Origins of Strength and Ductility in Mg Y Alloys. Xiaohui Jia ( Supervisor: Dr.Marek Niewczas ) 701 Graduate Seminar 18 th December,2012

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1 Origins of Strength and Ductility in Mg Y Alloys Xiaohui Jia ( Supervisor: Dr.Marek Niewczas ) 71 Graduate Seminar 18 th December,212

2 Outline 2 Introduction Background Slip systems and twin types in Magnesium alloys Strengthening mechanism Strain rate sensitivity Plastic instability Experimental procedure Preliminary results Conclusions & Future works

3 Introduction 3 Engine Cradle Mg alloys exhibit very high strength/density ratio which makes Mg the potential structure material in aerospace, aircraft and automotive industries. Two deficiencies in mechanical behavior: poor room temperature ductility relatively low strength Mg RE alloys have been given tremendous attention due to their higher specific strength and improved ductility.

4 Introduction 4 39 Y solid solution hardener recrystallization texture modifier Atomic radius 18 pm Density gcm 3 Electron configuration 4d 1 5s 2 2,8,18,9,2 Young s modulus 63.5 GPa Investigations of yttrium already created ground for development of a number of commercial alloys with high strength properties. It has been not very clear that how yttrium effects the mechanical properties of the magnesium alloys. Consequently, it is significant to investigate the effect of yttrium on mechanical properties and understand the origin of strength and ductility.

5 Slip systems and twin types in Magnesium alloys 5 Prism <a> slip c 2 nd Pyramidal <c+a> slip c a 3 a 3 {1 } [11 ] a a 2 Basal <a> slip (1) [11 ] a 2 a 1 {11 2} CRSS [MPa] {1 } {1 } Temperature [K] (F.Hiura (21))

6 Strengthening mechanism 6 Several strengthening mechanisms in polycrystalline materials: τ y = τ + τ gb + τ ss +(τ D2 + τ ppt2 ) 1/2 τ : intrinsic strengthening related to atomic bond nature τ gb : grain boundary strengthening τ gb =k*d 1/2 τ ss :solid solution strengthening τ ss =k*c n (n = 2/3 (Labusch) ) τ D : dislocation strengthening τ D = αgb ( α=.3 ( Ashby, 197) ) τ ppt : precipitation strengthening τ ppt =Gb/2R In order to focus on the effects of solid solution strengthening, the other contributions must be controlled as much as possible.

7 Strain rate sensitivity ( SRS ) 7 σ = C ε,t (1) T,Σ = m (2) ε : strain rate C : a constant m : engineering strain rate sensitivity T,Σ = [(σ σ )m d + σ s m s + σ t m t ] (3) σ : yield stress of the material d : dislocation s: solution t : thermally activatable species The experimental data of strain rate sensitivity measurements are analyzed by plotting as a function of (σ σ ), which is called Haasen plot. (B.J.Diak et al. Prog.Mat.Sci.43(1998) )

8 Strain rate sensitivity Haasen plot 8 T,Σ More thermal Strain rate sensitivity ( SRS ) The slope of the Hassen plot can be expanded as: Slope = T,Σ = m m = Slope * T T,Σ More athermal Forest dislocation only σ σ Schematics of the Haasen plot for metals (Diak et al.,1998) Y axis intercept A measure of the amount of thermally activatable components in the matrix. Positive : Thermal components Negative : Athermal components Origin : Dislocation dislocation interactions

9 Plastic instability 9 Adiabatic instability low temperature low thermal conductivity of the materials very high strain rate time for the thermal diffusivity is short Z. S. Basinski ( 1957) Portevin Le Chatelier ( PLC ) instability dynamic strain aging interaction between the moving dislocations and diffusing solute atoms. H.Halim (27)

10 Experimental procedure 1 Casting and Homogenization Casting: induction furnace (133K), argon atmosphere. Homogenization : 793K, 24h in air. Thermo mechanical processing Anneal at 753K for 1h, 1% thickness reduction by cold rolling procedure; machine compression and tension samples. Composition Grain Size (μm) Std.Dev. Mg.3at.%Y Mg.55at.%Y Rolling direction Mg.82at.%Y Mg 1.13at.%Y Mg 1.3at.%Y 75 1 Deformation axis

11 Experimental procedure 11 Mechanical testing Compressive and tensile testing at 298K, 78K and 4.2K to determine mechanical properties. Transverse section ND (RD for sheet) Texture measurements Texture was evaluated by XRD using a Bruker D8 diffractometer with a Co Kα source.

12 Flow stress ( Tension at 298K, 78K and 4.2K ) 12 True Stress (MPa) T=298K 1 8 Mg-.3at.% Y Mg-.55at.% Y 6 Mg-.82at.% Y 4 Mg-1.13at.% Y Mg-1.3at.% Y 2 Mg True Strain True Stress (MPa) True Stress (MPa) True Strain True Stress (MPa) Mg-.3at.% Y Mg-.55at.% Y 1 Mg-.82at.% Y Mg-1.13at.% Y 5 Mg-1.3at.% Y Mg True Strain 15 Mg-.3at.% Y Mg-.55at.% Y 1 Mg-.82at.% Y Mg-1.13at.% Y 5 Mg-1.3at.% Y Mg T=4.2K True Strain T=78K

13 Flow stress ( Tension at 298K, 78K and 4.2K ) 13 2 T=298K 25 T=78K True Stress (MPa) =.8 =.5 =.3 =.1 =.1 =.3 =.5 =.8 True Stress (MPa) =.3 =.1 =.1 = Solute Content, at% Solute Content, at% True Stress (MPa) =.7 =.5 =.3 =.1 =.3 =.5 =.7 5 =.1 T=4.2K Solute Content, at%

14 Flow stress ( Compression at 298K and 78K ) 14 True Stress (MPa) 3 Mg-.3at.% Y Mg-.55at.% Y Mg-.82at.% Y 25 Mg-1.13at.% Y Mg-1.3at.% Y 2 Mg T=298K True Stress (MPa) T=78K 2 Mg-.3at.% Y 15 Mg-.55at.% Y Mg-.82at.% Y 1 Mg-1.13at.% Y 5 Mg-1.3at.% Y Mg True Strain True Strain

15 Flow stress asymmetry Pure Mg Mg-.55 at.% Y True Stress (MPa) T=298K Tension Compression True Stress (MPa) T=298K Tension Compression True Strain True Strain

16 Yield stress 16 Tensile Yield Strength y (MPa) K 78K 298K 298K 78K 4.2K Compression Yield Strength y (MPa) K 78K 78K 298K Solute Content, at% Solute Content, at% τ y = τ + τ gb + τ ss +(τ D2 + τ ppt2 ) 1/2

17 Solid solution strengthening 17 s, y (MPa) T=298K s, y (MPa) C 2/3 (at.%) 2/ T=78K C 2/3 (at.%) 2/3 Δσ s = σ y Δσ g Δσ g = σ + kd 1/2 k=3 Mpa μm 1/2 σ = 11MPa (Caceres et al. Journal of light metals (21)) s, y (MPa) T=4.2K C 2/3 (at.%) 2/3 Solid solution strengthening rate Temperature dσ/dc 2/3 298K K K 2586

18 Solid solution strengthening 18 s (MPa) Mg-Y C 2/3 (at.%) 2/3 T=298K Mg-Al Solute Misfits Strengthening rate δ η ε dσ/dc 2/3 Y +11% Gd +11% Zn 17% Al 14% Size misfit Modulus misfit σ ys = σ y + Z L G( δ + β ) 4/3 c 2/3 ε = ( δ + β ) η= Valence difference :The bond energy between Mg and Y is always stronger than that of Mg and Al. (K.Chen et al.metall.mater.trans, (29 )) Solid solution strengthening parameters for Y, Gd, Zn and Al atoms in Mg Caceres et al. Journal of light metals (21) Caceres et al. Phys.stat.sol (22) Gao et al. Journal of alloys and compounds(29)

19 Strain rate sensitivity ( Tension 298K ) 19 /T ln. (MPa/K) Strain Rate Sensitivity Parameter m Mg-.3at.% Y Mg-.55at.% Y Mg-.82at.% Y Mg-1.13at.% Y Mg-1.3at.% Y Strain rate jump down T=298K Tension Effective Stress (MPa) T=298K Solute Concentration (at.%) Stress Drop Stress Rise /T ln. (MPa/K) Y-intercept of Haasen Plot T=298K Strain rate jump up Mg-.3at.% Y.25 Mg-.55at.% Y Mg-.82at.% Y Mg-1.13at.% Y Mg-1.3at.% Y Tension Effective Stress (MPa) T=298K.1 Stress Drop Stress Rise Solute Concentration (at.%)

20 Strain rate sensitivity ( Tension 78K ) 2 /T ln. (MPa/K) Mg-.3at.% Y Mg-.55at.% Y Mg-.82at.% Y Mg-1.13at.% Y Mg-1.3at.% Y Strain rate jump down T=78K /T ln. (MPa/K) Mg-.3at.% Y Mg-.55at.% Y Mg-.82at.% Y Mg-1.13at.% Y Mg-1.3at.% Y Strain rate jump up T=78K Strain Rate Sensitivity Parameter m Tension T=78K Effective Stress (MPa) Stress Drop Stress Rise Solute Concentration (at.%) Y-intercept of Haasen Plot Tension Effective Stress (MPa) T=78K Stress Drop Stress Rise Solute Concentration (at.%)

21 Texture measurements ND(RD for sheet) Annealed ( 773K \15 min ) 21 ND Pure Mg Mg.3at.%Y Mg.55at.%Y Mg.82at.%Y Mg 1.13at.%Y Mg 1.3at.%Y Compression (Maximum Strain, T=298K) ND Pure Mg Mg.3at.%Y Mg.55at.%Y Mg.82at.%Y Mg 1.13at.%Y Compression (Maximum Strain, T=78K) Mg 1.3at.%Y Pure Mg Mg.3at.%Y Mg.55at.%Y Mg.82at.%Y Mg 1.13at.%Y Mg 1.3at.%Y

22 Conclusions and Future works 22 After correcting for grain size strengthening effects, the yield strength increases linearly with c n, where c is the atom concentration and n is about 2/3.Comparing with the effect of Al, solid solution strengthening of Y is much higher. In addition to the size and/or modulus misfits between solute and solvent atoms, the valence effect is suggested to be possibly responsible for the enhanced solid solution strengthening of Y in Mg. At room temperature, the SRS decreases with the increase of Y content from positive to negative. The negative SRS may be attributed to the interaction between solute atmosphere and dislocations. At 78K, the SRS and Y intercept increases with the increase of Y content, suggesting the solute atom dislocation interaction dominates the plastic deformation. Texture measurements reveal that the initial texture with c axis perpendicular to compression axis (rolling direction) transferred to the twin texture with c axis aligned parallel to compression axis after compression. Compression test ( 4.2 K and strain rate sensitivity) Microstructure (TEM and EBSD analysis) Texture evolution ( XRD )

23 Acknowledgements 23 Dr. Niewczas Anna Kula Jim Britten Jim Garrett Doug Culley Ed McCaffery Xiaogang Li

24 24 Thank your for your attendance and attentiveness! Questions?