Cooperative Effect of Li Content and Equal-Channel Angular Pressing on Microstructure and Mechanical Properties of Al-Mg-Li Alloy

This paper investigated the cooperative effects of Li addition and warm Equal Channel Angular Pressing (ECAP) on improving the mechanical properties of an Al-Mg-Li alloy for further weight reduction. The results showed that more Li addition could obviously refine the grains of the as-cast alloy and reduce its density value to only 2.37 g/cm3 when Li content reached 2.5% (about 12.6% lighter than commercial Al alloys). It also had a pronounced influence on the precipitations in the Al-5.5Mg-xLi-0.1Zr alloy, i.e., the number density and size of Al2MgLi and δ’-Al3Li phases. The yield strength of the as-cast alloy was increased with increasing Li, but the elongation deteriorated due to primary network intergranular Al2MgLi and more δ’ particles in the high Li-containing alloy. The warm multi-pass ECAP process was found to simultaneously improve the strength and ductility of the Al-Mg-Li alloys, and greatly alleviate the detrimental effect of Li addition on the elongation, thereby making the ultrafine-grained (UFG) alloys, a good combination of lightweight and high performance, very attractive for aerospace applications for much higher specific strength than both Al matrix composites and Mg alloys.


Introduction
Since weight reduction has been considered as a more and more important contributor to performance and operating cost associated with fuel efficiency, the strength/weight ratio has become the prime driver for designers to select materials for aerospace systems [1]. Under such circumstances, aluminum-lithium alloys, with lower density, higher strength, and better stiffness than conventional aluminum alloys, are considered as promising structure materials to reduce cost in the aerospace industry [2,3]. It is well known that Al-Mg-Li alloys, as one of the industrially used categories of Al-Li alloys, have the lowest densities available for a commercial aluminum alloy. Therefore, it attracts specific interest in applications that have ever-increasing demands of being light-weight, weldability, and corrosion resistance, but require only moderate strength.
The significant weight reduction effect of Al-Mg-Li alloys is associated with the addition of Lithium (Li) and Magnesium (Mg). It has been reported that Mg contributes to the strength of Al alloys mainly by solid-solution strengthening. In Al-Li alloys, Mg addition also reduces in the solid solubility of Li to facilitate the precipitation of fine δ'-Al 3 Li phases and possibly incorporates into the δ' itself [4]. High Mg contents (>4 wt%) tend to favor the formation of the equilibrium cubic phase dimension of 19.5 mm × 19.5 mm × 45 mm cut from the as-cast alloys were heat-treated at 500 • C for 24 h and quenched in water. A rotational die ECAP (RD-ECAP) setup with a channel angle of 90 • was applied to process 16 passes of ECAP. More details of the RD-ECAP process have been illustrated in ref. [21]. Before the ECAP process, the billets were preheated at 300 • C for 20 min together with the die. During the ECAP process, the average press speed was kept at 5 mm/min. Finally, the ECAPed billets were removed from the die quickly and cooled in water.

Microstructure Observation
Microstructures and precipitate morphologies of the as-cast and as-pressed samples were observed by the optical microscopy (OM, Olympus BX51, Tokyo, Japan), scanning electron microscope (SEM, Hitachi-S3400N, Tokyo, Japan) with energy dispersive spectroscope (EDS) and transmission electron microscopy (TEM, FEI Tecnai G2 T20, Hillsboro, OR, USA). X-ray diffraction (XRD, D8 Advance, Karlsruhe, Germany) analyses were also used to investigate the phase existing in the as-cast samples with different Li content. The alloy density (g/cm 3 ) was measured using the standard Archimedes method with distilled water [22]. All the samples for observation were cut along the pressing direction. The samples for OM and SEM were polished and etched with a 0.5% hydrofluoric acid solution. Samples for TEM were prepared by mechanically polishing to~60 µm and subsequently, ion thinning. To characterize the initial grain size, the as-cast samples were investigated under polarized light after anodized by Barker's reagent (5 mL HBF 4 in 200 mL H 2 O, 25 V). Differential Scanning Calorimetry (DSC, NETZSCH SAT 449F3, Selb, Germany) was performed on the as-cast 1.6Li alloy, employing a constant heating rate of 10 • C/min within the temperature range 100 • C-500 • C.

Testing of Mechanical Properties
Tensile tests were carried out for all the samples at room temperature using a universal testing machine (UTM-4000, SUNS, Ningbo, China). The strain rate utilized was 5 × 10 −4 s −1 . Dog-bone shaped tensile specimens were cut from the central regions of the ECAPed billets parallel to the longitudinal axis, with a gauge length of 8 mm, a thickness of 2 mm, and a width of 2 mm.

Microstructures of As-Cast Al-5.5Mg-xLi-0.1Zr Alloy
The phase constitutions in the as-cast Al-5.5Mg-xLi-0.1Zr alloys are exhibited in Figure 1. It can be seen that all the samples consist of α-Al, δ'-Al 3 Li, and S 1 -Al 2 MgLi phases. According to the literature [23,24], the intermetallic phase δ'-Al 3 Li exist in all the as-cast alloys, owing to the decomposition of α-Al supersaturated solid solution during solidification. With the increase of Li addition, the peaks of δ' phase at 23 • (100) and 32 • (110) become more obvious [25]. According to [26], in an Al-Mg-Li alloy containing a Mg content >5% wt and a Li content >1.7 wt%, both the S 1 -Al 2 MgLi and δ-AlLi phases are formed.  It can be seen that the as-cast alloys consisted mainly of dendritic α-Al matrix with rod-like or network-like intermetallic compounds along the grain boundaries (indicated by white arrows). As shown in Figure 3, the EDS analysis suggest that these intergranular compounds contain as high as 20 wt% of Mg is Al 2 MgLi phase. Some black phases with coarse size and skeleton structure at the junction of boundaries (indicated by red arrows in Figure 2) were identified as Mg 2 Si, which is ascribed to the small quantity of Si introduced from the raw materials [5,27,28]. The two different phases are distributed in such a way that the Mg 2 Si phase, which appears black either completely or incompletely, surrounds the grains of the Al 2 MgLi phase, which appears light-grey. This may be connected with the so-called complete and incomplete wetting of grain boundaries by the melt or by the second solid phase in alloys [29]. The wetting of Al/Al GBs by the second solid phase such as Al 3 Mg 2 has been observed in Al-Mg alloys, and the transition from incomplete to complete wetting of Al/Al GBs [30,31]. More attention will be paid on this phenomenon in our future research.
Compared with the microstructure of 1.6Li in our previous work (Figure 2a,d) [32], the amount of Al 2 MgLi phases increases apparently with the increase of Li contents, eventually leading to a continuous network of intermetallic compounds around the grains in the 2.5Li alloy. Furthermore, the as-cast grains are obviously refined to about 87 µm for 2.5Li alloy compared to the large grain size of~135 µm for 1.6Li alloy (as shown in Figure 2d-f), which indicates that Li content has an obvious grain refinement effect on the Al-Mg-Li alloys. A similar finding was also reported by Adamczyk-Cieślak et al. [33], in which an increase of the Li content could enhance the SPD-induced microstructure refinement of binary Al-Li alloys.   Figure 4 illustrates the TEM dark-field images of the as-cast alloys. It presents that a high number density of spherical δ' precipitates are uniformly distributed throughout the α-Al matrix in the as-cast alloys. This is due to the small lattice misfit (about 0.08-0.3%) and the low surface energy (30 mJ/m 2 ) of δ'-Al 3 Li relative to the α-Al matrix, leading to a low thermodynamic nucleation barrier for δ' to homogeneous decomposition from the matrix during solidification [34][35][36]. As expected, δ' superlattice diffraction spots appear between the Al diffraction spots in the inset selected-area-diffraction (SAD) pattern taken from these areas, which is more and more apparent with increasing Li addition. The δ'-phase particles have an LI 2 cubic structure (α = 0.4 nm) with an orientation relationship (OR) (100)δ'//(100)Al, [100]δ'//[100]Al with the Al matrix [22,37]. As the Li content increases, the δ' phases become more and denser in the 2.0Li alloy, but obviously coarsen and scattered in the 2.5Li alloy. The addition of lithium has the greatest effect on the density of the alloy. Figure 5 exhibits the density change of Al-Mg-Li alloys with different Li content. Among three studied Al-Mg-Li alloys, 1.6Li alloy had the highest density value (2.52 g/cm 3 ), but still much lower than the commercial third-generation Al-Li alloys (2.63~2.72 g/cm 3 ) [4]. With increasing Li content, a very low density of 2.37 g/cm 3 could be achieved in the 2.5Li alloy, which has reduced its density by 12% compared with the commercial Al alloy. Therefore, Li content had a significant effect on the weight reduction of Al alloys, which could meet the increasing demand for weight-critical components in the aerospace field.

3.2.
Microstructures of ECAP 16-pass Al-5.5Mg-xLi-0.1Zr Alloy Figure 6 displays the scanning electron micrographs (SEM) of 1.6Li, 2.0Li, and 2.5Li alloy subjected to 16 passes of ECAP at 300 • C, respectively. After severe plastic deformation, the alloys with different Li content have similar microstructure. The dissolved Al 2 MgLi precipitations during heat treatment are precipitated again during ECAP process, with small size and uniform distribution. Some of the Mg 2 Si phases can still be found after ECAP 16 passes, but are much refined compared with those in the as-cast alloys. The TEM images of the ECAP 16-pass alloys are given in Figure 7 to gain more details of the microstructure. Similar to the early reports [20,38,39], ECAPed Al-Mg-Li-Zr alloys are characterized by ultrafine-grained structure (with grain size of~1 µm), relatively low density of dislocations, a lot of Al 2 MgLi particles, but few δ' phases. Therefore, apart from the volume and number, the density of the Al 2 MgLi precipitations gradually increase with increasing Li content (as shown by red arrows in Figures 6 and 7a-c.), there is little difference between the microstructure of as-pressed alloys with different Li addition. That is to say, ECAP reduces the effect of Li content on the microstructure of Al-Mg-Li-Zr alloys.  The mechanical properties and typical true stress-strain curves of Al-5.5Mg-xLi-0.1Zr alloys under different states are given in Figure 8. As shown in Figure 8a, the yield strength (YS) increases almost linearly with the gradual increase of Li content, whether in the as-cast alloys or in the ECAP processed alloys. As expected, the elongation (EL) decreases a lot with Li addition. This can be attributed to the enhanced precipitation in the Al-5.5Mg-xLi-0.1Zr alloy with increasing Li content. Although multiplied precipitations of Al 2 MgLi and δ' phase are beneficial to the modules and yield strength, they are detrimental to the plastic deformation ability of Al-Li alloys. Especially for the 2.5Li alloy, it manifests total brittle behavior in the as-cast state, which fractures immediately after the yield stage, as shown in Figure 8d. As a result, this great decrease in elongation with more Li addition results in dropped ultimate strength (UTS) (Figure 8b). Generally, in the as-quenched state, the UTS and EL show an obvious improvement compared with the as-cast state, while the YS reveals little change. After processing by 16-pass ECAP, both the YS and UTS of all the samples are increased further by more than 100 MPa. Meanwhile, the EL also increases or maintains. Take the 2.5Li alloy for example, it shows obvious plastic deformation and enhanced work-hardening ability after 16-pass ECAP, the YS, UTS, and EL of which is 84%, 73%, and 463% higher than those of the as-cast one, respectively (Figure 8f), indicating a transition from brittleness to toughness. Consequently, the brittle 2.5Li alloy is successfully fabricated into an Al-Li alloy exhibiting a good combination of strength (>410 MPa) and ductility (≈10%). This fact indicates that the harmful effect of high Li content on the mechanical properties of as-cast alloys could be greatly reduced by warm multi-pass ECAP process. That is to say, the warm multi-pass ECAP greatly contributes to both strength and elongation enhancement, making the fabrication of ultralight UFG Al-Li alloys with good mechanical properties possible [40][41][42].
Specific strength, defined as the ratio of ultimate strength to density of materials, is an important parameter in the design of light-weight structures, especially for aerospace applications. Figure 9 presents the specific strength of the current studied alloys after ECAP 16 passes and some other typical light materials [43][44][45][46]. It is well known that within light-weight materials, the biggest competitors of aluminum alloys are composite materials and magnesium alloys. Magnesium alloys, especially Mg-Li alloys, are well known for their extremely low density and high specific strength, so as to be highlighted as excellent potential candidates for lightweight structures. As shown in Figure 9, the investigated alloys possess much higher specific strength compared to that of the pure Al, aluminum matrix composite, and Mg-Li alloys. Besides, Al-Li alloys not only require much lower cost for the manufacturing and maintenance than composites but also exhibit higher corrosion resistance and better elongation compared with Mg and Mg-Li alloys. Consequently, the result puts forward great prospects for ECAPed Al-Li alloys in the aerospace field due to their lightweight and high specific strength characteristics.

Fracture Surface Observation
After tensile testing at room temperature, the fractured surfaces of the studied alloys with different Li content were observed by SEM. The typical fracture morphology with and without ECAP process are presented in Figure 10. The fracture surfaces of the as-cast samples (Figure 10a,c,e) reveal a predominantly quasi-cleavage fracture mode, in which tearing ridges, cleavage planes, and steps may be distinguished. According to the earlier report [47], intergranular compounds could offer potential sites for stress concentration and facilitate the formation and extension of microcracks. Thereafter, fracture would happen along the interface between the phases and the α-Al matrix, where the cleavage planes can be observed (indicated by white arrows). Moreover, it is well accepted that the coherent δ'-Al 3 Li particle is expected to provide a possible fracture path due to planar slip. As a result, these primary phases bring about low elongation, which is consistent with the mechanical properties of the as-cast alloys (Figure 8). With increased Li content, more intergranular intermetallic compounds appeared in the as-cast alloys, as previously shown in Figure 2. Therefore, the strain localization caused by the intergranular phases and the shearing of δ' particles will be intensified, leading to a sharply decreased ductility in the as-cast alloys. This is also demonstrated by more intermetallic compounds on the fracture surface (shown by red circles in Figure 10e) and the occurrence of both cleavage plans and grain boundaries, which is the fracture features of combined quasi-cleavage mode with intergranular mode.
After subjected to 16-pass ECAP, as shown in Figure 10b,d,f, a dimple-dominant fracture feature with a great number of dimples appeared on the fracture surface, which indicated a more ductile fracture mode comparing with the as-cast state. These dimples in the as-pressed alloys are the result of void growth and coalescence. During mechanical loading, disintegration between refined precipitates and α-Al matrix give rise to the nucleation of voids, which would gradually grow and coalesce at the precipitates [48]. As a consequence, the remarkable microstructural change by severe plastic deformation, mainly the homogeneously distributed and greatly refined precipitations, dissolution of δ' phases and ultrafine-grained microstructure, contributes to the transformation from brittle cleavage fracture to ductile dimple fracture after ECAP, thus significantly improving the ductility of the Li-containing alloys. However, in the as-pressed 2.5Li alloy as shown in Figure 10f, the dimples become sparser and shallower, accompanied by the appearance of small grain boundaries, revealing a signal of the decrease of elongation.

Discussion
On the basis of the above investigation, the cooperative effect of equal-channel angular pressing and Li content on the microstructure improvement of the Al-Mg-Li alloy is summarized in Table 2. For the as-cast alloy, Li addition has an obvious influence on the microstructure. That is, continuous network of more and coarser Al 2 MgLi along the boundaries, enhanced δ' precipitations and refined grains with increased Li addition. For the ECAPed alloys, the effect of Li addition on the microstructure of Al-Mg-Li alloys is greatly weakened by severe plastic deformation. The microstructure of alloys with different Li content has similar features. The only difference is that the Al 2 MgLi precipitated during ECAP are more and coarser in the high Li-containing alloy than those in the low Li-containing alloy. It is well known that the main intermetallic phases in the Al-Mg-Li alloys are δ'(Al 3 Li) and S 1 (Al 2 MgLi). Researches on precipitation in such alloys are mainly focused on the dominated precipitation of the metastable δ' phase after classical aging treatment. The ternary phase S 1 -Al 2 MgLi only tend to take over the δ' phase at high temperatures and long aging times [5]. But in our work, S 1 phase plays a more and more important role with increasing Li content. According to the quantitative analysis by S. Betsofen and M. Chizhikov [49], the phase ratio of the two main intermetallic phases δ' and S 1 phases can be calculated by the following balance equations.
Li , and X δ Li are the Al, Mg and Li contents in the alloy, α-Al, S 1 and δ' (%) phases, respectively. And W α , W S 1 and W δ represent the mass fractions of the α, S 1 and δ' (%) in the alloy, respectively. The values of X Li , and X δ Li are given in the reference [49]. We employ the value of X 0 Mg in our work as 5.5 and the substitution X 0 Al = 100 − X 0 Mg − X 0 Li , the solution indicating the relationship between the mass fraction of the phases with Li content in the alloy from the set of equation (1) can be obtained.
Although X α Al , X α Mg , and W α are unknown here, they have very small impact. From the solution, it can be found that Li content has a great effect on the mass fraction of S 1 phase but has a much smaller effect on the mass fraction of δ'. It is well consistent with our observation through OM, SEM, and TEM images. With the increasing Li addition, much more and coarser Al 2 MgLi phases can be observed in whether as-cast or as-pressed alloys.
After ECAP, the presence of S 1 -Al 2 MgLi particles can even hinder the precipitation of δ' phase. This phenomenon can be interpreted by the Differential Scanning Calorimetry (DSC) result of 1.6Li alloy exhibited in Figure 11. There are three endothermic peaks, "A", "B", and "C", at the temperature interval 160-200 • C, 350-370 • C and 420-440 • C, respectively. The first endothermic peak, "A", is related to the dissolution of δ´-particles, which already existed in the as-cast state [50]. The second and third endothermic peaks "B" and "C" are ascribed to the dissolution of the precipitation of S 1 -Al 2 MgLi. In [51], the S 1 -phase can be identified as two kinds differing from their crystal lattice character, with different dissolution temperature range. Davydov et al. [6] also found that the decomposition with S 1 -phase in temperature interval 250-375 • C begins with minimal used duration (15 s), and the formation of δ'-phase at the interval 100-200 • C. During the heat treatment at 500 • C in our works, the S 1 phases can be absolutely dissolved. After that, ECAP is performed processed at a temperature much higher than the ranges of the existence of δ' phases [52]. δ' phases dissolve, while greatly refined grains and large strains induced by ECAP bring about a lot of (sub)grain boundaries and a certain number of dislocations, which provides a large number of heterogeneous precipitation sites for S 1 -Al 2 MgLi phases. So, a lot of S 1 -phases precipitates from the matrix during ECAP, which consume most of the Li content, leading to a low driving force for the precipitation of δ' phases. From the mechanical properties in Figure 8, it is found that for the as-cast alloys, every 0.5 wt% Li addition provides 13% and 14% increase in YS, but 50% and 74% decrease in EL; for the ECAPed alloys, every 0.5 wt% Li addition provides 19% and 20% increase in YS, but 25% and 47% decrease in EL. Several things can be concluded from these data: Li addition has more influence on the mechanical properties when its content in the alloy is higher; Li has a positive effect on the strengthening of the Al-Mg-Li alloys, but makes a much more negative impact on the ductility. Last but not least, the ECAP process amplifies the positive effect, and furthermore, significantly diminishes the negative impact.
These conclusions are well related with the cooperative effect of Li addition and ECAP process on the microstructure of Al-Mg-Li alloys. In general, δ'-Al 3 Li is the main strengthening phase in the binary Al-Li alloy, and ternary Al-Mg-Li alloy, which attracts much attention in the existing researches, while S 1 -Al 2 MgLi phase is considered to play a less important role in the strength. This is also true in the studied as-cast alloys; enhanced precipitation of nanoscale δ'-Al 3 Li particles, with increased Li addition, play a major role in the increase of YS. The coarse S 1 -phases, present in the vicinity of grain boundaries in a strong heterogeneous manner, provided little contribution to strength. Nevertheless, both enhanced δ'-Al 3 Li and coarsen S 1 phases with Li addition were responsible for the decrease of EL, due to planar slip and intergranular stress concentration, respectively. As a result, Li addition has a more impact on the ductility of as-cast Al-Mg-Li alloy than on its strengthening. When Li content is higher, the much more S 1 -phases are tended to form network-like distribution, so as to affect the mechanical properties more.
After ECAP, the microstructure is greatly refined, with small S 1 particles precipitated uniformly, and the δ'-Al 3 Li dissolved in the matrix. It is not difficult to infer that the ultrafine grains and dispersed S 1 phases after ECAP provide more than a 60% increase in YS, and more than a 110% increase in EL compared with the as-cast state. Considering that the as-pressed alloys with different Li content have similar micron-scale grains, the formation of more and coarser particles of S 1 phases in the increased Li-containing ECAPed alloys lead to the increase in YS and decrease in EL. That is because the S 1 phase precipitated during ECAP is in the form of a colony composed of several small Al 2 MgLi crystallites, which may hinder the operation of grain boundary dislocation sources and pin the separate dislocations and sub-boundaries [51,53,54]. Similar to the as-cast alloys, the coarsening of the phase weakens the strengthening effect, but magnifies the negative effect on ductility with increasing Li content. However, ECAP process enlarges the positive effect of Li addition, but significantly reduces the negative impact, which is concerned with the small size of dispersed S 1 precipitations and enhanced work-hardening ability of the matrix through severe plastic deformation.

Conclusions
The cooperative effects of Li addition and ECAP on the microstructure improvement and mechanical properties of Al-5.5Mg-xLi-0.1Zr alloy have been investigated in the present study. The following conclusions can therefore be drawn: (1) Li addition apparently refines the grains of the as-cast Al-5.5Mg-xLi-0.1Zr alloy and greatly reduces the density value of the alloy. However, much more and coarser intergranular Al 2 MgLi phases, as well as coherent δ'-Al 3 Li particles occur in high Li alloys. This promotes quasi-cleavage fracture and intergranular fracture in as-cast state, thus leading to sharply decreased ductility. (2) Warm multi-pass ECAP provides a simple and effective process for microstructure refinement and homogeneity of Al-Mg-Li alloy. The grain size is reduced to~1 µm, and the precipitation of Al 2 MgLi is prompted after 16 passes of ECAP. With increased Li addition, the alloy is characterized by growing and increased S 1 precipitations, similar ultrafine grains, low density of dislocations, and a lack of δ' phases. (3) Li addition reduces the elongation and ultimate strength, but increases the yield strength of the alloys in both as-cast and as-pressed state. Cooperative ECAP process could remarkably improve the tensile mechanical properties of Al-Li alloys, thus effectively eliminating the harmful effect of Li addition. Warm multi-pass ECAP process offers a promising way to develop excellent light-weight Al-Mg-Li alloys with much higher specific strength than Al matrix composites and Mg alloys.