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The strengthening effect of icosahedral phase onas-extruded MgLi alloysD.K. Xu,a,bL. Liu,aY.B. Xuaand E.H. Hanb,*aShenyang National Laboratory for Materials Science, Institute of Metal Research, Chinese Academy of Sciences,Shenyang 110016, ChinabEnvironmental Corrosion Center, Institute of Metal Research, Chinese Academy of Sciences, Shenyang 110016, ChinaReceived 31 October 2006; revised 30 December 2006; accepted 7 March 2007Available online 11 May 2007Through investigating the mechanical properties of three kinds of MgLiZnY alloys, a strengthening method, i.e. introducingI-phase (Mg3Zn6Y, icosahedral quasicrystal structure) in the matrix of MgLi alloys, for as-extruded MgLi alloys has been dem-onstrated. The tensile results indicate that I-phase can effectively strengthen the alloys. The substantial enhancement of strength hasbeen explained by microstructure changes.? 2007 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.Keywords: I-phase; MgLi alloy; Mechanical propertiesAlloying magnesium with lithium of extremely lowdensity (0.534 g cm?3) can further reduce the weight ofMg alloys. However, based on the previous results listedin Table 1, the strength of MgLi alloys is very low 1,2.Generally, previous strengthening methods, such asadding Zn or Al alloying elements and severe plasticdeformation (hot extrusion or equal channel angularextrusion), cannot make the ultimate tensile strength(UTS) of MgLi alloys exceed 200 MPa.On the other hand, it has been reported that MgZnY alloys containing I-phase (Mg3Zn6Y, icosahedralquasicrystal structure) as a secondary phase exhibitgood mechanical properties at both room temperatureand elevated temperature 3. Depending on the volumefraction of I-phase, MgZnYZr alloys can have yieldstrength from 150 to 450 MPa at room temperature 4.Previous studies 59 indicated that the existence of I-phase in MgZnY or MgZnYZr alloys was closelydependent on the Zn/Y weight ratio. Literature 7 sug-gested that when the Zn/Y ratio exceeded 4.38, elementY would exist almost completely as I-phase. When theZn/Y ratio was above a certain value (4.38), withincreasing Y content, more I-phase would be formedin the Mg matrix.Based on the analysis of these two alloy systemsmentioned above, two questions can be asked: (i) canI-phase be introduced into MgLi alloys? (ii) If I-phasecan be introduced, will the mechanical properties ofMgLi alloys be greatly improved? Therefore, in thiswork, three alloys (with Zn/Y ratios higher than 5),namely Mg8% Li3% Zn0.6% Y, Mg8% Li6%Zn1.2% Y and Mg8% Li9% Zn1.8% Y, were pre-pared. By investigating the mechanical properties ofthe alloys, the two questions will be answered.The materials used in this study were as-extrudedMgLiZnY magnesium alloys with different Zn andY contents, which were prepared using specific techno-logy in the Magnesium Alloy Research Department ofIMR, China. Using inductively coupled plasma atomicemission spectrum apparatus, the chemical composi-tions of alloys IIII were determined, and these are listedin Table 2. The extrusion ratio was 10:1.Phase analysis was determined with a D/Max 2400 X-ray diffractometer (XRD). Microstructures of the as-castalloys IIII were examined by scanning electron micros-copy (SEM; XL30-FEG-ESEM). Tensile bars with agauge length of 25 mm and 5 mm in diameter were ma-chined from the alloys. The axial direction of the tensilespecimens was parallel to the extruded direction. Tensileexperiments were conducted on an MTS (858.01 M)testing machine with a constant strain rate of 1 10?3s?1at room temperature. SEM (XL30-FEG-ESEM)1359-6462/$ - see front matter ? 2007 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.doi:10.1016/j.scriptamat.2007.03.017*Corresponding author. E-mail: ehhanScripta Materialia 57 (2007) 285288/locate/scriptamatobservations using either secondary electron imaging orbackscattered electron imaging were made to determinethe fracture characteristics and cracked I-phase on thefracture surfaces.XRD analysis is shown in Figure 1. It reveals that foralloys IIII, the main phases are a-Mg, b-Li, LiMgZnand I-phase. Meanwhile, with the increase in Zn andY content, the diffraction peak of W-phase will be grad-ually intensified. In addition, it has been reported3,6,10 that I-phase could form interdendritic eutecticpockets with a-Mg. Therefore, an easy way to determineI-phase is by its morphology.The microstructure observations of as-cast alloysIIII are shown in Figure 2. The figure shows thatI-phase/a-Mg eutectic pockets preferentially form atthe a-Mg/b-Li phase interfaces. With increasing Znand Y contents, I-phase/a-Mg eutectic pockets can notonly coarsen at the a-Mg/b-Li phase interfaces but alsogradually form in the a-Mg matrix. Since element Yexists almost entirely in the form of I-phase, the quantityof I-phase for alloys is depended on Y content. There-fore, based on the variation of Y content, it can bededuced that the quantity of I-phase for alloy III is 3and two times as much as that of alloys I and II, respec-tively. In addition, with increasing Zn content, especiallyfor alloy III, many lamellar LiMgZn phases can beobserved in the a-Mg matrix, as shown in Figure 2(d).The stressstrain curves are shown in Figure 3. To de-scribe and compare these conveniently, the mechanicalproperties of 0.2% proof yield stress (r0.2), UTS andelongation to failure for the alloys are listed in Table2. It can be seen that the quantity of I-phase can effec-tively improve the yield strength and UTS of alloys.Comparing alloys I and III, with the quantity of I-phaseincreasing approximately 3-fold, the yield strength andUTS increase from 148 and 222 MPa to 166 and247 MPa, respectively. Meanwhile, the plasticity of alloyIII decreases greatly.Previous research of MgZnYZr alloys indicated5,7,9 that with the quantity of W-phase increasing,the strength of alloys decreased. X-ray analysis indicatesthat for alloy II, W-phase can hardly be detected. There-fore, it can effectively avoid the influence of W-phase.To indicate the effect of I-phase on the mechanical prop-erties of alloys, only the fracture of alloy II has beenchosen to be observed. Figure 4 shows the secondaryand backscattered SEM images of the fracture surfaces.The figure reveals that micro-cracks can form in theinterior of big bulk I-phases.Based on the MgLiZn ternary phase diagram 11,when Li content is between 6.0 and 9.5 wt.%, a-Mg andb-Li coexist and the Zn content in the solid solutionscannot exceed 2 wt.%. With decreasing solidificationtemperature, the solid solubility of Zn decreases gradu-ally. Meanwhile, the MgLiY ternary phase diagramreveals 12 that the Y content in the solid solutions isvery tiny. In addition, due to the interaction of elementsZn and Y, the solid solubility of Zn and Y is greatly de-creased 13. In this study, Li content of alloys IIII isabout 8 wt.%. Therefore, as the solidification processcontinues, redundant Zn and Y (with a Zn/Y ratio high-er than 4.38) will exist between a-Mg and b-Li phasesand preferentially form I-phase at the a-Mg/b-Li phaseinterface. Certainly, I-phase can also form in the interiorof a-Mg and b-Li matrix, as shown in Figure 2ac. Ithas been reported that the melting temperature ofI-phase eutectic pockets is about 450 ?C 3,8,14,15.Table 1. Summary of the mechanical properties of the different MgLi alloys (tested at room temperature)ConditionAs-extruded stateEqual channel angular extrusionr0.2(MPa)UTS (MPa)Elongation (%)r0.2(MPa)UTS (MPa)Elongation (%)Mg11% Li1% Zn 1961336015017535Mg9% Li1% Zn 11001415616018231Mg9% Li1% Zn0.2% Mn 1901307014016522Mg9% Li1% Zn1% Al0.2% Mn 11051506014518024Mg9% Li1% Zn3% Al0.2% Mn 11101615013018027Mg3.3% Li 2691601811320033Table 2. Chemical composition and the mechanical properties of the as-extruded MgLiZnY alloysNormal alloysChemical composition (wt.%)Zn/Y ratioMechanical propertiesMgZnYLir0.2(MPa)UTS (MPa)Elongation (%)Alloy IBal3.120.618.045.1114822230.7Alloy IIBal6.471.267.865.1315923920.4Alloy IIIBal9.251.797.675.1716624717.1Figure 1. X-ray diffraction patterns of as-extruded MgLiZnYalloys. The arrows in the figure indicate the intensifying tendency ofW-phase diffraction peak.286D. K. Xu et al. / Scripta Materialia 57 (2007) 285288Therefore, when the temperature is lower than 450 ?C,the forming of I-phase will retard the further diffusionof Zn and Y. Especially for alloys II and III, more I-phase can form during solidification process, which willeasily lead to the formation of areas with higher andlower Zn/Y ratios in the liquid phase. Therefore, thearea (with lower Zn/Y ratio) cannot fully meet therequirement of forming I-phase and W-phase will beformed, whereas the area (with higher Zn/Y ratio) cansuccessfully form I-phase and the redundant Zn willform a supersaturated solid solution in the a-Mg matrix.When the alloys are cooled down to room temperature,lamellar LiMgZn phases precipitate from the supersatu-ration solid solution, as shown in Figure 2(d). Previousresearch reported 16 that after T6 temper treatment(solid solution for 2.5 h at 500 ?C plus 15 h of artificialageing at 180 ?C), MgZnY phases (I-phase and W-phase) disappeared and rod-like MgZn0precipitatedfrom the supersaturated solid solution. In addition, theMgLiZn ternary phase diagram 11 indicates thatwhen the contents of Mg and Zn are higher than about40 at.%, Li will preferentially form LiMgZn phase withZn and Mg. Therefore, the formation of LiMgZn canbe divided into two steps: (i) the formation of anarea of higher Zn content caused by rod-like MgZn0;and (ii) the diffusion of Li and the formation ofLiMgZn. Based on the discussion above, I-phase canFigure 2. The microstructure of the as-cast MgLiZnY alloys: (a) alloy I, (b) alloy II, (c) alloy III and (d) high-magnification observation of thelocation squared in image (c).05101520253035050100150200250300Tensile strength (MPa)Strain (%)1 Alloy I2 Alloy II3 Alloy III123Figure 3. The stressstrain curves of the as-extruded MgLiZnYalloys.Figure 4. The SEM observation of cracked I-phase on the fracture surface for alloy II: (a) secondary and (b) backscattered SEM observation.D. K. Xu et al. / Scripta Materialia 57 (2007) 285288287be successfully introduced into the matrix of MgLi al-loys, which clearly answers the first question. Whether I-phase can effectively strengthen MgLi alloys or not willbe discussed below.It has been reported 3 that the interface layer ofa-Mg, 35 nm thick, still preserved the orientationrelationship with I-phase, and the coherency betweenI-phase and a-Mg could be achieved by introducingsteps and ledges periodically along the interface. There-fore, the atomic bonding between I-phase and the hex-agonal structure was rigid enough to be retainedduring severe plastic deformation. A study of as-castMgZnYZr alloys 5 suggested that the a-Mg/I-phase eutectic pockets could retard the basal slip andthat no cracks could be observed at the a-Mg/I-phaseinterfaces. Compared with the tensile properties listedin Tables 1 and 2, this clearly suggests that introducingI-phase into the Mg matrix can effectively improve thestrength of MgLi alloy. However, the stressstraincurves reveal that with the quantity of I-phase increas-ing, the difference in UTS between the alloys decreasesgreatly, as shown in Figure 3. The figure reveals thatthe difference of UTS between alloys I and II is abouttwice as great as that between alloys II and III, whichcan be ascribed to two main reasons. First, based onX-ray phase analysis (in Fig. 1) and the discussion ofthe phase-forming mechanism, the quantity of W-phaseincreases with increasing Zn and Y content, which de-grades the strength of alloys, especially for alloy III. Sec-ondly, due to the higher content of Zn and Y for alloysII and III, I-phase formed at the a-Mg/b-Li interfaceswill be coarsened, leading to a large bulk I-phase afterhot-extrusion processing. During the tensile testing,the higher stress concentration will occur around thelarge bulk I-phase, which degrades the strength of al-loys. Figure 4 shows that at a certain stress level, mi-cro-cracks will be formed in the interior of the largebulk I-phase to relieve the deformation incompatibilitybetween I-phase and a-Mg matrix. This provides furtherevidence that a-Mg/I-phase interfaces are very strong.Furthermore, it also indicates that the size of thecracked I-phase is larger than 10 lm. Therefore, to fullyexploit the potential strength of MgLi alloys, the quan-tity of existing W-phase and the size of I-phase must bestrictly controlled.By investigating three kinds of MgLi alloys, astrengthening method, i.e. introducing I-phase in thealloy matrix, has been demonstrated. The maximumUTS of the new exploited alloys can reach 250 MPa.Based on experimental results, two main factors influ-encing the strength of alloys can be determined: thequantity of existing W-phase and the size of I-phase.Therefore, it can be predicted that by controlling thesetwo factors, the potential strength of MgLi alloys canbe further improved.This work was supported by National Science Fundof China (NSFC) project under Grant No. 50431020.1 Tien-ChanChang,Jian-YihWang,Chun-LenChu,Shyong Lee, Mater. Lett. 60 (2006) 32723276.2 T. Liu, Y.D. Wang, S.D. Wu, R. Lin Peng, C.X. Huang,C.B. Jiang, S.X. Li, Scripta Mater. 51 (2004) 10571061.3 D.H. Bae, S.H. Kim, D.H. Kim, W.T. Kim, Acta Mater.50