Materials Transactions, Vol. 44, No. 4 (2003) pp.

562 to 570 Special Issue on Platform Science and Technology for Advanced Magnesium Alloys, II #2003 The Japan Institute of Metals

Development of New Die-castable MgZnAlCaRE Alloys for High Temperature Applications

Department of Mechanical Engineering, Nagaoka University of Technology, Nagaoka 940-2188, Japan Ahresty Corporation, Magnesium Products Manufacturing Division, Shimotsuga-gun, Tochigi 321-0215, Japan

New die-castable MgZnAlCaRE alloys are developed and evaluated in order to determine their suitability for high temperature applications. The crystallization of an AlCa compound along the grain boundaries and AlRE compounds both within the grains and along the grain boundaries helps to improve creep properties of the alloys. The creep resistance of diecast specimens of some of the investigated alloys is comparable to that of ADC12 aluminum alloy that is currently used for diecasting of automobile powertrain parts. (Received October 23, 2002; Accepted January 17, 2003) Keywords: magnesium alloys, heat resistance, diecasting, hot tearing

1.

Introduction

In recent times, energy conservation and environmental degradation problems have propelled the automobile industry to intensify the use of lightweight metallic alloys instead of the traditional automobile materials, such as iron and steel. This has led to a tremendous increase in the research and development of magnesium alloys for these applications. Magnesium, which has a density of 1.74 g/cm3 , is the lightest of all the metallic elements that are used for structural purposes and magnesium alloys have high specic strength and stiness. In addition to their intrinsic characteristics of lightness, magnesium alloys are also highly recyclable and this makes them favored to signicantly reduce social environmental burden.1) Consequently, lightweight magnesium alloys are increasingly being used as structural materials for automobile applications in order to lighten the weight of automobiles and improve fuel eciency.24) Reduction of the weight of powertrain components is particularly expected to be one of the most eective ways of improving fuel eciency. The need to build highly fuel-ecient cars with low emission of environmentally polluting gases has made the automobile industry to remain the highest provider of incentives for the development of lightweight magnesium alloys. Volkswagen of Germany was the foremost carmaker to practically utilize magnesium alloy components in the Beetle during a period that has been described as the rst magnesium age. That started after the Second World War and reached a peak in 1971 with an annual production volume of 42000 tonnes. During that period, air-cooled engines and gearboxes were the main components that were produced using the Mg alloys, AS41 and AZ81 and they made up roughly 20 kg of the vehicle weight.2) However, in the years that followed, there was a reversed trend in the use of magnesium alloys as inexpensive aluminum alloys that
*1Graduate *2Graduate

Student, Nagaoka University of Technology. Student, Nagaoka University of Technology. Present address: Internet Ware Corporation.

possess superior mechanical properties were preferred. Poor heat resistance of magnesium alloys is one of the major factors limiting the full utilization of the lightweight benets of magnesium applications in the automobile industries. Most of the current magnesium automobile components are made of AZ91D and AM60/50/20 alloy diecastings, which are only suitable for applications that do not require high heat resistance, like seat frames, steering wheels, instrument panels, valve covers, etc. Current magnesium alloys cannot be applied to automobile powertrain components, particularly automatic transmission cases that can operate at temperatures up to 175 C because they have poor creep resistance at temperatures above 130 C. The poor creep strength of such components can result in clamping load reduction in bolted joints and poor bearing/ housing contact leading to oil leakage and/or increased NVH (noise, vibration and harshness) in powertrains.4) Therefore, new alloys that are capable of meeting such high performance requirements at elevated temperatures must be developed. Mg alloys containing yttrium and heavy rare earth elements have been found to possess high heat and corrosion resistances.511) However, high contents of heavy rare earth elements (e.g. more than 10 mass% Gd or Dy) are required to achieve extraordinary mechanical properties even at temperatures above 200 C.12) Heat treatment is also required to optimize the strength of the alloys. As a result, the alloys are extremely expensive, and that poses a problem for mass production. This implies that low-cost alternatives, particularly diecasting alloys that could be used in the as-cast condition without further heat treatment are desired, since diecasting is the preferred manufacturing process for most automobile components due to its simplicity and high productivity. The basic task is to nd a suitable combination of alloying elements that have high solid solution strengthening eect and the elements should be capable of forming thermally stable compounds with magnesium or with other alloying additions. Thus the MgAlZn alloy system, which is one of the oldest and low-cost magnesium alloy systems that combine excellent casting characteristics with high

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strength at room temperature could be explored to raise the heat resistance of the alloys to the level required for high temperature applications. In addition to its benecial eect of suppressing the ignition of magnesium melt, calcium has been found to improve the heat resistance of MgAl based alloys.1316) However, such alloys are known to be prone to hot tearing,17) and their die-castability is poor. Although a certain level of calcium content could be tolerable in diecasting of thin walled components, calcium addition generally deteriorates the die-castability of MgAl alloys.18) It has been reported that high Zn content restore the die-castability of MgAlCa alloys and the resulting MgZnAlCa alloys oer appreciable heat resistance.19) Consequently, new alloy development programs based on MgZnAlCa alloy system are being pursued vigorously in search of a low-cost and die-castable heat resistant magnesium alloy. In a previous study,20) MgZnAlCa alloys having a wide range of zinc and aluminum contents were evaluated for possible application at relatively high temperatures of 150 to 200 C. It was found that while high temperature tensile strength increases with increasing Zn and Al content, creep resistance deteriorates in alloys containing large quantities of zinc and aluminum. Furthermore, the coarse eutectic compounds that form a network along the grain boundaries deteriorate the ductility of the alloys. Consequently, in order to improve the creep resistance and other mechanical properties of the MgZnAlCa alloys, 1 to 3 mass% of inexpensive Ce-rich mischmetal (RE) was added to Mg 8%Zn4%Al0.6%Ca and Mg6%Zn3%Al0.6%Ca alloys, which possess high strength and good creep resistance among the MgZnAlCa alloys. As a result, improved high temperature tensile properties and good creep resistance were obtained. However, although the MgZnAlCaRE alloys exhibit high uidity, diecasting of actual transmission cases was dicult due to the occurrence of hot tearing. Low melting point eutectics that form due to the high zinc content were found to be responsible for the occurrence of hot tearing in the alloys during diecasting.21) Therefore, in the present study, the solidication characteristics of new MgZnAlCaRE alloys with low zinc content were evaluated by carrying out DSC analysis in order to determine die-castable alloys. Subsequently, creep test results were used to evaluate the suitability of the alloys for high temperature applications. Furthermore, considering the preliminary results, Mg0.5%Zn46%Al1%Ca13%RE alloys were chosen for diecasting. Then the heat resistance of diecast samples of the new alloys was evaluated by carrying out tensile tests at temperatures varying from room temperature to 250 C, as well as creep tests at 175 to 200 C. Furthermore, diecasting of Mg0.7%Zn5%Al12%Ca 1%RE was carried out to investigate the eect of high calcium addition on diecasting characteristics in the alloys. 2. Experimental Procedure

Table 1

Chemical compositions of investigated alloys (mass%). Zn 0.47 0.91 0.53 0.94 0.55 1.0 0.58 1.06 0.50 0.49 0.53 Al 5.02 5.14 5.10 6.03 6.10 6.21 6.78 7.02 7.09 9.01 9.05 9.06 4.09 6.06 5.97 Ca 1.00 0.97 0.93 1.00 0.94 0.90 0.98 0.94 0.97 0.87 0.85 0.99 0.98 0.95 0.99 RE 2.64 2.56 2.56 2.61 2.59 2.57 2.68 2.49 2.43 2.38 2.66 2.24 0.98 1.10 2.94 Mn 0.19 0.17 0.18 0.18 0.18 0.18 0.15 0.15 0.16 0.15 0.16 0.14 0.1 0.1 0.24 Mg bal. bal. bal. bal. bal. bal. bal. bal. bal. bal. bal. bal. bal. bal. bal.

Alloy designation ZACE00513 ZACE05513 ZACE10513 ZACE00613 ZACE05613 ZACE10613 ZACE00713 ZACE05713 ZACE10713 ZACE00913 ZACE05913 ZACE10913 ZACE05411

ZACE05611 ZACE05613

Diecast

The investigated alloys were cast under atmospheric conditions using a mixed gas of CO2 and SF6 as the protection gas and the melt was poured into JIS standard (H5203) boat-type permanent mold. The diecast samples of

190 mm length, 115 mm width and 5 mm thickness were produced using cold chamber diecasting machine with die clamping force of 850 ton. The chemical compositions of the examined alloys are shown in Table 1. Rare earth elements are added in the form of mishmetal, which mainly consists of 50%Ce, 25%La, 20%Nd, 5%Pr. Thus, RE content indicates the sum of La, Ce, Nd and Pr contents. The marks given in Table 1, which indicate alloy compositions, are subsequently used as alloy names. DSC analysis was used to evaluate the solidication characteristics of the alloys in order to determine alloys that are not prone to hot tearing during diecasting. Cylindrical specimens weighing about 40 mg were heated in argon gas atmosphere from 30 to 650 C at a constant heating rate of 10 C/min, held for 5 min and then cooled at a constant cooling rate of 10 C/min. Alumina powder was used as the reference material. Microstructures of the alloys were observed using optical microscope after polishing and etching in 0.5%HF solution. The microstructures were also observed using Scanning Electron Microscope equipped with Wavelength Dispersive X-ray Spectrometer. X-ray diraction patterns of the alloys were obtained and analyzed using X-ray diractometer operating at 40 kV and 30 mA in order to characterize the eutectic compounds that crystallize in the alloys. Then Electron Probe Micro-Analysis (EPMA) was carried out to conrm the presence of the compounds. Creep tests of both as-cast and diecast specimens were carried out at 175 C and 200 C with a small uctuation of 2 C under a load of 50 MPa for 100 h. Flat specimens having reduced cross-sectional dimension of 10 mm 5 mm were tested using creep testing machine. High temperature tensile test of diecast samples was carried from room temperature to 250 C using specimens having a reduced cross-sectional dimension of 10 mm 5 mm and a gauge length of 30 mm. After tensile test, the fracture surfaces were observed with Scanning Electron Microscope (SEM), while the microstructures of the regions adjacent to the fracture surfaces were observed using optical microscope.

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3.

Results and Discussion

3.1 Microstructures of as-cast specimens During diecasting of MgZnAlCaRE alloys, hot tearing occurs even at a low zinc content of 2 mass%.21) Therefore, a new set of alloys with low zinc content, Mg 01%Zn59%Al1%Ca3%RE were prepared and evaluated. The aluminum content of the alloys was varied in the range which retains good casting characteristics even in low zinc content, while calcium and rare earth contents were maintained at levels that will oer good heat resistance. Figure 1 shows typical microstructures of as-cast specimens of the obtained alloys. In all of the examined alloys, ne eutectic compounds crystallize both within the grains and along the grain boundaries. However, as aluminum content increases, the size of the eutectic compounds tends to

increase and a new type of grain boundary phase is also observed. On the other hand, no signicant change is observed in the microstructures as zinc content increases. In order to properly classify the compounds that crystallize in the alloys, X-ray diraction was carried out. The diraction patterns obtained for Mg0.5%Zn59%Al 1%Ca3%RE alloys are shown in Fig. 2. Apart from magnesium solid solution, diraction peaks from Al2 Ca and Al11 RE3 compounds are also observed in all of the examined alloys. However, additional diraction peaks from Mg17 Al12 compound are also observed in alloys containing 9%Al. Figure 3 shows the results of EPMA analyses of ZACE05613 and ZACE05913 alloys. As shown in the gure, from the mol% ratio of Al content to Ca or RE, the quantitative analysis of the observed compounds conrms

0.5% Zn containing alloy 1.0% Zn containing alloy ZACE05513 ZACE10513 5%Al

ZACE05613 6%Al

ZACE10613

ZACE05713 7%Al

ZACE10713

ZACE05913 9%Al

ZACE10913

50m
Fig. 1 Microstructures of as-cast specimens of Mg0.51.0%Zn59%Al1%Ca3%RE alloys.

Development of New Die-castable MgZnAlCaRE Alloys for High Temperature Applications

565

ZACE05513

(Mg) Al2Ca Al11RE3 Mg17Al12

ZACE05613
Intensity (arbitrary unit)

ZACE05713

ZACE05913

20

30

50 60 40 2 ( /180rad)

70

80

Fig. 2 X-ray diraction patterns of Mg0.5%Zn59%Al1%Ca3%RE alloys.

Fig. 3 EPMA results of as-cast specimens of (a) ZACE05613 and (b) ZACE05913 alloys.

that the eutectic compounds A and B, which crystallize in all of the examined alloys, are most likely Al2 Ca and Al11 RE3 as obtained by X-ray diraction analysis. Furthermore, an additional compound, C is observed in ZACE05913 alloy (Fig. 3(b)). The quantitative analysis of compound C shows that it is most likely Mg17 Al12 phase. The high magnesium content obtained from the quantitative analysis of the compounds is thought to be from the surrounding magnesium matrix. Although it is possible for ternary MgAlCa or Mg AlRE compounds to form in the alloys, no such compounds are observed by X-ray diraction analysis. The SEM images of Fig. 3 show that the Ca-containing compound is mainly located along the grain boundaries, while the RE-containing compound is mainly distributed within the grains. 3.2 Solidication characteristics The results of DSC analysis of Mg0.5%Zn59%Al 1%Ca3%RE alloys are shown in Fig. 4. In ZACE05513 alloy, there are no crystallization peaks below 450 C. The crystallization reaction occurs at a temperature above 500 C, and this makes the alloy a good candidate for diecasting. The same results are obtained for the other Mg01%Zn5%Al 1%Ca3%RE alloys that contain 5 mass%Al. Similarly, no low temperature crystallization peaks are observed in ZACE05613 alloy. Also, no low temperature crystallization peaks are observed in ZACE00613 alloy that does not contain zinc. However, a small crystallization reaction occurs at about 400 C in ZACE10613 alloy that contains 1 mass%Zn. Therefore, only ZACE00613 and ZACE05613 alloys are

Fig. 4

DSC cooling curves of Mg0.5%Zn59%Al1%Ca3%RE alloys.

considered as good candidates for diecasting among Mg0 1%Zn6%Al1%Ca3%RE alloys. In ZACE05713 alloy shown in Fig. 4, and other Mg01%Zn7%Al1%Ca 3%RE alloys that contain 7 mass%Al, crystallization peaks

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that occur at about 400 C are observed. Therefore the alloys are not considered for diecasting. Figure 4 shows that the last crystallization reaction in ZACE05913 alloy occurs at about 450 C and similar results are obtained for other Mg0 1%Zn9%Al1%Ca3%RE alloys that contain 9 mass%Al. The crystallization reactions that occur at about 400 to 450 C may correspond to the crystallization of Mg17 Al12 compound. This implies that even if Mg01%Zn9%Al1%Ca 3%RE alloys have good diecasting characteristics, they are completely ruled out because of the presence of Mg17 Al12 compound, which could damage their high temperature strength. 3.3 Evaluation of high temperature applicability Having determined that some of the alloys exhibit solidication characteristics that could be suitable for diecasting, their high temperature applicability was assessed by carrying out creep test at 175 C for 100 h under a load of 50 MPa. Figure 5 shows the creep curves obtained for as-cast specimens of Mg0.5%Zn59%Al1%Ca3%RE alloys. As shown in Fig. 5, compared to conventional magnesium alloys of AZ91 and AS41, all the investigated alloys exhibit a good creep property. The total creep strain of as-cast specimen of ZACE05613 alloy is the lowest among the examined alloys and the creep strain increases as aluminum content increases. However, the creep strain of ZACE05513 alloy is higher than that of ZACE05613 alloy. The higher creep resistance of ZACE05613 alloy is probably due to the presence of larger amount of eutectic compounds. On the other hand, as aluminum content is increased in excess of 7 mass%, the presence of weak Mg17 Al12 compound deteriorates creep resistance. The good creep performance of the new diecast ZACE05613 alloy is due to the presence of Al2 Ca compound at grain boundaries, which controls grain boundary sliding and Al11 RE3 compounds that control dislocation slip within the grains. The new alloy has a better uidity than conventional diecasting heat resistant magnesium alloys, AS21 and AE42 alloys. Actual diecasting of transmission cases using the new alloy has been successfully carried out without encountering hot tearing problems.22)

Fig. 6 DSC cooling curves of diecast specimens of ZACE05411, ZACE05611 and ZACE05613 alloys.

Fig. 5 Creep curves obtained for as-cast specimens of Mg0.5%Zn5 9%Al1%Ca3%RE alloys tested under an applied stress of 50 MPa at 175 C.

3.4 Microstructures of diecast samples The above results show that ZACE05613 alloy exhibits a good combination of suitable solidication characteristics and high creep resistance. Therefore, using ZACE05613 alloy as the base alloy, Mg0.5%Zn46%Al1%Ca1 3%RE alloys were diecast and evaluated in order to determine the inuence of the major alloying elements on diecasting characteristics and heat resistance of the alloys. As expected, diecasting of the alloys was successfully carried out without the occurrence of hot tearing. The DSC cooling curves of the diecast specimens of the alloys, shown in Fig. 6, indicate that the last crystallization reaction occurs at temperatures higher than 500 C. Thus, there are no low temperature crystallization peaks that could cause hot tearing. Figure 7 shows the microstructures of diecast specimens of the alloys. In ZACE05411 and ZACE05611 alloys containing 1%RE, a network of eutectic compounds form a lamella structure along the grain boundaries. However, in ZACE05613 alloy that contains 3%RE, the network of eutectic compounds that form along the grain boundaries breaks up and acicular as well as massive compounds are nely distributed both within the grains and along the grain boundaries. Generally, the amount of eutectic compounds increases as aluminum content increases and it appears that dierent types of eutectic compounds crystallize in the alloys. X-ray diraction patterns of the diecast samples are shown in Fig. 8. The diraction peaks show that Al11 RE3 and Al2 Ca are the main compounds that crystallize in the investigated alloys. However, EPMA was also carried out using the DSC specimens. The EPMA result obtained for ZACE05613 alloy is shown in Fig. 9. As shown in the gure, acicular Al11 RE3 , massive Al2 RE and AlCa compounds crystallize in the alloy. Two AlRE compounds with dierent shapes have been observed in MgAlRE alloy by Powell et al.23) Similar results are obtained for ZACE05411 and ZACE05611 alloys except that additionally, a MgCa (Mg2 Ca) compound is observed in ZACE05411 alloy, while a MgAl (Mg17 Al12 ) compound is observed around AlCa compound in ZACE05611 alloy as shown in Fig. 10.

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ZACE05411

ZACE05611

ZACE05613

20m
Fig. 7 Microstructures of diecast specimens of ZACE05411, ZACE05611 and ZACE05613 alloys.

ZACE05411

(Mg) Al2Ca Al11RE3

Intensity (arbitrary unit)

20m

100 80 60 40 20 0

A:Al11RE3 (RE=La+Ce+Pr+Nd) 2.6 2.7 1

mol%

Mg Zn Al Ca RE
B:Al2RE 2.2 1 0.5

ZACE05611

100 80 60 40 20 0

C:Al-Ca

Mg Zn Al Ca RE

100 80 60 40 20 0

mol%

mol%

ZACE05613

Mg Zn Al Ca RE

Fig. 9 EPMA result obtained for ZACE05613 alloy using DSC specimen of the diecast sample.

20

30

40 50 2 ( /180rad)

60

70

80

ZACE05411

ZACE05611

Fig. 8 X-ray diraction patterns of diecast specimens of ZACE05411, ZACE05611 and ZACE05613 alloys.

D 10m

3.5 Heat resistance of diecast samples The suitability of the diecast samples for high temperature application was evaluated by carrying out creep tests, as well as high temperature tensile tests. Figure 11(a) shows the creep curves of the investigated alloys tested at 175 C for 100 h under an applied stress of 50 MPa. Those of conventional magnesium alloys, AZ91D, AS41 and AE42, and that of ADC12 aluminum alloy that is currently used for powertrain components of automobiles are also shown in the gure for comparison. All of the investigated alloys show excellent creep resistance that is much better than that of AZ91D and AS41 alloys. Furthermore, the total creep strain of ZACE05411 alloy is the same as that of the aluminum

100 80 60 40 20 0

mol%

Although diraction peaks from Mg2 Ca compound are not observed in the X-ray diraction patterns of ZACE05411 alloy, the crystallization of Mg2 Ca compound is expected because the reduced aluminum content implies that excess calcium or rare earths will combine with magnesium to form a new compound. Similarly, in ZACE05611 alloy, the reduction of rare earth content from 3 to 1% compared to ZACE05613 alloy implies that excess aluminum will combine with magnesium to form a new compound.

100 A1:Al2RE 80 60 (RE=La+Ce+Pr+Nd) 40 20 0 80 A2:Al11RE3 60 40 20 0 Mg Zn Al Ca RE

mol%

100 A1:Al 2RE 80 60 40 20 0 Mg Zn Al

mol%

Ca

RE

Mg

Zn

Al

Ca

RE

100 80 60 40 20 0 80 60 40 20 0
100 80 60 40 20 0

B1:Al-Ca

B2:Al-Ca

mol%

Mg

Zn

Al

Ca

RE

100 80 60 40 20 0

C:Mg-Ca

mol%

Mg

Zn

Al

Ca

RE

mol%

D:Mg-Al

Mg

Zn

Al

Ca

RE

Fig. 10 EPMA results obtained for ZACE05411 and ZACE05611 alloys using DSC specimens of the diecast samples.

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Fig. 11 Creep curves of investigated alloys tested under an applied stress 50 MPa for 100 h at (a) 175 C and (b) 200 C.

alloy, ADC12, while that of ZACE05613 alloy is quite close. However, AE42 alloy shows a higher creep resistance than ZACE05611. The relatively poor creep resistance of ZACE05611 alloy is due to smaller amounts of AlRE and AlCa compound and the likely presence of a MgAl compound as observed by EPMA. The dierence in creep resistance of the investigated alloys was conrmed by carrying out another creep test at a higher temperature of 200 C, and the result is shown in Fig. 11(b). Consequently, the eect of the major alloying

elements on the creep resistance of the alloys is clearly shown in Fig. 11(b). As shown in the gure, the creep resistance of ZACE05611 alloy is the lowest among the examined alloys. In contrast, ZACE05411 alloy is the most creep resistant. Apparently, the dierence of 2%Al content is responsible for this behavior. Additional crystallization of Mg2 Ca along the grain boundaries of ZACE05411 alloy, as shown in Fig. 10, helps to control grain boundary sliding, which is the main creep deformation mechanism in magnesium alloys.24) Furthermore, it seems that the excess aluminum content of ZACE05611 alloy results in creepinduced precipitation of Mg17 Al12 compound that deteriorates the creep resistance of MgAl based alloys.2325) Increasing the RE content to 3.0%, as in ZACE05613 alloy, results in creep resistance that is almost similar to ZACE05411 alloy. Thus by combining with excess Al to form more AlRE compounds, the amount of Al in solid solution is reduced and creep induced precipitation of Mg17 Al12 compound is prevented when the RE content is increased. Furthermore, the AlRE compounds eectively help to restrain dislocation motion and grain boundary sliding. Figure 12 shows the tensile properties of the investigated diecast samples as a function of temperature. Those of diecast AZ91D and AE42 magnesium alloys are also shown for comparison. As shown in the gure, the tensile strength of diecast specimens of the investigated alloys is comparable to that of AZ91D and AE42 diecasting magnesium alloys and it falls as the temperature increases. However, AZ91D alloy has slightly higher values of tensile strength and 0.2% proof stress at room temperature. The higher strength of AZ91D at room temperature is probably due to higher solid solution strengthening because of the high aluminum content of AZ91D alloy. On the other hand the 0.2% proof stress of the new alloys decreases gradually at a slower rate than that of AZ91D alloy, indicating the higher stability of the alloys at elevated temperatures. Furthermore, the investigated alloys exhibit good elongation of more than 5% at room tempera-

250
U.T.S.& 0.2%P.S., /MPa

U.T.S. 200 150 100 50 0 0 100 200 0

0.2%P.S.

ZACE05411 ZACE05613 ZACE05611 AZ91 AE42

30 Elongation 25 20 15 10 5 100 200 0 Temperature , T/C 100 200 0 300

Fig. 12 Tensile properties of the investigated diecast samples as a function of temperature.

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ture. This may be because thermally stable AlCa and AlRE compounds do not completely occupy the grain boundaries and they also crystallize within the grains. Generally, the elongation increases as the test temperature increases. This seems to be due to the activation of the secondary slip system of magnesium at high temperatures. Figure 13 shows the scanning electron micrographs of the tensile fracture surfaces and microstructures adjacent to the fracture surfaces of ZACE05411 alloy. Only the specimens tested at room temperature, 175 C and 250 C are shown. In all of the investigated alloys tested at room temperature and 175 C, cracks that originate at the grain boundary compounds and propagate along the grain boundaries are observed. This implies that elongation of the specimens depends on easiness of crack propagation along the grain boundaries. On the other hand, the grains of specimens tested

at 250 C are extended to the tensile direction and the SEM image reveals numerous dimples, characterizing ductile failure. This suggests that as the test temperature increases, the activation of the secondary slip system of magnesium occurs and the grains are easily deformed, resulting in large elongation. 4. Conclusions

(1) The size and type of the eutectic compounds increase as aluminum content increases. In all of the investigated alloys, Al2 Ca crystallizes mainly along the grain boundaries, while acicular Al11 RE3 and massive Al2 RE crystallize both within the grains and along the grain boundaries. However, Mg17 Al12 phase crystallizes in alloys containing high amount of aluminum.

Fig. 13 Scanning electron micrographs of the tensile fracture surfaces and microstructures adjacent to the fracture surfaces of diecast specimens of ZACE05411 alloy tested at (a) RT, (b) 175 C, and (c) 250 C.

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(2) Zinc addition improves uidity but high zinc content is not suitable for diecasting of the evaluated alloys due to the presence of eutectic compounds that crystallize at lower temperatures relative to the matrix, resulting in hot tearing during diecasting. (3) ZACE05613 is suitable for powertrain applications because it has good diecasting characteristics and satisfactory heat resistance. (4) The creep resistance of diecast specimens of the investigated alloys is much better than that of commercial AZ91D alloy, and with the exception of ZACE05611 alloy, the creep resistance of the investigated alloys is higher than that of conventional heat resistant AE42 alloy. ZACE05411 alloy exhibits similar creep resistance as ADC12 aluminum alloy that is currently used for pawertrain components. High Al and Zn contents reduce creep resistance of the alloys. On the other hand, high RE content increases their creep resistance. Acknowledgements This study is supported by New Energy and Industrial Technology Development Organization (NEDO) and Grantin-Aid for Scientic Research on Priority Area (B), Platform Science and Technology for Advanced Magnesium Alloys from the Ministry of Education, Culture, Sports, Science and Technology of Japan. REFERENCES
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