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Structure and properties of an ultra-high strength 7xxx aluminum alloy contained Sc and Zr
Journal of University of Science and Technology Beijing Volume 15, Number 3, June 2008, Page 276
Materials
Structure and properties of an ultra-high strength 7xxx aluminum alloy contained Sc and Zr Xiaoyuan Dai1, 2), Changqing Xia1), Xiaomin Peng1), and Ke Ma1) 1) School of Materials Science and Engineering, Central South University, Changsha 410083, China 2) School of Materials Science and Engineering, Changsha University of Science and Technology, Changsha 410076, China (Received 2007-05-10)
Abstract: An ultra-high strength aluminum alloy was produced by casting and then extruded to rods. The effect of heat treatment on the microstructure and mechanical properties of the alloy was investigated. After single ageing (120°C, 24 h), the tensile strength was 812.4 MPa and the elongation was 6.2%. After retrogression reaging (RRA), the tensile strength was 751.2 MPa and the elongation was 6.4%. The strengthening mechanism is considered as fine grain strengthening, substructure strengthening and dispersion strengthening by Al3(Sc, Zr). © 2008 University of Science and Technology Beijing. All rights reserved. Key words: aluminum alloy; scandium; zirconium; microstructure; mechanical properties; strengthening mechanism
[This work was financially supported by the National Key Fundamental Research and Development Program of China (No. 2005CB623706).]
1. Introduction The Al-Zn-Mg-Cu series alloys are one of the most important structural materials because of their high strength-to-weight-ratio, attractive specific stiffness, durability, low cost, machinability and easy formability [1], good corrosion resistance and high toughness. The more rapid the development of aerospace and nonmilitary industry is, the higher the requirement for the combination properties of the ultra-high strength aluminum alloys is. The requirement is from single high strength, to high strength and toughness, to high strength, toughness, and good stress corrosion cracking (SCC) resistance. To meet these aims, the design, casting, process, and heat treatment of the alloy have been investigated. Al-Zn-Mg-Cu series ultra-high strength aluminum alloys have been developed by increasing the contents of Zn and Cu to enhance ageing strengthening, reducing the contents of Fe and Si to enhance toughness and fatigue life for compensating the descending of toughness resulting from the content increasing of Zn Corresponding author: Xiaoyuan Dai, E-mail: [email protected] © 2008 University of Science and Technology Beijing. All rights reserved.
and Cu, restraining recrystallization and improving quenching susceptibility and SCC resistance by replacing Cr and Mn with Sc and Zr, adjusting grain boundary precipitation by T77 ageing process to improve SCC and exfoliation corrosion resistance with unchanged high strength [2]. Aiming at improving the synthetic properties of the studied aluminum alloy, the authors increased the content of Zn over 9.0wt%, added minor Sc and Zr, reduced the contents of Fe and Si and controlled the content of Cu below 1.5wt%. At present, Sc is the most efficient element to enhance the properties of aluminum alloys. Sc is a rare earth element; it is also a transition element. The rare earth element can refine and modify the microstructures of aluminum alloys. The transition element can restrain recrystallization. The effect of Sc is superior to that of the two latter. Zr is one of the most important transitional alloy elements used in aluminum alloys. It can improve the welding performance and resistance to stress corrosion, increase the thermal stability and enhance the temperature of recrystallization [3-18]. Also available online at www.sciencedirect.com
X.Y. Dai et al., Structure and properties of an ultra-high strength 7xxx aluminum alloy contained Sc and Zr
2. Experimental The staring materials were 99.96% Al, 99.8% Zn, 99.8% Mg, Al-2.38%Sc, Al-49.8%Cu and Mg-30%Zr master alloy. The composition of the studied alloy is Al-10.3Zn-3.2Mg-1.3Cu-0.16Zr-0.35Sc. The studied alloy was prepared in a crucible electric resistance furnace. The temperature of the alloy melt was maintained at 700-740°C. After homogenization at 450°C for 24 h, the ingots of 50-mm diameter were extruded at 350-420°C into 8-mm diameter bars. The extruded bars were solutionized and then quenched in room temperature water. The single ageing (T6) is 120°C × 24 h. T76 is 120°C × 8 h + 160°C × 16 h. RRA is 120°C × 24 h + 180°C × 20 min + 120°C × 24 h. Tensile samples with a gauge diameter of 3 mm and gauge length of 15 mm were tested on an Instron-8032 tensile test machine at room temperature and the tensile ratio is 2 mm/min. Microstructure examination of the samples was performed using optical microscopy, scanning electronic microscopy (JEOL-5600LV) and transmission electron microscopy (Tecnai G220). The preparation of TEM samples was done by twin-jet electrolytic polishing in a dilute perchloric acid solution.
3. Results and discussion 3.1. Tensile properties of the alloy The tensile properties of the studied alloy under different heat treatment conditions are shown in Table 1.
277
After single aged conditions (T6: 120°C × 24 h) the tensile strength was 812.4 MPa and elongation was 6.2%. After RRA, the tensile strength and elongation were 751.3 MPa and 6.4%, respectively. Simultaneously adding minor Sc and Zr to the Al-Zn-Mg-Cu series aluminum alloy can obviously increase the strength of the alloy, but the ductility remains on a higher level. Table 1.
Tensile properties of the alloy in different states State
σb / MPa
δ/%
Extruded Solution RRA T76 T6
476.2 680.2 751.3 578.6 812.4
9.0 9.9 6.4 10.8 6.2
3.2. Microstructure Figs. 1(a) and (b) show back-scattered electron SEM images of the studied alloy at as-cast. The result shows that the as-cast grains are equiaxed crystal grains, 40-50 µm in average without a dentritic structure. The microstructures of the homogenized ingot are shown in Fig 1(c). Fig. 1(d) shows that the bright, cuboidal shapes in Figs. 1(a) and (b) were identified as Al3(Sc, Zr) primary precipitation using energy dispersive spectroscopy (EDS). It shows that Al3(Sc, Zr) primary particles are precipitated from the melt during solidification and the particles, Al3(Sc, Zr), nucleated heterogeneously within α(Al) grains.
Fig. 1. Microstructures of the studied alloy: (a) and (b) as-cast; (c) homogenization; (d) composition analysis of the square particle in Fig. 1(b), EDS.
The structure can be described as anordered
face-centered cubic (fcc), although in crystallographic
278
terminology, the Al3Sc structure is actually a simple cubic lattice with four atoms (one Sc and three Al atoms) associated with each lattice point. This particular atomic arrangement is nominated L12 and is found in several intermetallic compounds, for instance Cu3Au [5]. According to the heterogeneous nucleation theory, the degree of cast grain refining, attributed to an addition of a modifier, is governed by two factors: number of particles nuclei in a unit of melt volume and effectiveness of the inoculate action of particles nuclei. The latter factor depends on the similarity of crystal lattices of a particle-nucleus and matrix in terms of size and structure. It is considered that the crystallographic similarity is a main cause of the dramatic modifying effect. Al3Sc particles have the same crystal structure as α(Al) matrix, fcc, whose lattice constant is 0.410 nm [17], very close to that of α(Al), 0.404 nm. Previous studies [7, 9, 11, 13] have demonstrated that adding Zr and Ti in conjunction with Sc can reduce the amount of Sc required to achieve grain refinement. Zr and Ti can substitute for Sc in Al3Sc phase. When Zr or Ti is dissolved in Al3Sc, it should be possible to obtain Al3(Sc, Zr) or Al3(Sc, Ti) substrates in the melt that match exactly the lattice parameter of the nucleating α(Al), the lattice mismatch is lower than the one between Al3Sc and α(Al), and thus further improves the grain refinement of the alloy [5]. The crystal lattice of scandium aluminide Al3(Sc, Zr), primary particles of which are the nuclei of solidifying aluminum grains, has unique similarity with that of aluminum in terms of size and structure. The discrepancy between lattice parameters is below 1.5%. Evidently, this cause is really important for refining as-cast grains of the alloys containing Sc. However, at the same time, the presence of a considerable number of the particles-nuclei of scandium aluminide in a unit of aluminum alloy melt volume must be taken into account. The dramatic modifying effect of scandium in aluminum alloys is caused equally by two factors: a considerable number of nuclei in the form of the Al3(Sc, Zr) primary particles in a unit of melt volume and high effectiveness of the inoculate action of these particles. Fig. 2 shows the microstructure of the Al-10.3Zn3.2Mg-1.3Cu-0.16Zr-0.35Sc alloy, which is not recrystallized after solid solution treatment and T6. The results of TEM observation of the studied alloys are shown in Fig. 3. It is shown that there are a great deal of fine precipitates and ungulate particles in Fig. 3(a) and (b). According to prior studies [2, 5, 13, 18], these fine ones are η′-phase (MgZn2). Indeed, a higher strength is required for dislocations to cross-cut large particles, and the Orowan mechanism starts to operate
J. Univ. Sci. Technol. Beijing, Vol.15, No.3, Jun 2008
when the η′-particle size exceeds 6 nm [13]. Transition to the Orowan mechanism generally increases ductility due to a more uniform distribution of dislocation in
Fig. 2.
Microstructure of the studied alloy after T6 (OM).
Fig. 3. RRA.
TEM microstructures of the alloy: (a) T6; (b)
comparison with the situation when the cutting mechanism operates; in the latter case, deformation is localized in planar bands, where the particles are cut. According to the published reports [5, 7, 13, 18], the ungulate particles in Fig. 3 are precipitated during homogenization. These particles are coherent to the matrix, strongly pin dislocations and sub-boundaries, restrain crystal boundary migration and subgrain growth, and enhance the temperature of recrystallization. Fig. 2 shows the microstructure of the studied alloy that remains fibrous after solid solution treat-
X.Y. Dai et al., Structure and properties of an ultra-high strength 7xxx aluminum alloy contained Sc and Zr
ment and T6. An addition of Sc increased both the tensile strength and ductility of extruded rods (Table 1). Probably, the increased number density and uniform distribution of the secondary Al3(Sc, Zr) particles are also important in preventing strain localization and in increasing strength when the cutting mechanism for strengthening operates. The strength of the studied alloy after T6 was 812 MPa. On the one hand, the high strength can be due to a large volume fraction and a smaller size of η′-particles in the studied alloy with high content of Zn. On the other hand, Sc addition causes fine grains and a large volume fraction and smaller size of Al3(Sc, Zr). These particles are precipitated and dispersed in matrix, and they can pin the grain boundaries and dislocations, and stop their moving and annexation. So the strengthening mechanisms of Sc are mainly sub-structure strengthening, precipitation strengthening, and solution strengthening. Sc is expensive, and this is the main reason that the aluminum alloys containing Sc cannot develop fast. But, Zr is cheap. Generally, Sc should be added simultaneously in aluminum alloys with Zr. Zr can substitutionally dissolve into Al3Sc phase (up to 50%) and form Al3(Sc, Zr) phase, whose crystal lattice type is the same as that of Al3Sc and whose lattice parameter is close to that of Al3Sc, which keeps all good properties of Al3Sc phase and owns new good properties [7, 13]. Simultaneous addition of Sc and Zr will reduce the manufacturing cost and provide a way to develop the aluminum alloys containing Sc.
4. Conclusions (1) Adding minor Sc and Zr to the Al-Zn-Mg-Cu series aluminum alloy, the primary Al3(Sc, Zr) precipitated from the melt during solidification is an ideal crystal nucleus and can greatly refine the grain size of the as-cast alloy. The secondary Al3(Sc, Zr) precipitated during homogenization strongly pins dislocations and sub-boundaries, restrains crystal boundary migration and subgrain growth, and enhances the temperature of recrystallization. (2) After single aging (T6: 120°C × 24 h), the tensile strength was 812 MPa and elongation was 6.2%. After RRA, the tensile strength was 751 MPa, and elongation was 6.4%, respectively. Simultaneous adding of Sc and Zr to the Al-Zn-Mg-Cu series aluminum alloy can obviously increase the strength of the alloy.
References [1] T.S. Srivatsan, Microstructure, tensile properties and frac-
[2]
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ture behavior of aluminum alloy 7150, J. Mater. Sci., 27(1992), No.17, p.4772. K.H. Chen and L.P. Huang, Strengthening-toughening of 7xxx series high strength aluminum alloys by heat treatment, Trans. Nonferrous Met. Soc. China, 13(2003), No.3, p.484. V.G. Davydov, V.I. Yelagin, V.V. Zakharov, and Y.A. Filatov, On prospects of application of new 01570 high-strength weldable Al-Mg-Sc alloy in aircraft industry, Mater. Sci. Forum, 217-222(1996), p.1840. O. Roder, O. Schauerte, G. Lüjering, and A. Gysler, Correlation between microstructure and mechanical properties of Al-Mg alloys without and with Scandium, Mater Sci. Forum, 217-222(1996), p.1835. J. Royset and N. Ryum. Scandium in aluminium alloys, Int. Mater. Rev., 50(2005), No.1, p.38. Y.A. Fiatov, V.I. Yelagin, and V.V. Zakharov, New Al-Mg-Sc alloy, Mater. Sci. Eng., A280(2000), p.97. Z.M. Yin, Q.L. Pan, Y.H. Zhang, et al., Effect of minor Sc and Zr on the microstructure and mechanical properties of Al-Mg based alloys, Mater. Sci. Eng. A, 280(2000), p.151. Y. Miura, T. Shioyama, and D. Hare, Recrystallization of Al-3Mg and Al-3Mg-0.2Sc alloys, Mater Sci. Forum, 217-222(1996), p.505. K.L. Kendig and D.B. Miracle, Strengthening mechanisms of Al-Mg-Sc-Zr alloy, Acta Mater., 50(2002), p.4165. V. Ocenasek and M. Slamova, Resistance to recrystallization due to Sc and Zr addition to Al-Mg alloys, Mater. Charact., 47(2001), p.157. V.G. Dvydov, T.D.Rostova, V.V. Zakharov, Y.A. Filatov, and V.I. Yelagin, Scientific principles of making an alloying addition of scandium to aluminum alloys, Mater. Sci. Eng. A, 280(2000), p.30. O.N. Senkov, D.B. Miracle, Y.V. Milman, et al., Low temperature mechanical properties of scandium-modified Al-Zn-Mg-Cu alloys, Mater Sci. Forum, 396-402(2002), p.664. Y.V. Milman, A.I. Sirko, D.V. Lotsko, et al., Microstructure and mechanical properties of cast and wrought Al-Zn-Mg-Cu alloys modified with Zr and Sc, Mater. Sci. Forum, 396-402(2002), p.1127. F.H. Zeng, C.Q. Xia, and Y. Gu, An assessment of Al-Mg-Sc-Zr system in aluminum-rich region, Mater. Rev. (in Chinese), 16(2002), No.6, p.16. C.Q. Xia, F.H. Zeng, and Y. Gu, Phase relations of Al-4Mg-Sc-Zr quaternary system in Al-rich range at 430°C, Trans. Nonferrous Met. Soc. China, 13(2003), No.3, p.547. X.Y. Dai, C.Q. Xia, and C.B. Liu., Effect of heat treatment on microstructures and mechanical properties of Al-Zn-Mg-Cu-Sc-Zr alloys, Hot Working Technol. (in Chinese), 11(2005), p.2. F.H. Zeng, C.Q. Xia, and Y. Gu, The isothermal section of the Al-4Mg-Sc-Zr quaternary system in Al-rich range. J. Alloys Compd., 363(2004), p.175. L.L. Rokhlin T.V. Dobatkina, N.R. Bochvar, E.V. Lysova, Investigation of phase equilibria in alloys of the Al-Zn-Mg-Cu-Z-Sc system, J. Alloys Compd., 367(2004), p.10.
Materials
Structure and properties of an ultra-high strength 7xxx aluminum alloy contained Sc and Zr Xiaoyuan Dai1, 2), Changqing Xia1), Xiaomin Peng1), and Ke Ma1) 1) School of Materials Science and Engineering, Central South University, Changsha 410083, China 2) School of Materials Science and Engineering, Changsha University of Science and Technology, Changsha 410076, China (Received 2007-05-10)
Abstract: An ultra-high strength aluminum alloy was produced by casting and then extruded to rods. The effect of heat treatment on the microstructure and mechanical properties of the alloy was investigated. After single ageing (120°C, 24 h), the tensile strength was 812.4 MPa and the elongation was 6.2%. After retrogression reaging (RRA), the tensile strength was 751.2 MPa and the elongation was 6.4%. The strengthening mechanism is considered as fine grain strengthening, substructure strengthening and dispersion strengthening by Al3(Sc, Zr). © 2008 University of Science and Technology Beijing. All rights reserved. Key words: aluminum alloy; scandium; zirconium; microstructure; mechanical properties; strengthening mechanism
[This work was financially supported by the National Key Fundamental Research and Development Program of China (No. 2005CB623706).]
1. Introduction The Al-Zn-Mg-Cu series alloys are one of the most important structural materials because of their high strength-to-weight-ratio, attractive specific stiffness, durability, low cost, machinability and easy formability [1], good corrosion resistance and high toughness. The more rapid the development of aerospace and nonmilitary industry is, the higher the requirement for the combination properties of the ultra-high strength aluminum alloys is. The requirement is from single high strength, to high strength and toughness, to high strength, toughness, and good stress corrosion cracking (SCC) resistance. To meet these aims, the design, casting, process, and heat treatment of the alloy have been investigated. Al-Zn-Mg-Cu series ultra-high strength aluminum alloys have been developed by increasing the contents of Zn and Cu to enhance ageing strengthening, reducing the contents of Fe and Si to enhance toughness and fatigue life for compensating the descending of toughness resulting from the content increasing of Zn Corresponding author: Xiaoyuan Dai, E-mail: [email protected] © 2008 University of Science and Technology Beijing. All rights reserved.
and Cu, restraining recrystallization and improving quenching susceptibility and SCC resistance by replacing Cr and Mn with Sc and Zr, adjusting grain boundary precipitation by T77 ageing process to improve SCC and exfoliation corrosion resistance with unchanged high strength [2]. Aiming at improving the synthetic properties of the studied aluminum alloy, the authors increased the content of Zn over 9.0wt%, added minor Sc and Zr, reduced the contents of Fe and Si and controlled the content of Cu below 1.5wt%. At present, Sc is the most efficient element to enhance the properties of aluminum alloys. Sc is a rare earth element; it is also a transition element. The rare earth element can refine and modify the microstructures of aluminum alloys. The transition element can restrain recrystallization. The effect of Sc is superior to that of the two latter. Zr is one of the most important transitional alloy elements used in aluminum alloys. It can improve the welding performance and resistance to stress corrosion, increase the thermal stability and enhance the temperature of recrystallization [3-18]. Also available online at www.sciencedirect.com
X.Y. Dai et al., Structure and properties of an ultra-high strength 7xxx aluminum alloy contained Sc and Zr
2. Experimental The staring materials were 99.96% Al, 99.8% Zn, 99.8% Mg, Al-2.38%Sc, Al-49.8%Cu and Mg-30%Zr master alloy. The composition of the studied alloy is Al-10.3Zn-3.2Mg-1.3Cu-0.16Zr-0.35Sc. The studied alloy was prepared in a crucible electric resistance furnace. The temperature of the alloy melt was maintained at 700-740°C. After homogenization at 450°C for 24 h, the ingots of 50-mm diameter were extruded at 350-420°C into 8-mm diameter bars. The extruded bars were solutionized and then quenched in room temperature water. The single ageing (T6) is 120°C × 24 h. T76 is 120°C × 8 h + 160°C × 16 h. RRA is 120°C × 24 h + 180°C × 20 min + 120°C × 24 h. Tensile samples with a gauge diameter of 3 mm and gauge length of 15 mm were tested on an Instron-8032 tensile test machine at room temperature and the tensile ratio is 2 mm/min. Microstructure examination of the samples was performed using optical microscopy, scanning electronic microscopy (JEOL-5600LV) and transmission electron microscopy (Tecnai G220). The preparation of TEM samples was done by twin-jet electrolytic polishing in a dilute perchloric acid solution.
3. Results and discussion 3.1. Tensile properties of the alloy The tensile properties of the studied alloy under different heat treatment conditions are shown in Table 1.
277
After single aged conditions (T6: 120°C × 24 h) the tensile strength was 812.4 MPa and elongation was 6.2%. After RRA, the tensile strength and elongation were 751.3 MPa and 6.4%, respectively. Simultaneously adding minor Sc and Zr to the Al-Zn-Mg-Cu series aluminum alloy can obviously increase the strength of the alloy, but the ductility remains on a higher level. Table 1.
Tensile properties of the alloy in different states State
σb / MPa
δ/%
Extruded Solution RRA T76 T6
476.2 680.2 751.3 578.6 812.4
9.0 9.9 6.4 10.8 6.2
3.2. Microstructure Figs. 1(a) and (b) show back-scattered electron SEM images of the studied alloy at as-cast. The result shows that the as-cast grains are equiaxed crystal grains, 40-50 µm in average without a dentritic structure. The microstructures of the homogenized ingot are shown in Fig 1(c). Fig. 1(d) shows that the bright, cuboidal shapes in Figs. 1(a) and (b) were identified as Al3(Sc, Zr) primary precipitation using energy dispersive spectroscopy (EDS). It shows that Al3(Sc, Zr) primary particles are precipitated from the melt during solidification and the particles, Al3(Sc, Zr), nucleated heterogeneously within α(Al) grains.
Fig. 1. Microstructures of the studied alloy: (a) and (b) as-cast; (c) homogenization; (d) composition analysis of the square particle in Fig. 1(b), EDS.
The structure can be described as anordered
face-centered cubic (fcc), although in crystallographic
278
terminology, the Al3Sc structure is actually a simple cubic lattice with four atoms (one Sc and three Al atoms) associated with each lattice point. This particular atomic arrangement is nominated L12 and is found in several intermetallic compounds, for instance Cu3Au [5]. According to the heterogeneous nucleation theory, the degree of cast grain refining, attributed to an addition of a modifier, is governed by two factors: number of particles nuclei in a unit of melt volume and effectiveness of the inoculate action of particles nuclei. The latter factor depends on the similarity of crystal lattices of a particle-nucleus and matrix in terms of size and structure. It is considered that the crystallographic similarity is a main cause of the dramatic modifying effect. Al3Sc particles have the same crystal structure as α(Al) matrix, fcc, whose lattice constant is 0.410 nm [17], very close to that of α(Al), 0.404 nm. Previous studies [7, 9, 11, 13] have demonstrated that adding Zr and Ti in conjunction with Sc can reduce the amount of Sc required to achieve grain refinement. Zr and Ti can substitute for Sc in Al3Sc phase. When Zr or Ti is dissolved in Al3Sc, it should be possible to obtain Al3(Sc, Zr) or Al3(Sc, Ti) substrates in the melt that match exactly the lattice parameter of the nucleating α(Al), the lattice mismatch is lower than the one between Al3Sc and α(Al), and thus further improves the grain refinement of the alloy [5]. The crystal lattice of scandium aluminide Al3(Sc, Zr), primary particles of which are the nuclei of solidifying aluminum grains, has unique similarity with that of aluminum in terms of size and structure. The discrepancy between lattice parameters is below 1.5%. Evidently, this cause is really important for refining as-cast grains of the alloys containing Sc. However, at the same time, the presence of a considerable number of the particles-nuclei of scandium aluminide in a unit of aluminum alloy melt volume must be taken into account. The dramatic modifying effect of scandium in aluminum alloys is caused equally by two factors: a considerable number of nuclei in the form of the Al3(Sc, Zr) primary particles in a unit of melt volume and high effectiveness of the inoculate action of these particles. Fig. 2 shows the microstructure of the Al-10.3Zn3.2Mg-1.3Cu-0.16Zr-0.35Sc alloy, which is not recrystallized after solid solution treatment and T6. The results of TEM observation of the studied alloys are shown in Fig. 3. It is shown that there are a great deal of fine precipitates and ungulate particles in Fig. 3(a) and (b). According to prior studies [2, 5, 13, 18], these fine ones are η′-phase (MgZn2). Indeed, a higher strength is required for dislocations to cross-cut large particles, and the Orowan mechanism starts to operate
J. Univ. Sci. Technol. Beijing, Vol.15, No.3, Jun 2008
when the η′-particle size exceeds 6 nm [13]. Transition to the Orowan mechanism generally increases ductility due to a more uniform distribution of dislocation in
Fig. 2.
Microstructure of the studied alloy after T6 (OM).
Fig. 3. RRA.
TEM microstructures of the alloy: (a) T6; (b)
comparison with the situation when the cutting mechanism operates; in the latter case, deformation is localized in planar bands, where the particles are cut. According to the published reports [5, 7, 13, 18], the ungulate particles in Fig. 3 are precipitated during homogenization. These particles are coherent to the matrix, strongly pin dislocations and sub-boundaries, restrain crystal boundary migration and subgrain growth, and enhance the temperature of recrystallization. Fig. 2 shows the microstructure of the studied alloy that remains fibrous after solid solution treat-
X.Y. Dai et al., Structure and properties of an ultra-high strength 7xxx aluminum alloy contained Sc and Zr
ment and T6. An addition of Sc increased both the tensile strength and ductility of extruded rods (Table 1). Probably, the increased number density and uniform distribution of the secondary Al3(Sc, Zr) particles are also important in preventing strain localization and in increasing strength when the cutting mechanism for strengthening operates. The strength of the studied alloy after T6 was 812 MPa. On the one hand, the high strength can be due to a large volume fraction and a smaller size of η′-particles in the studied alloy with high content of Zn. On the other hand, Sc addition causes fine grains and a large volume fraction and smaller size of Al3(Sc, Zr). These particles are precipitated and dispersed in matrix, and they can pin the grain boundaries and dislocations, and stop their moving and annexation. So the strengthening mechanisms of Sc are mainly sub-structure strengthening, precipitation strengthening, and solution strengthening. Sc is expensive, and this is the main reason that the aluminum alloys containing Sc cannot develop fast. But, Zr is cheap. Generally, Sc should be added simultaneously in aluminum alloys with Zr. Zr can substitutionally dissolve into Al3Sc phase (up to 50%) and form Al3(Sc, Zr) phase, whose crystal lattice type is the same as that of Al3Sc and whose lattice parameter is close to that of Al3Sc, which keeps all good properties of Al3Sc phase and owns new good properties [7, 13]. Simultaneous addition of Sc and Zr will reduce the manufacturing cost and provide a way to develop the aluminum alloys containing Sc.
4. Conclusions (1) Adding minor Sc and Zr to the Al-Zn-Mg-Cu series aluminum alloy, the primary Al3(Sc, Zr) precipitated from the melt during solidification is an ideal crystal nucleus and can greatly refine the grain size of the as-cast alloy. The secondary Al3(Sc, Zr) precipitated during homogenization strongly pins dislocations and sub-boundaries, restrains crystal boundary migration and subgrain growth, and enhances the temperature of recrystallization. (2) After single aging (T6: 120°C × 24 h), the tensile strength was 812 MPa and elongation was 6.2%. After RRA, the tensile strength was 751 MPa, and elongation was 6.4%, respectively. Simultaneous adding of Sc and Zr to the Al-Zn-Mg-Cu series aluminum alloy can obviously increase the strength of the alloy.
References [1] T.S. Srivatsan, Microstructure, tensile properties and frac-
[2]
[3]
[4]
[5] [6] [7]
[8]
[9] [10]
[11]
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[13]
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