M A T E R I A L S C H A R A C T E R I Z A T I O N 9 2 ( 2 0 1 4 ) 1 3 8 – 1 4 8

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Effect of Zr on microstructures and mechanical

properties of an Al\Mg\Si\Cu\Cr alloy preparedby low frequency electromagnetic casting

Yi Meng⁎, Jianzhong Cui, Zhihao Zhao, Lizi HeKey Laboratory of Electromagnetic Processing of Materials, Ministry of Education, Northeastern University, Shenyang 110819, PR China

A R T I C L E D A T A

⁎ Corresponding author. Tel.: +86 1359169020E-mail address: yimonmy@sina.com (Y. M

A B S T R A C T

Article history:Received 13 June 2013Received in revised form11 February 2014Accepted 22 February 2014Available online 3 March 2014

The Al\1.6Mg\1.2Si\1.1Cu\0.15Cr (all in wt. %) alloys with and without Zr addition preparedby low frequency electromagnetic casting process were investigated by using the opticalmicroscope, scanning electronmicroscope and transmission electronmicroscope equippedwithenergy dispersive analytical X-ray. The effects of Al3Zr phases on the microstructures andmechanical properties during solidification, homogenization, hot extrusion and solid solutionwere studied. The results show that Al3Zr phases reduce the grain size by ~29% and promote theformation of an equiaxed grain structure during solidification. Numerous spherical Al3Zrdispersoids with 35–60 nm in diameters precipitate during homogenization, and these finedispersoids change little during subsequent hot extrusion and solid solution. Adding 0.15 wt. %Zr results in no recrystallization after hot extrusion and partial recrystallization after solidsolution, while the recrystallized grain size is 400–550 μm in extrusion direction in the Zr-freealloy. In addition, adding 0.15 wt.%Zr canobviously promoteQ′phaseprecipitation,while theβ″phases are predominant in the alloy without Zr. Adding 0.15 wt. % Zr, the ultimate tensilestrength of the T6 treated alloy increases by 45 MPa, while the elongation remains about 16.7%.

© 2014 Elsevier Inc. All rights reserved.

Keywords:Aluminum alloyMechanical propertiesAl3ZrRecrystallizationPrecipitation

1. Introduction

Al\Mg\Si\Cu alloys have been widely used in the aircraft,automotive and construction industries due to their goodformability, corrosion resistance and weldability with mediumstrength and low cost [1–3]. Low frequency electromagneticcasting (LFEC) process developed recently by Cui et al. [4,5] wasused to solve the key problem of the direct chill (DC) castingprocess like hot tearing. The low frequency electromagneticfield could control the fluid flow and temperature field, and as aresult of the formation of a much finer and more uniformequiaxed grain structure.

8; fax: +86 024 83681758.eng).

Many literatures [6–9] reported that adding Zr into theAl\Zn\Mg\Cu alloys could not only improve their stresscorrosion resistance, exfoliation corrosion resistance, quenchsensitive and fracture toughness, but also refine the as-castgrains, inhibit the recrystallization and improve the mechan-ical properties as a result. Pourkia et al. [6] studied that theoptimal level of Zr as grain refiner added into the 7XXXaluminum alloy was 0.05–0.3 wt. %. Wagner et al. [7] foundthat Zr-bearing Al\Zn\Mg\Cu alloy exhibited higher fracturetoughness than that of the Cr-bearing alloys. Seyed Ebrahimi [8]found that the ultimate tensile strength (UTS), yield strengthand elongation of Zr-containing Al\Zn\Mg\Cu alloy were

Table 1 – Chemical composition of the alloys used in the present work (wt. %).

Alloy Si Mg Cu Ti Cr Zr Fe Al

Without Zr Nominal 1.2 1.6 1.1 0.03 0.15 – – Bal.Analyzed 1.22 1.58 1.11 0.033 0.153 <0.001 0.176 Bal.

With Zr Nominal 1.2 1.6 1.1 0.03 0.15 0.15 – Bal.Analyzed 1.17 1.54 1.17 0.035 0.168 0.154 0.168 Bal.

139M A T E R I A L S C H A R A C T E R I Z A T I O N 9 2 ( 2 0 1 4 ) 1 3 8 – 1 4 8

enhanced by nearly 13%, 15% and 16% respectively, comparedwith the Zr-free Al\Zn\Mg\Cu alloy. Eivani [9] studied thatthe Zener drag pressure increased with the increase of Zrcontent, which resulted in the decrease of volume fraction ofrecrystallized grains.

Yuan et al. [10,11] investigated the effect of Zr additionon properties of Al\Mg\Si alloys used for conductor. It wasdiscovered that higher thermal-resistant property of Al\Mg\Sialloys could be achieved through Zr addition. Litynska [12]found that both the cubic L12-type Al3Zr precipitates andtetragonal DO22-type Al3Zr precipitates were present in theAl\1.0Mg\0.6Si\0.5Zr (in wt. %) alloy, and Si addition resultedin the precipitation of the Al3Zr phase having structure of DO22

instead of DO23 structure.An ‘Air Slip’ direct chill cast (ASDC) Al\Mg\Si\Cu\Cr\V

alloy developed by Bergsma et al. [13–15] and accepted by theAluminum Association in 1997 has well been documented.The alloy with higher contents of Mg, Si, and Cu has goodweldability, higher strength and better plasticity, compared tothe traditional 6XXX aluminum alloys such as 6061 alloy.Bergsma found that the strength of hot-extruded roundbar with extrusion ratio of 8–12 was about 440 MPa, and thestrength reached ~480 MPa when the alloy was dynamicallyaged after equal channel angular extrusion (ECAE). This alloywill be a promising material used for bicycle manufacturing.According to our own research results, the alloy has thestrength of 390–400 MPa after hot extrusion at >450 °C withan extrusion ratio of >70 due to the coarse recrystallizationgrains, and thus can not meet the strength requirement of430 MPa for bicycle used materials. Additionally, the presenceof the coarse vanadium-containing phases has a deleteriouseffect on the mechanical properties of Al\Mg\Si\Cu\Cr\Valloy. So adding Zr instead of V into Al\Mg\Si\Cu\Cr alloywill be helpful to the improvement of its mechanical properties.However, the effects of Zr on the microstructures and mechan-ical properties of LFEC Al\Mg\Si\Cu\Cr alloys have not beenclearlyunderstood yet. In the present research, the evolutions of

Fig. 1 – Microstructures of as-cast alloys by p

Al3Zr phases during solidification, homogenization, hot extru-sion and solid solution in the LFEC Al\1.6Mg\1.2Si\1.1Cu\0.15Cr\0.15Zr (wt. %) alloy were studied in details.

2. Experiments

The chemical compositions of alloys used in the present workare listed in Table 1. The alloys were produced from lowfrequency electromagnetic cast ingots with the diameter of152 mm, using 99.97 wt. % pure aluminum, 99.99 wt. % copper,99.99 wt. % magnesium, Cr agent, Zr agent as well as masteralloys of Al-23 wt. % Si, Al-40 wt. % Ti. The alloysweremolten ina gas furnace, degassed by high purity argon gas at 750 °C andslag removed. The casting temperature was 750 °C. The castingspeed was 110 mm/min. The electromagnetic field was carriedout by an 80 turns water-cooled copper coil surrounding thealuminum alloy mold. The frequency of electromagnetic fieldwas fixed at 15 Hz and the current intensitywas 120 A,while theflow rate of cooling water was 80 l/min.

The JMatPro 5.0 software was used to calculate the variationof phase contents with the temperature. The temperature wasset from 750 °C to 20 °C with a step of 0.1 °C. The JMatPro 5.0 isthe commercial software developed by Sente Software Ltd. inBritain for material property simulation and metal materialphase equilibrium calculation.

The ingots were homogenized at 540 °C for 24 h. Thehomogenized alloys for hot extrusion with the diameter of127 mm and the length of 300 mm were then extruded intoØ15 mm bars at 450 °C and water quenched. The extruded rodswere solid solution treated at 550 °C for 2 h and then waterquenched. Artificial aging treatments were performed at 170 °Cfor 12 h (T6 heat treatment) and air-cooling.

Differential scanning calorimetry (DSC) analysis wasperformed in a purified argon atmosphere using a SETSYSEVOLUTION-16 DSC instrument with a scanning rate of10 °C/min from room temperature to 695 °C. The tensile

olarized light: (a) Without Zr. (b) With Zr.

140 M A T E R I A L S C H A R A C T E R I Z A T I O N 9 2 ( 2 0 1 4 ) 1 3 8 – 1 4 8

tests for as-extruded and T6 heat treated samples with thegauge diameter of 8 mm and the length of 45 mmwere carriedout in air at a constant crosshead velocity of 2 mm/min androom temperature, using a CMT5105 universal test machine.The tensile properties for each condition are the average of threesamples. Samples for optical examinations were polished bystandard techniques firstly. The as-cast samples were electro-chemically etched with a solution of 38 vol. % H2SO4, 43 vol. %H3PO4 and 19 vol. % H2O at 0.1–0.5 A and 20–30 V and thenobserved under polarized light on LECICA DM 5000 type opticalmicroscope (OM), while the as-extruded and T6 heat treatedsampleswere etchedbyKellers for 10 sand thenobservedon thismicroscope aswell. A JSM-6301F field emission scanning electronmicroscope (SEM) equipped with energy dispersive analyticalX-ray (EDAX) was used to observe the microstructure character-istics of ingots, homogenized alloys and the longitudinal sectionof as-extruded alloys and T6 heat treated specimens. A Philip EM420 transmission electron microscope (TEM) was used for highmagnificationobservation. The specimens for TEMwere cut fromthe homogenized, as-extruded and T6 heat treated samples,respectively. The thin foils were prepared by twin-jet thinningelectrolytically in a solution of 30% nitric acid and 70%methanolat −25 °C and 12 V.

500 550 600 650 700 7500

20

40

60

80

100

Phas

e co

nten

t (w

t. %

) Liquid α-Al

b

560.7279

6.66×10-6 wt. %

644.5823

1.19×10-5 wt. %

500 550 6

-4

-2

Hea

t Flo

w /m

W

Tempr

561

Tempreture / oC

a

Fig. 2 – DSC analysis result of the alloy with Zr addition and its calcuresult. (b) Dependence ofα-Al phase content on the temperature calcon the temperature calculated by JMatPro 5.0.

3. Results and Discussion

3.1. Effect of Zr on the As-cast Microstructures

Fig. 1 shows the as-cast microstructures of the Al\Mg\Si\Cu\Cr alloys without or with Zr. A coarse dendritic network isobserved in the alloy without Zr (Fig. 1(a)), and the mean grainsize is about 100 μm. A large number of equiaxed grains aredetected in the alloy with Zr addition (Fig. 1(b)), and its grain sizeis about 71 μm. Some literatures reported that the dendritemorphology did not vary much within a wide range of coolingrates [16,17], so it was difficult to refine the dendritic network bythe casting processes. According to our results, Zr addition, as aneffective method, can be used to refine as-cast grains ofaluminum alloys.

Fig. 2 shows the DSC analysis result of the alloy with Zraddition and its solidification sequence calculated by softwareJMatPro 5.0. The DSC analysis result shown in Fig. 2(a) indicatesthat the overburning temperature of the alloy with Zr additionis about 561 °C, which is closed to the one calculated by JMatPro5.0 (about 560.7 °C) shown in Fig. 2(b). Therefore, it can beconsidered that the calculation results got by software JMatPro

100 200 300 400 500 600 700

0.00

0.05

0.10

0.15

0.20

0.25

0.30

0.35

0.40

Al 3Z

r co

nten

t (w

t. %

)

c

645.1059

0.1542 wt. %

618.7

0.006321 wt. %

705.7735

2.53×10-6 wt. %

00 650 700eture / oC

647

Tempreture / oC

lation results done by the software JMatPro 5.0: (a) DSC analysisulated by JMatPro 5.0. (c) Dependence of theAl3Zr phase content

2 4 6 8 10 120

10

20

30

Inte

nsity

(C

ount

s)

E (KeV)

Al

Cu

Si

Zr

Cu

b

Element Weight% AT%

Al 64.80 80.10Si 2.33 2.76Cu 32.09 16.85Zr 0.78 0.29

a

b

Fig. 3 – SEM image and EDAX result of the as-cast alloy with Zr addition: (a) SEM image. (b) EDAX result of phase b in image (a).

141M A T E R I A L S C H A R A C T E R I Z A T I O N 9 2 ( 2 0 1 4 ) 1 3 8 – 1 4 8

5.0 in Fig. 2(b) and (c) should be authentic. It also can be seenfrom Fig. 2(c) that the formation of the metastable Al3Zr phasestarts at about 705 °C, and then the content of the Al3Zr phaseincreases almost linearly with decreasing temperature until theAl matrix starts to form at about 645 °C (Fig. 2(b)). So it can bededuced that the numerous extremely fine Al3Zr particlesbecome the heterogeneous nuclei of the Al matrix and thenrefine the as-cast microstructure of Al\Mg\Si\Cu\Cr alloy.The content of the Al3Zr phase decreases almost linearly with

a

b

c

2 40

10

20

30

Inte

nsity

(C

ount

s)

E (K

Al

Si

c

Element W

Al Si FeCu

b

Fig. 4 – SEM image and EDAX results of the alloy with Zr addition a(c) EDAX results of phase b and c in image (a) respectively.

decreasing temperature from645 °C to 619 °C. The reason couldbe closely related to the peritectic reaction in the aluminum-side corner of the Al\Zr equilibriumdiagram [18]. Some reports[18,19] also indicated that Zr mainly interacted with atomcluster, forming steady atom clusters, then growing up andfinally becoming nuclei.

The pisiform-like shaped particle present in the as-castalloy with Zr addition (Fig. 3(a)) should be the combination ofAl3Zr phase and Cu-containing phase. When the alloy with Zr

2 4 6 8 10 120

10

20

30

Inte

nsit

y (C

ount

s)

E (KeV)

Al

SiZr

Element Weight% AT%

Al 61.11 77.84Si 8.87 10.85Zr 30.02 11.31

6 8 10 12eV)

FeCu

eight% AT%

73.5 81.68.6 9.2

13.5 7.2 4.4 2.0

fter homogenization at 540 °C for 24 h: (a) SEM image. (b) and

Fig. 5 – Bright field TEMmicrograph of the Al3Zr phase presentin the homogenized alloy with Zr addition.

142 M A T E R I A L S C H A R A C T E R I Z A T I O N 9 2 ( 2 0 1 4 ) 1 3 8 – 1 4 8

is homogenized at 540 °C for 24 h, two types of Zr-containingphases will precipitate from the aluminum matrix. One is therodlike shaped DO22-type Al3Zr phase [12] containing Si withthe size about 1.48 μm long and 0.48 μm wide as shown inFig. 4(a) and (b). The substitution of Al by small amounts of Sican change the structure of the Al3Zr phase into the DO22

Fig. 6 – Micrographs of as-extruded alloys in longitud

DO22-Al3Zr

b

ba

Fig. 7 – SEM images and EDAX result of the as-extruded alloy wiupper left corner of image (a) is the magnified micrograph of theimage (a).

structure instead of equilibriumDO23 [20] which usually existsin other aluminum alloys and has less stability than thatof DO22. Additionally, the needle-like Fe-containing phasepresent in Fig. 4(a) and (c) can be considered as a coarseβ-AlFeSi phase whose brittleness and abrasive nature in 6XXXseries alloys severely reduce the hot workability and plastic-ity, resulting in pick-up formation during extrusion [21,22].The other is the fine and spherical shaped L12-type Al3Zr[23,24] phase with a mean diameter about 60 nm, as shown inFig. 5. These fine Al3Zr particles are considered to precipitatefrom the matrix during the homogenization.

3.2. Effect of Zr on Microstructures of As-extruded Alloys

Fig. 6 shows the micrographs of as-extruded alloys without orwith Zr in longitudinal direction etched with Kellers etch.Fig. 6(a) indicates that the grain structure of as-extruded alloywithout Zr is completely equiaxed. The grain structure of thealloy with Zr addition presents a complete fiber characteristicas shown in Fig. 6(b). It can be concluded that adding 0.15 wt. %Zr into the alloy can make a significant contribution to hinderthe formation of grain boundaries and their migration, so thedynamic recrystallization is completely suppressed.

Fig. 7 shows the SEM images and EDAX result of theas-extruded alloy with Zr addition. The rodlike DO22 Al3Zrphase marked by arrow in Fig. 7(a) is elongated and arranged

inal direction (etched): (a) Without Zr. (b) With Zr.

0 2 4 6 8 10 120

10

20

30

Inte

nsit

y (C

ount

s)

E (KeV)

Al

Si

Cr FeCu

Element Weight% AT%

Al 61.51 72.96Si 8.78 10.01Cr 7.29 4.49Fe 17.86 10.24Cu 4.56 2.30

th Zr addition (longitudinal direction): (a) SEM images, theAl3Zr phase indicated by arrow. (b) EDAX result of phase b in

a

a

b

b

Fig. 8 –Bright field TEMmicrographs of the as-extruded alloywith Zr addition: (a) Bright field TEMmicrograph of theAl3Zr phase inone location; (b) Bright field TEM micrograph of the Al3Zr phase in another location.

143M A T E R I A L S C H A R A C T E R I Z A T I O N 9 2 ( 2 0 1 4 ) 1 3 8 – 1 4 8

along the extrusion direction, and its size is about 1.26 μm longand 0.46 μm wide. This kind of particles can impede thenucleation and growth of recrystallization to some extent.Fig. 7(a) and (b) shows the Al(CrFe)Si phase (phase b) withrelatively large size and no tendency towards its alignment inspecific crystallographic directions [25,26]. Some literaturesreported that Cr-containing dispersoids promoted the formationof desirable α-AlFeSi during ingot solidification [21,27] as well asthe transformation of the α-AlFeSi phase from the β-AlFeSiphase during homogenization [28–30], and had strong effects onthe recovery and recrystallization due to their fairly high densityand high thermal stability [31]. However, based on Figs. 4 and 6,Cr has little influences on the transformation from the β-AlFeSiphase to the α-AlFeSi phase and the recrystallization inhibitionin theAl\Mg\Si\Cu\Cr-(Zr) alloys. Therefore, it is necessary toadd Zr into the Al\Mg\Si\Cu\Cr alloy to modify its micro-structures and mechanical properties.

Fig. 8 shows the bright field TEM micrographs of theas-extruded alloy with Zr addition in longitudinal direction.It is evident from Fig. 8 that both the fine spherical phase alocated nearby the grain boundary and phase b within thegrain are the metastable L12 Al3Zr phases with diameters about35–50 nm. Therefore, there are also two types of Zr-containingphases in the as-extruded alloy with Zr addition. One is a fewrodlike DO22 Al3Zr phases with relatively large sizes (Fig. 7),which is just similar to the one in Fig. 4(a); the other is numerousspherical Al3Zr phases with the small sizes of 35–50 nm indiameters (Fig. 8), and they are similar to the one shown inthe Fig. 5. Some reports [8,32] showed that the presence of

Fig. 9 – Microstructures of the T6 heat treated extrusions in l

metastable Al3Zr particles would strongly restrict the dynamicrecrystallization during hot extrusion. The inhibiting effects ofthe former one (DO22 Al3Zr phases) on the nucleation and thegrowth of recrystallization should be much smaller than thelatter one (L12 Al3Zr phases) due to their sizes, shapes andcoherency levels with aluminum matrix. The combined effectsof some DO22 type Al3Zr phases and a large number of finespherical L12 type Al3Zr phases result in no recrystallizationoccurring in the alloy with Zr addition during hot extrusion.

3.3. Effect of Trace Zr Addition on Microstructures ofExtrusions-T6

Fig. 9 shows the etched micrographs of T6 heat treatedextrusions of alloys without or with Zr addition on thelongitudinal section. Compared to the recrystallized grainsof as-extruded alloy without Zr (Fig. 6(a)), the grain growthwith the size of 400–550 μm is visible distinctly in the T6 heattreated alloy without Zr (Fig. 9(a)). On the contrary, only thepartial static recrystallization occurs in the T6 heat treatedalloywithZr additionas shown in Fig. 9(b) and the recrystallizedgrains are elongated along the extruding direction. Therefore, itis evident that 0.15 wt. % Zr addition to the Al\Mg\Si\Cu\Cralloy also plays an important role in restraining the formationof grain boundaries and their migration in the subsequent solidsolution at a high temperature (550 °C).

Fig. 10 illustrates the TEM bright-field images of sphericalZr-containing phases in the T6 heat treated extrusion of thealloy with Zr addition. The EDAX results of phases a and b

ongitudinal direction (etched): (a) Without Zr. (b) With Zr.

b

a

Fig. 10 – Bright field TEM micrograph of the T6 heat treatedextrusion of alloy with Zr addition.

144 M A T E R I A L S C H A R A C T E R I Z A T I O N 9 2 ( 2 0 1 4 ) 1 3 8 – 1 4 8

indicated by arrows in Fig. 10 are listed in Table 2. It was wellknown that a small addition of Zr (<0.5 wt. %) to aluminumresulted in theprecipitation of spherical Al3Zr particles coherentwith the aluminum matrix [9,23,24]. Nes et al. [23,24] identifiedthe Al\Zr particles in aluminum\zirconium binary alloys to bea metastable, cubic (L12) Al3Zr phase. Therefore, these sphericalphases a and b in Fig. 10 could be metastable L12-type Al3Zrphases which are coherent with the aluminum matrix, withdiameters about 60 nm and 56 nm respectively. They willlargely embarrass the movement of dislocations and hinderthe migration of recrystallized grain boundaries.

Fig. 11 also displays the TEM micrographs of anotherZr-containing particle in the T6 heat treated extrusion of thealloy with Zr addition. Fig. 11(a) is the bright fieldmicrograph ofthe particle indicated by arrow and its EDAX result is shown inFig. 11(b). Based on the EDAX result, as well as compared withthe diffraction patterns of the particle in Fig. 11(d) and the X-raydiffraction interplane distance data of DO22-Al3Zr [33], theparticle can be considered as DO22 type Al3Zr phase. Theselected dark field micrograph of the DO22-Al3Zr phase andaluminummatrix shown in Fig. 11(c) was carried out by the C#diffraction patterns in Fig. 11(d). The orientation relationshipbetween the DO22-Al3Zr phase and Al lattice is found to be[101] DO22 ‖ [367] Al from Fig. 11(c)–(e). Some literatures [12,33]showed that the DO22-Al3Zr phase had a tetragonal structure(the lattice parameters were a = 0.39 nm, c = 0.900 nm) and theorientation relationship of the DO22-Al3Zr phase with the Almatrix was close to <001> DO22 ‖ <001> Al and (100) DO22 ‖ (100)Al. The partial coherency of fine DO22-Al3Zr phases to thematrixwill also lead to the restriction of dislocationsmovementand recrystallization to some extent.

Fig. 12 shows the bright field TEM micrograph and thecorresponding selected area diffraction patterns of the T6 heat

Table 2 – Chemical composition of phases in the T6 heattreated extrusion of alloywith Zr addition in Fig. 10 (atom%).

Phase Al Si Cu Ti Zr

a 89.11 3.38 0.83 6.68b 86.67 3.82 0.7 0.57 8.24

treated alloy without Zr. Fig. 12(a) shows that many dotlikeprecipitates about 2–6 nm homogeneously distribute in thematrix. However, the diffraction patterns in Fig. 12(b) revealstreaks along [1 0 0]Al and [0 1 0]Al directions. Most dotlikeprecipitates are the end-on sections of needles, and the needlesalong [1 0 0]Al and [0 1 0]Al directions are not clearly visible inFig. 12(a). This is largely due to the small strain contrast of theseneedle precipitates which are coherent with the matrix. Manyliteratures [34–37] reported that Al\Mg\Si\(Cu) alloys couldachieve the maximize strength when the needle-like β″ phasesprecipitated from theAlmatrix. Previousworks done by authorsshowed that themaximize strengthwas carried out after thehotextruded Al\Mg\Si\Cu\Cr alloy aged at 170 °C for 12 h.Therefore, the fine dotlike precipitates in Fig. 12(a) should be β″phases.

Fig. 13 shows the bright field TEM micrographs of the T6heat treated alloy with Zr addition. Fig. 13(a) shows a greatnumber of precipitates delineated by strain-field contrast. It canbe seen from Fig. 13(b) and (c) that the needle-shaped β″ phases(about 2–4 nm in diameters) and the lath-shaped Q′ phases (theprecursor of Q phases) are present in the alloy with Zr addition.Some authors [38,39] considered that theQ′ precipitates, besidesβ″ phases, were also the important strengthening phases in theAl\Mg\Si\Cu alloys. The β″ precipitates are fully coherentwith the aluminum matrix along the b-axis and semi-coherentalong a and c. The lath-shapedQ' phases are recognized by theirrectangular end-on views and their long axis along the <1 0 0>direction of the aluminum matrix. So the appearances of theprecipitates shown in Fig. 13(b) and (c) are in agreement withprevious observations for the β″ phases and Q′ phases.

The types of precipitates in T6 treated alloys without or withZr are quite different. The fine and denser β″ precipitates arepresent in the T6 heat treated Zr-free alloy. The Zr additionpromotes the formation of Q′ precipitates, while not influencesthe volume fraction and size of β″ precipitates. Therefore, itcan be concluded that the addition of 0.15 wt. % Zr to theAl\Mg\Si\Cu\Cr alloy promotes Q′ phase precipitation, whilethe β″ precipitates are predominant in the Zr-free alloy.

3.4. Effect of Zr Addition on Mechanical Properties of Alloys

Fig. 14(a) shows themechanical properties of as-extrudedalloyswithout orwith Zr. TheUTS, yield strength and the elongation ofas-extruded alloy without Zr are 206 MPa, 93 MPa and 24.1%,respectively. Those of as-extruded alloy with Zr addition are348 MPa, 200 MPaand 18.31%, respectively. This ismainlydue tothe combine strengthening effects of Al3Zr dispersoids and thefiber structure.

Fig. 14(b) shows the mechanical properties of T6 heattreated alloys without or with Zr. The UTS, yield strength andthe elongation of T6 heat treated alloy without Zr are 395 MPa,319 MPa and 23.4%, respectively. Those of T6 heat treated alloywith Zr addition are 440 MPa, 361 MPa and 16.7%, respectively.Therefore, the Zr addition results in an increment by 45 MPa inUTS. This is mainly due to the combined effects of metastablefine Al3Zr dispersoids on the restriction of recrystallization anddislocation movement, a large quantities of β″ precipitates andthe promotionof Q′ precipitation in thealloywithZr addition. Inaddition, although some decrease in ductility is also observed inthe T6 heat treated alloy with Zr addition, it is well known that

a b

Fig. 12 – TEM micrographs of the T6 heat treated extrusion of alloy without Zr: (a) Bright field TEM micrograph taken with[0 0 1]Al. (b) The [0 0 1]Al diffraction patterns.

ab

Element Weight% AT%

Al 55.74 77.65Si 4.19 5.60Zr 38.8 15.99Cu 1.27 0.75

d

C#

(002)D022

(011)D022

(111)Al

(111)Al

(111)Al

(420)Al

(133)Al

C#

(000)

e

c

Fig. 11 – TEM micrographs of the T6 heat treated extrusion of alloy with Zr addition: (a) Bright field TEM micrograph. (b) EDAXresult of the phase indicated by arrow in image (a). (c) Dark field TEMmicrograph of image (a). (d) Electron diffraction patterns ofthe particle indicated by arrow in image (a). (e) Indexing of the diffraction patterns shown in image (d).

145M A T E R I A L S C H A R A C T E R I Z A T I O N 9 2 ( 2 0 1 4 ) 1 3 8 – 1 4 8

β''

Q'

b

β''

Q'

c

a

Fig. 13 – Bright field TEMmicrographs of the T6 heat treated extrusion of alloy with Zr addition: (a) TEMmicrograph taken with[0 0 1]Al. (b) and (c) HRTEM micrographs of neddle-shaped β″ and lath-shaped Q′ precipitates.

146 M A T E R I A L S C H A R A C T E R I Z A T I O N 9 2 ( 2 0 1 4 ) 1 3 8 – 1 4 8

the elongation of 16.7% is still good enough for the Al\Mg\Si\Cu alloys of 6XXX series.

Table 3 shows a comparison of mechanical propertiesbetween the T6 heat treated extrusion of the Al\Mg\Si\Cu\Cralloy with Zr addition studied in this paper and 7005 aluminumalloy after heat treatment. It can be concluded that the Al\Mg\Si\Cu\Cr\Zr alloy has the higher tensile strength and betterformability than those of 7005 alloy. In general, the Al\Mg\Si\Cu alloys (6XXX series aluminum alloys) also have the goodcorrosion resistance and weldability compared to the 7XXXseries aluminum alloys [1–3]. Therefore, the high strengthAl\Mg\Si\Cu\Cr\Zr alloy is a good substitution for 7005alloy in some respects. As a result, the Al\Mg\Si\Cu\Cr alloy

0

100

200

300

400

024681012141618202224262830

As-extruded alloy with ZrAs-extruded alloy without Zr

Elo

ngat

ion

/ %

UTS Yield Strength

Stre

ngth

/ M

Pa

Elongation

a b

Fig. 14 –Mechanical properties of as-extruded alloys and T6 heatalloys. (b) T6 heat treated extrusions.

will have a wider range of applications when it is added0.15 wt. % Zr.

4. Conclusions

(1) For the LFEC Al\1.6Mg\1.2Si\1.1Cu\0.15Cr (all inwt. %)alloy, 0.15 wt. % Zr addition can refine the as-castgrains becauseAl3Zr particles promote theheterogeneousnucleation during solidification.

(2) Numerous metastable L12-type Al3Zr dispersoids with thecross-section diameter of 35–60 nm precipitate duringhomogenization firstly and change little during the

300

350

400

450

02468101214161820222426

T6 treated alloy with ZrT6 treated alloy without Zr

Elo

ngat

ion

/ %

Stre

ngth

/ M

Pa

UTS Yield StrengthElongation

treated extrusion of alloys without or with Zr: (a) as-extruded

Table 3 – Comparison of the studied Al\1.6Mg\1.2Si\1.1Cu\0.15Cr\0.15Zr (all in wt. %) alloy and 7005 alloy [40].

Alloy Heat treatment UTS/MPa Yield strength/MPa Elongation/%

Experimental alloy + Zr 555 °C, 2 h + water quench + 170 °C, 12 h 440 361 16.77005 460 °C, 20 min + water quench + 120 °C, 24 h 420 395 12.3

147M A T E R I A L S C H A R A C T E R I Z A T I O N 9 2 ( 2 0 1 4 ) 1 3 8 – 1 4 8

subsequent hot extrusion and solid solution. In addition,some elongated DO22-type Al3Zr phases are also present inthe LFEC Al\1.6Mg\1.2Si\1.1Cu\0.15Cr\0.15Zr (all inwt. %) alloy. They result in no recrystallization after hotextrusion at 450 °C and partial static recrystallization aftersolid solution at 550 °C, while fully recrystallization andgrain growth are present in the LFEC Al\Mg\Si\Cu\Cralloy without Zr addition.

(3) 0.15 wt. % Zr addition can promote the Q′ phase precipi-tation of the LFEC Al\1.6Mg\1.2Si\1.1Cu\0.15Cr (all inwt. %) alloy, while β″ precipitations are predominant in theZr-free alloy.

(4) The strength of the LFEC Al\1.6Mg\1.2Si\1.1Cu\0.15Cr(all in wt. %) alloy is improved by the addition of 0.15 wt. %Zr largely, and the ductility is still maintained at a goodvalue. Themechanical properties (UTS, Yield Strength andElongationare 440 MPa, 361 MPaand16.7% respectively) ofthis Al\Mg\Si\Cu\Cr\Zr alloy are even better thanthose of 7005 aluminum alloy.

Acknowledgments

This work was financially supported by the FundamentalResearch Funds for the Central Universities (No. N110609002)and Fundamental Research Funds for National Project (No.N110408005).

Appendix A. Supplementary Data

Supplementary data to this article can be found online at http://dx.doi.org/10.1016/j.matchar.2014.02.013.

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