Effects of anisotropic microstructure of continuous cast Al–Cu eutectic alloys on their fatigue and tensile properties

International Journal of Fatigue 42 (2012) 45–56

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Effects of anisotropic microstructure of continuous cast Al–Cu eutectic alloys on their fatigue and tensile properties Mitsuhiro Okayasu ⇑, Ryo Sato, Satoshi Takasu Department of Machine Intelligence and Systems Engineering, Akita Prefectural University, 84-4 Aza Ebinokuchi, Tsuchiya, Yurihonjo City, Akita 015-0055, Japan

a r t i c l e

i n f o

Article history: Received 6 October 2010 Received in revised form 3 March 2011 Accepted 18 May 2011 Available online 30 May 2011 Keywords: Continuous casting Aluminum alloys Eutectic structure Mechanical property Fatigue property

a b s t r a c t The objective of this work is to explore the effects of anisotropic microstructure on the material properties of a continuous cast Al–33%Cu eutectic alloy, produced by the Ohno continuous casting technique (OCC). A clear anisotropic microstructure was obtained in the OCC samples, namely a fine lamellar eutectic structure with unidirectional growth along the axial direction. The eutectic structure was formed by a primary a-Al phase and secondary CuAl2 phase. The hardness of CuAl2 is about 2.8 times higher than that of the a-Al phase. Due to the anisotropic microstructure, the mechanical properties of the OCC samples depended on the loading direction. The tensile and fatigue properties of the OCC samples in the longitudinal direction were more than 30% higher than those in the perpendicular direction. In addition, the mechanical properties were influenced directly by the fine eutectic structure in the longitudinal direction. The ultimate tensile strength of the OCC sample in the longitudinal direction could be estimated theoretically using three different parameters: solid-solution strengthening, interlamellar eutectic structure and work hardening strengthening. Ó 2011 Elsevier Ltd. All rights reserved.

1. Introduction The favorable material properties of Al–33%Cu alloy have been investigated by several researchers. Specifically the Al–33%Cu alloy can possess high ductility although this depends on the formation of a eutectic structure. In the study by Chokshi and Langdon, the tensile tests were conducted using the Al–33%Cu eutectic alloy, in which there is sigmoidal relationship between flow stress and strain rate with the maximum ductility occurring at intermediate strain rates in the super-plastic. One of their samples after annealing process exhibited the maximum recorded elongation of 1475% at an initial strain rate of 1.3  105 s1 [1]. It is considered that duplex structures formed from the eutectic and lamellae fine grains can promote high ductility. Several techniques have been proposed to create the fine grain microstructure for this Al alloy. Lamellar eutectic structures are significant for the development of more adequate mechanical properties in metallic alloys. One representative eutectic material is the Al–33%Cu alloy, which can be highly ductile. The material ductility of Al–33%Cu alloy is affected by the loading conditions and microstructural characteristics. A representative technique to control the microstructural characteristics is the Ohno continuous casting (OCC) process. The OCC process is found to be reliable for the production of cast products with a unidirectional structure [2]. Moreover, the cast defects in

⇑ Corresponding author. Tel./fax: +81 184 27 2211. E-mail address: [email protected] (M. Okayasu). 0142-1123/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.ijfatigue.2011.05.015

the OCC samples are at a very low level. It is considered there are several types of cast defects in cast components, such as blow hole, shrinkage porosity and inclusion, which make a reduction in their mechanical properties [3]. Motoyasu et al. have attempted to produce OCC samples of Al–33%Cu alloys. The microstructure of the OCC samples consisted of Al–CuAl2 eutectic growth with a fine lamellar structure, for instance a groove spacing of about 100 nm [4]. Those grooves are found to grow along the direction parallel to the casting direction. They also reported that such microstructural characteristics in the OCC Al–Cu alloys showed high elongation, comparable with those of the as-extruded and the extruded and well-annealed materials. The materials with the lamellar eutectic structure are important for the development of improvements of the mechanical properties of metallic alloys. In our previous work, the tensile and fatigue properties of a lamellar eutectic OCC Al–33%Cu were investigated [5]. The ultimate tensile strength and fatigue strength for the OCC samples are apparently high compared to those for the Al–33%Cu alloy produced by gravity casting. The high mechanical strength in the OCC sample is associated with the fine lamellar structure in the unidirectional growth along the axial direction. The mechanical properties of the OCC samples have been examined by several researchers. However, information on the tensile and fatigue properties of the OCC samples is insufficient [6]. Moreover, although unidirectional growth of microstructure is obtained in the OCC Al–33%Cu samples, the influence of anisotropic microstructure on the material properties has not been examined. The aim of this work is, therefore, to investigate the effects of the

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Nomenclature a b c d, r, w dmax f h k, m ky r t z A C G Nf Pmax

face angle of Martens hardness indenter Burgers vector amount of copper in a-Al phase specimen configuration (see Fig. 2a) maximum indentation depth of Martens hardness test loading criterion of fatigue test penetration depth of Martens hardness indenter material constants to determine tensile property Hall–Petch coefficient radius of curvature thickness of CuAl2 eutectic fatigue exponent contact area of Martens hardness indenter material constant for notch sensitive factor shear modulus number of loading cycle to failure maximum applied load for Martens hardness measurement

microstructural characteristics on the tensile and fatigue properties of OCC Al–33%Cu alloy. 2. Experimental procedures 2.1. OCC condition In the present work, a Al–33%Cu alloy with low iron impurity concentration was employed, less than 0.1%. The test samples of

S V

interlamellar spacing of eutectic structure volume fraction of CuAl2 eutectic material constant to determine tensile property fatigue strength reduction factor stress concentration factor of notch effect elongation to failure plastic strain notch sensitive factor slope of loading vs. depth for Martens hardness test dislocation density stress amplitude for fatigue test fatigue strength coefficient flow stress nominal stress yield stress ultimate tensile strength

a b d

ef e g h

q ra rf r0 rn ry rUTS

the Al–Cu alloy were made by the OCC process. Fig. 1 gives a schematic diagram of the Ohno continuous casting apparatus, consisting of a graphite crucible with runner, a graphite die, a cooling device and pinch rolls for withdrawal of the cast metal. The die was machined to make cast samples in the shape of a long round bar (Ü4 mm  1 m). It should be noted that in this case the thin round bar was employed in order to make high quality cast aluminum alloy due to high cooling rate, e.g., fine microstructure and low defect rate. The casting pressure was controlled by the level

(a) Displacer block

Molten Al alloy Heater Tundish

Runner Graphite die OCC sample

Cooling

Pinch rolls

Crucible

(b)

Metal flow OCC sample

Die

Unidirectional growth Molten Al-Cu alloy

Water

Fig. 1. Schematic illustration of the Ohno continuous casting system: (a) overall view and (b) cooling system.

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(d) 0.2 (w)

2.0

1.5

2.5

(a) 4.0(OT), 5.0(OL) φ 0.85

0.2 (2r)

R0.1

(b) (i) Axial direction (OL) Metal flow φ4.0

(ii) Transverse direction (OT) Metal flow φ4.0

Fig. 2. (a) Schematic illustration of the specimens used for tensile and fatigue tests and (b) the position of the specimen in the OCC sample.

of molten metal in the crucible, controlled by furnace displacer block. In this casting process, about 10 kg of Al–33%Cu ingot was melted, and the temperature of the molten metal was maintained at about 843 K, which is 20 K above the melting point of the alloy. The molten metal was cast through the runner and graphite die before the cooling process. To make high quality OCC samples, the oxide impurities in the molten metal were removed in the tundish during the casting process. The die was heated to approximately 853 K, which is just above the liquidus of the Al–33%Cu alloy. For the solidification process, the aluminum alloy was cooled directly by water flowing to the exit just out of the graphite die (see Fig. 1b). In this way a unidirectional growth microstructure was created in the OCC sample. The cooling water was applied at a rate of approximately 150 ml/min. The details of the OCC process can be found in Ref. [7].

Fig. 2a displays the rectangular test specimens formed with a notch of radius r = 0.1 mm and a width of specimen w = 0.2 mm. Note the tiny special specimen was designed originally in this experiment, which is much smaller than that for standardized specimen, e.g., ASTM. Note the specimen width is approximately 40 times greater than the grain size of the OCC Al–33%Cu alloy. With this specimen, an attempt was made examine the anisotropic microstructual effect on the mechanical properties. In order to investigate the effect of the anisotropic microstructure on the mechanical properties, the specimens were machined from the OCC samples in different directions using electro-discharge machining (EDM) with a Ü0.2 mm wire. The samples are denoted as (i) axial direction (OL) and (ii) transverse direction (OT), as shown in Fig. 2b. Because the OCC sample was in the form of a thin rod (Ü4.0 mm), specimens were necessarily tiny (Fig. 2a).

2.2. Specimen preparation

2.3. Microstructural observation

Two major approaches were conducted to study the effect of the microstructural characteristics on the mechanical properties of the OCC Al–33%Cu alloy, namely the tensile and fatigue properties.

The microstructural observation of the OCC samples was carried out using a scanning electron microscope (SEM, JSM-7001F). SEM observations were carried out at 15 kV. In addition, high resolution

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(a)

(b)

Fig. 3. (a) FEA model used to determine the stress distribution in the specimen and (b) stress distribution to loading direction.

electron microscopy observations were conducted using a transmission electron microscope (TEM, H-8100). TEM samples were ground to about 100 nm thickness using a focused ion beam before the observation at 200 kV. The sample preparation to make the thin sample was done using a focused ion beam (FIB, FB2000A). 2.4. Mechanical properties The tensile and fatigue tests were performed at room temperature using a 10 kN screw driven universal testing machine. The loading speed for the tensile test was 1 mm/min to the final fracture. The tensile properties were monitored by a data acquisition system in conjunction with a computer through a standard load cell. It should be mentioned that in our specimen there would have technical difficulty to measure directly the material strain due to the small size. In this approach, the following method was conducted to examine the strain value: during the loading, the gauge length marked by a needle was periodically measured using a traveling light microscope with a resolution of 0.01 mm. Low cycle fatigue tests at 0.05 Hz were carried out to obtain the S–N curve, the relationship between the amplitude of the applied stress and cycle number to failure. The maximum cyclic loads were determined based upon the ultimate tensile strength (rUTS) of the appropriate alloy, e.g. 40–90% of rUTS. The cyclic loading was performed with load ratio of 0.05 up to 50,000 cycles under load control. In this examination, the low measurement frequency was

selected in order to reduce mechanical damage in our tiny specimen. In fact, in the beginning of high cycle fatigue tests, the applied cyclic loading level is unstable; furthermore, surge pressure sometimes generates. Low cyclic frequency (or low strain rate) has an advantage for the examination with tiny specimen, as the material strength is influenced by the strain rate, e.g., the higher the strain rate, the lower the material strength [8,9]. Details of the related information can be found in the previous works conducted by Wanjara et al. [9]. The material hardness was measured using a dynamic ultra-micro-hardness tester (DUH-211 Shimadzu). The hardness test was carried out on the sample surface after it had been polished to a mirror finish. The indentation load of this hardness test was 3 mN. The Martens hardness (HM) was calculated from the hardness measurement, which is defined as the maximum applied load, Pmax, divided by contact area A:

HM ¼

Pmax P max  AðhÞ 26:43h2

ð1Þ

pffiffiffi 3 3 tanðaÞ 2 h cosðaÞ

ð1aÞ

with

AðhÞ ¼

where h is the penetration depth, and the parameter a refers to the face angle of the indenter, which is 115°.

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Metal flow direction

Metal flow direction

(a)

(b)

5 µm

5 µm

2 µm

2 µm

(c)

Fig. 4. SEM images of the OCC Al–33%Cu alloys, showing microstructure: (a) axial direction and (b) transverse direction. (c) Schematic illustration of 3D shape of microstructure.

2.5. Finite element analysis (FEA) A finite element analysis (FEA) was performed to analyze the stress–strain distribution in our special specimen when the applied load was carried out. In this analysis, three-dimensional finite element simulation with 8-noded elements was employed. Fig. 3a displays the FEA models designed based upon the geometry of the aforementioned specimen, where the mesh size of the specimen adjacent to the notch was 0.02 mm. In this calculation, the plain stress criterion was selected due the thin specimen; moreover, the following material properties of the related aluminum alloy were selected: Young’s modulus E = 70 GPa and Poisson’s ratio for the material m = 0.33. The result of the FEA stress distribution on the loading direction (x-axis) is shown in Fig. 3b. As can be seen, the high stress level is uniformly distributed around the notch, as

indicated by the dashed circle. It is considered from this analysis that the material strength in this specimen is directly attributed to the notch effect.

3. Results and discussion 3.1. Microstructural characteristics Fig. 4a and b presents SEM micrographs of the axial and transverse Al–33%Cu samples. The eutectic structure of the primary a-Al phase is visible as a dark region and the CuAl2 phase is related to the white area. A fine lamellar eutectic structure with unidirectional growth along its axial direction can be observed. In contrast, several grains (about 5 lm in diameter) with lamellar eutectic

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500 nm Fig. 5. TEM micrographs of the OCC Al–33%Cu alloys, showing microstructure.

structure are seen in the transverse direction (Fig. 4b), being similar to ferrite and cementite phases as seen in pearlitic steel. On the basis of the microstructure shown in Fig. 4a and b, three-dimensional microstructural formation can be illustrated (Fig. 4c). Since it is difficult to obtain the volume fraction of Al–CuAl2 eutectic (V) due to lack of information, the V value was estimated using the formula for a hypereutectoid steel as a first approximation [10]. In this case, the function of the interlamellar spacing, S, and copper content, wt.%, of the Al alloy were considered for the approximation:

V¼

S  33ðwt:%Þ t

ð2Þ

where t is the thickness of the CuAl2 eutectic. In our sample, the nominal interlamellar spacing of eutectic structure (S) and thickness of CuAl2 eutectic structure (t) measured were S = 63 nm and t = 100 nm, respectively. Fig. 5 presents the TEM micrographs of the OCC Al–33%Cu alloys (OT), showing the microstructure. Similar to the SEM observation, the eutectic structure of the primary a-Al phase and the CuAl2 phase is observed. It is particular interest to mention that the darkness on the eutectic structure is different. As seen relatively dark areas are detected in several regions, which may be related to dislocation, since in the areas eutectic structure of CuAl2 phase seems to be deformed, as indicated by the dashed circles.

(a)

Microhardness measurements in the a-Al and CuAl2 phases were carried out. In this case, the hardness measurement was executed in the OCC samples after they had been heated to 510 °C for 10 h before furnace cooling. The reason for carrying out the heating process is due to the technical difficulty presented by measurement of the hardness in the fine lamellar structure, i.e., 42 nm spacing, using the ultra-micro-hardness tester. Fig. 6a displays the microstructure of the related samples after the heating process, in which the grown a-Al and CuAl2 phases with approximately 5 lm in thick were observed. Fig. 6b also shows the hardness indentation in the microstructure of the OCC sample after the heating process, where a tiny indentation, about 1 lm in diameter, is obtained, which size is apparently smaller than the a-Al and CuAl2 phases. Thus, it is confirmed that the ultra-micro-hardness test machine can be used to measure the material hardness in each microstructural phase. Fig. 7ab represents the Martens hardness (HM) data and the indentation load vs. displacement curves, respectively. It is clear that the hardness in the CuAl2 phase is about HM3770, and this level is about 2.8 times higher than that in a-Al phase, which is HM1340. Such different mechanical properties were also clear from the load vs. displacement relationship (Fig. 7b). As can be seen, the slope of the loading portion (h) and the maximum indentation depth (dmax) is altered, with the higher the hardness, the higher the slope and the lower the depth (for the CuAl2 phase).

3.2. Tensile properties Fig. 8 shows the stress–strain curves for the OL and OT samples. It can be seen that the relatively linear stress vs. strain relations are obtained and the tensile strength and elongation to failure are obviously high in the OL sample compared to the other one. On the basis of the stress–strain curves obtained, the tensile properties are summarized in Fig. 9. The tensile properties of both OCC samples (OL and OT) were examined and the measured ultimate tensile strength (rUTS) and elongation to failure (ef) are shown in Fig. 9. The average tensile properties of the OL and OT samples are rUTS = 489.8 MPa and ef = 4.9% and rUTS = 384.1 MPa and ef = 4.2%, respectively. It should be noted that these tensile strengths were evaluated by considering the effect of the specimen notch. Fig. 10 displays the theoretical stress concentration factors for the related specimen geometry [11]. On the basis of the specimen configuration, the notch effect (stress concentration factor) can be determined from elastic stress analysis and the specimen configuration, e.g., r/w and d/r, i.e. d = 1.6. Note the data for the tensile properties shown in Fig. 9 is relatively scattered [12]. The reason behind this may

(b)

30 µm

1 µm

Fig. 6. SEM micrographs of the OCC Al–33%Cu alloys after heating to 510 °C for 10 h before furnace cooling: (a) microstructure and (b) the indentation obtained by ultra-micro-hardness test.

M. Okayasu et al. / International Journal of Fatigue 42 (2012) 45–56

51

(a)

(b)

Fig. 7. Martens hardness (HM) data of the a-Al and CuAl2 phases in OCC Al–33%Cu alloy: (a) hardness data and (b) indentation load–depth curves.

Applied tensile stress, MPa

600 500

OL sample OT sample

400 300 200 100 0

0

2

4

6

8

Strain, % Fig. 8. Stress–strain curves for the OCC Al–33%Cu samples, obtained in the axial direction (OL) and transverse direction (OT).

Fig. 9. Tensile properties of the OCC Al–33%Cu samples measured along the axial (OL) and transverse (OT) directions.

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Stress concentration factor, δ

4.2

d w

d

d/r = 4

3.4

d/r = 2

2.6

d/r = 0.5 1.8

2r 1.0

0

0.2

0.4

0.6

0.8

r/w Fig. 10. Theoretical stress-concentration factors for our specimen.

(a) Al-α phase

(b) Loading

CuAl2 phase

Void Shear slip

50

(c)

(d)

Void growth

Void coalescence

Fig. 11. Schematic illustration showing the fracture characteristic of the materials along with the eutectic structures.

be attributed to the tiny special specimen with the high stress concentration as shown in Fig. 2. The standard deviations of the rUTS and ef for OT samples calculated are about 58.8 and 0.81, respectively. Although the above tensile properties were relatively scattered, the average rUTS for the OL samples is close to that obtained by the same aluminum alloy of the bigger size specimens [5]. The different tensile properties, shown in Fig. 9 (OL vs. OT), are reflected in the anisotropic microstructure of the OCC samples. In particular, the fine eutectic structure in the longitudinal direction can enhance the material properties (OL). This microstructure may be related to reinforcement by the fibrous structure. Furthermore, the high material ductility obtained for the OL samples can

be explained using the failure mechanism. Fig. 11 illustrates the failure process of the OL sample under tensile loading. With the applied load, the material failure occurs in the brittle CuAl2, formed parallel to the applied tensile stress, in advance (Fig. 11a). A concentrated shear slip at about 50° to the loading direction caused cracking of adjacent CuAl2 (Fig. 11b). The voids grew (Fig. 11c) and coalesced to form the ductile fracture [11]. On the other hand, the low tensile properties of the OT samples are caused by strain localization and stress concentration between the grains (grain boundary) and the layer of eutectic structures (a-phase and CuAl2). Fig. 12 presents SEM images of fracture surfaces for the OL and OT samples after tensile tests where different fracture characteristics

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(a) OL sample

(b) OT sample Smooth fracture face

Rough fracture face 5 μm

5 μm

S=87nm

2 μm

2 μm

Fig. 12. SEM images of fracture surfaces for the OCC Al–33% Cu alloys after tensile tests: (a) axial direction (OL) and (b) transverse direction (OT).

Dependence of the yield stress on interlamellar spacing may be postulated as following a Hall–Petch type equation [15]:

ry ¼ r0 þ ky Sm

ð3Þ

where r0 is the flow stress, ky is the Hall–Petch coefficient and m are material constants at a given strain. In this case, m = 0.5 is taken into consideration. Furthermore, the ultimate tensile strength can be assessed on the basis of the Hall–Petch consideration, adding two additional factors for the estimation, namely the solid solution strengthening and work hardening. In our Al–Cu alloy, both factors are important, due to the rapid cooling rate for the OCC process [5]. Consequently, the following equation can be expressed to estimate the value of rUTS [16].

pffiffiffi

pffiffiffiffi

rUTS ¼ r0 þ ky Sm þ k c þ aGb q

ð4Þ

with

q ¼ 5:3  109 e1:1 Fig. 13. Stress amplitude vs. cycles to failure curves for the OCC Al–33%Cu samples, obtained in the axial direction (OL) and transverse direction (OT).

are observed. For the OL sample, a rough fracture surface is obtained (Fig. 12a-(i)), whereas strip shape with along its casting direction can be observed in the OT sample (Fig. 12b-(iii)). A relatively smooth fracture surface in the stripe pattern can also be seen in the OT sample, which may be attributed to the crack growth in the grain boundaries and/or in eutectic structure. With the observation at high magnification, the groove shape of a fine lamellar eutectic structure was observed clearly in the fracture surface of both samples, Fig. 12 a and b (ii) and (iv). The interlamellar spacing of eutectic structure measured directly is about 87 nm, which is corresponding to the related span examined previously (63 nm). It is considered that mechanical properties are severely affected by the microstructural characteristics [13,14]. Based upon these tensile properties, the yield stress (ry) and ultimate tensile strength can be estimated theoretically. The value of ry for the OL sample is associated with the interlamellar spacing of the eutectic structure.

ð4aÞ

where c is the amount of copper in the a-Al phase, k and a are material constants, G is the shear modulus, b is the Burgers vector, q is the dislocation density and e is the strain. The material constants k and a, used in our samples, are taken to be 377.6 and 0.3, respectively [17,18]. If ky is 125 MPa lm0.5 [19], the value of ky Sm can be determined as 423.8 MPa. The solid solution strength of 151.0 MPa can be calculated for the amount of Cu solution in the a-Al grain (16.4 at.%) examined by EPMA. From the value of strain to failure (4.9%), the dislocation density can be determined according to Eq. (4a), i.e., q ¼ 5:3  109  0:0491:1 =m2 . With the material properties 0.3, 25.4  103 MPa and 0.286 nm for a, G and b, respectively, a vapffiffiffiffi lue of 0.03 MPa was taken for aGb q. By combining the above factors and the flow stress of 44.4 MPa [17,19], a theoretical ultimate tensile strength can be assessed as 619.2 MPa, which is relatively close to the experimental result of 489.8 MPa (see Fig. 9). Note, in this approach, various samples having different interlamellar spacing should have been prepared to obtain the accurate rUTS value. However, we could not make them due to the technical difficulty.

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(i) OL sample

(ii) OT sample Crack

Crack (grain boundary)

1 µm

µm 11 µm

Fig. 14a. Crack paths in the mid-section of the OCC Al–33% Cu alloys obtained by cyclic loading: (i) axial direction (OL) and (ii) transverse direction (OT). (Black region: a-Al, White region: CuAl2).

(ii)

(i) Al-α phase CuAl2 phase Crack

Crack

Grain boundary

Fig. 14b. Schematic illustration showing the fracture characteristic of the OCC Al–33%Cu alloys: (i) axial direction (OL) and (ii) transverse direction (OT).

In this study, the above theoretical estimation was conducted as first approximation. 3.3. Fatigue properties Fig. 13 shows the relationship between the stress amplitude and fatigue life for both the OL and OT samples. It should be noted first that the arrows in the S–N diagrams are the specimens which did not fracture within 50,000 cycles. Moreover, the fatigue stress was estimated for the notch effect of the specimen under cyclic loading process as follows [11]:

ra ¼ b  rn

ð5Þ

where ra is the stress amplitude, b is the fatigue strength reduction factor, rn is the nominal stress. The notch sensitivity factor (g) is used to characterize the effect of notches on a fatigue strength, and the value of g varies from 0 to 1 [20].

g¼ g¼

ðb  1Þ ðd  1Þ 1 1 þ C fr

ð5aÞ

ð5bÞ

where d is the stress concentration factor, C is the material constant, f is the loading criterion and r is the radius of curvature. As there is

no effect of a notch on fatigue strength, b = 1 and g = 0. The maximum notch effect occurs when b = d and g = 1 [20]. In this case, the parameters C and f are 0.63 mm (for Al alloy) and 1.0 (for tensile loading) [21], respectively. If tensile loading condition and aluminum alloy are considered, C = 0.63 mm and f = 1, g becomes a constant, i.e., g = 1/(1 + 0.63  10) = 0.137 according to Eq. (5b). On the basis of the above parameters, the value of fatigue strength reduction factor (b) calculated in terms of Eq. (5a) is 1.08. Note, in this approach, the microstructural characteristics ahead of notch tip, such as the grain size or eutectic lath size [22,23], were not considered although those factors are significant to accurately obtain the fatigue stress concentration factor [24]. This will be further examined in the future work. It is clear from the S–N diagrams that the S–N relationship for the OL is located at higher values compared to the OT sample. The endurance limit for the OL samples is approximately 400 MPa, which is about twice as high as that of the OT samples. It should be pointed out that S–N relationship for the OL is formed more plateau compared to the other one. This may be affected by the material brittleness for the OL samples. In fact, such S–N relations is sometimes seen in relatively brittle materials, e.g., ceramics [25,26]. However, the reason behind this in details will be discussed in the future. The S–N relationships shown in Fig. 13 can be represented by a power law dependence of the applied cyclic stress and cycle number to final fracture [27]:

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(a) OL sample

(b) OT sample

(i)

Metal flow

Metal flow

⊗

500 µm μm

500 μm µm 500

50 μm

50 μm

(ii)

Speckle

(iii)

Smooth fracture face Rough fracture face 5 μm

5 μm

2 μm

2 μm

(iv)

Fig. 15. SEM images of fatigue fracture surfaces for the OCC Al–33% Cu alloys: (a) axial direction (OL) and (b) transverse direction (OT).

ra ¼ rf Nzf ; MPa

ð6Þ

where rf is the fatigue strength coefficient, Nf is the cycle number to the failure and z is the fatigue exponent. The values of rf and z for both samples, estimated by least squares analysis, are (i) rf = 452.3 MPa and z = 0.02 for the OL sample and (ii) rf = 429.3 MPa and z = 0.07 for the OT sample. In this case, the fatigue life rises with increasing fatigue strength coefficient rf. To understand clearly the difference of the fatigue properties between OL and OT, crack growth characteristics were also investigated. Fig. 14a shows SEM images of the fatigue crack for both samples. A zigzag crack growth characteristic was seen in the OL sample because of the broken lamellar structures (a-Al phase and CuAl2 phase). Such a zigzag crack growth path could result in closure of

a more severe crack, resulting in long fatigue life. This closure characteristic is illustrated in Fig. 14b(i) [28]. In contrast, the crack path occurs along the grain boundaries for OT samples. This fracture characteristic leads to low fatigue strength, where the severity of crack closure is low and the stress concentration level is high (Fig. 14b(ii)). However, in our specimen, the fracture characteristics under the cyclic loading are similar to that for the static loading; moreover, the fatigue strength could be governed by a part of crack initiation or before propagation due to tiny specimen. Hence, the crack closure may not be so important factor. In this case, the difference in the crack growth rate for OL and OT samples may arise from the mechanical properties of Al eutectic cell and the eutectic boundaries, where the mechanical strength for eutectic cell (OL sample) would be stronger than that for the boundaries (OT sample). It is

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believed from this observation that different fracture characteristics lead to the different fatigue properties described above. Fig. 15 depicts SEM images of fracture surfaces for the OL and OT samples after fatigue tests where different fracture characteristics are observed. For the OL sample, a rough fracture surface is obtained with many speckles distributed randomly, as seen in Fig. 15a (ii). The speckle on the fracture surface seems to be related to the eutectic grains shown in Fig. 4b. Similar speckle fracture surface reflected by eutectic grains (or pearlite colony) of carbon steel is reported in the previous study [29]. However, in the present work, the average grain size of the eutectic structure examined in Fig. 4b is approximately 5 lm, which is not equal to the mean speckle diameter (about 50.6 lm). The reason for the large speckle size may be caused by the combining of several grains during the failure process. On the other hand, striped-like pattern with smooth fracture surface along its casting direction can be observed in the OT sample (see Fig. 15b (ii)), which is similar to the fracture surface obtained after the tensile tests. Such smooth faces may be attributed to the crack growth in the grain boundaries as described above. Like the microstructural characteristics in Figs. 4 and 12 the groove shape of a fine lamellar eutectic structure was observed clearly in the fracture surface of both samples (Fig. 15a(iv) and Fig. 15b (iv)). 4. Conclusions The effects of microstructural characteristics of OCC Al–33%Cu alloys on their tensile and fatigue properties have been investigated. The results obtained are as follows: (1) A clear anisotropic microstructure was obtained, namely a fine lamellar eutectic structure with unidirectional growth along its axial direction. The eutectic structure was formed by the primary a-Al phase and CuAl2 phase. (2) The tensile and fatigue properties of the samples in the longitudinal direction of the loading are more than 30% higher than those for the cast samples perpendicular to the casting direction. The excellent mechanical properties are directly influenced by the fine eutectic structure in the longitudinal direction. On the other hand, the poorer mechanical properties of the sample perpendicular to the casting direction are influenced by high stress concentration between the grain boundaries and the eutectic structures. (3) The ultimate tensile strength of the OCC sample can be estimated theoretically using three different parameters, the solid-solution strengthening, interlamellar eutectic structure and work hardening strengthening.

Acknowledgments The authors would like to acknowledge the technical support of Mr. Shigeki Yoshie at Osaka Fiji Corporation and Dr. Noriko Mutoh at Akita Prefectural University. References [1] Chokshi AH, Langdon TG. Cavitation and fracture in the superplastic Al–33%Cu eutectic alloy. J Mater Sci 1989;24(1):143–53.

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