Liquid Metal Induced Embrittlement Research Articles

Research on design and microstructure of hot-dip Zn-Al-Mg/Sn alloy

In this paper, based on the effect of different intermetallic compounds on liquid metal induced embrittlement (LMIE) and corrosion resistance of GI coating during hot-forming process, a low-aluminum, low-magnesium Zn-Al-Mg alloy with high corrosion resistance is designed by CALPHAD method. The equilibrium phase diagram and solidification path of Zn-Al-Mg(-Sn) zinc-rich corner are calculated, and the phase composition of the alloy is predicted and verified by experiments. The results show that the calculation results based on the CALPHAD method are highly reliable and can provide guidance for material design. The Mg-Zn phase in the melted Zn-Al-Mg alloy is Mg2Zn11, which is different from the MgZn2 phase in the traditional industrial field. This difference may be attributed to the difference of alloy element content and cooling rate. After the Sn is added on the basis of Zn-Al-Mg, hcp-Zn phase is initially precipitated in the alloy, then fcc-Al and Mg2Zn11 phases are precipitated at the same time, and then hcp-Zn + fcc-Al + Mg2Zn11 + Mg2Sn phase coexists. Finally, the phase is transformed into hcp-Zn + fcc-Al + Mg2Sn ternary eutectic. In addition, the corrosion resistance of the designed alloy shows that the Mg2Zn11 phase in eutectic structure can improve the corrosion resistance, while the improvement of the corrosion resistance of Mg2Sn is weaker than the Mg2Zn11 phase.

Performance Comparison of Zn-Based and Al-Si Based Coating on Boron Steel in Hot Stamping.

The coatings of boron steels play an important role in affecting the quality of hot stamping parts, so it is important to evaluate the hot stamping performance of coatings before designing processes. Taking the U-type hot stamping part of boron steel as research objects, the surface quality, microstructure and temperature variation of samples with GA (galvannealed), GI (galvanized) and Al–Si coatings were observed and analyzed to evaluate the anti-oxidation, forming and quenching performances of different coatings. The results show that all the GA, GI and Al–Si coatings could provide good oxidation protection and also act as the lubricants for avoiding the friction damage of sample substrates and die-surface. But the different compositions of GA, GI and Al–Si coatings will contribute the different colors. Under the same deformation degree, the Al–Si coating can provide the best substrate protection and the GI coating will induce cracks in the substrate because of the liquid metal-induced embrittlement phenomenon. There is no significant difference between the quenching performances of GA, GI and Al–Si coatings, and the thermal conductivity of the GI coating is slightly better than Al–Si and GA coatings.

Effect of Ni-interlayer on zinc-assisted liquid-metal-induced-embrittlement susceptibility of 22MnB5 galvanized steel

In this study, in-situ heat treatment and high-temperature tensile tests were performed to investigate the effect of an Ni-interlayer on the zinc-assisted liquid-metal-induced-embrittlement (LMIE) susceptibility of 22MnB5 galvanized steel. The heat treatment results indicated that the Ni-interlayer delays the diffusion between liquid Zn and steel substrate, and it can modify the phase composition of the coating. The high-temperature tensile test results showed that the strength and ductility of the 22MnB5 galvanized steel increased as the Ni-electroplating time increased, indicating that the Ni-interlayer could improve the strength and ductility of the 22MnB5 galvanized steel. The fracture type of the galvanized steel was brittle, while that of the Ni-interlayer galvanized steel was ductile, additionally, a U-shaped crack extending to the steel substrate was observed near the fracture of the galvanized steel, whereas no cracks extending to the steel substrate were observed near the fracture of the Ni-interlayer galvanized steel. In general, the Ni-interlayer can reduce the susceptibility of zinc-assisted LMIE; moreover, when the Ni layer was electrodeposited for 8 min, the property of the Ni-interlayer galvanized steel was found to be comparable to those of bare steel.

Effects of endogenous Al and Zn phases on mechanical properties of Sn58Bi eutectic alloy

Sn58-xBi-xAl-xZn were prepared by adding different addition (x = 0, 1.0, 2.0, and 3.0 wt%) of Al and Zn, and their response to melting point, microstructure, phase structure and mechanical properties were investigated. The presence of Sn-Bi-Zn eutectic significantly reduces the melting point of the alloy, but the heat fusion is increased with the increase of Zn and Al addition. The Al element is dispersed in the SnBi eutectic matrix in the form of dendritic Al-rich phase, and the Zn element is dispersed in the form of needle-like Zn-rich phase. Besides, the addition of Al and Zn leads to the refinement of the eutectic structure and the reduction of the interlaminar spacing. The Al, Zn, and Bi can be dissolved in Sn to form β-Sn phase. The competition of Al, Zn, and Bi solubilizes in Sn results in the diffraction peak of the alloy to shift. The doping of Al and Zn in Sn causes the diffraction peak of the alloy to shift to the right, but when the proportion of Bi solubilizes in Sn increases, the diffraction peak of the alloy shifts to the left. The phase interface of Sn and Bi is the non-coherent interface. The mismatch degrees between (200) Sn and (102¯) Bi is 27.5%. The liquid metal induced embrittlement (LMIE) phenomenon can be observed between Al grain and Sn phase, which leads to brittle fracture of Al grains. With the increase of Al and Zn content, the mechanical properties are significantly improved. When the Al and Zn addition reaches 2 wt%, the ultimate tensile strength (UTS) of Sn56Bi2Al2Zn reaches 68.5 MPa, while the elongation reaches 49.04%, which is 22.76% and 46.34% higher than that of Sn58Bi. However, when the Al and Zn addition reaches 3 wt%, the Sn56Bi3Al3Zn possess the best UTS (70.5 MPa), but the plasticity is reduced to 26.80%. Moreover, the macroscopic hardness of Sn55Bi3Al3Zn is 25.3 HV, which is 22.2% higher than that of Sn58Bi (20.7 HV). Results show that the endogenous Al and Zn phases plays an important role in the mechanical properties of Sn58Bi.

Effect of Al on phase evolution in Prior-Ni coated galvanized steel: Thermodynamic and kinetic assessment

Coating of advanced high strength steel and ultra-high strength steel is a burning issue due to the problem of surface oxidation of alloying elements and liquid metal induced embrittlement (LMIE). In last decade, prior coating of Ni before galvanizing has gained much interest as it has the potential to suppress the surface oxidation of advanced high strength steel with added benefits. Recently, the galvanized ultra-high strength steel with a prior-Ni coating also has been proved as a solution for hot stamping application. It has been observed that prior-Ni reacts with Zn at the stage of galvanizing and forms different Ni–Zn alloy phases with high melting temperatures and resist the LMIE at the time of hot stamping. Al is one of the most important alloying element in commercial galvanizing bath. However, effect of Al on the Ni–Zn phase formation in such coating has not been researched in detail. In the present study, phase formation in prior-Ni coated galvanized steel in the presence of Al in Zn bath has been thoroughly investigated. The influence of Al on the Ni–Zn phase formation has been assessed by thermodynamic and kinetic calculation and then validated by experiments. Al3Ni2 with dissolved Zn precipitated in the Zn overlay and of Ni–Zn alloy phases such as NiZn8 and gamma phase formed on prior-Ni layer in a Zn–Al bath up to 0.01 mass fraction of Al. However, Ni–Zn alloy phases such as NiZn8 and gamma phases were absent in coating achieved by Zn–Al bath above 0.01 mass fraction of Al. Instead a thin compact layer consisting high amount of Ni, Al and Zn formed on prior-Ni layer and hindered the formation of Ni–Zn alloy phase in coating. The mechanism of the phase formation in the prior-Ni coated galvanized steel in presence of Al has been described with the help dissolution, phase fraction, driving force calculations and experimental observations.

Diffusion and cracking behavior involved in hot press forming of Zn coated 22MnB5

Hot press forming of Zn coated 22MnB5 is an effective way of producing ultrahigh strength structural parts for automobiles. The austenization process in hot press forming was investigated in our work. The results showed that the interfacial Fe2Al5 inhibition layer in coated steel plays a critical role in diffusion during the austenization process. We proposed three stages of diffusion based on the level of stability of the inhibition layer (i.e., the non-diffusion, inhomogeneous diffusion and homogeneous diffusion stages). The progression of diffusion in the coating through the three diffusion stages was discussed using Fe–Zn binary phase diagram and the Znliquid+α-Fe(Zn) region was considered as the susceptible area to cracking. The penetration of liquid Zn into the substrate through grain boundary (GB) diffusion was observed after the breakdown of the interfacial Fe2Al5 inhibition layer. In addition, we identified a transition diffusion layer at the interface which we assumed to be a result of the relatively long-term bulk diffusion of liquid Zn. The transition diffusion layer was analyzed following hot tensile tests. The structure of the layer was identified as body-centered cubic (BCC) α-Fe with large amounts of dissolved Zn. Although GB diffusion of liquid Zn into substrate was not observed because of the presence of the transition diffusion layer at the interface, the liquid metal induced embrittlement (LMIE) was not avoided for the Zn coated 22MnB5 in the hot tensile test. The results suggest that the BCC α-Fe phase is also sensitive to liquid Zn.

Microstructural Evolution of the 55 Wt Pct Al-Zn Coating During Press Hardening

Press hardening is increasingly being used to produce ultra-high strength steel parts for passenger cars. Al-Si, Zn, and Zn-alloy coatings have been used to provide corrosion protection to press hardening steel grades. The use of coatings has drawbacks such as coating delamination or liquid metal-induced embrittlement. In the present work, the microstructural evolution of Al-Zn coating during press hardening was studied. The 55 wt pct Al-Zn coating can in principle provide both Al barrier protection and Zn cathodic protection to press hardened steel. During the heat treatment associated with the press hardening, the 55 wt pct Al-Zn alloy coating is converted to an intermetallic surface layer of Fe2Al5 and a FeAl intermetallic diffusion layer. The Zn is separated from both intermetallic compounds and accumulates at grain boundaries and at the surface. This Zn separation process is beneficial in terms of providing cathodic protection to Al-Zn coated press hardening steel.

Liquid-Metal-Induced Embrittlement of Zn-Coated Hot Stamping Steel

Liquid-metal-induced embrittlement (LMIE) of galvanized hot stamping steel occurs due to the simultaneous application of stress and the presence of a liquid Zn surface layer during the hot stamping process. The mechanism specific to the liquid metal induced embrittlement occurring during hot stamping was investigated in detail. It was found that when a tensile stress was applied, liquid Zn could penetrate along grain boundaries in the steel matrix at temperatures above the Liquid + α-Fe (Zn) → Γ1 peritectic transformation temperature of 1055 K (782 °C). The results show that an increase of the annealing time prior to hot stamping is an effective way to prevent LMIE by the elimination of the liquid phase.

Liquid-Metal-Induced Embrittlement in Turbine Casing Segment Screws of an Aeroengine

One of the 12 screws meant to fasten the diffuser segment to the flange of a turbine casing in an aeroengine was found broken in-service. Cracks were also observed in some of the remaining screws. The screws made of 35NC6 steel material were plated with cadmium. The chemistry, hardness, and microstructure (heat-treatment condition) of the screws met the drawing requirements. Heavy oxidation was noticed on the surface as well as on the fracture surface of the screws. Fractographic studies showed brittle intergranular features. Scanning electron microscope–energy dispersive x-ray spectroscopy (SEM–EDS) analysis detected cadmium on the entire fracture surface of the screws. Micro-examination and SEM–EDS analysis of the cracked screws showed the presence of cadmium all along the path of crack propagation including the crack tip. From microscopic and fractographic studies, it was concluded that the screws had failed by liquid-metal-induced embrittlement (LMIE). It appeared that the screws had experienced high service temperature that had resulted in melting of the cadmium plating on the screws. The molten cadmium had penetrated (adsorbed) along the grain boundaries of the screw material causing the screws to eventually fail in the brittle (intergranular) manner by the phenomenon of LMIE.

Evidence for a Diffusion-Based Mechanism of Liquid Metal Intergranular Penetration: Case Study of a Ni-Bi Model System

A model Ni-Bi system has been used to investigate intergranular penetration (IGP) phenomenon. All experiments have been done on Ni 26°<110> bicrystal at 700°C using bismuth vapour condensation as a source of liquid bismuth. Such a procedure results at room temperature in either partial or total Liquid Metal Induced Embrittlement (LMIE) of a unique grain boundary, depending on the duration of liquid Bi / solid Ni contact at 700°C. Auger Electron Spectrometry (AES) and Rutherford Backscattering Spectrometry (RBS) have been used to measure the Bi concentration profile between the source of liquid bismuth and the penetration front. Two zones have been clearly identified : the first one of almost constant Bi concentration called nanometrethick film which is interpreted in terms of Fowler-Guggenheim multi-layer segregation under local equilibrium conditions and the second one with a progressive decrease of Bi concentration over a distance of the order of 20-200µm. Such a long transition zone, together with parabolic diffusion kinetics indicates diffusion-based mechanism of intergranular penetration as opposed to the direct grain boundary wetting.

Liquid metal induced embrittlement in fuel line braze joints

This paper documents an instance of liquid metal induced embrittlement (LMIE) that occurred during the manufacture of automobile engine fuel lines. The fuel lines were constructed of low-carbon steel fittings press fit onto 304 stainless steel tube. The joints were subsequently furnace brazed with copper braze paste. The carbon steel fittings were occasionally observed lying separately as the part exited the brazing furnace. Failure analysis using metallographic techniques, scanning electron microscopy, and energy-dispersive spectroscopy indicated that the fractures were intergranular with no observable ductility. Copper from the braze filler metal was observed on the stainless steel tube fracture surfaces and within the tube wall grain boundaries. Copper and austenitic stainless steel are a well-known LMIE couple. Subsequent investigation indicated the stress required for rapid fracture was due to impacts of the parts against one another and the brazing furnace stationary components while the braze metal was liquid.

Liquid metal embrittlement of tensile specimens of En19 steel by tin

Liquid metal induced embrittlement (LMIE) of En19 steel was investigated by tensile testing specimens coated with liquid pure tin. Testing was carried out over a range of steel hardnesses and test temperatures, and for each material hardness the temperature above which recovery of the unembrittled tensile behaviour of the specimens occurred was measured. Metallographic examination of each of the specimens was carried out and the results of this examination were compared with the mechanical behaviour. This work illustrates the importance of dissolution of the steel by tin on the behaviour of the specimens tested at the higher temperatures.

A fluid flow based model to predict liquid metal induced embrittlement crack propagation rates

An analytical model is developed for the rate of supply of embrittler to the crack tip in liquid metal induced embrittlement (LMIE), assuming that crack propagation rate is controlled by viscous losses in the liquid metal. The results of the modelling are compared with experimentally determined data for crack propagation rates in LMIE. This study indicates that in several systems the LMIE crack propagation rate is not controlled by viscous losses in the crack and that a study of crack propagation rates in LMIE may yield useful insight into the micromechanisms of LMIE fracture.

Mechanism of embrittlement of Al alloy by liquid metal (Ga)

The liquid metal-induced embrittlement (LME) of 7075 Al alloy by Ga was investigated by using modified wedge open loaded (WOL) specimens. The results showed that the adsorption of Ga atoms could facilitate local plastic deformation of the base metal and there was an observable concave plastic zone at the notch tip. A microcrack then initiated when the plastic deformation reached a critical condition. The threshold stress intensity of LME under tensile applied stress is 1.7 MPa m 1/2. When the specimens are loaded in the opposite direction, the stress is compressive at the notch tip and is tensile some distance away from the notch tip. Ga atoms diffused to the tensile zone and resulted in microcrack initiation. The microcrack propagated outwards in the tensile stress zone and was arrested at the compressive stress zone near the notch tip. The nominal compressive threshold stress intensity K ∗ IC(L ) is equal to 3.5 MPa m 1/2. The fracture surfaces of LME for all the specimens are intergranular.

Mechanisms of liquid metal induced embrittlement

Liquid metal induced embrittlement (LMIE) refers to the loss of ductility of normally ductile metals and alloys when stressed while in contact with a liquid metal. Various models have been proposed to describe the mechanisms by which LMIE failure takes place. In the present study the most promising of these are discussed in detail and analysed critically in the light of experimental results. The advantages and limitations of each are highlighted and, where possible, comparisons between the characteristics of each are presented. The diverse results obtained in previous investigations and the difficulties encountered in LMIE testing have resulted in numerous and often contradictory models.

Failure by liquid metal induced embrittlement

Liquid metal induced embrittlement (LMIE) refers to the loss of ductility of normally ductile metals and alloys when stressed whilst in contact with a liquod metal. This paper describes the characteristics of LMIE with particular emphasis on how these may assist in the process of failure analysis and presents the distinctive features of LMIE fracture. A comprehensive review of the incidence of LMIE failures in practice illustrates the range of circumstances in which this phenomenon has manifested itself. Finally, the technological uses of LMIE are discussed.

Embrittlement of 2 1/4Cr-1Mo Steel by Lithium and a Lead-Lithium Liquid

The embrittlement of 2 1/4Cr-1Mo steel by lithium or lead-lithium liquids can occur when loading conditions and microstructural strengthening effects limit plastic relaxation at points of high stress, and a critical liquid metal induced embrittlement (LMIE) stress is reached. This paper presents the LMIE results of both constant displacement rate uniaxial tensile testing and fatigue crack propagation studies. The temperature for the onset of LMIE susceptibility at a given localized strain rate is shown to be predictable based on a critical value of flow stress, calculated by means of the Zener-Holloman parameter.

Metal-Induced embrittlement of metals—an evaluation of embrittler transport mechanisms

An analysis is made of the possible embrittler transport mechanisms in liquid metal-induced embrittlement (LMIE) and solid metal-induced embrittlement (SMIE). The analy-sis and comparison with experiments makes it appear likely that a) in LMIE bulk liquid flow is the transport mechanism; liquid metals which “wet” the base metalcan penetrate to the tips of even very sharp cracks, contrary to some statements in the literature; b) in SMIE surface self-diffusion of the embrittler is the transport mechanism; and c) vapor transport could play a role for a few high-vapor pressure embrittlers—such as Zn, Cd, and possibly Hg—but is not a viable transport mechanism for most embrittlers.