Toshiaki Horiuchi a, Atsushi Hamaya a, Satoshi Minamoto b,Sukeharu Nomoto b and Seiji Miura c

O14: Phase EquilibriaIncluding X or W Phase

in Mg-Zn-Y Ternary System

The Discussion Meeting on Thermodynamics of Alloys (TOFA 2010)12-16 September 2010, University of Porto, Portugal

a) Department of Mechanical Systems Engineering,Hokkaido Institute of Technology, Sapporo 006-8585, Japan

b) Science & Engineering Systems Division,ITOCHU Techno-Solutions Corporation (CTC), Tokyo 100-6080, Japan

c) Division of Materials Science and Engineering, Graduate Schoolof Engineering, Hokkaido University, Sapporo 060-8628, Japan

TOFA 2010No.1Mg-Y-Zn ternary alloy

0.2% proof stress: 610MPaRupture elongation: 5%Mg97Zn1Y2 Alloy

High heat resistance and fatigue strength

Mg solid solution(α phase)

Long Period Stacking Ordered (LPSO) phaseFormation process and stability of X phase is almost uncertain.

α +X+W : 3 phases Further improvement?

Y. Kawamura, K. Hayashi, A. Inoue and T. Masumoto: Materials Transactions, 42 (2001), 1172-1176.

Intermetallic compound(X phase: Mg12Zn1Y1)+

Higher specific strength than extra super duralumin

TOFA 2010No.2Equilibrium composition of W phase

55 50 45 40 35 30 25 2060

15

25

20

30

35

40

45

10

5035

50

45

40

55

60

65

70

30

Mg (at.%)Mg Zn

Y

(3) G.Shao et al.: Calphad , 30 (2006), 286-295.(1) E. M. Padezhnova et al.: Russian Metallurgy, 4 (1982), 185-188.(2) A. P. Tsai, Y. Murakami, A. Niikura: Philosophical Magazine, A 80 (2000) 1043-1054.

Shao et al. (773K)(3)

discrepancy

Tsai et al. (773K)(2)Padezhnova et al. (573K)(1)

ObjectiveClarifying phaseequilibria in thevicinity of theX and W phases

TOFA 2010No.3Experimental procedure

Melted approximately 100g in φ 30.5mm graphitecrucible using a high-frequency induction furnaceunder high-purity Ar atmosphere

20m

m

4mm

6mm

Mg

Zn

Y

Mg70Y30(wt.%)

Mg97Zn1.3Y1.7, Mg87Zn7Y6Mg84Zn7Y9, Mg81Zn8.3Y10.7Mg53Zn36Y11

Mg53Zn36Y11: As cast

20µmChemical composition check(ICP-OES, EDXRF)

Isothermal heat treatment

Cut and encapsulated in lidded φ 6mmgraphite crucible and encapsulated inquartz tube with high-purity Ar gas

TOFA 2010No.4Isothermal heat treatment conditions

Nominalcomposition (at.%)

Holding time (h)833K 793K 723K 673K

Mg97.0Zn1.3Y1.7 2.0 72 － －

Mg87.0Zn7.0Y6.0 2.0 72 1000 －

Mg84.0Zn7.0Y9.0 － － － 2000Mg81.0Zn8.3Y10.7 － 4.0 － －

Mg53.0Zn36.0Y11.0 2.0 － 1000 －

W.Q.Observation of microstructure (OM, EPMA-SEM)Analysis of chemical compositions for equilibrium phases (EPMA-WDS)Analysis of crystal structure of the W phase (XRD, EBSD)

TOFA 2010No.5Analyzed chemical compositions

Nominalcomposition (at.%)

Analyzedcomposition (at.%)

Mg97.0Zn1.3Y1.7 Mg97.2Zn1.3Y1.5

Mg87.0Zn7.0Y6.0 Mg83.1Zn9.0Y7.9

Mg84.0Zn7.0Y9.0 Mg84.7Zn6.7Y8.6

Mg81.0Zn8.3Y10.7 Mg81.7Zn8.4Y9.9

Mg53.0Zn36.0Y11.0 Mg50.7Zn37.4Y11.9

TOFA 2010No.6

10µmgray region

500µmsolidification contraction

Mg97.2Zn1.3Y1.5 at 833K

98.4

6.587.2 6.3

81.2 7.811.0

BEI (at.％)Mg Zn Y

X

2 phases

black region

α

L

85.8 7.17.1X(1)

(1) E. M. Padezhnova et al. :Russian Metallurgy, 4 (1982), 185-188.

Formation of the X phase isnot eutectic, but peritectic.

5µm

mesh texture

TOFA 2010No.7Mg50.7Zn37.4Y11.9 at 723K

(at.％)Mg Zn Y

(1) E. M. Padezhnova et al. : Russian Metallurgy, 4 (1982), 185-188.(2) G. Shao et al.: Calphad, 30 (2006), 286-295.

10µm

97.2 2.8

25.7 58.3 16.0

26.3 49.5 24.2

IWα

3 phases

I (2) 30 60 10

W (2) 4020 2040

60

W (1) 31.0 43.5 25.5

BEI

Chemical compositions forthe W and I phases arevery similar.

TOFA 2010No.8EBSD analysis for Mg50.7Zn37.4Y11.9 at 723K

111

101001

bcc

20µm

IPFBEI

20µm

Only some white regions areidentified as bcc based phase.

26.6 57.2 16.2I : icosahedral(2)

(1) E. M. Padezhnova et al.:Russian Metallurgy, 4 (1982),185-188.

(2) A. P. Tsai et al.: Philosophical Magazine, A 80 (2000),1043-1054.

Zn and Y contents are slightlydifferent between the W andI phases.

W : bcc based (L21)(1)

26.7 48.8 24.5

(at.％)Mg Zn Y

TOFA 2010No.9Determined equilibrium phases

Analyzedcomposition (at.%)

Holding time (h)

833K 793K 723K 673K

Mg97.2Zn1.3Y1.5 L+α α+X － －

Mg83.1Zn9.0Y7.9 L+X+W α+X+W α+X+W －

Mg84.7Zn6.7Y8.6 － － － X+W

Mg81.7Zn8.4Y9.9 － L+X+W － －

Mg50.7Zn37.4Y11.9 L+W － α+W+I －

The W phase appeared as an equilibrium phase at alltemperatures examined except for Mg97.2Zn1.3Y1.5.

TOFA 2010No.10Equilibrium composition of W phase

Y 30

Y 25

Y 20

(at.%)

Padezhnova et al. (573K)(1)

Tsai et al. (773K)(2)

Shao et al. (773K)(3)

Mg83.1Zn9.0Y7.9 (833K)

Mg83.1Zn9.0Y7.9 (793K)

Mg83.1Zn9.0Y7.9 (723K) Mg50.7Zn37.4Y11.9 (723K)

Mg84.7Zn6.7Y8.6 (673K)Mg81.7Zn8.4Y9.9 (793K)Mg50.7Zn37.4Y11.9 (833K)

(3) G.Shao et al.: Calphad , 30 (2006), 286-295.(1) E. M. Padezhnova et al.: Russian Metallurgy, 4 (1982), 185-188.(2) A. P. Tsai, Y. Murakami, A. Niikura: Philosophical Magazine, A 80 (2000) 1043-1054.

The Y content in the W phase isalmost constant approximately 25 at.%.

The Mg and Zn contents vary with bothalloy compositions and temperatures.

TOFA 2010No.11

2θ (degree)

Inte

nsity

20 30 8050 706040 90

Mg50.7Zn37.4Y11.9833K

Crystal structure of W phase

111

220

331

422

440Mg3Zn3Y2(W phase)

Heusler (L21)

Mg or Zn

Zn or MgY

Pearson Symbol: cF16Space Group: Fm3m

Superlattice reflection of L21 is observed.

Padezhnova et al.Stoichiometric composition of the W phase

Mg1Zn2Y1

Mg3Zn3Y2

X-ray diffraction pattern for the W phase

TOFA 2010No.12Comparison of calculated phase diagrams

Isothermal section at 573K

Mg Mg (at.%) Zn

Y

10

40

30

20

50

60

70

80

90

90

60

70

80

50

40

30

20

10

90 80 70 60 50 40 30 20 10

WI

Z

H

(Mg,Zn)Y

X

Shao et al.

G. Shao et al.: Calphad, 30 (2006), 286-295.

Ternary phase diagrams can be calculated by the CALPHADtechnique for any temperatures and alloy compositions.

Mg Mg (at.%) Zn

Y

10

40

30

20

50

60

70

80

90

90

60

70

80

50

40

30

20

10

90 80 70 60 50 40 30 20 10

WI

Z

H

(Mg,Zn)Y

X

Presentstudy

TOFA 2010No.13Summary

• Morphology of the X phase and indeterminate boundary shapes between α and X phases support the idea that formation of the X phase does not occur by a eutectic reaction, but rather by a peritectic reaction.

• The Y content in the W phase is almost constant approximately 25 at.%, and only Mg and Zn may be substituted for each other.

• The W phase has the Heusler (L21) type crystal structure with a stoichiometric composition of Mg1Zn2Y1.

• Mg-Zn-Y ternary phase diagrams can be calculated using the Thermo-Calc software package with assessed CALPHAD thermodynamic database with incorporating the experimental results of the present study.

Phase Equilibria for 5 Mg-Zn-Y alloys including the Xand/or W phases were investigated to improve the phase diagram in Mg-Zn-Y ternary system. It is clarified that;