersoids cannot be precipitated to form PFZs when Zr atom solubility is insufficient in some dislocation-free regions, as shown in Figures 2(a) and 4(a). The PF1S and PF2S samples were pretreated by hot deformation, and the number of dislocations increased, which promoted the precipitation of Al3Zr and the decrease in the local PFZs, as shown in Figures 2(b), (c) and 4(b), (c). Compared with the PF1S sample, the PF2S sample showed a greater deformation degree, and the number density of dislocations was the highest. Therefore, the Al3Zr dispersoid number density was the highest, and the local PFZs was eliminated, as shown in Figures 2(c) and 4(c). The increased number density of the precipitated phase leads to a decrease in particle distances, which shortens the diffusion distance of the solute, accelerates the solute consumption, reduces the saturation of the supersaturated solid solution faster, and shortens the average radius of the dispersoids, as shown in Figure 3.

The dispersoids retarded recrystallization can be measured by the Zener pinning formula [25-26]:

Pz=3fvγGB2r

(1)

where γGB is the interface energy (~0.3 J/m2) [27]; fv is the volume fraction; and r is the average radius of the dispersoids. The higher the ratio of fv/r, the stronger the resistance to recrystallization. After substituting the relevant data in Table 3 into Eq. (1), it can be seen that the Pz value of the PF2S sample has higher value of 192.3 kJ/m3 compared with the PAS and PF1S samples with 127.9 and 145.6 kJ/m3, respectively. Among the three homogenized samples, the PF2S sample has the highest number density of Al3Zr dispersoids, the smallest size, and the best uniform distribution. Therefore, the recrystallization resistance of the PF2S samples is the largest, and more deformed grains are retained. For the PAS samples, the Al3Zr dispersoids had the lowest number density, the largest size, and the highest nonuniformity, and there was a large amount of local PFZs. Therefore, the recrystallization resistance is the lowest.

Table 3  The volume fraction (fv) of Al3Zr particles and Zener pinning pressure (Pz) of three different pretreatment samples

Sample fv/% Pz/(kJ·m-3)

PAS 0.423 127.9

PF1S 0.440 145.6

PF2S 0.475 192.3

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5 Conclusions

In summary, three different pretreatments before homogenization on the precipitation behavior of Al3Zr precipitates and their effect on the recrystallization of spray-deposited Al-Cu-Li alloy are investigated by transmission electron microscopy and electron backscatter diffraction. From the study conducted in this paper, the following was found:

1) The distribution of Al3Zr dispersoids in the spray-deposited Al-Cu-Li alloy is heterogeneous. The number density of dispersoids is higher in the intragranular where the Zr atom supersaturation is higher, and it is lower near the grain boundary where the supersaturation is lower. There is a local PFZs in the intragranular and near grain boundaries, and there were linear clusters near the grain boundaries.

2) The thermal deformation pretreatment before homogenization increased the number of dislocations in the matrix, promoted the precipitation of Al3Zr dispersoids, improved particle distribution, and inhibited local PFZs. Compared with PAS and PF1S, the Al3Zr particles in the PF2S sample have a higher number density, more uniform distribution, and smaller size.

3) After the hot compression treatment, the HAGBs of the three hot compression samples were 30.3%, 21.6% and 17.3%, respectively. The rank of recrystallization resistance is PAS<PF1S<PF2S.