A New Kinetic Mode During the Austenite-to-Ferrite Transformation in Fe–Mn and Fe–Mn–Mo Alloys

The kinetics of isothermal austenite(γ)-to-ferrite(α) transformation at various temperatures in Fe-2 Mn and Fe-2 Mn-0.5 Mo(wt%) alloys is investigated via dilatometry experiments and phase-field modeling.It was interestingly found that Mo addition has a marginal effect on the transformation kinetics.Besides the well-known partitioning and partitionless transformation modes,a new kinetic mode,in which interface migration is controlled by interfacial diffusion,was identified.The phase-field model with considering interfacial segregation could well predict the transformation kinetics and the kinetic mode transition.     

Influence of Mo in the structure of rapidly solidified Fe–Mo–Cr–Mn–Si–C alloy

Abstract

An Fe–Mo–Cr–Mn–Si–C alloy was prepared in an induction furnace and was cast into cylindrical rod in a copper mould in castmatic equipment (low pressure casting). A single phase non-equilibrium featureless (no visible microstructures after deep etching) phase was observed over a certain range of thickness of the rod. In this present work, the extent of the featureless phase was studied with different concentrations of Mo (5–25 wt-%) for 5·5 mm diameter of cylindrical rod at a cooling rate of 1100 K s^–1. Light optical microscopy, scanning electron Microscopy and Vickers hardness tests were used to analyse the samples. The amount of the featureless area varies as the Mo content changes and the maximum featureless area was obtained for 7 wt-% of Mo. This single phase featureless structure exhibits very high hardness (>1350 HV) which can be used in many interesting applications with or without suitable heat treatments.     

Phase selection in undercooled peritectic Fe–Mo alloys

The change of the primary solidification mode of undercooled peritectic Fe–Mo melts has been studied by in situ observation of recalescence events during electromagnetic levitation. A maximum melt undercooling up to 380 K has been achieved. Levitated drops of controlled undercooling were quenched onto chill substrates and subjected to phase and microstructure analysis. A transition from the primary bcc-Mo to the peritectic σ-phase solidification mode beyond a critical undercooling of 345 K was revealed for the Fe₄₇Mo₅₃ alloy and in a similar way for other compositions between Fe₄₅Mo₅₅ and Fe₅₄Mo₄₆. The suppression of the properitectic bcc-Mo phase was also achieved for subcritical undercooling in substrate-quenched Fe₄₅Mo₅₅ samples. In Fe₆₁Mo₃₉ a transition from the primary σ- to the peritectic R-phase solidification mode beyond a critical undercooling of 150 K was inferred from recalescence processes and X-ray investigation of as-quenched undercooled samples.     

Microstructure and Properties of Electrodeposited nc-TiO2/Ni–Fe and Ni–Fe Coatings

Metals and Materials International - In this study, nc-TiO2/Ni–Fe composite coatings, and Ni–Fe alloys as equivalents to their matrices, were obtained from citrate-sulphate baths in the...     

Three-dimensional morphology of degenerate ferrite in an Fe–C–Mo alloy

The three-dimensional morphology of degenerate ferrite formed below the bay of the TTT diagram in an Fe–C–Mo alloy was observed by serial sectioning and computer-aided reconstruction. The degenerate ferrite apparently consists of rod-like subunits, each several micron in length and 1–2 μm in diameter.     

Effect of Nb and Mo on the Microstructure,Mechanical Properties and Ductility-Dip Cracking of Ni–Cr–Fe Weld Metals

A series of Ni–Cr–Fe welding wires with different Nb and Mo contents were designed to investigate the effect of Nb and Mo on the microstructure, mechanical properties and the ductility-dip cracking susceptibility of the weld metals by optical microscopy(OM), scanning electron microscopy, X-ray diffraction as well as the tensile and impact tests. Results showed that large Laves phases formed and distributed along the interdendritic regions with high Nb or Mo addition. The Cr-carbide(M_(23)C_6) was suppressed to precipitate at the grain boundaries with high Nb addition. Tensile testing indicates that the ultimate strength of weld metals increases with Nb or Mo addition. However, the voids formed easily around the large Laves phases in the interdendritic area during tensile testing for the weld metal with high Mo content. It is found that the tensile fractographs of high Mo weld metals show a typical feature of interdendritic fracture. The high Nb or Mo addition, which leads to the formation of large Laves phases, exposes a great weakening effect on the impact toughness of weld metals. In addition, the ductility-dip cracking was not found by OM in the selected cross sections of weld metals with different Nb additions. High Nb addition can eliminate the ductility-dip cracking from the Ni–Cr–Fe weld metals effectively.     

Densification and grain growth during isothermal sintering of Mo and mechanically alloyed Mo–TZM

Isothermal sintering behavior of pure molybdenum (Mo) and mechanically alloyed Mo–TZM (Mo–0.6Ti–0.2Zr–0.02C) has been investigated in the temperature range 1000–1800 °C. A linear relationship has been found to exist between logarithms of increment in density and time. Although the volume diffusion has been found to be the dominant sintering mechanism, a significant contribution from grain boundary diffusion is also identified. Both the diffusion coefficients (D_v) obtained from shrinkage data and the grain boundary mobility (M_b) during grain growth are found to be lower for Mo–TZM due to the presence of carbides in the microstructure. The grain boundary migration is restricted due to the presence of carbides and porosities in the microstructure.     

Environmental Resistance of Mo–Si–B Alloys and Coatings

For high temperature application beyond the range of Ni-base superalloys, multiphase Mo–Si–B alloys with compositions, that yield the ternary intermetallic Mo₅SiB₂ (T₂) phase as a key microstructure constituent together with the Mo and Mo₃Si phases, offer an attractive balance of high melting temperature, oxidation resistance and mechanical properties. The investigation of reaction kinetics involving the T₂ phase enables the analysis of oxidation in terms of diffusion pathways and the design of effective coatings. From this basis kinetic biasing is used together with pack cementation to develop multilayered coatings and in situ diffusion barriers with self-healing characteristics for enhanced oxidation resistance. While a combustion environment contains water vapor that can accelerate attack of silica based coatings, the current pack cementation coatings provide oxidation resistance in water vapor up to at least 1500 °C. An exposure to hot ionized gas species generated in an arc jet confirms the robust coating performance in extreme environments.     

Theoretical Analysis of Anomalies in High-Temperature Oxidation of Fe–Cr, Fe–Ni, and Fe–Ni–Cr Alloys

The following anomalies are theoretically analyzed: weakening of the protective ability of dense Cr₂O₃ film during its long-term thermal exposure (because of iron oxidation under the film); lowering of the heat resistance of Fe–Cr and Fe–Ni–Cr alloys during the oxidation (800°C) with an increase in the chromium content over 40 at. %; improving of the protective ability of the films formed at Fe–Ni alloys because of nickel oxidation under the dense FeO film; and the internal oxidation of the Fe 30Ni alloys under the FeO films with the internal formation of FeO oxides and spinel of NiFe₂O₄ type. It is shown that these anomalies can be explained, and the composition of the most heat-resistant alloys calculated, if one takes into account that associates with significantly stronger interatomic bonds than those in ideal solutions can form in solid solutions and cause unlimited solubility of the metallic components in each other.     

Dynamic recrystallization behavior of Fe–20Cr–30Ni–0.6Nb–2Al–Mo alloy

Single-pass compression tests of an aluminaforming austenite(AFA) alloy(Fe–20Cr–30Ni–0.6Nb–2Al–Mo) were performed using a Gleeble-3500 thermal–mechanical simulator. By combining techniques of electron back-scattered diffraction(EBSD) and transmission electron microscopy(TEM), the dynamic recrystallization(DRX) behavior of the alloy at temperatures of 950–1100 ℃ and strain rates of 0.01–1.00 s~(-1) was investigated. The regression method was adopted to determine the thermal deformation activation energy and apparent stress index and to construct a thermal deformation constitutive model. Results reveal that the flow stress is strongly dependent on temperature and strain rate and it increases with temperature decreasing and strain rate increasing. The DRX phenomenon occurs more easily at comparably higher deformation temperatures and lower strain rates. Based on the method for solving the inflection point via cubic polynomial fitting of strain hardening rate(h) versus strain(e) curves, the ratio of critical strain(ec) to peak strain(ep) during DRX was precisely predicted. The nucleation mechanisms of DRX during thermal deformation mainly include the strain-induced grain boundary(GB)migration, grain fragmentation, and subgrain coalescence.     

Synthesis of Fe–Cr–Mo–C–B–P bulk metallic glasses with high corrosion resistance

Bulk metallic glasses with a maximum thickness (t_max) of 1.0–2.7 mm were synthesized in the Fe₄₃Cr₁₆Mo₁₆(C, B, P)₂₅ system over a wide composition range by copper mold casting. They exhibit a large supercooled liquid region (ΔT_x) of 40–90 K and a high reduced glass transition temperature (T_g/T_l) of 0.54–0.60, indicating high glass-forming ability (GFA) and high thermal stability of the supercooled liquid. The critical cooling rate for glass formation was evaluated to be of the order of 10² K s⁻¹. The bulk metallic glasses exhibited high corrosion resistance in aggressive HCl solutions. The alloying element P has a beneficial effect on corrosion resistance.     

Electrodeposition and properties of Zn,Zn–Ni,Zn–Fe and Zn–Fe–Ni alloys from acidic chloride–sulphate electrolytes

Abstract

Zn, Zn–Ni, Zn–Fe and Zn–Fe–Ni films have been deposited by electrochemical deposition technique onto steel plate substrates. The objective of this study was to characterise the corrosion properties of these alloys in saline solution for the application as new environmentally friendly sacrificial coatings in the protection of steel structures. The morphological and structural properties of the alloys were systematically studied using XRD and SEM techniques. Cyclic voltammetry of the individual metals was performed to help understand the electroplating process of the films. Grain sizes of the films were calculated using Scherrer's formula. Partial substitution of Zn to Fe and Ni leads to an improvement in the corrosion resistance. Compared with other zinc alloys, the Zn–Ni alloy deposit was the noblest.     

Extrusion and selected engineering properties of Mo–Si–B intermetallics

In this study, an extrusion process has been developed to produce defect free, high-density rods of Mo–Si–B material. An initial powder composition (53.5 vol.%, 91 wt.%) of 66 vol.% Mo₅Si₃B_x (T1)–16 vol.% MoB–18 vol.% MoSi₂ was mixed with a paraffin-wax based binder (46.5 vol.%, 9 wt.%) and extruded using a twin-screw extruder. Following binder removal by a combination process of wicking and thermal degradation, the material was sintered at 1800°C. The bulk density of the sintered material was 90–92% of theoretical. Thorough binder removal was evidenced by low impurity levels: 258±6 ppm carbon and 772±10 ppm oxygen. The material demonstrated excellent high temperature oxidation resistance. The calculated parabolic rate constant is 1.1×10⁻² mg²/cm⁴/h at 1600°C. The extruded material was also successfully tested as a resistance heating element. These materials show promise for the development of heating elements with enhanced performance compared to current MoSi₂-based heating elements.     

Microstructural evolution and strain hardening of Fe–24Mn and Fe–30Mn alloys during tensile deformation

High Mn steels demonstrate an exceptional combination of high strength and ductility owing to their sustained high work hardening rate during deformation. In the present work, the microstructural evolution and work hardening of Fe–30Mn and Fe–24Mn alloys during uniaxial tensile testing at 293 K and 77 K were investigated. The Fe–30Mn alloy did not undergo significant strain-induced phase transformations or twinning during deformation at 293 K, whereas these transformations were observed during deformation at 77 K. A modified Kocks–Mecking model was successfully applied to describe the strain hardening behavior of Fe–30Mn at both temperatures, and quantitatively identified the influence of stacking fault energy and strain-induced phase transformations on dynamic recovery. The Fe–24Mn alloy underwent extensive ε martensite transformation during deformation at both test temperatures. An analytical micromechanical model was successfully used to describe the work hardening of Fe–24Mn and permitted the calculation of the ε martensite stress–strain curve and tensile properties.     

Au–Fe alloy solidification and solid-state transformations

In order to better understand the microstructure that forms during laser welding of an 18 carat gold and an austenitic stainless steel, solidification of the Au–Fe binary analog has been studied using thermal analysis and interrupted Bridgman experiments. For a hypoperitectic composition, the formation of the primary phase, its coarsening and the peculiar macrosegregation associated with the large density difference between the elements have been studied. Just after the peritectic phase forms around the primary dendrites, continuous and discontinuous precipitation has been shown to occur as a result of the immiscibility of the two face-centered cubic phases below the peritectic temperature. Finally, the solid-state transformations associated with the eutectoid have been characterized.     

Preparation and characterisation of melt-spun Al–Cu–Fe quasicrystals

Three Al–Cu–Fe alloys with compositions of Al_60–65Cu_20–27.5Fe_12.5–15 were prepared by conventional casting and further processed by melt-spinning. The structures formed were examined to get an insight into the interrelated effects of synthesis, processing and microstructure of Al–Cu–Fe alloys. The study aimed at answering the questions such as whether the production of single-phase quasicrystalline ribbons is possible by the melt-spinning process and what is the role of the degree of undercooling in the development of microstructure in melt-spun ribbons.The icosahedral ψ-Al₆₅Cu₂₀Fe₁₅ phase forms by a peritectic reaction between the primary β-AlFe phase and the liquid, as the temperature decreases. At the later stages of cooling, the monoclinic λ-Al₁₃Fe₄ phase and the tetragonal θ-Al₂Cu phase are formed in the cast alloys, as a result of peritectic reactions. In the rapidly solidified alloys, the formation of the tetragonal θ-Al₂Cu phase and, in the case of alloy Al₆₀Cu₂₅Fe₁₅, the monoclinic λ-Al₁₃Fe₄ phase is avoided, apparently due to high degree of undercooling. Thus, the production of single-phase quasicrystalline ribbons is not possible by the melt-spinning process, at least by using the cooling rate of 5–7×10⁴ °C/s. In addition to phase selection, the degree on undercooling influences, for example, the composition of the ψ-Al₆₅Cu₂₀Fe₁₅ phase and the grain morphology in melt-spun ribbons.     

Interfacial reaction between Co–Cr–Mo alloy and liquid Al

The interfacial reaction between Co–Cr–Mo alloy and liquid Al was investigated using immersion tests. Microstructure characterization indicated that the Co–Cr–Mo alloy was corroded by liquid Al homogeneously, with the formation of a (Co,Cr,Mo)₂Al₉ layer close to alloy matrix and “(Cr,Mo)₇Al₄₅ + Al” layer close to Al. Kinetics analysis showed that the corrosion of the Co–Cr–Mo alloy followed a linear relationship with the immersion duration. Compared with pure Co–liquid Al reaction system, the alloying of Cr and Mo changed the solid–liquid interface structure, but the corrosion of the solid metal was still dominated by the dissolution of an intermetallic layer.