ALMA, 6 August 2016, Kyoto, Japan

Fatigue of ultrafine grained light metals

ALEXEI VINOGRADOVN O RW EG I A N U N I V E RS I T Y O F S C I E N C E A N D T EC H N O LO GY – N T N U, T RO N D H E I M , N O RWAY

1

Acknowledgements
Yuri Estrin, Professor, Monash University, Melbourne, Australia
Kwang Seon Shin, Professor, Seoul National University, Seoul, Korea
Yoshihito Kawamura, Kumamoto University, Japan
Dmitry Orlov, Professor, University of Lund, Sweden2OutlineStrength, ductility and fatigue
Severe plastic deformation techniques for grain refinement
Improvement of fatigue properties of light metals
Fatigue of Mg alloys – effect of severe plastic deformation andgrain refinement3100Fatigue Life and Hysteresis

80

 t   el   pl pl2

  (2 N f )'fc- Coffin-Manson law

 'f el  a(2 N f )b- Basquin law
2EE
 t (2 N f )b   'f (2 N f )c
E
2'f

[H. Mughrabi, Int. J. of Fatigue (1999)]

Stress,  / MPa60
40200-20-40-60-80-100-0.002-0.001

0.000

0.001

0.002

Strain,   pl   el t4

Cyclic and Monotonic Strength

 t  f(2 N f )b   'f (2 N f )c
2E' In order to enhance the fatigue
performance in both high cyclic
and low cyclic regimes we need to
improve both
STRENGTH and DUCTILITY One of these properties is often
improved at the expense of theother SPD, in principle, can give rise toimprovement of both Strength andDuctility

Mg !!

[Y. Estrin, A. Vinogradov, Int. J. Fatigue (2009)]5

Hall‐Petch relationship: clue for strength enhancement
Grain size reduction

 y   0  k y d 1/ 21mDislocation
StrengtheningSevere Plastic Deformation –technique of choice for grain refinement◦ 1972, V.M.Segal, Minsk, USSR ‐ Equal Channel Angular Pressing◦ 1986, V.V. Rybin, Large Plastic Deformations and Fracture of Metals – Mechanisms of grain refinement◦ 1988, R. Z. Valiev et al., Dokl Akad Nauk SSSR

ECAP is much more efficient in straining than rolling andfurthermore the dimensions of the working billet do not alter!

Severe Plastic Deformation Techniques• A broad variety of severe plastic
deformation is available nowadays• Up‐scaling is still an issue, but
technologically available solutions
have been proposed[Y. Estrin, A. Vinogradov, Acta Mater (2011)]

8Enhancement of fatigue of Titanium
1000

Stress Amplitude, a / MPa

Ti
800
G4, ECAP + CW
Ti-6Al-4V ELI
Ti-13Nb-13Zr

600

Ti-6Al-7Nb
Ti, G4 ECAP+CW
Ti, G2 ECAP+CW

400

ECAP Ti, G2 ECAPCG Ti, G2, d=9m[Turner, Roberts 1968]

200103104105106

Number of cycle to failure, Nf107

Ultrafine grained Ti‐6Al‐4V (Eli)Ti64

Pure Ti[L. Saitova et al. , Mat Forum (2008)]

10Mg Alloys

11Fine grain AZ31 Alloy[Zuberova et al., Met Mat Trans A (2007)]

12

Processing of Mg‐alloys
Integrated Extrusion + ECAP

Multi‐axial Isothermal Forging (MIF)350C
ε=2300 ‐400C
ε=3.4[Orlov et al., Acta Mat (2011)][D. Nugmanov et al. , Metals (2015) ]Severe Plastic Deformation of Mg‐Zn‐Zr alloy ZK60.
Integrated Extrusion +ECAP
Orlov et al, Acta Mater. 2011

Weak texture ,70 m grain size (coarse grain)Strong texture,
2 m grain size after IE (fine grain)
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Improvement of monotonic and fatigue
properties

7075‐T7651[Orlov et al., Acta Mater, 2011][Vinogradov et al, Scripta.Mat. (2012)]

15

Fatigue Fractography
Stable crack
Basal slip

RuptureDimples

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Fatigue crack propagation

Key importance oftwinning‐dislocation
Interaction
200Stress,  / MPa

150
10050
0-50

-100-150-200-1.00 -0.75 -0.50 -0.25

0.00

0.25

Total Strain,  / %0.50

0.75

1.00
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Microstructure (Extrusion)

ZX50WZ21Microstructure
ZX40

Mg‐4Zn‐0.56Ca
ECAP (320 C)

Mg‐6Zn‐0.5Zr
MIF (400 C)

ZK60

Spectrum of Tensile Stress‐Strain Curves
ZK60
Effect ofprocessing

MIF

D2 ST
D22 ECAP 2p
D24 ECAP 4p

300

Stress,  MPa

ECAP.
=0.001/s

350250as castas castU4U5xU5z
U6xU6z
ZK60

200

150

10050

005101520253035

Strain, %)4045505560

Stress
Stress Amplitude,
Amplitude,
//MPa
MPa

Effect of warm SPD on Fatigue of ZK60 alloy
200200160160

ZK60
ZK60120120

Standard Saline solution ,pH=7.0T=37 oC

8080

4040

1033
10400ooC
C400o300oC
C-X
300
300ooC
C-Z300as castcastasas extruded,extruded, Constantinescu
Constantinescu etet al.
al. (2009)(2009)
as1044
101055101066101077
10

Number ofof Cycles
Cycles toto Failure,
Failure, N
Nf
Numberf

1088
1022

Properties of biodegradable alloys
300

ZX50

Stress Amplitude,  / MPa

ZX40.
WZ21 =0.001/s
160

250

Stress,  / MPa

200

200

ZK60 WZ21120

ZX50

ZX40

ZX40-B ECAP #U1
ZX40-A 80
as cast
ZX40-B as cast
Mg-98 (XHP)
ZX40-B SPD #U1
WZ21 extruded
ZX50 (XHP) extruded
ZX40-A ECAP #U240Mg-4Zn ECAP #U3150100HP Mg

5000510152025Strain,  (%)

303540103

104105106107

Number of Cycles to Failure, Nf108

Corrosion Properties
Impedance techniqueStandard Saline solution , pH=7.0
T=37 oC
Alloy

Composition

Corrosion rate,mm/year

Surface roughness,Sq, m2

ZK60 Integrated
Extrusion + ECAP

Mg‐6Zn‐0.5Zr

1.01±0.04

3.431

ZK60 MIF 400 C

Mg‐6Zn‐0.5Zr

0.95±0.01

4.665

ZK60 MIF 300 CMg‐6Zn‐0.5Zr

0.73±0.02

3.344

Mg‐4ZnMg‐4Zn0.71±0.01

7.168

ZX40 (A) ECAP

Mg‐4Zn‐0.16Ca

1.36±0.02

3.344

ZX40 (B) ECAP

Mg‐4Zn‐0.56Ca

1.30±0.12

5.084

ZX50 (XHP) Extruded

Mg‐5Zn‐0.25Ca

0.24±0.02

1.245

WZ21 (extruded)

Mg‐1.65Y‐0.85Zn‐0.25Ca

0.85±0.07

3.246

Stress Amplitude,  / MPa

Fatigue and Corrosion Fatigue of
Biodegradable Alloys
200160120

ZX60
ZK60 WZ21ZX50

ZX4080Strong effect ofcorrosion on crack
initiation

40103

104105106107Number of Cycles to Failure, Nf108

Corrosion Fatigue Fractography• Environment‐affected fatigue crack initiates at the surface• Corrosion process under cyclic stress promotes crack initiation even at very low stress• Once initiated, the crack dominates the distribution of local stresses and propagates very
fast regardless of a particular microstructure of the alloy
500 µm26

LPSO Mg97Y2Zn1

Tsushida et al, Mat Sci Forum (2007)][M. Okayasu et al, MSEA (2016)]

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Conclusions and Open Questions
 HCF performance of magnesium alloy ZK60 after processing by integrated
extrusion+ECAP improves significantly to of 160 MPa, which is about three times
larger than the value for the initial material.
 The observed enhancement of endurance and fatigue strength is related to theincrease of ductility and ultimate tensile strength as a result of the integrated
processing and grain refinement.
 Nature of fatigue in Mg alloys
o Asymmetry of the plastic hysteresis ???
o Role of dislocation –twinning interaction ?
o De‐twinning??
o Nature of the Bauschinger effect and the role of microsplasticity ????
o LPSO???
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