O14: Phase Equilibria Including X or W Phase in Mg-Zn-Y Ternary System

Transcription

1 The Discussion Meeting on Thermodynamics of Alloys () September 2010, University of Porto, Portugal O14: Phase Equilibria Including X or W Phase in Mg-Zn-Y Ternary System Toshiaki Horiuchi a, Atsushi Hamaya a, Satoshi Minamoto b, Sukeharu Nomoto b and Seiji Miura c a) Department of Mechanical Systems Engineering, Hokkaido Institute of Technology, Sapporo , Japan b) Science & Engineering Systems Division, ITOCHU Techno-Solutions Corporation (CTC), Tokyo , Japan c) Division of Materials Science and Engineering, Graduate School of Engineering, Hokkaido University, Sapporo , Japan

2 Mg 97 Zn 1 Y 2 Alloy Mg-Y-Zn ternary alloy No.1 0.2% proof stress: 610MPa Rupture elongation: 5% Higher specific strength than extra super duralumin High heat resistance and fatigue strength Y. Kawamura, K. Hayashi, A. Inoue and T. Masumoto: Materials Transactions, 42 (2001), Mg solid solution (α phase) + Intermetallic compound (X phase: Mg 12 Zn 1 Y 1 ) Long Period Stacking Ordered (LPSO) phase Formation process and stability of X phase is almost uncertain. α +X+W : 3 phases Further improvement?

3 Equilibrium composition of W phase Padezhnova et al. (573K) (1) Tsai et al. (773K) (2) Shao et al. (773K) (3) Objective Clarifying phase equilibria in the vicinity of the X and W phases 10 Mg Y discrepancy Mg (at.%) No.2 (1) E. M. Padezhnova et al.: Russian Metallurgy, 4 (1982), (3) G.Shao et al.: Calphad, 30 (2006), (2) A. P. Tsai, Y. Murakami, A. Niikura: Philosophical Magazine, A 80 (2000) Zn

4 Mg Zn Experimental procedure No.3 Melted approximately 100g in φ 30.5mm graphite crucible using a high-frequency induction furnace under high-purity Ar atmosphere Mg 97 Zn 1.3 Y 1.7, Mg 87 Zn 7 Y 6 Mg 84 Zn 7 Y 9, Mg 81 Zn 8.3 Y 10.7 Mg 53 Zn 36 Y 11 Mg 53 Zn 36 Y 11 : As cast Y Chemical composition check (ICP-OES, EDXRF) 20µm Mg 70 Y 30 (wt.%) Cut and encapsulated in lidded φ 6mm graphite crucible and encapsulated in quartz tube with high-purity Ar gas 20mm 6mm Isothermal heat treatment 4mm

5 Isothermal heat treatment conditions No.4 Nominal composition (at.%) Holding time (h) 833K 793K 723K 673K Mg 97.0 Zn 1.3 Y Mg 87.0 Zn 7.0 Y Mg 84.0 Zn 7.0 Y Mg 81.0 Zn 8.3 Y Mg 53.0 Zn 36.0 Y 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)

6 Analyzed chemical compositions No.5 Nominal composition (at.%) Analyzed composition (at.%) Mg 97.0 Zn 1.3 Y 1.7 Mg 97.2 Zn 1.3 Y 1.5 Mg 87.0 Zn 7.0 Y 6.0 Mg 83.1 Zn 9.0 Y 7.9 Mg 84.0 Zn 7.0 Y 9.0 Mg 84.7 Zn 6.7 Y 8.6 Mg 81.0 Zn 8.3 Y 10.7 Mg 81.7 Zn 8.4 Y 9.9 Mg 53.0 Zn 36.0 Y 11.0 Mg 50.7 Zn 37.4 Y 11.9

7 BEI Mg 97.2 Zn 1.3 Y 1.5 at 833K α X L Mg Zn Y 98.4 No.6 (at.%) µm solidification contraction black region X (1) Formation of the X phase is not eutectic, but peritectic. mesh texture 2 phases gray region 10µm 5µm (1) E. M. Padezhnova et al. : Russian Metallurgy, 4 (1982),

8 Mg 50.7 Zn 37.4 Y 11.9 at 723K BEI α W I Mg Zn Y No.7 (at.%) Chemical compositions for the W and I phases are very similar. 10µm 3 phases W (1) W (2) I (2) (1) E. M. Padezhnova et al. : Russian Metallurgy, 4 (1982), (2) G. Shao et al.: Calphad, 30 (2006),

9 20µm No.8 EBSD analysis for Mg 50.7 Zn 37.4 Y 11.9 at 723K BEI 20µm IPF Only some white regions are identified as bcc based phase. bcc 001 Mg Zn Y W : bcc based (L2 1 ) (1) I : icosahedral (2) Zn and Y contents are slightly different between the W and I phases (at.%) (1) E. M. Padezhnova et al.: Russian Metallurgy, 4 (1982), (2) A. P. Tsai et al.: Philosophical Magazine, A 80 (2000),

10 Determined equilibrium phases No.9 Analyzed composition (at.%) Holding time (h) 833K 793K 723K 673K Mg 97.2 Zn 1.3 Y 1.5 L+α α+x - - Mg 83.1 Zn 9.0 Y 7.9 L+X+W α+x+w α+x+w - Mg 84.7 Zn 6.7 Y X+W Mg 81.7 Zn 8.4 Y L+X+W - - Mg 50.7 Zn 37.4 Y 11.9 L+W - α+w+i - The W phase appeared as an equilibrium phase at all temperatures examined except for Mg 97.2 Zn 1.3 Y 1.5.

11 Equilibrium composition of W phase Mg 83.1 Zn 9.0 Y 7.9 (833K) Mg 83.1 Zn 9.0 Y 7.9 (793K) Mg 83.1 Zn 9.0 Y 7.9 (723K) Y 30 Mg 84.7 Zn 6.7 Y 8.6 (673K) Mg 81.7 Zn 8.4 Y 9.9 (793K) Mg 50.7 Zn 37.4 Y 11.9 (833K) Mg 50.7 Zn 37.4 Y 11.9 (723K) No.10 Padezhnova et al. (573K) (1) Tsai et al. (773K) (2) Shao et al. (773K) (3) Y 25 Y 20 The Y content in the W phase is almost constant approximately 25 at.%. The Mg and Zn contents vary with both alloy compositions and temperatures. (at.%) (1) E. M. Padezhnova et al.: Russian Metallurgy, 4 (1982), (3) G.Shao et al.: Calphad, 30 (2006), (2) A. P. Tsai, Y. Murakami, A. Niikura: Philosophical Magazine, A 80 (2000)

12 Crystal structure of W phase No.11 Intensity Mg 3 Zn 3 Y 2 (W phase) Mg 50.7 Zn 37.4 Y K 440 Heusler (L2 1 ) θ (degree) X-ray diffraction pattern for the W phase Superlattice reflection of L2 1 is observed. Stoichiometric composition of the W phase Padezhnova et al. Mg 3 Zn 3 Y 2 Mg 1 Zn 2 Y 1 Mg or Zn Y Zn or Mg Space Group: Fm3m Pearson Symbol: cf16

13 10 20 Mg 30 Comparison of calculated phase diagrams No X Y Isothermal section at 573K 10 (Mg,Zn)Y W 20 Mg (at.%) I Z Shao et al. G. Shao et al.: Calphad, 30 (2006), H 90 Zn Mg Mg (at.%) Ternary phase diagrams can be calculated by the CALPHAD technique for any temperatures and alloy compositions X Y 10 (Mg,Zn)Y W I Z Present study H Zn

14 Summary No.13 Phase Equilibria for 5 Mg-Zn-Y alloys including the X and/or W phases were investigated to improve the phase diagram in Mg-Zn-Y ternary system. It is clarified that; 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 (L2 1 ) type crystal structure with a stoichiometric composition of Mg 1 Zn 2 Y 1. 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.