Mechanical Properties andMicrostructures of Mg97Zn1MM2 Alloys
Asian Forum on Light Metals
ALMA Forum 2014 ,November 14, 2014, Shinagawa, Japan
Jian-Yih Wang (王建義)
Department of Materials Science and Technology,
National Dong Hwa University (NDHU;国立東華大学),Taiwan

Research purpose The rapid solidified Mg alloy (Mg97Zn1Y2; Kumadai alloy)
with long-period stacking ordered (LPSO) phasepossesses good combination of mechanical properties
and corrosion resistance. It is attracted considerableattention.
 However, the expense of Y addition and inconvenienceof rapid solidification process restricts the applicationof this alloy.
 Low priced misch metals were used to replace the Y,and alloys were cast by conventional VIM method.

 The room temperature and elevated temperature
mechanical properties of Mg97Zn1MM2(La, Ce-rich mischmetal) alloys were investigated.
 The Mg97Zn1Y2 alloy is also investigated for acomparison with Mg97Zn1MM2 alloys in mechanicalproperties and microstructure.

Experimental proceduresCast ingot
The ingot washomogenized at 500oC
for 10h.

Homogenization

The ingot was extruded
at 400oC.

Hot rolling at 400oC.

Ingot was cast by VIM method.

Rapid solidification

Extrusion
Hot rolling

Annealing

Canning

Cold rolling

Extrusion

Mechanical test

Microstructure analysis

Tensile, compressive test andmicrohardness measurement

FE-SEM, XRD, EDS and OM,
image analysis

The chemical compositions of alloys
The chemical composition of the alloys were measured by ICP-OES
and listed below.
Alloys (wt.%)

Mg

ZnY

La

CeAl

Si

Mg97Zn1Y2

85.8

2.49

6.42

0.186

0.077

0.030

0.047

Mg97Zn1(MM-La)2

75.4

2.59

<0.009

6.80

2.83

0.067

0.062

Mg97Zn1(MM-Ce)2

68.9

2.13

<0.009

2.98

5.08

0.034

0.032

Two different kinds of misch metals (La-rich and Ce-rich) were added inthe Mg97Zn1MM2 alloys, respectively.
The ratios of lanthanum and cerium in the Mg97Zn1MM2 alloys aredifferent, but other added elements are almost the same.
Both misch metals contain some minor elements of Nd and Pr.
The total amounts of minor elements below 1wt.%.

OM images of as-cast alloys(a) Mg97Zn1(MM-La)2(b) Mg97Zn1(MM-Ce)2(c) Mg97Zn1Y2

Mg grain: 196.6μm

Mg grain: 257.6μm

All of the alloys demonstrate a dendrite structure with large
grain sizes as well as second phase particles.

Mechanical properties of as-cast alloys

The yield strengths of both Mg97Zn1MM2 alloys are higher than that ofMgZnY2 alloy.

OM images of extruded alloys(a) Mg97Zn1(MM-La)2(b) Mg97Zn1(MM-Ce)2(c) Mg97Zn1Y2

Mg grain: 10.2μm

Mg grain: 11.5μm

Mg grain: 16.8μm

The grain size of Mg matrix as well as second phase particles can be refined byextrusion remarkably. However, the shape of second phase particles is irregular andthey are distributed unevenly.

OM images of as-extruded Mg97Zn1MM2 alloys(a) Mg97Zn1(MM-La)2
Mg grain: 10.2μm(b) Mg97Zn1(MM-Ce)
Mg grain: 11.5μm

The volume fractions of second phase particles in Mg97Zn1(MM-La)2 and Mg97Zn1(MMCe)2 alloys are 24.9% and 23.1%, respectively.

Mechanical properties of extruded alloys
400U.T.S.

350

20Yield strength15

Elongation

250
200

10150
1005Elongation (%)

UTS
Yield stress300

strength (MPa)Elongation

5000

Mg97Zn1Y1 23
Mg972Zn1(MM-La)
2045Mg97Zn1(MM-Ce)2

The yield strengths of all alloys are improved due to the contribution of finestructure. And strengths are increasing by almost two times comparing withas-cast ones.

Microharnesses of extruded alloys(b) Mg97Zn1(MM-Ce)2
2nd phase:135.5Hv

Mg:60.3Hv(a) Mg97Zn1(MM-La)2
2nd phase:146.2Hv
Mg : 52.8Hv(c) Mg97Zn1Y2
2nd phase:86.9Hv

The dispersion
strengthening effect isvery obvious in theMg97Zn1MM2 alloys.

Mg: 64.2Hv

Nanoindentation of extruded Mg97Zn1MM2 alloys

The hardness of second phase in Mg97Zn1MM2 alloys is about 5 times
higher than that of matrix.

H.T. mechanical properties of extruded alloys
Mg97Zn1Y2
Mg97Zn1 (MM-La)2
Mg97Zn1 (MM-Ce)2

300

200

70150

60

100
50001R.T.

2373

3473

Temperature (K)4573

Elongation (%)

Yield strength (MPa)250

5Mg97Zn1Y2
Mg97Zn1(MM-La)2
Mg97Zn1(MM-Ce)2504030
20
10001R.T.

2373

3473

4573

Temperature (K)The high temperature mechanical properties of both Mg97Zn1MM2 alloys area slightly worse than that of Mg97Zn1Y2. That means the LPSO phase have astronger heat resistance than that of second phase in Mg97Zn1MM2 alloys.

5Microstructures of extruded alloys after
annealing at 773K(a) Mg97Zn1(MM-La)2
The shape of second
phase particles in bothMgZnMM2 alloys isgetting circular anduniform distribution.

(b) Mg97Zn1(MM-Ce)2The lamellarstructure of LPSO
is more obvious.
(c) Mg97Zn1Y2

Microstructures of extruded alloys after
annealing at 773K + 10% cold rolling(d) Mg97Zn1(MM-La)2
The microstructures
did not have bigchange after coldrolling.

(e) Mg97Zn1(MM-Ce)2
The brittle second phaseparticles is easily broken
during cold rolling.

(f) Mg97Zn1Y2

Microstructures of extruded alloys after
80% hot rolling at 673K(a) Mg97Zn1(MM-La)2(b) Mg97Zn1(MM-Ce)2(c) Mg97Zn1Y2

The second phase particles and LPSO are refined very remarkably, and consequentlythe high strengths can be obtained.

Yield strength of alloys after different
processing
400

Yield strength (MPa)

350
300
250
200
150
Mg97Zn1Y2
Mg97Zn1MM(MM-La)2
Mg97Zn1MM(MM-Ce)2100
50
00As-cast

extrusion

Annealing at773K

773K annealed+10% cold rolled

1880% hot rolled
at 673K

The extrusion improves the yield strength, annealing declines the strength
due to the recrystallization, and cold or hot rolling can increase the yieldstrength because the particle size of second phase was refined.

FESEM and EDS results of as-cast alloys(a) Mg97Zn1(MM-La)2(b) Mg97Zn1(MM-Ce)2(c) Mg97Zn1Y2bbbaa100 μmab100 μm

Mg

Zn

La

Ce

99.4

0.4

0.10.1

Mg

Zn

La

Ce

87.6

4.3

6.1

2.0aba60 μm

Mg

Zn

La

Ce

99.5

0.3

0.10.1

Mg

Zn

La

Ce

87.3

4.5

3.7

4.5

ab

Mg

Zn

Y

99.1

0.2

0.7

Mg

Zn

Y

87.1

6.1

6.8

(at%)

(Mg,Zn)12(La,Ce)(Mg,Zn)12(La,Ce)(Mn12Th)-type structure

Mg12ZnYLPSO

30(1011)α354045(1012)α(101,16)18R(101,13)18R

Mg97Zn1Y2 extruded alloy(101,10)18R(1018)18R(0002)α (000,18)18R(1015)18R(1010)α

(1010)18R

Intensity (Arb. unit)

XRD pattern of extruded Mg97Zn1Y2 alloy5055

2 θ (degree)

The second phase in Mg97Zn1Y2 alloy is confirmed as 18R LPSO

(102)α

Mg97Zn1(MM-La)2 extruded alloy(101)α(202)CeMg12(002)α(100)α(002)CeMg12Intensity (Arb. unit)

XRD patterns of extruded Mg97Zn1(MM)2 alloys

Mg97Zn1(MM-Ce)2 extruded alloy

303540455055

2 θ (degree)
The second phase in both Mg97Zn1MM2 alloys is confirmed as (Mg,Zn)12(La,Ce).

TEM image and diffraction pattern of as-extruded
Mg97Zn1(MM-La)2(a) TEM image, (b) SADP of particle, (c)SADP of matrix, and (d) EDS result ofparticle.

The particle was identified as(Mg, Zn)12(La, Ce).

HR-TEM image of as-extruded Mg97Zn1(MM-La)2Mg(La, Ce)(Zn, Mg)12(a) HR-TEM lattice image and diffraction
pattern, and (b) HRTEM image of interface
between (Mg, Zn)12 (La, Ce) and matrix.
No explicit orientation relationship between
Mg matrix and compound.

Fractography of extruded alloys after R.T.
tensile test

(a)

(b)

(a), Mg97Zn1(MM-La)2,(b) Mg97Zn1(MM-Ce)2,(c)(c) Mg97Zn1Y2

The second phase particles in both Mg97Zn1MM2 alloys arebroken perpendicular to tensile direction.

Fractography of extruded alloys after 473K
tensile test
(a)

(b)

(a), Mg97Zn1(MM-La)2,(b) Mg97Zn1(MM-Ce)2,(c)(c) Mg97Zn1Y2

The second phase particles are so brittle to be easily broken even duringhigh temperature tensile test, and consequently weaken the elevatedtemperature strength. However, if we can refine the second phase particles
by high hot rolling, the dispersion strengthening can be expected.

Fractography of extruded alloys after 573K
tensile test
(a)

(b)

(a), Mg97Zn1(MM-La)2,(c)(b) Mg97Zn1(MM-Ce)2,(c) Mg97Zn1Y2

The cracks are no more existing in the second phase particles,
instead of getting rounder and finer shape.

True stress-true strain curves of as-extruded, hotrolled and annealed Mg97Zn1(MM-La)2.

The high temperature rolling greatly
increased the strength.

High temperature tensile test results of as-extruded
and 80% hot rolled at 773K of Mg97Zn1MM2.

DL : degree of dispersion.

The degrees of dispersion in Mg97Zn1(MM-La)2

DL/μm-1 : dispersion level

LV is the length of a vertical line segment drawn perpendicular to theextrusion direction, and NV is the number of second phase intersected bysegment LV. The total length of segments must be more than 5mm.

DL/um-1

As-extruded

High temperatureannealed

High temperaturerolled

0.013

0.05

0.06

High temperature rolling promoted the uniform dispersion andrefinement of the second phase. Therefore, dispersion strengthening
improved the mechanical properties the Mg97Zn1MM2 alloys.

Conclusions(1) The Mg97Zn1MM2 alloys possess second phase(Mg,Zn)12(La,Ce) instead of LPSO phase in Kumadai
alloy.
(2) The particles size of (Mg,Zn)12(La,Ce) in
Mg97Zn1MM2 alloys can be refined by cold rolling andeven in hot rolling remarkably. Consequently, themechanical properties can be improved.
(3) Due to the existence of distributed second phases,
the high temperature strength of MgZnMM2 alloys arecomparable with that of Mg97Zn1Y2 alloy.
(4)The dispersion strengthening is very effective in
Mg97Zn1MM2 alloys due to the fine and hard particle(Mg,Zn)12(La,Ce) phase.