Liquid Metal Embrittlement Research Articles

Comparative Study on the Liquid Metal Embrittlement Susceptibility of the High-Si Advanced High-Strength Steel with EG and GA Zn Coatings

The Zn-coated high-Si advanced high-strength steel (AHSS) tends to suffer Zn-assisted liquid metal embrittlement (LME) during the resistance spot welding (RSW) process. In this study, the LME behaviors of electrogalvanized (EG) and galvannealed (GA) high-Si steels were comparatively investigated. The maximum lengths of the LME cracks at the shoulder and center of the spot weld were approximately 366.6 μm and 1486.5 μm, respectively, for the EG yet 137.0 μm and 1533.3 μm, respectively, for the GA high-Si steels. Additionally, all EG and GA welded joints were etched to measure the nugget size. It was found that the increased welding current could aggravate the formation tendency of the LME cracks for both the EG and GA high-Si steels. Furthermore, the statistical results revealed that the electrogalvanized high-Si AHSS exhibited a relatively higher LME susceptibility than the galvannealed high-Si AHSS. It was deemed that the internal oxidation produced during the annealing before the Zn coating was the crucial factor that led to the difference in the LME susceptibilities for the EG and GA high-Si steels.

Effect of Steel Subsurface Structure on the Liquid Metal Embrittlement Behavior of Third-Generation Advanced High-Strength Steels

Effect of Steel Subsurface Structure on the Liquid Metal Embrittlement Behavior of Third-Generation Advanced High-Strength Steels

A study on the fretting corrosion of 316L in static lead-bismuth eutectic (LBE): The role of slip amplitude and normal force on damage mechanism at 350 °C

Fretting corrosion of stainless steel in the LBE affects the safety of lead-cooled fast reactors. Slip amplitude and normal load are the main mechanical factors affecting fretting wear behavior. Thus, the damage mechanism of 316L stainless steel at 350 °C LBE influenced by slip amplitude and normal load was investigated by jointly utilizing multiple characterization methods. The results indicate that the normal load and slip amplitude essentially affect the tangential stress and relative sliding value in the contact area, leading to different slip regions and damage mechanisms. In the mixed slip region, the damage mechanism is adhesion and delamination cracks. The increase in tangential stress leads to decrease in relative sliding. The thick wear debris layer attached to the worn surface can protect the substrate from being attacked by the LBE. In the gross slip region, the damage mechanism is abrasive wear and dissolution corrosion. The increase in relative sliding causes more damage and Ni dissolution, leading to the transformation from austenite to ferrite and internal strain, making the substrate more susceptible to damage and increasing the risk of liquid metal embrittlement (LME) of austenitic stainless steel at 350 °C. Accordingly, a model for different damage mechanisms was proposed. These results can provide important information on the fretting damage related to the LBE environment.

Influence of liquid lead and lead-bismuth eutectic on three alumina forming austenitic (AFA) steels through slow strain rate testing

Liquid metal embrittlement (LME) in three newly developed alumina-forming austenitic (AFA) alloys, two 50 kg batches and one 5-ton heat, was studied in the temperature range 350–600 °C in liquid Pb and 140–600 °C in LBE using slow strain rate testing (SSRT) in a low-oxygen environment. No significant decrease in the engineering strain was observed in either environment. However, the presence of secondary cracks along the length of the specimen and brittle intergranular areas on the fracture surfaces indicates that the AFA alloys do show a minor degree of embrittlement above 570 °C. This appears to be related to grain boundary wetting by Pb/LBE. At temperatures below 570 °C, this wetting effect does not seem to be strong enough to induce LME in the alloys, and their ability to form a sufficiently protective oxide means that they remain unaffected by LME. The results indicate that the AFA alloy group can perform sufficiently well in liquid Pb/LBE environments, and long-term testing should be carried out to determine their viability as candidate materials for use in Pb- and LBE-based cooling systems.

Liquid metal embrittlement and fracture behavior of three Mo metals in liquid lead-bismuth eutectic

In this work, liquid metal embrittlement (LME) and the related fracture behavior of Mo, TZM and Mo-14Re were assessed in oxygen-saturated and -depleted LBE at 350°C by means of slow strain rate tensile (SSRT) tests. Neither tensile mechanical property data nor fractographic micrographs show strong evidence of LME. Nevertheless, the observation of deep secondary cracking at the fiber-like grain boundaries of Mo and Mo-14Re tested in LBE suggests occurrence of grain boundary wetting (GBW). Moreover, oxidation-assisted crack initiation and propagation were also identified. Both fracture modes may be a concern when Mo metals with a fiber-like grain structure are used in high-oxygen LBE environment.

The High-Cycle Tensile-Shear Fatigue Properties and Failure Mechanism of Resistance Spot-Welded Advanced High-Strength Steel with a Zn Coating.

Advanced high-strength steels (AHSSs) with Zn coatings are commonly joined by the resistance spot welding (RSW) technique. However, Zn coatings could possibly cause the formation of liquid metal embrittlement (LME) cracks during the RSW process. The role of a Zn coating in the tensile-shear fatigue properties of a welding joint has not been systematically explored. In this study, the fatigue properties of tensile-shear RSW joints for bare and Zn-coated advanced high-strength steel (AHSS) specimens were comparatively studied. In particular, more severe LME cracks were triggered by employing a tilted welding electrode because much more stress was caused in the joint. LME cracks had clearly occurred in the Zn-coated steel RSW joints, as observed via optical microscopy. On the contrary, no LME cracks could be found in the RSW joints prepared with the bare steel sheets. The fatigue test results showed that the tensile-shear fatigue properties remained nearly unchanged, regardless of whether bare or Zn-coated steel was used for the RSW joints. Furthermore, Zn mapping adjacent to the crack initiation source was obtained by an electron probe micro-analyzer (EPMA), and it showed no segregation of the Zn element. Thus, the failure of the RSW joints with the Zn coating had not initiated from the LME cracks. It was concluded that the fatigue cracks were initiated by the stress concentration in the notch position between the two bonded steel sheets.

Simulation Of Mechanical Stress On A Solution-Annealed 15-15Ti Steel Using ABAQUS CAE Program

In addressing the problem of Ti steel (15-15Ti) proposed as the main candidate material for the manufacture of coatings and fuel wrappers for liquid LBE-cooled fast reactors at high temperatures related to material degradation, such as liquid metal embrittlement (LME) and liquid metal corrosion (LMC), Gong et al. conducted research related to the creep failure behavior of solution-annealed 15-15Ti steel exposed to LBE at temperatures of 550 and 600oC using a creep test facility. However, in this study, testing the mechanical properties of 15-15Ti steel through tensile testing was not really discussed, even though the mechanical properties of a material are one of the most important things in determining structural design. The mechanical properties obtained from previous research were then simulated using ABAQUS CAE software to determine the stress distribution profile (initial and final) and the mechanical stress-strain performance used to understand more about the 15–15Ti material. From the simulation results, it was found that the peak force received by the specimen for a strain rate of 1.1 x 10-5s-1 was 6.0 kN, while for a strain rate of 5 x 10-5s-1, it was 6.2 kN. This means that the specimen used cannot accept a force greater than the peak force value. A stress-strain difference graph was also obtained in the experimental results, with simulation results showing a decrease in the value of the fracture point. This is because the mesh setting in the simulation is not close to a more detailed value.

Zinc-Induced Liquid Metal Embrittlement in Austenitic Microstructures

The problem of liquid metal embrittlement (LME) is one of the main concerns to be addressed when developing high-strength automotive steels. Although LME has been extensively investigated over the past decade, the role of LME-induced cracking on the failure mechanism of steel substrates has not been adequately explored. This study investigates the influence of LME cracking on the tensile properties and failure mechanism of an austenitic microstructure. An in-depth analysis of the LME crack propagation path revealed that the crack propagated predominantly along highangle, random grain boundaries. The detailed failure analysis showed that the Zn-coated austenitic steel failed in an intergranular mechanism without any plastic strain being applied to the grains during deformation. The results of the study indicate that stress-assisted grain boundary diffusion is responsible for the premature failure of Zn-coated austenitic microstructures at much lower tensile stresses than that of the uncoated specimen. It was found that the application of thermomechanical conditions characterized by low stress and low temperature, combined with microstructures featuring a low Zn grain boundary diffusion rate, can effectively reduce LME cracking.

The liquid metal embrittlement of a reactive system at room temperature: α-brasses in contact with the liquid eutectic Ga-In

We studied the liquid metal embrittlement (LME) at room temperature of α-brasses by the liquid eutectic Ga-In (EGaIn). The EGaIn partially wets the α-brasses and forms the CuGa2 intermetallic. 3-point bending tests, Finite Element Analysis, and in situ testing were conducted. When LME occurs, there is a ductile fracture followed by an intergranular brittle fracture that propagates through the zones with the highest plastic strain. The LME sensitivity increases with higher strain rates, higher Zn content, and higher work hardening rates, suggesting a significant role of dislocation mobility in LME and a lack of influence from the CuGa2 intermetallic.

Effects of CuCr1Zr contamination on the tensile properties and microstructure of stainless steel 316L produced via laser powder bed fusion

AbstractCopper contamination has a negative effect on the tensile properties of certain stainless steel grades due to a weakening of grain boundaries via liquid metal embrittlement. This is especially problematic given current trends in laser powder bed fusion (L-PBF) that elevate contamination risks, such as multi-material processing or the use of recycled materials. As such, it is critical to establish composition limits for use in standard specifications. This study investigates the changes in tensile properties and cracking behavior in stainless steel alloy 316L contaminated with copper alloy CuCr1Zr at concentrations of 0–10 particle percent (pt.%) in horizontal, diagonal, and vertical build orientations. It is found that microcracks are already present at 1 pt.% Cu alloy and increase in density with contamination. The cracks are generally vertically oriented along columnar grain boundaries and are associated with high local Cu content, thus exacerbating the anisotropy of the as-built material. The contamination decreases the elastic modulus, yield strength (YS), ultimate tensile strength (UTS), and uniform elongation, eventually transitioning from ductile to brittle fracture modes. The build orientation relative to the tensile loading axis is shown to be a critical design parameter due to the preferential crack initiation and growth direction. The fracture surfaces at 10 pt.% contamination show regularly spaced, smooth brick-like cleavage patterns that correspond to the columnar grain dimensions. Even so, the measured YS and UTS exceeded the ASTM F3184-16 standard for CuCr1Zr contaminations up to 5 pt.%. As a conservative limit, it is proposed that a maximum content of 1 wt% Cu be specified for L-PBF SS316L.

Silicon effect on retardation of Fe-Zn alloying behavior: Towards an explanation of liquid zinc embrittlement susceptibility of third generation advanced high strength steels

Silicon (Si) aggravates liquid metal embrittlement (LME) susceptibility of Zn-coated advanced high strength steels (AHSS) by inhibiting intermetallic formation between liquid Zn and the AHSS substrate at elevated temperatures. This study performed detailed microstructural and elemental characterization of an LME-susceptible Si-bearing AHSS to reveal the fundamental mechanism by which Si inhibits Fe-Zn intermetallic reactions. Notably, the interaction between Si-alloyed AHSS and liquid Zn at elevated temperatures results in pronounced Si enrichment in a thin region of the substrate in contact with the liquid. This Si enrichment behavior is at the core of the retarded Fe-Zn reactions, and is explained from a fundamental standpoint by analyses of the phase equilibria in the Fe-Zn-Si ternary system and diffusion kinetics using DICTRA®. Specifically, the liquid dissolves the surface layers of the AHSS substrate upon initial contact at elevated temperatures. The liquid phase, however, has no appreciable solubility for Si; hence, almost all of the Si atoms from the dissolved substrate layers are “back diffused” into the substrate leading to Si enrichment. The Si-enrichment of the substrate reduces the thermodynamic driving force for nucleating intermetallic phases at the steel/coating interface, making it difficult to form a “protective” interfacial layer barrier between the liquid and the substrate. Further, Si enrichment results in an increase in the chemical potential of Zn and reduction in the chemical potential of Fe in the substrate; both these factors slow the kinetics of substrate dissolution and Fe-Zn alloying, leading to an increased liquid fraction in the coating available to activate LME.

Experimental Investigation of Hot Forming of Zn‐Coated Hot‐Stamping Steel

An investigation on the thermal property, fracture mechanism of substrate and coating layer, liquid‐metal embrittlement (LME) crack, coating layer morphology, microstructure, and hardness of Zn‐coated hot‐stamping steel is presented. The effect of forming temperature on the LME is studied. The crack mechanism of substrate at elevated temperature is ductile fracture with various‐sized dimples. The inclusions in the dimples are also detected and researched. However, the fracture mechanism of coating layer and weld spot is different from that of substrate. At high temperature, the liquid Zn invades into the substrate with oxidation reaction, which reduces the thermal formability. The complex chemical reaction and different phase formation occur in the coating layer. The coating layer breaks and peels off from the substrate with sharp edge, indicating brittle fracture. The phase of substrate mainly transforms into martensite phase with the microhardness Vickers hardness 0.2 of 410–490.

Effect of alloying elements on zinc-induced liquid metal embrittlement in steels: A first-principles study

Liquid metal embrittlement (LME) induced by Zn melt in galvanized steels during spot-welding poses a significant challenge to the advancement of the automotive industry. A critical strategy to address this challenge is identifying promising alloying elements capable of mitigating LME. High-throughput first-principles calculations were employed to investigate the segregation behavior of 22 common alloying elements at the Σ5(310) grain boundary of iron and their impact on the grain boundary energy. The results indicate that volume distortion upon doping of the alloying elements predominantly influences their segregation behavior, while the weak bonding between Zn and Fe is responsible for the grain boundary embrittlement. Exploration of solute interaction between Zn and the other alloying element in the Fe grain boundary model reveals that the attraction between metallic doping elements and Zn correlates with both the elastic effects and the magnetic properties of the dopant, while the repulsion between non-metallic elements and Zn is primarily governed by the elastic effects. Separating the grain boundary strengthening energy into mechanical and chemical contributions shows that they correlate linearly with variations in volume of the doping site and changes in bond energy for bonds associated with the doping site at the grain boundary, respectively. Accordingly, the strengthening effect of the doping elements can be estimated by evaluating the induced changes in local volume and bond energies on grain boundaries. Furthermore, potential alloying elements to mitigate the Zn-induced LME are screened and discussed, with B and Mo being the most promising candidates.

Influence of microstructural morphologies on liquid metal embrittlement of Zn-coated dual-phase steels

Dual-phase (DP, ferrite+martensite) steels are crucial for automotive components, predominantly spot-welded using thin Zn-coated cold-rolled sheets. However, the effect of martensite morphologies, namely fine-lamellar (LD) or coarse-globular (GD) shapes, on Zn-assisted liquid metal embrittlement (LME) remains underexplored. This study investigates LME mechanisms during resistance spot welding of Zn-coated DP steel sheets with LD or GD microstructures. Analysis focuses on cracks within the weld shoulder zone, categorized into sub-critical HAZ (SCHAZ), inter-critical HAZ (ICHAZ), and upper-critical HAZ (UCHAZ). The LME susceptibility index, calculated as total crack length divided by total contact length, increased in the order of LD-SCHAZ, LD-ICHAZ, GD-ICHAZ, and GD-SCHAZ. These results indicated that the microstructure of the GD sheet itself was very vulnerable to the cracking because the initial microstructure was almost retained in the SCHAZ. Cracking analysis in the LD-SCHAZ and GD-SCHAZ suggested that martensite grains near ferrite/ferrite boundaries impeded crack growth by deflecting propagation, and that the LD sheet showed higher crack resistance due to unfavorably oriented ferrite/martensite boundaries.

Unraveling the impact of the substitution of Si by Al on liquid metal embrittlement behavior of 3rd generation AHSS

This study addresses one of the most crucial ongoing discussions concerning the mitigation of Zinc-induced Liquid Metal Embrittlement (LME) in third-generation Advanced High-Strength Steels (AHSS). The role of Silicon (Si) and Aluminum (Al), both common alloying elements in AHSS, in triggering LME-induced cracking is thoroughly investigated. Two chemical compositions, 0.2C–3Mn-0.8Si and 0.2C–3Mn–1Al, featuring martensitic microstructures, were selected. Hot tensile tests were conducted in the temperature range of 600–900 °C to simulate the thermomechanical cycle during spot welding.This resulted in a significant reduction in ductility for both Si and Al-alloyed steels in the coated samples compared to the uncoated ones. The loss of ductility was notably more pronounced in Si-alloyed steel than in Al-alloyed steel. These findings were also evident in a specialized LME-triggering spot-welding test, suggesting that Si, as an alloy element in steel, leads to higher LME susceptibility compared to Al. This can be attributed, as indicated by thermodynamic equilibrium calculations and investigations of the Zn–Fe layer, to an increased destabilization and retardation of the protective intermetallic Zn–Fe phases by Si. The insolubility of Si in intermetallic phases and liquid Zn is considered a thermodynamic constraint.In contrast, the theoretically possible full solubility of 1% Al in intermetallic phases as well as in liquid Zn was experimentally demonstrated and supported by thermodynamic simulation. Furthermore, Al, by increasing the Ac3 temperature, results in lower amounts of austenite in the microstructure. It is known that austenite plays a detrimental role in triggering LME. Based on this comprehensive study, it is reasonable to conclude that the formation of intermetallic phases plays a critical role in controlling the contact time between steel and liquid Zn as an embrittling agent, and this is strongly influenced by the Si and Al content of the steel.

Effect of Si doping on elastic-plastic fracture toughness (JIC) of T91 ferritic/martensitic steel in liquid lead-bismuth eutectic

This work was devoted to understanding the effect of Si doping (1.27 wt%) on elastic-plastic fracture toughness (JIC) of T91 ferritic/martensitic (F/M) steel in liquid lead-bismuth eutectic (LBE, Pb44.5Bi55.5, wt%) under various parameters. The results showed that Si doping significantly promoted occurrence of liquid metal embrittlement (LME) and led to more severe degradation of fracture toughness at 350 °C, especially in oxygen-poor LBE. The LME susceptibility of T91-Si steel was found to be nearly independent on the loading speeds, while T91 F/M steel exhibited a stronger LME effect when the loading speed was decreased from 0.02 mm/min to 0.002 mm/min. Oxygen concentration had a mild effect. Detailed microstructural analyses showed that the fracture behavior of the T91-Si steel in LBE environment at 350 °C was analogous to cleavage fracture and the crack propagation was mainly transgranular. The crack wall was quite rough and locally faceted to low-index crystallographic planes of {110}. Numerous dislocations were also present in the vicinity of the crack tip. No solid evidence of diffusion of Pb and Bi into the steel bulk was discovered even at the atomic scale, which strongly suggests a crack tip surface adsorption-based LME mechanism.

The impact of interfacial characteristics on the interfacial properties of 316 L/CuSn10 multi-material manufactured by laser powder bed fusion

Multi-material additive manufacturing (MMAM) enables the simultaneous creation of multi-material components in a single process, accomplishing customized different performances in specific regions of the same components. In particular, the copper-steel multi-material structure can provide considerable strength and high thermal conductivity, thus playing a crucial role in electronic components, heat exchangers and injection molds. However, few studies have explored the relationship between the microstructure and properties of the copper-steel fusion zone. Therefore, this study aims to reveal the heterogeneous evolution of the microstructure in the copper/steel fusion zone and its impact on the interfacial properties of 316 L/CuSn10 multi-material structure fabricated by laser powder bed fusion (L-PBF). We characterized the multi-layer specimens using laser confocal microscopy and SEM to obtain the macroscopic and microscopic microstructure, liquid metal embrittlement (LME) microcrack characteristics and precipitation evolution of the copper-steel fusion zone. The generation of liquid phase separation (LPS) was explored based on block specimens, and the textural characteristics of the fusion zone were analyzed using EBSD. As a result, because of multiple element precipitation and the high thermal conductivity of copper alloy, abrupt grain size changes occur in the fusion zone, affecting the fusion zone's corrosion resistance and nano-hardness. In tensile tests analyzing the anisotropy, the results show that LME microcracks have little impact on the interfacial bonding properties of the copper-steel bimetallic structure, among which the ultimate tensile strength of the specimens formed at 30° to the substrate reached a maximum of 562.9 MPa. These findings add substantially to our understanding of the relationship between the microstructure and properties of copper-steel bimetallic interface, and provide a theoretical reference for additive manufacturing copper-steel multi-material components.

Effects of Pb and Bi on mechanical properties and fracture modes of bcc iron symmetrical tilt grain boundaries

The internal structural material T91 in the accelerator driven subcritical reactor system (ADS) has encountered the challenge of liquid metal embrittlement caused by Pb and Bi. Therefore, it is of utmost importance to gain a comprehensive understanding of their embrittlement mechanism. The effects of Pb and Bi on the surface energy of surfaces and separation work and fracture toughness of grain boundaries in body center cubic (BCC) Fe are examined using first-principles calculation. Pb and Bi atoms reduce the surface energy and grain boundary cohesion strength of BCC Fe, with a more significant decrease in surface energy. Meanwhile, the segregation of Pb and Bi in the grain boundary reduces the fracture toughness of the grain boundary. By comparing different surface energy and separation work, it is found that BCC pure Fe is more prone to intergranular fracture, while the presence of Pb and Bi causes cleavage fracture. This elucidates the low-temperature fracture mode of BCC-Fe and reveals the potential mechanism of brittle cleavage fracture caused by Pb and Bi in BCC-Fe. The derived results are discussed in terms of electronic structure and show good agreement with experimental results in the literature, ultimately contributing to a thorough understanding of the intrinsic mechanism of liquid metal embrittlement of T91 steel.

Uncovering the mechanism of the LME susceptibility for the galvanized and electrogalvanized DP 1180 steels

Zn-based coatings generally protect dual-phase (DP) 1180 advanced high-strength steel in automobiles from corrosion. However, liquid metal embrittlement (LME) tends to occur in Zn-coated DP 1180 steel during resistance spot welding. In this study, the LME phenomena of the galvanized (GI) and electrogalvanized (EG) DP 1180 steels were compared based on the LME susceptibility evaluation standard. For DP 1180 steel coated with GI, the maximum length of the LME crack was approximately 121.2 μm at the shoulder of the welding spot. In contrast, for the specimen coated with EG, the maximum length of the LME crack was 792.6 μm at the center of the welding spot. Owing to the different galvanizing processes, the microstructure differed during annealing, thereby influencing the occurrence of the LME. Furthermore, the microstructural observations indicated that the protection of the interfacial oxide layer in the GI specimen could prevent the formation of the LME to some extent. Therefore, the presence of the interfacial oxide layer could account for the different LME susceptibilities of the GI and EG DP 1180 steels.

Numerical and experimental assessment of liquid metal embrittlement in externally loaded spot welds

Zinc-based surface coatings are widely applied with high-strength steels in automotive industry. Some of these base materials show an increased brittle cracking risk during loading. It is necessary to examine electrogalvanized and uncoated samples of a high strength steel susceptible to liquid metal embrittlement during spot welding with applied external load. Therefore, a newly developed tensile test method with a simultaneously applied spot weld is conducted. A fully coupled 3D electrical, thermal, metallurgical and mechanical finite element model depicting the resistant spot welding process combined with the tensile test conducted is mandatory to correct geometric influences of the sample geometry and provides insights into the sample’s time dependent local loading. With increasing external loads, the morphology of the brittle cracks formed is affected more than the crack depth. The validated finite element model applies newly developed damage indicators to predict and explain the liquid metal embrittlement cracking onset and development as well as even ductile failure.