Soft Impingement Effect Research Articles

Stabilizing Fe-based nanocrystalline alloys via a high-entropy strategy

Fe-based nanocrystalline alloys with unique dual-nano-phase structure and superior magnetic softness have aroused tremendous interest, yet they generally suffer from the harsh annealing process due to the poor thermal stability. In this study, a high-entropy strategy was proposed to enhance the soft-magnetic property and nanostructure stability of Fe-based nanocrystalline alloys by adding P and C elements in the FeCuSiBPC alloy system. This alloying approach by complicating the composition will greatly increase the mixing entropy, which significantly increase the frequency factor for the formation of α-Fe grains and activation energy for the formation of compounds, which on one hand enhance the competition and soft-impingement effects due to the increased number density of α-Fe grains, and on the other hand hinder the precipitation of compounds in the intergranular amorphous interphase due to the improved crystallization resistance. These both contribute to a thermodynamically and kinetically stable dual-nano-phase structure with fine α-Fe grains embedded in amorphous matrix. Following this strategy of nanostructure stabilization by tuning the compositional complexity, FeCuSiBPC alloy with enlarged processing window and enhanced soft-magnetic properties were successfully obtained. This high-entropy strategy can be applied in a verity of alloy systems to develop high performance nanocrystalline alloys with excellent thermal stability suitable for large-scale industrial processing.

Grain refinement mechanism of soft-magnetic alloys with nanocrystals embedded in amorphous matrix

To obtain uniform and stable nanostructure with fine α-Fe grains is very important for the wide applications of Fe-based nanocrystalline soft-magnetic alloys. In this study, the nanostructure evolution of the Fe84.75Si2B9P3C0.5Cu0.75 (at.%) alloy after annealing under different conditions was characterized in detail. It is found that the alloy exhibits excellent structural stability, which can maintain small α-Fe grains for a prolonged annealing time at low temperature. The increase of annealing temperature and/or annealing time will lead to the precipitation of compound phases in the intergranular amorphous interphase, which affects the α-Fe grains size greatly and determines the structural stability. The elemental mappings of the nanostructured alloys reveal that metalloid elements are enriched in the intergranular amorphous interphase, wrapping around α-Fe grains. The grain refinement and nanostructure stability of these alloys are derived from the shielding and soft-impingement effects of the core-shell like structure. The nanostructure stability is lost with the precipitation of compound phases in the intergranular amorphous interphase, owing to the break-down of the shielding layer, which results in the rapid coarsening of α-Fe grains by coalescence.

Formation mechanism of lamellar alpha in titanium through accurate simulation

The lamellar α, transformed β matrix and spheroidization products of them combine to form all typical microstructures in titanium alloys. Accurately predicting the formation of lamellar α and revealing the precipitation mechanism therefore become vital for tailoring the microstructure and thus optimizing the properties of titanium alloys. A mixed-mode model combined with cellular automaton is proposed to simulate the process of nucleation and growth of lamellar α phase in isothermal treatment of Ti alloys. In this model, the driving force that is dependent on temperature, the solute concentration in two phases and interface is derived; the soft impingement effect, considering the microscopic morphology evolution of lamellar α phase in two dimensions, is well described by not only approximating the diffusion field function in front of the interface as a polynomial but also introducing the area and interface length of lamellar α into the mass conservation law. A comparision with the interface-controlled and diffusion-controlled models shows that the mixed-mode model gives obviously more accurate description of β-to-α phase transformation kinetics. Furthermore, it is also found that the interfacial migration coefficient, and the microscopic morphology of lamellar α phase to be precipitated have great effects on the mixed mode character (in especial the change in controlled mode) and the kinetics of entire phase transformation process.

Influence of M23C6 dissolution on the kinetics of ferrite to austenite transformation in Fe-11Cr-0.06C stainless steel

The design of high-strength martensitic stainless steels requires an accurate control over the stability of undesired phases, like carbides and ferrite, which can hamper strength and ductility. Here, the ferrite to austenite transformation in Fe-11Cr-0.06C has been studied with a combined experimental-modelling approach. Experimental observations of the austenization process indicate that austenite growth proceeds in multiple steps, each one characterized by a different transformation rate. DICTRA based modelling reveals that the dissolution of the M23C6 Cr-rich carbides leads to Cr partitioning between austenite and parent phases, which controls the rate of transformation through (i) a soft-impingement effect and (ii) consequent stabilization of the ferrite, which remains untransformed inside chromium-enriched-zones even after prolonged austenization stage. Slow heating rate and smaller initial particle sizes allow the design of ferrite-free microstructure.

Growth mechanism of primary needles during the solidification of chromium carbide overlays

A mechanism for modelling the growth of faceted primary M7C3 needles during solidification of chromium carbide overlays is presented for the first time. In contrast with existing models that consider diffusion-controlled growth over the length of the needle, this work disaggregates the flow of solute at the tip (diffusion-controlled) and the faceted sides (interface-controlled). The proposed solution predicts an initial transient followed by a steady state longitudinal growth. The outward growth of the needle prism faces is linear and slow, consistent with interface-dominated kinetics. Abrupt decreases in longitudinal growth rate was observed when the tip of the carbides approached other carbides, consistent a soft-impingement effect when the diffusion field of two carbides a overlap. The model was compared to in-situ visualisation of the solidification of an alloy with primary chromium carbide needles, and was found to match closely to observations.

Microstructural evolution and phase transformation kinetics of MnBi alloys

The low temperature phase (LTP) compound of MnBi is a promising, rare-earth metal free, and permanent magnetic material. However, it is difficult to obtain pure LTP MnBi, which results from a peritectic reaction between Mn and Bi atoms. In this study, the microstructural evolution and phase transformation kinetics of MnxBi100-x (x = 50, 55, 60) alloys were systematically investigated. Choosing an appropriate Mn content (Mn55Bi45) was found to promote the formation of the LTP MnBi. The results of the phase transformation kinetics indicated that the nucleation, growth and soft impingement processes are involved in the phase transformation process. In addition, in the initial stage of the LTP phase transformation, it was shown that the diffusional transformation was governed by two- and three-dimensional nucleation and growth, as well as a single mechanism of diffusion-controlled growth, which describes the entire transformation. At the final stage of the phase transformation, diffusion controlled growth, with soft impingement effects, dominates the process. The phase transformation kinetics analysis was confirmed with the X-ray diffraction and scanning electron microscopy results. This work implies that the local activation energy, Ec(α), and local Avrami exponent, n(α), can be applied to characterize the phase transformation behaviors of the LTP MnBi alloy.

AUSTENITE TRANSFORMING IN CONTINUOUS COOLING PROCESS UNDER DIFFUSION CONTROL MODEL

Austenite-ferrite transformation in low carbon steels has a fundamental role in phase transforma- tion and is industrial importance. The kinetics of austenite transformation can be described by the kinetics of aus- tenite-ferrite interface migration. Two classical models, the diffusion-controlled growth model and the interface- controlled model, can be used to describe the growth of proeutectoid ferrite during g→a isothermal transformation. The austenite transformation in continuous cooling process is more common in production. In continuous cooling process, the equilibrium carbon concentrations in austenite and ferrite change with temperature and the kinetics of austenite transformation is different from that in isothermal process. Based on the models for g→a isothermal trans- formation, a diffusion control model is established for the growth of proeutectoid ferrite during the decomposition of supersaturated austenite in continuous cooling process. The interface position of proeutectoid ferrite varying with temperature is described with the model. The soft impingement effect at the later stage of transformation is considered. The carbon concentration at the austenite side of interface is difficult to reach the equilibrium carbon concentration when the cooling rate is high. A parameter as the function of cooling rate is proposed to modify the carbon concentration at the austenite side of interface. The polynomial diffusion field approximation is assumed in front of the interface. Simulation is done by utilizing the model to analyze the growth of proeutectoid ferrite in con- tinuous cooling process with different bulk concentrations, austenite grain sizes and cooling rates. The interface po- sition of proeutectoid ferrite as a function of temperature or time is obtained under different cooling conditions. Al- so, carbon diffusion length at the austenite side of interface as a function of time and carbon profile as a function of

Nanocrystallization in spark plasma sintered Fe48Cr15Mo14Y2C15B6 bulk amorphous alloy

Spark plasma sintering (SPS) is evolving as an attractive process for the processing of multi-component Fe-based bulk amorphous alloys and their in-situ nanocomposites with controlled primary nanocrystallization. Extended Q-range small angle neutron scattering (EQ-SANS) analysis, complemented by x-ray diffraction and transmission electron microscopy, was performed to characterize nanocrystallization behavior of SPS sintered Fe-based bulk amorphous alloys. The SANS experiments show significant scattering for the samples sintered in the supercooled region indicating local structural/compositional changes associated with the profuse nucleation of nanoclusters (∼4 nm). For the samples spark plasma sintered near and above crystallization temperature (>653 °C), the SANS data show the formation of interference maximum indicating the formation and growth of (Fe,Cr)23C6 crystallites. The SANS data also indicate the evolution of bimodal crystallite distribution at higher sintering temperatures (above Tx1). The growth of primary nanocrystallites results in impingement of concentration gradient fields (soft impingement effect), leading to non-random nucleation of crystallites near the primary crystallization.

Preparation and characterization of poly(amic acid)-stabilized silver nanoparticles

Stable and monodispersed silver nanoparticles were produced through a mild, convenient, one-pot method based on the reduction of silver nitrate in the presence of poly(amic acid) (PAA) as a stabilizer. The surface plasma band transition was monitored along with time in the reaction mixture for three sets of experiments by ultraviolet-visible spectroscopy. Analysis of the data with the Avrami equation yielded n exponent with values between 0.5 and 1.5, demonstrating three-dimensional heterogeneous nucleation and diffusion-controlled growth, accompanied by soft impingement effect. XRD and TEM analyses show a softly agglomerated polycrystalline state and a nearly spherical morphology (<50 nm) of nanoparticles. The FT-IR result indicates that the PAA molecular structure could be hardly influenced by the formation of nanoparticles.

A Mixed-Mode Model Considering Soft Impingement Effects for Solid-State Partitioning Phase Transformations

The original mixed-mode model is reformulated by considering the soft impingement effect and applying a general polynomial method of dealing with the concentration gradient in front of the interface. Comparison with the numerical solution shows that the reformulated mixed-mode model is more precise than the original model. The effect of soft impingement on the kinetics of partitioning phase transformation depends on both the growth mode and the degree of super-saturation.

Modeling of soft impingement effect during solid-state partitioning phase transformations in binary alloys

The soft impingement effect at the later stage of partitioning phase transformations has been modeled both for the diffusion-controlled growth model and for the mixed-mode model. Instead of the linear and exponential approximations for the concentration gradient in front of the interface used in the past, a general polynomial method of dealing with the diffusion field is proposed. The linear and exponential diffusion field approximations are two specific cases of the polynomial diffusion field approximation. The effect of soft impingement on the overall partitioning phase transformation is only related to the degree of the super-saturation in case of the diffusion-controlled growth model, while it is determined by both the growth mode and the degree of super-saturation in case of the mixed-mode model.

Application of the cyclic phase transformation concept for investigating growth kinetics of solid-state partitioning phase transformations

A heat treatment involving cyclic partial phase transformations has been analyzed to determining growth kinetics of the austenite to ferrite phase transformation and vice versa more accurately. The mixed-mode model and the diffusion-controlled phase transformation model, including the soft impingement effects at the later stage of phase transformation, are reformulated for the cyclic austenite to ferrite and ferrite to austenite transformation. A new growth mode parameter H is defined for the mixed-mode model. It shows that not all partitioning phase transformations starts as pure interface-controlled growth but all of them shift gradually towards pure diffusion control when thermodynamic equilibrium is approached. The diffusion-controlled model predicts that the ferrite to austenite transformation should be faster than the austenite to ferrite, while the simulation of the mixed-mode model shows that the transformation rate ratio is a function of the ratio of interface mobility.

A JMAK-like approach for isochronal austenite–ferrite transformation kinetics in Fe–0·055 wt-%N alloy

The kinetics of the isochronal austenite γ to ferrite α transformation of Fe–0·055 wt-%N alloy was investigated for cooling rates in the range of 5–15 K min− by high resolution dilatometry. In accordance with thermodynamic characteristics of γ→α transformation investigated in this study and previous kinetic theory, a Johnson–Mehl–Avrami–Kologoromov (JMAK)-like approach for the kinetics of isochronal phase transformations was developed that incorporates three overlapping processes: site saturation nucleation, alternate growth modes (transition from interface- to diffusion-controlled growth) as well as impingement for random distribution nuclei. The JMAK-like approach has been employed to fit the experimental results, and the fitting results show that the γ→α transformation of Fe–0·055 wt-%N alloy does have two stages: a first, short interface-controlled growth stage and a second, long diffusion-controlled growth stage. In addition, soft impingement effect has been recognised to become serious in the later part of the second stage.

Phase-field modelling of microstructural evolution in primary crystallization

One of the main routes to obtain nanostructured materials is through the primary crystallization of metallic glasses. In such transformations, crystallites with a different composition than the amorphous precursor grow with a diffusion-controlled regime. Particle growth is slowed and eventually halted by the impingement between the concentration gradients of surrounding particles. Primary crystallization kinetics is not well described by the KJMA equation, and this fact was generally ascribed to both the soft-impingement effect and the non-random nucleation. However, recent phase-field simulations showed that the underlying physical reason is the change in the local diffusion properties of the amorphous precursor due to the variation of the composition during the transformation. The kinetics of primary crystallization is thus well described by considering a diffusion coefficient of the slowest diffusing species dependent on the local concentration. The nanostructure developed in such transformations is a key point to explain the macroscopic properties of these materials. In this work the grain size distributions obtained in realistic phase-field simulations of transformations with continuous nucleation and both constant and variable diffusion coefficient are presented. The obtained distributions are analyzed and the physical mechanisms responsible of their different features are recognized.

Phase Transformation Modeling of Medium-Carbon Forging Steel

The kinetics of phase transformations in medium-carbon forging steels (MCFS) have been modeled based on CALPHAD multicomponent thermodynamics and the classical nucleation-growth theory. New treatments include the time dependency of parabolic growth rate of proeutectoid ferrite (α) , which account for the soft impingement effect by carbon enrichment in austenite (γ). And a potential transition of γ/α interface equilibrium has also been considered depending on temperatures and velocity of the moving interface. To make a realistic prediction of the onset of pearlite (P) transformation, a normal distribution of γ grain size has been assumed and successive α→P transformation kinetics in each grain size have been summated. The developed program coupled with thermodynamic solver, 'ThermoCalc', calculated the isothermal kinetics of MCFS and has been found to predict well the effect of minor difference of chemical composition / holding temperatures.

Phase-field modeling of glass crystallization: Change of the transport properties and crystallization kinetic

In this work we present many-particle phase-field simulations of primary crystallization with a diffusion coefficient dependent on the local composition of the untransformed matrix. The results show that the experimental kinetics observed in primary crystallization of many metallic systems cannot be described by the soft-impingement effect but to the change of the transport properties of the matrix as the transformation proceeds.

Modelling of primary bcc-Fe crystal growth in a Fe 85B 15 amorphous alloy

A kinetic modelling of primary crystallization in metallic glasses, based on the CALPHAD approach and the moving boundary model, has been applied to the Fe–B system. The DICTRA software has been used to perform numerical calculations. Kinetic and thermodynamic parameters (atomic mobilities and thermodynamic factors) are required and they have been obtained from the literature. Various simulations have been performed in order to evaluate the influence of different parameters choice. The soft impingement effect has been discussed. Furthermore, amorphous Fe 85B 15 samples have been prepared and examined by differential scanning calorimetry. Calculated and experimental results, both on continuous heating and isothermal conditions, have been compared.

Modelización de la formación isotérmica de ferrita alotriomórfica a temperaturas inferiores a la del eutectoide en un acero microaleado de medio carbono

The present paper is concerned to the theoretical and experimental description of the isothermal growth kinetics of allotriomorphic ferrite in a medium carbon vanadiumtitanium microalloyed steel at temperatures below its eutectoid temperature and above any adiffusional transformation start temperature. Since pearlite is formed at these temperatures, the carbon enrichment of austenite far from the α/γ interface remains the same as the overall carbon content of the steel, and the soft-impingement effect can be neglected. Therefore, allotriomorphic ferrite can be considered to grow under a parabolic law, and the assumption of a semi-infinite extent austenite with constant boundary conditions is suitable for the kinetics of the isothermal decomposition of austenite at temperatures below eutectoid and above any adiffusional transformation. Moreover, the influence of the austenite grain size and isothermal temperature on the kinetics of the allotriomorphic ferrite transformation has been analyzed.

Modeling of kinetics of austenite-to-allotriomorphic ferrite transformation in 0.37C-1.45Mn-0.11V microalloyed steel

The present article is concerned with the theoretical and experimental study of the growth kinetics of allotriomorphic ferrite in medium carbon vanadium-titanium microalloyed steel. A theoretical model is presented in this work to calculate the evolution of austenite-to-allotriomorphic ferrite transformation with time at a very wide temperature range. At temperatures above eutectoid temperature, where allotriomorphic ferrite is the only austenite transformation product, the soft-impingement effect should be taken into account in the modeling. In that case, the Gilmour et al. analysis reliably predicts the progress of austenite-to-allotriomorphic ferrite transformation in this steel. By contrast, since pearlite acts as a carbon sink, the carbon enrichment of austenite due to the previous ferrite formation is avoided, and carbon concentration in austenite far from the α/γ interface remains the same as the overall carbon content of the steel. Hence, the soft-impingement effect should be neglected, and allotriomorphic ferrite is considered to grow under a parabolic law. Therefore, assumption of a semi-infinite extent austenite with constant boundary conditions is suitable for the kinetics of the isothermal decomposition of austenite. An excellent agreement (higher than 93 pct in R2) has been obtained between the experimental and predicted values of the volume fraction of ferrite in all of the ranges of temperature studied.

Modeling of kinetics of austenite-to-allotriomorphic ferrite transformation in 0.37C-1.45Mn-0.11V microalloyed steel

The present article is concerned with the theoretical and experimental study of the growth kinetics of allotriomorphic ferrite in medium carbon vanadium-titanium microalloyed steel. A theoretical model is presented in this work to calculate the evolution of austenite-to-allotriomorphic ferrite transformation with time at a very wide temperature range. At temperatures above eutectoid temperature, where allotriomorphic ferrite is the only austenite transformation product, the soft-impingement effect should be taken into account in the modeling. In that case, the Gilmour et al. analysis reliably predicts the progress of austenite-to-allotriomorphic ferrite transformation in this steel. By contrast, since pearlite acts as a carbon sink, the carbon enrichment of austenite due to the previous ferrite formation is avoided, and carbon concentration in austenite far from the α/γ interface remains the same as the overall carbon content of the steel. Hence, the soft-impingement effect should be neglected, and allotriomorphic ferrite is considered to grow under a parabolic law. Therefore, assumption of a semi-infinite extent austenite with constant boundary conditions is suitable for the kinetics of the isothermal decomposition of austenite. An excellent agreement (higher than 93 pct in R 2) has been obtained between the experimental and predicted values of the volume fraction of ferrite in all of the ranges of temperature studied.