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Zr ratio on the microstructure and mechanical properties of new type of Al–Zn–Mg–Sc–Zr alloys
Materials Science & Engineering A 617 (2014) 219–227
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Effect of Sc/Zr ratio on the microstructure and mechanical properties of new type of Al–Zn–Mg–Sc–Zr alloys Gen Li a, Naiqin Zhao a,b,n, Tao Liu a, Jiajun Li a,b, Chunnian He a,b, Chunsheng Shi a,b, Enzuo Liu a,b, Junwei Sha a a b
School of Materials Science and Engineering, Tianjin University, Tianjin 300072, China Tianjin Key Laboratory of Composite and Functional Materials, Tianjin 300072, China
art ic l e i nf o
a b s t r a c t
Article history: Received 7 May 2014 Received in revised form 5 July 2014 Accepted 18 August 2014 Available online 26 August 2014
The rare earth scandium (Sc) as a microalloying element has attracted an increasing interest in aluminum alloys for achieving excellent mechanical properties. Combining with zirconium, high strength and low price Al–Sc alloys are expected. The effects of Sc and Zr on the grain refinement, recrystallization resistance and precipitation hardening were investigated in new type of Al–Zn–Mg–Sc– Zr alloys by rolling, annealing and aging processes. Scandium addition into the Al–Zn–Mg alloys can efficiently refine the grain size and increase recrystallization resistance, especially together with zirconium addition. The maximum value of the yield-to-tensile strength (627 MPa/667 MPa) was obtained with 0.2Sc/0.4Zr ratio of the alloy after solution-aging treatment. The additional strengthening of the alloys is attributed to the grain refinement and the precipitation-strengthening effect of Al3Sc, Al3Zr or Al3(Sc, Zr) in the proper ratio of Sc/Zr during aging. & 2014 Elsevier B.V. All rights reserved.
Keywords: Aluminum alloys Sc/Zr ratio Microstructure Mechanical properties
1. Introduction The addition of rare-earth elements into aluminum alloys is an effective approach to improve their microstructure and mechanical properties. Scandium, as one of the rare-earths, is a very promising element in aluminum alloys via the formation of a high density of fine Al3Sc (L12) particles. It is found that Al3Sc particles can serve as the grain refiner for the solidification process of aluminum melt, the dispersions for controlling the grain structure of the alloys and the strengthening precipitates by pinning dislocation [1,2]. Norman et al. [3] found that the refinement effect on grain size in as-cast Al–Sc alloys with hypereutectic compositions was much greater than that of conventional aluminum grain refiners. Seidman et al. [4] indicated that the strengthening mechanism for Al3Sc precipitates transformed from the precipitate shearing mechanism to the Orowan dislocation bypass mechanism with a critical particle radius of 2.1 nm. Since scandium element can help in improving the performance of aluminum alloys, aluminum alloys containing scandium have drawn a great attraction [5,6].
n Corresponding author at: School of Materials Science and Engineering, Tianjin University, Tianjin 300072, China. Tel./fax: þ86 22 27891371. E-mail address: [email protected] (N. Zhao).
http://dx.doi.org/10.1016/j.msea.2014.08.041 0921-5093/& 2014 Elsevier B.V. All rights reserved.
However, the high price of scandium restricts their wide application. It is of great importance to investigate the addition of scandium with other elements into aluminum alloys to reduce cost and improve mechanical properties of the alloys. Zirconium as another efficient and common alloying element has proved to decrease average grain size and increase tensile strength due to the formation of Al3Zr particles. Thus, combining Sc with Zr, high strength and low price Al alloys are expected. Recently, aluminum alloys with minor Sc and Zr additions have been broadly studied [7–9]. The formation of extremely fine, coherent Al3(Sc, Zr) (L12) precipitates, consisting of a core containing Al and Sc surrounded by a Zr-rich shell [10,11], can significantly inhibit the dislocation motion and result in an additional strengthening effect. Based on these effects of Sc and Zr elements in aluminum alloys, microalloying addition of Sc and Zr into commercial aluminum alloys has attracted increasing attention. Al–Zn–Mg alloys have extensively been used for structural applications in the aerospace industry due to their low mass and excellent mechanical properties [12–14]. However, requirements for further improving alloy performance have been proposed in the high-tech fields. It has been demonstrated that the introduction of minor Sc and Zr into Al–Zn–Mg alloys can achieve excellent mechanical performance [15–20]. Deng [21] focused on the Al–Zn– Mg alloy with different amounts and ratios of Sc and Zr. The results showed that the yield strength increased by 66 MPa with 0.1 wt% Sc and 0.1 wt% Zr, and improved by 96 MPa with 0.25 wt%
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Sc and 0.10 wt% Zr, respectively, compared with the strength of the peak-aged Al–Zn–Mg alloy without Sc and Zr. Due to the beneficial effect of scandium and zirconium on the properties of Al–Zn–Mg alloys, systematic research about the effect of Sc/Zr ratio on the microstructure and properties of the alloys, especially on the recrystallization and aging precipitation behaviors should be carried out. Besides, it is necessary to study the mechanism of interaction between scandium and zirconium in Al–Zn–Mg alloys for guiding the design of the alloy compositions. However, few reports have focused on these issues. In this work, we investigated the influence of Sc/Zr ratio on the mechanical behavior and microstructure of Al–Zn–Mg–Sc–Zr alloys. Al–Zn–Mg–0.6 wt% (Scþ Zr) alloys with different Sc/Zr ratios were designed to obtain the optimum Sc/Zr ratio for addition and to study the mechanism of interaction between scandium and zirconium in Al–Zn–Mg alloys. Discussion about the strengthening mechanism in the alloys is also present.
2. Experimental procedure Five kinds of Al–7.0Zn–2.5Mg–0.2Cu–0.6(ScþZr) (in wt%) alloys with different Sc/Zr ratios were provided by Hunan Rare-Earth Metal Research Institute. The chemical compositions of the alloys are presented in Table 1. The homogenization treatment of the alloys was carried on at 470 1C for 12 h. The temperature was determined by simultaneous thermal analysis. Then, the ingots with a thickness of 20 mm, were pre-annealed at 450 1C for 2 h, followed by hot rolling to 6 mm thick plates immediately. Then the hot rolled plates were interannealed at 400 1C for 2 h, and cold rolled to 3.7 mm sheets. The sheets were subjected to solution treatment at 470 1C for 1 h followed by water quenching, and then artificial aging at 120 1C for various times. Metallography microscopy, scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS) and transmission electron microscopy (TEM) methods were utilized to investigate the effects of different Sc/Zr ratios on the microstructure of Al–Zn–Mg alloys. Grain size of the cast alloys was counted with at least 100 grains. Hardness measurements and tensile tests were used to study the effects of different Sc/Zr ratios additions on the mechanical behavior of Al–Zn–Mg alloys. Vickers hardness tests were carried out on an EveroneMH-6 machine. The tensile specimens were prepared along the rolling direction. Tensile testing was performed on a CSS44100 electronic universal testing machine with 1 mm/min loading speed. The yield strength of the materials was identified at 0.2% plastic strain. A minimum of ten times hardness measurements was measured on each sample, and the experimental errors here represented one standard deviation from the mean. SEM observations were performed on a Hitachi SU1510 and S4800 at low and high magnification severally, both operating at 15 kV. Thin foils for TEM observations were prepared by twin-jet electro-polishing at Table 1 Actual composition of studied Al–7.0Zn–2.5Mg–0.2Cu–0.6(Sc þ Zr) alloys with different Sc/Zr ratios (in wt%). Alloysa
Zn
Mg
Cu
Sc
Zr
Sc/Zr ratio
SZ06 SZ24 SZ33 SZ42 SZ60
7.03 7.11 6.98 6.84 6.94
2.63 2.49 2.55 2.60 2.58
0.21 0.22 0.19 0.21 0.17
– 0.22 0.27 0.37 0.48
0.51 0.40 0.29 0.18 –
0 0.5 1 2 1
a SZ06 implies that the nominal composition of ‘S’(Sc) and ‘Z’(Zr) are 0% and 0.6%, respectively. The definition of the other alloys is similar to the one stated above.
100 mA in a solution of 30% nitric acid and 70% methanol solution cooled to 30 1C and observed on a JEM-2100F electron microscope and TECNAIG G2 F20 electron microscope.
3. Results and discussion 3.1. Effect of Sc/Zr ratio on the microstructures of cast alloys The microstructures of the as-cast Al–Zn–Mg alloys with different Sc/Zr ratios are shown in Fig. 1. All the alloys exhibit equiaxed grains. As shown in the upper right corner of every image in Fig. 1, grain size distribution (red rectangle) was measured and fitted by Gaussian function (blue line). The order of grain size from coarse to fine is as follows: SZ064SZ244SZ604SZ424SZ33. Fig. 2 shows the hardness and mean grain size of the as-cast alloys. The maximum hardness (HV136) obtained by SZ33 is attributed to its minimum mean grain size (16.1 μm). Conversely, the maximum mean grain size and minimum hardness are obtained by SZ06 alloy with a value of 111.0 μm and HV104, respectively. According to Figs. 1 and 2, the refining effect of scandium is better than that of zirconium with the same amount of additions in the as-cast alloys. Fig. 3(a) presents the back scattered electron images of as-cast SZ24 alloy. According to the results of element line distribution analysis in Fig. 3(b), the non-equilibrium precipitates in the grain boundary are rich in Cu. Other main alloy elements, such as Zn and Mg, are distributed relatively uniformly. Sc and Zr are merely distributed in the white square precipitates, which are primary Al3(Sc, Zr) precipitates. Since the non-equilibrium phases are harmful to the subsequent process ability, homogenization treatment is needed to eliminate these precipitates. Back scattered electron images (Fig. 3(c) and (d)) of SZ24 alloy show that nonequilibrium precipitates rich in Cu have disappeared after homogenization treatment. Besides, precipitates in the grain boundary have transformed from the continuous coarse ones in as-cast state to discontinuous fine ones due to the coarse precipitates dissolving into the matrix during homogenization treatment. Therefore, homogenization treatment has a positive effect on the microstuctures of the alloys. According to the Al–Sc binary diagram, α-Al and Al3Sc form from the eutectic reaction at 655 1C [22]. However, as α-Al and eutectic Al3Sc form simultaneously, eutectic Al3Sc particles cannot act as nuclei sites for α-Al grains. Primary Al3Sc particles can form prior to the solidification of the α-Al phase with Sc content higher than the eutectic composition (0.6 wt%). Therefore, primary Al3Sc particles are effective nuclei during the solidification process and the non-dendritic structure exists only in hypereutectic Al–Sc alloys, whereas, due to the high price of scandium, hypereutectic Al–Sc alloys cannot be widely applied. In our work, in view of the non-equilibrium solidification situation, equiaxed grains existed in SZ06 alloy with an actual Sc content of 0.48 wt%. Furthermore, when the Sc content is far less than the eutectic point (SZ24 alloy), nondendritic structure in as-cast state is still present. This illustrates that the addition of Zr can reduce the minimum Sc content required to form primary particles. In this case, primary Al3(Sc, Zr) particles substituted as new nucleation centers for aluminum grains. The refining effect from the primary Al3(Sc, Zr) particles is determined by the density and the quantity of the primary particles. Fine grains are the result of the mass formation of primary Al3(Sc, Zr) particles. The amount of Sc and Zr left in the solid solution decline simultaneously. Davydov [23] indicated that for grain refiner, there were two factors responsible for its degree of cast grain refining: volume density of primary particles and effectiveness of inoculation of these particles. Furthermore, the latter factor was determined by the similarity of crystal lattices of a particle-nucleus and matrix in terms of size and structure. Owing
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Fig. 1. Metallographic images and grain size distribution of the as-cast alloys. (a) SZ06; (b) SZ24; (c) SZ33; (d) SZ42; and (e) SZ60. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
to the similar lattice type and very low lattice misfit between the intermetallic particles and the matrix phase, primary Al3Sc or Al3Zr particles could act as heterogeneous nucleants. However, since scandium atoms diffuse faster than zirconium atoms in aluminum melt, the ratio of the diffusion coefficients DSc/DZr varies from 1800 to 190 between 400 and 550 1C [24], primary Al3Sc particles can separate out more quickly than Al3Zr particles during the early stage of formation of the primary phases. Therefore, the grain refinement effect derived from primary Al3Sc particles (SZ60) is more effective than that of primary Al3Zr particles (SZ06). Besides, it is strange that SZ42 and SZ24 show similar hardness but different grain size. This might be due to the similar reinforcement effect between solution strengthening from the Sc and Zr in the solid solution and fine-grain strengthening from the primary Al3(Sc, Zr).
3.2. Effect of Sc/Zr ratio on recrystallization behavior The grain shapes of different sections in SZ24 alloy after the rolling process are illustrated in Fig. 4. Resulting from the stress during the rolling process, the grains are deformed along the rolling direction. Fig. 4(a) describes the characters of grain shape in different sections. There are obvious deformed grains on the rolling section (Fig. 4(b)) and slender fibrous grains are found on the longitudinal section (Fig. 4(c)). In addition, fine grains without noticeable preferred orientation appear on the transverse section (Fig. 4(d)). By comparing the hardness of the cold-rolled alloys with that of the alloys in solid–solution state (Fig. (5)), the latter is generally lower than the former one. This may be due to the fact that the softening effects caused by the dissolution of precipitated phases exceeded the solution strengthening effect.
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In order to compare the recrystallization behaviors of the alloys with different Sc/Zr ratios, the slender fibrous grains in the longitudinal section after annealing at 470 1C for 1 h were investigated, as shown in Fig. 6. There is obvious recrystallization in the alloy without scandium (SZ06) after solution treatment. The alloy containing only scandium also has a certain tendency of recrystallization, whereas the recrystallization behavior is effectively suppressed when adding zirconium together with scandium. It is established that the Zener drag caused by the distributed precipitates is responsible for the inhibition of the advancing grain boundaries during the annealing process. Zener's theory shows that increasing the ratio between the volume fraction and mean radius of the precipitates, and maintaining a coherent interface between the precipitates and matrix both increase the resistance
Fig. 2. Hardness and mean grain sizes of the as-cast alloys.
to recrystallization [2,19,25]. In our work, more effective antirecrystallization behaviors are present in the Al–Zn–Mg–Sc and Al– Zn–Mg–Sc–Zr alloys compared to the Al–Zn–Mg–Zr alloy in solid– solution states. Strong antirecrystallized effect of Sc and Zr addition is attributed to the fine, secondary Al3(Sc, Zr) particles with a high density appearing in the solid–solution SZ24 alloy, as shown in Fig. 6(f). As they strongly inhibit the motion of subgrain boundaries via the Zener drag during the heat treatment process, deformed microstructures still retain in the Al–Zn–Mg–Sc–Zr alloys after solution treatment. 3.3. Effect of Sc/Zr ratio on age hardening and tensile properties Aging hardening curves of the alloys with different Sc/Zr ratios at 120 1C are presented in Fig. 7. The hardness of the alloys increases with aging time significantly (under-aging state) from the beginning until reaching the maximum value (peak-aging state). Then, the hardness varies in a range of 10–20 HV without dropping with the aging time prolonging. The peak hardness, HV235, is obtained by SZ24 alloy with an aging time of 20 h at 120 1C. With the extension aging time, the hardness of all alloys does not decline sharply. Furthermore, zirconium addition contributes additional stabilization effect to the secondary Al3Sc particles due to the lower susceptibility of Al3(Sc, Zr) particles to coarsen [23]. To demonstrate the influence of Sc/Zr ratio on the mechanical properties in Al–Zn–Mg alloys, tensile tests were carried out and their results are shown in Table 2. Both the highest value of tensile strength and yield strength are acquired by SZ24 alloy in different processes of heat treatment. In solid–solution state, the value of tensile strength and yield strength acquired by SZ24 alloy are 489 MPa and 319 MPa, separately. Moreover, the elongations of all the alloys in the solution state are all over 15%. In the aged alloys, the ultimate tensile strength and yield strength increase obviously,
Fig. 3. Back scattered electron and element line distribution analysis images of the SZ24 alloy. (a) and (b): as-cast; (c) and (d): homogenized at 470 1C for 12 h. The elements in order top-down are Mg (red), Al (green), Zr (blue), Sc (turquoise), Cu (pink) and Zn (violet), respectively. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
G. Li et al. / Materials Science & Engineering A 617 (2014) 219–227
but the elongation declines. In general, a good balance between the tensile strength (667 MPa) and elongation (8–10%) can be obtained by SZ24 alloy after aging at 120 1C for 24 h. With the prolongation of aging time, there is no remarkable change in strength, which is in agreement with the results of aging harding curves. In addition, the yielding-to-tensile ratios of alloys exhibit a significant increase from the solution state to the aging state. The variation range of yield ratio after solution treatment is from 0.55 to 0.70. However, all the yield ratios of alloys in aging state are over 0.90. In view of engineering applications, high-strength weldable aluminum alloys with high yield ratios have a good application prospect in the field of automobile body materials, which requires lightweight alloys with strong abilities to resist deformation. The morphology of tensile fracture surface is shown in Fig. 8. As shown in Fig. 8(a) and (b), obvious deep toughening dimples are distributed on the fracture surface of the SZ24 alloy in solid– solution state, indicating a ductile fracture. After the alloy ages at 120 1C for 24 h (Fig. 8(c)) and 32 h (Fig. 8(d)), dimples become much smaller and shallower than those of the alloys in the solid– solution state, and the brittle characteristics appear in both fractures surfaces, which can be confirmed by the lamellar and transgranular fracture in fractures.
aluminum matrix. These Al3Sc or Al3 (Sc, Zr) precipitates, which range in size from 20 to 30 nm, help in stabilizing the fine grained microstructure and strengthening the alloys by pinning the grain boundary and hindering the dislocation motion (Fig. 9(e)). According to the dispersion-strengthening theory, an increase in the yield strength caused by the Al3(Sc, Zr) precipitates is controlled by a shearing mechanism for smaller sizes, and the Orowan dislocation bypass mechanism for larger sizes. Seidman [4] investigated that the critical precipitate radius for this transition was 2.1 nm in dilute Al–Sc alloy. Therefore, Al3(Sc, Zr) precipitates which range in diameter from 20 nm to 30 nm contribute additional strength enhancement to the aged alloys through Orowan bypass mechanism. According to the Orowan-mechanism theory, the effectiveness of Orowan strengthening derived from the precipitates is
3.4. Characterization of precipitates in the aged Al–Zn–Mg–Sc–Zr alloys TEM images in Fig. 9 show the secondary Al3Zr, Al3(Sc, Zr) and Al3Sc precipitates in Al–Zn–Mg–Sc–Zr alloys after aging at 120 1C for 24 h. The density of Al3Zr particles separated out in the SZ06 alloy (Fig. 9(a)) is much less than that of Al3(Sc, Zr) (Fig. 9(b)–(d)) or Al3Sc (Fig. 9(e)) particles in the other alloys. The presence of Ashby–Brown contrast in the bright-field images as well as the superstructure reflections confirm that these particles are Al3Sc or Al3(Sc, Zr) particles, and these particles are coherent with the
223
Fig. 5. Hardness of the cold-rolled and solid–solution alloys.
Fig. 4. Metallographic images of different sections in cold-rolled SZ24 alloy.(a) sketch map; (b) rolling plane; (c) longitudinal section and (d) transverse section.
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Fig. 6. Metallographic and TEM images of the alloys in solid–solution state. (a) SZ06; (b) SZ24; (c) SZ33; (d) SZ42; (e) SZ60; and (f) precipitates in the SZ24 alloy.
Fig. 7. Age harding curves of hardness in the alloys with different Sc/Zr ratios.
approximately inversely proportional to the mean precipitate spacing. In our work, the mean particle spacing of secondary Al3(Sc, Zr) particles separating out in Al–Zn–Mg–Sc–Zr alloys is significantly smaller than that of Al3Sc or Al3Zr in SZ60 and SZ06 alloys during the heat-treating process. Therefore, the additional strengthening from Al3(Sc, Zr) precipitates via Orowan mechanism is larger than that of Al3Sc or Al3Zr precipitates, which can explain that the higher tensile strength was obtained by Al–Zn–Mg–Sc–Zr alloy rather than by the alloy without Sc (SZ06 alloy) or Zr (SZ60 alloy). The amount of Sc and Zr left in the solid solution of the alloys with fine grains (SZ33, SZ42 and SZ60 alloys) is less than that of the alloys with larger grains (SZ24 alloy). Therefore, the secondary Al3(Sc, Zr) precipitates separating out in SZ33, SZ42 and SZ60 alloys are less than the secondary precipitates in SZ24 alloy during heat treatment. TEM images in Fig. 10 show the precipitates in SZ24 and SZ60 alloys after aging at 120 1C for 24 h. The high-density ultrafine η0 precipitates are distributed homogenously within the matrix and some coarsen rod precipitates are distributed discontinuously along the grain boundaries (Fig. 10(a)). HRTEM image of
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an Al3(Sc, Zr) dispersoid, as shown in Fig. 10(b), reveals lattice fringes with an interplanar spacing of 2.041 Å corresponding to the (200) lattice planes of the L12-ordered Al3(Sc, Zr) particles. Through comparative observations, there is no significant difference in the microstructure of η0 phases in the alloys with different Sc/Zr ratios. As described in Fig. 10(c), the selected area diffraction (SAD) pattern in the sample of the aged SZ60 alloy taken along [001] zone axis of Al matrix shows characteristic super-lattice spots from coherent Al3Sc precipitates. Fig. 10(d) shows the darkfield image of the diffraction spot selected by the dotted circle in Fig. 10(c). Previous studies have manifested that the precipitation sequence of the Al–Zn–Mg alloys is usually described as over-saturated solid
Table 2 Mechanical properties of Al–Zn–Mg alloys with different Sc/Zr ratios. Alloys
Condition
UTS (σb/MPa)
0.2% YS (σ0.2/MPa)
EI (δ/%)
Yield ratio (σ0.2/σb)
SZ06
Solution Aged 24 h 32 h
404 607 648
235 552 597
17.1 8.6 10.6
0.58 0.91 0.92
SZ24
Solution Aged 24 h 32 h
489 667 667
319 627 629
19.2 8.3 9.9
0.65 0.94 0.94
SZ33
Solution Aged 24 h 32 h
468 643 632
311 599 597
17.2 9.0 8.6
0.66 0.93 0.94
SZ42
Solution Aged 24 h 32 h
478 616 650
307 573 608
15.9 11.0 8.4
0.64 0.94 0.94
SZ60
Solution Aged 24 h 32 h
445 641 638
264 594 604
19.3 7.9 8.3
0.59 0.93 0.95
225
solution (α) -GP zones-metastable η0 phases -stable η phases (MgZn2) [26,27]. During the initial stage of the aging process, the precipitation reaction starts with the formation of GP zones, which later transform into metastable η0 transition phases, which are more thermally stable than GP zones. The η0 phase will transform into the equilibrium η phase at higher temperature or after longer aging time [17,20]. Besides, the strengthening from minor Sc and Zr is independent of strengthening caused by other aging precipitates in Al–Zn–Mg alloy. The formation dynamics of aging precipitates in the Al–Zn–Mg– Sc–Zr alloys are not affected by the Sc and Zr microalloying additions [21]. Through strengthening calculations conducted on 7XXX series alloys, namely, Al 7050 and Al 7055 based on the existing models, η0 phase is found to be present in significant amount during peak-aged condition [28]. In our studies (Fig. 9 and Fig. 10), η0 phases and secondary coherent Al3(Sc, Zr) precipitates are the main phases in the Al–Zn–Mg–Sc–Zr alloys aged at 120 1C for 24 h. In addition, the higher strength of the alloys with minor additions of Sc and Zr was also derived from the fine-grain strengthening of primary Al3(Sc, Zr) particles formed during solidification and sub-grain strengthening due to retention of a nonrecrystallized structure caused by the secondary Al3(Sc, Zr) precipitates during heat treatment. In our work, the grain refinement is derived from the primary Al3(Sc, Zr) particles. The effects of antirecrystallization and precipitation strengthening are derived from the secondary Al3(Sc, Zr) precipitates. What is more, according to the results of mechanics test, the strength increments caused by refining strengthening of primary Al3(Sc, Zr) particles were less stronger than the strengthening effect caused by the precipitation strengthening from the secondary Al3(Sc, Zr) precipitates.
4. Conclusions New type of Al–Zn–Mg–Sc–Zr alloys with different Sc/Zr ratios has been studied in the aspects of microstructure and mechanical
Fig. 8. Fracture morphology of SZ24 alloy after solution state (a) and (b), and aged for 24 h (c) and for 32 h (d) at 120 1C.
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Fig. 9. TEM micrographs of secondary precipitates in alloys with different Sc/Zr ratios aged at 120 1C for 24 h. (a) SZ06; (b) SZ24; (c) SZ33; (d) SZ42 and (e) SZ60 alloy.
performance. The conclusions below could be drawn from the experimental investigations: 1. Scandium and zirconium are both effective grain refines in Al– Zn–Mg alloys, and the refining ability of scandium is better than that of zirconium with the same addition.
2. Scandium possesses a strong antirecrystallized effectiveness in rolled Al–Zn–Mg–Sc alloy after annealing at 470 1C for 1 h, especially together with zirconium addition. 3. The addition of scandium and zirconium to Al–Zn–Mg alloys improved the strength of the alloys evidently. The highest value of tensile strength was acquired by adding 0.2Sc/0.4Zr(wt%)
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Fig. 10. TEM images of the alloys aged at 120 1C for 24 h (a) bright field image of SZ24 alloy and (b) HRTEM images of Al3 (Sc, Zr) precipitates with the dashed frame in (a); (c) SAD of Fig. 9(e), [001] projection of the Al matrix in SZ60 alloy and (d) dark-field image of Al3Sc precipitates from the diffraction spot in the dashed circle in Fig. 10(c).
with the largest distribution density of Al3(Sc, Zr) precipitate among the designed alloys.
Acknowledgments This work was financially supported by the National High-tech Research & Development Program of China (2013AA031002). References [1] [2] [3] [4] [5] [6] [7]
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Effect of Sc/Zr ratio on the microstructure and mechanical properties of new type of Al–Zn–Mg–Sc–Zr alloys Gen Li a, Naiqin Zhao a,b,n, Tao Liu a, Jiajun Li a,b, Chunnian He a,b, Chunsheng Shi a,b, Enzuo Liu a,b, Junwei Sha a a b
School of Materials Science and Engineering, Tianjin University, Tianjin 300072, China Tianjin Key Laboratory of Composite and Functional Materials, Tianjin 300072, China
art ic l e i nf o
a b s t r a c t
Article history: Received 7 May 2014 Received in revised form 5 July 2014 Accepted 18 August 2014 Available online 26 August 2014
The rare earth scandium (Sc) as a microalloying element has attracted an increasing interest in aluminum alloys for achieving excellent mechanical properties. Combining with zirconium, high strength and low price Al–Sc alloys are expected. The effects of Sc and Zr on the grain refinement, recrystallization resistance and precipitation hardening were investigated in new type of Al–Zn–Mg–Sc– Zr alloys by rolling, annealing and aging processes. Scandium addition into the Al–Zn–Mg alloys can efficiently refine the grain size and increase recrystallization resistance, especially together with zirconium addition. The maximum value of the yield-to-tensile strength (627 MPa/667 MPa) was obtained with 0.2Sc/0.4Zr ratio of the alloy after solution-aging treatment. The additional strengthening of the alloys is attributed to the grain refinement and the precipitation-strengthening effect of Al3Sc, Al3Zr or Al3(Sc, Zr) in the proper ratio of Sc/Zr during aging. & 2014 Elsevier B.V. All rights reserved.
Keywords: Aluminum alloys Sc/Zr ratio Microstructure Mechanical properties
1. Introduction The addition of rare-earth elements into aluminum alloys is an effective approach to improve their microstructure and mechanical properties. Scandium, as one of the rare-earths, is a very promising element in aluminum alloys via the formation of a high density of fine Al3Sc (L12) particles. It is found that Al3Sc particles can serve as the grain refiner for the solidification process of aluminum melt, the dispersions for controlling the grain structure of the alloys and the strengthening precipitates by pinning dislocation [1,2]. Norman et al. [3] found that the refinement effect on grain size in as-cast Al–Sc alloys with hypereutectic compositions was much greater than that of conventional aluminum grain refiners. Seidman et al. [4] indicated that the strengthening mechanism for Al3Sc precipitates transformed from the precipitate shearing mechanism to the Orowan dislocation bypass mechanism with a critical particle radius of 2.1 nm. Since scandium element can help in improving the performance of aluminum alloys, aluminum alloys containing scandium have drawn a great attraction [5,6].
n Corresponding author at: School of Materials Science and Engineering, Tianjin University, Tianjin 300072, China. Tel./fax: þ86 22 27891371. E-mail address: [email protected] (N. Zhao).
http://dx.doi.org/10.1016/j.msea.2014.08.041 0921-5093/& 2014 Elsevier B.V. All rights reserved.
However, the high price of scandium restricts their wide application. It is of great importance to investigate the addition of scandium with other elements into aluminum alloys to reduce cost and improve mechanical properties of the alloys. Zirconium as another efficient and common alloying element has proved to decrease average grain size and increase tensile strength due to the formation of Al3Zr particles. Thus, combining Sc with Zr, high strength and low price Al alloys are expected. Recently, aluminum alloys with minor Sc and Zr additions have been broadly studied [7–9]. The formation of extremely fine, coherent Al3(Sc, Zr) (L12) precipitates, consisting of a core containing Al and Sc surrounded by a Zr-rich shell [10,11], can significantly inhibit the dislocation motion and result in an additional strengthening effect. Based on these effects of Sc and Zr elements in aluminum alloys, microalloying addition of Sc and Zr into commercial aluminum alloys has attracted increasing attention. Al–Zn–Mg alloys have extensively been used for structural applications in the aerospace industry due to their low mass and excellent mechanical properties [12–14]. However, requirements for further improving alloy performance have been proposed in the high-tech fields. It has been demonstrated that the introduction of minor Sc and Zr into Al–Zn–Mg alloys can achieve excellent mechanical performance [15–20]. Deng [21] focused on the Al–Zn– Mg alloy with different amounts and ratios of Sc and Zr. The results showed that the yield strength increased by 66 MPa with 0.1 wt% Sc and 0.1 wt% Zr, and improved by 96 MPa with 0.25 wt%
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Sc and 0.10 wt% Zr, respectively, compared with the strength of the peak-aged Al–Zn–Mg alloy without Sc and Zr. Due to the beneficial effect of scandium and zirconium on the properties of Al–Zn–Mg alloys, systematic research about the effect of Sc/Zr ratio on the microstructure and properties of the alloys, especially on the recrystallization and aging precipitation behaviors should be carried out. Besides, it is necessary to study the mechanism of interaction between scandium and zirconium in Al–Zn–Mg alloys for guiding the design of the alloy compositions. However, few reports have focused on these issues. In this work, we investigated the influence of Sc/Zr ratio on the mechanical behavior and microstructure of Al–Zn–Mg–Sc–Zr alloys. Al–Zn–Mg–0.6 wt% (Scþ Zr) alloys with different Sc/Zr ratios were designed to obtain the optimum Sc/Zr ratio for addition and to study the mechanism of interaction between scandium and zirconium in Al–Zn–Mg alloys. Discussion about the strengthening mechanism in the alloys is also present.
2. Experimental procedure Five kinds of Al–7.0Zn–2.5Mg–0.2Cu–0.6(ScþZr) (in wt%) alloys with different Sc/Zr ratios were provided by Hunan Rare-Earth Metal Research Institute. The chemical compositions of the alloys are presented in Table 1. The homogenization treatment of the alloys was carried on at 470 1C for 12 h. The temperature was determined by simultaneous thermal analysis. Then, the ingots with a thickness of 20 mm, were pre-annealed at 450 1C for 2 h, followed by hot rolling to 6 mm thick plates immediately. Then the hot rolled plates were interannealed at 400 1C for 2 h, and cold rolled to 3.7 mm sheets. The sheets were subjected to solution treatment at 470 1C for 1 h followed by water quenching, and then artificial aging at 120 1C for various times. Metallography microscopy, scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS) and transmission electron microscopy (TEM) methods were utilized to investigate the effects of different Sc/Zr ratios on the microstructure of Al–Zn–Mg alloys. Grain size of the cast alloys was counted with at least 100 grains. Hardness measurements and tensile tests were used to study the effects of different Sc/Zr ratios additions on the mechanical behavior of Al–Zn–Mg alloys. Vickers hardness tests were carried out on an EveroneMH-6 machine. The tensile specimens were prepared along the rolling direction. Tensile testing was performed on a CSS44100 electronic universal testing machine with 1 mm/min loading speed. The yield strength of the materials was identified at 0.2% plastic strain. A minimum of ten times hardness measurements was measured on each sample, and the experimental errors here represented one standard deviation from the mean. SEM observations were performed on a Hitachi SU1510 and S4800 at low and high magnification severally, both operating at 15 kV. Thin foils for TEM observations were prepared by twin-jet electro-polishing at Table 1 Actual composition of studied Al–7.0Zn–2.5Mg–0.2Cu–0.6(Sc þ Zr) alloys with different Sc/Zr ratios (in wt%). Alloysa
Zn
Mg
Cu
Sc
Zr
Sc/Zr ratio
SZ06 SZ24 SZ33 SZ42 SZ60
7.03 7.11 6.98 6.84 6.94
2.63 2.49 2.55 2.60 2.58
0.21 0.22 0.19 0.21 0.17
– 0.22 0.27 0.37 0.48
0.51 0.40 0.29 0.18 –
0 0.5 1 2 1
a SZ06 implies that the nominal composition of ‘S’(Sc) and ‘Z’(Zr) are 0% and 0.6%, respectively. The definition of the other alloys is similar to the one stated above.
100 mA in a solution of 30% nitric acid and 70% methanol solution cooled to 30 1C and observed on a JEM-2100F electron microscope and TECNAIG G2 F20 electron microscope.
3. Results and discussion 3.1. Effect of Sc/Zr ratio on the microstructures of cast alloys The microstructures of the as-cast Al–Zn–Mg alloys with different Sc/Zr ratios are shown in Fig. 1. All the alloys exhibit equiaxed grains. As shown in the upper right corner of every image in Fig. 1, grain size distribution (red rectangle) was measured and fitted by Gaussian function (blue line). The order of grain size from coarse to fine is as follows: SZ064SZ244SZ604SZ424SZ33. Fig. 2 shows the hardness and mean grain size of the as-cast alloys. The maximum hardness (HV136) obtained by SZ33 is attributed to its minimum mean grain size (16.1 μm). Conversely, the maximum mean grain size and minimum hardness are obtained by SZ06 alloy with a value of 111.0 μm and HV104, respectively. According to Figs. 1 and 2, the refining effect of scandium is better than that of zirconium with the same amount of additions in the as-cast alloys. Fig. 3(a) presents the back scattered electron images of as-cast SZ24 alloy. According to the results of element line distribution analysis in Fig. 3(b), the non-equilibrium precipitates in the grain boundary are rich in Cu. Other main alloy elements, such as Zn and Mg, are distributed relatively uniformly. Sc and Zr are merely distributed in the white square precipitates, which are primary Al3(Sc, Zr) precipitates. Since the non-equilibrium phases are harmful to the subsequent process ability, homogenization treatment is needed to eliminate these precipitates. Back scattered electron images (Fig. 3(c) and (d)) of SZ24 alloy show that nonequilibrium precipitates rich in Cu have disappeared after homogenization treatment. Besides, precipitates in the grain boundary have transformed from the continuous coarse ones in as-cast state to discontinuous fine ones due to the coarse precipitates dissolving into the matrix during homogenization treatment. Therefore, homogenization treatment has a positive effect on the microstuctures of the alloys. According to the Al–Sc binary diagram, α-Al and Al3Sc form from the eutectic reaction at 655 1C [22]. However, as α-Al and eutectic Al3Sc form simultaneously, eutectic Al3Sc particles cannot act as nuclei sites for α-Al grains. Primary Al3Sc particles can form prior to the solidification of the α-Al phase with Sc content higher than the eutectic composition (0.6 wt%). Therefore, primary Al3Sc particles are effective nuclei during the solidification process and the non-dendritic structure exists only in hypereutectic Al–Sc alloys, whereas, due to the high price of scandium, hypereutectic Al–Sc alloys cannot be widely applied. In our work, in view of the non-equilibrium solidification situation, equiaxed grains existed in SZ06 alloy with an actual Sc content of 0.48 wt%. Furthermore, when the Sc content is far less than the eutectic point (SZ24 alloy), nondendritic structure in as-cast state is still present. This illustrates that the addition of Zr can reduce the minimum Sc content required to form primary particles. In this case, primary Al3(Sc, Zr) particles substituted as new nucleation centers for aluminum grains. The refining effect from the primary Al3(Sc, Zr) particles is determined by the density and the quantity of the primary particles. Fine grains are the result of the mass formation of primary Al3(Sc, Zr) particles. The amount of Sc and Zr left in the solid solution decline simultaneously. Davydov [23] indicated that for grain refiner, there were two factors responsible for its degree of cast grain refining: volume density of primary particles and effectiveness of inoculation of these particles. Furthermore, the latter factor was determined by the similarity of crystal lattices of a particle-nucleus and matrix in terms of size and structure. Owing
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Fig. 1. Metallographic images and grain size distribution of the as-cast alloys. (a) SZ06; (b) SZ24; (c) SZ33; (d) SZ42; and (e) SZ60. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
to the similar lattice type and very low lattice misfit between the intermetallic particles and the matrix phase, primary Al3Sc or Al3Zr particles could act as heterogeneous nucleants. However, since scandium atoms diffuse faster than zirconium atoms in aluminum melt, the ratio of the diffusion coefficients DSc/DZr varies from 1800 to 190 between 400 and 550 1C [24], primary Al3Sc particles can separate out more quickly than Al3Zr particles during the early stage of formation of the primary phases. Therefore, the grain refinement effect derived from primary Al3Sc particles (SZ60) is more effective than that of primary Al3Zr particles (SZ06). Besides, it is strange that SZ42 and SZ24 show similar hardness but different grain size. This might be due to the similar reinforcement effect between solution strengthening from the Sc and Zr in the solid solution and fine-grain strengthening from the primary Al3(Sc, Zr).
3.2. Effect of Sc/Zr ratio on recrystallization behavior The grain shapes of different sections in SZ24 alloy after the rolling process are illustrated in Fig. 4. Resulting from the stress during the rolling process, the grains are deformed along the rolling direction. Fig. 4(a) describes the characters of grain shape in different sections. There are obvious deformed grains on the rolling section (Fig. 4(b)) and slender fibrous grains are found on the longitudinal section (Fig. 4(c)). In addition, fine grains without noticeable preferred orientation appear on the transverse section (Fig. 4(d)). By comparing the hardness of the cold-rolled alloys with that of the alloys in solid–solution state (Fig. (5)), the latter is generally lower than the former one. This may be due to the fact that the softening effects caused by the dissolution of precipitated phases exceeded the solution strengthening effect.
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In order to compare the recrystallization behaviors of the alloys with different Sc/Zr ratios, the slender fibrous grains in the longitudinal section after annealing at 470 1C for 1 h were investigated, as shown in Fig. 6. There is obvious recrystallization in the alloy without scandium (SZ06) after solution treatment. The alloy containing only scandium also has a certain tendency of recrystallization, whereas the recrystallization behavior is effectively suppressed when adding zirconium together with scandium. It is established that the Zener drag caused by the distributed precipitates is responsible for the inhibition of the advancing grain boundaries during the annealing process. Zener's theory shows that increasing the ratio between the volume fraction and mean radius of the precipitates, and maintaining a coherent interface between the precipitates and matrix both increase the resistance
Fig. 2. Hardness and mean grain sizes of the as-cast alloys.
to recrystallization [2,19,25]. In our work, more effective antirecrystallization behaviors are present in the Al–Zn–Mg–Sc and Al– Zn–Mg–Sc–Zr alloys compared to the Al–Zn–Mg–Zr alloy in solid– solution states. Strong antirecrystallized effect of Sc and Zr addition is attributed to the fine, secondary Al3(Sc, Zr) particles with a high density appearing in the solid–solution SZ24 alloy, as shown in Fig. 6(f). As they strongly inhibit the motion of subgrain boundaries via the Zener drag during the heat treatment process, deformed microstructures still retain in the Al–Zn–Mg–Sc–Zr alloys after solution treatment. 3.3. Effect of Sc/Zr ratio on age hardening and tensile properties Aging hardening curves of the alloys with different Sc/Zr ratios at 120 1C are presented in Fig. 7. The hardness of the alloys increases with aging time significantly (under-aging state) from the beginning until reaching the maximum value (peak-aging state). Then, the hardness varies in a range of 10–20 HV without dropping with the aging time prolonging. The peak hardness, HV235, is obtained by SZ24 alloy with an aging time of 20 h at 120 1C. With the extension aging time, the hardness of all alloys does not decline sharply. Furthermore, zirconium addition contributes additional stabilization effect to the secondary Al3Sc particles due to the lower susceptibility of Al3(Sc, Zr) particles to coarsen [23]. To demonstrate the influence of Sc/Zr ratio on the mechanical properties in Al–Zn–Mg alloys, tensile tests were carried out and their results are shown in Table 2. Both the highest value of tensile strength and yield strength are acquired by SZ24 alloy in different processes of heat treatment. In solid–solution state, the value of tensile strength and yield strength acquired by SZ24 alloy are 489 MPa and 319 MPa, separately. Moreover, the elongations of all the alloys in the solution state are all over 15%. In the aged alloys, the ultimate tensile strength and yield strength increase obviously,
Fig. 3. Back scattered electron and element line distribution analysis images of the SZ24 alloy. (a) and (b): as-cast; (c) and (d): homogenized at 470 1C for 12 h. The elements in order top-down are Mg (red), Al (green), Zr (blue), Sc (turquoise), Cu (pink) and Zn (violet), respectively. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
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but the elongation declines. In general, a good balance between the tensile strength (667 MPa) and elongation (8–10%) can be obtained by SZ24 alloy after aging at 120 1C for 24 h. With the prolongation of aging time, there is no remarkable change in strength, which is in agreement with the results of aging harding curves. In addition, the yielding-to-tensile ratios of alloys exhibit a significant increase from the solution state to the aging state. The variation range of yield ratio after solution treatment is from 0.55 to 0.70. However, all the yield ratios of alloys in aging state are over 0.90. In view of engineering applications, high-strength weldable aluminum alloys with high yield ratios have a good application prospect in the field of automobile body materials, which requires lightweight alloys with strong abilities to resist deformation. The morphology of tensile fracture surface is shown in Fig. 8. As shown in Fig. 8(a) and (b), obvious deep toughening dimples are distributed on the fracture surface of the SZ24 alloy in solid– solution state, indicating a ductile fracture. After the alloy ages at 120 1C for 24 h (Fig. 8(c)) and 32 h (Fig. 8(d)), dimples become much smaller and shallower than those of the alloys in the solid– solution state, and the brittle characteristics appear in both fractures surfaces, which can be confirmed by the lamellar and transgranular fracture in fractures.
aluminum matrix. These Al3Sc or Al3 (Sc, Zr) precipitates, which range in size from 20 to 30 nm, help in stabilizing the fine grained microstructure and strengthening the alloys by pinning the grain boundary and hindering the dislocation motion (Fig. 9(e)). According to the dispersion-strengthening theory, an increase in the yield strength caused by the Al3(Sc, Zr) precipitates is controlled by a shearing mechanism for smaller sizes, and the Orowan dislocation bypass mechanism for larger sizes. Seidman [4] investigated that the critical precipitate radius for this transition was 2.1 nm in dilute Al–Sc alloy. Therefore, Al3(Sc, Zr) precipitates which range in diameter from 20 nm to 30 nm contribute additional strength enhancement to the aged alloys through Orowan bypass mechanism. According to the Orowan-mechanism theory, the effectiveness of Orowan strengthening derived from the precipitates is
3.4. Characterization of precipitates in the aged Al–Zn–Mg–Sc–Zr alloys TEM images in Fig. 9 show the secondary Al3Zr, Al3(Sc, Zr) and Al3Sc precipitates in Al–Zn–Mg–Sc–Zr alloys after aging at 120 1C for 24 h. The density of Al3Zr particles separated out in the SZ06 alloy (Fig. 9(a)) is much less than that of Al3(Sc, Zr) (Fig. 9(b)–(d)) or Al3Sc (Fig. 9(e)) particles in the other alloys. The presence of Ashby–Brown contrast in the bright-field images as well as the superstructure reflections confirm that these particles are Al3Sc or Al3(Sc, Zr) particles, and these particles are coherent with the
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Fig. 5. Hardness of the cold-rolled and solid–solution alloys.
Fig. 4. Metallographic images of different sections in cold-rolled SZ24 alloy.(a) sketch map; (b) rolling plane; (c) longitudinal section and (d) transverse section.
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Fig. 6. Metallographic and TEM images of the alloys in solid–solution state. (a) SZ06; (b) SZ24; (c) SZ33; (d) SZ42; (e) SZ60; and (f) precipitates in the SZ24 alloy.
Fig. 7. Age harding curves of hardness in the alloys with different Sc/Zr ratios.
approximately inversely proportional to the mean precipitate spacing. In our work, the mean particle spacing of secondary Al3(Sc, Zr) particles separating out in Al–Zn–Mg–Sc–Zr alloys is significantly smaller than that of Al3Sc or Al3Zr in SZ60 and SZ06 alloys during the heat-treating process. Therefore, the additional strengthening from Al3(Sc, Zr) precipitates via Orowan mechanism is larger than that of Al3Sc or Al3Zr precipitates, which can explain that the higher tensile strength was obtained by Al–Zn–Mg–Sc–Zr alloy rather than by the alloy without Sc (SZ06 alloy) or Zr (SZ60 alloy). The amount of Sc and Zr left in the solid solution of the alloys with fine grains (SZ33, SZ42 and SZ60 alloys) is less than that of the alloys with larger grains (SZ24 alloy). Therefore, the secondary Al3(Sc, Zr) precipitates separating out in SZ33, SZ42 and SZ60 alloys are less than the secondary precipitates in SZ24 alloy during heat treatment. TEM images in Fig. 10 show the precipitates in SZ24 and SZ60 alloys after aging at 120 1C for 24 h. The high-density ultrafine η0 precipitates are distributed homogenously within the matrix and some coarsen rod precipitates are distributed discontinuously along the grain boundaries (Fig. 10(a)). HRTEM image of
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an Al3(Sc, Zr) dispersoid, as shown in Fig. 10(b), reveals lattice fringes with an interplanar spacing of 2.041 Å corresponding to the (200) lattice planes of the L12-ordered Al3(Sc, Zr) particles. Through comparative observations, there is no significant difference in the microstructure of η0 phases in the alloys with different Sc/Zr ratios. As described in Fig. 10(c), the selected area diffraction (SAD) pattern in the sample of the aged SZ60 alloy taken along [001] zone axis of Al matrix shows characteristic super-lattice spots from coherent Al3Sc precipitates. Fig. 10(d) shows the darkfield image of the diffraction spot selected by the dotted circle in Fig. 10(c). Previous studies have manifested that the precipitation sequence of the Al–Zn–Mg alloys is usually described as over-saturated solid
Table 2 Mechanical properties of Al–Zn–Mg alloys with different Sc/Zr ratios. Alloys
Condition
UTS (σb/MPa)
0.2% YS (σ0.2/MPa)
EI (δ/%)
Yield ratio (σ0.2/σb)
SZ06
Solution Aged 24 h 32 h
404 607 648
235 552 597
17.1 8.6 10.6
0.58 0.91 0.92
SZ24
Solution Aged 24 h 32 h
489 667 667
319 627 629
19.2 8.3 9.9
0.65 0.94 0.94
SZ33
Solution Aged 24 h 32 h
468 643 632
311 599 597
17.2 9.0 8.6
0.66 0.93 0.94
SZ42
Solution Aged 24 h 32 h
478 616 650
307 573 608
15.9 11.0 8.4
0.64 0.94 0.94
SZ60
Solution Aged 24 h 32 h
445 641 638
264 594 604
19.3 7.9 8.3
0.59 0.93 0.95
225
solution (α) -GP zones-metastable η0 phases -stable η phases (MgZn2) [26,27]. During the initial stage of the aging process, the precipitation reaction starts with the formation of GP zones, which later transform into metastable η0 transition phases, which are more thermally stable than GP zones. The η0 phase will transform into the equilibrium η phase at higher temperature or after longer aging time [17,20]. Besides, the strengthening from minor Sc and Zr is independent of strengthening caused by other aging precipitates in Al–Zn–Mg alloy. The formation dynamics of aging precipitates in the Al–Zn–Mg– Sc–Zr alloys are not affected by the Sc and Zr microalloying additions [21]. Through strengthening calculations conducted on 7XXX series alloys, namely, Al 7050 and Al 7055 based on the existing models, η0 phase is found to be present in significant amount during peak-aged condition [28]. In our studies (Fig. 9 and Fig. 10), η0 phases and secondary coherent Al3(Sc, Zr) precipitates are the main phases in the Al–Zn–Mg–Sc–Zr alloys aged at 120 1C for 24 h. In addition, the higher strength of the alloys with minor additions of Sc and Zr was also derived from the fine-grain strengthening of primary Al3(Sc, Zr) particles formed during solidification and sub-grain strengthening due to retention of a nonrecrystallized structure caused by the secondary Al3(Sc, Zr) precipitates during heat treatment. In our work, the grain refinement is derived from the primary Al3(Sc, Zr) particles. The effects of antirecrystallization and precipitation strengthening are derived from the secondary Al3(Sc, Zr) precipitates. What is more, according to the results of mechanics test, the strength increments caused by refining strengthening of primary Al3(Sc, Zr) particles were less stronger than the strengthening effect caused by the precipitation strengthening from the secondary Al3(Sc, Zr) precipitates.
4. Conclusions New type of Al–Zn–Mg–Sc–Zr alloys with different Sc/Zr ratios has been studied in the aspects of microstructure and mechanical
Fig. 8. Fracture morphology of SZ24 alloy after solution state (a) and (b), and aged for 24 h (c) and for 32 h (d) at 120 1C.
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Fig. 9. TEM micrographs of secondary precipitates in alloys with different Sc/Zr ratios aged at 120 1C for 24 h. (a) SZ06; (b) SZ24; (c) SZ33; (d) SZ42 and (e) SZ60 alloy.
performance. The conclusions below could be drawn from the experimental investigations: 1. Scandium and zirconium are both effective grain refines in Al– Zn–Mg alloys, and the refining ability of scandium is better than that of zirconium with the same addition.
2. Scandium possesses a strong antirecrystallized effectiveness in rolled Al–Zn–Mg–Sc alloy after annealing at 470 1C for 1 h, especially together with zirconium addition. 3. The addition of scandium and zirconium to Al–Zn–Mg alloys improved the strength of the alloys evidently. The highest value of tensile strength was acquired by adding 0.2Sc/0.4Zr(wt%)
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Fig. 10. TEM images of the alloys aged at 120 1C for 24 h (a) bright field image of SZ24 alloy and (b) HRTEM images of Al3 (Sc, Zr) precipitates with the dashed frame in (a); (c) SAD of Fig. 9(e), [001] projection of the Al matrix in SZ60 alloy and (d) dark-field image of Al3Sc precipitates from the diffraction spot in the dashed circle in Fig. 10(c).
with the largest distribution density of Al3(Sc, Zr) precipitate among the designed alloys.
Acknowledgments This work was financially supported by the National High-tech Research & Development Program of China (2013AA031002). References [1] [2] [3] [4] [5] [6] [7]
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