Phase Transition In Iron Research Articles

Effect of thermal treatment of chromium iron melts on the structure and properties of castings

The article describes the results of experimental studies for the effect of thermal treatment (TTM) of G-X300CrMo27-1 high-chromium cast iron samples in the solid–liquid and liquid state on the structure, phase composition, and properties of ingots. For ingots with a carbon content of 2.8 to 4.5 wt%, cooled at a rate of 3.3 * 10−2 deg s−1, the dependencies of the structure, phase composition, composition of primary, eutectic carbides and matrix, hardness HV and microhardness of its individual phases and resistance to abrasive wear from the temperature of isothermal holding at TTM. A significant effect of the TTM temperature of melts on the structure and properties of high-chromium cast iron ingots was detected. The temperatures of inflection on the plotted curves for the characteristics of the structure and properties of the ingots were associated with a phase transition in iron at 1400 °C and with a point on the phase diagram liquidus of the Fe-Cr system. This as well as an increase in the concentration of chromium in the composition of primary carbides with an increase in the TTM temperature up to 1480 °C, made it possible to assume that (Cr, Fe)7C3 clusters stable in composition were formed in the melt of such cast irons below ∼1500 °C. The reasons for the decrease in the size of primary carbides during G-X300CrMo27-1 molten chromium iron overheating above 1500 °C were substantiated based on these data. During the TTM of fine-crystalline ingots made of hypereutectic cast iron in the temperature range between the liquidus and solidus lines, it was detected that the primary carbides (Cr, Fe)7C3 recrystallized resulting in a significant decrease in the chromium content and an increase in the iron content in them. At the same time, their sizes did not change significantly. Their share increased, and the share of eutectic carbides decreased. Such a process also had a significant impact on the properties of the resulting ingots. The most preferable temperatures of hot metal cast irons in the liquid and solid–liquid states were identified based on the results of the studies.

Influence of spin fluctuations on structural phase transitions of iron

The effect of spin fluctuations on the $\ensuremath{\alpha}$ (bcc)-$\ensuremath{\gamma}$ (fcc)-$\ensuremath{\delta}$ (bcc) structural phase transitions in iron is investigated with a tight-binding (TB) model. The orthogonal $d$-valent TB model is combined with thermodynamic integration, spin-space averaging, and Hamiltonian Monte Carlo to compute the temperature-dependent free-energy difference between bcc and fcc iron. We demonstrate that the TB model captures experimentally observed phonon spectra of bcc iron at elevated temperatures. Our calculations show that spin fluctuations are crucial for both the $\ensuremath{\alpha}\text{\ensuremath{-}}\ensuremath{\gamma}$ and $\ensuremath{\gamma}\text{\ensuremath{-}}\ensuremath{\delta}$ phase transitions but they enter through different mechanisms. Spin fluctuations impact the $\ensuremath{\alpha}\text{\ensuremath{-}}\ensuremath{\gamma}$ phase transition mainly via the magnetic/electronic free-energy difference between bcc and fcc iron. The $\ensuremath{\gamma}\text{\ensuremath{-}}\ensuremath{\delta}$ phase transition, in contrast, is influenced by spin fluctuations only indirectly via the spin-lattice coupling. Combining the two mechanisms, we obtain both the $\ensuremath{\alpha}\text{\ensuremath{-}}\ensuremath{\gamma}$ and $\ensuremath{\gamma}\text{\ensuremath{-}}\ensuremath{\delta}$ phase transitions with our TB model. The calculated transition temperatures are in very good agreement with experimental values.

Decoupling spin-crossover and structural phase transition in iron(II) molecular complex

Decoupling spin-crossover and structural phase transition in iron(II) molecular complex

Design of high-pressure iron Rayleigh–Taylor strength experiments for the National Ignition Facility

Iron is an important metal, scientifically and technologically. It is a common metal on Earth, forming the main constituent of the planet's inner core, where it is believed to be in solid state at high pressure and high temperature. It is also the main component of many important structural materials used in quasistatic and dynamic conditions. Laser-driven Rayleigh–Taylor instability provides a means of probing material strength at high pressure and high temperature. The unavoidable phase transition in iron at relatively low pressure induces microstructural changes that ultimately affect its strength in this extreme regime. This inevitable progression can make it difficult to design experiments and understand their results. Here, we address this challenge with the introduction of a new approach: a direct-drive design for Rayleigh–Taylor strength experiments capable of reaching up to 400 GPa over a broad range of temperatures. We use 1D and 2D hydrodynamic simulations to optimize target components and laser pulse shape to induce the phase transition and compress the iron to high pressure and high temperature. At the simulated pressure–temperature state of 350 GPa and 4000 K, we predict a ripple growth factor of 3–10 depending on the strength with minimal sensitivity to the equation of state model used. The growth factor is the primary observable, and the measured value will be compared to simulations to enable the extraction of the strength under these conditions. These experiments conducted at high-energy laser facilities will provide a unique way to study an important metal.

The mechanism of plasticity and phase transition in iron single crystals under cylindrically divergent shock loading

Cylindrically divergent shock loading significantly influences the plasticity and phase transition of iron. In this study, the mechanisms of plasticity and α→ε phase transition are investigated using large-scale nonequilibrium molecular dynamics simulation. We find that iron undergoes a local bcc–hcp–bcc–hcp (bcc: body-centered cubic; hcp: hexagonal close-packed) phase transition under cylindrically divergent shock loading with cylinder axis lying in [001] and [110] directions. A bcc lattice reorientation driven by resolved shear stress occurs during the hcp–bcc reverse transition and results in bcc twins, which transform into hcp twin structures with an unexpected c-axis, as compared with planar shock. Moreover, we analyze the coupling behavior between plasticity and phase transition and find that collective movement and rearrangement of atoms induced by plastic slip on the shuffle plane play a key role in the activation of phase transition, which significantly differs from that of planar shock.

Phase Transition in Iron Thin Films Containing Coherent Twin Boundaries: A Molecular Dynamics Approach.

Using molecular dynamics (MD) simulation, the austenitic and martensitic phase transitions in pure iron (Fe) thin films containing coherent twin boundaries (TBs) have been studied. Twelve thin films with various crystalline structures, thicknesses and TB fractions were investigated to study the roles of the free surface and TB in the phase transition. In the austenitic phase transition, the new phase nucleates mainly at the (112)bcc TB in the thicker films. The ()bcc free surface only attends to the nucleation, when the film is extremely thin. The austenitic transition temperature shows weak dependence on the film thickness in thicker films, while an obvious transition temperature decrease is found in a thinner film. TB fraction has only slight influence on the austenitic temperature. In the martensitic phase transition, both the (fcc free surface and (111)fcc TB attribute to the new body-center-cubic (bcc) phase nucleation. The martensitic transition temperature increases with decreased film thickness and TB fraction does not influent the transition temperature. In addition, the transition pathways were analyzed. The austenitic transition obeys the Burgers pathway while both the Kurdjumov–Sachs (K–S) and Nishiyama–Wassermann (N–W) relationship are observed in the martensitic phase transition. This work may help to understand the mechanism of phase transition in the Fe nanoscaled system containing a pre-existing defect.

The model of iron properties for plate impact and explosive compression simulations

The material model of iron and steel is developed for fluid dynamics simulations of samples under extreme loading induced by an impact or explosion. The model is validated on a set of plate impact tests using the contact smoothed particles hydrodynamics method. The model takes into account the polymorphic α–ε phase transition in iron which is shown to be correctly reproduced, including the hysteresis effect at unloading. The equation of state for steel is shown to be very close to that of iron on a set of tests with spherical shells under explosive compression; however the yield strength of steel is greater.

Imaging stress and magnetism at high pressures using a nanoscale quantum sensor.

Pressure alters the physical, chemical, and electronic properties of matter. The diamond anvil cell enables tabletop experiments to investigate a diverse landscape of high-pressure phenomena. Here, we introduce and use a nanoscale sensing platform that integrates nitrogen-vacancy (NV) color centers directly into the culet of diamond anvils. We demonstrate the versatility of this platform by performing diffraction-limited imaging of both stress fields and magnetism as a function of pressure and temperature. We quantify all normal and shear stress components and demonstrate vector magnetic field imaging, enabling measurement of the pressure-driven [Formula: see text] phase transition in iron and the complex pressure-temperature phase diagram of gadolinium. A complementary NV-sensing modality using noise spectroscopy enables the characterization of phase transitions even in the absence of static magnetic signatures.

Influence of grain boundaries on the austenitic and martensitic phase transitions in iron

Using classical molecular dynamics simulations, we study the martensitic and austenitic phase transformation in an iron crystal containing a symmetric tilt grain boundary (GB). Without a GB, the system does not transform. The presence of a GB enables the transformation. The new phase nucleates at the GB. The austenitic transition temperature decreases approximately linearly with the GB energy. Here, the GB inherits its inherent periodicity to the microstructure of the forming austenite phase. The martensitic transformation proceeds via a two-step pathway resulting in a twinned microstructure.

Influence of the Crystal Surface on the Austenitic and Martensitic Phase Transition in Pure Iron

Using classical molecular dynamics simulations, we studied the influence that free surfaces exert on the austenitic and martensitic phase transition in iron. For several single-indexed surfaces—such as ( 100 ) bcc and ( 110 ) bcc as well as ( 100 ) fcc and ( 110 ) fcc surfaces—appropriate pathways exist that allow for the transformation of the surface structure. These are the Bain, Mao, Pitsch, and Kurdjumov–Sachs pathways, respectively. Tilted surfaces follow the pathway of the neighboring single-indexed plane. The austenitic transformation temperature follows the dependence of the specific surface energy of the native bcc phase; here, the new phase nucleates at the surface. In contrast, the martensitic transformation temperature steadily decreases when tilting the surface from the (100) fcc to the (110) fcc orientation. This dependence is caused by the strong out-of-plane deformation that (110) fcc facets experience under the transformation; here, the new phase also nucleates in the bulk rather than at the surface.

Atomistic studies of shock-induced plasticity and phase transition in iron-based single crystal with edge dislocation

In engineering applications, various types of defects inevitably exist in materials which would critically affect their properties. By introducing an edge dislocation into the model of single-crystal iron, the plasticity and α→ε phase transition in iron under shock loading have been investigated by means of non-equilibrium molecular dynamics (NEMD) simulations with a modified analytic embedded-atom model potential. Plastic slip is clearly observed in our work, which is induced by the initial edge dislocation under shock loading. The activated slip systems are considered as {110}<111> at the nucleation stage of dislocation generation, and then a self-adaptive transformation process occurs, which results in transforming the activated slip systems from {110}<111> into {112}<111>. The α→ε phase transition only occurs after the activation of dislocation slip, which indicates there is a strong connection between plasticity and the phase transition. The process of this plasticity-controlled phase transition is illustrated. By calculating the resolved shear stress along the shuffle plane of the shocked crystal iron at different moments, the mechanism of the plasticity-controlled phase transition is deeply discussed. Our findings could contribute to the better understanding of the behaviors of iron or iron-based alloy under high pressure conditions.

Shock-induced phase transition of iron studied with phase field method

To develop phase field method in high strain rate loading, it derived the governing equations of phase field model in the foundation of entropy functional, and the model adopted a diffusion interface, which made automatic tracking on interface possible. Added the order parameter to characterize the different phases of materials, and simulated the soften and phase transition in shock-induced loading. According to the conservation theorems and principle of entropy increasing, we established the governing equations of shock-induced phase transition in metal dielectric. Then the work discretized phase field model by using high order difference scheme, introducing the artificial viscosity, and obtained the numerical solution of the phase field model. Compared the numerical value with the theoretical solution, the accuracy of the developed phase field model was verified. Based on the model described above, the work study the a®e polymorphic phase transition of iron-base alloy induced by one-dimensional shock wave. Considering the spatial and temporal distribution of each parameter field and the Hugoniot data for iron, it further analyzed the coupling effect of wave and phase transition by tracking the changes of pressure and speed in each unit. The simulation results fitted well with the experimental results, which indicated that the phase field model developed was successful, and it would help people to understand the microstructural evolution of metallic materials under the shock wave.

Anomalous Behavior of an α → γ Phase Transition in Iron: Results of In Situ Neutron Diffraction Experiment

Neutron diffraction studies of a α → γ structural phase transition (Tc ≈ 911°C) in iron samples at different initial states have been performed. The anomalous disappearance of a diffraction pattern is observed in iron in the region of the α → γ phase transition. At repeated heating processes of an as cast sample, this effect becomes less pronounced and it is absent at the heating of iron powder. The observed effects have been assumingly attributed to the formation of small paracrystalline clusters with a large dispersion of the cell parameters in the process of phase transition. The kinetics of formation of clusters is slowed in as cast samples, which leads to the temporal disappearance of the diffraction pattern. These results make it possible to reconsider the atomic processes in the α → γ phase transition in iron.

Development of Fe-C interatomic potential for carbon impurities in α-iron

Two versions of interatomic potentials for iron-carbon system based on the analytic bond-order potential formalism were developed and tested on diffusion of carbon in body-centered cubic (BCC) iron. By using these potentials, we found that carbon diffuses from octahedral site to the next nearest neighbor octahedral site via tetrahedral site, with energy barriers of 0.74 and 0.70 eV, respectively. These values are in good agreement with the transition from octahedral site to octahedral site via tetrahedral site with energy barriers of about 0.81–0.87 eV, observed in experiment. Other theoretical works found similar diffusion mechanism with energy barriers of about 0.72–0.92 eV. We also verified the transferability of the newly developed potential in the study of the effect of carbon on the body-centered cubic/face-centered cubic (BCC/FCC) phase transition in iron following the phase transition in the Bain mechanism. We found that the presence of carbon leads to the decrease of the BCC to FCC transformation energy barrier, consistent with first-principles calculations. Thus, the new potentials can be good candidates for modeling carbon diffusion and the transformation processes in iron-carbon alloys.

Influence of Phase Transitions on the Macrokinetics of the Gaseous Reduction of Iron Oxide

The reduction of magnetite pellets is studied by thermogravimetry in a hydrogen flow upon linear heating. A multiple decrease in the reduction rate is observed at the degrees of metallization higher than 70% in a temperature range of 920–950°C. The observed effect of a decrease in the reduction rate by four to five times on heating is related to the α → γ phase transition of iron. Temperature cycling (heating–holding–heating–holding…) in the phase transition range shows that this effect is reversible; i.e., the reduction rate increases on cooling, unlike that on heating. The average temperature of the beginning of the effect in heating–cooling cycles (913°C) turns out to be close to the thermodynamic temperature (911°C) of the α → γ phase transition of iron. Isothermal studies of the reduction rate of pellets in a temperature range of 900–1000°C also confirm this phenomenon. The effect of a decrease in the reduction rate is assumed to appear due to a decrease in the effective coefficient of gas diffusion in γ-iron as compared to α-iron.

High‐Pressure Phase Transition of Iron: A Combined Magnetic Remanence and Mössbauer Study

AbstractWe measured Mössbauer spectra and the acquisition of saturation isothermal remanent magnetization in alternating steps on the same sample of polycrystalline, multidiron metal powder in a diamond anvil cell across the body centered cubic (bcc) to hexagonal closed packed (hcp) phase transition at room temperature up to 19.2 GPa. Within the bcc stability field indicated by the presence of magnetic hyperfine splitting, saturation remanent magnetization and sextet area were well correlated during compression and decompression. The areas and dips of the outer (first and sixth) and middle (second and fifth) components of the sextet changed in relative proportion as a function of pressure, which was attributed to rotation of the magnetization direction perpendicular to the gamma‐ray source. Sextet peaks disappeared above ∼15 GPa, yet magnetic remanence persisted. Magnetic remanence intensity divided by the fractional area of the sextet, taken to represent bcc Fe, attained maxima at pressures near the boundaries of the hysteretic transition, which we attribute to strain‐related magnetostriction effects associated with a distorted bcc‐hcp phase. Magnetic remanence observed within the hcp stability field, as defined by the absence of sextet peaks, could be due to a previously described, distorted bcc‐hcp phase whose hyperfine field was below detection limits of Mössbauer spectroscopy. Our study suggests that distorted bcc‐hcp Fe holds magnetic remanence and leaves open the possibility that this phase carries magnetic remanence into the pressure range where only pure hcp Fe is considered stable.

Interactions between coherent twin boundaries and phase transition of iron under dynamic loading and unloading

Phase transitions and deformation twins are constantly reported in many BCC metals under high pressure, whose interactions are of fundamental importance to understand the strengthening mechanism of these metals under extreme conditions. However, the interactions between twins and phase transition in BCC metals remain largely unexplored. In this work, interactions between coherent twin boundaries and α ↔ ε phase transition of iron are investigated using both non-equilibrium molecular dynamics simulations and the nudged elastic band method. Mechanisms of both twin-assisted phase transition and reverse phase transition are studied, and orientation relationships between BCC and HCP phases are found to be 111¯BCC||1¯21¯0HCP and 11¯0BCC||0001HCP for both cases. The twin boundary corresponds to 101¯0HCP after the phase transition. It is amazing that the reverse transition seems to be able to “memorize” and recover the initial BCC twins. The memory would be partly lost when plastic slips take place in the HCP phase before the reverse transition. In the recovered initial BCC twins, three major twin spacings are observed, which are well explained in terms of energy barriers of transition from the HCP phase to the BCC twin. Besides, the variant selection rule of the twin assisted phase transition is also discussed. The results of present work could be expected to give some clues for producing ultra-fine grain structures in materials exhibiting martensitic phase transition.

Phase transition of iron-based single crystals under ramp compressions with extreme strain rates

Recent widespread interests on strain rate effects of phase transition under dynamic loadings bring out present studies. α→ε phase transition of iron, as a prototype of martensite phase transition under dynamic loadings, exhibits huge diverges in its transition pressure (TP) among experiments with different pressure medium and loading rates, even in the same initial samples. Great achievements are made in understanding strain or stress dependence of the TP under dynamic loadings. However, present understandings on the strain rate dependence of the TP are far from clear, even a virgin for extreme high strain rates. In this work, large scale nonequilibrium molecular dynamics simulations are conducted to study the effects of strain rates on the phase transition of iron-based single crystals. Our results show that the phase transition is preceded by lattice instabilities under ramp compressions, but present theory, represented by modified Born criteria, cannot correctly predict observed onsets of the instability. Through considering both strain and strain gradient disturbances, new instability criteria are proposed, which could be generally applied for studying instabilities under either static or dynamic loadings. For the ramp with a strain rate smaller than about 1010 s−1, the observed onset of instabilities is indeed equal to the one predicted by the new instability criteria under small gradient disturbances. The observed onsets deviates from the predicted one at lager strain rates because of finite strain gradient effect —— nonzero higher order stresses and work conjugates of the strain gradient. Interestingly, the strain rate (ε˙) dependence of the TP also exhibits an obvious change at the same strain rate, i.e., 1010 s−1. When ε˙ ≤ 1010 s−1, a certain power law is obeyed, but it is not applicable at larger strain rates. This strain rate effect on the TP is well interpreted with nucleation time and the finite strain gradient effect. According to these basic understandings, the roles of strain rates on nucleation and growth of the phase transition are studied. Besides, initial shock formation time at extreme strain rates is analyzed, which also exhibits a deviation from a scaling law.

Study of the magnetite to maghemite transition using microwave permittivity and permeability measurements

The microwave cavity perturbation (MCP) technique is used to identify the transition from magnetite (Fe3O4) to the meta-stable form of maghemite (γ-Fe2O3). In this study Fe3O4 was annealed at temperatures from 60 to 300 °C to vary the oxidation. Subsequent to annealing, the complex permittivity and magnetic permeability of the iron oxide powders were measured. The transition to γ-Fe2O3 was corroborated with x-ray diffraction (XRD), x-ray photoelectron spectroscopy (XPS) and vibrating sample magnetometry (VSM). XRD, XPS and VSM implied that the starting powder was consistent with Fe3O4 and the powders annealed at more than 200 °C were transitioning to γ-Fe2O3. The MCP measurements gave large differences in both complex permittivity and magnetic permeability of the two phases in the frequency range of 2.5–10.2 GHz. Magnetic permeability decreased with annealing temperature, though magnetic losses showed frequency dependent behaviour. Complex permittivity measurements showed a large decrease in both dielectric constant and losses at all measurement frequencies, as well as a prominent loss peak centred around the phase transition temperatures. We interpret the loss peak as being a consequence of field effects due to an intermediate multi-phase mixture. Additionally, almost no frequency dependence was observed. The reduction in complex permittivity implies that the cations in the lattice provide a significant contribution to polarization at microwave frequencies and the effects of are nominal in comparison. The change in loss can be explained as a combination of the differences in the effective conductivity of the two phases (i.e. Fe3O4 exhibits electron-hopping conduction whereas the presence of vacancies in γ-Fe2O3 nullifies this). This shows that the non-invasive MCP measurements serve as a highly sensitive and versatile method for looking at this phase transition in iron and potentially the effects of oxidation states on the polarization in other iron oxides.

Pressure-induced Structures and Structural Evolution in Iron

Molecular-dynamic simulations have been used to study the structure evolution in iron melts rapidly cooled under different pressures. An extreme cooling rate (4×1012K/s) was adopted in the cooling process. The simulation results show that at the ambient pressure, martensitic transformation happened. However, at a pressure of 1.4GPa, the system passes from bcc structure to a less closed packed structure which is composed of both full icosahedra clusters and bcc structure. With the increase of pressures, an amorphous state was observed, and a compacted local structure with more defected icosahedra is obtained. This work contributes to a better understanding about the dynamics of phase transitions in iron under high pressure, especially during the extremely fast cooling process.