Effect of microstructure on the mechanical properties and fracture of ...

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LAoRCHAN and J. GRUSLE sKr, ibid. 100 (1992) 53. 6. M. cIGLOTTI, Metall. Trans. 3 (1972) 933. 37. A. HELLAWELL, "Progres...

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JOURNAL OF MATERTALS SCTENCE 34 (1999) 3s5s_3564

Effect of microstructure on the mechanical properties and fracture of commercial hypoeutectic Al-Si alloy modified with Na, Sb and Sr N. FATAHALLA, M. HAFIZ Mechanical Department, Faculty of Engineering, Al Azhar University, Nasr City, Cairo, Egypt (Postal Code 11371)

M. ABDULKHALEK S-enior Engineer, lndustrial Engineering Company for Construction and Development (ICON),

Cairo, Egypt E-mai I : nfata h al I [email protected]

u

n.eg

The microstructure, hardness, tensile properties and fracture have been studied for the non-modified and modified aluminium (Al) silicon (Si)commercial hypoeutectic alloy. Three modifiers were used being sodium (Na), antimony (Sb) and strontium (Sr). The Sb-mod'itieO structure revealed small plate-like Si morphology. The Na and Sr-modified structures exhibited fibrous Si. A slight increase in the hardness values (HV) due to modification was observed. A general increase in the tensile properties was observed due to modification. The tensile properties of the sand mould Sr-modified alloy were significantly higher than those of the Na-modified alloy by 12.7% in proof stress, 16.3% in ductility and 33.3% in toughness. For the metal mould ingots the increase in tensile properties of Sr-modified alloy were respectively: 16.70/o, 32.5% and 41 .7o/" compared to a Na-modified alloy. Optical fractography on longitudinal sections near the fracture surfaces of the modified alloys revealed that the crack propagates in the eutectic thus, circumventing the Al-dendrites. The dimple and smooth ripple patterns observed by scanning electron microscope (SEM) on the fracture surface of the Na and Sr-modified alloys suggest a transgranular type of fracture across the grains of the eutectic matrix. @ tggg Kluwer Academic publishers

1. lntroduction

I

I J

In spite of the well-known [1] numerous advantages of Al-Si alloys and their widespread applications [2, 3], however, their use in industry as structural materials have been limited due to lack of ductility. The poor ductility refers to the microstructure, which contains platelike Si particles, embedded in an Al-matrix. Considerable work has been done concerning the microstructural modification of Al-Si alloys. The modification process

good wear resistance...) besides improving its ductility and consequently its toughness. The Na, Sb and Sr modifiers were selected to achieve this goal. The investigation implied studying the microstructure, hardness, tensile properties and fracture of the non-modified and modified-alloys. The effect of the three modifying processes on the properties of the investigated all,oy is discussed. Correlation among microstructure, mechanical properties and fracture is presented.

of these alloys has been carried out using several mod-

iflers. The most widely used commercial modiflers are Na [4-8] and Sr [4,'7 ,8,10, and 13-20]. The numerous publications on modification pointed out that each of these modifiers has a beneficial influence on the structure of the eutectic. However, the choice of the best modifler in order to obtain parts having optimum properties seems to be still an open question. In the present investigation an Al-5.5mass%Si alloy (commercially produced in Egypt for production of automobile pistons) was used as a reference material. The research aimed at retaining the good properties of this alloy (e.g. low density, relatively high strength and

0022-2461 @ 1999 Kluwer Academic Puhli,sher.t

2. Experimental The chemical analysis of the reference material used is listed in Thble L The solidification cooling rates (SCRs) were measured using a chromel-alumel thermocouple connected to a digital counter and an x-t recorder. Table II presents the characteristics ofthe two types of moulds used to allow two distinct SCRs. Firstly, the modiflcation process with sodium was

of a mixture of Na-salts (two thirds of Na-fluoride and one third of Na-chloride) [14] to the molten metal. These salts were preheated to

carried out by adding 1%

3555

TABLE I

Chemical analysis of the reference material used in the

present investigation

Element

Si

Mg

Fe

Cu

MassTo 5.5 0.005 0.46 0.009 0.006 0.02 0.0037 Balance Ni, Zn and Pb are almost nil.

TAB LE

II

Type of mould and corresponding SCR

Type of mould

SCR (I(/s)

Sand

0.6

Metal (steel)

l6

523K prior to adding to the molten metal at 1023K. The melt was then superheated to 1053 K prior to pouring into sand and metal moulds. The melting operation was carried out in a vertical electric furnace connected with a millivolt device for controlling the temperature. The charges were melted in a steel-crucible lined with graphite and liquid glass. The crucible cavity-size was about 500 ml and the mass of the melt was 1250 g ap-

proximately. Secondly, the Sb-treatment was performed by adding of pure metal to the Al-Si alloy [9]. The Sb was added at993 K and the molten metal was held for 300 s inside an electric furnace at993 K and poured directly into the moulds. Thirdly, modiflcation was performed by adding 0.0157oSr in the form of extruded rods (9 mm diameter) of Al-5VoSr master alloy 116-19,25,26). Small pieces were cut from the extruded rod for addition into the

0.8Vo

melt at 993 K. The molten metal was manually stirred for few seconds and then held, prior to casting, in the furnace at the same temperature of 993K, to ensure homogeneity 1271. At the end of the holding period, the molten alioy was taken out of the furnace, skimmed and poured directly into sand and metal moulds. Standard technique [28] was used for the metallographic study. Metallographic examination was carried out using the SEM after specimens had been deeply etched for 1.8 ks with a 5ToNaOH solution. The volume fraction (VF) of the constituent phases has been detected using the point counting technique [29]. Macrohardness values'i'were obtained using the Vickers test. A square-based'pyramid indenter with an angle of 136' was used. A loail of 15 .625 kg was applied for a period of 30 s. Standard tensile specimens were machined to gauge dimensions of 5 mm diameter and 25 mm length. The tensile tests were conducted on a motor driven tensometer type "'\ry'". Tensile tests were perforned at ambient temperature of 300 K at a strain rate of 4 x 10-a s-1. The tensile fracture characteristics were examined

using both optical microscope and SEM. The optical microscope was used to investigate the longitudinal sections near the fractured surfaces. Additionally, the SEM was employed to reveal the fractography of the fractured surfaces.

3. Results 3.1. Microstructure Fig. 1 shows the eutectic-Si size and morphology as revealed by the SEM of the non-modifled alloy cast

l0

pm

Figure 1 Effect of mould-type (solidiflcation cooling rate) on the eutectic-Si size and morphology for nonmodified alloy; (a) sand mould and (b) metal mould.

ni \;l rnorliflerl allor

(s,lrrrrl rrroultl)

b modified alloy (sand mould)

h)

Nn

rttotlifi etl :rlloy (lrrclnl rrrorrltl)

alloy

d) sb

L)

e) Sr modified alloy (sand mould) Figure

2

f) Sr rnodified alloy

Silicon morphology of an Al-5.5VaSi alloy modified respectively with Na, Sb and Sr.

into both sand and metal moulds. It can be seen that the eutectic-Si is of plate-like morphology. The dimensions of these platelets are indicative of the SCR. The effect of Na-modification on the Si-morphology is shown in Frg.2aand b for both sand and metal mould cast ingots. The eutectic-Si shows, generally, a flbrous morphology. However, the size was smaller for the metal mould ingot (Fig. 2b) tn comparison to the sand mould ingots

(Fig.2a). A three dimensional observation of the Sb-modified alloy (Frg. 2c and d) exhibits a lamellar form of the eutectic-Si. Sb addition did not result in a flbrous Si morphology, as was the case with Na-modiflcation. The microstructure of the Sr-modifled alloy is shown in Fig. 2e andf. Fibrous eutectic-Si morphology can be seen for both SCRs. This fibrous morphology resembles that of a Na-modifled alloy (cf. Fig. 2a andb). However, in the case of Sr the Si-particles size was scaled down to flner fibres.

For an Al-5.5VoSi binary alloy it can be determined from the equilibrium diagram [17] that the volume fraction of Al-solid solution (Vf") is 567o. The results obtained from the microstructures for the Vfo are listed in Table III for both non-modified and modifled alloys. A general increase in the values of Vf* for the modifled alloys can be observed. Additionally, it seems that the SCR plays also a role, which affects the value of Vfo. TAB LE

III

Effect of Na, Sb and Sr modification and type of mould on the volume fraction of Al-solid solution Volume fraction of Al-solid solution (7o)

Alloy condition

Sand mould

Metal mould

Nonmodified Na modified

57

59

59

60

Sb modified Sr modified

58

59.6

60

6T

3557

TABLE IV Vickers hardness of

non-modifled and modified

on the ductility of the investigated Al-Si alloy. The ductility of the Sr-modified alloy reached a value of 5%o which is 3 to 4 times the value for non-modified alloy (l.4Vo) and, in addition to that, it is higher than the ductility of the Na-modifled alloy (4.3Vo) and Sb-

Al-5.57oSi alloy Vickers Hardness (MPa)

Alloy condition

Sand mould

Metal mould

Non-modified

445 448 468 482

460 462

Na-modified Sb-modified Sr-modifled

modifled alloy (3.7Vo).In consistence with strength and ductility results the toughness of the Sr-modified alloy (5.2 MPa) was sdperior to values for non-modifled alloy (0.95 MPa), Na-modifled alloy (3.9 MPa) and Sbmodified alloy (3.7 MPa). By analogy, the tensile properties of the metal mould ingots resembled those explained in the previous para' graph for the sand mould ingots. The op., UTS, EVo and toughness of the Sr-modifled alloy showed the highest values (70 MPa, l42MPa, L0.6Vo and 11.9 MPa respectively), Na-modifled (60 MPa, 96MPa,2.57o and2

475

49t

3.2. Hardness The Vickers hardness values are given in Table

IV for

both the non-modifled and modifled alloys. A slight increase in hardness can be observed due to modiflcation process. The hardness of the Sr-modifled alloy

MPa respectively) and Sb-modified (56MPa,ll5 MPa, 7.3Vo and7.4 MPa respectively). It should be indicated here that the increase in the mechanical properties (under consideration) due to modiflcation, which is higher for the metal mould ingots,

reached the maximum value of all tested specimens (cf. Table IV). However, the hardness of the Sr-modified alloy increased only by about 87o for sand mould ingots and 6.77o for metal mould ingots in comparison with hardness of the respective non-modified ingots.

compared'to sand mould ingots. The higher SCR appears to be the main reason.

3.3. Tensile properties

3.4. Fracture characteristics

Table V lists the results of the tensile test for both modified and non-modifled alloys. The 0.27o proof stress

(opr), ultimate tensile strength (UTS), elongation percent (E%o) and toughness, which is deflned by the area under the engineering stress-strain diagram, were the tensile properties under consideration. It should be noted that the modification processes have, generally, improved the mechanical properties (strength, ductilityand toughness) relative to the non-modified alloy. The Sr-modifled alloy showed the highest properties, followed by Na-modified one, and thirdly the Sb-modified alloy. opr of the Sr-modified alloy cast in sand mould (62 MPa) represents 1687o of the value for the non-modified alloy (37 MPa). It is also higher than opr of the alloy modifled with Na (55 MPa) and Sb (50 MPa). Additionally, the UTS of the Sr-modifled alloy (I24 MPa) is approximately 168%o of the value for

the non-modifled alloy. Likewise, the UTS of the Srmodified alloy is higher than that for the Na-modifled (114 MPa) and for Sb-modified (115 MPa). Naturally, the UTS of the Sr-modified alloy was 1687o of the value for the non-modified alloy (7 4MPa). On the other hand, a more pronounced effect of modiflcation was observed

TABLE V

-

As has been previously indicated [18], the examination by the optical microscopy of the longitudinal section near the fracture surfaces of the non=modified alloys cast in both sand and metal moulds revealed an intergranular mode of fracture passing along the eutectic ribbon around the cy-particles. Fig. 3a-f shows the longitudinal section near the fracture surfaces of the alloys modified with Na (Fig. 3a and b), Sb (Fig. 3c and d) and Sr (Fig. 3e and f). In Fig. 3, it is a general feature that the fracture paths are propagating along the eutectic phase circumventing the relatively ductile Al-solid solution dendrites. However, it is not clear from Fig. 3 whether this fracture path propagates transgranularly or intergranularly in the interior of the eutectic matrix. This will be more clearly demonstrated by SEM observations.

Fractographs showing the fracture surfaces of the non-modifled alloy are given in Fig. 4a and b. The similarity between these photographs gives the impression that the variation in the SCR did not have an impact on the fracture process of the non-modifled alloy. Apparently, Si-particles occupy a considerable

Effect of solidification cooling rate (type of mould) and Na, Sb and Sr-modiflcation on the tensile properties of non-modifled and

modified Al-5.5%Si alloy Tensile properties

Alloy condition

Non-modified Na-modified Sb-modified Sr-modified

Mould type

UTS (MPa)

EVo

Toughness (MPa)

Sand mould

37

14

1.4

0.95

Metal mould

48

96

2.5

2.0

Sand mould

55

tt4

4.3

3.9

Metal mould

60

tzt

8.0

8.4

Sand mould

50

109

3.7

3.7

Metal mould

56

115

7.3

7.4

Sand mould

62 70

t24

5.0 10.6

II.9

Metal mould

3558

opr(MPa)

142

5.2

n) N,r nr()(lifietl :rllo1' (s:rrrrl rrrorrlrl)

lr) N:r nrorlifierl nlloy (nretnl nroultl)

c) Sb modified alloy (sand mould)

d) Sb modifled alloy (nretnl rporrlt!)

e) Sr rnodifled alloy (sartd Figure

-, Longitudinal

mould)

f) Sr rnodified alloy (nre(nl nrorrltl)

section near the fracture surfaces of modified-alloys.

portion of the fracture surfaces in Fig. 4. A broken eutectic-Si is the characteristic of the fracture surface of both sand and metal mould cast specimens. Broken Si-particles could be detected on the fracture surfaces, as indicated by area " A" in Fig. 4a. It should also be noted that no separation of Si-particles from the matrix was observed. The features of Fig. 4 are typical of brittle fracture lL6 and l7l. Fig. 5a and b shows the effect of Na addition on the fracture characteristics of the alloy. It can be seen that the form of the Si-particles of the Na-modifled alloy has a significant effect on the fracture process. Ripple pattern of fracture can be observed on the fracture surface. A broken Si-particle indicated by an arrow is observed in Fig. 5b. Typical SEM fractographs of Sb-modifled alloy are shown in Fig. 5c and d. These fractographs exhibit a mixed mode of fracture. The fracture surface is characterised by areas of typical ductile fracture and others

show cleavage-type of fracture. However, both types of fracture are dominated by the eutectic-Si particles.

Fractographs showing

the fracture surfaces of

Sr-modifled alloy cast into sand and metal moulds are given in Fig. 5e and f. Regardless of the SCR, two features characterise the fracture surfaces of Sr-modifled alloy being dimple and ripple pattern of fracture. It is interesting to note that the dimples of metal mould cast ingots are shallower and larger than those of sand mould cast ingots.

4. Discussion 4.1. Microstructure In accordance with the results obtained previously by the present authors U6-27), the microstructure of the non-modified sand mould ingot revealed, generally, coarse dendrites and eutectic structure. This phenomenon could be seen by the optical microscopy 3559

alloy (Fig. 2a and b). This result implies that Sr can be considered as an alternative modifler to Na. It can also be noticed that the eutectic-Si particles in the Srmodified alloy are much flner than that of both Na and Sb modified microstructure (Fig. 2a-f).

4.2. Yolume fraction The Al-Si phase diagram represents an ideal

case,

where phase transformations are allowed to take place inflnitely slowly where as in casting transformations take place at finite rates. In most binary Al-Si alloys the deviation from the equilibrium eutectic temperature is a few degrees Kelvin 133, 34). Therefore, during solidification the primary Al-solid solution grows

quickly into the liquid. Thus, the liquid composition

Figure 4 Features of the fracture surfaces of the nonmodifled alloy as revealed by SEM (a) sand mould, and (b) metal mould.

previously 116, 171 and can be seen by the SEM in the present study (Fig. 1a) which reveals thick platelike Si-morphology. The Si-platelets were smaller in the case of fast SCR (Fig. 1b). Therefore, it seems that-_SCR, in the domain of variation considered here, controls the structure refinement but it does have an impact on the morphology of eutectic-Si particles 122,301.

The effect of Na-addition on the microstructure of the alloy is shown in Fig. 2a and b. The change in the morphology of Si-particles from plate-like to fibrous is achieved at both SCRs. The Si-particle size was found to be smaller for the metal mould ingots than that for its counterpart cast into sand mould. This is clearly observed by comparing Fig. 2a and b. The similarity of the Si-morphology between sand and metal mould cast of Na-modified alloy maybe explained in terms of the effect of Na on the nucleation and growth of Si [31, 321. The similarity inthe Si-particle size can onlybe due to the difference in the SCR. It is therefore, concluded that the application of high SCR in conjunction with Naaddition results in Si-particles which are signiflcantly smaller than those obtained either by modiflcation or by high SCR. Sb is observed to have a different effect on the eutectic-Si than Na does (Fig.2a and b). This lamellar structure (Fig.2c and d) is flner than the eutectic structure in non-modifled alloy. Thus, it is believed that Sb

has a reflnement effect on the microstructure rather than a modifying effect. The refining action of Sb is not as pronounced as the Na-modifying effect.

Fibrous-Si morphology was obtained after modiflcation of the alloy with Sr as shown in Fig. 2e and f. The morphology of Si-particles is similar to a Na-modified 3560

moves down following the liquidus line until the point of Si nucleation. This point is dependent on the SCR and the corresponding undercooling. As a consequence, more primary Al-solid solution is found in the metal mould cast alloy than in a similar alloy cast into sand mould (Table III). This result confirms the suggestion that the high SCR shifts the eutectic point to the right side of the phase-diagram, i.e. higher Si levels 135,361. Using thermal analysis [5], Radhakrishna showed that the modified alloy solidification temperature is lower than that of a non-modified one. Consequently, the eutectic-point is shifted to a higher Si level [37]. The new location of the eutectic-point is somewhere on the extension of the liquidus line, depending on the amount of the modifler and the SCR [37]. It is interesting to note that a higher amount of the Al-solid solution is the characteristic of the Sr-modifled alloy, especially if cast in a metal mould. On the other

hand, the values obtained for the Sb-modified alloy were, forboth SCRs, lower. This maybe attributed to the

relative amount of undercooling associated with each modifier. Therefore, the effectiveness of each modifier used resulting in the displacement of the eutectic-point towards a higher Si-content is in the following order: Sb, Na and Sr. Fig. 6 shows the location of the eutecticpoint based on the Vf of the constituent-phases given

in Table III.

4.3. Hardness The slight increase in hardness with an increase in the SCR (Table IV) can be attributed to the microstructure refinement. Recalling that the microstructure of thepresent alloy comprises Al-solid solution andbinary Al/Si eutectic (Si-particles embedded in Al-matrix). It is well known that Si-phase is harder than Al-phase.

Thus, the size, morphology and distribution of the Si-particles could affect the hardness of the eutecticmixture. Consequently, the hardest alloy should have a microstructure in which fine and uniformly distributed Si-particles are present in the Al-matrix. Such microstructure seems to be obtained in the case of Sr-modified alloy (Table IV). These results given in Table IV are conflrmed by Mondolfo [38] in his report about the effect of modiflcation on the hardness of some Al-Si alloys having Si-contents ranging from 5 to 7Vo by weight. His non-modified alloys showed

Figure

5

n) Na modified alloy (snnrl mould)

lt) Nn rnodified nlloy (mrtnl mould)

c) Sb modlfied alloy (sand mouldl

d) Sb modified alloy {='.c:,1 rnoutd}

e) Sr modified alloy (sand mould)

f) Sr modlfled

Features of the fracture surfaces of modified alloys as revealed by SEM.

hardness values of 400 to 500 HV (MPa) depending on the Si-content. The modified alloys showed almost the same hardness values as the non-modifled alloys for the respective Si-contents. This phenomenon was valid for both sand and pernanent-mould castings [38]. The hardness values obtained in the present study ranging from 445 to 460 HV (MPa) for the non-modified alloys, and from 448 to 491 HV (MPa) for the modified alloys correspond well with the hardness values reported above by Mondolfo for both non-modified and modifled Al-Si alloys.

4.4. Tensile properties An increase in tensile strength, ductility and toughness was observed due to modiflcation (Table V). The increase in the tensile properties (opr, EVo and toughness) is shown in Table VI for the modifled alloys. All the tensile properties of Sr-modifled alloy are superior to others. It is interesting to note that the increases in

tensile properties for Sr-modifled sand cast alloy were higherby l2.7Vo forcrpr, 16.3Vo forductility, and 33.37o for toughness to that of Na-modifled alloy. For metal mould ingots the increases were respectively: 16.7%o, 32.5Vo, and4l%o.ltcanbe seen that the impact of modiflcation on the tensile properties is more pronounced for the higher SCR (metal mould). Fig. 7 exhibits a comparison among the stress/plastic strain diagrams for the modifled and non-modified alloys. Superiority of the

tensile properties of Sr-modifled alloy is that clearly demonstrated by curves 3 and 4 in Fig. 7. These values of the tensile properties conflrm those reported by Mondolfo [38] for sand and metal mould ingots, although he did not mention the type of modifier used.

4.5. Fracture characteristics The SEM fractographs presented in Fig. 4 characterise the nature of fracture in the non-modifled version of the present alloy. Generally, features typical of 3561

TABL

E

V

I

Percentage change of the tensile properties due to Na, Sb and Sr modification 7o increase w.r.ta

Vo changew.r.t Na modified alloy

nonmodified alloy

Mould type

Modifier

op,

Sand

Na

48.6

207

3t4.9

Sb

35.1

164

293.6

Sr Na

-9

-13.9

-5.1

67 _6

257

453.2

+12.7

+16.3

tJJ.J

25.O

220

sb

t6.7

r92

Sr

45.8

324

320 270 495

Metal

a

Toughness

opr

Toughness

Vo increase

op,

l0

-6.7

-8.7

+16.7

+32.5

-r., +41.7

w.r.t

Sb modified alloy Toughness

16.2

7.1

35.1 9.6

25

45.2

24

5.4 40.s 13.s 60.8

w.r.t. with respect to.

ll0

;g

gro

th

E 6

TYPE OF ALLOY

1 3 5 7 Figure

6

Nonmodified alloy (sand mouldl Na modified alloy (sand mould) Sb modified alloy (sand mould) Sr modified alloy (sand mould)

Nonmodified alloy (metal mould) Na modified alloy (metal mould) Sb modified alloy (metal mouldl Sr modified alloy (metal mould)

Effect of solidification cooling rate and Na, Sb and Sr modification on the shift of the eutectic point.

cleavage fracture could be identified. Thus, the fracture pattern shown in Fig. 4 can be rated as a typical brittle fracture. These features explain the poor tensile properties of the non-modifled alloy (Table V and

3562

2 4 6 I

Fig. 7). The broken Si-particles detected on the fracture surface suggest that cracks be initiated in the Siparticles. It can thus be concluded that in non-modified alloys, where the Si-particles have a coarse plate-like

4

.e.---!t

:-t-.tsi.'

/'2 ,re /.u

'r't d

= o u)

o,

v)

50

l-ls 2

W

1- Non-modified atloy 2- Non-modified alloy 3- Sr modified alloy rL Sr modified alloy 5- Sb modified alloy 6- Sb modified alloy 7- Na modified allo,"' 8- Na modified alloy

i/

Plasric Strain

(Saod_modd}

(metal moutd)

(sand mould)

(metal mould)

(sand mould)

(Metal moutd)

(Sana mo&a)-

(Metal mould)

%

Figure 7 Comparison among the stress/plastic strain curves of Na, Sb and Sr modified alloys.

form, fracture initiation and propagation through these platelets is more likely to occur. The fracture characteristics of Na-modifled alloy described earlier in the article 3.4 and shown in Fig. 5a and b are great evidence that the eutectic-Si morphology influences significantly the fracture behaviour of the present alloy. The fracture mechanism changes

from brittle (Fig. 4a and b) to ductile (Fig. 5a and b) due to the change of Si-particles shape from aplate-like to a fibrous form. The ripple pattern observed on the fracture surface (cf. Fig. 5a and b) suggests that a considerable amount of plastic deformation occurs, likely, prior to fracture. The features of fracture surfaces of the Sb-modifled alloy were presented in Fig. 5c and d. The mixed mode of fracture clearly shows that the fracture occurs by the growth of internal cavities and that cavity nucleation occurred around the Si-particles. The fracture pattern (brittle/ductile) seems to be determined by the Siparticle size and morphology, which may influence the damage initiation and the level of strain in the matrix-

mateial123). Typical dimple and ripple patterns were observed on the fracture surfaces of the Sr-modifled alloy (Fig. 5e and f). Therefore, the fracture in Sr-modified alloy can

be rated as a ductile fracture. The light portions in Fig. 5a suggest that the Al-matrix takes part in the fracture process. It also tells that the fracture is preceded by plastic deformation. It is interesting to note that the dimple size is larger for metal mould ingots (Fig. 5f) than that of sand mould cast ingots (Fig. 5e). In addition to that, the dimple depth of the former is a smaller (shallower dimple). Large-size and shallow dimples reflect

the higher ductility of the metal mould alloy studied (Table V).

The optical fractography of the longitudinal section near the fracture surfaces of Na, Sb, and Sr-modifled alloys, which were shown in Fig. 3e and f, revealed that the crack preferred to propagate along the eutectic phase, circumventing the Al-dendrites. The dimple and smooth ripple patterns observed by SEM on the fracture surfaces of the Na and Sr-modifled alloys (Fig. 5a, b, c and f) suggest a transgranular type of fracture across the grains of the eutectic matrix.

5. Conclusion 1. The eutectic-Si is found in different morphologies depending upon the presence or absence of the modifying agent and the type of the modifier used. plate-like eutectic-Si characterises the non-modifled alloys. Sbmodifled version displays a lamellar eutectic-Si. Modiflcation with Na or Sr resulted in changing the coarse plate-like morphology into a flne fibrous one. 2. Detection of the location of the eutectic-point on the phase-diagram has been carried out based on the determination of the volume fraction of the constituent phases. Modification resulted in shifting the eutecticpoint to higher Si side at temperatures lower than that of the binary eutectic temperature (850 K). 3. High solidiflcation cooling rate when combined with Sr-modification enhances the hardness of the Al5.SVoSi alloy. HV reached a value of 491Mpa, which represents an increase of about6.TVo with respect to the non-modified one. 4. The increase in the tensile properties of sand

mould Sr-modified alloy compared to those of 3563

Na-modifled alloy were 12.77o for the proof stress. 16.3%o for the ductility and33.3 for the toughness. For metal mould ingots, the increases were 16.7Vo,32.57o

4l .7 Vo respectively. 5. The fracture characteristics were found to be strongly related to the microstructural features. For non-modifled alloys, where the Si-particles have a coarse, plate-like form, the fracture pattern was of brittle appearance. The miniature lamellar structure of eutectic-Si in Sb-modifled alloy resulted in a mixed mode of fracture (brittle/ductile). Typical ductile fracture (dimple * ripple) were found to be the characteristic of Na and Sr-modified alloys, with flbrous Siand

particles.

6. An intergranular mode of fracture has been observed on the longitudinal section near the fracture surface of non-modifled alloys. However, the dimple and smooth ripple patterns of fracture observed on the frac-

t4.

M. HAQUE, Met. Forum.6 (i983) 54. Idem., Foundry Trade. J. U (1983) 387.

15.

B.

13.

CLOSSET andJ. GRUZLESKI, Metall.Trans.l3A (1982)

945. N . F A T A H A L L A, in Proc. of the 3rd Int. Conf. Cairo Univ., Cairo, Egypt, Dec. 28-30, 1985, edited by S. Bayoumi (Pergamon Press, 1985) p. 239. 17. N. FATAHALLA,J. Mater Sci.22 (1987) 1013. 18. Idem., in Proc. of the 5th Int. Conf. on Mech. Behaviour of Mater., Beijin, China, 2, June 3-6, 1987,YoL.2, p. 1419. 19. N. FATAHALLA, P. SECORDEL andM. SUERY, J. MateT Sci.23 (1988)2419.

16.

20.

M. HAFIZ, T. KOBAYASHI

ANd

N. FATAHALLA,

N. FATAHALLA,'A

Treatise on Materials Sci. and Eng.," (Al Ahram, Cairo, Egypt, 1995). 22. M. HAFIZ and T. KOBAYASHI,TTans. J. Foundrymen's Soci-

21.

ety.l(1993)

tl5.

23. Idem., Scripta Metall.

24.

et

Mater.3l (1994)701. and S. BAHI,

M. HAFIZ, N. FATAHALLA

Z.

25. N. FATAHALLA, J. Mater Sci.25 (1990)339626. Idem., J. Mater Sci.24 (1989) 2488.

trans granular fracture.

27_

M. HAFIZ, N. FATAHALLA 2nd Int. Conf.

The authors would like to thank Prof. M. A. AlNawawy and Prof. S. ElGemae for their help and advice.

References

Q.

Aluminium.49 (1979) 499. NAGEL and R. PoRTALIER, AFS Int. Cast Metals

GARAT,

A. TELLI

3564

. RAUTA, in Proc. of the 2nd Int. Conf. on Molten Aluminium (AFS, 5, 1966) p.396. 35. D. GRANGER, R. SAWTELL and M. KERSKER, AFS Trans.92 (1984) 579. 36. w. LAoRCHAN and J. GRUSLE sKr, ibid. 100 (1992) 53. 37. A. HELLAWELL, "Progress in Mater. Sci.," (Pergamon Press, 22,1977) p-

38.

l.

L. MoNDoLFo,

'Aluminm Alloys: Structure and Properties,"

(Butterworth, England, 1976) p.767

Rev.

J.5

(1980) 2.

12.

Development, Cairo, Egypt, Dec. 29-31, 1987, Vol. 2, p. 277 . 28. ASM, Metals Handbook (Metals Park, Ohio, USA. 7, 1972)p.241. 29. Idem., (Metals Park, Ohio, USA.9, 1985) p. 123. 30. S . S PEAR and G. GARDNER, AFS Trans. 7l (1963) 2O9. 31. c. srcwoF.rH, ibid. 91 (1983) 7. 32. B. GALLOIS and G. SIGWORTH, in Proc. of the AFACMI Conf., USA, Dec. 11-12,1984, p. 101. JJ. A. HELLAWELL, "Progress in Mater. Sci.," (Pergamon Press,

34. Y

2. J. JoRSTAD, AFS Trans. 92 (1984) 573. 3, B . THALL and B . CHALMERS, "f. Inst. Met.77 (1950) 79. 4. B . GUNTHER and H. JURGENS , Giessesrel6T(1) (1980) 8. 5. r. RR o H A KRI S HN A, Indian Foundry J. 29 (1983) Il5. 6. M. cIGLOTTI, Metall. Trans. 3 (1972) 933. 7. P. HESS andE. BLACKUMUN,AF,S Trans. 83 (1975) 87. 8. J. CHARBoNNIER, AFS Int. Cast Met. J. 4 (1979) 39. 9. s. HoLECEK and A. BRANDEIS,Slevarents.l(1979)28.

N.

and S. BAHI, in Proc. of the Ain Shams Univ. on Production Eng. and Design for

15. 1970) p. 3.

1. ASIM, Metals Handbook (Metals Park, Ohio, USA, 1948).

10. 11.

Metallkde.Sl

(1990) 7s.

ture surfaces of Na and Sr modifled alloys suggest a

Acknowledgement

CASI

Metals J.7 (1994) 103.

and

S. KISAKUREK,Scripta Metall.20(1986)

1657.

Received 3l May 1995 and accepted 14 January 1999

.

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