Effect of Heat Treatment and Chemical Composition on the Mechanical Properties of a 357 Semi-solid Alloy using SEED

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Effect of Heat Treatment and Chemical Composition on the Mechanical

Properties of a 357 Semi-solid Alloy using SEED

Samuel, E.; Zheng, C.-Q.

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E ffe c t o f H e a t T re a tm e n t

a n d C h e m ic a l C o m p o s itio n

o n th e M e c h a n ic a l

P ro p e rtie s

o f a 3 5 7 S e m i-s o lid

A llo y u s in g S E E D

E. Samuel and C-Q. Zheng

National Research Council of Canada, Aluminum Technology Centre (NRC-ATC), 501 Boulevard de l'Universite, Chicoutimi, Canada

A B S T R A C T

The importance of aluminum-silicon casting alloys in the automotive industry has been well-documented. Fewer studies, however, have devoted themselves to the use of semi-solid Al-Si alloys as an alternative to conventionally cast Al-Si alloys. ln recent years, the National Research Council Canada - Aluminum Technology Centre (NRC-ATC) has investigated the use of semi-solid aluminum alloys, notably 357, using the SEED rheocasting method, as developed by Rio Tinto A1can in collaboration with NRC-ATC. SEED (Swirled Enthalpy Equilibration Deviee) is a novel process which relies on the mechanical agitation (swirling) and cooling of molten aluminum to yield a semi-solid billet. The current work consists of a two-part study wherein the semi-solid 357 alloy is subjected to several heat treatments (Part (i) and chemical composition changes (Part (ii», in an attempt to further maximize the already favourable mechanical properties of the rheocast alloy. Typical mechanical pro pert y values observed for the SEED processed 357 alloy inc1ude -210-250 MPa yield strength, -300-320 MPa ultimate tensile strength and -12-17% elongation for Part (i) and -265-280 MPa yield strength, -330-350 MPa ultimate tensile strength and -12-15% elongation for Part (ii).

IN T R O D U C T IO N

The inherent strength and ductility in aluminum alloys can often be augmented by the use of heat treatment or chemical modification. However, the use of alternative casting procedures should also be considered. Given that casting defects are greater in size than microstructural defects', adjustments in the way a part is cast prior to heat treatrnent, for example, presents a greater challenge than simply modifying the heat treatment if casting defects are prevalent in the cast part. Semi-solid alloys, for example, have been reported to demonstrate ease of material flow and good die-filling capabilities/, owing to their dual liquid-solid nature and characteristic globular network. ln terms of semi-solid casting, two routes are often considered: (a) rheocasting and (b) thixocasting.

Rheocasting involves the preparation of a semi-solid slurry beside a diecasting machine followed by the immediate casting of the slurry into parts, whereas thixocasting requires that ingots be cast first. These ingots are then sectioned into slugs, reheated into the semi-solid temperature range and cast into parts using a diecasting machine.'. By virtue of this reduction in process cornplexity", extensive work has been carried out at NRC-ATC using the SEED rheocasting method in an attempt to enhance the already favourable values of mechanical properties in the 357 semi-solid aluminum alloy.

The SEED process is a simple one relying on the proper enthalpy discharge from the molten aluminum alloy to bring the liquid metal down to the temperature range associated with semi-solids. Initial studies at NRC-ATC quickly confirmed a marked reduction in porosity observed in semi-solid cast parts and tensile samples, when compared to conventional casting methods. Furthermore, no transportation, storage or handling of the semi-solid billets is necessary, making SEED a suitable alternative to thixocasting. Although a novel process, a noteworthy portion of the literature with respect to SEED focuses primarily on rheology and microstructureî", as opposed to the optimization of mechanical properties

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u s i n g the SEED method.

E X P E R IM E N T A L P R O C E D U R E

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The 357 alloy (7±O.lwt% Si, 0.59±0.03wt% Mg, :sO.lOwt% Fe, 0.09±0.0Iwt% Ti, 0.0025±0.0005wt% Sr, Al balance) used in this study was rheocast using SEED at NRC-ATC; the resulting semi-solid billets were then cast into wedge shape plates via a high pressure die-cast press (Figure 1). As can be seen, the Iiquid aluminum is poured into a confined cylinder which is then subjected to swirling. The swirling action brings about a reduction in the temperature of the alloy, until all that remains is a semi-solid slurry. The drainage step allows one to adjust the fraction solid of the semi-solid billet, by the removal of excess liquid. The resulting semi-solid billet is removed and fed into a high-pressure die-cast press and cast into the desired shape. ln the case of the present study,

wedge

shape plates (Pigure

1(b))

were produced.

Tilting and wirling Drainage

~_!_(O~')_

De-molding

and transfer

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F ig u r e 1 ( a ) T h e S E E D p r o c e s s a t t a c h e d to ah ig h p r e s s u r e d ie - c a s t ( H P D C ) o r e e s " .

Wedgeplate

ASTM tensile samples

F ig u r e 1 ( b ) W e d g e s h a p e p la t e s o f t h e s e m i- s o lid 357a l/ o y , o b t a in e d f r o m t h e H P D C p r e s s a b o v e .

The dimensions of the wedge plates are 16 cm in length by 10 cm in width by 0.7-1 cm in thickness. Ail heat treatments (T6) were carried out prior to machining round bar ASTM ESM tensile samples. Tensile testing was performed using a SO-kN capacity MTS servohydraulic tensile tester. Elongation was recorded using an extensometer attached to the testing apparatus. Optical microscopy was carried out using an Olympus

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B X S lM microscope coupled with a Clemex camera and image analysis system. Scanning electron microscopy was done using a Hitachi SU-70 field emission scanning electron microscope coupled with an Inca 300 series EDX detector.

PART 1- EFFECT OF HEAT TREATMENT

ln the first part of this study, the wedge plates were subjected to seven T6 heat treatments (Table 1). The nominal heat treatment used for 357 aluminum alloy studies at NRC-ATC is that detailed in HTI. Solution heat treating at 535°-540°C and artificially aging at 160o_{-1S0°C greatly encourages the formation of the Mg}

2Si hardening phase': 9-11. Due to lower diffusion

rates at 160°C, however, longer aging times would be required to attain a similar level of strength at IS0°C12. ln the current

work, the aging time was kept constant while the aging temperature was varied. Changes in temperatures and times resulted in heat treatments HTI to HT6. It should be pointed out that 'aging' in this paper refers to artificial aging. Ten to fifteen tensile samples were tested for each heat treatment, with the mechanical properties being recorded.

T a b le 1.H e a t t r e a t m e n t s ( H T ) u s e d in P a r t ( i ) Heat Treatment (HT) Heat Treatment Schedule

HT1 SHT: 520°C/4h, Q: 20°C, NA: 3h, AA: 140°C/6h HT2 SHT: 520°C/4h, Q: 20°C, NA: 3h, AA: 150°C/6h HT3 SHT: 520°C/6h, Q: 20°C, NA: 3h, AA: 150°C/6h HT4 SHT: 535°C/2h, Q: 20°C, NA: 3h, AA: 150°C/6h HT5 SHT: 535°C/4h, Q: 20°C, NA: 3h, AA: 150°C/6h HT6 SHT: 535°C/2h, Q: 20°C, NA: 3h, AA: 160°C/6h HT7 SHT: 540°C/2h, Q: 65°C, AA: 170°C/6h ... 'S H T : s o lu t io n h e a t t r e a t m e n t , Q:q u e n c h , N A : n a t u r a l a g m g , A A : e r t i î i c i e l a g m g

PART Il - EFFECT OF CHEMICAL COMPOSITION

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ln the second part of this study, the 357 aluminum alloy chemical composition was altered by varying the silicon, magnesium and strontium contents (Table 2). Prior to machining round bar ASTM E8M tensile samples, the wedge plates were T6 heat-treated using heat treatment HT7 (Part (i». Alloy 8 represents the nominal composition of the 357 alloy from Part (i). Ten to

fifteen tensile samples were tested for each composition, with the mechanical properties being recorded .

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T a b le 2. C h e m ic a l c o m p o s it io n s ( w t % ) o f a l/ o y s u s e d in Alloy Number Alloy Composition (wt"lo)

1 5%Si, 0.45%Mq, 75 ppm Sr 2 5%Si, O.6%MÇj, 0ppm Sr 3 5%Si, O.6%Mq, 75 ppm Sr 4 6%Si, 0.45%Mq, 0 ppm Sr 5 6%Si, 0.45%MÇj, 75 ppm Sr 6 6%Si, O.6%Mq, 0 corn Sr 7 6%Si, O.6%Mq, 75 nom Sr 8 7%Si, O.6%MÇj, 25 ppm Sr

P a r t ( ii)

RESUL TS AND DISCUSSION

PART 1- EFFECT OF HEAT TREATMENT

Figure 2 and Table 3 illustrate the mechanical pro pert y results obtained, with error bars, as a function of the given heat treatment.

360

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/ / / BYield Strength .Tensile Strength .Percent Elongation

4

160

o

HTl SHT: 5ZO'C/4h 0:20'( NA:3h AA: 140'C!6h

HT2

SHT: 520'C/4h 0:20'( NA:3h AA: 150'C/6h HT3 HT4 SHT: 520'C!6h SHT: S3So C/2h Q:20'C Q:ZOoC NA: 3h NA: 3h AA: 150'C!6h AA: 150'C!6h

HTS

SHT: S3S'C/4h 0:20'C NA:3h AA: lS0'C/6h

HT6

SHT: S35"C/2h 0:20'C NA:3h AA: 160"C!6h

HT7

SHT: S40'C/2h 0: 65'( NA:O AA: 170'C!6h

Alloy

F ig u r e 2.M e c h a n ic a l p r o p e r t ie s o f a S E E D p r o c e s s e d 357s e m i- s o lid a l/ o y , u s in g v a r io u s T 6 h e a t t r e a t m e n t s .

T.a

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b le .3 Me chs r u c e t. 1p r o p e r t i e s 0f a S E

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E D o r o c e s s e d 357s e m i- s o lid a l/ o v , u s in a v a r io u s T 6 h e a t t r e a t m e n t s

Heat Treatment (HT) Yield Strength (MPa) Ultimate Tensile Strenqth (MPa) Percent Elongation (%)

1 169±1 280±1 17±3 2 207±4 301±1 15±3 3 214±7 306±6 14±3 4 219±4 316±4 14±3 5 231±9 321±7 13±2 6 256±5 331±3 12±2

7

,~6;t7

354±5 11±1

As can be seen from Figure 2, the yield strength (YS) and ultimate tensile strength (UTS) of the 357 alloy increase for each subsequent heat treatment. For simplicity, the aging sequence with regards to the 357 alloy system'?' 14 is:

Supersaturated solid solution ---> GP zones ---> Mg2Si

The efficiency with which the Mg2Si phase precipitates depends on the amount available for precipitation, i.e. the amount of

Mg and Si that was properly dissolved. A change in the aging temperature will subsequently affect the amount of Mg2Si that

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c a n precipitate out of solution. Therefore, it is important to maintain a proper balance between the solution heat treatment,

quench and aging steps in the T6 temfer. A proper solution heat treatment step also modifies the morphology of the otherwise acicular eutectic Si phase '" 1 • However, with increased solution temperature/time, these spheroidized partic1es

coarsen, as shown in Figure 3. Consider, for example, alloy samples taken at HT2 and HT5. Both have been aged at 150°C/6 hl', yet solutionized at 520°C and 535°C, respectively (solution treatment time =4 hr for both alloys). As can be seen in Figure 3(b), the eutectic Si partic1es appear coarser than in Figure 3(a).

..~

251olm-F ig u r e 3.O p t ic a l m ic r o g r a p h s o f a 357-T 6 a l/ o y a t ( a ) H T 2 a n d ( b ) H T 5 , at5 0 0 X .

A closer examination of Figure 2 indicates that the aging temperature has the most effect on the alloy mechanical properties. Consider, for example, the graduai increase in both the average YS and UTS from HT2 to HT5. The solution heat treatment changes from 520°C/4h at HT2 to 535°C/4h at HT5, with aging being maintained at 150°C/6h. Now, consider the increase in the average YS from HTI to HT2, HT5 to HT6 and HT6 to HTI. ln each of these three cases, the aging temperature has been increased by 10°C, which is shown to markedly increase the YS (the UTS continues to increase gradually).

For an aging treatment of 150°C/6h, a solution heat treatment of 535°C/2h (HT4) yields a slightly higher, yet ultimately comparable, value of average YS and UTS compared to a solution heat treatment of 520°C/6h (HT3). Therefore, an increase in solution temperature of 15°C can save up to 4 hours of solution treatment time. The microsegregation of Mg and Si in AI-Si-Mg alloys requires only a short time to place the Mg2Si in solution 17. Given the size of a wedge plate casting, a 2-hour

solution treatment period is sufficient to homogenize the piece.

The percent elongation is shown to decrease gradually with heat treatment, yet still maintains a favourable range of 11±1 % to 17±3% (from HTI to HTl). At HT7, the alloy exhibits a maximum average YS of -300 MPa, maximum average UTS of -355 MPa, with an associated average %EI. of -II %. Table 4 presents a comparison of mechanical pro pert y values between the SEED processed 357-T6 alloy in this work and the findings of other authors. It should be pointed out that these values will change with specimen size, heat treatment, chemical composition, casting method and so on. However, it can be seen that the SEED rheocast alloys from this work as weil as the work of Brochu e t a l .10 demonstrate a very favourable

a e4.

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e c a m c a p r o p e r t t e s 0 a e a t - t r e a t e c a s a o y s

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Alloy Heat Treatment Vield Strength Ultimate Tensile Percent (MPa) Strength (MPa) Elongation (" 1 0 )

SEED rheocast 357 - current work

(AI-7"1oSi-O.59"1oMg, with 25ppm Sr) 298±7 354±5 11 ±1 SEED rheocast 35710 (AI-7.32"1oSi-0.6"1oMg, with 25ppm Sr) 288 339 8.8 CSIR rheocast 35711 (AI-7"1oSi-0.62"1oMg) 307 356 6.1 Semi-solidPONMLKJIHGFEDCBA3 5 i8 T6 (AI-(6.55-6.73)"IoSi-(0.5-0.56)"IoMg) 288 339 6.3 USE rheocast 35719 _{221±5 to 238±7 296±5 to 307 ±23 12.3±1.1 to 15.6±5.5} Formcast thixocast 35719 260±6 to 265±5 306±4 8.9±0.8 to 9.6±1.3 Squeeze cast 35719 266±6 301±46 4.2±3.4 Conventionally (sand) cast 35720

296 345 2

Forged 35721 280 340 9

Permanent mold cast 35710

(AI-7.8"1oSi-0.55"1oMg, with <20ppm Sr) 264 325 8.3 SEED rheocast 35710

(AI-7.32"1oSi-0.6"1oMg, with 25ppm Sr) 185 278 11.7 SEED rheocast 35710

(AI-8.1 "IoSi-0.62"1oMg, with <20ppm Sr) 182 263 5.8 USE rheocast 35719 188±3 t a 211 ±4 265±3 to 288±2 6.7±1.6 to 7.7±0.4 Formcast thixocast 35719 T5 _{215±6 3291±4 7.9±2} Semi-solid 35718 (AI-(6.55-6.73)"IoSi-(0.5-0.56)"IoMg) 219 289 5.5 Semi-solid 35722 180 255 5-10 Semi-solid 35722 228 277 6-10 Squeeze cast 35721 186 262 5

Conventionally (sand) cast 35720 117 179 3

Squeeze cast 35721 T51 138 186 2

Semi-solid 35721 T7 260 310 9

Semi-solid 35721 T4 130 250 20

SEED rheocast 3 5 io

(AI-8.1 %Si-0.62%Mg, with <20ppm Sr) 111 213 9.1

Semi-solid 35721 110 220 14

Semi-solid 35722 F 110 220 13

Permanent mold cast 35710

(AI-7.8%Si-O.55"1oMg, with <20ppm Sr) 90 177 7.4

T. b l M h . 1 f h d 3 5 7 t 1 /

As the highest value of UIS (and lowest value of elongation) occurs at HI7, it can be assumed that an aging temperature of

170°C falls either at, or just before, the point of peak aging. Therefore, ail aging temperatures from HTl to HT6 fall in the underaging region.

When dealing with serni-solids, one must also take into account the type of microstructure found. The mechanical properties of a serni-solid improve as the microstructure approaches a globular network, more so than a dendritic one. Depending on the rate of swirIing or stirring, existing dendrites can be decomposed into globules. Therefore, in addition to the heat treatments

applied, having a globular microstructure helps to maintain a favourable level of strength and ductility in the alloy. Figure 4 demonstrates an ex ample of a globular microstructure found in our 357- T6 alloy.

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F ig u r e 4.T e n s ile s a m p le ( H T 7 ) d e m o n s t r a t in g ag lo b u la r m ic r o s t r u c t u r e .

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PART Il - EFFECT OF CHEMICAL COMPOSITION

Figure 5 and Table 5 illustrate the mechanical pro pert y results obtained, with error, as a function of the alloy composition. As can be seen from the figure, the YS and UTS behaviours are very similar. Moreover, it is seen that the alloys displaying the greatest values of strength are those containing O.6%Mg. As highlighted in Part (i), additions of Mg help strengthen this alloy by forming Mg2Si precipitates. Alloys 1, 4 and 5 (ail having 0.45%Mg) display comparable levels of YS and UTS, while

Alloys 2, 3, 5 and 6 (ail having O.6%Mg) demonstrate comparable levels of YS and UTS. It has been suggested that excess Si does not contribute to hardening':', which may explain why the alloys at 5% and 6%Si exhibit comparable levels of strength for the same Mg content. Alloy 8, our standard 357 alloy composition, however, displays much higher YS and UTS. ln turn, it also exhibits the lowest elongation value, though still 11±1%.

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AlLOYl 5%5i 0.45%MR 75 ppm Sr ALLOY7 6%Si O.6%MR 75ppm Sr ALLOY8 7%Si O.6%MR 25 ppm Sr A.!...I..QYl. 5%Si O.6%MR Oppm Sr AlLOY3 5%Si O.6%MR 75 ppm Sr AllOY4 6%Si 0.45%MR Oppm Sr A!JQti 6~lSi O.45%MR 75 ppm Sr AllOY6 6%5i O.6%Mg Oppm Sr F ig u r e 5.M e c h a n ic a l p r o p e r t ie s o f aS E E D p r o c e s s e d 357-T 6 s e m i- s o lid a l/ o y , f o r d if f e r e n t c h e m ic a l c o m p o s it io n s .

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A llo y

T a b le 5 . M e c h a m c a l o r o p e r t i e s

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0 a p r o c e s s e

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357- 6PONMLKJIHGFEDCBAs e m t - s o / a o v , o r / e r e n t c e m ic e t c o m a

AIIoy Yield Strength (MPa) Ultimate Tensile Strength (MPa) Percent Elongation (%)

1 266±8 334±4 13±3 2 277±5 344±5 12±3 3 281±8 346±6 13±1 4 255±6 328±5 12±2 5 258±5 329±3 15±3 6 274±5 342±4 13±2 7 279±6 347±4 13±1 8 298±7 354±5 11±1 f S E E D d r d 1 / h o s it io n s

An increase in Mg can also lead to an increase in the formation of certain Fe-bearing intermetallic phases'", notably the n-phase (AlgMg3FeSi6)1l, 13,23,24.This intermetallic phase was observed primarily as fragmented particles, although in certain cases, the traditional Chinese-script morphology was observed. ln alloys containing 0.45%Mg, only the Mg-free ~-phase (AI5FeSi) was observed, yet with an increase in Mg content to 0.6%, both the ~- and n-phase were both noted to occur. It has

been reported that solution heat treatment brings about a transformation of the n-phase into the ~-phase, accompanied by a release of Mg into the matrix, in low Mg-containing AI-Si-Mg alloys!'. This will affect the precipitation of Mg2Si during the

aging step.

Given the excellent mechanical properties observed in our alloy, along with its low Fe content (::;0.10 wt%), it can be assumed that any detrimental effects of Fe-intermetallics on the mechanical properties was minimal. From Figure 4, it can be seen that the elongation levels vary between ll±l % to 15±3%, which, in spite of the presence of the Fe phases, is still very favourable. Examples of the se phases are given in Figure 6 .

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-

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Needle-like •.• Il-phase

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a-AImatrix •

•

•

y

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a-AI matrix F ig u r e 6.E x a m p le s o f p h a s e s o b s e r v e d in t h e 357- T6a l/ o y ( A I / o y 6:6 % S i, O . 6 % M g , 0 p p m S r ) , at5 0 0 X . CONCLUSIONS

ln this work, the mechanical pro pert y response of a SEED-processed 357-T6 serni-solid alloy was observed with respect to various applied heat t:reatments (Part (i)) and changes in chemical composition (Part (ii)). After having carried out this study, it was found that the SEED produced 357-T6 serni-solid alloy displayed (i) very favourable levels of mechanical properties with acceptable margins of error, (ii) a globular microstructure, which aids in maintaining favourable levels of mechanical properties, and (iii) the castings and tensile samples displayed no signs of major casting defects.

ln Part (i), it was shown that the alloy exhibits a maximum average YS and UTS of 298±7 and 354±5 MPa, respectively, with an associated average elongation of Il ± 1%. Although elongation decreases as the strength (YS and UTS) increased, typical elongation values fell in the range of -12-17%. Moreover, the average UTS increased by 25% from 280 MPa (HTl) to 350 MPa (HT7), and the average YS is nearly doubled from 170 MPa (HTl) to 300 MPa (HT7). These levels of strength are attributed to the uniform globular network in the semi-solid alloy, proper solution heat treatment and aging techniques in order to rnaximize the amount of Mg2Si precipitation as weil as a lack of porosity and other major casting defects.

ln Part (ii), it was shown the alloy still exhibits favourable levels of mechanical properties, especially at the higher Mg content (i.e. 0.6%Mg, as opposed to 0.45%). Samples containing

5

or

6%Si

displayed comparable values of

YS, UTS and

%El., for a given Mg level. Alloy 8 (i.e. HTI, our 'standard' 357-T6 alloy) maintained the best combination of properties. Although Fe-bearing phases were observed, their presence did not result in a drop in properties. Moreover, the iron content in ail tested was low (:::;0.10wt%).

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A C K N O W L E D G M E N T S

The authors gratefully acknowledge support from the National Research Council Canada (NRC), Rio Tinto Alcan and STAS, as weil as Dany Drolet, Marie-Eve Larouche, Stephane Lamontagne, Helene Gregoire, Genevieve Simard and Alain Simard of the Aluminum Technology Centre for their support and expertise.

R E F E R E N C E S

1. Wang, Q.G., Apelian, D. and Lados, D.A.,

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1.

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L i g h t M e t a l s , vol. 1, pp. 85-97 (2001). 2. Lemieux, A., Langlais, J., Bouchard, D. and Chen, X.G., T r a n s . N o n f e r r o u s M e t . S o c . C h i n a , vol. 20, pp. 1555-1560

(2010).

3. Young, K. and Eisen, P., M e t . S e i . T e c h . , vol. 18, pp. 11-15 (2000). 4. Basner, T.,S A E T e c h n i c a l P a p e r S e r i e s , 2000-01-0059 (2000).

5. Nafisi, S., Lashkari, O., Ghomashchi, R, Ajersch, F. and Charette, A.,A c t a M a t e r . , vol. 54, pp. 3503-3511 (2006). 6. Lashkari, O. and Ghomashchi, R, M a t e r . S e i . E n g . A , vol. 454-455, pp. 30-36 (2007).

7. Nafisi, S. and Ghornashchi, R.,M a t e r . S e i . E n g . A , vol. 415, pp. 273-285 (2006).

8. da Silva, M., Lernieux, A. and Chen, X.G.,

YXWVUTSRQPONMLKJIHGFEDCBA

J . M a t e r . P r o e . T e c h . , vol. 209, pp. 5892-5901 (2009).

9. Gan, Y.X. and Overfelt, RA., J .M a t e r . s e i , vol. 41, pp. 7537-7544 (2006).

10. Brochu, M., Verreman, Y., Ajersch, F. and Bouchard, D., I n t . J . F a t i g u e , pp. 1233-1242 (2010).

Il. Moller, H., Govender, G., Stumpf, W.E. and Pistorius, P.c., I n t . J . C a s t M e t . R e s e a r c h , vol. 23, pp. 37-43 (2010). 12. Moller, H., Govender, G. and Stumpf, W.E., M a t e r . S e i . F o r u m , vol. 618-619, pp. 365-369 (2009).

13. Imurai, S., Kajornchaiyakui, J., Thanachayanont, C., Pearce, J.T.H. and Chairuangsri, T., C h i a n g M a i 1.S e i , vol. 37, pp.

269-281 (2010).

14. Li, R.X., Li, R.D., Zhao, Y.H., He, L.Z., Li, C.X., Guan, H.R and Hu, Z.Q., M a t e r . L e t t e r s , vol. 58, pp. 2096-2101 (2004).

15. El Sebaie, 0,Samuel, A.M., Samuel, F.H. and Dot y, H.W., M a t e r . S e i . E n g . A , vol. 480, pp. 342-355 (2008). 16. Eshaghi, A., Ghasemi, H.M. and Rassizadehghani, J.,M a t e r . D e s i g n , vol. 32, pp. 1520-1525 (2011).

17. ASM Handbook V o l u m e 4:H e a t T r e a t i n g , pp. 823-879 (1991).

18. Bergsma,

s.c,

Li, X. and Kassner, M.E., M a t e r . S e i. E n g . A , vol. 297, pp. 69-77 (2001).

19. Druschitz, A.P., Prucha, T.E., Kopper, A.E. and Chadwick, T.A., S A E T e c h n i c a l P a p e r S e r i e s , 2001-01-0406 (2001).

20. Davis, J.R., Aluminum and Aluminum Alloys, ASM Metals, OH (1993).

21. Chiarmetta, G., 4th International Conference on Semi-Solid Processing Alloys and Composites, England (1996). 22. Winterbottom, W.L., M e t . S e i . T e c h . , vol. 18, pp. 5-10 (2000)

23. Caceres, C.H., Davidson,

c.r.,

Griftïths, J.R and Wang, Q.G., M e t . M a t e r . T r a n s . A , vol. 30A, pp. 2611-2618 (1999).