Microstructual Stability of Magnesium Alloys at High Temperature

Transcription

1 Microstructual Stability of Magnesium Alloys at High Temperature M.A.Sc. Candidate : Zhan (Maggie) Gao Supervisor: Prof David S. Wilkinson January 21st, 2009 MSE 701 Graduate Seminar

2 Outline Introduction Literature Review Research Objectives Experimental Results and Discussions Part I: Microstructural Characterization Part II: Mechanical Behaviour Charaterization Summary Future work Acknowledgements 2

3 Introduction Why Mg alloys? Element Density g/cm 3 Magnesium 1.74 Aluminum 2.70 Iron 7.87 Comparisons of density Average magnesium usage per vehicle NA (kg) Auto parts of magnesium Comparisons of properties Tadataka et al, Materials Science Forum, (2003)

4 Literature Review -slip system Why high temperature? - Improve ductility CRSS: non-basal slip >> basal slip at room temperature HCP crystal structure Two independent slip systems [100](001) and [010](001) Not satisfy Taylor criterion: 5 independent slip systems B Bhattacharya, PhD thesis, Mat Sci and Eng (2006), 4

5 Literature Review -slip system Why high temperature? - Improve ductility Temperature increase CRSS: non-basal slip σ y τ = CRSS ( cosφ cosλ) max decrease Slip systems activated Ductility improved % El l = f Temperature dependence of the yield stress (full circles) and the ultimate tensile stress (empty circles) l l Taylor criterion obeyed: 5 independent slip systems The variation of ductility as a function of temperature A. Jager, P et al. Journal of Alloys and Compounds, 378 (2004),

6 Literature Review-SPF SPF: Super Plastic Forming technique Strain-rate-sensitivity m σ = k ε m < 0.2 m m > 0.33 most metals and alloys superplastic alloys SPF advantage for Mg alloys Complex-shaped components directly made from rods and plates Cost-effective compared with diecasting Mechanical properties are superior to die-casting parts The variation in flow stress and elongationto-failure as a function of strain rate in AZ31 (d=130μm) H. Watanabe, H. et al. International Journal of Plasticity, 17 (2001), W. J. KIM, S. W. et al. Acta mater, 49 (2001),

7 SPF Application SPF of magnesium sheet: SP formation of Mg sheet AZ31B Sketch of gas pressure forming Business Jet service door inner panel 7

8 Literature Review-FSS FSS: Fine Structure Superplasticity The variation in elongation-to-failure as a function of strain rate for magnesium alloys and composites Rate equation for creep deformation, p σ ε Gb b = A kt d n G D n = 1 m The variation in strain rate as a function of the reciprocal grain size at 473K in ZK61 alloys T.G. Nieh, O.D. Sherby, J. Wadsworth, Superplasticity in metals and ceramics, (1997), Cambridge H. Watanabe et al, Acta mater, 49 (2001),

9 Literature Review-DRX DRX: Dynamic Recrystallization Diffusion necking Localization necking Strain hardening Strain softening (a) (e) ε=5% ε=100% Static grain growth (b) ε=20% (c) ε=40% (d) ε=60% Dynamic grain growth d=25μ m DRX start d=15μ m DRX d=10μ m J. C. Tan et al., Materials Science and Engineering, A339 (2003), DRX complete d=7μm d>7μm 9

10 Literature Review-GBS GBS: Grain Boundary Sliding In high temperature creep, the total strain can be expressed by, ε = ε + ε + ε ε Rate equation for grain boundary sliding, 6 Gb b σ = kt d G 2 πδ Deff = DL D d ε g gbs dc H. Watanabe et al. Acta mater, 49 (2001), J. C. Tan et al. Materials Science and Engineering, A339 (2003), gbs 2 w1 = Φ L1 Contribution of GBS to overall strain, ε gbs ξ = ε 2 gb D eff Contribution of GBS to overall strain Illustration of GBS and dislocation creep mechanism 10

11 Literature Review -Abnormal grain growth Deformation mechanism Fine grain Grain boundary sliding n = 2 m = 0.5; p = 2 Coarse/fine interface fine Coarse = ε Gb A kt b d p σ G n D 0 exp Q RT Microcracking decrease Microstructure of deformed ZK60 at uniform region n = 3 m = 0.33; p = 0 Superplasticity Coarse grain Glide-controlled dislocation creep Avoid abnormal grain growth H. Watanabe, H. et al. International Journal of Plasticity, 17 (2001), A. Bussiba et al. Materials Science and Engineering, 302 (2001),

12 Research Objectives Fine grain structure Superplastic behavior Part I- Microstructural characterization Grain growth function (temperature) Particle dissolution function (temperature) Part II- Mechanical properties characterization Ductility function (temperature, strain rate) m value function (flow stress, strain rate) DRX characterization GBS deformation mechanism 12

13 Experimental results & Discussions-Part I Materials: as-extruded AZ31 Al Zn Mn Fe Ce Cu Zr Ni mass % Temperature 550 C 500 C 450 C 400 C 350 C 300 C 1 hr Water quenching Sketch routine of heat treatment Table: chemical compositions of as-extruded AZ31 Time Heat treatment Argon atmosphere Sample preparation OM: cold mounted, polished, etched in acetic picral solution SEM & EDS: coated with silver paste 13

14 Part I Microstructual characterization of AZ31 Microstructure and average grain size (μm) 50 µm 1000 µm As-received 500 o C 50 µm 50 µm 300 o C 50 µm 400 o C 450 o C 14

15 Part I Microstructual characterization of AZ31 Particles observation EDS analysis Al GB Mn 2 μm Mn SEM images after AZ31 annealed at 400 C for 1hr & EDS analysis E 2 ~ 3 U = = 20keV Ec - E electron beam energy - Ec critical ionization energy Mg Al Mn Zn Atomic number K Shell Critical Ionization Energies for Mg, Al, Mn, Zn (unit: kev) 15

16 Part I Microstructual characterization of AZ31 Phase analysis XRD measurement Heat treatment temperature: 400 C Heat treatment temperature: 500 C XRD result of AZ31 after annealed at 400 o C XRD result of AZ31 after annealed at 500 o C 16

17 Part I Microstructual characterization of AZ31 Phase analysis phase diagram calculations AZ31 Temperature, o C Liqiud+Mg+Al 8 Mn 5 Mg+Al 8 Mn 5 Mg+Al 8 Mn 5 +Al 11 Mn 4 Equilibrium phase 400 C Mg+Al 8 Mn 5 +Al 11 Mn C Mg+Al 8 Mn 5 Mg+Al 11 Mn Manganese, wt% 17

18 Part I Microstructual characterization of AZ31 Quantify the volume fraction of phases Sample Heat treatment Average Grain Size 1 RT (as-received) μm C μm C μm C μm C μm C 2891 μm C 2993 μm Relative phase volume fraction after annealed at 400 C and 500 C dr dt Table: summary of microstructure after different heat treatment αγ 3γ = M ( P Pz ) = M f R 2r P: driving pressure for grain growth Pz: pinning pressure due to particles 18

19 Experimental results & Discussions-Part II Materials under study and experimental 25 mm 6 mm 19 mm Gauge section Clamp section 25 mm Sample preparation 15 mm Extrusion Direction ASTM dimensions of tensile test coupon Anneal at 345 C for 15 minutes to remove residual stress Raise the temperature to desired value, then hold for 10 minutes to ensure thermal equilibrium Equipment MTS Series 810 Material Testing Machine, 250 kn lbf Instron Environmental Chamber ARAMIS 6.1 System 19

20 Part II Mechanical properties characterization of AZ31 Elongation-to-failure tensile test σ-ε curve Temp: 200 C; strain rate: /s ~ /s Engineering stress True strain σ E = εt F A Stress Vs Strain curve against different strain rate 0 Measured by Testing machine Measured by Aramis system True stress Engineering strain σ T = σ E (1 + ε E ) εt ε E = e 1 20

21 Part II Mechanical properties characterization of AZ31 Elongation-to-failure tensile test σ-ε curve Temp: 300 C; strain rate: /s ~ /s Steady Steady state state Stress Vs Strain curve against different strain rate Steady state flow stress observed at strain rate of /s. 21

22 Part II Mechanical properties characterization of AZ31 Elongation-to-failure,% l f 19mm % El = mm Elongation-to-failure variation against different strain rate 22

23 Part II Mechanical properties characterization of AZ31 Strain-rate-sensitivity m σ = k ε m m = d logσ d log ε Temp: 200 C,300 C; strain rate: /s ~ /s Strain rate sensitivity, m C 300 C (a) (b) Variation of (a) flow stress and (b) strain rate sensitivity m as function of strain rate with definite ε= Strain Rate

24 Part II Mechanical properties characterization of AZ31 Average grain size (μm) near fracture tip (a) (b) Average grain size (μm) near fracture tip against (a) strain rate (b) temperature 200 C, average grain size increases as strain rate increases. 300 C, average grain size decreases as strain rate increases. 24

25 Part II Mechanical properties characterization of AZ C /s (a) 50 µm (b) /s (c) 50 µm (d) OM of tensile samples near fracture region at different strain rate and according grain size distribution 25

26 Part II Mechanical properties characterization of AZ C /s (a) 50 µm (b) /s (c) 50 µm (d) OM of tensile samples near fracture region at different strain rate and according grain size distribution 26

27 Part II Mechanical properties characterization of AZ31 Effect of temperature and strain rate on DRX 50 μm Volume fraction of fine grains ( 10 μm) at fracture against temperature. Microstructure and grain size distribution of as-extruded AZ31 Clemex Image Analysis: grain size distribution. Uniform magnification: 200 C 1000 times; 300 C 500 times. 27

28 Part II Mechanical properties characterization of AZ31 Effect of deformation on DRX, 200 C 200 C Variation of average grain size against position Variation of true strain against position 0 distance Variation fracture of tip average grain size against true strain at 200 C 28

29 Part II Mechanical properties characterization of AZ31 Effect of deformation on DRX, 300 C 300 C Variation of average grain size against position Variation of true strain against position Variation of average grain size against true strain at 300 C 29

30 Part II Mechanical properties characterization of AZ31 a b refining grains tearing edges Fracture surface of AZ31 after superplastic deformation at 300 C and strain rate of /s 30

31 Part II Mechanical properties characterization of AZ31 refining grains tearing edges intergranular cracking a b transgranular cracking Surface near fracture tip of AZ31 after superplastic deformation at 300 C and strain rate of /s 31

32 Part II Mechanical properties characterization of AZ31 GBS (n=2) and Wedge cracking m=0.48 Wedge cracking at triple grain boundary Surface microstructure of AZ31 after superplastic deformation at 300 C and strain rate of /s 32

33 Summary-Part I Microstructure characterization of AZ31 Grain growth as function of temperature Fine grain size heat treated up till 400 C Abnormal grain growth heat treated at 450 C Rapid grain growth heat treated above 500 C Particle dissolution as function of temperature Al 11 Mn 4 dissolved rapidly at 500 C Second phase particles show pinning effect 33

34 Summary-Part II Mechanical properties characterization of AZ31 Superplastic behaviour observed Temperature: 300 C higher temperature Strain rate: /s lower strain rate Elongation-to-failure: %, m=0.48 Dynamic Recrystallization mechanism observed 200 C: strain rate V f % DRX dominate 300 C: strain rate V f % Time Grain growth dominate GBS deformation mechanism accommodated with dislocation creep observed 34

35 Future Work Further investigation of superplastic behaviour Temperature: 400 C Elongation-to-failure test: /s ~ /s m value measurement: /s ~ /s Deformation mechanism Exploring grain refinement of AZ31 (<10μm) by asymmetrical rolling Static recrystallization Dynamic recrystallization Effect to microstructure, texture 35

36 Acknowlegement Supervisor Prof David S. Wilkinson GM for providing materials MSE Dept. Support Prof Tony Petric, Mr. Doug Culley, Ms. Connie Barry Researchers and Grad. Students Dr. Jidong Kang, Dr. Xiaohua Hu, Dr. Xiang Wang, Kendal Dunnett, Mechanical Dept. Support Dr. Michael Bruhis, Dr. Xiaochun Zeng BIMR and CCEM Support Mr. Jim Garret, Mr. Chris Butcher, Dr. Steve Koprich, Mr. Wenhe Gong 36

37 Thank You! 37