Microstructural evolution of Al-8.59Zn-2.00Mg-2.44Cu during homogenization

Abstract

The microstructural evolution and phase transformations of a high-alloyed Al-Zn-Mg-Cu alloy (Al-8.59Zn-2.00Mg-2.44Cu, wt%) during homogenization were investigated. The results show that the as-cast microstructure mainly contains dendritic α(Al), non-equilibrium eutectics (α(Al) + Mg(Zn,Al,Cu)₂), and the θ (Al₂Cu) phase. Neither the T (Al₂Mg₃Zn₃) phase nor the S (Al₂CuMg) phase was found in the as-cast alloy. The calculated phase components according to the Scheil model are in agreement with experimental results. During homogenization at 460°C, all of the θ phase and most of the Mg(Zn,Al,Cu)₂ phase were dissolved, whereas a portion of the Mg(Zn,Al,Cu)₂ phase was transformed into the S phase. The type and amount of residual phases remaining after homogenization at 460°C for 168 h and by a two-step homogenization process conducted at 460°C for 24 h and 475°C for 24 h (460°C/24 h + 475°C/24 h) are in good accord with the calculated phase diagrams. It is concluded that the Al-8.59Zn-2.00Mg-2.44Cu alloy can be homogenized adequately under the 460°C/24 h + 475°C/24 h treatment.

Keywords

aluminum alloys / homogenization / microstructural evolution / phase composition / thermodynamic calculations

References

[1] Starke EA, Staley JT. Application of modern aluminum alloys to aircraft. Prog. Aerosp. Sci., 1996, 32(2–3): 131.

[2] Liu J. Advanced aluminium and hybrid aerostructures for future aircraft. Mater. Sci. Forum, 2006, 519–521, 1233.

[3] Warren AS. Developments and challenges for aluminum: a Boeing perspective. Mater. Forum, 2004, 28, 24.

[4] Yu JJ, Li XM. Modelling of the precipitated phases and properties of Al-Zn-Mg-Cu alloys. J. Phase Equilib. Diffus., 2011, 32(4): 350.

[5] Li WB, Pan QL, Xiao YP, He YB, Liu XY. Microstructural evolution of ultra-high strength Al-Zn-Cu-Mg-Zr alloy containing Sc during homogenization. Trans. Nonferrous Met. Soc. China, 2011, 21(10): 2127.

[6] Mulherin JH, Rosenthal H. Influence of nonequilibrium second-phase particles formed during solidification upon the mechanical behavior of an aluminum alloy. Metall. Trans., 1971, 2, 427.

[7] Lim ST, Lee YY, Eun IS. Dilute alloy designs of 7xxx aluminum alloys for thick forging applications. Mater. Sci. Forum, 2005, 475–479, 369.

[8] Deng YL, Wan L, Wu LH, Zhang YY, Zhang XM. Microstructural evolution of Al-Zn-Mg-Cu alloy during homogenization. J. Mater. Sci., 2011, 46(4): 875.

[9] Fan XG, Jiang DM, Meng QC, Li NK, Sun ZX. Evolution of intermetallic phases of Al-Zn-Mg-Cu alloy during heat treatment. Trans. Nonferrous Met. Soc. China, 2006, 16(Suppl.3): 1247.

[10] Deng Y, Yin ZM, Cong FG. Intermetallic phase evolution of 7050 aluminum alloy during homogenization. Intermetallics, 2012, 26, 114.

[11] Li XM, Starink MJ. The effect of compositional variations on the characteristics of coarse intermetallic particles in overaged 7xxx Al alloys. Mater. Sci. Technol., 2001, 17, 1324.

[12] Raghavan V. Al-Cu-Mg-Zn (aluminum-copper-magnesium-zinc). J. Phase Equilib. Diffus., 2007, 28(2): 211.

[13] Belov NA, Eskin DG, Aksenov AA. Multicomponent Phase Diagrams: Applications for Commercial Aluminum Alloys, 2005, Amsterdam, Elsevier, 212.

[14] Li YX, Li P, Zhao G, Liu XT, Cui JZ. The constituents in Al-10Zn-2.5Mg-2.5Cu aluminum alloy. Mater. Sci. Eng. A, 2005, 397(1–2): 204.

[15] Mondal C, Mukhopadhyay AK. On the nature of T(Al₂Mg₃Zn₃) and S(Al₂CuMg) phases present in as-cast and annealed 7055 aluminum alloy. Mater. Sci. Eng. A, 2005, 391(1–2): 367.

[16] Xie FY, Yan XY, Ding L, Zhang F, Chen SL, Chu MG, Chang YA. A study of microstructure and microsegregation of aluminum 7050 alloy. Mater. Sci. Eng. A, 2003, 355(1–2): 144.

[17] Mukhopadhyay AK. Selection and design principles of wrought aluminium alloys for structural applications. Mater. Sci. Forum, 2012, 710, 50.

[18] Xu DK, Rometsch PA, Birbilis N. Improved solution treatment for an as-rolled Al-Zn-Mg-Cu alloy: Part I. Characterisation of constituent particles and overheating. Mater. Sci. Eng. A, 2012, 534, 234.

[19] Richard D, Adler PN. Calorimetric studies of 7000 series aluminum alloys: I. Matrix precipitate characterization of 7075. Metall. Trans. A, 1977, 8(7): 1177.

[20] Chen KH, Liu HW, Zhang Z, Li S, Todd RI. The improvement of constituent dissolution and mechanical properties of 7055 aluminum alloy by stepped heat treatments. J. Mater. Process. Technol., 2003, 142(1): 190.

[21] Gao FH, Zhao G, Bian WM, Tian N. TEM in-situ investigation on non-equilibrium eutectics in semicontinuous casting ingot of Al-6.2Zn-2.3Mg-2.3Cu super-high strength aluminum alloy. Mater. Sci. Forum, 2010, 638–642, 384.

[22] Han X L, Xiong BQ, Zhang YA, Li ZH, Zhu BH, Wang F. Microstructure of as-cast and as-homogenized 7150 aluminum alloy. Trans. Mater. Heat Treat., 2010, 31(11): 104.

[23] Zhong H, Han Y, Deng ZZ, Le YK. Study on the microstructure of as-cast and homogenized 7150 aluminum alloy. Heat Treat. Met., 2008, 33(4): 36.