Materials Science and Engineering A308 (2001) 225–232

Fatigue crack growth behavior in semi-liquid die-castAl-7%Si-0.4%Mg alloys with fine effective grain structure

Sang-Won Han, Shinji Kumai *, Akikazu SatoDepartment of Materials Science and Engineering, Tokyo Institute of Technology, 4259 Nagatsuta, Midori-ku, Yokohama 226-8502, Japan

Received 21 August 2000; received in revised form 23 October 2000

Abstract

Structural control was performed on Al-7%Si-0.4%Mg alloy castings by three different fabrication routes: sand mold cast andHIP, permanent mold cast and semi-liquid die-cast. In contrast to the ordinary dendrite structure in the sand and permanent moldcast alloys, the microstructure of the semi-liquid die-cast alloy was characterized by colonies consisting of single or severaldendrite cells. Crystallographic misorientation among these colonies is large and they can be regarded as ‘effective grains’. Fatiguecrack growth tests were performed using CT specimens at a stress ratio of 0.1 and effects of microstructure were examined.Difference in the overall growth rates was not evident in three samples. However, crack growth path and fatigue fracture surfaceof the semi-liquid die-cast alloys was different from those in the others. The fatigue crack grew straight even at small DK levels.Large area of the fatigue fracture surface was covered with striations. These characteristic features are correlated with therelationship between the plastic zone size at the crack tip and the effective grain size in the semi-liquid die-cast alloys. © 2001Elsevier Science B.V. All rights reserved.

Keywords: Al-Si-Mg alloys; Semi-liquid die-cast; Grain refinement; Effective grain; Fatigue crack growth tests; Striation

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1. Introduction

Aluminum alloy castings have become a viable alter-native in structural applications for aircraft and groundvehicles because of potential economic advantage forprocessing and great benefits of weight reductions [1–3]. In such nontraditional use, fatigue resistance is ofimportance, as well as monotonic strength and ductil-ity. It has been reported that fatigue life of the castalloy is controlled by casting defects such as shrinkageand gas porosity [1,3]. Microstructural effect on fatigueproperties of castings has been left disregarded since theeffect is generally undetectable owing to the over-whelming contribution of casting defects. However,recent progress in casting technique and application ofhot iso-static pressing (HIP) to the cast products suc-ceeded in providing near-defect-free castings. Such asituation facilitates promoting a basic research concern-ing microstructure/fatigue relationship of the cast al-loys. Not only fatigue life but also fatigue crack growth

properties for such high quality castings are full ofrecent interest [1,3–5].

The Al-7%Si-0.4%Mg alloy, which is known as A356(JIS AC4CH) is commonly used for engineering appli-cation as mentioned above. The alloy usually showedso-called ‘ordinary dendrite structure’, which basicallyconsists of primary a-Al dendrites and interdendriticeutectic Si particles. Effects of dendrite cell size andeutectic Si particle morphology on fatigue crack growthin the cast and HIPed alloy were studied by Kumai etal. [5].

Recently, the present authors [6] produced Al-7%Si-0.4%Mg alloy castings by three different fabricationroutes: sand mold cast and HIP, permanent mold castand semi-liquid die-cast. A unique microstructure wasobtained for the semi-liquid die-cast alloy, which wascharacterized by refined ‘effective grain’ and eutectic Siparticles. It was found that the unique microstructureprovided improved ductility compared with the ordi-nary dendrite structure. The purpose of the presentstudy is to examine the effect of this characteristicmicrostructure on fatigue crack growth behavior of thealloy.

* Corresponding author. Fax: +81-045-9245173.E-mail address: kumai@materia.titech.ac.jp (S. Kumai).

0921-5093/01/$ - see front matter © 2001 Elsevier Science B.V. All rights reserved.PII: S0921 -5093 (00 )01981 -X

S.-W. Han et al. / Materials Science and Engineering A308 (2001) 225–232226

Table 1Chemical composition (mass%) of samples a

Si Fe Cu Mn Mg Ti Sr Al

0.010– bal.7.10 0.10 – 0.34 0.16Sm0.004 0.37 0.14 0.011 bal.0.10Pm 0.0087.31

0.14 0.010 0.39 0.13 0.014 bal.–SS 7.49

a Sm, sand mold cast; Pm, permanent mold cast; SS, semi-liquiddie-cast.

Table 2Quantitative data on microstructural factors a

GS (mm) D (mm) d (mm) a AIS (mm)

8.7 13.82.151370 52 (D1)Sm3.5Pm 1.453.6980 28 (D1)2.71.31SS 130* 46 (D2) 2.5

a GS, grain size;* for SS means effective grainsize D, primary a-Aldendrite size (D1, dendrite arm spacing; D2, dendrite cell size), d,eutectic Si particle size; a, aspect ratio of Si particle; AlS, Al-richintermetallic compound size.

2. Experimental procedures

2.1. Materials

2.1.1. MicrostructureThree cast materials were fabricated using a foundry

ingot of A356 alloys. Modification of eutectic Si wasachieved by the addition of Sr. For convenience thesematerials are called Sm, Pm and SS, hereafter. Sm wasfabricated by sand mold casting followed by HIP treat-ment. Pm was permanent mold cast material. SS wasfabricated by the semi-liquid die-cast method. Theirchemical compositions and optical micrographs areshown in Table 1 and Fig. 1.

Fig. 1(a) and (b) show the microstructure of Sm andPm. Both of them exhibit ordinary dendrite structureconsisting of primary a-Al dendrite, modified eutecticSi particles and small amount of Al-Fe and Al-Fe-Sibase intermetallic particles.

The HIP treatment for Sm was made under theapplied pressure of 100 MPa for l h at 773 K in Aratomosphere. It was found from quantitative measure-ment using an image analyzer that the area fraction ofporosity appeared on the polished specimen surface wasreduced from 2.2% for as cast material to 0.22% forpost-HIP material. Increase in cooling rate for the Pmspecimen brought about refinement of microstructure,as well as reduction in casting defects.

Quantitative data on microstructural parameters issummarized in Table 2. Dendrite arm spacing (DAS)and Si particle size for Pm are half of those for Sm.Aspect ratio of the Si particles is also smaller in Pm.

The fabrication process of SS is as follows: the meltwas poured into a special sleeve equipped with aninduction coil and a cooling jacket. The melt wasstirred by electromagenetic force in the course of cool-ing. Crystallized primary a-Al dendrites were brokenand the broken dendrite branches became globularprimary crystals while they were held at the solid-liquidcoexisting temperature. The partially solidified (semi-liquid) melt was injected into a permanent mold andcooled rapidly.

The microstructure of SS is characterized by globularcell structure and fairly refined eutectic Si particles as

Fig. 1. Optical micrographs of cast samples. (a) Sm; (b) Pm; and (c)SS.

S.-W. Han et al. / Materials Science and Engineering A308 (2001) 225–232 227

shown in Fig. 1(c). The intermetallic particles were alsorefined. For such a structure, cell size was used as aparameter instead of DAS. Here, a cell with differentaspect ratio was converted to an equivalent circle withthe same area. Its diameter was defined as cell size. Theaverage cell size of SS is larger than DAS of Pm andcomparable to that of Sm as shown in Table 2.

Fig. 2(a–c) are optical micrographs of the anodizedspecimens showing grain structure. Average grain size

Fig. 3. EBSP image showing effective grain structure in SS. (a) SEMimage; (b) inverse-pole figure of (a); and (c) reference for orientation.

Fig. 2. Optical microscopy under a polarized light for anodizedspecimens. (a) Sm; (b) Pm; and (c) SS.

of Sm and Pm were about 1400 and 1000 mm, respec-tively. Colonies consisting of several dendrite cells arerecognized as units of different color tones for (c). Asshown in Fig. 3 the EBSP analysis confirmed that theseveral broken dendrite cells forming a colony have thesame crystal orientation and large misorientation existsacross neighboring colonies. Therefore, each colony issuitable for being called as ‘effective grain’ in the sensethat the colonies act as usual grains in mechanicalbehaviors. The effective grain size of SS, which isindicated by * is also shown in Table 2 with theordinary grain size for Sm and Pm.

S.-W. Han et al. / Materials Science and Engineering A308 (2001) 225–232228

2.1.2. Heat treatmentThe castings were cut into rectangular bars with

shoulders (gage section: 16×5×4 mm3) for tensiletests and compact tension (CT) specimens (thickness,B:8.5 mm; width, W:30 mm) for fatigue crack growthtests. They were solution treated at 813 K for 8 h andwater quenched and then aged at 443 K for 48 h. Thisis the peak-aged condition referring to the age harden-ing curves obtained in advance.

2.1.3. Tensile propertiesTensile tests were performed using an Instron type

testing machine at a constant strain rate of 8.3×10−5

s−1 at a room temperature in air. The obtained tensileproperties for three materials are summarized in Table3.

2.2. Obser6ation of fatigue crack growth andfractography

2.2.1. Crack growth ratesConstant load amplitude fatigue crack growth tests

were performed in general accordance with ASTME647–93. The tests were made using a servo-hydraulictesting machine under sinusoidal waveform at a fre-quency of 10 Hz with a stress ratio of R=0.1. Crack

length measurement was made by two ways, i.e. bytaking replica from the specimen surface and by moni-toring a compliance change of the specimen using aback-face strain gage. Crack growth rates (crack lengthincrement per cycle) were calculated for the crackwhose length is beyond 1 mm from the artificial notchtip since the ASTM E647–93 requires introduction ofthe pre-fatigue crack which is longer than 10% of thespecimen thickness, B, i.e. 0.85 mm.

2.2.2. FractographyThe CT specimen after fatigue crack growth test was

divided into halves at the mid-thickness of the speci-men. The section was metallographically polished andanodized to reveal grain structure. The relationshipbetween the crack growth path profile and the grainstructure was examined. Fatigue fracture surfacesformed at different DK levels were examined usingSEM JEOL JSM-5310 at the operation voltage of 15kV. Measurements of the area fractions of striationsand dimples were made at the magnification of 5000X.

3. Experimental results

3.1. Fatigue crack growth rates

Fig. 4 shows the da/dN−DK curves for the threesamples. Fluctuation of the growth rate was observedin Sm and Pm at low DK levels (8–10 MPam). Thisis in contrast to the continuous increase in growth ratefor SS. The crack growth rate of Sm is larger than thoseof others over the whole range of DK and Sm specimenreached final fracture at a lower DK than the others.Pm and SS exhibit essentially the same growth rate.The Paris exponent m, which is the gradient of thelinear region of the da/dN−DK curve was 7.5 andcomparable among three samples. Such a high m valueis in accordance with earlier reports for A356 alloys [5].

Crack growth rate in the Paris regime is often de-scribed by the geometrical model, which has been devel-oped by many researchers [7]. The model is based onthe geometrical relationship between the striation spac-ing and the crack tip blunting process [8]. Experimentalcorrelations of striation spacings with growth rates arealso a fundamental base of the model. As will be shownin Section 3.3, the spacing of striations formed in thepresent alloys were comparable to the crack growthrates of the corresponding DK.

The cyclic crack tip opening displacement Dd isrelated to the DK by Eq. (1).

Dd

2=

dadN

=C (DK)2

sY% E(1)

where, sY% is the cyclic proof stress, E is Young’smodulus in plane strain and C is a constant [7]. What

Table 3Mechanical properties of three samples

e (%)UTS (MPa)s0.2 (MPa)

182 217Sm 2.3Pm 285 322 6.3

342SS 11.3283

Fig. 4. Fatigue crack growth rates for three samples at R=0.1.

S.-W. Han et al. / Materials Science and Engineering A308 (2001) 225–232 229

Fig. 5. Fatigue crack growth paths in Pm (a) and SS (b).

the model means is that the crack growth rate isinversely proportional to sY% . Therefore, decrease in sY%will increase the crack growth rate.

It is considered that the larger da/dN in Sm thanother two is due to the decrease in sY% in Sm. Cyclicproof stress was not obtained in the present study.However, smaller sY% in Sm than other two is antici-pated from the monotonic 0.2% proof stress shown inTable 3.

3.2. Fatigue crack growth path

Several fatigue tests were interrupted before finalfracture for Pm and SS. The CT specimen subjected tofatigue cracking was sliced into several pieces parallel tothe crack growth direction for the crack growth pathobservation. Crack growth path at the central section isshown in Fig. 5. The low magnification pictures coverthe whole length of the cracks. A tip of the starternotch is shown at a left-hand side. The crack growthpath in each grain in Pm looks flat as shown in Fig.5(a). However, a slight change in the growth directiontakes place at grain boundaries and results in a macro-scopic crack deflection with a large wavelength.

In contrast, for SS, the crack growth path is generallystraight through the whole length, although it stillfluctuated with a small wavelength as shown in Fig.5(b). No specific relationship is observed between thecrack deflection and the effective grain structure.

3.3. Fatigue fracture surface formed at 6arious DKranges

Fig. 6 (a) and (b) show fatigue fracture surface of Pmand SS produced at the small DK of 8–9 MPam. Twograins are shown in the fracture surface of Pm in (a). Agrain boundary runs from upper left to lower rightdiagonally in the picture. The fracture surface of eachgrain is divided into elongated columns. These columnscorrespond to primary a-Al dendrite cells. The fracture

surface looks flat, but it consists of many steps of slipplanes in fine scale. On the other hand, the fracturesurface of SS exhibits rougher topographical appear-ance as shown in (b). The fracture surface consists ofequiaxed granular regions containing the slip steps ofdifferent height and direction in each region.

Fatigue fracture surfaces of Pm and SS at a large DKof 13–l4 MPam are compared in Fig. 7(a and b). Thefracture surface is covered with two different topo-graphic features. One is the flat island-like region elon-gated along the crack growth direction. The other one

Fig. 6. Fatigue fracture surface at small DK of 8–9 MPam for Pm(a) and SS (b). Crack growth direction is from top to bottom.

S.-W. Han et al. / Materials Science and Engineering A308 (2001) 225–232230

Fig. 7. Fatigue fracture surface at DK of 13–14 MPam for Pm (a)and SS (b). Crack growth direction is from top to bottom. Striationsformed in the plateau region (indicated by �) are shown in thehigh-magnification picture.

Fig. 8 shows the fatigue fracture surface of SSformed at DK of 18 MPam. Striations are clearlyobserved over the large area of the fracture surface.Their average spacing is about l�2 mm and this valuealso agrees with the crack growth rate at the DK of 18MPam as shown in the da/dN−DK curves in Fig. 4.It was found from careful observation that striationsare formed not only at the plateau region but also atthe immediate vicinity of the eutectic Si particles.

The fractographic appearance for Sm at the small DKof 8�9 MPam is similar to that of Pm shown in Fig.6(a) despite that the dendrite cell size is different.Plateau region was also observed in the fracture surfaceat the DK of 13�14 MPam, but in the very limitedarea. Consequently the areal fraction of striations isvery small. Pseudo-cleavage-like morphology with bro-ken Si particles dominantly covers the fracture surface.The areal fraction of striations was quantitatively mea-sured for three samples and plotted against DK for Sm,Pm and SS in Fig. 9.

4. Discussion

Effects of microstructure on fatigue crack growthhave been examined for the Al-7%Si-0.4%Mg cast al-loy. Microstructural features which may affect the fa-tigue crack growth behavior are grain size, dendritearm spacing (DAS), size and distribution of eutectic Siparticles, intermetallic particles originated from impuri-ties, casting defects and non-metallic inclusions. Mi-crostructural control of the cast alloy is normallyachieved by changing cooling rates. The problem is thatincrease of the cooling rate results in refinement ofthese microstructures all together, as well as in reduc-

Fig. 8. Fatigue fracture surface of SS at DK=18 MPam showingstriations. Crack growth direction is from top to bottom.

Fig. 9. Change in areal fraction of striations with DK.

is the region consisting of dimples. Such a flat region iscalled ‘plateau’ or ‘patch’, where striations form, asshown in the high-magnification picture included in thefigures. The measured striation spacing at this DK levelwas about 0.1 mm. This value is in good agreement withthe crack growth rate expected by the da/dN−DKcurves in Fig. 4.

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Table 4Relationship between characteristic microstructural dimension, l*and DKtr

sY (MPa)l* (mm) DKtr (MPam)

285D1 (Pm) 5.028D2 (SS) 46 283 6.8GS (Pm) 285980 31.1

283130 11.4GS (SS)

tigue crack growth (Paris regime in general) accompa-nies a noticeable change from a microstructure-sensitiveto a microstructure-insensitive fracture behavior [7].The fracture surface concerned possibly corresponds tothe microstructure-insensitive fracture behavior. Thereare many experimental observations of such transitionsin the past [11–13]. There is an idea that transitions infatigue failure modes from the slow growth rate regimeto the Paris regime typically occur when the size of thecyclic plastic zone rc becomes comparable to the char-acteristic microstructural dimension l* of the alloy sys-tem [7].

Since the cyclic plastic zone size rc is generally givenby

rc=1p

�DK2sy

�2

(2)

The relationship between DKtr at which the transitionis expected and the characteristic microstructural di-mension l* is given by

DKtr=2(pl*)sy (3)

Here, sy is an yield stress of the material.If we take the grain size and dendrite cell size shown

in Table 2 as the characteristic microstructural dimen-sion l*, the value of DKtr is estimated by using s0.2

shown in Table 3. The obtained DKtr values are shownin Table 4. DK=11 MPam is considered to be areasonable value in explaining the present experimentalresults. This value is obtained when the effective grainsize in SS is taken as the characteristic microstructuraldimension l*. Neither the grain or dendrite cell size issuitable to explain the transition in Pm.

4.2. Effects of Si particle refinement on the crackgrowth beha6ior

In the present study, striations are observed in arelatively large area in Pm and SS. The areal fraction ofstriations increases with DK and then decreases showingthe maximum at around DK=14–16 MPam asshown in Fig. 9.

It is difficult to classify all fractographic features intowell-known typical fracture modes since the fatiguefracture surface is complicated. However, we can saythat the initial increase in the striation area is balancedwith the decrease in the stage I-like fracture mode, asshown in Fig. 6.

On the other hand, the decrease in the striation areafraction at the higher DK levels results from the in-crease in dimple fracture surface. The fatigue fracturesurface formed by rapid crack growth at high DK levelsclose to 20 MPam is almost covered with dimplestructure, which is quite similar to the monotonic ten-sile fracture surface. This means that monotonic frac-ture mode significantly contributes to the fatigue crackgrowth at the high DK region (high Kmax).

tion of casting defects. It is not easy to control individ-ual factors at will in general. In the present study,however, the semi-liquid die-cast process worked suc-cessfully to achieve refinement of grains (effectivegrains) and eutectic Si particles. In the followings, theeffect of these microstructural factors on the fatiguecrack growth behavior is discussed on the basis of theabove experimental results.

4.1. Effect of grain refinement on the fatigue crackgrowth beha6ior

It has been demonstrated that the fatigue fracturesurface produced at a low DK levels near thresholdexhibits crystallographic features as shown in Fig. 6(a).Threshold value was estimated by employing a load-shedding technique with load reduction of 5% in theearlier study. [9,10] The obtained value at R=0.1 is6–7 MPam. It is known that the fracture surfaceshown in (a) is formed by the slip plane separationmechanism. [7] Similar fracture surface is observed atthe stage of crack initiation from the smooth surface,which is so-called ‘stage I’ crack growth. The dendritecells in a single grain generally have the same crystalorientation. Therefore, the slip step morphology shouldbe similar among the dendrite cells contained in onegrain. This is confirmed in the fatigue fracture surfaceof Pm which consists of fine DAS and large grain size(see Table 2).

Fracture surface morphology of SS may also beformed by the slip plane separation mechanism. While,it consists of equiaxed granular regions as shown inFig. 6(b). Height and direction of the slip steps aredifferent in each region. It should be mentioned thatthe average size of the regions corresponds to theeffective grain size consisting of single or several globu-lar dendrite cells.

Fatigue fracture surface of Pm and SS formed at DKof 13–l4 MPam is covered with plateaus and dimplesas shown in Fig. 7. The ‘stage I’ type fracture surface isno longer observed here. In addition, there is no signifi-cant difference between Pm and SS in spite of theirmicrostructural difference.

It has been known that the transition from thenear-threshold regime to the intermediate stage of fa-

S.-W. Han et al. / Materials Science and Engineering A308 (2001) 225–232232

Origin of the dimple is considered to be dispersedeutectic Si particles and Al-Fe and Al-Fe-Si base inter-metallic compound particles for the present materials.Propensity of particle fracture as well as decohesion issize and morphology dependent. The particle, which islarger in size and aspect ratio is more susceptive tocracking or debonding. As shown in Fig. 8, for SS,striations are clearly observed over the large area of thefracture surface even at the DK of 18 MPam. Stria-tions are formed not only at the plateau region but alsoat the immediate vicinity of the eutectic Si particles.Such fractographic evidence suggests that particlesahead of the growing fatigue crack tip experienced fewdamage. Refinement of eutectic Si and other intermetal-lic particles in SS contributes effectively to reduce thepropensity of particle fracture and sustain the ductilestriation formation.

5. Conclusions

Fatigue crack growth behavior was examined forAl-7%Si-0.4%Mg alloy castings with different mi-crostructures using CT specimens at R=0.1. Presenceof refined effective grains in the semi-liquid die-castalloys brought about no significant change in thegrowth rate for the long fatigue crack compared withthe castings with ordinary dendrite structure. However,the refined effective grain structure resulted in differentcrack growth path. Crack growth direction was influ-enced by crystallographic orientation at small DK levelsfor the alloy with ordinary dendrite structure. This isbecause the plastic zone size at the crack tip is smallerthan the grain size and the crack growth occurs pre-dominantly by the single shear mode. In contrast tothat, the fatigue crack grew normal to the loadingdirection in the semi-liquid die-cast because the plasticzone at the crack tip can encompass many refinedeffective grains. At high DK levels refinement of eutecticSi particles in the semi-liquid die-cast alloys reduces the

propensity of particle fracture/decohesion ahead of thecrack tip. Presence of refined effective grains and Siparticles is considered to promote homogeneous cyclicdeformation at the crack tip to form ductile fatiguestriations.

Acknowledgements

Thanks are due to Dr Ryoichi Shibata, HitachiMetals Ltd., for supplying materials. The authors arepleased to thank the Suzuki foundation and the LightMetal Education Foundation, Inc. for providing thefinancial support for a part of this work.

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