matgen-iv.2 february 2 - 7, 2009, kiruna, sweden
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MATGEN-IV.2 February 2 - 7, 2009, Kiruna, Sweden
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MATGEN-IV.2 February 2 - 7, 2009, Kiruna, Sweden
Liquid Metal Embrittlement
Dominique Gorse (presented by Vassilis Pontikis)
CEA-IRAMIS / CNRS, Laboratoire des Solides Irradiés91191 Gif-sur-Yvette, FRANCE
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MATGEN-IV.2 February 2 - 7, 2009, Kiruna, Sweden
Outline
• Environment & Mechanical behavior• What is LME ?• Surfaces, Liquids & Wetting• Experimental facts• Empirical criteria• Modeling• Conclusive remarks
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MATGEN-IV.2 February 2 - 7, 2009, Kiruna, Sweden
Surfaces and mechanical behavior
Surfaces control the nucleation of cracks and dislocations and the annihilation of dislocations as well. However, bulk phenomena are predominent in plasticity.
“If dislocations are nucleated principally at the surface, we may expect that irradiation with alpha particles, which penetrate only a short distance, will affect the mechanical properties. The evidence is contradictory”
F. R. N. Nabbarro in “Theory of Crystal Dislocations ”, Clarendon, 1967 P. 281
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MATGEN-IV.2 February 2 - 7, 2009, Kiruna, Sweden
Joffe effect, irradiated KCl, Rebinder et al., 1944
air
60 s H2O, dried, air
H2O
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MATGEN-IV.2 February 2 - 7, 2009, Kiruna, Sweden
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Steel 316L + Hg, Medina-Almazan et al. (2005)
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MATGEN-IV.2 February 2 - 7, 2009, Kiruna, Sweden
good wetting
<90°
bad wetting
>90°
LV
SV SL
LV
S
GB /2
SLSL
SLSL
GB
S1LS1L
S2LS2L
S1P + S2P = GB
2 SP cos/2 = GB
(SV, LV, SL)
mechanical equilibrium
chemical equilibrium
SV - SL = LV cos(Young)
Grain boundaries
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MATGEN-IV.2 February 2 - 7, 2009, Kiruna, Sweden
Liquid Metal Embrittlement: what is it ?
Ensemble of phenomena, including wetting, whereby materials suffer a marked decrease in their ability to deform (loss of ductility) or in their ability to absorb energy during fracture (loss of toughness), with little change in other mechanical properties.
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MATGEN-IV.2 February 2 - 7, 2009, Kiruna, Sweden
• Cd: ductile in inert environment is brittle in presence of Ga at room T.
• Zn: brittle at ≈150 °C in presence of Hg or Ga
(Rozhanski et al.,1957, Goryunov et al. 1978).
• Al: can be cleaved in presence of Bi (Lynch, 1985).
LME: Single crystals or polycrystals ?
Al-T4/2024 Hardrath & McEvily (1961)
The liquid metal may invade grain & interphase
boundaries
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MATGEN-IV.2 February 2 - 7, 2009, Kiruna, Sweden
Macroscopic features - I
Ratio of fracturestresses of 4145 steel with & without Pb(0.3%)
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MATGEN-IV.2 February 2 - 7, 2009, Kiruna, Sweden
Macroscopic features - II
Room T
400 < T < 620 °F
Steel 4145Mostovoy & Breyer, (1968)
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MATGEN-IV.2 February 2 - 7, 2009, Kiruna, Sweden
LME&Chemistry The effect of impurities
Westwood et al., (1971)
Polycrystalline Al
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MATGEN-IV.2 February 2 - 7, 2009, Kiruna, Sweden
LMEDelayed failure
2024-T4 Al alloy + Hg amalgam
Rostoker et al. (1960)
Static fatigue
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MATGEN-IV.2 February 2 - 7, 2009, Kiruna, Sweden
LME and Grain Boundaries: Al/Ga
Ludwig et al. (2006)
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MATGEN-IV.2 February 2 - 7, 2009, Kiruna, Sweden
LME and Grain Boundaries
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MATGEN-IV.2 February 2 - 7, 2009, Kiruna, Sweden
LME: consensus about facts
• Instantaneous failure under applied or residual stresses
• Delayed failure at a static stress level below the tensile strength
• Microstructure is important BUT LM’s embrittle amorphous alloys too ! Ashok et al. (1981)
• Plasticity is often present• Stress independent (?) grain boundary
penetration• High temperature “corrosion”
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MATGEN-IV.2 February 2 - 7, 2009, Kiruna, Sweden
From consensus: prerequisites & empirical criteria of occurrence
However factors determining which liquid metal will embrittle which solid metal still remain unclear
!
• Presence of an external stress• A pre-existing crack or plasticity (at least limited) & presence of obstacles to dislocation motion e.g. grain boundaries, twins, precipitates• Adsorption of the active species at the obstacles and at the tip(s) of propagating crack(s)• Limited mutual solubility of the liquid and the solid
• Little or no tendency of stable high Tm compounds
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MATGEN-IV.2 February 2 - 7, 2009, Kiruna, Sweden
Is modeling understanding ?
An heuristic approach (sometimes a random walk)
Experiments
Analytic theory Numerical approaches
LME ?
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MATGEN-IV.2 February 2 - 7, 2009, Kiruna, Sweden
Modeling LME & Crack propagation: Elasticity
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dU =πadaσ 2
E
G =dUda
=πaσ 2
EGc =2s
σ c =2γ sE
π a(Griffith −1921)
Adsorption of the liquid at the crack tipdecreases s and thus σc
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MATGEN-IV.2 February 2 - 7, 2009, Kiruna, Sweden
LME & Crack propagation: ....adding plasticity
Griffith’s model is elastic (Orowan, Stoloff & Johnson, …)
σ c =2γ eff E
π a(Orowan −1950)
eff = γ p + γ s (Al : γ p ≈ 4200γ s , Kargol et al., 1977)But…
eff = Aγ s , A ≈ σ 0 / σ y ≈ 100 for steel (Gilman, 1960)However…
Small changes in s large influence on eff
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MATGEN-IV.2 February 2 - 7, 2009, Kiruna, Sweden
“Thermodynamic” model: Successes & limitations
• Cu/Pb-melt Eborall et al. (1956)• Al bicrystals/HgGa solution Kargoll et al. (1977)• Al 6061/inclusions Bi, Cd, Pb Roth et al. (1980, 1982)
• No mechanisms• No microscopic description of the SL interface• No kinetics• No understanding of plasticity at the crack tip
BUT …
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MATGEN-IV.2 February 2 - 7, 2009, Kiruna, Sweden
LME & Cracks & DuctileBrittleTransitionTemp
System s(J/m2)
Al 1.0
Al-Pb 0.344
Al-Cd 0.298
Al-Bi 0.288
Old & Trevena (1979)
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MATGEN-IV.2 February 2 - 7, 2009, Kiruna, Sweden
Limitations of this approach
• Lack of surface energy data (ab-initio ?)• Physical parameters influencing LME do not
appear in the model (e.g. microstructure)• No insight into the mechanisms (atomistic scale)
Consensus: reduction in s is necessary but not sufficient
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MATGEN-IV.2 February 2 - 7, 2009, Kiruna, Sweden
plane
Slipplane
Slipplane
σ
σ
σm =γ sE
x0
- Bond breaking
σ =2σ a
a
ρ
- Crack length 2a Stress at the tip
-x0 σ prop =γ sE
4a
σ prop(blunted ) =Eγ fracture
4a=
Eγ sξ
4a
AIRC: the atoms of the liquid (black) reduce/increase the bond strength
AIRC Adsorption Induced Reduction in Cohesion
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MATGEN-IV.2 February 2 - 7, 2009, Kiruna, Sweden
• Hardening is absent• Enhanced plasticity at the crack tip and HT
LME are not explained• Transport mechanisms are not considered• No realistic prediction of the crack velocity
AIRC: present situation
• AIRC is widely accepted• Consistent with several experimental findings (Zn-Hg, Zn-Ga, hydrogen, …)• Explains impurity action
BUT ...
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MATGEN-IV.2 February 2 - 7, 2009, Kiruna, Sweden
AIRC variants & improvements
Wetting effects ?
Rabkin et al, (19..)
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MATGEN-IV.2 February 2 - 7, 2009, Kiruna, Sweden
[010]
[001]
Modeling:Crystal-Liquid
interface
Geysermans et al. (2005) %(x) = δ x − xi( )
i∑
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MATGEN-IV.2 February 2 - 7, 2009, Kiruna, Sweden
Conclusive remarks
• LME can affect several structural materials and be a major cause of damage
• Understanding of LME is still limited• No general agreement exists about mechanisms• There is lack of atomic scale description• No predictions can be made i.e. which liquid
metal embrittles which solid metal
• Research on LME could result in decisive advances in the physics & chemistry of interfaces
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MATGEN-IV.2 February 2 - 7, 2009, Kiruna, Sweden
Origins of Illustrations1. http://www.tech.plym.ac.uk/sme/Interactive_Resources/
tutorials/FailureAnalysis/Fractography/Fractography_Resource4.htm