KIMS Research Activities on Light Metals

KIMS Research Activities on Light Metals

ALMA, 6 August 2016, Kyoto, Japan
Fatigue of ultrafine grained light metals
ALEXEI VINOGRADOV
N 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, Sweden
2
Outline
Strength, 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 and grain refinement 3
100
Fatigue Life and Hysteresis
80
 t   el   pl
 pl
2
  (2 N f )
'
f
c
-
Coffin-Manson law
 'f
 el  a


(2 N f )b
- Basquin law
2
E
E
 t 
(2 N f )b   'f (2 N f )c

E
2
'
f
[H. Mughrabi, Int. J. of Fatigue (1999)]
Stress,  / MPa
60
40
20
0
-20
-40
-60
-80
-100
-0.002
-0.001
0.000
0.001
0.002
Strain, 
  pl   el
 t
4
Cyclic and Monotonic Strength
 t  f
(2 N f )b   'f (2 N f )c

2
E
'
 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 the other
 SPD, in principle, can give rise to improvement of both Strength and Ductility 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/ 2
1m
Dislocation 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 and
furthermore 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 ECAP
CG Ti, G2, d=9m
[Turner, Roberts 1968]
200
103
104
105
106
Number of cycle to failure, Nf
107
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=2
300 ‐
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)
14
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
Rupture
Dimples
16
Fatigue crack propagation
Key importance of twinning‐dislocation
Interaction
200
Stress,  / MPa
150
100
50
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
18
Microstructure (Extrusion)
ZX50
WZ21
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 of
processing
MIF
D2 ST
D22 ECAP 2p
D24 ECAP 4p
300
Stress,  MPa
ECAP
.
=0.001/s
350
250
as cast
as cast
U4
U5x
U5z
U6x
U6z
ZK60
200
150
100
50
0
0
5
10
15
20
25
30
35
Strain, %)
40
45
50
55
60
Stress
Stress Amplitude,
Amplitude,
//MPa
MPa
Effect of warm SPD on Fatigue of ZK60 alloy
200
200
160
160
ZK60
ZK60
120
120
Standard Saline solution , pH=7.0
T=37 oC
80
80
40
40
1033
10
400ooC
C
400
o
300oC
C-X
300
300ooC
C-Z
300
as cast
cast
as
as extruded,
extruded, Constantinescu
Constantinescu et
et al.
al. (2009)
(2009)
as
1044
10
1055
10
1066
10
1077
10
Number of
of Cycles
Cycles to
to Failure,
Failure, N
Nf
Number
f
1088
10
22
300
ZX40
ZX50
WZ21
Stress,  / MPa
250
200
.
Stress Amplitude,  / MPa
Properties of biodegradable alloys
200
160
=0.001/s
ZK60 WZ21
120
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 #U2
40
Mg-4Zn ECAP #U3
150
100
HP Mg
50
0
0
5
10
15
20
25
Strain,  (%)
30
35
40
103
104
105
106
107
Number of Cycles to Failure, Nf
108
Corrosion Properties
Impedance technique
Standard 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 C
Mg‐6Zn‐0.5Zr
0.73±0.02
3.344
Mg‐4Zn
Mg‐4Zn
0.71±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
200
160
120
ZX60
ZK60 WZ21
ZX50
ZX40
80
Strong effect of corrosion on crack initiation
40
103
104
105
106
107
Number of Cycles to Failure, Nf
108
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 µm 26
LPSO Mg97Y2Zn1
Tsushida et al, Mat Sci Forum (2007)]
[M. Okayasu et al, MSEA (2016)]
27
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 the increase 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??? 28