Vol.26 No.1 YI Yong et al: Effect of Lanthanum-praseodymium-cerium…

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DOI 10.1007/s11595-011-0177-5

Effect of Lanthanum-praseodymium-cerium Mischmetal on

Mechanical Properties and Microstructure of Mg-A1 Alloys

YI Yong1, 2, FAN Yongge2, TANG Yongjian1

(1. Research Center of Laser Fusion, China Academy of Engineering Physics, Mianyang 621900,China;

2.School of Materials Science & Engineering, Southwest University of Science & Technology, Mianyang 621010, China)

Abstract: The effects of lanthanum-praseodymium-cerium mischmetal (LPC) on the micro-

structure and mechanical properties of Mg-Al alloy were investigated. With the addition of LPC, an

additional rod-like Al11La3 phase was deposited in the alloy. LPC greatly improves the tensile strength

of cast Mg-Al alloys but negatively affects the elongation of cast alloys above 473 K. Cast alloys are

strengthened by both precipitation strengthening and dispersion strengthening at ambient temperature.

When the temperature exceeds 473 K, only the dispersion operates as a strengthening mechanism.

Key words: lanthanum-praseodymium-cerium mischmetal; Mg-alloy; microstructure; me-

chanical properties

1 Introduction

Magnesium alloys are attractive for space, aeronau-

tical, automobile and leisure applications because of their

low density, high specific strength, good machinability

and availability[1-3]

. The use of low cost Mg alloys is

limited due to their poor mechanical and creep properties

at elevated and high temperatures. The commercially

available highly creep resistant alloys for applications at

elevated and high temperatures, e g, QE22 and WE54,

WE43, often fulfills the specifications but not at an

economical price. A similar situation exists for the ex-

perimental high temperature creep resistant alloys de-

veloped by alloying Mg with various rare earths and

combinations thereof, e g, Mg–Gd, Mg–Gd–Y,

Mg–Gd–Nd, or with the addition of Sc and Mn, e g,

Mg–Sc–Mn, Mg–Sc–Gd (or Y, Ce)–Mn[4-8]

.

On the other hand, due to the low ductility at room

temperature, the main method in the industrial manu-

facturing of magnesium alloy products is casting, espe-

cially die casting and thixocasting. But these processes

tend to bring about defects in the products’ special 3C

shells such as pin holes, cold shuts, low strength that

would need supplemental processing[9]

. Now the semi-

solid methods such as thixomolding and rheomolding can

increase the quality, but due to their extremely high

temperature, corrosion of molds and defects are difficult

to overcome. It is proved that forming at lower tem-

perature such as forging and extrusion can heighten effi-

ciency and improve surface quality of products[10-12]

.

In this work rare-earth metal mixture (LPC, Lan-

thanum-praseodymium-cerium mischmetal) was added,

which was very cheap and whose industrial application

needs to be extended in China[10-12]

, and incorporating hot

extrusion was applied to improve the mechanic proper-

ties of Mg-Al alloy at an acceptable cost.

2 Experimental

The composition of magnesium alloy and LPC is

shown in Tables 1 and Tables 2. The alloys were pre-

pared by melting and casting in a vacuum induction

furnace in the argon atmosphere.

Tensile tests were performed on an INSTRON 5566

type machine and at a temperature range of 298 to 523 K.

The specimens were in the atmosphere furnace and were

kept 10 minutes to equilibrate at the test temperatures

before they were strained.

Table 1 Composition of alloys/wt %

Alloy Al Zn LPC Mg

Mg-Al 8.00 0.62 - Remains

Mg-Al-Re 8.00 0.62 1.0 Remains

Mg-Al-2Re 8.00 0.62 2.0 Remains

Mg-Al-3Re 8.00 0.62 3.0 Remains

Table 2 Composition of LPC

Element La Pr Ce Nd Other

wt % 83.8 6.2 9.0 0.8 Remains

©Wuhan University of Technology and Springer-Verlag Berlin Heidelberg 2011

(Received: Sept. 19, 2009; Accepted: Nov. 12, 2010)

YI Yong(易易): Ph D; E-mail: yiyong2000@gmail.com

Journal of Wuhan University of Technology-Mater. Sci. Ed. Feb.2011

103

The microstructures were observed using an optical

microscope (Olympus BX51) after polishing and etching

in 0.5% HF solution. The microstructures of the alloys

were also observed using a scanning electron microscope

(HITACHI TM-100). X-ray diffraction patterns of the

alloys were obtained and analyzed using an X-ray dif-

fractometer operating at 35 kV and 60 mA (Rigaku

DMAX/RB-II).

3 Results and Discussion

3.1 Analysis of microstructures X-ray diffraction spectra of some cast and hot extru-

sion specimens are presented in Fig.1. It could be seen that

the main intermetallic phases in the alloys are Al11La3 and

Al12Mg17 (γ phase). According to the research results of

some groups[6-8]

, due to the good chemical stability and

higher precipitation temperature, Al11La3 prior to

Al12Mg17 would precipitate until RE was used up, and no

Mg-RE and Mg-Al-RE phases were found.

Fig.1 indicates also that the precipitation of Al11La3

will reduce γ-phase.Fig.2 and Fig.3 display the micro-

structures of the as-cast specimens. The morphologies of

as-cast Mg-Al alloy consist of dendritic α-phase and

mixture of irregular divorced eutectic α and

coarse-crystalline γ along the grain boundaries.

(Fig.2(a)); in addition a discontinuous lamellar structure

of α could be seen alongside of dendritic crystal and γ

around as well as superfine AlMn (Fig.3 (a)). With the

addition of LPC, acicular Al1La3 phase precipitates more

and more in the alloy. For this reason it was more difficult

to observe AlMn (Fig2.(b, c) and Fig.3 (b, c)).

3.2 Effect on mechanical properties

Fig.4 reveals the variation between the content of

LPC and the mechanical properties of alloys such as the

tension, yield strength and elongation from 298-523 K.

In Fig.4(a) at ambient temperature and 473 K it can

be found that the ultimate strength of as-cast decreases

with the addition of LPC. This was due to the concentrate

of stress and strain caused by the acicular precipitation of

Al11La3. At high temperature (above 473 K) the ultimate

strength of all alloys increased slightly with the amount

of LPC. In Fig.4 (b), for the as-cast the addition of LPC

would evidently improve the 0.2 proof stresses (R0.2) at

all temperatures studied. The R0.2 of cast Mg-Al-2RE was

34% more than that of Mg-Al at ambient temperature.

This was due to the Al11La3; on the one hand, it could

prevent holes from forming and growing, on the other

hand, it reduced the amount of γ phase, which would

produce holes. There was a noticeable phenomenon that

the optimal content of LPC for R0.2 increased with the

temperature. In Fig.4 (c), it shows the variation in the

elongation of alloys at the studied temperature. It would

be found that for the cast specimens the addition of LPC

had negative effect on the elongation at 523 K, but little

effect at ambient temperature and 473 K.

Fig.1 XRD patterns of (a) as-cast Mg-Al; (b)as-cast Mg-Al-2RE

Fig.3 SEM images of (a) as-cast Mg-Al; (b) as-cast Mg-Al-1RE; (c) as-cast Mg-Al-2RE

Fig.2 Optical micrographs of (a) as-cast Mg-Al; (b) as-cast Mg-Al-1RE; (c) as-cast Mg-Al-2RE

Vol.26 No.1 YI Yong et al: Effect of Lanthanum-praseodymium-cerium…

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4 Conclusions

a) The microstructures of the present as-cast Mg-Al

alloy consist of α Mg matrix and irregular γ precipi-

tates with two different morphologies along the grain

boundaries. With the addition of LPC, the additional

rod-like Al11La3 phase precipitates in the alloy.

b) LPC greatly improves the tensile strength of cast

Mg-Al alloys above 473 K. LPC negatively affects the

elongation of cast alloys at 523 K. Cast alloys are

strengthened by both precipitation strengthening (γ

precipitate) and dispersion strengthening (Al11La3 pre-

cipitate) at ambient temperature. When the temperature

exceeds 473 K, only the dispersion operates as a

strengthening mechanism.

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Fig.4 Variation between LPC content and property of (a) ultimate strength; (b) 0.2% proof stress; (c) elongation (◇523 K; △473 K; □298 K)