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Journal of Alloys and Compounds 476 (2009) 118124

Contents lists available at ScienceDirect

Journal of Alloys and Compounds

journa l homepage: www.e lsev ier .com/ locate / ja l l com

ging behaviour and precipitate morphologies in Mg7.7Al0.5Zn0.3Mnwt.%) alloy

ei-Jen Laia,, Yi-Yun Lia, Yung-Fu Hsub, Shan Trongc, Wen-Hsiung Wanga

Department of Materials Science and Engineering, National Taiwan University, Taipei 106, Taiwan, ROCDepartment of Materials Science and Mineral Resources Engineering, National Taipei University of Technology, Taipei 106, Taiwan, ROCMaterials and Electro-Optics Research Division, Chung-Shan Institute of Science and Technology, Lung-Tan 325, Taiwan, ROC

r t i c l e i n f o

rticle history:eceived 26 July 2008

a b s t r a c t

All the precipitate morphologies of Mg17Al12 in AZ80 for a range of aging temperatures are investigatedin detail using TEM and SEM. The results show that Mg17Al12 is the dominant precipitate in AZ80 andccepted 25 August 2008vailable online 16 October 2008

eywords:etalsechanical alloying

can be divided into different types which can be discriminated by their morphologies. To date there havebeen few papers in the literature focused on the relationship between the aging behaviour and thesemorphologies. This study elaborates on the sequence of morphological evolution of Mg17Al12 precipitatesas a function of temperature and investigates the associated age-hardening response.

2008 Elsevier B.V. All rights reserved.

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canning and transmission electronicroscopy

. Introduction

In recent years magnesium alloys have become one of the mostmportant commercial alloys. The major advantage of magnesiumlloys is their high strength to weight ratio, which makes themopular with mobile device manufacturers, whose main goal is toeduce the weight of their products. Conventionally, magnesiumlloys are fabricated largely by casting and in particular die-casting.owever, the major problem with this processing route is the lowield rate. Therefore, manufacturers are striving to develop betterroughtmagnesiumalloys to address aspects of this problem. ThusZ80 has become one of the prominent newly developed wroughtagnesium alloys.There are several kinds of magnesium alloy systems. The AZ

eries is primarily based on the MgAl binary alloy system andominates most of the magnesium alloys. Aluminium, which caneduce the grain size and greatly enhance the mechanical proper-ies ofmagnesium alloys, is themost significant elemental additiono magnesium alloys. Besides, when the addition of Al exceeds theritical limit (6wt.%) [1], Mg17Al12 intermetallic compounds will

recipitate and themechanical properties of the AZ series alloy areurther enhanced.

AZ80 is based on the Mg8wt.% Al with the addition of about.5wt.% Zn and a small amount of Mn. Manganese can improve

Corresponding author. Tel.: +886 22 5915343.E-mail address: william721225@hotmail.com (W.-J. Lai).

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925-8388/$ see front matter 2008 Elsevier B.V. All rights reserved.oi:10.1016/j.jallcom.2008.08.043he corrosion resistance by adding MnCl2 to the melt to precipi-ate any dissolved iron as a complex compound, thus removing theron and enhancing the corrosion resistance of the resulting alloy.esides, manganese will also form intermetallic compounds withluminium and magnesium [2].A small amount of zinc in magnesium alloys will not form any

ntermetallic compound with either magnesium or aluminium,ut will reduce the solid solubility of aluminium in magnesiumnd increase the amount of Mg17Al12 precipitate. It will thereforemprove the strength of the alloy [2]. Besides, Zn is also a potentolid solution strengthener.

This study will focus on the identification of the diverse rangefMg17Al12 precipitatemorphologies formed under different agingonditions and thedeterminationof the relationshipbetween theseorphologies and the associated aging behaviour.

. Experimental procedure

The as-extruded AZ80 alloy was received from Chung-Shan Institute of Sciencend Technology (Taiwan). An ingot of 150mm in diameterwas produced by vertical-irect-chill-casting and was extruded at 340 C. The composition was measured byPMA and is shown in Table 1. The as-extrudedmaterial was in the form of a slab of00mm in width, 6mm in thickness and 1000mm in length. The thickness of theaterial was then further reduced to 2mm by hot rolling at 400 C. The rolled plate

as thensolution-treatedat420 C for1handwaterquenched to roomtemperature.amples were cut from the plate and then aged at 125, 150, 175, 200, 250, and00 C, respectively. Samples aged between 125 and 200 C were heat treated in ailiconeoil bathand forhigher temperatures inanair furnace.All aging temperaturesere calibrated within 2 C. The aged samples were then water quenched to roomemperature.

http://www.sciencedirect.com/science/journal/09258388http://www.elsevier.com/locate/jallcommailto:william721225@hotmail.comdx.doi.org/10.1016/j.jallcom.2008.08.043

W.-J. Lai et al. / Journal of Alloys and Compounds 476 (2009) 118124 119

Table 1The composition (wt.%) of the as-received AZ80 alloy

Al 7.7ZMM

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Hardnesswasmeasuredusing amicro-Vickersmachine. Themicrostructurewasbserved after polishing and etching with a solution comprising of 3 g picric acid,ml acetic acid, 5ml water and 50ml ethanol. Materal for TEM samples was firstround to about 100m thickness and then punched to a disk of 3mm in diameter.he disks were subsequent twin-jet electro-polished at 15 C and 20V with a 10ol.%perchloric acid90vol.% ethanol electrolyte. TEMobservationswere conductedith a JEOL100CXII electron microscope operated at 100kV.

. Results and discussion

.1. Hardness

The hardness curves of the AZ80 aged at different tempera-ures for various periods of time are shown in Fig. 1. No significantncrease in hardness was observed for AZ80 aged at 125 C upo 128h. The hardness curves are obviously composed of threeegions, which is similar to most of the precipitation-hardenablelloys. First there is an incubation period, with the higher the agingemperature the shorter the length of time. The second region isefined by a steady increase to the peak hardness, and the value ofhich also depends on the aging temperature. After the peak hard-ess, the hardness became steady and does not fall abruptly, whichs contrast to the behaviour of precipitation-hardenable aluminiumlloys.At aging temperatures above 150 C, the alloy started to show

ignificant increase in hardness. As can be seen from Fig. 1, whenging at 150 C, the hardness began to increase significantly after2h and did not reach its peak hardness during the time periodxamined, 256h. When aging at 175 C, a peak hardness of HV82t about 240h was observed. When aging at 200 C, the hardnesslateaued at about HV76 after 64h. The peak hardness drops toV70 at 32h for aging at 250 C. Aging at 300 C has no significant

ardening effect.The difference in peak hardness observed at different aging

emperatures is in part associated with the difference in volumeraction of the precipitates formed. But the other important reason

Fig. 1. The aging curve of AZ80 magnesium alloy.

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Fig. 2. Discontinuous lamellar structure in AZ80 aged at 200 C for 16h.

or this difference is the morphology of the corresponding precipi-ates.

.2. Morphologies of Mg17Al12 precipitates in AZ80 alloy

The Mg17Al12 phase has a BCC structure with a lattice parame-er of 1.056nm [3]. The Mg17Al12 precipitates can be divided intowo distinct categories according to how they are formed, beingitherdiscontinuousor continuous. Bothof these categoriesprecip-tate fromthemagnesiummatrixdirectly to the-phase (Mg17Al12)ithout any intermediate phases or GP zones [4]. The discontinu-us precipitates can subsequently be divided into three differentroups according to their growth morphology. One is a cellulartructure which consists of lamellae. The lamellar structure alwaysnitiates at grainboundaries andgrowsperpendicular to thebound-ry, as shown in Fig. 2. The growth will stop only if the grains full of the precipitates or the continuous precipitates start toncrease significantly and impede the growth of the discontinu-us precipitates. According to the literature, the growth region ofhe cellular structure can be treated as a part of the grain in thepposite side of the grain it grows into [5]. The cellular structureegion and the grain which it grows from have the same matrix

tructures.

Another kind of discontinuous precipitate is an oval-shapedllipsoid, hereafter referred to as the elliptical structure. The ellip-ical structure and the cellular structure usually formed at the same

Fig. 3. Discontinuous elliptical structure in AZ80 aged at 150 C for 64h.

120 W.-J. Lai et al. / Journal of Alloys and Compounds 476 (2009) 118124

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2cttataap2npahelliptical structure is rather smaller in size and has a larger numberdensity, which corresponds to a higher peak hardness. The resultFig. 4. Discontinuous intergranular structure in AZ80 aged at 300 C for 1h.

ime, as shown in Fig. 3. The size of the elliptical structure is about.11.5m and rarely exceeds 1.5m.These two kinds of precipitate constitute about 90% of the

iscontinuous precipitates observed. Once these precipitates areormed, they do not change in their morphology significantly. Theynly grow outward to consume regions which are not occupied byther precipitates.The last minor kind of discontinuous precipitate is the inter-

ranular precipitate, as shown in Fig. 4. This kind of precipitates detrimental to mechanical properties of AZ80 and causes theaterial to fracture intergranularly.The continuous precipitates occur in two distinctmorphologies.

ne is the Widmansttten structure (Fig. 5), which has some dif-erent precipitation orientations with the matrix [3,611], and thether being an irregular slab structure (Fig. 6), which is highly ori-nted to onedirectionwithin a grain. Sometimes the two structuresill also mix with each other, as can be seen in Fig. 7.Table 2 contains data on the size of precipitates as a function of

ime at different aging temperatures. Because the observed precip-tate size varieswidely, a range of size is given instead of an averagealue. The range is calculated by counting at least 20 precipitatesnd eliminating the extreme cases. The thickness of the lamellae inhe lamellar structures showed no significant change with differ-

nt aging times or temperatures and therefore is omitted from theable. FromTable2 it canbe seen that continuousanddiscontinuousrecipitates appear at almost the same time.Discontinuous precipitates stop growing relatively earlier than

ontinuous ones. But continuous precipitate will still keep grow-

ig. 5. Dark field image of continuous Widmansttten structure in AZ80 aged at00 C for 8h.

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Fig. 6. Continuous irregular slab structure in AZ80 aged at 250 C for 32h.

ng and occupy the rest of the unoccupied areas. The size of thesewo precipitates will not change significantly during the overagingrocess.

.3. Variation of discontinuous precipitate morphology

The lamellar and elliptical structures appear mostly under00 C, with the amount of these two kinds of discontinuous pre-ipitate decreasing as the aging temperature increases. It is hardo see them above 250 C. Table 3 shows the different precipi-atemorphologies associatedwith different aging temperatures. Inddition, different aging temperatures will result in different ellip-ical structures, not in the shape of the precipitate, but in the sizend number density. Comparing the 150 C-256h, 200 C-64h (T6)nd 250 C-32h (T6) samples, it was observed that the ellipticalrecipitate density at 150 C was higher than that at both 200 and50 C, as indicated in Fig. 8(a), (b) and (c) respectively. The lowerumber density translates into a larger interspacing between therecipitates. The larger the interspacing will result in the lower thebility of the precipitate to resist dislocation movement, and theardness will then decrease [12]. In the 150 C-256h sample thes from lots of observations of the SEM and TEM samples, not justrom particular sites.

ig. 7. Irregular slab and Widmansttten structure intermix with each other. Spec-men aged at 250 C for 32h.

W.-J. Lai et al. / Journal of Alloys and Compounds 476 (2009) 118124 121

Table 2Dimensions (m) of the precipitates in AZ80 aged at various temperatures (C) and times (h)

Aging condition Continuous precipitate Discontinuous precipitate

Temperature (C) Time (h) Widmansttten Irregular slab Elliptical Intergranular

Length Width Length Width Diameter Diameter

150 64 0.20.4 0.030.05 0.10.3 0.10.5256 0.51 0.10.15 0.10.3 0.30.6

200 4 0.20.5 0.020.05 0.20.4 0.10.38 0.20.7 0.050.07 0.20.5 0.10.3

16 0.41.0 0.050.1 0.20.5 0.20.432 0.71.5 0.070.1 0.20.5 0.50.764 1.01.7 0.150.3 0.20.5 0.60.9

256 1.52.0 0.30.4 0.20.7 1.01.4

250 2 0.61.0 0.10.15 0.51.5 0.30.5

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00 1 0.20.7 0.050.116 1.52.5 0.5

From Table 2 it can be seen that the size ranges for the variousrecipitate morphologies observed in the T6 condition for the dif-erent aging temperatures. The size may contribute a little benefito the mechanical properties because smaller and more numerousrecipitation particles will have better abilities to resist disloca-ion slip. Despite a large amount of discontinuous precipitationn the material, the hardening effect is relatively poor because ofhe large precipitate size compared with precipitation-hardenable

luminium alloys.The last kind of discontinuous precipitation is the intergranu-

ar precipitate. It appears for a wide temperature range and formsimultaneously with the other two kinds of discontinuous precipi-

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Fig. 8. The morphologies of elliptical precipitates: (a) 150 C0.52.5 0.30.6 0.51.5 1.02.0

0.30.5

ates. The size of the intergranular precipitate changes widely withothaging timeand temperatureas indicated inTable2. This kindofrecipitate prefers to nucleate at grain boundary triple points, butill also nucleate discretely along the grain boundaries, as shown

n Fig. 4. This type of precipitate grows into large and irregularhape which is undesired and therefore should be avoided as fars possible. When the aging temperature is raised up to 300 C, thentergranular precipitates become the dominant precipitate and

row extensively. The discrete precipitation particles along grainoundaries also grow, coalesce and form a massive bulk structurehich is also detrimental to the material, as shown in Fig. 9. Thisay be the primary factor that AZ80 at this aging temperature

-256h, (b) 200 C-64h (T6), and (c) 250 C-32h (T6).

122 W.-J. Lai et al. / Journal of Alloys and Compounds 476 (2009) 118124

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Table 3The temperature ranges (C) of various precipitates

Widmanstatten 150300Slab-shaped 250LEI

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wilalong a particular direction while keeping the other dimensionsig. 9. Coalescent intergranular precipitates of specimen aged at 300 C for 16h.

ields no increase in hardness after a significant aging time. Thentergranular precipitates also consume a significant proportion ofhe solute content and this results in a depletion of other precipi-ation like continuous precipitates, as shown in Fig. 9.

.4. Variation of continuous precipitate morphologyContinuousprecipitationprecipitateswithin thewholematerialniformly but does not form on the grain boundaries. Its growth iscontinuous process which the composition of the matrix is con-inuously changingwith the growth of the continuous precipitates.

uslh

Fig. 10. Widmansttten structures of (a) 150 C-256h, (b) 2amellar 150250lliptical 150250ntergranular 150300

The Widmansttten structure is the most dominant continu-us precipitate morphology in the aging temperatures between50 and 250 C. From the work of Celotto [3], it is stated that pre-ipitates of the Widmansttten structure are the major hardeningactor in AZ91 and different morphologies of the Widmanstttentructure were also observed. The size changes of the precipitatesith aging temperatures are listed in Table 2, but the distribu-

ion densities of the precipitate are different as well. From thebservations of 150 C-256h, 200 C-64h (T6), and 250 C-32h (T6)amples (Fig. 10(a)(c)), itwas found that thedistributiondensity ofrecipitates in the 150 C-256h samplewasmuchhigher and yieldsigher hardness than for the other two conditions. Lower agingemperatures also yield smaller precipitates, which are beneficialo the mechanical properties.

The morphologies of Widmansttten structures which changeith aging time were also investigated in Celottos work [3]. Tak-

ng 200 C aging for example, initially the precipitate nucleate as aozenge shape about 0.20.5m in length (Fig. 5), and then growsnchanged. The precipitate is now called an asymmetric lozengehape (Fig. 10(b)). The precipitate keeps growing to its maximumength, which corresponds to the peak hardness. After the peakardness, the precipitate grows in thickness. The thickness at this

00 C-64h (T6), and (c) 250 C-32h (T6) specimens.

W.-J. Lai et al. / Journal of Alloys and C

Fig. 11. Specimen aged at 200 C for 256h shows an asymmetric hexagon structure.

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Fig. 12. TEM image and the corresponding diffraction pattern of Widmansttten structdiffraction pattern of the Widmansttten precipitate lying in (0001) zone axis, (c) the recvariants of the precipitate.ompounds 476 (2009) 118124 123

tage is about 0.30.4mwhile the length varies from 1.52.0m.fter sufficient time of aging, the sharp corners of theWidmanstt-en structures will become blunt and are shaped like asymmetricexagons (Fig. 11).According to the literature [611], the predominant orien-

ation relationship between Widmansttten structure and theagnesium matrix is (0 001)m//(1 10)p and [1 2 10]//[1 1 1]. Somerecipitates have precipitation orientations perpendicular to theasal plane. The orientation relationship of the precipitate is0001)m//(1 1 1)p and [1 2 10]m//[1 1 2]p, which is reported byrawley and Lagowski [6] and Crawley and Milliken [7]. Andome precipitates have its growing direction lie an angle to theasal plane. The orientation relationship of the precipitate is1 2 11)m//(1 10)p and [10 1 0]m//[1 10]p [8].Fig. 12(a) and (b) is the bright field image and correspond-ng diffraction pattern from the Widmansttten structure aged at50 C for 32h. The diffraction pattern is from the precipitate indi-ated in Fig. 12(a). The spots frommatrix and precipitate are easilyistinguished. The zone axis of the matrix diffraction pattern is

ure of the alloy aged at 250 C for 32h. (a) Bright field image, (b) correspondingonstructed pattern, (d) the computed symmetric pattern and (e) the computed six

1 and C

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[8] D. Duly, W.-Z. Zhang, M. Audier, Phil. Mag. A 71 (1) (1995) 187.[9] D. Duly, Y. Brechet, Acta Metall. Mater. 42 (9) (1994) 3036.24 W.-J. Lai et al. / Journal of Alloys

0001]m and the zone axis of the precipitate diffraction patterns [110]p. Because of the two-fold symmetry of the (110)p pre-ipitate diffraction pattern, the above diffraction pattern has twoymmetric forms. Fig. 12(c) is thediffractionpatterns reconstructedrom Fig. 12(b), and Fig. 12(d) is the symmetric pattern computedrom Fig. 12(c). Open circles indicate the matrix reflections andlack spots indicate the precipitate reflections. From Fig. 12(c) itan be seen that [1 2 10]m is parallel to [1 1 1]p and [10 1 0]m isarallel to [1 1 2]p. On the other hand, the symmetric pattern inig. 12(d) has [1 2 10]m parallel to [1 1 1]p and [10 1 0]m parallel to1 1 2]p. Fig. 12(e) shows the relative position of the precipitate inig. 12(a). The white one in the middle represents the precipitatendicated by an arrow in Fig. 12(a). The other white ones representhe precipitates resulted from the six-fold symmetry of the (0001)atrix diffraction pattern. The black ones represent the precipi-

ates resulted from the two-fold symmetry of (110)p stated above.herefore, the Widmansttten structure has totally six variants [3]s indicated in Fig. 12(e).All the precipitates in Fig. 12(a) have their shapes and directions

xactly the sameas inFig. 12(e). This further confirms thecomputedesult of the precipitate orientation. Most of the Widmanstttentructures have a length 48 times greater than the width. Somere extremely long, and some have almost equal sides.Inmagnesium alloys,most of the slips occurmainly on the basal

lanes (0001) at room temperature. Only at high temperature theigh index slip systems are initiated.Since the most Widmansttten structures precipitate parallel

o the basal planes, it is hard for precipitation plates to resist theislocationmovement. Only the precipitates lying perpendicular tohe basal plane and having an angle to the basal plane have betterbility to impede dislocation slip.The irregular slab structures form above 200 C and appear

xtensively around 250 C, as indicated in Table 2. These precipi-ates have a length that varies widely, from 0.5 to 2.5m in length,.3 to 0.6m in width, as shown in Fig. 6. The special feature ofhis precipitate is that it orients along a specific direction whichemains undetermined. The precipitates are usually accompaniedy Widmansttten structures.The irregular slab structures are usually relatively large, so the

echanical properties are not good. From the aging curve of 250 Cn Fig. 1, it was found that the hardness of the material has only 10V higher than that of the solution-treated state. At this aging tem-erature the dominant precipitates are Widmansttten and slabtructures. When irregular slab structure starts to form, the Wid-ansttten structurewill coarsensignificantly, and resulted inpoorechanical properties.. Conclusions

1. Thehardness incrementofAZ80 is about38% for theT6 conditionat 175 C. This hardening ability is extremely poor compared to

[[[

ompounds 476 (2009) 118124

precipitation-hardenable aluminium alloys, which is influencedby the morphology, the size, and the distribution density of theMg17Al12 precipitates.

. All the morphologies of Mg17Al12 precipitate in AZ80 are dis-cussed. There are three kinds of discontinuous precipitateincluding a lamellar structure, an elliptical structure, and anintergranular precipitate. In addition, there are two kinds of con-tinuous precipitate, a Widmansttten structure and an irregularslab structure.

. The size and distribution density of precipitates depend onthe aging temperature. Different aging temperatures also yielddifferent precipitate morphologies. Basically, the Widmanstt-ten structure appears at all aging temperatures; the lamellarand elliptical structures appear under 250 C; the irregular slabstructure appears around 250 C and the intergranular precipi-tates appear at all temperatures but grows extensively around300 C.

. Both continuous and discontinuous precipitates have effect onthehardnessof thealloy.At loweraging temperatures, theellipti-cal andWidmansttten structures have a higher number densityand smaller size, which will significantly increase the hardnessof the alloy.

. The Widmansttten structure has three different orientationswith respect to basal plane. Most of them are lying in the basalplane.

. At around 250 C both the irregular slab and Widmanstttenmorphologies coarsen extensively. So the hardness is poor com-pared with other aging temperatures.

cknowledgement

The authors gratefully acknowledge the financial support forhis research by Chung-Shan Institute of Science and TechnologyTaiwan) under grant no. BV96E06P.

eferences

[1] C.H. Caceres, C.J. Davidson, J.R. Griffiths, C.L. Newton, Mater. Sci. Eng. A 325(2002) 344355.

[2] C.R. Brooks, Heat Treatment, Structure and Properties of Nonferrous Alloys,ASM, Metals Park, OH, 1984.

[3] S. Celotto, Acta mater. 48 (2000) 17751787.[4] S. Celotto, T.J. Bastow, Acta Mater. 49 (2001) 4151.[5] D.A. Porter, K.E. Easterling, Phase Transformations inMetals andAlloys, 2nd ed.,

CRC Press, 2004, pp. 322326.[6] A.F. Crawley, B Lagowski, Metall. Trans. 5 (1974) 949.[7] A.F. Crawley, K.S. Milliken, Acta Metall. 22 (1974) 557.10] D. Duly, M.C. Cheynet, Y. Brechet, Acta Metall. Mater. 42 (11) (1994) 3843.11] D. Duly, J.P. Simon, Y. Brechet, Acta Metall. Mater. 43 (1) (1995) 101.12] R.E. Reed-Hill, R. Abbaschian, Physical Metallurgy Principles, 3rd ed., PWS Pub-

lishing Company, 1994, pp. 532534.

Aging behaviour and precipitate morphologies in Mg-7.7Al-0.5Zn-0.3Mn (wt.%) alloyIntroductionExperimental procedureResults and discussionHardnessMorphologies of Mg17Al12 precipitates in AZ80 alloyVariation of discontinuous precipitate morphologyVariation of continuous precipitate morphology

ConclusionsAcknowledgementReferences