Onset Pressure Of Transition Research Articles

Kinetics of the bcc to hcp phase transition in [formula omitted]-oriented single crystal iron under ramp compression

Non-Equilibrium Molecular Dynamics simulations have been used to investigate the kinetics of the bcc to hcp phase transition in ramp-compressed [001]-oriented single crystal iron in the presence of defects consisting in micro-voids. Depending on the ramp maximum pressure, the microstructure of the daughter hcp phase is observed to be strongly affected by the plastic activity in the parent bcc phase. The hcp phase is first nucleated around the pre-existing microvoids, then it grows along preferential crystal directions and the subsequent volume reduction induces a local relaxation of the pressure. The velocity of the bcc-hcp phase boundary is shown to increase gradually with maximum pressure up to supersonic values. For high over-compression beyond the transition onset pressure, the phase transformation becomes almost instantaneous and essentially independent of the initial defects. Meanwhile, the ramp wave steepens with propagation distance and evolves into a shock front, up to the over-driven regime classically observed in both experiments and hydrodynamic, macro-scale simulations.

Optical full-field strain measurement within a diamond anvil cell.

Digital image correlation computations are run on optical images of iron samples within a diamond anvil cell to obtain in-plane strain components at the surface of the sample up to 17GPa. The α-Fe → ε-Fe transition onset pressure and phase coexistence pressure domain can be identified from the evolution of the surface average of strain components. Strain fields exhibit localizations for both direct and reverse transition; they coincide with the approximate boundary locations of reversion variants inside the microstructure of a single crystal sample. The so-called DICDAC (Digital Image Correlation within a Diamond Anvil Cell) setup is then a suitable tool for the investigation of phase transformations strains under pressure. In addition, specific volumes that are deduced from strain out of the transition pressure domains agree within ΔV/V = 0.4% with the equation of state data from the literature.

Influence of point defects and grain boundaries on plasticity and phase transition in uniaxially-compressed iron

Using Molecular Dynamics (MD) computations we have reported recently a shear stiffening (hardening-like) effect in [001]-oriented defect-free iron single crystal. This effect makes shift the structural phase transition to a particularly high pressure level under ramp compression, but not under shock compression, so that the question arises about the essential role of the compression path and wave evolution in the solid state response including phase transition. Here, plasticity and phase transition are studied for iron samples under non-equilibrium uniaxial compression. Our focus is put on the effects of grain size and point defects such as atoms vacancy. Consistently with our previous results, it is found that, after yielding via twinning, both defect-free and defective iron single crystals exhibit a hardening-like effect while the twins progressively recede upon further compression and no dislocations activity is detected. This leads to mounting of the deviatoric stress. Yet, the transition onset pressure shift and temperature behavior are found to be defect-density dependent. Considering polycrystalline lattice, plasticity consists in complex interplay of intra-grain twinning and dislocation activities from grain boundaries. The elastic stiffening is attenuated now as compared to single crystal so that the phase transition is found to occur at much lower pressure in polycrystalline iron.

Kinetics of iron α-εphase transition under thermodynamic path of multiple shock loading-unloading

The dynamics of iron under extreme conditions like high temperature and high pressure has been well studied for several decades. But, there have been not many reports about the phase transition kinetics coupled with complicated thermodynamic paths, especially loading-unloading-reloading path, which is closer to the real applications. A three-layer structure impactor with five stages performed in the front-surface experiment is made up to approach the special path. We choose epoxy to be the adhesive as it has low impedance and high strength. Tantalum, the standard material of high impedance which also has single wave structure, is selected for reloading process. The wave profile shows a 3-wave structure in the first unloading period and the inverse phase transition threshold is calculated to be about 11.3 GPa. This onset pressure of reverse phase transition is not consistent with Barker’s result, higher than his result (about 2.5 GPa). By comparing with recalculated result of Jensen’s data, we find that our result is consistent with theirs.In this work the inverse phase transition ends at about 10 GPa, the value from this way which is higher than Barker’s finding, even higher than his result of the threshold pressure of reverse phase transition. And at this state there remains 12%–15% of ε phase. So it cannot be seen as the completed reverse phase transformation. The phase transition onset pressure is 10–12 GPa on the reloading path and it is about 1–2 GPa lower than the first phase transition. By simulating the wave profile, the discrepancy of using different phase transformation characteristic time <i>τ</i> as 30 ns and 5 ns is analyzed. It can be seen that the phase transition rate of reloading is faster than that of the first loading process. These phenomena may be caused by the twins and the dislocations which are produced by the inverse phase transition. Also, as unloading time becomes longer, the mass fraction of ε phase becomes lesser and the onset pressure of α → ε phase transition becomes lower. This because with more ε phases transforming into α phase, more twins and dislocations will be produced in material. Therefore, it brings the lower onset pressure.

Experimental investigation on dynamic lattice response by in-situ Xray diffraction method

Structure evolution under dynamic compression condition (high temperature, high pressure and high strain rate) is one of the most important problems in engineering and applied physics, which is vital for understanding the kinetic mechanism of shock-induced phase transition. In this work, an in-situ dynamic X-ray diffraction (DXRD) diagnostic method is established to probe the lattice response driven by shock waves. The geometry is suitable for the study of laser-shocked crystals. In order to eliminate the measurement error arising from the difference in experimental setup, the static and dynamic lattice diffraction signals are measured simultaneously in one shot by using a nanosecond burst of X-ray emitted from a laser-produced plasma. Experimental details in our investigation are as follows. 1) The laser driven shock wave transit time △ tShock and the shock pressure in sample are accurately determined from the shock-wave profile measurement by dual laser heterodyne velocimetry. 2) A laser pump-and-probe technique for adjusting the time-delay of DXRD diagnosis during △ tShock, with a series of repeated shock loadings is then employed to generate and measure the dynamic structure evolution. Using this method, the dynamic lattice response of[111] single-crystal iron is studied on Shenguang-Ⅱ facility. Single-shot diffraction patterns from both unshocked and shocked crystal are successfully obtained. An elastic-plastic transition process –elastic wave followed by a plastic wave– is observed in shocked[111] single-crystal iron on a lattice scale. The lattice compressibility values of the elastic wave and plastic wave are in agreement with those derived from the wave profiles. It is found that the Hugoniot elastic limit is measured to be about 6 GPa under nanosecond-pulsed laser shock compression. Such a high yield strength is consistent with recent laser ramp compression experimental results in polycrystalline Fe[Smith et al. 2011 J. Appl. Phys. 110 123515], suggesting that the peak pressure of elastic wave is dependent on the loading rate and the thickness of sample. Based on the analysis of diffraction patterns, the BCC phase is determined to be stable till 23.9 GPa, the highest pressure explored in this work, which might indicate that the phase transition strongly couples with the crystal orientation and loading rate. Some possible physical mechanisms remain to be further studied:whether the transition time hysteresis occurs or the metastable FCC phase exists in shocked[111] single crystal Fe, or the phase transition onset pressure increases under high strain-rate compression. Our DXRD results provide a primary experimental reference for the follow-up study on the phase kinetics.

Kinetics of the ironα−ɛphase transition at high-strain rates: Experiment and model

In this article, we investigate the kinetics of the iron $\ensuremath{\alpha}\ensuremath{-}\ensuremath{\varepsilon}$ transition under laser-driven ramp compression for deformation rates ranging from 3 to $9\ifmmode\times\else\texttimes\fi{}{10}^{7}\phantom{\rule{0.28em}{0ex}}{\mathrm{s}}^{\ensuremath{-}1}$. As in previous work, we observe a plateau in the rear surface velocity profile at the transition. With increasing deformation rate the transition onset pressure raises from 11 to 25 GPa, while the plateau duration decreases. These kinetic effects are well reproduced by an Avrami-type kinetics model of nucleation and growth with a constant, nanosecond scale completion time, which suggests an isokinetic regime over the explored range of strain rates.

MD modeling of screw dislocation influence upon initiation and mechanism of BCC-HCP polymorphous transition in iron

The present work is devoted to classical molecular dynamics investigation into microscopic mechanisms of the bcc-hcp transition in iron. The interatomic potential of EAM type used in the calculations was tested for the capability to reproduce ab initio data on energy evolution along the bcc-hcp transformation path (Burgers deformation + shuffe) and then used in the large-scale MD simulations. The large-scale simulations included constant volume deformation along the Burgers path to study the origin and nature of the plasticity, hydrostatic volume compression of defect free samples above the bcc to hcp transition threshold to observe the formation of new phase embryos, and the volume compression of samples containing screw dislocations to study the effect of the dislocations on the probability of the new phase critical embryo formation. The volume compression demonstrated high level of metastability. The transition starts at pressure much higher than the equilibrium one. Dislocations strongly affect the probability of the critical embryo formation and significantly reduce the onset pressure of transition. The dislocations affect also the resulting structure of the samples upon the transition. The formation of layered structure is typical for the samples containing the dislocations. The results of the simulations were compared with the in-situ experimental data on the mechanism of the bcc-hcp transition in iron.

Shock compressibility and shock-induced phase transitions of C 60 fullerite

Shock compressibility of C 60 fullerite and sound velocities in shock-compressed fullerite were experimentally studied at the pressures range up to ∼50 GPa. In our experiments, we used polycrystalline C 60 specimens with a density of 1.64 g/cc. The Hugoniot of C 60 fullerite had a set of peculiarities, which may be attributed to a series of polymorphic phase transitions. The jump of sound velocity in shocked C 60 at pressure ∼9 GPa indicates the formation of a rather hard carbon phase. It is possible to assume that it is a polymerized C 60 phase. In the region of pressures 9–25 GPa, destruction of this phase and formation of a graphite-like carbon occurs. With further growth of shock pressure, phase transition of the graphite-like carbon to a diamond-like phase is observed with a transition onset pressure ∼25 GPa. If shock pressures are higher than ∼33 GPa, Hugoniot of C 60 fullerite is determined by the thermodynamic properties of the diamond-like phase.

Shock-induced phase transition of oriented pyrolytic graphite to diamond at pressures up to 15 GPa

In studies of shock-induced phase transition of ordered pyrolytic graphite to a diamond-like phase, the lowest transition onset pressure was observed at 19.6 GPa. The phase transition in that case was considered to be martensitic. In the present study ordered pyrolytic graphite with voids between particles was loaded at pressures up to 15 GPa using a planar shock wave propagating along the basal plane of the graphitic crystal structure. As a result, both diamond-like carbon and diamond were observed in the postshock sample. The phase transition of graphite to diamond was assumed to occur by the release of distortional energy stored in the graphite particles, that is, diffusional-controlled reconstructive mechanism, on the basis of the data by high resolution electron microscopy together with electron energy loss spectroscopy.

Electrical Resistance of CdSe and CdTe at Elevated Temperatures and Pressures

The electrical resistance of polycrystalline CdSe and CdTe was measured as a function of temperature (up to 560 K) and pressure (up to 5.5 GPa). The resistance change across the phase transition and the transition onset pressure were found to decrease linearly with increasing temperature. The band gap of the low-pressure phases of CdSe and CdTe decreases with increasing pressure. The rapid reduction in the resistance change with increasing temperature restricts the use of CdSe and CdTe as high-temperature pressure calibration standards.

Shock-induced martensitic transformation of highly oriented graphite to diamond

Shock-wave profiles of highly ordered pyrolytic graphite shocked normal to the basal plane of the graphite crystal structure have been measured. For graphite with sufficient orientational order a martensitic transformation to a diamond-like phase is observed with a transition onset pressure 19.6±0.7 GPa, the stability limit of the graphite structure under shock compression. The minimum overpressure required for the transformation is not more than 6 GPa and the two-wave structure of the transition is overdriven to a single wave above 40 GPa.

Shock-Induced Martensitlc Transformation of Highly Oriented Graphite to Diamond

ABSTRACTShock-wave profiles of highly ordered pyrolytic graphite shocked normal to the basal plane of the graphite crystal structure have been measured. For graphite with sufficient orientational order a martensitic transformation to a diamond-like phase is observed with a transition onset pressure 19.6±0.7 GPa, the stability limit of the graphite structure under shock compression. The minimum overpressure required for the transformation is not more than 6 GPa.