design of high strength wrought magnesium...

Joseph Robson!School of Materials!

University of Manchester UK!joseph.robson@manchester.ac.uk!

Design of High Strength Wrought Magnesium Alloys!

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!

!•  Pure Al and pure Mg have similar strength (AlUTS=80MPa,

MgUTS=90MPa)!•  Poor age hardening of Mg limits maximum strength!

–  Elektron-675: 15% strength increase on ageing (F to T5)!–  AA7449: >500% strength increase on ageing (F to T6)!

Mg! Al!

Strength

Mg! Al!

Density

Mg! Al!

Specific strength

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)!

Fundamentals of Deformation •  Magnesium has a hexagonal close packed (hcp)

crystal structure!•  This has important implications for its strength and

deformation!Slip mode Relative CRSS

(at RT)

Basal 1

Prismatic 40

Pyramidal 50

c-axis!

<1120>!_!

_!<1123>!

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!

Basal plane!

Prismatic plane!

1st order pyramidal plane!

2nd order pyramidal plane!

Prismatic

Slip modes providing deformation in <a> direction only!

Slip mode providing deformation in <c+a> direction, but very high CRSS!

Twinning mode providing deformation in <c+a>, low CRSS !

•  Problem in Mg is accommodating <c> axis deformation!

•  Twinning produces <c> deformation BUT!–  Inherently

asymmetric!–  Accommodates

limited and fixed (low) strain!

•  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!

Effect of Temperature (T)

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>!

prism <a>

<c+a> basal <a>

•  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!

Deformation of Polycrystals

[Hutchinson and Barnett, Scripta Mater.]

CR

SS

Single x-tal pure Mg

Polycrystalline alloy

x40

x1.5

AZ31 ECAP >25% elongation

•  Limited deformation systems typically leads to strong textures during deformation to produce wrought alloys!

•  Most wrought Mg alloys show basal texture!

•  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!

Importance of Texture

c-axis

RD

TD

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!

σy

d-0.5!

Mg!

Al!

k = 0.35 MPam-0.5!

k = 0.14 MPam-0.5!

•  Age hardenable Mg alloys typically derive~50% of strength from Hall-Petch (grain size) strengthening. !

σy

d-0.5!

Mg non-basal slip!

Mg twinning!

Slip easier than twinning (small grains)!

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!

Akhtar and Teghtsoonian, Acta Metall., 17, p. 1351-1356, 1969 (single crystal study)

More Zn, prismatic <a> CRSS reduced

Age hardening: Current limtations!

0.1µm

WE43 Mg-Y-RE σy~200MPa

AA2198 Al-Cu-Mg-Li σy~500MPa

•  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

Effect of Precipitate Shape and Habit on Strengthening - 1!

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!

•  Summary: prismatic plate shaped precipitates are best!

Effect of Precipitate Shape and Habit on Strengthening - 3!

Precipitation Strengthening: Twinning!

Bowing stress (Orowan)

Measured ΔCRSS

CRSS gap

•  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!

Precipitate/Twin interactions

Precipitate/Twin: Schematic

Twin

Twin

Twin

Twin

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!

Contributions to strengthening against twinning

M. R. Barnett

~25%

~75%

•  Unsheared precipitate generates a misfit leading to a back-stress 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!

Precipitate Induced Backstress

(a)! (b)!

(c)! (d)!

Misfit Stresses: Plastic Relaxation (a)! (b)!

(c)! (d)!

Basal plates!

c-axis rods! Prismatic plates!

x

y

x

y z

M. A. Gharghouri, G. C. Weatherly, J. D. Embury, Phil. Mag. A, 78 (1998) pp. 1137-1149

Precipitation: Asymmetry •  Use strengthening models for slip (Orowan) and twinning

(backstress) to predict effect of precipitate shape/habit on asymmetry!

Alloy (ppt) AR (before age) AR (after age)

AZ91 (basal plate) 0.65 0.95

Z5 (rods) 061 0.53

Measured asymmetry ratios (AR) before and after precipitation (basal plates vs c-axis rods)!

Model predictions of effect of precipitate shape/habit on asymmetry!

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

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 σy=230MPa)!–  Mg-Zn-Ca (+ Ag): σy=325MPa, low asymmetry!–  Mg-Sn-Al-Zr (+ Na): σy~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!

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)!

Twin

Basal

Prismatic

Mg-Sn-Zn (+MA)

No single phase region

0.45

0.50

0.60

0.70

0 = isotropic

σy>230MPa

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!

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!

{10-12} twin in magnesium