the discussion meeting on thermodynamics of alloys (tofa...
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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;
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