J. Cent. South Univ. (2013) 20: 2083−2089 DOI: 10.1007/s11771-013-1710-9

Effects of solution treatment on mechanical properties and microstructures of Al-Li-Cu-Mg-Ag alloy

YU Cheng(余成)1, YIN Deng-feng(尹登峰)1, ZHENG Feng(郑峰)1, 2, YU Xin-xiang(余鑫祥)1

1. School of Materials Science and Engineering, Central South University, Changsha 410083, China;

2. Key Laboratory of Nonferrous Materials of Ministry of Education (Central South University), Changsha 410083, China

© Central South University Press and Springer-Verlag Berlin Heidelberg 2013

Abstract: Mechanical properties and microstructures of Al-Li-Cu-Mg-Ag alloy after solution treatments were investigated by means of optical microscopy (OM), tensile test, hardness measurement and electrical conductivity test, differential scanning calorimetric (DSC), energy dispersive X-ray (EDX), scanning electron microscopy (SEM) and transition electron microscopy (TEM), respectively. The results show that both tensile strength and hardness increase first and then decrease with temperature at constant holding time of 30 min with maximum strength and hardness appearing at 520 °C. Tensile strength, hardness and elongation of samples treated at 520 °C for 30 min are 566 MPa (σb), 512 MPa (σ0.2), HB 148 and 8.23% (δ), respectively. There are certain amount of fine T1 (Al2CuLi) phase dispersing among Al substrates according to TEM images. This may result in mixed fracture morphology with trans-granular and inter-granular delamination cracks observed in SEM images. Key words: Al-Li-Cu-Mg-Ag alloy; solution treatment; microstructure; mechanical properties

1 Introduction

Of high specific modulus, high strength, high stiffness and low density, commercial aluminum-lithium- copper alloy has drawn extensive attention among materials scientists [1−2] with special applications in aerospace and aircraft industries. Since 1980s, many researches [3] have been conducted on Al-Li-Cu alloy with focus on its physical and mechanical properties as a function of solution treatment.

Mechanical properties and microstructure evolutions of Al-Li-Cu alloy are very sensitive to solidification, heat treatment and deformation [4]. Solution treatment is often the first step used in heat treatments [5−6]. During solution treatment, the alloy is exposed to a predefined temperature approaching to the lowest melting point. At such a high temperature, soluble atoms including Cu, Li, Mg will dissolve into matrix to create supersaturated solid solutions which are beneficial for subsequent aging precipitations and to promote mechanical property for Al-Li-Cu-Mg-Ag alloy.

Recently, many works on relationship between principle of heat treatment and microstructure of Al-Li-Cu-Mg-Ag alloy have their aims focusing on aging procedure or deformation. To our knowledge, the effect of solution treatment of such alloy is not very well

understood [7]. It is then the objective of our work to study the effect of solution treatment on microstructure evolution and physical properties of Al-Li-Cu-Mg-Ag alloy through coupling of mechanical tests, SEM and TEM analysis as well. Optimal processing parameters of solution treatment and their associated results will provide indispensable information for further study of Al-Li-Cu-Mg-Ag alloy. 2 Experimental

Commercially available pure Al, Li, Mg and master alloys Al-Cu, Al-Zr, Al-Ce etc (listed in Table 1) were used and melted inside induction graphite crucible under protection of flowing Ar and cast into Cu mold for testing samples. The cast ingots were homogenized at 450 °C for 6 h and at 500 °C for 8 h in salt bath furnace prior to hot and cold rolling to 2 mm in thickness. Tensile specimens were cut parallel to rolling direction. Samples were solution treated at 510, 515, 520 and 525 °C for various time as shown in Table 2, followed by artificial aging at 160 °C for 20 h.

Table 1 Nominal composition of testing alloy (mass fraction, %)

Cu Li Mg Ag Zr Ce Al

5.8 1.3 0.4 0.4 0.14 0.3 Bal.

Foundation item: Project(6140506) supported by GAD (General Armament Department), China Received date: 2012−05−31; Accepted date: 2012−10−29 Corresponding author: YIN Deng-feng, Associate Professor, PhD; Tel: +86−731−88879341; E-mail: dfyin@126.com

J. Cent. South Univ. (2013) 20: 2083−2089

2084

Table 2 Solution treatment of Al-Li-Cu-Mg-Ag alloy

Temperature/°C Holding time/min

510 — 30 — —

515 — 30 — —

520 20 30 40 50

525 — 30 — —

Tensile tests were carried out on a WD-10A

universal tensile machine at rate of 2 mm/min. Hardness tests were performed on HBE-3000 machine under loading force of 2.5 kN for 30 s. Specimens were ground, polished, rewashed and etched with Keller solution (1 mL HF+1.5 mL HCl+2.5 mL HNO3+95 mL H2O) for microstructure observation using POLYVER-MET optical microscopy. Fracture morphology of tensile samples was examined by KYKY-1000 scanning electron microscope (SEM). Specimens for transmission electron microscope (TEM) analysis were prepared by twin-jet electro-polishing using 75% methanol and 25% nitric solution at −30 °C and examined using TECNAl G220 microscope. Composition of large secondary particles was measured by energy dispersive X-ray (EDX) analyzer. Differential scanning calorimetric (DSC) analysis and electrical conductivity test were carried out on SDT-Q600 differential scanning calorimetric and

D60K digital metallic conductivity machine, respectively.

3 Results

3.1 Mechanical properties of aged Al-Li-Cu-Mg-Ag alloy samples after solution treatment Figures 1 and 2 show the mechanical properties of

aged Al-Li-Cu-Mg alloy samples after solution heat treatments. Figure 1 shows that tensile strengths (σb and σ0.2) first increase then decrease with temperature and time; while the behavior of elongation (δ) keeps dropping from 9.9% to 7.24%. Moreover, σb and σ0.2 increase rapidly with temperature between 500 and 520 °C, followed by slightly decrease above 520 °C. The maximum σb and σ0.2 are achieved around 520 °C for 30 min with values of 563.4 MPa and 506.2 MPa, respectively. Those results show that the strength and ductility of Al-Li-Cu-Mg alloy containing trace Ag, Zr and Ce can be optimized by solution treatment (520 °C for 30 min). Similar to tensile strength, hardness increases rapidly before 520 °C and 30 min (Fig. 2(a)). Electrical conductivity not only reflects the conductivity of materials, but also shows relationship between solubility of soluble atoms and solution treatment [8−9]. We find that the electrical conductivities of our samples

Fig. 1 Tensile properties of aged alloy after solution treatments: (a) Effect of temperature; (b) Effect of time

Fig. 2 Hardness and electrical conductivity of aged alloy after solution treatments: (a) Effect of temperature; (b) Effect of time

J. Cent. South Univ. (2013) 20: 2083−2089

2085

decrease once solution treatment is less than 30 min and keep unchanged between 30 and 50 min, as shown in Fig. 2(b). Many researchers [10] have reported this phenomenon but have not given clear explanation. Optimal solution treatment parameters deduced from our results are 520 °C and 30 min, which should have significant impact on alloy for further artificial aging. 3.2 Effect of solution treatment on microstructure 3.2.1 EDX and DSC analysis of residual phases

EDX analysis and corresponding DSC curve of one as-cast sample after solution treatment are presented in Fig. 3. We can find several phases with different morphologies remain in matrix (Fig. 3(a)) and their corresponding EDX analyses are shown in Figs. 3(b) and 3(c). The spherical particle pointed by arrow (Point 1) is Fe-containing phase (Al7Cu2Fe) forming during solidification, which does not dissolve into Al matrix after homogenization and solution treatments. While the content of iron is limited in Fig. 3(c), and the residual phase with rod shape pointed by arrow (Point 2) is Cu-riched phase (θ′, Al2Cu, see Fig. 3(b)). θ′ is the main strengthening phase in aged Al-Cu alloy [1, 4, 8−9].

Figure 3(d) shows DSC curve of as-cast sample. There is no heat reaction between 180 °C and 490 °C. Endothermic and exothermic reactions occur around 518−550 °C, as marked by A, B, C and D, respectively.

A clear peak of endothermic reaction is shown at A (525 °C) referring to dissolution of either atom clusters, secondary phases or particles of impurity forming during solidification. Note here, samples maybe over-burn at temperature exceeding 525 °C. A small peak at B results from re-precipitation of metastable phases, such as δ′ (Al3Li) or GP zones due to the effect of heat flow from DSC test. Eutectic structures with low melt point melt to form liquid at D (655−660 °C). According to DSC test, we find out that solution temperature should be below 525 °C. Then, we select 520 °C and 30 min as solution treatment parameters, which is consistent with optimum mechanical properties shown in Figs. 1 and 2. 3.2.2 Optical images, SEM and TEM analysis

Figure 4 shows typical optical images of tested samples after solution treatment. Figures 4(a)−4(d) are those treated between 510 and 520 °C for 15 min, while Figs. 4(e)−4(h) show those held at 520 °C for 20, 30, 40 and 50 min, respectively. According to Fig. 4(a), there are many residual phases and coarse secondary particles distributing along sub-grain and grain boundaries. We also find that grain size is not homogeneous and dislocations are tangled around sub-grain boundaries. And also, solute atoms and solvable secondary phases [11] redissolve into aluminum matrix gradually with temperature increasing, accompanied by reduction of

Fig. 3 EDX graphs of residual phases and DSC curve of casted alloy: (a) SEM image; (b) EDX of Point 1; (c) EDX of Point 2;

(d) DSC curve

J. Cent. South Univ. (2013) 20: 2083−2089

2086

Fig. 4 Optical microscopy structures of Al-Li alloy samples: (a) 510 °C, 15 min; (b) 515 °C, 15 min; (c) 520 °C, 15 min; (d) 525 °C,

15 min; (e) 520 °C, 20 min; (f) 520 °C, 30 min; (g) 520 °C, 40 min; (h) 520 °C, 50 min

density of dislocations (Figs. 4(b)−4(d)). Similar to the effect of temperature, size and concentration of secondary phases and density of dislocations decrease

after samples are treated at 520 °C for 20, 30, 40 and 50 min (Fig. 4(e)−4(h)). In conclusion, microstructural evolutions of samples are susceptible to solution

J. Cent. South Univ. (2013) 20: 2083−2089

2087

treatment, which has important influence on mechanical properties of aged samples as shown in Figs. 1 and 2.

Figure 5 gives SEM images of tensile fracture surfaces of aged Al-Li-Cu-Mg-Ag samples after various solution treatments. Features of tensile fracture with clear cleavage fracture and delamination cracks can be observed clearly and dimples with different sizes and shapes are also seen around delaminated structures as well. Of course, lamellar structures get clear and fracture morphology evolves from trans-granular crack to a combination of trans-granular and inter-granular fracture with temperature increasing. However, the lamellar structures and dimples (see Fig. 5(b)) become thinner, smaller and deeper than those shown in Figs. 5(a) and 5(c), respectively, and fracture morphology of samples tends to be smooth crystal-sugar shapes as temperature approaches 525 °C. The magnified fracture surface of aged sample after being treated at optimal processing, 520 °C and 30 min (Fig. 5(d)), shows that those thin lamellar structures and small dimples make many contributions to improve strength and ductility of alloy, which is consistent with the mechanical property tests shown in Figs. 1 and 2.

Figure 6 shows TEM micrographs of aged samples after solution treatments. Plenty of aging precipitates can be observed, such as T1 (Al2CuLi) phase detected by selected area electronic diffraction patterns in the [100] direction (Fig. 6(f)), precipitating homogeneously in Al matrix. Certainly, θ′ (Al2Cu) phases can also be observed. Although the density and size of precipitates are small,

they distribute dispersedly when solution temperature is 510 °C (see Fig. 6(a)). The densities of these major strengthening phases become greater and precipitates distribute more homogeneously with temperature increasing, accompanied with growing up to large size as shown in Figs. 6(b) and 6(c). There are little precipitates observed in the narrow precipitate free zone around grain boundary in aged alloy after treated at 520 °C, 30 min (see Fig. 6(e)), which can promote fracture toughness and improve fracture mechanisms of alloy. 4 Analysis and discussion

As has been discussed before, improvement of aged alloy in tensile strength and toughness can be obtained through suitable solution treatment making alloy elements and precipitates dissolve into Al matrix mostly [12−13], which can refine grains and facilitate further artificial aging properties. The most important processing solution parameters are holding time and temperature which have significant effects on the mechanical properties of alloys. Moreover, the greater the supersaturated solubility of soluble atoms is, the higher the strength of aged alloy after solution treatment will be obtained. The strengthening mechanisms of Al-Li-Cu- Mg-Ag alloy are mainly fine grain strengthening and precipitation strengthening [14]. The precipitation strengthening mechanism is mainly attributed to large supersaturated solubility of soluble atoms after solution treatment, which is beneficial for subsequent aging

Fig. 5 SEM fracture surface morphologies of aged alloy solution treated: (a) 510 °C, 30 min; (b) 515 °C, 30 min; (c) 525 °C, 30 min;

(d) 520 °C, 30 min

J. Cent. South Univ. (2013) 20: 2083−2089

2088

Fig. 6 TEM micrographs of aged alloy solution treated: (a) 510 °C, 30 min; (b) 515 °C, 30 min; (c) 525 °C, 30 min; (d) 520 °C,

30 min; (e) PFZ at 520 °C, 30 min; (f) [100] direction

precipitation [15]. With temperature approaching to over-burnt point, more residual phases redissolve into Al matrix, which creates higher kinetics and dynamics for aging precipitation strengthening of Al-Li-Cu-Mg-Ag alloy. And the extent of lattice distortion and probability of electron diffraction will also increase quickly, accompanied by reduction of alloy mechanical and physical properties. According to Hall-Petch relationships [16], both fraction of microstructure recrystallization and grain size increase greatly with temperature increasing, which deteriorates the effects of the fine grain strengthening. These conclusions are well consistent with results of tensile strength and hardness tests as shown in Fig. 1. At the same time, electrical

conductivity of tested samples decreases quickly first and then keeps at a stable stage around 22.00 S/m with temperature up to 520 °C (Fig. 2), which means that solubility of soluble atoms approaches supersaturation and good combination of strength, ductility and toughness can be obtained. Therefore, optimal processing parameters of solution treatment are very important to benefit for subsequent aging precipitation strengthening.

Obtained from the TEM images, aged Al-Li-Cu-Mg-Ag alloy samples treated at 520 °C, 30 min precipitate a lot of fine uniformly distributed T1(Al2CuLi) and a few needle-like phases θ′(Al2Cu). When alloy samples are treated at high solution

J. Cent. South Univ. (2013) 20: 2083−2089

2089

temperature limited to over-burnt point, the concentration of vacancies and soluble atoms, such as Cu and Li in Al substrates [17−18] will both increase and get supersaturated to benefit for increasing kinetics of aging precipitation (T1 and θ′) after quenching [19]. On one hand, dynamics and kinetics of secondary phases transformation and soluble atoms diffusion are promoted with solution temperature increasing; on the other hand, when alloy samples are held for suitable solution time, the secondary particles can dissolve into Al matrix perfectly accompanied by fine phases precipitating around grain boundary or sub-grain boundary [20−21]. 5 Conclusions

1) The optimal solution temperature and time obtained in this work for Al-1.3Li-5.8Cu-0.4Mg-0.4Ag- 0.14Zr-0.3Ce (mass fraction, %) alloy are 520 °C and 30 min, respectively.

2) Solution parameters (temperature and time) have a significant influence on mechanical properties and microstructure evolutions of alloys.

3) Under the solution condition (520 °C, 30 min), aged alloy samples can achieve excellent microstructures, in which a large amount of fine uniformly distributed T1

(Al2CuLi) and a few needle-like phases θ′(Al2Cu) can be observed, and also attain corresponding mechanical properties: σb=566 MPa, σ0.2=512 MPa, δ=8.23% and hardness HB148. References [1] YIN Deng-feng, ZHENG Zi-qiao. History and current status of

aluminum-lithium alloy [J]. Materials Review, 2003, 17(2): 18−20.

(in Chinese)

[2] HUANG Lan-ping, ZHENG Zi-qiao, LI Shi-chen, WEI Xiu-yu,

HUANG Bi-ping. Study and application of aluminum-lithium alloy

[J]. Materials Review, 2002, 16(5): 20−23. (in Chinese)

[3] ZHANG Xin-ming, ZHENG Da-wei, YE Ling-ying, TANG Jian-guo.

Superplastic deformation behavior and mechanism of 1420 Al-Li

alloy sheets with elongated grains [J]. Journal of Central South

University of Technology, 2010, 17(1): 659−665.

[4] FAN Yun-qiang, CHEN Zhi-guo, ZHENG Zi-qiao, TAN Cheng-gu,

LI Shi-chen, LI Ming-hong. Effects of multi-stage ageing treatments

on microstructure and mechanical properties of Al-Cu-Li-Mg-Mn-Zr

alloy [J]. The Chinese Journal of Nonferrous Metals, 2005, 15(4):

590−595. (in Chinese)

[5] ZHANG Xin-ming, LIU Sheng-dan. Effect of solution treatment on

the strength and fracture toughness of aluminum alloy 7050 [J].

Journal of Alloys and Compounds, 2011, 509(25): 4138−4145.

[6] YUAN Zhi-shan, LU Zheng, XIE You-hua, WU Ming, CUI

Wen-hua, ZHANG Zhe-xiang. Effect of plastic deformation on

microstructure and properties of high strength Al-Cu-Li-X

aluminum-lithium alloy [J]. Rare Metal Materials and Engineering,

2007, 36(3): 493−495. (in Chinese)

[7] ZHANG Xin-ming, YE Ling-ying, DU Yu-xuan, LUO Zhi-hui.

Particle stimulated nucleation of recrystallization in 01420 Al-Li

alloy [J]. Journal of Central South University: Science and

Technology, 2007, 38(1): 19−23. (in Chinese)

[8] SHERCLIFF H R, ASHBY M F. A process model for age hardening

of aluminum alloy [J]. Acta Metallurgy, 1990, 38(10): 1789−1802.

[9] GHOSH K S, DAS K, CHATTERJEE U K. Characterization of

retrogression and re-aging behavior of 8090 Al-Cu-Li-Mg-Zr alloy

[J]. Metallurgical and Materials Transactions A, 2004, 35(8): 3681− 3691.

[10] SONG Mei, HE Yuan, XIAO Dong, HUANG Bo. Effect of

thermomechanical treatment on the mechanical properties of an

Al-Cu-Mg alloy [J]. Mater Des, 2009, 30(3): 857−861.

[11] SENKOV O N, SHAGIEV M R, SENKOVA S V, MIRACLE D B.

Precipitation of Al3(Sc, Zr) particles in an Al-Zn-Mg-Cu-Sc-Zr alloy

during conventional solution heat treatment and its effect on tensile

properties [J]. Acta Mater, 2008, 56(15): 3723−3729.

[12] ZHU Zhi, STARINK M J. Solution strengthening and age hardening

capability of Al-Mg-Mn alloys with small additions of Cu [J].

Material Science and Engineering A, 2008, 488(12): 125−133.

[13] STARINK M J, HOBSON A J, SINCLAIR I, JOHNSTON W.

Embrittlement of Al-Li-Cu-Zr alloys at slightly elevated

temperatures: Microstructural mechanisms of hardening [J].

Materials Science and Engineering A, 2000, 289(6): 130−142.

[14] EDDAHBI M, JIMÉNEZ J A, RUANO O A. Microstructure and

creep behaviour of an osprey processed and extruded

Al-Cu-Mg-Ti-Ag alloy [J]. Journal of Alloys and Compound, 2007,

43(13): 97−107.

[15] BAKAVOS D, PRANGNELL P B, BES B, EBERL F. The effect of

silver on micro-structural evolution in two 2xxx series Al-alloys with

a high Cu:Mg ratio during ageing to a T8 temper [J]. Material

Science and Engineering A, 2008, 491(9): 214−223.

[16] XIAO Dong-hong, SONG Mei. Super-plastic deformation of an

as-rolled Al-Cu-Mg-Ag alloy [J]. Material Designs, 2009, 30(2):

424−429.

[17] ZHANG Da-long, ZHNEG Li-hui, STJOHN D H. Effect of a short

solution treatment time on microstructure and mechanical properties

of modified Al-7wt.%Si-0.3wt.% Mg alloy [J]. Light Metals, 2002,

2(1): 27−36.

[18] LI Hong-ying, TANG Yi, ZENG Zai-de. Effect of aging time on

strength and microstructures of an Al-Cu-Li-Zn-Mg-Mn-Zr alloy [J].

Materials Science and Engineering A, 2008, 498: 314−320.

[19] ROBSON J D. Optimizing the homogenization of zirconium

containing commercial aluminium alloys using a novel process

model [J]. Materials Science and Engineering A, 2002, 338(14):

219−229.

[20] WLOKA J, HACK T, VIRTANEN S. Influence of temper and

surface condition on the exfoliation behavior of high strength

Al-Zn-Mg-Cu alloys [J]. Corrosion Science, 2007, 49(3): 1437− 1449.

[21] ROBINSON J S, TANNE D A. The influence of aluminum alloy

quench sensitivity on the magnitude of heat treatment induced

residual stress [J]. Materials Science Forum, 2006, 524−525:

305−310.

(Edited by HE Yun-bin)