Phase Transition In Iron Research Articles

High-pressure phase transition detection in diamond anvil cell using the method of ellipsometry

It is suggested to use the ellipsometric technique to measure the electronic properties of static highly compressed matter. The information derived from this technique is important for the theoretical and phenomenological research for the equation of state. This diagnostic was found to be useful and sensitive for measuring pressure induced phase transitions. In particular, high-pressure ellipsometry with a diamond anvil cell was used to detect the α⇄ε phase transition in iron. The polarization of the light reflected from an iron foil was analyzed in the pressure domain of 0–24 GPa and the α⇄ε transition in compression and decompression was detected. These results are in good agreement with Mössbauer spectroscopy, x-ray diffraction, and resistivity measurements.

Mössbauer study of the polymorphism in iron and iron-nickel alloys under deformation and high pressure

Structural and phase transitions in iron and binary iron-nickel alloys containing 6–32 at % nickel have been investigated by Mossbauer spectroscopy in situ under quasi-hydrostatic compression and compression shear. It is shown that the deformation decreases the onset pressure of the α-phase transition to the high-pressure ∈ and γ phases. Shear changes the transformation mechanism due to the formation of a large number of structural defects; it is an independent parameter of the phase transition kinetics. Compression shear leads to an irregular change in the hyperfine parameters of the initial and newly formed phases, which is due to the stress relaxation as in the trip effect.

In situ diffraction measurements of lattice response due to shock loading, including direct observation of the α– ε phase transition in iron

In situ diffraction is a technique to probe directly the lattice response of materials during the shock loading process. It is used to record diffraction patterns from multiple lattice planes simultaneously. The application of this technique is described for laser-based shock experiments. The approach to analyze in situ wide-angle diffraction data is discussed. This is presented in the context of single crystal [001] iron shock experiments where uniaxial compression of the bcc lattice by up to 6% was observed. Above the α– ε transition pressure, the lattice showed a collapse along the [001] direction by 15–18%. Additional diffraction lines appear that confirm the transformation of the iron crystal from the initial bcc phase to the hcp phase.

Effect of shear strain on the α–ε phase transition of iron: a new approach in the rotational diamond anvil cell

The effect of shear strain on the ironα–ε phase transformation has been studied using a rotational diamond anvil cell (RDAC). Theinitial transition is observed to take place at the reduced pressure of 10.8 GPa underpressure and shear operation. Complete phase transformation was observed at 15.4 GPa.The rotation of an anvil causes limited pressure elevation and makes the pressuredistribution symmetric in the sample chamber before the phase transition. However,it causes a significant pressure increase at the centre of the sample and bringsabout a large pressure gradient during the phase transformation. The resistanceto the phase interface motion is enhanced due to strain hardening during thepressure and shear operations on iron and this further increases the transitionpressure. The work of macroscopic shear stress and the work of the pressure andshear stress at the defect tips account for the pressure reduction of the iron phasetransition.

Femtosecond Laser Synthesis of the High-Pressure Phase of Iron

The synthesis of the high-pressure ε phase of iron, which has not been observed under a conventional shock compression, was attained using a femtosecond laser. The lower pressure and temperature α phase (bcc) transforms to the γ phase (fcc) at higher temperatures and to the ε phase (hcp) at higher pressures. A shock induced α to ε phase transition in iron is one of the most famous transitions under high pressure. The induced high-pressure ε phase by a conventional shock loading returns to the α phase and it is not quenched after the shock release because this transition is considered to be diffusionless. Crystalline structures in a recovered iron sample after the femtosecond laser (800 nm, 120 fs, 1014 W/cm2) irradiation were determined using the electron diffraction and the synchrotron X-ray diffraction methods. These results show the existence of the ε phase and the fcc structure in the recovered iron. The femtosecond laser-driven shock wave may have the potential to synthesis high-pressure phases of other materials that has not been done using the conventional shock wave.

X-ray diffraction study of phase transitions in iron(II) trisnioximate hexadecylboronate clathrochelate complex

Crystals of the iron(II) nioximate hexadecylboronate clathrochelate complex—FeNx3(BHd)2 [tris(μ-1,2-cyclohexanedionedioximato-O:O′)di-n-hexadecyldiborato(2−)-N,N′, N″,N‴, N⁗,N″‴]iron(II)—are investigated by differential scanning calorimetry and X-ray diffraction. Two structural phase transitions are revealed at Tcr1 = 290(3) K and Tcr2 = 190(3) K. The crystal structures of phases I, II, and III are determined by X-ray diffraction analysis at 303, 243, and 153 K, respectively. It is demonstrated that the I ↔ II phase transition is due to a change in the system of translations, and the II ↔ III phase transition is accompanied only by a jumpwise change in the unit cell parameters. The possible mechanisms of phase transitions are discussed in terms of geometry and molecular packing of FeNx3(BHd)2 in all three phases.

In situ X-Ray study of thermal expansion and phase transition of iron at multimegabar pressure

The density of varepsilon-iron has been calculated at pressures and temperatures up to 300 GPa and 1300 K, respectively. We observe varepsilon to beta phase transition at pressures between 135 and 300 GPa and temperature above 1350 K; the pattern can be interpreted in terms of double hexagonal close-packed structure. The density calculated at high pressure and temperature (330-360 GPa and 5000-7000 K) closely matches with preliminary reference Earth model density, thereby imposing constraint on the composition of the Earth's inner core.

FCC-BCC Phase Transition in Iron under a Periodic Boundary Condition

By using isobaric-isothermal molecular dynamics (MD) technique and many-body effective potential, we investigate the martensitic transformation in iron. To describe structural phase transitions, we adopt the Parrinello-Rahman scheme and its invariant modification, in which the MD cell with periodic borders is allowed to change in size and shape with time. The main observed path from fcc to bcc is the Bain transformation, but this process seems to be induced by the tetragonal distortion of the MD cell. Accordingly, the dynamical trajectory of the transformation does strongly depend on the size and shape of the MD cell and on the MD scheme itself. The unwanted contribution from the cell dynamics cannot be eliminated even in the modified schemes based on the Parrinello-Rahman method.

Optical and magneto-optical properties of Fe/Cu multilayered films: Influence of the modulation period and the bcc-fcc phase transition in iron

The optical and magneto-optical properties of multilayered film samples of the Fe/Cu system prepared by high-frequency sputtering on an Si(100) substrate are studied by ellipsometry and by measuring the equatorial Kerr effect (the δp effect) in the spectral range 0.25–7 μm. The optical characteristics, the plasma frequency ωp and the relaxation frequency γ0 of the conduction electrons, and the δp effect are found as functions of the modulation period D=12.5–100 A. Anomalous behavior of the optical and magneto-optical characteristics is discovered in short-period Fe/Cu structures. The results are discussed within a phenomenological theory of optical and magneto-optical properties for layered structures. Several factors, such as the indirect exchange interaction between the iron layers, the presence of a transition layer on the internal boundaries, the possible “magnetizing” of copper, and the formation of an fcc iron phase in the thin layers, are taken into account in the analysis of the experimental data.

Metastability and dynamics of the shock-induced phase transition in iron

The shock-induced \ensuremath{\alpha}(bcc)\ensuremath{\rightarrow}\ensuremath{\varepsilon}(hcp) transition in iron begins at 13 GPa on the Hugoniot. In the two-phase region above 13 GPa, the Hugoniot lies well above the equilibrium surface defined by ${\mathrm{G}}_{\mathrm{\ensuremath{\alpha}}}$ =${\mathrm{G}}_{\mathrm{\ensuremath{\varepsilon}}}$ , with G the Gibbs free energy. Also, the phase transition relaxation time \ensuremath{\tau} is uncertain, with estimates ranging from 50 ns to \ensuremath{\approx}180 ns. Here we present an extensive study of these important aspects, metastability and dynamics, of the \ensuremath{\alpha}-\ensuremath{\varepsilon} transition in iron. Our primary theoretical tools are (a) accurate theoretically based free energies for \ensuremath{\alpha} and \ensuremath{\varepsilon} phases of iron and (b) accurate calculations of the wave evolution following planar impacts. We define metastable surfaces for forward and reverse transitions by the condition that the thermodynamic driving force ${\mathrm{G}}_{\mathrm{\ensuremath{\alpha}}}$ -${\mathrm{G}}_{\mathrm{\ensuremath{\varepsilon}}}$ is just balanced by an opposing force resulting from elastic stresses, and we calibrate the forward surface from the Hugoniot and the reverse surface from the phase interface reflection feature of shock profiles. These metastable surfaces, corresponding to \ensuremath{\alpha}\ensuremath{\leftrightarrows}\ensuremath{\varepsilon} transitions proceeding at a rate of tens of nanoseconds, are in remarkable agreement with quasistatic diamond cell measurements. When the relaxation time \ensuremath{\tau} is calibrated from the rise time of the P2 wave, our calculated wave profiles are in good agreement with VISAR data. The overall comparison of theory and experiment indicates that (a) \ensuremath{\tau} depends on shock strength and is approximately 60\ensuremath{\rightarrow}12 ns for shocks of 17\ensuremath{\rightarrow}30 GPa, and (b) while \ensuremath{\tau} expresses linear irreversible-thermodynamic relaxation, some nonlinear relaxation must also be present in the shock process in iron.

Mechanism of the Body-Centered Cubic—Hexagonal Close-Packed Phase Transition in Iron

The transition from body-centered cubic to hexagonal close-packed phase in iron has been studied in a diamond anvil cell with synchrotron radiation. The hexagonal close-packed phase, when it first appears, has a ratio of lattice parameters that is significantly larger than normal. This is attributed to a displacive mechanism that causes a distortion of the hexagonal close-packed structure in a body-centered cubic matrix. The hexagonal close-packed phase adjacent to a boundary with the body-centered cubic phase is stretched in the c direction and compressed in the a direction when it first forms.

Thermodynamically complete equations of state for ?-, ?-, and ?-iron

The numerical model of α ↔ e phase transition in iron in stress waves described in [1] contains equations of state with a limited range of applicability. They do not consider thermal excitation of conduction electrons and the presence of and α — e-triple point on the phase equilibrium curve, the effect of which should appear in shock loading of porous or preheated specimens. The present study will offer thermodynamically complete equations of state for the α-, e-, γ-phases of iron, free of these shortcomings.

Structure of shock-wave flows with phase transitions in iron near a free surface

Structure of shock-wave flows with phase transitions in iron near a free surface

A magnetic field induced first-order phase transition in iron based alloy

We investigate the influence of high steady and pulsed magnetic fields on the phase composition of a quenched multicomponent iron based alloy (Fe-18% W-4% Cr-1% V-0.7% C) at 77 and 293 K. In all experiments the retained austenite decreases with the increase of the applied field, following a straight line relationship.

The Curie Point and the α–γ transition of iron to 53 Kbar — A reexamination

We have completed a study of the ferromagnetic‐paramagnetic phase transition (Curie transition) and the α‐γ phase transition in iron by differential thermal analysis. The investigation was carried out in a piston‐cylinder apparatus to 53 kbar. A low‐friction cell was employed that allowed the phase transitions to be detected by monitoring the change in compressibility. The ferromagnetic‐paramagnetic phase boundary was found to have a small negative slope and to intersect the α‐γ phase boundary in a triple point at 20±1.2 kbar. The precision of the measurements is discussed.

Thermodynamics of phase transitions in iron

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Shock waves and phase transitions in iron

Shock waves and phase transitions in iron

Shock wave study of the α ⇄ ε phase transition in iron

Plate impact experiments producing plane waves of up to 40 GPa (400 kbar) peak stress were performed using Armco iron specimens and impactors. Highly accurate time-resolved measurements of the resulting free-surface velocities of the specimens were obtained with the VISAR laser interferometer instrumentation system. The free-surface velocity profiles provide new information concerning the rate effects associated with the α → ε polymorphic phase transition at 13 GPa, the material strength and release wave speeds at 10 and 40 GPa, and the stress level at which the iron reverts back to the α phase on unloading. A strong magnetic field was found to produce no measurable change in the phase-transition stress. The accuracy of the ``factor-of-2'' assumption relating free-surface velocity to particle velocity in iron was also evaluated experimentally.

Dynamic Electrical Resistivity of Iron: Evidence for a New High Pressure Phase

A CAREFUL re-investigation of the response of α-iron (body-centred cubic) to shock loading, using an improved technique for the measurement of electrical resistivity during shock compression, has provided data that are in substantial disagreement with the phase transition results obtained by Fuller and Price1. In addition to describing this disagreement, we report here the possible existence of a new dynamic phase transition in iron.