Elsevier

Journal of Alloys and Compounds

Volume 715, 25 August 2017, Pages 421-431
Journal of Alloys and Compounds

Influences of Mn content on the microstructures and mechanical properties of cast Al-3Li-2Cu-0.2Zr alloy

https://doi.org/10.1016/j.jallcom.2017.05.030Get rights and content

Highlights

  • Microstructures-property relationship of cast Al-3Li-2Cu-0.2Zr alloy with different Mn addition was systematically studied.

  • The Mn addition reduced the strength of Al-3Li-2Cu-0.2Zr alloy during ageing.

  • The alloy ductility exhibits a complex behavior after Mn addition in various ageing stage.

  • The optimized Mn addition into Al-3Li-2Cu-0.2Zr alloy was 0.3 wt.%.

Abstract

In this work, the influences of Mn content on the microstructures and mechanical properties of cast Al-3Li-2Cu-0.2Zr alloy were investigated. The results showed that with the increase of Mn addition, the as-cast grain size was gradually reduced, and the primary Al20Cu2Mn3 phase was formed in as-cast higher Mn alloys (0.8Mn and 1.2Mn alloy). The formation of Al20Cu2Mn3 dispersoids can restrict the grain growth during solution treatment, but decrease the number density of Cu-rich precipitates during ageing treatment (mainly T1-Al2CuLi) because they consume the solute Cu available for precipitation. The tensile property results showed that Mn addition had little effect on the yield strength (YS) but a detrimental effect on the ductility of the as-quenched alloys due to the presence of Al20Cu2Mn3 dispersoids and/or primary Al20Cu2Mn3 phase. With increasing the Mn content, the YS of ageing-treated alloys was continuously decreased, and the highest elongation of 3.5% was obtained in 0.3Mn-bearing alloy after ageing for 32 h at 175 °C. From the comprehensive consideration of ductility and strength, the optimal Mn addition in cast Al-3Li-2Cu-0.2Zr alloy was 0.3 wt.%.

Introduction

Lithium (Li) is not only the lightest element (with specific mass of 0.534 g/cm3) among the components used for alloying commercial Al alloys, but also has a substantial solubility in solid Al (4.2 wt.% in the binary Al-Li alloy, all compositions quoted in this work are in wt.% unless otherwise noted) [1]. This is significant because that the introduction of each 1 wt.% Li (up to 4 wt.%) decreases the specific weight of Al alloys by 3% and simultaneously increases the elastic modulus by about 6% [2]. In addition to the enhanced rigidity, Al-Li alloys also possess high strength and resistance to fatigue loads in combination with good corrosion resistance and satisfactory weldability [3], [4]. Possessing such unique properties, Al-Li alloys are hence found increased use in aerospace and military industries where strongly require lightweight-critical and stiffness-critical structures.

At present, Al-Li alloys used in the aerospace industries are mainly wrought alloys such as AA2196 (Airbus A380) and AA2297 (F16 aircraft), while data on cast Al-Li alloys are still limited. This situation is mainly due to the superior mechanical properties of wrought Al-Li alloys to the cast counterparts. Casting is not only a low-cost way to manufacture products with complex shapes, but also an effective approach to mitigate the anisotropy of mechanical property. More importantly, the weight reduction could be much more effective in cast Al-Li alloys, since the limitation of Li addition could be much higher than that in the wrought alloys. Therefore, in order to broaden the application of Al-Li alloys, it is of great importance to investigate the cast Al-Li alloys. In our earlier work, the microstructures and mechanical properties of cast Al-3Li-0.2Zr alloy with different Cu contents were systematically studied [5]. It was found that the cast Al-3Li-2Cu-0.2Zr alloy possessed a relatively high tensile property (YS and UTS) with a slightly low elongation after solution and aging at 175 °C for 32h. The low ductility was mainly ascribed to the presence of a large volume fraction of δ′ (Al3Li) particles due to the extremely high Li content (3%), resulting in planar slip and strain localization. Thus, in an attempt to improve the ductility, some alloying elements are sought for use in cast Al-Li alloys.

As known, the planar slip caused by shearing of δ′ (Al3Li) phases is an important reason for the low ductility of Al-Li alloys [6]. Unfortunately, this situation will become more seriously in cast Al-Li alloys due to their higher Li content. Zr and Sc additions to Al-Li alloys could not only refine the grain size, but also result in the formation of core/shell coherent Al3(M, Li) (M = Zr, Sc, Zr/Sc) precipitates. These composite core/shell Al3(M, Li) particles are resistant to be cut by dislocations during plastic deformation, and can disperse the planar slip arising from shearing of δ′ phase, contributing to enhanced ductility [7]. Our recent work successfully improved the ductility of cast Al-3Li-1.5Cu-0.15Zr alloy by adding trace amount of Sc [8]. However, the high cost of Sc limits its commercial use. Addition of elements that form dispersoids during solidification and/or high temperature homogenization treatment was reported to be an attractive method to improve the ductility in Al-Li alloys [2]. Mn addition into Cu-containing Al-Li alloys may result in the formation of Al20Cu2Mn3 dispersoids during solution treatment. These dispersoids are also resistant to be cut by dislocations, and could effectively inhibit the grain growth during solution [9]. Furthermore, Mn is also used extensively in casting Al-Cu alloys to neutralize the detrimental effect of Fe [10], [11]. Owing to the low ductility, Al-Li alloys are more sensitive to Fe than other conventional Al alloys. Therefore, Mn might be a promising cheap element in promoting the ductility of cast Al-Li-Cu alloy.

Although Mn addition was assumed to be beneficial to the ductility of cast Al-Li-Cu alloys, primary coarse Al20Cu2Mn3 phase will form during casting if the Mn addition exceeds a certain level. These Al20Cu2Mn3 primary phases could not be dissolved into α-Al matrix during solution and have a deleterious effect on the ductility of Al alloys [12]. On the other hand, the Al20Cu2Mn3 dispersoids formed during solution will inevitably consume some Cu element, leading to a reduction in tensile properties as a result of the decreased precipitation hardening. Therefore, it is necessary to optimize the Mn addition in the Al-3Li-2Cu-0.2Zr alloy to obtain a good combination of strength and ductility. In this paper, the microstructure-mechanical property relationship of cast Al-3Li-2Cu-0.2Zr alloy with different Mn content was systematically studied to optimize the Mn addition.

Section snippets

Materials and methods

Al-3Li-2Cu-xMn-0.2Zr alloys were prepared from commercially pure Al and Li, and master alloys of Al-50%Cu, Al-10%Zr and Al-10%Mn using an electric resistance furnace at 750 °C under the protection of LiCl-LiF flux mixture. After the alloy was completely melted, 0.3% C2Cl6 was used for degassing at 740 °C. The melt was fully stirred to ensure the complete homogenization of alloying elements and isothermally held at 720 °C for about 10 min. The melt was subsequently poured into a permanent mould

Microstructures of as-cast Al-3Li-2Cu-xMn-0.2Zr alloy

Fig. 1 exhibits the XRD spectra of as-cast Al-3Li-2Cu-0.2Zr-xMn alloys. It can be seen that all the alloys consist of α-Al, δ (AlLi), δ′ (Al3Li), θ (Al2Cu) and T2 (Al6CuLi3) phases, and among them, the 0.3Mn alloy has an extra TB (Al7.5Cu4Li) phase which is unexpected since the amount of TB is usually extremely low due to the characteristics of peritectic reaction (L + Al2Cu → α-Al + TB) [13]. The presence of Al3Li phase is because that the small misfit between δ′-Al3Li and the matrix results

Conclusions

Based on the results obtained, the following conclusions can be drawn.

  • 1)

    Mn addition into cast Al-3Li-2Cu-0.2Zr alloy could refine the as-cast grain size especially when the Mn content exceeds 0.5%. But primary Al20Cu2Mn3 phase could form in the high Mn alloys, promoting low-energy intergranular fracture and low ductility. During solution, Al20Cu2Mn3 dispersoids were precipitated in these Mn-containing alloys, which could remarkably inhibit the grain growth.

  • 2)

    A reduction and slowing down of ageing

Acknowledgments

This project is supported in part by National Natural Science Foundation of China (No. 51404153) and Shanghai Yang-fan Program (No. 14YF1402000). The authors would like to thank Dr. Jichun Dai at Baosteel Research Institute for his kind help in discussing the manuscript.

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