Fe-Zn System Research Articles

Giant segregation transition as origin of liquid metal embrittlement in the Fe-Zn system

A giant Zn segregation transition is revealed using CALPHAD-integrated density-based modeling of segregation into Fe grain boundaries (GBs). The results show that above a threshold of only a few atomic percent Zn in the alloy, a substantial amount of up to 60 at.% Zn can segregate to the GB. We found that the amount of segregation abruptly increases with decreasing temperature, while the Zn content in the alloy required for triggering the segregation transition decreases. Direct evidence of the Zn segregation transition is obtained using high-resolution scanning transmission electron microscopy. Base on the model, we trace the origin of the segregation transition back to the low cohesive energy of Zn and a miscibility gap in Fe-Zn GB, arising from the magnetic ordering effect, which is confirmed by ab-initio calculations. We also show that the massive Zn segregation resulting from the segregation transition greatly assists with liquid wetting and reduces the work of separation along the GB. The current predictions suggest that control over Zn segregation, by both alloy design and optimizing the galvanization and welding processes, may offer preventive strategies against liquid metal embrittlement.

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.

Plastic deformation of single crystals of the δ1p and δ1k intermetallic compounds in the Fe–Zn system by micropillar compression

The plastic deformation behavior of single crystals of the δ1p and δ1k phase compounds in the Fe–Zn system, which are the major constituent phases in the coating layer of galvannealed (GA) steels, has been investigated by micropillar compression tests at room temperature as a function of crystal orientation and specimen size. (0001)[112‾0] basal slip and (101‾0)[1‾21‾0] prism slip are observed to operate in the δ1p phase compound while (0001)[112‾0] basal slip is observed in the δ1k phase compound. For all cases, a significant strain burst occurs immediately after yielding, forming giant steps on the slip planes along the slip direction and leading in many cases to instantaneous ‘slip plane (shear) failure’. The extent of such strain burst as well as instantaneous ‘slip plane failure’ is reduced by introducing dislocations (pre-straining) prior to micropillar testing, accompanied by the reduction in the stress at which such strain burst occurs as well as that in the extent of strain burst. The critical resolved shear stress (CRSS) for basal and prism slip in the δ1p and δ1k phase compounds all show an inverse power-law scaling against the specimen size with an exponent in the range of 0.02–0.06. The bulk CRSS values are estimated respectively to be approximately 320, 430, and 765 MPa for the basal slip in the δ1p and δ1k compounds and the prism slip in the δ1p compound by taking into account the specimen size effects of CRSS.

Zn-induced liquid metal embrittlement of galvanized high-Mn steel: Strain-rate dependency

Zn-coated steel sheets suffer from Zn-induced liquid metal embrittlement (LME) during resistance spot welding and hot tensile deformation. By tensile testing at 700 and 800 °C, it was found that LME of an electro-galvanized high-Mn steel sheet was suppressed at low strain rate. Such strain-rate dependency of LME has been observed in various embrittlement metallic couples. Conventional ductile-to-brittle transition theory successfully explains the strain-rate reliance of LME in other embrittlement couples while it is invalid for Fe–Zn system. Owing to the high mutual solubility and several intermetallic compounds in Zn-rich regime of Fe–Zn system, the strain-rate dependency of Zn-induced LME was highly correlated with the existence of liquid Zn, which was verified by analyzing the cross section of the steel sheets.

Phase equilibria and diffusion coefficients in the Fe-Zn binary system

Phase diagram and diffusion coefficients of the Fe-Zn binary system are required to predict and control the microstructure of galvanized zinc coatings and thus were systematically investigated in the temperature range from 700 to 1100 °C using nine novel Fe/Zn liquid-solid diffusion couples (LSDCs). The equilibrium compositions of the α and Γ phases of the Fe-Zn system were determined and agree well with the recently established Fe-Zn phase diagram. The extracted interdiffusion coefficients in α-Fe at temperatures between 700 and 1100 °C and in Γ at 750 and 700 °C using the forward-simulation analysis (FSA) extend the experimental interdiffusivity measurements to the whole solubility range of these two phases. Three impurity diffusion coefficients of Zn in paramagnetic α-Fe were also determined by the FSA and show good agreement with the literature data. The temperature dependence of the Zn impurity diffusion coefficients in both the paramagnetic and ferromagnetic α-Fe phases across the Curie temperature TC was established by analyzing the reliable experimental results as well as by density functional theory (DFT) calculations. The newly measured phase equilibrium and diffusivity data provide reliable inputs for future thermodynamic and kinetic modeling of the Fe-Zn system.

Crystal Structures and Mechanical Properties of the Intermetallic Compounds in the Fe-Zn System Comprising the Coating of Galvannealed Steel Sheets

Crystal Structures and Mechanical Properties of the Intermetallic Compounds in the Fe-Zn System Comprising the Coating of Galvannealed Steel Sheets

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.

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.

Preparation of nanosized zinc oxide by vacuum oxidation and behaviors of impurity elements

Nanosized zinc oxide with the purity ≥99.98 was prepared with a vacuum oxidation method based on hot-dip galvanizing slag by using air as the oxygen source. The effects of reaction temperature and vacuum degree on the product morphologies were investigated, and the influences of main impurity elements in raw materials on the quality of products were evaluated under different oxidation conditions. Reaction temperature arid vacuum degree dramatically affect the product morphologies. At appropriate temperature and vacuum degree, the main products are wurtzite crystals. The crystal needles are elongated and long in length. Impurity Fe and Pb elements influence the quality of products through various patterns under different oxidation conditions. Fe remains in the products mainly relying on the mechanical entrainment of Zn vapor and the positive deviation of Fe-Zn system, while Pb is retained mainly based on the vapor pressure and the positive deviation of Pb-Zn system.

Part I. Thermodynamic and Kinetic Aspects of the Hot Dip (Zn) - Coating Formation

Abstract A hot dip (Zn) – coating formation is carried out in the industry conditions. Two types of the steel substrate are applied to the experiment. Two morphologically different coatings are obtained, accordingly. A hot dip (Zn) – coating formation is also carried out in the laboratory conditions for making some additional explanations of the revealed phenomena. The thickening of the - phase sub-layer is observed in details to determine time of the transition from stable into meta-stable solidification. The Fe-Zn phase diagrams for stable and meta-stable equilibrium are calculated, respectively. The phase diagram for dissolution is also determined. The criterion of the higher temperature of the solid/liquid (s/l) interface is successfully applied to the Fe-Zn system to justify the competition between stable and meta-stable solidification. The mass balance verification is performed for the (Zn) - coating in order to define the nominal Zn–solute concentration required by dissolution and next by solidification. The Zn – solute concentration in the dissolution, super-saturation and saturation zones are determined thermodynamically. The growth kinetics is described for all the sub-layers in the (Zn) – coating.

Order–disorder structure of the δ1k phase in the Fe–Zn system determined by scanning transmission electron microscopy

The crystal structure of the δ1k phase in the Fe–Zn system was investigated by scanning transmission electron microscopy. The δ1k phase has a superlattice structure based on the δ1p phase having a tripled periodicity along the a-axis direction of the δ1p phase, accompanied by one-dimensional stacking disorder of structural blocks (called order–disorder (OD) packets) along the c-axis direction. The crystal structure can be described crystallographically in terms of the OD theory so as to belong to the category IV OD structures composed of two types of non-polar OD layers (L2n and M2n+1). The tripled periodicity along the a-axis direction is due to chemical ordering of the constituent Fe and Zn atoms in the OD layer M2n+1. Because of the tripled periodicity of the OD layer M2n+1, three different equivalent stacking positions are generated in stacking an OD layer M2n+1 on top of the OD layer L2n. Depending on the stacking order, the crystal structure of the δ1k phase can be ordered with various periodicities along the stacking direction or completely disordered. Based on the OD theory, two maximum degree of order (MDO) polytypes belonging to the space groups of P63/mcm (MDO1) and R3c (MDO2) are deduced for the δ1k phase. The most stable MDO polytype in the OD family of the δ1k phase is determined experimentally to be the MDO2 polytype.

Compression of Single-Crystal Micropillars of the Γ Intermetallic Phase in the Fe-Zn System

The deformation behavior of the Γ (gamma) phase in the Fe-Zn system has been investigated via room-temperature compression tests of single-crystal micropillar specimens fabricated by the focused ion beam method. Trace analysis of slip lines indicates that {110} slip occurs for the specimens investigated in the present study. Although the slip direction has not been uniquely determined, the slip direction might be <111> in consideration of the crystal structure of the Γ phase (bcc).

Structure refinement of the δ1pphase in the Fe–Zn system by single-crystal X-ray diffraction combined with scanning transmission electron microscopy

The structure of the δ1p phase in the iron-zinc system has been refined by single-crystal synchrotron X-ray diffraction combined with scanning transmission electron microscopy. The large hexagonal unit cell of the δ1p phase with the space group of P63/mmc comprises more or less regular (normal) Zn12 icosahedra, disordered Zn12 icosahedra, Zn16 icosioctahedra and dangling Zn atoms that do not constitute any polyhedra. The unit cell contains 52 Fe and 504 Zn atoms so that the compound is expressed with the chemical formula of Fe13Zn126. All Fe atoms exclusively occupy the centre of normal and disordered icosahedra. Iron-centred normal icosahedra are linked to one another by face- and vertex-sharing forming two types of basal slabs, which are bridged with each other by face-sharing with icosioctahedra, whereas disordered icosahedra with positional disorder at their vertex sites are isolated from other polyhedra. The bonding features in the δ1p phase are discussed in comparison with those in the Γ and ζ phases in the iron-zinc system.

Micropillar compression deformation of single crystals of the intermetallic compound ζ-FeZn13

The deformation behavior of the ζ phase in the Fe–Zn system has been investigated by micropillar compression tests at room temperature with the use of single crystals with 13 different crystal orientations prepared by the focused ion beam method. Two different slip systems, {110}〈11¯2〉 and (100)[001], are observed to operate. The critical resolved shear stresses (CRSS) value for {110}〈11¯2〉 slip is more than three times smaller than that for (100)[001] slip. From the anisotropy in CRSS for these two slip systems, {110}〈11¯2〉 slip is predicted to operate for most crystal orientations, except for a narrow orientation region around [3¯05] where (100)[001] slip operates. The CRSS for {110}〈11¯2〉 slip shows an inverse power-law scaling against the specimen size with an exponent of −0.517. The bulk CRSS value for {110}〈11¯2〉 slip is estimated to be 62–76MPa by taking into account the specimen size effects of CRSS. The reasons why {110}〈11¯2〉 slip with a rather long Burgers vector (0.7700nm) is selected as the easiest slip system are discussed in terms of the nature of atomic bonding in the crystal structure, especially the rigid atomic bonding within an Fe-centered Zn12 icosahedron (for slip plane selection), and the energetic barrier height along the slip direction and the resultant possible dissociation schemes (for slip direction selection). Some implications are made on how the deformability of the ζ phase can be improved in the textured coating layer in galvannealed steels based on the results obtained.

Compression deformability of Γ and ζ Fe–Zn intermetallics to mitigate detachment of brittle intermetallic coating of galvannealed steels

Micropillar compression tests on each of the five intermetallic phases of the Fe–Zn system, which constitute the coating of galvannealed steels, have revealed that the Γ phase formed in direct contact with the steel substrate and the ζ phase formed on the outermost surface are ductile, sandwiching the other three brittle phases (Γ1, δ1k and δ1p). Compression deformability of these ductile phases is considered to mitigate the coating failure through sustaining ruptured fragments of the brittle phases during the forming process.

Electrical, magnetic, and thermal properties of theδ-FeZn10complex intermetallic phase

We report the electrical, magnetic, and thermal properties of the \ensuremath{\delta}-FeZn${}_{10}$ phase in the zinc-rich domain of the Fe-Zn system. The \ensuremath{\delta}-FeZn${}_{10}$ phase possesses high structural complexity typical of complex metallic alloys: a giant unit cell comprising 556 atoms, polyhedral atomic order with icosahedrally coordinated environments, fractionally occupied lattice sites, and statistically disordered atomic clusters that introduce intrinsic disorder into the structure. Structural disorder results in suppression of the electrical and heat transport phenomena, making \ensuremath{\delta}-FeZn${}_{10}$ a poor electrical and thermal conductor. Structural complexity results in a complex electronic structure that is reflected in the opposite signs of the thermoelectric power and the Hall coefficient. The \ensuremath{\delta}-FeZn${}_{10}$ phase is paramagnetic down to the lowest investigated temperature of 2 K with a significant interspin coupling of antiferromagnetic type. Specific heat indicates the formation of short-range-ordered spin clusters at low temperatures, very likely a precursor of a phase transition to a collective magnetic state that would take place below 2 K. The magnetoresistance of \ensuremath{\delta}-FeZn${}_{10}$ is sizeable, amounting to 1.5$%$ at 2 K in a 9-T field. The electrical resistivity exhibits a maximum at about 220 K, and its temperature dependence could be explained by the theory of slow charge carriers, applicable to metallic systems with weak dispersion of the electronic bands, where the electron motion changes from ballistic to diffusive upon heating.

Compression of Single-Crystal Micropillars of the ζ Intermetallic Phase in the Fe-Zn System

ABSTRACTThe deformation behaviour of the ζ (zeta) phase in the Fe-Zn system has been investigated via room-temperature compression tests of single-crystal micropillar specimens prepared by the focused ion beam method. Trace analysis of slip lines indicates that {110} slip occurs for the specimens investigated in the present study. Although the slip direction has not been uniquely determined, comparison of Schmid factors and yield stress values suggests that the slip direction might be <1$\overline 1 $2>, which is inconsistent with the easiest slip system {110}[001] predicted on the basis of the primitive Peierls-Nabarro model.

Study of growth mechanism of zinc hot dip galvanising coatings

Zinc hot dip galvanising is one of the most effective methods for the corrosion protection of ferrous materials. However, although its efficiency is undisputable, the exact growth mechanism of these coatings is still under question. In the present work, the diffusivity of liquid zinc in carbon steels was measured with electron microscopy. Furthermore, the change of the free Gibbs energy was calculated for each phase of the coating. Nevertheless, the coating structure was microscopically studied for an extremely short immersion time (about 1 s). In this way, it was deduced that the coating growth begins with the nucleation of the ζ phase of the Fe–Zn system. The γ and λ phases grow with zinc diffusion through the ζ phase. The η phase is mechanically drifted due to the surface tension of the liquid metal.

Analyzing Fe–Zn system using molecular dynamics, evolutionary neural nets and multi-objective genetic algorithms

Failure behavior of Zn coated Fe is simulated through molecular dynamics (MD) and the energy absorbed at the onset of failure along with the corresponding strain of the Zn lattice are computed for different levels of applied shear rate, temperature and thickness. Data-driven models are constructed by feeding the MD results to an evolutionary neural network. The outputs of these neural networks are utilized to carry out a multi-objective optimization through genetic algorithms, where the best possible tradeoffs between two conflicting requirements, minimum deformation and maximum energy absorption at the onset of failure, are determined by constructing a Pareto frontier.

Thermodynamic investigation of the galvanizing systems, I: Refinement of the thermodynamic description for the Fe–Zn system

The thermodynamic description for the Fe–Zn system was updated using CALPHAD approach. A set of self-consistent thermodynamic model parameters for this system was obtained by considering the available experimental data. Compared with the previous thermodynamic modeling, the present assessment with fewer parameters shows not only a better agreement with the experiments but also sounder physical meaning. The present CALPHAD modeling coupled with the ab initio calculations were used to predict the enthalpies of formation of the solid phases in the Fe–Zn system.