Fe-Zn Intermetallic Compounds Research Articles

Effects of Si Addition on Interfacial Microstructure and Corrosion Resistance of Hot-Dip Zn–Al–Mg–Si Alloy-Coated Steel

Alloy coatings protect steel from corrosion in various applications. We investigated the effects of Si addition on the microstructure, electrochemical behavior, and corrosion resistance of steel sheets coated with a hot-dip Zn–Mg–Al–Si alloy using a batch-type galvanization process. Microstructural analysis revealed that the Zn–Al–Mg alloy coating layer contained a significant amount of Fe that diffused from the substrate, leading to delamination due to the formation of brittle Fe–Zn intermetallic compounds. However, the introduction of Si resulted in the formation of a stable Fe2Al3Si inhibition layer at the substrate–coating interface; this layer prevented interdiffusion of Fe as well as enhanced the coating adhesion. Additionally, the formation of acicular Mg2Si phases on the coating surface improved the surface roughness. As the Si content increased, the corrosion resistance of the coating improved. Specifically, the Zn–Al–Mg coating layer with 0.5 wt.% Si exhibited excellent anti-corrosion performance, without red rust formation on its surface even after 2600 h, during a salt spray test.

Control of various Zn-based weld seam/steel interface structures in AA5083/FH36 steel welded joint

Replacing steel with aluminum alloy is appealing to realize the lightweight of ship structure, so it is inevitable to weld these two materials in products. When joining AA5083 aluminum alloy to FH36 steel with the thickness of 6 mm by tungsten inert gas (TIG) arc welding process, the liquid Zn-based filler metal reacted with steel matrix to form Fe-Zn intermetallic compounds (IMCs) and Fe-Al IMCs. A novel interface structure composed of spinous Г-Fe3Zn10, thinner η-Fe2Al5Zn0.4 and a small number of dispersed δ-FeZn10 with a relatively high bonding strength can be obtained at the weld seam/steel interface by optimizing the backing welding current. When the backing welding current was 120 A, the maximum tensile strength of joint could reach 98 MPa. Simulation indicated that a large stress concentration developed at the weld seam/steel interface, within crack active temperature range of Fe-Zn and Fe-Al IMCs, during the backing welding process. The lattice misfit of δ-FeZn10/η-Fe2Al5Zn0.4 and Г-Fe3Zn10/η-Fe2Al5Zn0.4 interfaces were 19.9% and 9.4%, which revealed that dislocations were more likely to accumulate around δ-FeZn10, providing the location of crack initiation in aluminum alloy/steel welded joints.

Role of silicon on formation and growth of intermetallic phases during rapid Fe–Zn alloying reaction

The study of liquid metal embrittlement in Fe–Zn systems is challenging because of the high temperature and vapor pressure of Zn, which hinders in-situ investigations with sufficiently high spatial resolution. This is typically associated with subsecond processing steps and the coexistence of a solid substrate and a liquid Zn phase, which renders direct observations at the microstructural scale difficult. In this study, we comprehensively investigate the reactions occurring during the rapid heating and cooling stages of Fe–Zn systems using synchrotron X-ray diffraction. The phase transformation is analyzed for specimens with different interfacial structures, with focus on changes in the coating microstructure above and below the peritectic temperature (782 °C) of the Fe–Zn system. Advanced high-strength steel (AHSS) variants with 1.5 wt% Si content show a prominent destabilizing effect at the onset of Fe–Zn intermetallic compound formation and a simultaneous deceleration of the liquid Zn depletion rate compared with other AHSS alloys containing 0 wt% Si. Furthermore, the addition of Si suppresses the formation of the Γ phase, which is due to turbulence in the outburst Zn at temperatures above 773 K. Consequently, the fraction of Γ phase compounds with accumulated Zn deceases as the exposure time of the liquid Zn to the ferritic matrix increases owing to the solute redistribution of Si in the liquid Zn during rapid heating. Meanwhile, the absence of silicon in the substrate causes the formation of a ζ phase with a low melting point, which delays the formation of liquid Zn via an increase in the melting point of the Zn layer in the early stage of heating. Additionally, the amount of residual liquid Zn decreases due to the rapid depletion of Zn during the liquid initiation stage via an equilibrium Fe/Zn binary phase transformation. The results of this study provide a deeper understanding of the Fe–Zn intermetallic phase transformation and the sensitivity to liquid metal embrittlement of third-generation AHSSs based on silicon content.

Microstructural evolution of liquid metal embrittlement in 1.5-GPa-grade Zn-coated hot-press-forming steels

Zn-assisted liquid metal embrittlement (LME) in hot press forming (HPF) steels is attributed mainly to the cracking inside both Zn coating and HPF substrate, but very few studies on how the Zn coating composed of Fe–Zn intermetallic compound of Γ-Fe3Zn10 and Zn-containing ferrite (α-Fe(Zn)) and alloying elements influence the LME cracking have been conducted. In this study, mechanisms of LME were investigated by using laboratory-scale HPF simulation tests in relation with microstructural evolutions of Zn coating and LME cracking. The coating and steel substrate were composed mostly of α-Fe(Zn) and full martensite, respectively, in both reference (Ref) and 1-wt.%-Si-containing (1Si) HPF steels, while the Γ phase was additionally formed at the 1Si steel. The high-temperature tensile test results indicated that the loss of elongation increased with decreasing exposure time near or at 900 °C and with increasing test temperature or Si content. Based on the equilibrium Fe–Zn binary phase diagram, the amount of liquid remained after the austenitization was larger in the 1Si steel than in the Ref steel because the addition of 1 wt% Si retarded the Fe–Zn alloying reaction between the Zn coating and steel substrate to raise the susceptibility to LME. Thus, the Si content should be carefully controlled by appropriately using the HPF process parameters such as the time of austenitization at 900 °C to sufficiently achieve the Fe–Zn alloying reaction.

Zn penetration and its coupled interaction with the grain boundary during the resistance spot welding of the QP980 steel

The liquid metal embrittlement (LME) defect in the Zn-coated high-strength steels is intimately associated with the formation of Fe-Zn intermetallic compounds (IMCs). The relationship between LME crack propagation and Fe-Zn IMCs during the resistance spot welding of the galvanized QP980 steel was systematically investigated. The results showed that LME cracks were mainly generated in the center of the weld (type I crack) and the shoulders of the weld (type II crack) for the galvanized QP980 resistance spot welded joint. The longest type Ⅰ crack was around 1014.8 and 95.7 μm for type Ⅱ crack. Moreover, the details of the microstructure and element distribution near the LME crack were analyzed. The brittle Fe-Zn IMCs were traced at the grain boundaries to initiate the cracks. In addition, these brittle IMCs could further weaken the grain boundaries of the steel substrate and resultantly promote the formation and further propagation of LME cracks.

Diffusion Bonding of Al-Mg-Si Alloy and 301L Stainless Steel by Friction Stir Lap Welding Using a Zn Interlayer.

Friction stir lap welding (FSLW) is expected to join the hybrid structure of aluminum alloy and steel. In this study, the Al-Mg-Si aluminum alloy and 301L stainless steel were diffusion bonded by FSLDW with the addition of 0.1 mm thick pure Zn interlayer, when the tool pin did not penetrate the upper aluminum sheet. The characteristics of lap interface and mechanical properties of the joint were analyzed. Under the addition of Zn interlayer, the diffusion layer structure at lap interface changed from continuous to uneven and segmented. The components of the diffusion layer were more complex, including Fe-Al intermetallic compounds (IMCs), Fe-Zn IMCs and Al-Zn eutectic. The largely changed composition and thickness of uneven and segmented diffusion layer at the lap interface played a significant role in the joint strength. The tensile shear load of Zn-added joint was 6.26 kN, increasing by 41.3% than that of Zn-not-added joint. These two joints exhibited interfacial shear fracture, while the Zn interlayer enhanced the strength of diffusion bonding by extending the propagation path of cracks.

Role of liquid Zn and α-Fe(Zn) on liquid metal embrittlement of medium Mn steel: An ex-situ microstructural analysis of galvannealed coating during high temperature tensile test

Zn assisted liquid metal embrittlement (LME) has characteristics that differ from the general characteristics of other LME systems: the formation of Fe-Zn intermetallic compounds and the high solubility of Zn in Fe. Consequently, there is a need to investigate the influence of the Fe-Zn interaction and resulting phase composition on LME behavior. However, little research has been conducted to reveal such relationships. This paper explores the influence of high temperature phases in galvannealed coating, specifically liquid Zn and α-Fe(Zn) on the LME sensitivity of medium Mn steel using a high temperature tensile test with eight combinations of temperatures (700 °C to 900 °C) and hold times (1 s, 50 s, and 100 s). The test results are categorized as LME, microcrack and LME free cases in terms of LME behavior. The study reveals that the LME sensitivity is governed by the fraction of effective liquid Zn, which is a consequence of isothermal solidification owing to the thermodynamic stability of liquid Zn, diffusivity of Zn in Fe and the partitioning coefficient of liquid Zn. Moreover, LME is influenced by the breaking of α-Fe(Zn) layer, which directs the liquid Zn into the substrate steel. Finally, a water-tank analogy is proposed to demonstrate the effect of the fraction of liquid Zn and the breaking of α-Fe(Zn) layer on LME.

Effect of Si/Mn Ratio on Galvannealing Behavior of Si-added Steel

The influence of the Si/Mn ratio on the galvannealing behavior of 1.5 wt% Si -1.5~2.5 wt% Mn-added steel in the Fe oxidation-reduction process was investigated. The Si/Mn ratio of the steel affected the formation of Si-containing oxides during the annealing process. The amount of SiO2 formed on the steel surface decreased with as the Si/Mn ratio decreased, while the amount of Mn2SiO4 increased. In addition, the internal oxide formed in a relatively narrow area near the surface in the lower Si/Mn ratio sample, which indicated that the content of solute Si near the surface was lower in the lower Si/Mn ratio sample. The galvannealing reaction was accelerated by decreasing the Si/Mn ratio of the steel. The species and morphology of the Si-containing oxides determined the galvannealing behavior of the Si-added steel. The Si-containing selective surface oxide affected the formation of the initial Fe-Zn intermetallic compounds (IMC) during hot-dipping in molten Zn. The formation of SiO2 was suppressed in the sample with the lower Si/Mn ratio, which resulted in accelerated Fe-Zn IMC formation. On the other hand, solute Si in the steel affected the growth of the Fe-Zn IMC during heating in the galvannealing process. The content of solute Si was assumed to be lower in the lower Si/Mn ratio sample, which resulted in acceleration of Fe-Zn IMC growth.

Crystal Structures and Mechanical Properties of Fe–Zn Intermetallic Compounds Formed in the Coating Layer of Galvannealed Steels

Crystal Structures and Mechanical Properties of Fe–Zn Intermetallic Compounds Formed in the Coating Layer of Galvannealed Steels

Effect of Si/Mn Ratio on Galvannealing Behavior of Si-added Steel

The influence of the Si/Mn ratio on the galvannealing behavior of 1.5 wt% Si −1.5~2.5 wt% Mn-added steel in the Fe oxidation-reduction process was investigated. The Si/Mn ratio of the steel affected the formation of Si-containing oxides during the annealing process. The amount of SiO2 formed on the steel surface decreased with as the Si/Mn ratio decreased, while the amount of Mn2SiO4 increased. In addition, the internal oxide formed in a relatively narrow area near the surface in the lower Si/Mn ratio sample, which indicated that the content of solute Si near the surface was lower in the lower Si/Mn ratio sample. The galvannealing reaction was accelerated by decreasing the Si/Mn ratio of the steel. The species and morphology of the Si-containing oxides determined the galvannealing behavior of the Si-added steel. The Si-containing selective surface oxide affected the formation of the initial Fe–Zn intermetallic compounds (IMC) during hot-dipping in molten Zn. The formation of SiO2 was suppressed in the sample with the lower Si/Mn ratio, which resulted in accelerated Fe–Zn IMC formation. On the other hand, solute Si in the steel affected the growth of the Fe–Zn IMC during heating in the galvannealing process. The content of solute Si was assumed to be lower in the lower Si/Mn ratio sample, which resulted in acceleration of Fe–Zn IMC growth.

Fracture Toughness of the Fe–Zn Intermetallic Compounds Measured by Bend Testing of Chevron-Notched Single-Crystal Microbeams

Fracture Toughness of the Fe–Zn Intermetallic Compounds Measured by Bend Testing of Chevron-Notched Single-Crystal Microbeams

Fe-Zn reaction and its influence on microcracks during hot tensile deformation of galvanized 22MnB5 steel

Fe-Zn reaction was investigated by annealing a galvanized 22MnB5 steel sheet at 500–900 ° C for 5 min. The observed Fe-Zn intermetallic compounds within the coating depended on the annealing temperature. As the temperature increased, the coating phase changed from δ via Γ1 to Zn-rich ferrite. At the temperature lower than or equal to 700 ° C, Zn seems to have diffused along the grain boundaries of substrate, which formed fine Zn-rich ferrite grains. Annealing at higher temperature produced coarse Zn-rich ferrite grains which contributed to significant thickening of the coating layer. By tensile straining at the annealing temperature, it was found that the fine Zn-rich ferrite along the grain boundaries was the origin of the microcracks in the substrate.

Heating rate effect on liquid Zn-assisted embrittlement of high Mn austenitic steel

The effect of heating rate on liquid metal embrittlement (LME) of a galvanized twinning induced plasticity (TWIP) steel was investigated by hot tensile testing. The specimens were heated with different heating rates to 600, 700 and 800 °C, and elongated to 40%. During the hot tensile tests, various Fe-Zn intermetallic compounds were formed depending on the heating rate and the deformation temperature, and they were distinguished by X-ray diffraction and energy dispersive spectroscopy. δ and Γ were observed in the coating after 600 °C deformation regardless of heating rate. Since both phases did not liquefy at 600 °C, the steel was not embrittled. Heating to 700 °C sometimes produced δ which would be liquid at 700 °C. It was only produced at the highest heating rate and embrittled the steel. Γ was produced at 800 °C regardless of heating rate. Hence, the steel suffered from LME in all heating rate condition. Based on the results, the importance of selecting temperature and heating rate to study LME by hot deformation testing is discussed.

Characterization of interfaces in tri-layered brass/steel/brass composites fabricated by casting and hot dipping

AbstractTri-layered brass steel/brass laminated composites were fabricated through hot dipping as well as casting. The chosen temperatures for hot dipping were 1 150 °C and 1 350 °C and for casting was 1 150 °C in addition to performing a heat treatment cycle at 900 °C for 90 min. The characterization results revealed that in the as-cast and the heat-treated samples, the formation of Fe-Zn intermetallic compounds occurred. In addition, with increasing the hot dipping temperature, stronger interfaces were achieved due to easier diffusion at higher temperatures. Moreover, casting without heat treatment was enough to obtain good bonding since the amount of Fe3Zn10 intermetallic compound increased and grain growth occurred.

Crystal structure refinement of the Γ- and Γ1-phase compounds in the Fe-Zn system and orientation relationships among α-Fe, Γ and Γ1 phases in the coating layer of galvannealed steel

The crystal structures of the Γ- and Γ1-phase compounds in the Fe-Zn system have been refined by single-crystal synchrotron X-ray diffraction. The Γ-phase compound consists of four Fe-centred (Zn,Fe)12 icosahedra connected with one another by face-sharing with the vertex sites of the shared triangle face being occupied by both Fe and Zn atoms. The Fe occupancy at the vertex sites increases with increasing Fe composition of the compound. The stoichiometry of the Γ phase is described as Fe4Zn9 (Zn-30.77 at.%Fe), realized when all the vertex sites of the shared triangle face are occupied exclusively by Fe atoms. The Γ1-phase compound consists of Zn12 and (Zn,Fe)12 icosahedra whose central site is exclusively occupied by Fe atoms, which is different from the previously reported structural model where some centres of these icosahedra are occupied by both Fe and Zn atoms. The crystal structures of all Fe-Zn intermetallic compounds can thus be best understood by considering the packing of the common structural unit; Fe-centred Zn12 and/or Fe-centred (Zn,Fe)12 icosahedron. Although the crystal structures of the α-Fe, Γ and Γ1 phases with cubic symmetries are related with each other in terms of the arrangement of structural vacancies, there is no orientation relationship either between the Γ and Γ1 phases or between the α-Fe and Γ phases in the coating layer of galvannealed steel sheet.

Si添加鋼のFe-Zn合金化反応に及ぼすMn添加量の影響

The influence of the Mn content of Si-added steel sheets on the Fe-Zn galvannealing reaction was investigated. Three steel sheets, 1.5 mass%Si-1.4, 1.9 and 2.7 mass%Mn, were annealed in a 10vol%H2-90vol%N2 atmosphere. Si and Mn oxides were analyzed by reflectance Fourier transform infrared spectroscopy, X-ray photoelectron spectroscopy, scanning electron microscopy and energy dispersive X-ray spectrometry. SiO2 and Mn2SiO4 formed as selective oxides at the steel surface after recrystallization annealing. The ratio of the oxide species changed depending on the Mn content in the steel. When the Mn content was lower, formation of SiO2 was promoted and that of Mn2SiO4 was suppressed. In the selective oxide layer which formed on the surface of the 1.5 mass%Si-1.4 mass%Mn steel sheets, Mn2SiO4 formed at the outer side, and SiO2 formed at the inner side. This can be explained by consideration of the thermodynamic oxygen potential gradient. Furthermore, areas where SiO2 mainly formed and those where Mn2SiO4 mainly formed were distributed on the surface of the 1.5 mass%Si-1.4 mass%Mn steel sheets. In this case, the Fe-Zn intermetallic compound (IMC) formed preferentially on the Mn2SiO4 between the zinc coating and the substrate steel after galvanizing, and the Fe-Zn galvannealing reaction was suppressed on the SiO2 layer. It is considered that a dense and continuous protective SiO2 layer acted as a barrier to the Fe-Zn galvannealing reaction.

Low-temperature joining of Fe-based amorphous foil with aluminum by ultrasonic-assisted soldering with Sn-based fillers

Abstract Low temperature joining of amorphous alloys has big challenges to prevent them from crystallization. An ultrasonic-assisted soldering method was proposed to join the Fe-based amorphous alloys with the aluminum plates in air by using Sn-based solders. No obvious defects were observed and the joining ratio was more than 90%. At the interface of solder/aluminum, scallop α-Al solid solutions were formed and large fretting (pits) with a size of 50 μm on the aluminum plates was found due to ultrasonic vibrations. At the interface of solder/amorphous, Fe–Zn intermetallic compound (FeZn 13 ) was probably formed. Its formation activation energy was approximately 30.9 kJ/mol between the amorphous iron and the Sn–9Zn during the solid state diffusion, which was much lower compared with crystallized couples. The enhanced diffusion/reaction and the much low activation energy in this work are probably attributed to the amorphous state of the Fe-based foils. Adding nickel in the Sn–Zn solder can improve the hardness of the joints by formation of NiAl 3 intermetallic particles and depress the corrosion on the aluminum. Both the initially interfacial wetting and the microstructure refinement during solidification of the filler solder were improved by ultrasonic vibrations.

Arrangements of Fe-Centered Zn12 Icosahedra in Fe-Zn Intermetallic Compounds Determined by Ultra-High Resolution Scanning Transmission Electron Microscopy

ABSTRACTThe crystal structure of the δ1p phase in the Fe-Zn system has been refined by single-crystal synchrotron X-ray diffraction combined with ultra-high resolution scanning transmission electron microscopy. The crystal structure can be described to build up with Fe-centered Zn12 icosahedra. The deformation properties obtained by single-crystal micropillar compression tests of the δ1p phase is discussed in terms of the arrangement of the Fe-centered Zn12 icosahedra in contrast with the ζ phase in the Fe-Zn system.

Fe-Zn系金属間化合物皮膜の破壊靭性評価

The Fe-Zn intermetallic compounds (IMC) layers composed of δp, δk, and Γ phases were fabricated using two different kinds of Fe/Zn diffusion couple (DC), and the fracture toughness of the constituent phases was estimated from the toughness of the IMC layers. In the DCs with sufficient Zn supply, the IMC layers were mainly composed of δp phase after isothermal holding at 450-600 °C for 60 s, while in the DCs with limited Zn supply, the IMC layers were composed of δp, δk, and Γ phases at the early stage of isothermal holding, which was later dominated by Γ phase with further isothermal holding. The average grain size of the IMC phases was found to increase with increased holding temperature. From the comparisons between the IMC layers composed mainly by either δp or Γ phases, toughness of Γ phase was found inferior to that of δp phase. In addition, it was demonstrated that the toughness of δp and Γ was found to increase with decreased average grain size. Whereas, from the results using the multi-phase IMC layers, the toughness of δk phase was found almost equivalent to that of Γ phase.

純亜鉛融液／鋼界面におけるFe-Zn系金属間化合物形成に及ぼす鋼中Siの影響

The effect of Si additions to the steel on the microstructure and growth kinetics of the Fe-Zn intermetallic compound (IMC) in the Fe/Zn diffusion couple with limited Zn supply was experimentally investigated. Three different steels with Si contents of less than 0.01 wt%, 0.44 wt%, and 1.85 wt% were machined in cylindrical shape of 8 mmφ×6 mmh, and their bottom surfaces were polished chemically and mechanically to remove oxide and Si depletion layer. Pure Zn (99.99%) film was inserted between two steel cylinders, and enclosed in the steels in vacuum. The samples were heated at 450 °C for various holding time. Without Si, an IMC layer composed of δp, δk, and Γ phases were formed first, and it was later replaced by an IMC layer composed only of Γ phase by the growth of δk and Γ phases. The addition of Si in steel suppresses the formation of Γ phase as well as the growth rate of δk phase, and IMC layers composed mainly of either δp or δk phases were successfully fabricated. Si was found to accumulate in α-Fe at the α-Fe/δk interface, which indicates that Si partitioning at the α-Fe/δk interface is responsible for the instability of Γ phase and slow supply of Fe from steel substrates. Finally, the fracture toughness and adhesion of IMC layer composed mainly of δk phase were demonstrated inferior to those of δp phase.