Liquid Metal Embrittlement

2. Liquid Metal Embrittlement Dominique Gorse (presented by Vassilis Pontikis) CEA-IRAMIS / CNRS, Laboratoire des Solides Irradiés 91191 Gif-sur-Yvette, FRANCE

3. Outline • Environment & Mechanical behavior • What is LME ? • Surfaces, Liquids & Wetting • Experimental facts • Empirical criteria • Modeling • Conclusive remarks

4. 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

5. air 60 s H2O, dried, air H2O Joffe effect, irradiated KCl, Rebinder et al., 1944

6. Steel 316L + Hg, Medina-Almazan et al. (2005)

7. SL GB /2 SL S1L GB S2L LV V L  SV SL S bad wetting >90° good wetting<90° Grain boundaries 2 gSP cos/2=gGB (SV, LV, SL) mechanical equilibrium chemical equilibrium gS1P+gS2P=gGB gSV- gSL= gLV cosq (Young)

8. 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.

9. LME: Single crystals or polycrystals ? • 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). Al-T4/2024 Hardrath & McEvily (1961) The liquid metal may invade grain & interphase boundaries

10. Macroscopic features - I Ratio of fracture stresses of 4145 steel with & without Pb(0.3%)

11. Macroscopic features - II Room T Steel 4145 Mostovoy & Breyer, (1968) 400 < T < 620 °F

12. Polycrystalline Al LME&ChemistryThe effect of impurities Westwood et al., (1971)

13. LMEDelayed failure 2024-T4 Al alloy + Hg amalgam Static fatigue Rostoker et al. (1960)

14. Ludwig et al. (2006) LME and Grain Boundaries: Al/Ga

15. LME and Grain Boundaries

16. 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”

17. From consensus: prerequisites & empirical criteria of occurrence • 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 However factors determining which liquid metal will embrittle which solid metal still remain unclear !

18. Is modeling understanding ? Experiments LME ? Analytic theory Numerical approaches An heuristic approach (sometimes a random walk)

19. Modeling LME & Crack propagation: Elasticity Adsorption of the liquid at the crack tip decreases s and thus c

20. LME & Crack propagation: ....adding plasticity Griffith’s model is elastic (Orowan, Stoloff & Johnson, …) But… However… Small changes in s large influence on eff

21. “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) BUT … •  No mechanisms •  No microscopic description of the SL interface •  No kinetics •  No understanding of plasticity at the crack tip

22. LME & Cracks &DuctileBrittleTransitionTemp Old & Trevena (1979)

23. 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 gs is necessary but not sufficient

24. AIRC Adsorption Induced Reduction in Cohesion Slip plane  - Bond breaking - Crack length 2a   Stress at the tip  plane -x0  Slip plane AIRC: the atoms of the liquid (black) reduce/increase the bond strength

25. AIRC: present situation •   AIRC is widely accepted •   Consistent with several experimental findings (Zn-Hg, Zn-Ga,        hydrogen, …) •   Explains impurity action • 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 BUT ...

26. AIRC variants & improvements Wetting effects ? Rabkin et al, (19..)

27. Modeling:Crystal-Liquid interface Geysermans et al. (2005)

28. 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

29. Origins of Illustrations • http://www.tech.plym.ac.uk/sme/Interactive_Resources/tutorials/FailureAnalysis/Fractography/Fractography_Resource4.htm