The phase-field approach and solute drag modeling of the transition to massive γ → α transformation in binary Fe-C alloys

The transition between diffusion controlled and massive transformation γ →α in Fe–C alloys is investigated by means of phase-field simulations and thermodynamic functions assessed by the Calphad technique as well as diffusional mobilities available in the literature. A gradual variation in properties over an incoherent interface, having a thickness around 1 nm, is assumed. The phase-field simulations are compared with a newly developed technique to model solute drag during phase transformations. Both approaches show qualitatively the same behavior and predict a transition to a massive transformation at a critical temperature below the T₀ line and close to the α/α + γ phase boundary. It is concluded that the quantitative difference between the two predictions stems from different assumptions on how the properties vary across the phase interface yielding a lower dissipation of Gibbs energy by diffusion in the phase-field simulations. The need for more detailed information about the actual variation in interfacial properties is emphasized.     

奥氏体形成机制

以T8等钢为例研究并综合分析了珠光体逆共析转变为奥氏体的过程和机制,指出,奥氏体是碳或各种化学元素溶入γ-Fe中所形成的固溶体。奥氏体在铁素体／渗碳体相界面形核,也可在珠光体领域交界处和原奥氏体晶界上形核。奥氏体以体扩散形式长大。奥氏体形核及长大过程中可同时吞噬铁素体和渗碳体,或只吞噬铁素体片长大,或吞噬铁素体较快,而吞噬渗碳体较慢,完成逆共析转变,最后剩下部分渗碳体。为减少相变积累的畸变和缓和应变能,而调整生长方向,形成了孪晶。奥氏体中存在未溶碳化物,需继续溶解和均匀化。     

The influence of lattice strain on pearlite formation in Fe–C

The effect of stress and strain on the transformation kinetics of pearlite is investigated by phase-field simulation. Strain is considered in terms of expansion/contraction during transformation and due to concentration gradients in austenite. It is demonstrated that due to the concentration dependence of the eigenstrain, an inhomogeneous stress distribution ahead of the transformation front enhances diffusion in the austenitic phase and reduces chemical supersaturation in both austenite and ferrite. The main result of the investigation is that transformation strain inhibits the cooperative growth mode of cementite and ferrite, as considered by the Zener–Hillert model, and provokes the salient growth of cementite needles ahead of the ferrite front, which we call “staggered growth”. The predicted growth velocities give the right order of magnitude compared to the experiment and close the gap between theoretical models based on diffusion only, and experimental observations.     

A combined model for the description of austenitization,homogenization and grain growth in hypoeutectoid Fe–C steels during heating

A combined model which allows one to simulate all the steps of the reaustenitization process of ferrito-pearlitic plain carbon steel has been developed. The dissolution of pearlite, the transformation of ferrite into austenite and the homogenization of the carbon distribution is described with a finite volume method. The simulation is performed on a bidimensional domain where ferrite (α), pearlite (P) and austenite (γ) grains are represented. The dissolution of pearlite is described by the growth of spherical grains and simple nucleation and growth laws. The movement of α/γ interfaces is calculated by solving the diffusion equation for carbon in the α and γ phases and accounting for the solute flux balance at the interface using a pseudo-front tracking method. The diffusion model is coupled with a Monte Carlo simulation which describes the grain growth occurring in austenite at a later stage of austenitization. The evolution of the volume fractions of pearlite and ferrite, the maximum and minimum carbon concentrations in the domain and the mean austenite grain size are represented as a function of the temperature for a typical case of constant heating rate. The influence of the different steps of the austenitization process on the global kinetics is discussed.     

GCr15钢离异共析转变前沿生长速度的计算

通过中断淬火试验,采用扫描电子显微镜研究了GCr15钢的离异共析转变。采用相变动力学方法计算了片状珠光体转变和离异共析转变前沿的生长速度。结果表明：过冷奥氏体剩余碳化物颗粒间距越小,离异共析转变临界过冷度就越大。     

合金元素对铸铁共析转变的影响

介绍了铸铁共析转变的概念、特点和转变产物;说明了没有其他合金的铸铁共析转变时,奥氏体中富余碳的去向问题;阐明了合金元素对铸铁共析转变温度、转变产物的影响情况和珠光体形成与生长机理;提出了降低铸件断面敏感性的途径.     

On the role of martensitic transformation on damage and cracking resistance in TRIP-assisted multiphase steels

The damage resistance, fracture toughness and austenite transformation rate in transformation-induced plasticity (TRIP)-assisted multiphase steel sheets were comparatively characterised on two steel grades differing by the volume fractions of the phases (i.e. ferrite, bainite, retained austenite) and by the mechanical stability of retained austenite. The influence of stress triaxiality on austenite transformation kinetics and the coupling between martensitic transformation and damage were investigated using double edge notched (or cracked) plate specimens tested in tension. The map of the distribution of transformation rates measured locally around the notch (or the crack) was compared with the map of the effective plastic strains and stress triaxialities computed by finite element simulations of the tests. The mechanically-activated martensitic transformation was found to progress continuously with plastic straining and to be strongly influenced by stress triaxiality. Fracture resistance was characterised by means of J_R curves and CTOD measurements using DENT specimens. The fracture toughness at cracking initiation was found to be lower for the steel with higher tensile strength and ductility. The contrasted influence of the TRIP effect, which improves formability by delaying plastic localisation but reduces fracture toughness at cracking initiation, is shown to result from parameters such as the volume fraction of non-intercritical ferrite phases or the mechanical properties of martensite.     

Direct evidence for the formation of ordered carbides in a ferrite-based low-density Fe–Mn–Al–C alloy studied by transmission electron microscopy and atom probe tomography

We study the structure and chemical composition of the κ-carbide formed as a result of isothermal transformation in an Fe–3.0Mn–5.5Al–0.3C alloy using transmission electron microscopy and atom probe tomography. Both methods reveal the evolution of κ-particle morphology as well as the partitioning of solutes. We propose that the κ-phase is formed by a eutectoid reaction associated with nucleation growth. The nucleation of κ-carbide is controlled by both the ordering of Al partitioned to austenite and the carbon diffusion at elevated temperatures.     

9%Cr-1%Mo耐热钢焊缝金属连续冷却组织转变

利用Ｆｏｒｍａｓｔｏｒ－Ｄ全自动热膨胀记录仪测定了三种改进型９％Ｃｒ－１％Ｍｏ耐耐热钢焊缝金属的连续冷却组织转变相图,并用光学显微镜和透射是分析了各种冷却条件下的组织转变特点。结果表明,在很大的冷却范围内,三种焊缝金属的工体均只发生马氏体转变,只有当冷速足够慢时才发生先共析铁素体转变和共析转变。其中,含合金元素较少的２号焊缝金属具有最快的先共析铁素体形成冷速。研究发现,三种焊缝金属的奥氏体均形成板     

High-energy X-ray diffraction study on the temperature-dependent mechanical stability of retained austenite in low-alloyed TRIP steels

The stability of the retained austenite has been studied in situ in low-alloyed transformation-induced-plasticity (TRIP) steels using high-energy X-ray diffraction during tensile tests at variable temperatures down to 153 K. A detailed powder diffraction analysis has been performed to probe the austenite-to-martensite transformation by characterizing the evolution of the phase fraction, load partitioning and texture of the constituent phases simultaneously. Our results show that at lower temperatures the mechanically induced austenite transformation is significantly enhanced and extends over a wider deformation range, resulting in a higher elongation at fracture. Low carbon content grains transform first, leading to an initial increase in average carbon concentration of the remaining austenite. Later the carbon content saturates while the austenite still continues to transform. In the elastic regime the probed {h k l} planes develop different strains reflecting the elastic anisotropy of the constituent phases. The observed texture evolution indicates that the austenite grains oriented with the {2 0 0} plane along the loading direction are transformed preferentially as they show the highest resolved shear stress. For increasing degrees of plastic deformation the combined preferential transformation and grain rotation results in the standard deformation texture for austenite with the {1 1 1} component along the loading direction. The mechanical stability of retained austenite in TRIP steel is found to be a complex interplay between carbon concentration in the austenite, grain orientation, load partitioning and temperature.     

Transformation of austenite in Fe-Cu alloys. II: Kinetics of a two-stage transformation of austenite in the Fe-5.5% Cu alloy

The decomposition of austenite in the hypereutectoid Fe-5.5% Cu alloy has been considered as a complex process that includes the precipitation of ε-phase particles and a eutectoid transformation, during which particles of a granular pearlite are nucleated. The theory of the growth of particles has been considered, and the kinetic equations for each stage of the austenite transformation have been introduced.     

Three-dimensional multi-phase-field simulation of eutectoid alloy based on OpenCL parallel

Seeking high-performance computing methods to solve the problem of a large amount of calculation, low calculation efficiency, and small simulation scale on the traditional single central processing unit (CPU) platform is of great value to the simulation study of micro-structure. In this study, based on the three-dimensional multi-phase-field model of KKSO coupling phase-field and solute field, the open computing language (OpenCL) + graphics processing unit (GPU) heterogeneous parallel computing technology is used to simulate the eutectoid growth of Fe-C alloy and the end growth process of pearlite under pure diffusion. The effects of initial supercooling and different diffusion coefficients on the growth morphology of lamellar pearlite were investigated. The results show that ferrite and cementite are perpendicular to the front of the solid-solid interface and are coupled and coordinated to grow, and there is no leading phase under the initial supercooling degree of 20 K. With the continuous increase of the initial supercooling degree (19 K–22 K), the morphology changes of the eutectoid layer are as follows: cementite stops growing → slice amplitude increases → regular symmetric growth → oblique growth → layer merge. With the increase of the diffusion coefficient from 3×10⁻¹³ m²·s⁻¹ to 15×10⁻¹³ m²·s⁻¹, the growth rate of the microstructure of the lamellar pearlite increases linearly, and there is no obvious change in the frontal appearance of the pearlite.

     

The Mechanism of Phase Transformation in Thermally-Grown FeO Scale Formed on Pure-Fe in Air

The isothermal phase transformation behavior of thermally grown oxide scale of FeO, which was formed on Fe at 700 °C in air for 16 min, was investigated at 320, 450, 500, 520, and 560 °C in air. The phase transformation of FeO was found to consist of four transformation modes: (1) growth of outer Fe₃O₄ layer; (2) precipitation of Fe₃O₄; (3) formation of magnetite seam; and (4) eutectoid decomposition of FeO. The transformation was always completed by the eutectoid decomposition at all temperatures in the present study; however, the proportion of transformation mode (1) and (2) strongly depended on temperature. At higher temperatures growth of the outer Fe₃O₄ layer initially predominates, but the precipitation of Fe₃O₄ controls the initial transformation at lower temperature before the eutectoid reaction. The eutectoid reaction was found to be initiated by Fe nucleation from Fe-saturated FeO. Fe saturation in FeO was due to growth and/or precipitation of Fe₃O₄ and formation of the magnetite seam layer, which acts as a diffusion barrier for Fe inward diffusion into Fe substrate. It was proposed that these transformation modes, growth and/or precipitation of Fe₃O₄ and magnetite seam formation, are necessary to begin the eutectoid reaction, i.e., completion of FeO scale transformation.     

Giant magnetocaloric effect in a Heusler Mn50Ni40In10 unidirectional crystal

A modified high-pressure optical zone-melting technique was used to grow a Mn-rich Heusler Mn₅₀Ni₄₀In₁₀ unidirectional crystal. Experimental results showed that the produced unidirectional crystal underwent a magnetic transition in austenite, followed with a martensitic transformation from a ferromagnetic austenite to a ferromagnetic martensite upon cooling. Under a magnetic field change of 30 kOe, the total effective refrigeration capacities (RC_total) reached as high as 231.58 J/kg when the magnetic field was applied along parallel to the crystal growth direction, or 246.79 J/kg when the magnetic field was applied along perpendicular to the crystal growth direction. It was suggested that this unidirectional crystal growing technique may provide an effective approach to enhance the magnetocaloric effect of Mn-rich Heusler materials.     

Modeling austenite decomposition into ferrite at different cooling rate in low-carbon steel with cellular automaton method

The austenite decomposition into ferrite during continuous cooling in low-carbon steel has been investigated with a two-dimensional cellular automaton (CA) approach. In this model, the growth of ferrite grain is controlled by both carbon diffusion and γ–α interface dynamics. In order to predict the growth kinetics of ferrite grain, the coupled carbon diffusion behavior in untransformed austenite and γ–α interface dynamics are numerically resolved. The simulation provides an insight into the carbon diffusion process in retained austenite and microstructure evolution during the transformation. The predicted ferrite growth kinetics and average grain size at different cooling rates are compared with experimental results in the literature and the simulated results show that the final grain size and newly formed ferrite fraction vary with cooling rate. The γ–α interface is stable in the studied cooling rate range (up to 58 °C s⁻¹) in this work, so the simulated morphology of ferrite grain is almost equiaxed, which is not influenced by the anisotropy of the hexagonal mesh in this CA model.     

Phase field study on crystal orientation effects in eutectoid phase transformation

In this present work, a multi-phase field model was used to simulate the eutectoid transformation process, and on the basis of the nucleation model that was previously proposed by our research team, anisotropic and orientation relationship models were introduced to study the growth mechanism of the pearlite lamellae with anisotropy. It was found that the growth direction of the pearlite lamellae is related to its orientation and spacing. In the process of lamellar growth, deflection growth of pearlite will appear along with the adjustment of lamellar spacing, and the deflection angle is equal to the orientation difference between the austenite and the pearlite. Comparison between experimental and numerical results indicates a good consistency in pearlite morphology.     

A study of the influence of Mn and Ni on the kinetics of the proeutectoid ferrite reaction in steels

The kinetic transition between partitioned and unpartitioned growth of proeutectoid ferrite has been studied for high-purity Fe–C–Mn and Fe–C–Ni alloys, and for temperatures just above the eutectoid. These results (and certain results of previous investigations) are compared with computed paraequilibrium and equilibrium ternary phase diagrams, and it is shown that the transition occurs well within the paraequilibrium two-phase regions, but significantly outside the limit predicted by the local equilibrium analysis of the ternary precipitation reaction. These observations are interpreted in terms of solute drag theory. It is inferred that both Mn and Ni exert a drag on the moving ferrite/austenite interfaces, and that this drag force is due to substitutional solute diffusion within the moving interface. The equilibrium binding energies of each of the substitutional solutes to the boundary are expected to be of order RT.     

钢中珠光体转变（一）

钢中珠光体转变属典型的扩散型共析转变。其典型的显微组织为层片状珠光体。本文吸取了国内外近一二十年中关于珠光体转变的主要研究成果,阐述了珠光体形成热力学、动力学、形成机制、影响因素和产物的力学性能等。最后讲述了珠光体生长的台阶机制及一些非铁碳合金的共析组织。     

Crystallography of grain boundary cementite dendrites

The crystallography and morphology of proeutectoid grain boundary cementite precipitates were studied in an isothermally transformed Fe-1.3%C-12%Mn steel. Grain boundary cementite precipitates develop with complex variations of a morphology that can be described as fern-like dendrites growing preferentially within and along austenite grain boundaries. The dendritic morphology was made most readily apparent by deep etching specimens of the alloy whose growth had been arrested after relatively short times during the solid state austenite→cementite transformation. Trace analysis revealed that these precipitates usually have no crystallographically preferred primary or secondary dendrite arm growth directions. Electron Backscattered Diffraction pattern analysis showed that most of the grain boundary cementite precipitates approximated one of the known cementite–austenite crystallographic orientation relationships (OR) with at least one of its adjacent austenite grains. Occasionally, the grain boundary cementite exhibited an OR that achieved ‘best fit’ directions between the cementite and austenite in both adjacent austenite grains.     

Growth of austenite from as-quenched martensite during intercritical annealing in an Fe–0.1C–3Mn–1.5Si alloy

The growth of austenite from as-quenched martensite during intercritical annealing was studied in a quaternary Fe–0.1C–3Mn–1.5Si alloy. Fine austenite grains either grew from interlath-retained austenite films or were newly nucleated at lath and martensite packet boundaries. Both types grew to a size comparable to the width of the martensite lath. It was found both metallographically and by dilatometry that the austenite grew to an amount in excess of the volume fraction at final equilibrium. Simulation by DICTRA, which assumed local equilibrium at the α/γ boundary, confirmed that the development of austenite is composed of three stages: initial negligible-partitioning growth controlled by rapid carbon diffusion in ferrite, which is gradually replaced by carbon diffusion in austenite; intermediate slow growth, controlled by diffusion of Mn and/or Si in ferrite; and a final stage controlled by diffusion of substitutional elements in austenite for final equilibration, which may result in the shrinkage of austenite. The formation of austenite in excess of the equilibrium amount is considered to occur due to very slow substitutional diffusion in the growing austenite compared to the boundary migration.