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.     

Analysis of transformation stasis during the isothermal bainitic ferrite formation in Fe–C–Mn and Fe–C–Mn–Si alloys

The “transformation stasis” phenomenon during the isothermal bainitic ferrite formation has been investigated in a series of Fe–C–Mn and Fe–C–Mn–Si alloys. The Gibbs energy balance (GEB) approach is applied to model the transformation stasis phenomenon, and theoretical predictions are compared with experimental observations. The good agreement over several alloy systems demonstrates that the transformation stasis is caused by diffusion of alloying elements into the migrating austenite/bainitic ferrite interfaces. It is found that the occurrence of transformation stasis in the Fe–C–Mn and Fe–C–Mn–Si alloys depends on the concentration of Mn, while the addition of Si has a negligible effect on transformation stasis. The GEB model clearly outperforms the diffusionless T₀ model.     

Grain boundary embrittlement by Mn and eutectoid reaction in binary Fe–12Mn steel

The grain boundary embrittlement in a binary Fe–12Mn is due to the grain boundary segregation of Mn. During tempering at 400 °C (higher than the equilibrium eutectoid reaction temperature 247 °C), reverted austenite particles were formed at lath and grain boundaries through the equilibrium reaction of lath martensite to ferrite + austenite. Surprisingly, hydrostatic pressure, which is induced by the transformation of epsilon martensite to austenite during heating at the tempering temperature, resulted in the nonequilibrium eutectoid reaction producing α-Mn precipitates at the interface between lath martensite and the transformed austenite during the tempering. The segregation concentration kinetics of Mn formed a convex profile due to the active grain boundary precipitation of the reverted austenite particles and the α-Mn particles, which act as a sink for the segregated Mn. Finally, the convex segregation profile of Mn corresponded to the concave profile of intergranular fracture strength.     

Hot Deformation and Dynamic Recrystallization Behavior of Austenite-Based Low-Density Fe–Mn–Al–C Steel

The hot deformation and dynamic recrystallization(DRX) behavior of austenite-based Fe–27Mn–11.5Al–0.95 C steel with a density of 6.55 g cm-3were investigated by compressive deformation at the temperature range of900–1150 °C and strain rate of 0.01–10 s-1. Typical DRX behavior was observed under chosen deformation conditions and yield-point-elongation-like effect caused by DRX of d-ferrite. The flow stress characteristics were determined by DRX of the d-ferrite at early stage and the austenite at later stage, respectively. On the basis of hyperbolic sine function and linear fitting, the calculated thermal activation energy for the experimental steel was 294.204 k J mol-1. The occurrence of DRX for both the austenite and the d-ferrite was estimated and plotted by related Zener–Hollomon equations. A DRX kinetic model of the steel was established by flow stress and peak strain without considering dynamic recovery and d-ferrite DRX. The effects of deformation temperature and strain rate on DRX volume fraction were discussed in detail. Increasing deformation temperature or strain rate contributes to DRX of both the austenite and the d-ferrite, whereas a lower strain rate leads to the austenite grains growth and the d-ferrite evolution, from banded to island-like structure.     

Microstructure evolution during homogenization of Al–Mn–Fe–Si alloys: Modeling and experimental results

Microstructure evolution during the homogenization heat treatment of Al–Mn–Fe–Si, or AA3xxx, alloys has been investigated using a combination of modeling and experimental studies. The model is fully coupled to CALculation PHAse Diagram (CALPHAD) software and has explicitly taken into account the two different length scales for diffusion encountered in modeling the homogenization process. The model is able to predict the evolution of all the important microstructural features during homogenization, including the inhomogeneous spatial distribution of dispersoids and alloying elements in solution, the dispersoid number density and the size distribution, and the type and fraction of intergranular constituent particles. Experiments were conducted using four direct chill (DC) cast AA3xxx alloys subjected to various homogenization treatments. The resulting microstructures were then characterized using a range of characterization techniques, including optical and electron microscopy, electron micro probe analysis, field emission gun scanning electron microscopy, and electrical resistivity measurements. The model predictions have been compared with the experimental measurements to validate the model. Further, it has been demonstrated that the validated model is able to predict the effects of alloying elements (e.g. Si and Mn) on microstructure evolution. It is concluded that the model provides a time and cost effective tool in optimizing and designing industrial AA3xxx alloy chemistries and homogenization heat treatments.     

Sluggish diffusion in Co–Cr–Fe–Mn–Ni high-entropy alloys

Sluggish diffusion kinetics is an important contributor to the outstanding properties of high-entropy alloys. However, the diffusion kinetics in high-entropy alloys has never been probed directly. Here, the diffusion couple method was used to measure the diffusion parameters of Co, Cr, Fe, Mn and Ni in ideal-solution-like Co–Cr–Fe–Mn–Ni alloys. These parameters were compared with those in various conventional face-centered cubic metals. The results show that the diffusion coefficients in the Co–Cr–Fe–Mn–Ni alloys are indeed lower than those in the reference metals. Correspondingly, the activation energies in the high-entropy alloys are higher than those in the reference metals. Moreover, the trend of the normalized activation energy is positively related to the number of composing elements in the matrix. A quasi-chemical model is proposed to analyze the fluctuation of lattice potential energy in different matrices and to explain the observed trend in activation energies. Greater fluctuation of lattice potential energy produces more significant atomic traps and blocks, leading to higher activation energies, and thus accounts for the sluggish diffusion in high-entropy alloys.     

Deformation-intensified atomic separation in bcc Fe–Mn alloys

The deformation-intensified atomic Mn-related separation of the bcc solid solution has been found in Fe_100–xMn_x alloys (x = 4.5–9.9) subjected to ball milling using Mössbauer spectroscopy. In the near surrounding of iron atoms, the atomic separation is similar to that observed upon the annealing of the alloys in a temperature range of 400–500°С. It has been found that the deformation-intensified atomic separation leads to the stabilization of the bcc phase with regard to the α → γ transformation, as well as to the expansion of the field of the existence of the bcc phase during heating.     

Microstructure and high temperature tensile properties of Al–Si–Cu–Mn–Fe alloys prepared by semi-solid thixoforming

The differences in the microstructure and elevated temperature tensile properties of gravity die cast, squeeze cast, and semi-solid thixoformed Al–Si–Cu–Mn–Fe alloys after thermal exposure at 300 °C were discussed. The results demonstrate that the elevated temperature tensile properties of semi-solid thixoformed alloys were significantly higher than those of gravity die cast and squeeze cast alloys, especially after thermal exposure for 100 h. The ultimate tensile strength (UTS) of semi-solid thixoformed alloys after thermal exposure at 300 °C for 0.5, 10 and 100 h were 181, 122 and 110 MPa, respectively. The UTS values of semi-solid thixoformed alloys were higher than those of heat resistant aluminum alloys used in commercial applications. The enhanced elevated temperature tensile properties of semi-solid thixoformed experimental alloys after thermal exposure can be attributed to the combined reinforcement of precipitation strengthening and grain boundary strengthening due to thermally stable intermetallic phases as well as suitable grain size.     

Effect of niobium and grain boundary density on the fire resistance of Fe–C–Mn steel

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Effects of carbon addition and aging on the shape memory effect of Fe–Mn–Si–Cr–Ni alloys

The effects of carbon content and aging treatment on the microstructures and shape recovery ratio of Fe–Mn–Si–Cr–Ni alloys were studied. It was found that the carbon content possessed significant effects on the shape recovery ratio of Fe–Mn–Si–Cr–Ni alloys. The shape memory effect of alloys containing different carbon amount could be improved through aging.     

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...     

Studying recovery processes in a strain-hardened Al–Mg–Mn–Fe–Si alloy

The material of a shell structure subjected to 20-year use under ambient conditions has been studied. The structure and mechanical characteristics of a strain-hardened AMg6 alloy, as well as the effect of subsequent holdings of this alloy for 10–3000 h at temperatures of 50, 70, 80, 100, 130, 150, 180, and 220°C, on changes in its dislocation structure and mechanical characteristics have been investigated. It has been shown that, in the structures under study, the AMg6 alloy has a cellular structure with a high density of dislocations and the ultimate strength σ_u = 445.5 ± 2.5 MPa, the proof stress σ_0.2 = 326.5 ± 3.5 MPa, and the relative elongation δ = 11.7 ± 0.5%. Polygonization in the alloy occurs at a temperature of 220°C and the initial stage of the recovery process corresponds to a temperature range of 50–100°С in which the softening process can be divided into two stages, i.e., stage (1) of active softening due to the interaction of point defects with each other and stage (2) of the stabilization of the characteristics of the alloy.     

Fe–Mn mechanically alloyed powders characterised by local probes

High-energy ball milling of Fe–Mn elemental powder mixtures has been carried out for Mn atomic concentrations in the range 10–90%. X-ray diffraction (XRD), Mössbauer spectroscopy (MS) and perturbed angular correlations (PAC) have been used to investigate the crystalline structure in the milled samples. It is found that ball milling gives rise to concentration ranges of existence of terminal solid solutions more extended than in the equilibrium phase diagram. Particular attention has been paid to the environment of ⁵⁷Fe (MS) and ¹¹¹In (PAC) probe atoms: ⁵⁷Fe atoms are a constituent of the system, while ¹¹¹In atoms have been implanted at 400 keV into pills made from the milled powders. The PAC spectra of milled Fe show the ferromagnetic α-phase with a high fraction of a single vacancy next to the probe. Accordingly, the Mössbauer spectrum is a sextet with the characteristic splitting of α-Fe and a broadening which indicates a certain degree of atomic structure disorder. With an amount of Mn up to 15% PAC probes with one and two Mn next neighbours are identified by their smaller magnetic hyperfine field, corresponding to two distinct magnetic components in the Mössbauer spectra. At 20 and 30% of Mn the PAC magnetic signal of the a-phase disappears in favour of a distorted cubic apparently non-magnetic signal. From 40 to 70% of Mn a broad distribution of hyperfine magnetic fields is observed by PAC. Mössbauer spectra in the 20–70% of Mn range can be fitted with an unresolved low field magnetic sextet. Finally, the PAC spectra at 80 and 90% are quite similar to the one which is obtained after milling pure Mn, while the corresponding Mössbauer data can be roughly approximated to those of the α-Mn(Fe) solid solution.     

Recrystallization behavior,microstructure evolution and mechanical properties of biodegradable Fe–Mn–C(–Pd) TWIP alloys

In this study the interplay between recrystallization and precipitation in a biodegradable TWIP (twinning-induced plasticity) steel developed for use in temporary implants was investigated. Microstructural and mechanical properties were studied and a thermomechanical treatment was designed with the aim of achieving an overall performance suitable for the intended application as temporary implant material. The formation of Pd-rich precipitates in the cold-worked state was found to considerably retard recrystallization during an annealing treatment. The formation, morphology and interaction with dislocations of these precipitates were studied by means of scanning and transmission electron microscopy. Grain boundary pinning by Pd-rich precipitates (Zener drag) and reduced dislocation mobility due to a solute drag effect caused by the enrichment of dislocation cores with Pd were both identified as mechanisms which impede recrystallization. A model is reported which explains the interplay between recrystallization and precipitation, and provides the basis for the optimized thermomechanical treatment then presented. The resulting mechanical properties, in particular the combination of high strength and ductility with a pronounced strain-hardening response, exceed the performance of other TWIP steels and alloys typically used in biomedical implants, such as stainless steel, titanium or cobalt–chromium alloys. The specific property profile developed is especially advantageous for the production and deployment of cardiovascular stents.     

Wear Resistance of Austenitic Steel Fe–17Mn–6Si–0.3C with High Silicon and High Manganese

To solve the problem of poor wear resistance in conventional Hadfield steels under medium and low stress,a new kind of steel with high silicon and high manganese Fe–17Mn–6Si–0.3C was designed and its wear resistance was studied.The results showed that it exhibited better wear resistance than conventional Hadfield steel in both dry friction and abrasive friction.The better wear resistance of the new steel with high silicon and high manganese resulted from the stressinduced γ→ε martensitic transformation.     

Microstructure,martensitic transformation and mechanical properties of Ni–Mn–Sn alloys by substituting Fe for Ni

The effects of partial substitution of Fe element for Ni element on the structure, martensitic transformation and mechanical properties of Ni_50–xFe_xMn₃₈Sn₁₂ (x=0 and 3%, molar fraction) ferromagnetic shape memory alloys were investigated. Experimental results indicate that by substitution of Fe for Ni, the microstructure and crystal structure of the alloys change at room temperature. Compared with Ni₅₀Mn₃₈Sn₁₂ alloy, the martensitic transformation starting temperature of Ni₄₇Fe₃Mn₃₈Sn₁₂ alloy is decreased by 32.5 K. It is also found that martensitic transformation occurs over a broad temperature window from 288.9 to 352.2 K. It is found that the mechanical properties of Ni–Mn–Sn alloy can be significantly improved by Fe addition. The Ni₄₇Fe₃Mn₃₈Sn₁₂ alloy achieves a maximum compressive strength of 855 MPa with a fracture strain of 11%. Moreover, the mechanism of the mechanical property improvement is clarified. Fe doping changes the fracture type from intergranular fracture of Ni₅₀Mn₃₈Sn₁₂ alloy to transgranular cleavage fracture of Ni₄₇Fe₃Mn₃₈Sn₁₂ alloys.     

Hydrogen embrittlement in a Fe–Mn–C ternary twinning-induced plasticity steel

The influence of hydrogen entry on ductility was evaluated in a ternary twinning-induced plasticity (TWIP) steel with a composition of Fe–18Mn–0.6C in wt.% using tensile tests. The samples with a thickness of 1.2 mm were charged with hydrogen galvanostatically during the tensile tests. Significant hydrogen content was introduced by the hydrogen-charging. The total elongation was significantly deteriorated from approx. 60% to 30% by the hydrogen-charging. A clear intergranular fracture surface was observed in a vicinity of the sample surface in the hydrogen-charged samples.     

Methods for controlling the composition and morphology of electrodeposited Fe–Mo and Fe–Co–Mo coatings

The effect of the electrolyte composition and the stationary electrolysis parameters on the composition and morphology of Fe–Mo and Fe–Co–Mo coatings deposited from complex citrate electrolytes based on Fe(III) is studied. It is shown that, at a constant component ratio of с(Fe³⁺): с(Co²⁺): с(MoO^2? ₄): с(Cit^3–) = 2: 2: 1: 4, an increase in the electrolyte concentration leads to a decrease in the pH of the solution in a range of 4.85–4.30 and in the molybdenum content in the coating. An increase in the current density contributes to the molybdenum enrichment of the electrodeposited alloy in the entire range of electrolyte concentrations. The Fe–Mo alloy coatings have a rough microporous surface; an increase in the current density does not lead to significant changes in the surface topography. It is found that the formation of ternary coatings is characterized by the competitive reduction of iron and cobalt in the alloy; the molybdenum content depends on the current density. At a metal ratio of 3: 2: 1 and a molybdenum content of up to 17 at % in the Fe–Co–Mo alloy, the surface has a fine-grained needlelike structure typical of cobalt. With an increase in i _c, the atomic fraction of molybdenum increases, while the surface becomes microglobular. The Fe–Co–Mo electrodeposits with a metal ratio of 2.5: 1.5: 1.0 and a molybdenum content of 19–20 at % have a more developed surface with a high density of spheroids.     

Reduction of Fe concentration in Al–4Si–1Fe–1Cu–0.5Zn–0.5Mn alloys with S

In this article,the effect of sulfur on the reduction of Fe concentration in aluminum alloy scraps was investigated.The iron content decreases from1.224 wt% to \0.854 wt% and achieves an optimal ratio of30 % when the sulfur addition is 3 %.Thermodynamic calculations,the X-ray diffraction(XRD),scanning electron microscope(SEM),and differential scanning calorimetry(DSC)analyses of the sample indicate that the formation of Fe S can occur spontaneously in molten aluminum with the addition of sulfur.The mechanism of Feremoving purification process was also discussed.