J. Cent. South Univ. Technol. (2010) 17: 19−23 DOI: 10.1007/s11771−010−0004−8

Existing form and action mechanism of minor scandium and zirconium in Al-Cu-Mg alloy

JIANG Feng(姜锋), WEN Kang(文康), JIAN Hai-gen(蹇海根),

JIANG Chun-li(蒋春丽), ZHAO Juan(赵娟), JIANG Long(蒋龙)

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

© Central South University Press and Springer-Verlag Berlin Heidelberg 2010

Abstract: In order to investigate the existing form and action mechanism of minor scandium (Sc) and zirconium (Zr) in Al-Cu-Mg alloy, microstructures of Al-4Cu-1Mg-Sc-Zr alloy under different conditions, including states of as-cast, homogenized, hot-rolled, as-solution and natural aged, were observed by scanning electron microscopy (SEM), X-ray diffractometry (XRD) and transmission electron microscopy (TEM). It is revealed that Sc and Zr are completely dissolved into the supersaturated solid solution in as-cast ingot, but grain refinement is not observed. Coffee-bean-like Al3(Sc, Zr) particles deposit during homogenization of ingot induce an increase in hardness. Al3(Sc, Zr) particles are slightly coarsened in as-solution samples, but they still maintain coherent to matrix, which indicates a high thermal stability of these particles. Good coherency of Al3(Sc, Zr) particles makes some benefits for inhibiting recrystallization and reserving work-hardening. Key words: Al-Cu-Mg-Sc-Zr alloy; Sc; Zr; existing form; action mechanism 1 Introduction

Al-Cu-Mg alloy is a kind of typical heat-treatable strengthened aluminum alloys with high strength, high heat resistance and nice mechanical work properties, which is widely used in aerospace industry. In the past aircraft design only focused on static strength of structural materials, but nowadays damage tolerance, toughness and fatigue resistance are considered as well [1−3]. In recent years, a series of Al-Cu-Mg alloys with high strength and toughness have been developed by purifying and micro-alloying techniques. Scientists in USA developed 2124, 2224, 2324, 2424 and 2524 alloys based on 2024 Al alloy and scientists in USSR developed Д16ч and 1163 alloys based on Д16 alloy by decreasing the contents of impurities Fe, Si and optimizing the alloy components. It was found that resistance of crack propagation can be further improved by adding minor transition elements. Then scientists in USSR developed 1161 and 1163+Zr alloys [3−4]. Meanwhile, it was reported that adding minor Sc is a new prospective method to improve fatigue durability of Al-Cu-Mg alloy [5−8].

The action mechanism of minor Sc in Al-Mg and Al-Zn-Mg alloys are well-known [9−12], and these alloys are strengthened and their weld abilities can also be improved with the addition of Sc. Moreover, XU et al

[13] carried out a series of investigations on variation of microstructure and properties of Al-Cu binary alloy after adding elements of Sc. They found that the dendritic structure is refined after adding Sc. However, information about existing form and action mechanism of minor Sc and Zr in Al-Cu-Mg alloy is still absent.

In order to understand the existing form and action mechanism of minor Sc and Zr in Al-Cu-Mg alloy, microstructures of Al-Cu-Mg-Sc-Zr alloy in different states, including as-cast, homogenized, hot-rolled, as-solution and natural aged, were studied by scanning electron microscopy (SEM), X-ray diffractometry (XRD) and transmission electron microscopy (TEM). 2 Experimental

The nominal chemical composition of Al-4Cu-1Mg- Sc-Zr alloy is shown in Table 1. Table 1 Chemical composition of studied alloy (mass fraction, %)

Cu Mg Zr Sc Al 3.8−4.2 1.0−1.5 0.05−0.10 0.05−0.08 Bal.

Industrial pure aluminum of 99.98%, magnesium of

99.92%, Al-50Cu, Mg-20Zr and Al-2Sc master alloys were used to produce the experimental alloy. Al-2Sc master alloy was made by aluminothermic reduction of

Foundation item: Project(2005DFA50550) supported by International Science and Technology Cooperation Program of China; Project(2005CB623705)

supported by the National Basic Research Program of China Received date: 2009−06−17; Accepted date: 2009−08−31 Corresponding author: JIANG Feng, Professor; Tel: +86−731−88877682; E-mail: jfeng@mail.csu.edu.cn

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the Sc-containing compound molten salt. The raw materials (300 kg in total) were put into the electric resistance furnace and heated at 720−780 ℃ till they were completely melted. After refinement and degassing with argon, the alloy was cast into flat section ingots with dimensions of 300 mm×85 mm by semi-continuous casting method.

Blocks of ingot with dimensions of 12 mm×

12 mm×20 mm were heated at 350, 400, 450, 490 and 500 ℃ respectively for 24 h, and hardnesses were determined to identify the optimal homogenizing process. After homogenizing at the optimal temperature, the flat ingots with dimensions of 300 mm× 85 mm were subjected to a appropriately surface treatment, and then they were hot- rolled into plates of 6 mm. After intermediate annealing, the plates were cold-rolled into sheets of 2 mm. The sheets as-cold-rolled were heated at 495 ℃ for 40 min, quenched in cold water, and then naturally aged for 96 h.

Samples were etched with Keller’s reagent and observed by optical metallography and SEM. The components of precipitates at the grain boundary were examined by energy dispersive spectroscopy (EDS), which was attached to SEM. Intermetallic phases in homogenized samples were identified by a D/Max−2500 X-ray diffractometer. The foil specimens for TEM observation were prepared by twin-jet eletropolishing with an electrolyte solution consisting of 30% HNO3 and 70% methanol. The temperature of electrolyte solution should be lower than −25 ℃, and the working voltage and current of the device were 75 V and 20−30 mA, respectively. TEM observation was taken on Tecnai G2 20 transmission electron microscope. 3 Results and discussion 3.1 Microstructures of Al-4Cu-1Mg-Sc-Zr alloy ingot

Microstructures of Al-4Cu-1Mg and Al-4Cu-1Mg- Sc-Zr alloy ingots are shown in Fig.1. It is apparent that as-cast microstructure of Al-4Cu-1Mg-Sc-Zr alloy, similar to 2024 alloy [14−15], is α-Al solid solution matrix with some dendritic segregations. The size of the solid solution grain is large and discontinuous, and the thickness of the interdendritic net is uneven (Fig.1(b)). EDS identifies that the eutectic blocks at grain boundaries are layers of α+S(Al2CuMg) (point A in Fig.1(c)) and cellular α+θ(CuAl2)+S(point B in Fig.1(c)). Comparing Fig.1(b) with Fig.1(a), it can be found that the addition of 0.08%Sc and 0.1%Zr does not lead to obvious refinement of the grains.

Sc is the strongest inoculant of the grain structure of aluminum alloy ingots. Al3(Sc, Zr) tiny particles deposited in Al alloy containing Sc and Zr act as the best heterogeneous nucleus and grains of ingot are refined,

Fig.1 Microstructures of as-cast alloy ingot: (a) Matrix with dendritic segregation (without addition of Sc and Zr); (b) Matrix with segregation (with addition of Sc and Zr); (c) Eutectic phases at grain boundaries because the tiny particles have small lattice mismatch (about 1.5%) with the matrix. At eutectic temperature the maximum solubility of Sc in aluminum is 0.35% and the content of Sc is 0.55%. The inoculating action of Sc manifests itself at a content exceeding 0.55% (i.e., only in hypereutectic alloys), which may decrease to about 0.18% when Zr exists [16−17]. But the contents of Sc and Zr in this studies alloy are 0.05%−0.08% and 0.05%−0.10%, respectively, and primary Al3(Sc, Zr) particles acting as crystallization nucleus are rare during the casting process. To sum up, there is not an obvious grain refinement. 3.2 Precipitation of Sc and Zr during ingot homogeni-

zation Microstructures of Al-4Cu-1Mg-Sc-Zr alloy ingot

under different homogenization conditions are shown in Fig.2. Comparing Fig.1 with Fig.2(a), it can be found that grain boundary is broadened and the dendritic segregation is obvious after homogenizing at 350 ℃ for 24 h. By contrary, Fig.2(b) shows that grain boundary becomes thinner and the dendritic structures are eliminated after homogenizing at 490 ℃ for 24 h. Taking XRD analysis results into account (Fig.3), it can be explained that, dominant tendency is decomposition of

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Fig.2 Microstructures of Al-4Cu-1Mg-Sc-Zr alloy under different homogenization conditions: (a) 350 ℃, 24 h (SEM); (b) 490 ℃, 24 h (SEM); (c) 490 ℃, 24 h (TEM)

Fig.3 XRD patterns of Al-4Cu-1Mg-Sc-Zr alloy at different homogenization temperatures the supersaturated solid solution and precipitation of intermetallics (CuAl2 and Al2CuMg) at lower homogenization temperature, while at higher temperatures dominant tendency is re-dissolution of coarse intermetallic.

Al3(Sc, Zr) particles precipitated during the process of homogenization can be clearly observed under TEM,

and the sizes of these dispersive coffee-bean-like particles are 10−20 nm (Fig.2(c)). Since the sensitivity limitations of XRD method in identifying micro-phases, Al3(Sc, Zr) phase (which is marked by arrow in Fig.2(c)) is hardly demonstrated in XRD patterns (Fig.3).

Al3(Sc, Zr) particles can still keep complete coherent relationship with the matrix at higher homogenization temperature of 490 ℃. The increase of the hardness of ingots is attributed to the formation of Al3(Sc, Zr) particles. W phase reported in Ref.[16] was not identified in this work. 3.3 High thermal stability of Al3(Sc, Zr) particles

Fig.4 shows microstructures of the alloy in conditions of hot-rolled, solution (495 ℃, 40 min) and natural aging. After hot rolling, unlike typical hot-rolled products, the alloy is of a non-recrystallization structure (Fig.4(a)). Large amounts of dislocation cells and subgrain structures can be observed under TEM (Fig.4(a)). This indicates that Sc and Zr are strong suppressors of dynamic recrystallization during the process of thermal deformation. Al3(Sc, Zr) particles can be observed under TEM in alloys subjected to solution treatment at 495 ℃ for 40 min (Fig.4(b)) and natural aging (Fig.4(c)), respectively. The presence of strain contrast demonstrates the coherent relationship between these particles and the matrix. During solution treatment, Al3(Sc, Zr) particles are hard to dissolve but get slightly coarsened. And after natural aging dispersively distributed Al3(Sc, Zr) particles still keep coherent with matrix. This verifies the high thermal stability of Al3(Sc, Zr) particles, which is consistent with the results in Ref.[18].

Fig.5 illustrates the composition of three particles marked in Fig.4(c). According to the EDS result and morphology, particles 1 and 3 are rod-shaped T phases (Al20Cu2Mn3), while particle 2 shows a coarsened Al3(Sc, Zr) particle.

When adding Sc and Zr into aluminum alloys simultaneously, part of Sc in the Al3Sc (or Al3Zr) phase is substituted by Zr (or Sc). Thus, Al3(Sc, Zr) complex particles are formed. These particles have higher thermal stability and are susceptible to coagulation to a less extent during heating. Furthermore they would be effective in hindering coagulation of second-phase particles during the process of hot-working [16−17].

The high thermal stability of Al3(Sc, Zr) particles makes some benefits for inhibiting recrystallization and reserving work-hardening. There is no re-dissolution of Al3(Sc, Zr) particles during the hot working process. These particles inhibit recrystallization by pinning the dislocations and subgrain boundaries. Consequently, the alloy is strengthened and its recrystallization temperature increases significantly [19−20].

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Fig.4 TEM images of Al-4Cu-1Mg-Sc-Zr alloy under different conditions: (a) Hot rolling; (b) Solution treatment at 495 ℃ for 40 min; (c) Natural aging

Fig.5 Phase compositions of particles 1, 2 and 3 in Fig.4(c) respectively

4 Conclusions

(1) The contents of Sc and Zr in this alloy are both below 0.1% and far lower than the critical value for inoculating. Sc and Zr exist in the form of supersaturated solid solution during solidification, so there is an absence of “crystallization nucleus” (Al3(Sc, Zr) particles). Therefore, the refinement of casting grains in this alloy is not obvious.

(2) Coffee-bean-like Al3(Sc, Zr) deposition particles can be apparently observed during homogenization. Their dispersively distribution and coherent relationship with the matrix strengthen the alloy and improve its hardness.

(3) Al3(Sc, Zr) particles (10−20 nm) precipitated from the supersaturated solid solution possess high thermal stability. They are slightly coarsened and remain coherent with matrix of alloy after solution treatment (495 ℃, 40 min) and natural aging (96 h). Al3(Sc, Zr) particles make some benefits for inhibiting recrystallization and reserving work-hardening with its pinning effect on dislocations and sub-boundaries. References [1] POLMEAR I J. Aluminum alloys—A century of age hardening [J].

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(Edited by YANG You-ping)