Design of High Strength Wrought Magnesium Alloys!

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1 Design of High Strength Wrought Magnesium Alloys! Joseph Robson! School of Materials! University of Manchester UK!

2 ! Strengthening of Mg! Mg has low density (2/3 Al, 1/4 Fe)! Specific strength of commercial strong wrought Mg alloys is less than commercial strong wrought Al alloys! Density Strength Specific strength Mg! Al! Pure Al and pure Mg have similar strength (Al UTS =80MPa, Mg UTS =90MPa)! Poor age hardening of Mg limits maximum strength! Mg! Elektron-675: 15% strength increase on ageing (F to T5)! AA7449: >500% strength increase on ageing (F to T6)! Al! Mg! Al!

3 Strengthening Mg: Issues! Require higher strength Mg alloys with reduced mechanical anisotropy and asymmetry! Scientific issues! Fundamental deformation mechanisms of Mg! Role of texture! Grain size strengthening! Solute strengthening and softening! Strengthening against deformation twinning! Optimizing precipitation for strengthening (slip and twinning)!

4 Fundamentals of Deformation Magnesium has a hexagonal close packed (hcp) crystal structure! This has important implications for its strength and deformation! 1 st order pyramidal plane! c-axis! Prismatic plane! Slip mode Relative CRSS (at RT) Basal 1 Prismatic 40 Pyramidal 50 2 nd order pyramidal plane! _! <1123>! _! <1120>! Basal plane! Slip systems in Mg! Mg only has 2 easily activated independent slip systems at room temperature!! At least 5 independent slip systems are needed for to accommodate general deformation in a single crystal!

5 Slip modes providing deformation in <a> direction only! Prismatic Slip mode providing deformation in <c+a> direction, but very high CRSS! Problem in Mg is accommodating <c> axis deformation! Twinning produces <c> deformation BUT! Inherently asymmetric! Accommodates limited and fixed (low) strain! Twinning mode providing deformation in <c+a>, low CRSS!

6 Effect of Temperature (T) Non-basal slip in Mg usually initated by thermally activated cross-slip from basal slip easier at higher T! CRSS for slip systems converge at higher T: easier activation of non-basal modes and greater ductility! CRSS for twinning relatively insensitive to T! prism <a> One proposed mechanism for<c+a> slip [Yoo]: Cross slip of basal <a> to prismatic <a>. Combination of prismatic <a> with sessile <c> = glissle <c+a>! basal <a> <c+a>

7 Deformation of Polycrystals x1.5 For engineering applications, we use polycrystals - single crystal limitations are relaxed! 5 independent slip systems not necessarily needed (grains can accommodate deformation cooperatively)! Relative CRSS values for different modes converge! Polycrystalline Mg alloys generally show quite good uniaxial ductility! CRSS x40 Single x-tal pure Mg [Hutchinson and Barnett, Scripta Mater.] Polycrystalline alloy AZ31 ECAP >25% elongation

8 Importance of Texture Limited deformation systems typically leads to strong textures during deformation to produce wrought alloys! Most wrought Mg alloys show basal texture! c-axis TD RD Ductility and isotropy can be greatly enhanced by weakening/changing texture! Alloying: RE additions (and others)! Processing: ECAP, asymmetric rolling! Reduce aniostropy but at expense of strength!

9 Grain Size! Strengthening Grain refinement is potent strengthening mechanism in Mg! Grain refinement below critical level can also suppress twinning (good!)! Critical in Mg to obtain and retain fine grain structure! T6 not used for Mg to avoid recrystallization/grain growth! T5 retains fine grains but reduces age hardening potential! σ y σ y Al! Mg! k = 0.14 MPam -0.5! k = 0.35 MPam -0.5! d -0.5! Mg twinning! Mg nonbasal slip! Slip easier than twinning (small grains)! d -0.5! Age hardenable Mg alloys typically derive~50% of strength from Hall- Petch (grain size) strengthening.!

10 Solute Strengthening Effect of solutes in Mg on strengthening is not fully understood! Solutes strengthen against basal slip (expected)! Some solutes can soften non-basal modes (but not always!) by promoting cross-slip from basal plane! First principles methods (Yasi, Trinkle et al.) have shown good potential to predict this behaviour: It may be useful in producing more isotropic and more formable Mg alloys! Solute softening to improve isotropy will reduce strength! More Zn, prismatic <a> CRSS reduced Akhtar and Teghtsoonian, Acta Metall., 17, p , 1969 (single crystal study)

11 Age hardening: Current limtations! 0.1µm AA2198 Al-Cu-Mg-Li σ y ~500MPa WE43 Mg-Y-RE σ y ~200MPa Low nucleation rate of precipitates (compared with Al)! Precipitates poorly oriented to block basal slip! Current Mg alloys poorly designed to optimize ageing! Cannot fully solutionize strengtheing elements! Interaction between grain refiner and strengthener! Solution treatment not possible (excessive grain growth)! Need to strengthen against both slip and twinning

12 Effect of Precipitate Shape and Habit on Strengthening - 1!

13 Effect of Precipitate Shape and Habit on Strengthening - 2! Strengthening against slip controlled by gap between precipitates (Orowan strengthening)! Two factors control gap! Number of particles/area on slip plane! Mean planar diameter on slip plane! Since basal slip is easiest mode in Mg, gap on basal plane most critical!

14 Effect of Precipitate Shape and Habit on Strengthening - 3! Summary: prismatic plate shaped precipitates are best!

15 Precipitation Strengthening: Twinning! Compressive yield of extrusions (basal texture) controlled by twinning! Precipitation can significantly increase yield strength in compression! Precipitates can be strong obstacles for twin growth! CRSS for twin growth increased by 20-50MPa! Increased CRSS for twin growth not well predicted by Orowan! Measured ΔCRSS CRSS gap Bowing stress (Orowan)

16 Precipitate/Twin interactions

17 Precipitate/Twin: Schematic Twin Twin Twin Twin

18 Contributions to Strengthening Contributions to strengthening against twinning due to unsheared precipitates! Orowan stress required to loop twinning dislocations and leave precipitate unsheared! Back-stress arising from strain incompatibility between unsheared precipitate and sheared matrix! ~25% Contributions to strengthening against twinning ~75% M. R. Barnett

19 Precipitate Induced Backstress Unsheared precipitate generates a misfit leading to a backstress when embedded in twin! Back-stress acts against twin growth harder to twin! Basal and prismatic plates produce maximum back-stress! Accommodation tensor (fn of particle shape/orientation)! Strain discontinuity tensor!

20 Misfit Stresses: Plastic Relaxation Basal plates! c-axis rods! Prismatic plates! x x M. A. Gharghouri, G. C. Weatherly, J. D. Embury, Phil. Mag. A, 78 (1998) pp y y z

21 Precipitation: Asymmetry Use strengthening models for slip (Orowan) and twinning (backstress) to predict effect of precipitate shape/habit on asymmetry! Model predictions of effect of precipitate shape/habit on asymmetry! Measured asymmetry ratios (AR) before and after precipitation (basal plates vs c-axis rods)! Alloy (ppt) AR (before age) AR (after age) AZ91 (basal plate) Z5 (rods)

22 Ideal High Strength Mg Alloy Ideal high strength wrought Mg alloy would have:! 1 Weak/random texture to minimize asymmetry/anistropy! 2 Effective precipitates for strengthening against both slip and twinning (prismatic plates, finely distributed)! 3 Fine grain size for strength, ductility, resistance to twinning! 4 Other desirable properties: corrosion resistance, low flamability! 5 Low cost! Requirement Mg-Al-Zn (AZ) Mg-RE (WE, E675 ) Texture N (strong basal) Y (RE-texture) Effective pptn N (basal plates) Y (prismatic plates) Fine grain size Y/N Y/N Other props N Y Low cost Y N

23 Research to Replace REs RE containing Mg alloys have best properties but high cost/security of supply issues! Large research effort to find replacement to RE alloys that match strength and low anisotropy! Most promising systems (e.g. to replace Mg-RE Elektron-675-T5 σ y =230MPa)! Mg-Zn-Ca (+ Ag): σ y =325MPa, low asymmetry! Mg-Sn-Al-Zr (+ Na): σ y ~300MPa! Key ingredients of such an alloy! Element to induce texture weakening (e.g. Ca)! Elements capable of strong age hardening response! Microalloying to promote precipitate nucleation (Ag, Na)! Element to pin grain boundaries retain fine grains after TMP!

24 High Strength Mg Alloy Design! Alec Davis, CDT PhD student Prismatic plates most effective but common only in Mg-RE alloys! Non-RE alloys can form mix of basal plates/c-axis rods! Design alloy using mix of basal plates/c-axis rods to both! Maximize strengthening (inhibit basal slip)! Minimize asymmetry/anisotropy (suppress twinning)! Prismatic Twin Basal Mg-Sn-Zn (+MA) σ y >230MPa No single phase region 0 = isotropic

25 Summary In specific strength limited applications, high strength Mg alloys underperform high strength Al alloys due to relatively poor ageing response! Strengthening of Mg requires different approaches to strengthening of Al! Increased importance of grain size strengthening! Highly anisotropic strengthening from precipitation! Critical role of texture in anisotropy and asymmetry! Understanding contributions to stengthening can lead to design of reduced cost, higher strength Mg alloys!

26 Acknowledgements Matt Barnett (Deakin), Nikki Stanford (Monash)! EPSRC Light Alloys for Sustainable Transport (LATEST-2), CDT in Advanced Metallic Systems! Magnesium Elektron! Thanks for listening!

27 {10-12} twin in magnesium