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Microstructures and Mechanical Properties ofHeat-treated Al-5.0Cu-0.5Fe Squeeze CastAlloys with Different Mn/Fe Ratio

Weiwen Zhang, Bo Lin, Jianlei Fan, DatongZhang, Yuanyuan Li

PII: S0921-5093(13)01019-8DOI: http://dx.doi.org/10.1016/j.msea.2013.09.043Reference: MSA30296

To appear in: Materials Science & Engineering A

Cite this article as: Weiwen Zhang, Bo Lin, Jianlei Fan, Datong Zhang,Yuanyuan Li, Microstructures and Mechanical Properties of Heat-treated Al-5.0Cu-0.5Fe Squeeze Cast Alloys with Different Mn/Fe Ratio,Materials Science &Engineering A, http://dx.doi.org/10.1016/j.msea.2013.09.043

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1  

Microstructures and Mechanical Properties of Heat-treated

Al-5.0Cu-0.5Fe Squeeze Cast Alloys with Different Mn/Fe Ratio Weiwen Zhang*, Bo Lin, Jianlei Fan, Datong Zhang, Yuanyuan Li

(School of Mechanical and Automotive Engineering, South China University of

Technology, Guangzhou, 510640, China)

*Corresponding author.

Tel:+86-20-87112272,Fax:+86-20-87112111,E-mail:mewzhang@scut.edu.cn

Abstract: The Al - 5.0 wt % Cu - 0.5 wt % Fe alloys with different Mn/Fe ratio

were prepared by squeeze casting. Various test techniques, including tensile test,

image analysis, scanning electron microscope (SEM), X-ray diffraction (XRD),

electron probe micro-analyzer (EPMA) and transmission electron microscopy

(TEM) was used to examine the microstructures and mechanical properties of the

alloys in T5 heat-treated condition. The results show that the β-Fe (Al7Cu2Fe) is

stable and its needle-like morphology is maintained after T5 heat treatment.

However, the Chinese script AlmFe, α-Fe or Al6(FeMn) partially transform to a new

Chinese script Cu-rich α(CuFe) (Al7Cu2Fe or Al7Cu2(FeMn)), which is harmful to

the mechanical properties of the alloys due to the decrease of the Cu content in α(Al)

matrix. The optimal Mn/Fe ratio is determined by the morphology of Fe-rich

intermetallics, volume fraction of θ’ and T (Al20Cu2Mn3), size of α(Al) dendrite and

porosity. Excessive Mn/Fe ratio will deteriorate the mechanical properties of the

alloys due to the increase of the total amount of porosity and the Fe-rich

intermetallics. When the Mn/Fe ratio is 1.6 and 1.2 for the applied pressure of 0 MPa

and 75 MPa, respectively, the needle-like β-Fe phase is completely converted to the

Chinese script Fe-rich intermetallics. The ultimate tensile strength, yield strength and

elongation of the T5 heat-treated alloy with the Mn/Fe ratio of 1.2 and applied

pressure of 75MPa reach 395MPa, 335MPa and 14%, respectively.

Keywords: Squeeze casting; Fe-rich intermetallics; Microstructures; Mechanical

properties; Mn/Fe ratio

2  

1. Introduction

The heat-treatable Al-Cu cast alloys have been widely used in the transportation,

aerospace and military industry owing to their excellent mechanical and physical

properties. However, the tolerance of Fe content in the high performance Al-Cu alloys

is very poor. For example, in the 206 cast alloy family, the maximum Fe content is

usually limited to 0.15% (206.0) or less than 0.10% for (206.2) for general purpose

use. In the aerospace applications, the Fe content is even required below 0.07 %

(A206.2) [1] (all compositions quoted in this work are in weight percent unless

indicated otherwise). However，with the increasing use of the recycled aluminum

alloys for the purpose of energy saving and environment protection, developing high

performance cast Al-Cu alloy with high tolerance of Fe content has become a great

challenge.

The solubility of Fe in pure aluminum is very low (0.048%) and decreases by about

5 times when 5% copper is added [2]. Therefore, almost all the Fe will precipitate

from liquid Al alloys in the form of Fe-rich intermetallic phases, mainly as the

needle-like β-Fe (Al7Cu2Fe) phase. These needle-like Fe-rich intermetallics have been

considered most detrimental to the mechanical properties due to the brittle features

and stress concentration caused by the needle-like morphology [3-4]. Tseng studied

A206 alloy with different Fe content in T7 condition [3]. They concluded that the

strength and ductility decreased linearly with increasing Fe content, since the

needle-like Al7Cu2Fe phase acting as crack initiation sites. To extend the tolerance of

Fe content in Al-Cu cast alloys and reduce the detrimental effects of needle-like β-Fe,

Mn and Si additions are used to modify the needle-like Fe-rich intermetallics by

replacing it with less detrimental Chinese script Fe-rich intermetallics. Kamga studied

the effect of Si addition on the mechanical properties of B206 alloy, and it was found

that the best properties were obtained with a Fe/Si ratio close to 1 [4]. Tseng et al also

reported that most of the needle-like β-Fe is completely converted to Chinese script

α-Fe when 0.66 % Mn is added into the A206 alloy with 0.30 % Fe [5]. It was found

that Mn addition can improve the ultimate tensile strength (UTS), especially the

3  

elongation. Recently, the formation of Fe-rich intermetallics in 206 Al-Cu cast alloys

at 0.15 % and 0.3% Fe has been systematically investigated [2, 6]. The Mn and/or Si

additions as well as high cooling rate favor the formation of Chinese script α-Fe and

prevent the formation of β-Fe. Therefore, acceptable mechanical properties can still

be achieved for the 206 cast alloys even at high Fe level of 0.3% [4]. However, with

the increasing use of recycled aluminum alloys, it is necessary to extend the tolerance

of Fe content in Al-Cu cast alloys as high as possible. Recently, Liu investigated

systematically the formation of Fe-rich intermetallics of A206 alloys at a higher level

Fe content of 0.5% [7-8]. A new Chinese script AlmFe presents as the dominate

Fe-rich intermetallics in Al-4.6Cu-0.5Fe-0.1Si alloy at a low level of Mn content

(0.03%), and the needle-like Al3(FeMn) becomes the major Fe-rich intermetallics in

Al-4.6Cu-0.5Mn-0.5Fe-0.1Si 206 cast alloy. According to the studies mentioned

above, it is expected that new Al-Cu cast alloys with high Fe content of 0.5% can be

developed through composition design and processing improvement.

Squeeze casting is a method of producing near-net-shape components with very

low defects level [9-11], its application on the aluminum alloys has drawn many

interests. Dong [12] and Maeng [13] studied the effect of Fe-rich intermetallics on the

mechanical properties of Al-7Si-0.3Mg and B390 alloys prepared by squeeze casting.

They found that the squeeze cast alloys had superior mechanical properties compared

to the gravity die cast alloy due to the formation of smaller Fe-rich phases caused by

applied pressure. Until now, studying the effect of Fe-rich intermetallics on the

microstructures and mechanical properties is mainly focused on gravity die cast Al-Cu

alloys with low Fe content. Consequently, it is necessary to systematically investigate

the squeeze cast Al-Cu alloys with high Fe content and the effects of the various

iron-rich intermetallics on the mechanical properties. Since the squeeze cast Al-Cu

alloys are usually used in heat treatment condition, it is important to investigate the

relationship between the Fe-rich intermetallics and mechanical properties in the

heat-treated Al-Cu alloys. In this paper, the effect of Mn/Fe ratio and applied pressure

on microstructures and mechanical properties of squeeze cast Al-5.0Cu-0.5Fe alloys

in T5 heat treatment condition were investigated.

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2. Experimental procedures

Commercially pure Al (99.5%), Al-50% Cu, Al-10% Mn, and Al-5% Fe master

alloys were used to prepare the experimental alloys and the chemical composition

analyzed by an optical emission spectrometer were shown in Table 1. The raw

materials were melted at 1053 K in a clay-graphite crucible using an electric

resistance furnace. The melts about 10 Kg were degassed by 0.5% C2Cl6 to minimize

hydrogen content. Then the melt was poured into a cylindrical die under different

applied pressure ranging from 0 MPa to 75 MPa. The die temperature was set at

approximately 250 ℃ and the pouring temperature was set at 710 ℃ before

squeeze casting. Finally, the samples were obtained with the size of 65 mm in height

and 68 mm in diameter. All samples for tensile test were cut into the dimension of

Φ10 mm × 65 mm by line-cutting machine from the same radius of the castings. Then

the samples were solution treated at 538 ℃ for 12 h, quenched into water (25 ℃)

and then aged to a T5 condition at 155 ℃ for 8 h. The tensile test was carried out on

a SANS CMT5105 standard testing machine and the reported values were tested at

least three samples. Samples for metallographic observation were cut in the gauge

length part from selected tensile specimens. Metallographic samples were etched with

1ml HF+16 ml HNO3+3g CrO3+83 ml H2O solution for 30 seconds. A Leica optical

microscopy equipped with the image analysis software Leica Materials workstation

V3.6.1 was used to quantitatively analyze the intermetallic compounds and α(Al)

dendrite size. In order to get statistically significant data, approximate 50 different

regions each at magnification of 500 times around the centre of the etched specimen

were measured. The average compositions of the phases and fracture surfaces of

tensile specimens were analyzed using Nova Nano SEM 430, equipped with an

energy-dispersive X-ray analyzer (EDX). X-ray diffraction (XRD) was performed

with Bruker D8 ADVANCE to analyze the phase transformation after T5 heat

treatment. The Fe-rich intermetallic was analysis by selected area diffraction pattern

(SDAP) in the JEOL JEM-3010 transmission electron microscopy (TEM) at 200 kV.

5  

In order to obtain more accurate solid solubility of Cu in the α(Al) matrix, the

minimum Cu content in the α(Al) matrix of the heat-treated alloys was taken from 5

data by EPMA-1600 electron probe micro-analyzer.

3. Results

3.1 Microstructures

A. Microstructures in as-cast condition

Fig. 1 shows the microstructures of the alloys in as-cast condition without applied

pressure. Microstructures of as-cast samples consist of α(Al), θ(Al2Cu) and Fe-rich

intermetallics. Four kinds of Fe-rich intermetallics are observed in the cast

Al-5.0Cu-0.5Fe alloys with different Mn/Fe ratio: Chinese script AlmFe,

α-Fe(Al15(FeMn)3(CuSi)2) and Al6(FeMn), needle-like β-Fe(Al7Cu2Fe). Both of the

Chinese script AlmFe and needle-like β-Fe are observed in Mn0 alloy (Mn/Fe=0).

With increasing the Mn/Fe ratio, the needle-like β-Fe decreases and Chinese script

α-Fe increases. A bit of Al6(FeMn) phase is observed when the Mn/Fe ratio increases

to 2.0.

B. Microstructures in T5 condition

Fig. 2 shows the microstructures of the T5 treated alloys with different Mn/Fe

ratio and applied pressures. The Cu-rich intermetallics θ (Al2Cu) are dissolved and the

needle-like or Chinese script Fe-rich intermetallics still maintain compared with the

as-cast alloys. However, the morphology and size of Fe-rich intermetallics change

obviously with the increase of Mn/Fe ratio and applied pressure. The volume percent

of Fe-rich intermetallics has been measured by image analysis, as shown in Fig.3. In

the gravity die cast alloys, it can be seen that the total volume percent of Fe-rich

intermetallics and the needle-like β-Fe decrease obviously with increasing Mn/Fe

ratio. When the Mn/Fe ratio is 0, the Chinese script Fe-rich intermetallics and

needle-like β-Fe coexist. When the Mn/Fe ratio is 0.8, the Chinese script Fe-rich

intermetallics begin to dominate, but a bit of needle-like Fe-rich intermetallics still

exist. The relative volume percent of needle-like β-Fe to the total Fe-rich

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intermetallics decreases from 38% to 17% when the Mn/Fe ratio increases from 0 to

0.8. Only Chinese script Fe-rich intermetallics are observed in Mn8 alloy

(Mn/Fe=1.6). However, excessive Mn/Fe ratio (Mn/Fe=2) beyond the need to

eliminate the needle-like β-Fe leads to an even greater volume fraction of the Fe-rich

intermetallics and porosity (Fig. 2g).

When the applied pressure is 75 MPa, the morphology of the Fe-rich

intermetallics change significantly compared to the alloys without applied pressure.

Both the amount and the size of the Fe-rich intermetallic phases in the α(Al) dendritic

boundary decrease and their distribution becomes discontinuously. Most of the

Fe-rich intermetallics become small Chinese script shape one, although a little

needle-like β-Fe are still observed in the Mn 0 alloy(Mn/Fe=0). When the Mn/Fe ratio

is 0.8, all Fe-rich intermetallics become Chinese script shape, at the same time the

needle-like one disappears (Fig. 2d).

The SEM images and EDS analysis of Fe-rich intermetallics are shown in Fig.4

and Table 2. The needle-like phase is β-Fe (Al7Cu2Fe) (Fig.4a). Furthermore, the

Chinese script Fe-rich intermetallic is partially converted into a new bright phase. The

SEM-EDS results show that the chemical composition of this new Chinese script

phase is close to that of β-Fe (Al7Cu2Fe). Since this phase with Chinese script

morphology is different from the needle-like β-Fe, this phase is labeled as α(CuFe) in

Fig. 4 and Table 2. Fig.5 shows the XRD pattern of Mn 10 alloy (Mn/Fe=2) without

applied pressure. The T5 treated Mn 10 alloy consists of α(Al) and Al7Cu2(FeMn)

phases, which proves the α(CuFe) phase is Al7Cu2(FeMn). In order to further confirm

the Chinese script Al7Cu2(FeMn), TEM was performed on the sample of T5 treated

Mn 10 alloy. Fig.6a presents the micro-image of Chinese script Al7Cu2(FeMn). It

seems that the Chinese script Al7Cu2(FeMn) nucleates and grows on the α-Fe. The

result of SADP confirm the crystal structure of Chinese script Al7Cu2(FeMn), which

has a tetragonal unit cell with lattice parameters a = b = 0.634 nm, c = 1.488 nm

[10].The EDS result further confirms this new Chinese script phase is Al7Cu2(FeMn)

(The SADP and EDS location is marked with A in Fig.6a).

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3.2 Mechanical properties

Fig. 7 shows the mechanical properties of the T5 heat-treated alloys with different

applied pressures and Mn/Fe ratios. Fig.7d also present the typical load curves of

some samples with different Mn/Fe ratios and applied pressures. As shown in Fig. 7,

the ultimate tensile strength (UTS), yield strength (YS) and elongation of the gravity

die cast alloy increase significantly when the Mn/Fe ratio raises to 1.6. When the

applied pressure is over 25 MPa, the great increase of the UTS and YS in the squeeze

cast alloys occurs in the range of Mn/Fe ratio from 0 to 1.2. As for the elongation of

the squeeze cast alloys, there is a slightly reduction with the increase of Mn/Fe ratio.

It is also found that all the UTS, YS and elongation of the alloys increase significantly

as the applied pressures increase from 0 MPa to 75 MPa. It should be noted that the

mechanical properties of both squeeze cast alloys and gravity die cast alloys are

worsen when the Mn/Fe ratio is excessive.  For the squeeze cast alloy, the ultimate

tensile strength of 395MPa, yield strength of 335MPa and elongation of 14% are

achieved when the alloy with the Mn/Fe ratio of 1.2 and applied pressure of 75MPa is

T5 heat-treated.

4 Discussions

4.1 Effect of T5 heat treatment on the Fe-rich intermetallics

The Chinese script Fe-rich intermetallics in T5 treated Al-5.0Cu-0.5Fe alloys

with different Mn/Fe ratio and applied pressures partially change to a new Chinese

script Fe-rich intermetallics. That means the Chinese script AlmFe, α-Fe or Al6(FeMn)

partially change to Chinese script Al7Cu2Fe or Al7Cu2(FeMn). Similar phenomenon is

also reported in Kamga’s study. They found that the Chinese script α-Fe becomes an

unknown Chinese script post-α phase, which has a composition close to β-Fe [4].

Pannaray also pointed out that Al12(FeMn)3Si2，Al3(FeMn)，Al6(FeMn) which

presented as the dominate Fe-rich intermetallics in 2024 aluminum alloy would

transform to β-Fe during heat treatment [14]. Furthermore, Mukhopadhyay found that

there is a darker Al-Fe-Cu-Mn phase surrounded by a bright Fe-rich intermetallic

(β-Fe) in the heat treated 7075 alloy [15].

8  

In Al-Cu cast alloys, the needle-like β-Fe is the frequently observed Fe-rich

intermetallics. It was even reported to be the only Fe-rich intermetallic presented in

the fully solidified microstructures of the most Al-Cu cast alloys [16-17]. As shown in

Fig.8a, the occurrence of Chinese script Al7Cu2(FeMn) in association with AlmFe,

α-Fe and Al6(FeMn) phases tends to suggest that the AlmFe, α-Fe or Al6(FeMn) phases

may act as the heterogeneous nucleation site for the former. This phenomenon has

been proved by Liu’s study. They found that β-Fe can potentially nucleate on the

early precipitated AlmFe and α-Fe in the solidification processing of A206 alloys

[6-10]. During further cooling, only partial transformation of AlmFe, α-Fe and

Al6(FeMn) to β-Fe takes place because of the limited diffusion time at low

temperature. However, this transformation may implement completely during solution

heat treatment due to the fully diffusion of the elements at high temperature for

enough time. Fig.8 presents the distribution of elements along Chinese script Fe-rich

intermetallics in Mn6 T5 treated alloy (Mn/Fe=1.2). As shown in Fig. 8c, the content

of Cu in the bright phase α(CuFe) is higher than that of the darker Fe-rich

intermetallics α-Fe. However, the content of Si in the α-Fe is higher than that of the

α(CuFe) (Fig. 8f). Table 3 shows the chemical compositions of α-Fe in as-cast and T5

condition. It can be found that the Si content in T5 heat treated α-Fe is higher than

that in the as-cast one. All these results prove that the transformation of the α-Fe into

α(CuFe) is due to the diffusion of the Cu and Si between α-Fe and the α(CuFe).

According to the EDS results shown in Table 2 and Table 3, the Cu content in Chinese

script Al7Cu2Fe or Al7Cu2(FeMn) is higher than that in α-Fe [6-10], which indicates

the transformation of the Fe-rich intermetallics during T5 heat treatment will consume

the Cu content in the α(Al) matrix. This means the solid-soluted copper in α(Al)

matrix decreases after T5 heat treatment.

4.2 Effect of Mn/Fe ratio on the mechanical properties in gravity die cast alloys

As mentioned above, most Chinese script Fe-rich intermetallic phases have

partially changed to a new Chinese script Al7Cu2Fe or Al7Cu2(FeMn) after T5 heat

treatment. However, the needle-like morphology of β-Fe is preserved, which is the

9  

most detrimental phase to the mechanical properties of the alloys [3-4]. Mn or Si is

usually added to reduce its negative effects by replacing the needle-like β-Fe with less

detrimental Chinese script α-Fe phase in Al-Cu cast alloys [4-5]. However, excessive

Mn addition increases the total amount of the Fe-rich intermetallics and porosity.

Therefore, the Mn/Fe ratio should be controlled seriously. Fig.9 shows the fracture

surfaces of the gravity die cast alloys with different Mn/Fe ratio. When the Mn/Fe

ratio is 0, a lot of plate β-Fe that generally act as crack initiation sites can be observed

clearly. The fracture surfaces possess cleavage fracture characteristics (Fig.9a). When

the Mn/Fe ratio is 0.8, some plate β-Fe can also be observed in the fracture surface

because of the insufficient Mn/Fe ratio to convert the β-Fe. It seems that the

quasi-cleavage and toughing fracture characteristics occurs simultaneously (Fig.9b).

The typical characteristics of ductile failure are observed when the Mn/Fe ratio is 1.6

because the more and deeper dimples are present on the fracture surface (Fig.9c). The

increase in UTS and elongation with Mn/Fe ratio is clearly related to the reduction of

the β-Fe. However, excessive amount of Mn addition (Mn/Fe=2) will lead to the

increase of the amount of Fe-rich intermetallics and porosity (Fig.9d). Therefore,

there is an optimal Mn/Fe ratio (Mn/Fe=1.6) for the best mechanical properties of the

alloy, as shown in Fig.7. The similar results are also reported in the Al-Si-Cu cast

alloys [18].

The yield strength of the Al-Cu alloys depends mainly on the plastic deformation

capacity of the α(Al) matrix. Therefore, the size of α(Al) dendrite, the dislocation and

the small size of dispersive precipitated second phases in α(Al) matrix dominate the

yield strength of T5 heat-treated Al-5.0Cu-0.5Fe alloys [19-20]. Fig.10 presents the

average size of α(Al) dendrite in T5 heat-treated alloys with different Mn/Fe ratios

and applied pressures. The decrease of the α(Al) dendrite size with increasing Mn/Fe

ratio is attributed to the precipitation of T(Al20Cu2Mn3) particles during the solid

solution, which could effectively inhibit the grain growth in high Mn content Al-Cu

system alloys [5, 19-20]. Furthermore, more Mn-containing dispersoids are found in

high Mn/Fe ratio alloys after solution treatment, such as Mn6 alloy (Mn/Fe=1.2)

shown in Fig.11a. Most of these dispersoids are 0.5 μm to1 μm in size. The EDS

result in Fig.11b shows that the precipitation particles are T (Al20Cu2Mn3). Moreover,

the T (Al20Cu2Mn3) particles increase with increasing the Mn/Fe ratio, which is

10  

beneficial to the improvement of yield strength [19]. Finally, the small particles of θ’

phase with a size of 50 nm after T5 heat treatment (in Fig.11c) will also contribute to

the yield strength. However, with the addition of more Mn, the increase of T

(Al20Cu2Mn3) and Fe-rich intermetallics will cause the decrease of copper content in

α(Al) matrix. The transformation of Chinese script Fe-rich intermetallics into Cu–rich

α(CuFe) will consume the copper in α(Al) matrix, too. The Cu content in α(Al) matrix

decrease in solution treatment with the increase of Mn/Fe ratio can also be proved by

EPMA, as shown in Fig.12. Similar result was reported that addition of Fe and Mn

decreased the amount and precipitation kinetics of θ’ phase, which caused a loss in

yield strength [5].Therefore, there is an optimum Mn/Fe ratio for the yield strength.

According to Fig.7b, 1.6 is considered to be the best Mn/Fe ratio for the gravity die

cast alloy. The yield strength decreases when the Mn/Fe ratio reaches to 2.0, which is

due to the porosity and the reduction of copper content in α(Al) matrix caused by

excessive amount of the Chinese script Fe-rich intermetallics.

4.3 Effect of applied pressure on the mechanical properties of the alloys

The fracture morphology of the alloys with different Mn/Fe ratio at the applied

pressure of 75 MPa is shown in Fig. 13. The squeeze cast alloys present the typical

feature of ductile failure. The fracture surfaces of squeeze cast alloys have more and

deeper dimples and presents more plastic deformation feature compared with the

fracture surfaces of gravity die cast alloys. This means the elongation of the squeeze

cast alloys can be significantly improved. Since most of the Fe-rich intermetallic

become small Chinese script shape one in the high applied pressure condition, there

are no much differences in the fracture morphology of squeeze cast alloys with

different Mn/Fe ratio. Fig. 14 shows the microstructures beneath the fracture surfaces

of Mn0 (Mn/Fe=0) alloy at different applied pressures. It can be found that the Fe-rich

intermetallics provide the crack initiation sites when the applied pressure is 0 MPa.

And the Fe-rich intermetallics may also lead to the formation of secondary cracks (Fig.

14a). Furthermore, some porosity can be frequently found in the 0 MPa sample

(Fig.14b). As to squeeze cast alloys, although Fe-rich intermetallics act as the crack

11  

initiation sites (Fig. 14c), it is difficult to initiate the secondary cracks along the

Fe-rich intermetallics because the Fe-rich intermetallics become smaller and more

dispersive and the porosity decreases significantly.

As shown in Fig.7, the Mn/Fe ratio is 1.2 for the maximum ultimate strength and

yield strength of the T5 heat-treat alloy when the applied pressure is over 25 MPa.

However, the Mn/Fe ratio is 1.6 when the T5 heat-treat alloy is prepared by gravity

die casting. This indicates that less Mn content is demanded for the alloy prepared by

squeeze casting because needle-like β-Fe is easier to convert to Chinese script α-Fe in

squeeze cast alloy.

5. Conclusion

The effects of Mn/Fe ratio and applied pressures on the microstructures and

mechanical properties of T5 heat-treated Al-5.0Cu-0.5Fe alloys have been examined.

The conclusions are as follows:

1) The chemical composition and the needle-like shape of β-Fe (Al7Cu2Fe) have no

change after T5 heat treatment. However, the Chinese script AlmFe, α-Fe or Al6(FeMn)

transform partially to a new Chinese script α(CuFe) (Al7Cu2Fe or Al7Cu2(FeMn)).

2) The optimal Mn/Fe ratio for the best mechanical properties of heat-treated alloy

is determined by the morphology of Fe-rich intermetallics, volume fraction of θ’ and

T (Al20Cu2Mn3), size of α(Al) dendrite and porosity. Excessive Mn/Fe ratio will

deteriorate the mechanical properties of the alloys due to the increase of the total

amount of porosity and the Fe-rich intermetallics.

3) When the Mn/Fe ratio is 1.6 and 1.2 for the applied pressures of 0 MPa and 75

MPa, respectively, the needle-like β-Fe phase is completely converted to the Chinese

script Fe-rich intermetallics. The ultimate tensile strength, yield strength and

elongation of the T5 heat-treated alloy with the Mn/Fe ratio of 1.2 and applied

pressure of 75MPa reach 395MPa, 335MPa and 14%, respectively.

Acknowledgement

The financial support from the Guangdong – Natural Science Foundation of China

12  

(GD-NSFC) Foundation (U1034001), Key Projects in the National Science and

Technology Support Program of China (2011BAE21B00) and Specialized Research

Fund for Doctoral Program of Higher Education (20120172110045) is acknowledged.

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Table 1 Chemical composition of the experimental alloys, wt % 

Alloys  Cu  Mn  Fe  Si  Mg  Al  Mn/Fe ratio 

Mn0  5.03  0.01  0.51 0.12 0.008 Balance 0 

Mn2  5.02  0.22  0.50 0.08 0.008 Balance 0.4 

Mn4  4.89  0.39  0.46 0.07 0.008 Balance 0.8 

Mn6  4.92  0.59  0.46 0.08 0.008 Balance 1.2 

Mn8  5.07  0.78  0.47 0.07 0.008 Balance 1.6 

Mn10  5.00  0.93  0.45 0.07 0.008 Balance 2.0 

   

14  

 

Table 2 Average chemical composition of Chinese script α(CuFe) phase, at%   

Condition  Alloys  Al  Cu  Mn  Fe  Si  Phases 

T5 Mn0  73.04 18.02 ‐  8.94  ‐  α(CuFe) 

Mn8  72.13 18.46 1.99  7.37  0.05  α(CuFe) 

 

15  

 

Table 3 Chemical composition of α‐Fe in as‐cast and T5 heat‐treated alloys, at%   

Condition  Alloys  Al  Cu  Mn  Fe  Si  Phases 

As‐cast 

 

 

T5 

M4  72.41 9.71 3.74 13.31 1.39

α‐Fe   

M6  75.28 9.62 3.98 10.13 1.41

M8  77.44 8.16 5.19 8.65 1.15

M10  74.84 11.34 5.15 7.47 1.20

M4  80.72 2.07 4.91 7.09 5.21

M6  81.75 2.10 5.05 6.40 4.70

M8  81.65 3.92 5.86 4.02 4.56

M10  78.95 2.64 6.00 7.25 5.17

 

1

Fig.1 Fe-rich phases in as-cast condition without applied pressure：

(a) Mn/Fe=0; (b) Mn/Fe=1.2; (c) Mn/Fe=2;

AlmFe

(a)

Al6(FeMn)

α-Fe (c)

-Fe

α-Fe

-Fe

(b)

θ

θ

θ

Figure1

1

Fig. 2 Optical microstructures of the T5 heat-treat alloys:

(a) Mn/Fe=0, 0MPa; (b) Mn/Fe=0, 75MPa; (c) Mn/Fe=0.8, 0MPa; (d) Mn/Fe=0.8, 75MPa;

(e) Mn/Fe=1.6, 0MPa; (f) Mn/Fe=1.6, 75MPa; (g) Mn/Fe=2, 0MPa; (h) Mn/Fe=2, 75MPa;

porosity

(a) (b)

(c) (d)

(e) (f)

(g) (h)

Figure2

1

0

1

2

3

4

0, 750, 750, 750, 750, 750, 75

2.01.61.20.80.40

Applied pressure:

vo

lum

e p

ercen

t (%

)

Mn/Fe ratio:

Needle-like

Chinese script

Fig. 3 Volume percent of the Fe-rich phases after T5 heat treatment

Figure3

1

Fig. 4 Chinese script phases in T5 heat-treat alloys without applied pressure:

(a) Mn/Fe=0; (b) Mn/Fe=1.6;

α(CuFe)

(a) (b)

α(CuFe)

Figure4

1

20 40 60 80

0

2000

4000

6000

8000

10000

Al7Cu2(FeMn)

Inte

nsi

ty

2, CuK

(Al)

Fig.5 XRD pattern of the Mn10 alloy (Mn/Fe=2) after T5 heat treatment at 0 MPa

Figure5

1

0 2000 4000 6000 8000 100000

400

800

1200

1600

Cu

Al

Fe

Mn

Mn Fe Cu

Co

un

ts

Energy (ev)

Cu

Fig.6 TEM image of Chinese script α(CuFe) phase in the Mn10 alloy (Mn/Fe=2) after

T5 heat treatment at 0 MPa

Elements at%

Al: 72.78

Cu:18.33

Mn:1.64

Fe: 7.23

Si: 0.09

(a) (b)

(c)

α(CuFe)

α-Fe

A

Figure6

1

-0.4 0.0 0.4 0.8 1.2 1.6 2.0 2.4150

200

250

300

350

400

450

Ult

ima

te t

en

sile

str

en

gth

(M

Pa

)

Mn/Fe ratio

0 MPa

25 MPa

75 MPa

-0.4 0.0 0.4 0.8 1.2 1.6 2.0 2.4150

200

250

300

350

Yie

ld s

tren

gth

(M

Pa

)

Mn/Fe ratio

0 MPa

25 MPa

75 MPa

(a)

(b)

Figure(s)

2

-0.4 0.0 0.4 0.8 1.2 1.6 2.0 2.40

4

8

12

16

20

0MPa

25MPa

75MPa

Elo

ng

ati

on

(%

)

Mn/Fe ratio

0 5 10 15 200

50

100

150

200

250

300

350

Mn0 75MPa

Mn0 25MPaMn06 0MPa

Str

ees

(MP

a)

Strain (%)

Mn0 0MPa

Mn10 0MPa

Fig.7 Mechanical properties of the alloys with different Mn/Fe ratios

(c)

(d)

1

Distance (μm)

Distance (μm) Distance (μm)

Distance (μm) Distance (μm)

Fig.8 Distribution of elements along the red line of Fe-rich phases for the Mn6 alloy

at 0 MPa

(b)

(c) (d)

(e) (f)

Al

Mn Cu

Fe Si

(a)

Cou

nt

Cou

nt

Co

un

t

Cou

nt

Cou

nt

α(CuFe)

α-Fe

Figure8

1

Fig.9 SEM of the fracture surface of the alloys with different Mn/Fe ratio at the

applied pressure of 0 MPa:

(a) Mn/Fe=0; (b) Mn/Fe=0.8; (c) Mn/Fe=1.6; (d) Mn/Fe=2.0;

β-Fe

β-Fe

(a) (b)

(c) (d)

Figure9

1

0.0 0.4 0.8 1.2 1.6 2.00

50

100

150

200

250

Siz

e o

f

(Al)

den

drit

e (

m)

Mn/Fe ratio

0 MPa

75 MPa

Fig.10. Size of α(Al) dendrite in T5 heat-treated alloys at different applied pressures

Figure10

1

0 2000 4000 6000 8000 100000

100

200

300

400

500

Fe

Fe

Cu

Si Mn

Al

Mn

Cu

Co

un

ts

Energy (ev)

Cu

Fig.11 TEM images of Al20Cu2Mn3 (a) and θ’ phase (c) in Mn6 alloy

Elements at% Al 78.41 Cu 7.46 Mn 12.54 Fe 1.26 Si 0.30

(a)

(b)

(c)

500nm

Figure11

1

0

1

2

3

4

5

6

Mn10Mn6Mn2

Cu

co

nte

nt

in

Al)

m

atr

ix (

wt%

)

Mn0

Fig.12 Cu content in the α(Al) matrix in T5 heat-treated alloys at 0MPa

Figure12

1

Fig.13 Fracture surface of the alloys with the applied pressure of 75 MPa:

(a) Mn/Fe=0; (b) Mn/Fe=2

(a) (b)

Figure13

1

Fig.14 Microstructures beneath the fracture surfaces of the Mn0 (Mn/Fe=0) alloy

at the applied pressures of 0 MPa (a, b) and 75 MPa (c)

Secondary crack

porosities

microcracks

(a) (b)

(c)

Figure14