Development of Heat Treatable
Magnesium-Lithium Alloys

Asian Forum on Light Metals
ALMA Forum 2018 ,9-10 November, 2018, Tokyo, Japan
Cheng-Yu Wang1, Jian-Yih Wang2, Po-Ding Wu2
1, Department of Materials Science and Engineering, National Chiao Tung University, Taiwan
2, Department of Materials Science and Engineering, National Dong Hwa University, Taiwan

Contents• Background• Motivation• Experimental• Results and Discussion• Summary1

Background• Magnesium-Lithium Alloy– Advantages• Low density, light weight• Damping capacity• Thermal conductivity• Formability, machinability• Electron-magnetic interference shielding• Recyclable– Disadvantage• Low recrystallization temperature leads to quick grain growth.
• Compared to other light alloys, Mg alloy has low mechanicalstrength, which makes it improper for structural application
purpose.
2

Mg Alloy Strengthening Mechanisms

Mg-Li-Si

Mg-Li-Al

Mg-Li-Ag
Mg-Li-Ca
Mg-Li-Zn

Mg-Li-Y

Compared to commercial magnesium alloy LZ91 (Mg-9wt%Li0.7wt%Zn), three strengthening methods may be applicable toMg-Li-Al alloy
3S. Oki, M. Kamida, M. Tsutikawa, 䓊⬠忋㓄䞼䨞㍐忚ḳ㤕ㆸ㝄⟙⏲㚠, ⸛ㆸ 20 ⸜⹎, P507-515.

Work Hardening vs. Solid Solution
Strengthening
Under aging? Peak?

Over aging?

The variation of the hardness with the room temperature aging time of 80% cold rolled
and solid-solution treated LAZ1010 specimens.
4S.K. Wu, Y.H. Li, K.T. Chien, C. Chien, C.S. Yang, X-ray diffraction studies on cold-rolled/solid–solution treated α+β Mg–10.2Li–1.2Al–0.4Zn alloy,
Journal of Alloys and Compounds 563 (2013) 234–241

Grain Refining Strengthening

Yield strength (σy) and mean grain size obtained from the assqueeze Mg-15Li-xAl (x = 0, 1, 3, and 5 wt%) alloys

5Park, G. H., et al. (2016). "Development of lightweight Mg Li Al alloys with high specific strength." Journal of Alloys and Compounds 680: 116-120.

Motivation• Mg - Li - Al - Zn alloy– Try to elucidate the best strengthening method/mechanism– Different heat treatments were applied• Parameters– As-rolled alloy– Different cooling rates after heat treatment (solid solution)– Aging after water quench (WQ)
– WQ after two stage heating (TSH)– Different heat treatments after WQ and TSH

6Experimental
As-rolledMgLiAl
LAZ941alloy(single stage)
Heating荛Two stage
heating (TSH)荜Waterquench(WQ)

Annealing(FC)Annealing(AC)荞荝with/without
aging

OM
10XRD

SEM

WQ

Tensiletest荟Reheating

HardnesstestMicrostructures of as-rolled MgLiAl alloy
OM

SEM

αη

β11Hardness

HV

α Phase (Mg-rich)66

β Phase (Li-rich)56

η Phase (AlLi)

77

Element
Mg K
Al K
Totals

Weight%87.16
12.84
100.00Element
Mg K
Al K
Totals

Weight%97.70
2.30
100.00Element
Mg K
Al K
Totals

Weight%96.70
3.30
100.00XRD of as-rolled MgLiAl alloy

12Experimental
As-rolledMgLiAl alloy(single stage)
Heating荛

Two stage
heating (TSH)荜Waterquench(WQ)

Annealing(FC)

Annealing(AC)荞荝with/without
aging

OM
13

XRD

SEM

WQ

Tensiletest荟ReheatingHardness
testEffect of cooling rate on microstructure
Water quench (WQ)

Air cooling (AC)Furnace cooling (FC)

(a)

(b)

(c)

(d)

(e)

(f)

2 phases in WQ (fast cooling)
Cooling from 400℃ x 30min
14

3 phases in AC and FC (slow cooling)(a-c) is lower magnification and higher magnification (d-f).

Effect of cooling rate on XRD spectra

15

Better mechanical strength with more η from slow cooling rate?

Effect of cooling rate on mechanical property

16

solid solution vs. precipitation?

Experimental
As-rolledMgLiAl alloy(single stage)
Heating荛

Two stage
heating (TSH)荜Waterquench(WQ)

Annealing(FC)

Annealing(AC)荞荝with/without
aging

OM
17

XRD

SEM

WQ

Tensiletest荟ReheatingHardness
testNatural aging of water quenched MgLiAlalloy
aged at room temperature for (a) 0hr, (b) 50hrs and (c)100hrs.

(a)

18(b)

(c)

Artificial aging (100濎) of water quenched
MgLiAl alloy
aged at 100濎 for (a) 0hr, (b) 50hrs and (c) 100hrs.

(b)

(a)

(c)

(HV)

0hr

50hrs

100hrs

α Phase787472β Phase14576

77

η phaseη precipitation softening19

OM of MgLiAl alloy after WQ from different
temperatures(a) WQ from 400濎(b) WQ from 450濎

(b)

(a)

(HV)

400℃

450℃

500℃

α Phase787576

β Phase

145

142

144

• Hardness is kept
• Grain growth @ 500濎• Due to stored energy– How to remove?
20(c) WQ from 500濎(c)

Experimental
As-rolled
MgLiAl alloy(single stage)
Heating荛

Two stage
heating (TSH)荜Waterquench(WQ)

Annealing(FC)

Annealing(AC)荞荝
with/without
aging

OM
21

XRD

SEM

WQ

Tensiletest荟Reheating

Hardnesstest

OM of MgLiAl alloy after 2-stage heatingand then quench
(a)
(b)

200μm

Higher magnification

Heating to 100℃ for 1h and then heating to 400℃ x 30min

22

Nanoindentation of MgLiAl alloy
after 2-stage heating and then quenchβαWidmanstätten pattern?
23

Young’s modulus
α phase53 Ʋ 5 GPa

β phase46 Ʋ 6 GPaXRD of MgLiAl alloy after 2-stage heatingand then quench

25

Natural aging of 2-stage heating andquenched MgLiAl alloy

26

Mechanical property of natural aging of2-stage heating and quenched MgLiAl alloy

27

Fracture surfaces of WQ and 2-stage heating
and quenched MgLiAl alloy(a) WQ(a)(b)

Cleavage-like (brittle)

28(b) TSH(WQ)

Dimple-like (ductile)

OM of MgLiAl alloy WQ from different
temperatures in 2-stage heating(a)

450濎

(b)

500濎

(c)

200μm

200μm

550濎

Massive alpha phase ʽʿ almost no alpha phase(e)(d)(f)

40μm

29200μm

40μm

(HV)

450℃

500℃

550℃

α Phase76

78-β Phase

141

145

148

40μm

Widmanstätten pattern?

WIDMANSTӒTTEN structures(A) Low-mobility semicoherent interfaces.
(B) High-mobility incoherent interfaces

ββSlow cooling30

αRapid cooling

Porter, D. A., et al. (2009). Phase Transformations in Metals and Alloys, (Revised Reprint), CRC press.

Experimental
As-rolled
MgLiAl alloy(single stage)
Heating荛

Two stageheating (TSH)荜Waterquench(WQ)

Annealing(FC)

Annealing(AC)荞荝with/withoutaging

OM
31

XRD

SEM

WQ

Tensiletest荟ReheatingHardnesstest

Mechanical property of reheated MgLiAl alloy
after 2-stage heating and quench

32Summary• Compared to commercial LZ91, the MgLiAl alloy– has doubled UTS (183 → 357 MPa)
– with reasonable elongation (8%)• Aging softening– Solid solution strengthening is more prominent than
precipitation strengthening.

• Under higher solid solution temperature– Water quenching leads to Widmanstätten pattern.

• TSH + WQ vs. WQ– The former has slightly reduced strength, but improvedelongation.

• Reheating after TSH + WQ
– The elongation can be greatly improved with strength sacrificed.
33

National Chiao Tung University

National Dong Hwa University

34