Modeling of austenite to ferrite transformation

In this research, an algorithm based on the $\boldsymbol{Q}$ -state Potts model is presented for modeling the austenite to ferrite transformation. In the algorithm, it is possible to exactly track boundary migration of the phase formed during transformation. In the algorithm, effects of changes in chemical free energy, strain free energy and interfacial energies of austenite?Caustenite, ferrite?Cferrite and austenite?Cferrite during transformation are considered. From the algorithm, the kinetics of transformation and mean ferrite grain size for different cooling rates are calculated. It is found that there is a good agreement between the calculated and experimental results.     

Intragranular formation of austenite during delta ferrite decomposition in a duplex stainless steel

A Fe–22.5%Cr–4.53%Ni–3.0%Mo duplex stainless steel was solution treated at 1,325 °C for 1 h, quenched in water and isothermally treated at 900 °C for 5,000 s. The crystallography of austenite was studied using EBSD technique. Intragranular austenite particles formed from delta ferrite are shown to nucleate on inclusions, and to be subdivided in twin-related sub-particles. Intragranular austenite appears to have planar-only orientation relationships with the ferrite matrix, close to Kurdjumov–Sachs and Nishyiama–Wassermann, but not related to a conjugate direction. Samples treated at 900 °C underwent sparse formation of sigma phase and pronounced growth of elongated austenite particles, very similar to acicular ferrite.     

Dual orientation and variant selection during diffusional transformation of austenite to allotriomorphic ferrite

The crystallographic relationship between austenite and grain boundary nucleated allotriomorphic ferrite has been investigated using electron back-scattered diffraction with a view to establishing a mechanism of variant selection. It is possible in some circumstances for the ferrite to adopt a favoured orientation relationship with both of the austenite grains with which it is in contact. However, the theoretical probability for the development of such a dual orientation has in previous work been shown to be very small, although experiments indicate otherwise. In this work, we have discovered experimentally that the probability of dual orientations is significantly increased when adjacent austenite grains are connected by special high-angle boundaries. Crystallographic calculations validate these observations and lead to the conclusion that simultaneous lattice matching between ferrite and its parent austenite grains is more likely in the presence of certain kinds of microscopic texture in the austenite. The phenomenon of dual orientation provides a criterion for crystallographic variant selection during diffusional transformation.     

Small-scale mechanical property characterization of ferrite formed during deformation of super-cooled austenite by nanoindentation

The mechanical properties of dynamically and statically transformed ferrites were analyzed using a nanoindentater-EBSD (Electron BackScattered Diffraction) correlation technique, which can distinguish indenting positions according to the grains in the specimen. The dilatometry and the band slope and contrast maps by EBSD were used to evaluate the volume fractions of two kinds of ferrite and pearlite. Fine ferrites induced by a dynamic transformation had higher nano-hardness than the statically transformed coarse ferrites. Transmission electron microscopy (TEM) showed the dynamic ferrites to have a higher dislocation density than the statically transformed ferrites.     

Tempering of hard mixture of bainitic ferrite and austenite

Abstract

Recent work has shown that bainitic ferrite plates produced by transformation at low temperatures can be as thin as 20 nm with a hardness in excess of 650 HV30, tensile strength ~2.3 GPa and toughness ~30 MPa m^1/2. Because these properties rely on the fine scale of the microstructure, a study has been carried out in relation to the tempering resistance of steel over the temperature range 350 – 750°C. It is found that significant softening occurs only after the plates of ferrite begin to coarsen. The coarsening process is hindered by the intense precipitation of carbides resulting from decomposition of the carbon enriched retained austenite. The carbides themselves lead to some precipitation strengthening during the early stages of tempering. The ferrite is found to contain excess carbon, beyond its solubility limit, and X-ray analysis indicates that the carbon is associated with heterogeneous strains in the microstructure. It does not readily precipitate until the onset of substantial recovery during annealing.     

Auger electron spectroscopy analysis of interface roughness of Fe/Cr bilayers

In this work we investigate the surface and interfacial properties of Fe/Cr and Cr/Fe bilayers before and after annealing using Auger electron spectroscopy (AES). The roughness of the interface is also determined with the X-ray reflection method. The fitted values of inelastic mean free path λ_{Cr in Fe} reproduce the calculated value for Cr in Fe well, whereas the values of λ_{Fe in Cr} are significantly larger than the calculated ones, suggesting mutual segregation of atoms during growth. The low-energy range Auger spectra demonstrated that the MNN lines of Cr covered with Fe and Fe covered with Cr disappear after the deposition of 1 nm overlayer, this being an indication of continuous deposited film, but not excluding mixing at interfaces. The results of X-ray reflectometry measurements, which give the values of Fe/Cr and Cr/Fe roughness, are in accordance with this observation. The LMM Auger spectra of annealed samples showed that at the largest applied temperature, Cr diffuses into Fe, but the reverse effect of Fe diffusion into Cr is not observed.     

Transformation of austenite remaining after intercritical rolling to ferrite

Abstract

The impact of austenite deformation in the intercritical range on the rate of transformation in continuous cooling to ferrite, pearlite, bainite or martensite has been studied. The austenite associated with the rolled ferrite is much higher in carbon content, which does not influence the pearlite transformation but retards bainite and martensite. Furthermore, in comparison with rolling of stable austenite the increased strain hardening of the intercritically cooled austenite accelerates the formation of ferrite and pearlite (+ 10–30°C) and refines them but retards the bainite and martensite transformations (?20–40°C). At the intermediate cooling rate near 16 K s^?1, these several influences combined with near doubling of the ferrite production give rise to the suppression of bainite formation and to maximum increased delay of martensite start.     

Numerical analysis of composite systems by using interphase/interface models

The paper considers two classes of approaches for the numerical analysis of composite systems: the first one discretizes the assumed interphase (between matrix and fibre) as volumic elements and uses material models that degenerate from Continuum Damage Mechanics. The second one introduces interface elements that relate non linearly the normal and tangential tractions to the corresponding displacement discontinuities, incorporating a progressive decohesion, following the lines of Needleman (1987) and Tvergaard (1990). The respective capabilities of these two approaches are discussed on the basis of some numerical results obtained for a unidirectional metal matrix composite system. When the models are consistently adjusted they are able to reproduce the same kind of results. The advantages of the second class of method is underlined and two new versions of interface models are proposed that guarantee the continuity and the monotonicity of the shear stiffness between the progressive decohesion phase and the subsequent contact/friction law that plays role under compressive shear after complete separation.     

Numerical analysis of ferrite recording heads with complex permeability

Numerical methods based on finite differences are applied to the analysis of ferrite recording heads in which the permeability is complex and flux leakage effects are significant. The methods described allow computation of head efficiency, inductance, loss angle, and phase shift. As a specific example, the analysis is performed for a reduced track width version of the Memorex 3650 head for a wide range of the real and imaginary parts of the permeability. Using published values for the frequency dependence of the complex permeability, the frequency dependence of the head performance is estimated. Some experimental data are presented and compared to the theoretical predictions.     

Effect of Nb on dynamic strain induced austenite to ferrite transformation

Abstract

Dynamic strain induced transformation (DSIT) is an interesting processing route to obtain ultrafine ferrite grains. In the present work, the effect of Nb on DSIT was investigated. Samples of low C–Mn steels, with and without Nb, were intensively deformed in hot torsion, aiming at the production of ultrafine ferrite grains. After soaking at 1200°C, the samples were cooled to 1100°C, submitted to hot torsion deformation to decrease the grain size and then cooled to 900, 850 or 800°C for further hot torsion deformation. In the steel without Nb, recrystallisation took place before enough deformation could be accumulated to induce ferrite formation, so DSIT would only take place at the lowest temperature investigated, 800°C. In the Nb steel, Nb addition delayed austenite recrystallisation, allowing DSIT ferrite to form at higher temperature than in the steel without Nb, 850°C.     

Acicular ferrite transformation in deformed austenite of an alloy-steel weld metal

The effect of different amounts (5, 10 and 15%) of compressive deformation of austenite on the isothermal transformation of acicular ferrite in an alloy-steel weld metal has been investigated. It was found that prior deformation of austenite significantly enhanced acicular ferrite transformation. At the same isothermal transformation temperature, as a higher amount of prior deformation was applied, a greater quantity of acicular ferrite could be obtained and the size of acicular ferrite plates became much finer. These results implied that the effective nucleation sites of acicular ferrite increased with increasing amount of prior deformation. The other results also emphasized that the accumulated strain (due to prior deformation of austenite) could trigger acicular ferrite to nucleate on inclusions at high temperatures, where undeformed austenite remained stable. The acicular ferrite start temperature was found to be raised continuously by increasing the amount of prior deformation of austenite. Further evidence suggests that the application of deformation can boost the driving force for acicular ferrite formation. This phenomenon is similar to the case in which martensite forms under the influence of deformation.     

Dispersion characteristics of magnetostatic waves propagating at a ferrite/perovskite interface

The propagation of magnetostatic waves at the interface between an yttrium-iron-garnet (YIG) film and a layer of perovskite compound LaAlO₃, SrTiO₃, or YBa₂Cu₃O_7−x (YBCO) was experimentally studied. The microwave dispersion characteristics were measured at room temperature and at 77 K. An effective method for controlling the propagation speed and the phaseshift of magnetostatic waves in the YIG/YBCO structures is suggested. Specifically, the phaseshift at 3 GHz is changed by more than 1.5π by reducing the critical current density from 1 to 0.7× 10⁶ A/cm².     

基于数值仿真的冲压成形界面接触压力

为探究冲压成形过程中板料-凹模圆角区界面接触压力,采用有限元静力算法建立了U形件小圆角半径弯曲成形过程的数值仿真模型,完成了板料和模具界面接触压力数值模拟,并参数分析了钢板强度、相对圆角半径、压边力和摩擦系数对板料界面接触压力分布的影响.研究表明:与压边力和摩擦系数相比,钢板强度和相对圆角半径更明显地影响着板料界面接触压力,并随着材料强度增加和相对模具圆角半径减小界面接触压力明显增加;随着压边力和摩擦系数的增加,界面接触压力宽度也随之增大.     

Phase evolution and magnetic properties study of Fe/Cobalt ferrite nanocomposites

In the work presented here attempt is made to investigate phase evolution and magnetic properties of Co/α-Fe₂O₃:1:6 (molar ratio) powder mixtures subjected to high energy milling (30 h) followed by annealing in air and subsequent heat-treatment in a reducing atmosphere at 400 °C for 20 min. The latter process gave rise to the formation of a nanocomposoite compound composed of CoFe₂O₄, Fe₃O₄ and α-Fe phases, as evidenced by Mössbauer and XRD results. Rather high maximum magnetization M_max (88 emu/g) and reasonable _iH_C (1.03 kOe) values were obtained for the nanocomposite sample prepared. This is possibly attributed to the existence of well exchange coupled soft and hard magnetic phases. Further, the structural-magnetic properties relationship of the various powders prepared is discussed in detail.     

The influence of diffusion of carbon in ferrite as well as in austenite on a model of reaustenitization from ferrite/cementite mixtures in Fe-C steels

The kinetics of formation of austenite from ferrite and cementite mixtures has been modelled by assuming the local equilibrium at the planar phase interfaces. The exact solutions to the diffusion equations governing the volume diffusion of carbon in austenite and ferrite are presented. The concurrent motions of the two interfaces are calculated via solving a set of transcendental equations derived from the flux balance conditions. At low isothermal transformation temperatures, it is found that the time required for reaustenitization is slightly greater than the time previously calculated with no diffusion of carbon in ferrite.     

Forming interface in Pd/Fe/GaAs/InGaAs structure for optical detector of free-electron spin

Conditions necessary for the formation of a Fe/GaAs interface have been established and the electrical, magnetic, and optical properties of Pd/Fe/GaAs heterostructures with InGaAs quantum wells have been studied. The possibility of obtaining an epitaxial layer of Fe on GaAs(001) surface at room temperature is demonstrated. The magnetization curve of Fe layer exhibits hysteresis with an easy axis in plane of the sample. Iron exhibits surface segregation by diffusion through a 4-nm-thick Pd layer. The properties of obtained Pd/Fe/GaAs/InGaAs structures show evidence for their possible use in optical detectors of free-electron spin.