Liquid metal embrittlement

Summary

Liquid metal embrittlement (also known as LME and liquid metal induced embrittlement) is a phenomenon of practical importance, where certain ductile metals experience drastic loss in tensile ductility or undergo brittle fracture when exposed to specific liquid metals. Generally, a tensile stress, either externally applied or internally present, is needed to induce embrittlement. Exceptions to this rule have been observed, as in the case of aluminium in the presence of liquid gallium. This phenomenon has been studied since the beginning of the 20th century. Many of its phenomenological characteristics are known and several mechanisms have been proposed to explain it. The practical significance of liquid metal embrittlement is revealed by the observation that several steels experience ductility losses and cracking during hot-dip galvanizing or during subsequent fabrication. Cracking can occur catastrophically and very high crack growth rates have been measured. Similar metal embrittleme

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Acquisition des notions fondamentales liées à la réactivité des molécules organiques, identification de la structure de petites molécules organiques au moyen des techniques de spectrométrie de masse, spectroscopie infra-rouge et résonance magnétique nucléaire.

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In liquid metal spallation targets such as the MEGAPIE (the megawatt pilot experiment) target and the future ADS (accelerator driven system) spallation targets, which utilize liquid lead-bismuth eutectic (LBE) as the target material, liquid metal embrittlement (LME) effects on the target structural materials are considered as one of the critical issues that determine the lifetime of the targets. However, nowadays, the LBE embrittlement effects are not yet well understood, particularly under irradiation conditions. The aim of this work was: (a) to investigate the effects of LBE embrittlement on the mechanical properties of ferritic-martensitic (FM) steels (candidate materials for applications in spallation targets) in various states, and (b) to study the mechanisms of LBE embrittlement effects on the mechanical properties of FM steels. These two goals have been reached by studying LBE embrittlement effects on the mechanical properties of the T91 steel in various conditions: (i) the standard normalized and tempered condition, (ii) hardened by tempering at lower temperatures, and (iii) after irradiation under high energy proton and spallation neutron mixed spectrum. Mechanical tests such as slow-strain-rate tensile (SSRT) tests and 3-point bending tests have been performed to characterize the mechanical properties of the T91 steel and to determine the effects of LBE embrittlement on these mechanical properties. Microstructural analyses such as transition electron microscopy (TEM) and scanning electron mirocopy (SEM) observations have been conducted to obtain the microstructural information needed for understanding the embrittlement mechanisms. SSRT tests on the T91 steel in the standard metallurgical (SM) state, tempered at 760°C and denoted as HT760, revealed that it may encounter LBE embrittlement in the temperature range of 300 to 425°C. For the first time, a well defined "ductility trough" for a FM steel – LBE system was evidenced from the reduction of the fracture strain (or total elongation) of the T91 FM steel in this temperature range. SEM observations of the fracture surfaces showed that the fracture mode of the specimens suffering embrittlement effects is brittle transgranular cleavage. SSRT tests on the hardened T91 steel, tempered at 500 or 600°C and denoted as HT500 and HT600, respectively, showed more pronounced LBE embrittlement effects. In comparison to that of HT760, the "ductility troughs" of HT600 and HT500 cover a wider temperature range. The LME onset temperature TE is not exactly known but most probably lower than 150°C, which means close to the melting point temperature (125°C) of LBE. The ductility recovery temperature TR of HT500 is higher than that of HT760 and HT600. The results of SSRT tests on the HT600 and HT500 materials demonstrate clearly that the LBE embrittlement effects on the tensile properties of FM steels are strongly enhanced by hardening. Both the influenced temperature range and the degradation level increase with the hardening amount. The fracture mode of HT600 and HT500 materials tested in liquid LBE at T ≤ 450°C showed mainly brittle transgranular cleavage. SSRT tests on the T91 and F82H FM steels irradiated at temperatures in the range of 140-500°C to doses in the range of 0.048 to 20 dpa revealed significant irradiation-induced hardening and embrittlement effects (loss of ductility) as compared to the unirradiated ones. The SSRT tests performed on irradiated specimens in liquid LBE revealed that the specimens undergo a further reduction in ductility. The ductility of some irradiated specimens is reduced to a very low level of about 2-3%. 3-point bending tests on the T91 steel showed that the load-displacement curves of specimens tested in liquid LBE are different from those of specimens tested in Ar. The evaluated J-integral values of specimens tested in liquid LBE are always lower that of the specimens tested in Ar. It means that the crack propagation in a specimen tested in liquid LBE needs less energy as compared to a specimen tested in Ar. These results suggest that the LBE embrittlement is so-called "crack propagation controlled" rather than "crack nucleation controlled". SEM observations revealed that the crack propagation mode undergoes a ductile-to-brittle transition. In liquid LBE the fracture toughness of FM steels can be reduced by about 20% as compared to that in inert Ar environment. 3-point bending tests on the hardened T91 materials HT500 and HT600 demonstrated that the LBE embrittlement effects are much more drastic for hardened materials, similarly to what was observed as a result from SSRT tests. The JQ value is reduced by as much as 95% for both HT600 and HT500 materials tested in liquid LBE. The hardened materials exhibit unstable crack propagation behavior in liquid LBE in a wide temperature range of 150 to 500°C. 3-point bending tests on the irradiated T91 steel revealed not only irradiation-induced embrittlement effects but also LBE embrittlement effects. The fracture toughness of the irradiated specimens was found to decrease further as a result from testing in liquid LBE. SEM observations of the fracture surface showed a mixture of intergranular and transgranular fracture. The LME phenomena evidenced for the T91 steel – LBE couple could be interpreted using the model of adsorption-induced reduction in cohesion of atomic bonds combined with the Kelly-Tyson-Cottrell (KTC) criterion (σ/τ ≥ σmax/τmax). According to the KTC criterion, the tendency to fail by cleavage increases as (σmax/τmax) decreases. The σmax value can be greatly reduced by adsorption of liquid Pb or Bi atoms at crack tips, while the τmax value can be increased by hardening the steel by means of either deformation, or tempering, or irradiation. Practically, the present results indicate that a great attention should be paid to application of FM steels in liquid LBE, since the mechanical properties of FM steels, such as the ductility and the fracture toughness, can be substantially degraded by LBE embrittlement effects, particularly under irradiation conditions as the LBE embrittlement are enhanced by irradiation-induced hardening. Fundamentally, the observations made in this work suggest that the mechanism responsible for liquid LBE-induced embrittlement effects on FM steels can be essentially interpreted by using the adsorption-induced reduction in cohesion of atomic bonds model combined with the KTC criterion for brittle cleavage fracture.

EPFL2009

Intermediate temperature grain boundary phenomena in copper alloys

Doris Empl

Copper and copper alloys typically experience a sharp loss in ductility at intermediate temperature, induced by grain boundary damage and fracture processes. The susceptibility of the material to intermediate temperature embrittlement is, as a result, strongly affected by the presence, along the grain boundary, of segregated atoms or low-melting phase inclusions, the former potentially weakening atomic bonding and the second causing liquid metal embrittlement along the boundaries. This thesis is an exploration of underlying phenomena that govern the intermediate temperature embrittlement of Cu-Pb alloys, addressing in turn capillary phenomena and then their relation to the tensile strength and ductility of leaded copper alloys. The dihedral angle shown by intergranular lead inclusions in Cu-1Pb alloys is measured varying the purity of the metal and the temperature. Several measurement methods are used and compared, namely classical 2D methods based on metallurgical cross section analysis, and a 3D stereoscopic method that yields the true three-dimensional angle value for individual inclusions straddling a flat grain boundary (first presented by Felberbaum et al., J. Mat. Sci., vol. 40 (2005) p. 3121). Data confirm and extend earlier measurements using the newer 3D method. It is shown that a discrepancy found between literature data and stereoscopic 3D dihedral angle measurements is not caused by impurity effects, but the data indicate that its origin lies in a difference in average dihedral angle values measured between inclusions straddling two grains, and values found at inclusions located where three or more grains meet. The method is then extended towards the simultaneous measurement of relative grain boundary energy and all five macroscopic grain boundary degrees of freedom in a copper polycrystal containing quenched lead inclusions. Data are compared with a model for grain boundary energy in pure fcc metals. The comparison suggests (i) that relaxation can lower the grain boundary energy significantly when equal numbers of broken bonds are present on either side of the grain boundary and (ii) that lead segregation to grain boundaries affects the orientation-dependence of large-angle grain boundary energies in copper. A study of the tensile strength of two Cu-1Pb alloys of different purity and of different microstructure tested at 400 °C and at a strain rate of 10 s-1 is then presented. Data show that the alloy fracture stress is consistent with a Griffith-type fracture criterion that results from assimilation of the inclusions to a void. Measured fracture stresses suggest a 25-fold increase in fracture energy dissipation over surface tension contributions, likely caused by plastic deformation of the ductile copper phase. Tensile tests conducted at 400 °C and at a strain rate of 10-2 s-1 are then presented for a Cu-Ni-Sn-Pb alloy, which exhibits high mechanical strength at room temperature coupled with good machinability. Additions to this alloy of less than 1 wt% are investigated, namely Al, Mn, Zr, B or P. It is found that, unlike the other alloying additions explored, B and P improve the strength and ductility of the leaded alloy and also that these form second phase particles in the alloy; it is proposed that the improvement results from a capillary and mechanical interaction between these particles and molten lead inclusions in the alloy, consistent with the theory advanced to explain results found for the Cu-1Pb alloy. The thesis concludes with a preliminary exploration of grain boundary engineering strategies towards the improvement of the 400 °C strength of Cu-1%Pb, showing that marginal improvements in both strength and ductility can be achieved.

EPFL2009

Microstructure and embrittlement of leaded copper alloys

Laurent Felberbaum

Mechanisms responsible for the embrittlement of leaded "free-machining" copper alloys at intermediate temperatures (i.e. around 400 °C) are examined using a combination of microstructural and mechanical characterization. Tensile tests are conducted on pure copper and on industrial high-strength Cu-Ni-Sn alloys, comparing leaded and unleaded versions, at strain rates ranging from 10-4 to 10 s-1, and various temperatures between 25 °C and 800 °C. It is found that, in the leaded samples, both liquid metal embrittlement and grain boundary embrittlement operate at intermediate temperature,i.e., near the melting point of lead. Their respective importance depends mainly on the strain rate: fracture at high strain rate (10 s-1) displays characteristic signatures of liquid metal embrittlement, while at low strain rate (10-4 s-1) the ductility trough behaviour can be attributed to grain boundary embrittlement. In leaded copper and copper alloys, lead is mostly present as discrete inclusions, frequently located along grain boundaries. The dihedral angle of intergranular lead inclusions is then the main microstructural parameter besides their size. A new technique is proposed and demonstrated for the measurement of inclusion dihedral angle in a high-purity Cu-1 wt.% Pb alloy, based on room temperature characterization of samples that were held at elevated temperature at times sufficiently long for shape equilibration of the inclusions and then rapidly quenched. Quantitative scanning electron microscopic analysis of metallographic surfaces along which individual inclusions have been dissolved is used to deduce the dihedral angle using a mathematical fit of their solid/liquid interface. For a specific temperature, we show that the dihedral angle is not unique; this reflects the fact that high-angle grain boundary energies are not constant in copper. Existing micromechanical analyses of shape equilibrium for intergranular inclusions in the presence of external stress are adapted to produce an improved description at low stress and to take into account the inclusion bulk compressibility. The derivation is based on minimization of the global capillary and elastic strain energy associated with such inclusions. An adimensional parameter that depends on the applied stress, the inclusion volume, the elastic properties of the solid, and the relevant interfacial energies emerges from the analysis as the sole factor governing the shape of the inclusion. If void nucleation occurs easily within the inclusion, such that its apparent bulk modulus is nil, the inclusion become unstable, degenerating to a crack when this adimensional parameter exceeds a critical value. Measurements of the dihedral angle on samples previously subjected to a remote stress at 400 °C show that applied stress causes a reduction in the apparent dihedral angle of liquid lead inclusions. Calculated critical stresses for both leaded pure copper and leaded copper-nickel-tin alloys that fail at high strain rate by liquid metal embrittlement are calculated for individual inclusions in the metal. These calculated critical stresses correlate relatively well with the measured fracture stress, suggesting that shape instability of the inclusions cause fracture of the alloys. This result is applied towards the suggestion of general strategies for reduction of the susceptibility of leaded copper alloys to liquid metal embrittlement. A case study addressing the problem of quench cracking of leaded Cu-Ni-Sn alloys on the basis of experimentation and finite element thermomechanical simulation, showing how the problem can be alleviated in production, concludes the thesis.

EPFL2005

Microstructure and embrittlement of leaded copper alloys

Laurent Felberbaum

EPFL2005

Intermediate temperature grain boundary phenomena in copper alloys

Doris Empl

EPFL2009