Experimental studies on the effects of pressure on plastic properties are reviewed with an emphasis on those on geological materials. High pressures may affect plastic properties through two different ways: (i) through the effects of pressure on plastic deformation of a material with a given crystal structure and (ii) through the effects of pressure-induced phase transformations on plastic deformation. It is emphasized that the manner in which pressure affects plastic properties of solids depends on the microscopic mechanisms of plastic deformation and therefore it is essential to identify the mechanisms of deformation involved in a particular experimental condition. Three mechanisms are considered: (i) the Peierls mechanism, (ii) dislocation creep (power-law creep) and (iii) diffusion creep (or superplasticity). Creep strength of materials changes with pressure through the change in shear modulus and in the activation enthalpy for the Peierls mechanism. Structural phase transformations can dramatically change the creep strength determined by the Peierls mechanism. Changes can be either positive (increase in strength in a high pressure phase) or negative (decrease in strength in a high pressure phase). Creep strength controlled by dislocation creep usually increases exponentially with pressure through the increase in the activation enthalpy with pressure. Phase transformations can change the creep strength either positively or negatively for this mechanism depending on the manner in which dislocation mobility changes with crystal structure. Creep strength corresponding to diffusion creep changes with pressure in a similar fashion as dislocation creep.

High pressures in the deep interior of Earth and other terrestrial planets could lead to a significant increase in viscosity with depth, resulting in high temperatures due to the feedback between viscosity and temperature. Also phase transformations in constituent minerals that occur in certain depth ranges can cause dramatic changes in plastic properties. Both of them are likely to have important influence on the style of convection in the mantle of terrestrial planets and thus affect their geological history and tectonic activities.

We discuss the behavior of two minerals of the same composition, Mg2SiO4 but different structures: forsterite and ringwoodite. Ab initio plane-wave pseudopotential results are discussed in terms of the full elastic constant tensors of these phases and their elastic anisotropy. The structures of the two minerals, based on pseudo-hep and pseudo-fee packing of oxygens, respectively, show very different behavior at high pressure. While the elastic anisotropy of olivine depends weakly on pressure between 0 and 25 GPa, the anisotropy of ringwoodite decreases with pressure initially, vanishing at 17 GPa before increasing again at higher pressure. This unusual behavior is understood in terms of a change of sign of the combination of elastic constants c2+2c44-c11, and a resulting interchange of fast and slow directions of acoustic wave propagation. To gain insight into the origin of their elastic behavior, the ab initio results are compared with a simple model based on the O sub-lattice.

Linearized augmented plane wave (LAPW) results are presented for FeO at high pressures using the Generalized Gradient Approximation (GGA) to study the high-spin low-spin transition previously predicted by LAPW with the Local Density Approximation (LDA) and Linear Muffin Tin Orbital (LMTO-ASA) methods within the GGA. We find a first-order transition at a pressure of about 105 GPa for the cubic lattice, consistent with earlier LAPW results, but much lower than obtained with the LMTO. The results are generally consistent with recent Mössbauer experiments that show a transition at about 100 GPa. We also discuss the origin of the transition, and show that it is not due to electrostatic crystal-field effects, but is rather due to hybridization and band widening with pressure. Examination of experimental data and computations suggest that the high pressure hexagonal phase of FeO is likely a polytype between the B8 NiAs and anti-B8 AsNi structures. The former is predicted to be an antiferromagnetic metal, and the latter an antiferromagnetic insulator. Implications for geophysics are discussed.

Shock-wave experiments on pure iron preheated to 1573 K were conducted in the 17–73 GPa range. The shock-wave equation of state of γ-iron at an initial temperature of 1573 K can be fit with us = 4.102 (0.015) km/s + 1.610(0.014) up with ρo = 7.413±0.012 Mg/m3 We obtain for γ-iron's bulk modulus and pressure derivative the values: 124.7±1.1 GPa and 5.44±0.06, respectively.

We present new data for sound velocities in the γ- and liquid-phases. In the γ-phase, to a first approximation, the longitudinal sound velocity is linear with respect to density: Vp = −3.13 (0.72) + 1.119(0.084) p(units for Vp and p are km/s and Mg/m3, respectively). Melting was observed in the highest pressure (about 70–73 GPa) experiments at a calculated shock temperature of 2775±160 K. This result is consistent with a previously calculated melting curve (for ε-iron) which is close to those measured by Boehler [1] and Saxena et al. [2]. The liquid iron sound velocity data yields a Grüneisen parameter value of 1.63±0.28 at 9.37±0.02 Mg/m3 at 71.6 GPa. The quantity γρ is 15.2±2.6 Mg/m3, which agrees with the uncertainty bounds of Brown and McQueen [3] (13.3–19.6 Mg/m3). Based on upward pressure and temperature extrapolation of the melting curve of γ-iron, the estimated inner core-outer core boundary temperature is 5500±400 K, the temperature at the core-mantle boundary on the outer core side is 3930±630 K.

Radio-frequency eddy current heating of metallic samples in shock wave, ultrasonic and diamond anvil apparatus provides a methodology for obtaining hot samples or hot metallic gaskets (containing a sample) and yet not heating the entire target, ultrasonic assembly or diamond cell. Analysis of a previous design of a radio-frequency (∼0.5 MHz) coil demonstrated that the center of 13 mm diameter shock wave experiment sample discs were underheated and the experimental temperatures achieved resulted from conduction of heat from the overheated sample edges relative to the center of the sample. We show, using the Biot-Savant law, that the ratio of magnetic field in the center to that at the edge of the sample, f, can be maximized to a value of f=1.8, by decreasing the radius of the heater coil, relative to the 13 mm diameter of sample. A simple analysis provides the thermal gradient across the sample thickness. An electric skin depth for iron of ∼1 mm for the 0.45 MHz frequency of the heater power supply of our system is obtained. This leads to a maximum temperature gradient of ∼52 K over the 4 mm thickness of the shock wave sample discs.

Laser shot peening, a surface treatment for metals, is shown to induce compressive residual stresses of over 0.040 inch to provide improved resistance to vairous forms of failure. A solid state laser technology employing Nd:glass slabs and phase conjugation enables this process to move into high throughput production processing. Details of the technology are discussed.

In this paper examples of the shock-induced defects produced during shock compression which correlate with microstructure / mechanical property changes induced in materials due to shock prestraining are discussed. The characteristics of the shock impulse(peak shock pressure, pulse duration, and rarefaction rate) imparted to the material under investigation and the shock-induced defects produced in numerous metals and alloys are compared with their deformation behavior at ordinary rates of deformation. Examples of the range of defects observed in shock-recovered metals and alloys, include: dislocations, deformation twins, point defects, and residual metastable remnants from pressure-induced phase transformations. Results concerning the influence of interstitial content on the propensity of ω-phase formation and its structure in high-purity and A-70 Ti are presented. The influence of shock-wave deformation on the phase stability and substructure evolution of high-purity (low-interstitial) titanium and A-70 (3700 ppm oxygen) titanium were probed utilizing real-time velocity interferometry (VISAR) and “soft” shock-recovery techniques. Suppression of the α-ω pressure-induced phase transformation in A-70 Ti, containing a high interstitial oxygen content, is seen to simultaneously correspond with the suppression of deformation twinning.

Investigations of Hugoniots the diamond-type carbides(various SiC) and NaCl-type carbides such as TiC give some insights into the high-pressure carbon behaviors. The experimental results of phase transitions of a-SiC and β-SiC, together with those of diamond-structure Si, imply that the candidate as post-diamond phase has sixfold coordination and that a possible transition pressure is about 1–2 TPa. The NaCl-type carbide Hugoniots indicate that sixfold coordinated C is very stable at high pressures. The partial molar volume of carbon in the NaCl-type carbides ranges between 1.4 to 2.6 cnvVg-atom C at 1 atm and reaches about 2.8 cm3/g-atom C at 100 GPa. Taking into account structural variations of the corresponding metals, the volume of the sixfold coordinated C is estimated to be 1.7 cm3/g-atom C, about half of that of diamond, and the post-diamond phase appears to be extremely hard.

Recent advances in computational condensed matter theory have yielded accurate calculations of properties of materials. These calculations have, for the most part, focused on the low temperature (T=0) limit. An accurate determination of the equation of state (EOS) at finite temperature also requires knowledge of the behavior of the electron and ion thermal pressure as a function of T. Current approaches often interpolate between calculated T=0 results and approximations valid in the high T limit. Plasma physics-based approaches are accurate in the high temperature limit, but lose accuracy below T∼Tfermi. We seek to “connect up” these two regimes by using ab initio finite temperature methods (including linear-response[l] based phonon calculations) to derive an equation of state of condensed matter for T<Tfermi.

We will present theoretical results for the principal Hugoniot of shocked materials, including carbon and aluminum, up to pressures P>100 GPa and temperatures T> 104K, and compare our results with available experimental data.

Compacts of polycrystalline diamond were made in toroid-type high-pressure camera under the pressure of 8 GPa using temperatures between 800 to 2150°C without the use of additive components. Nanocrystalline commercial DALAN, and microcrystalline ASM diamond powders were used. The compacts were characterized by helium pycnometry, Vickers hardness measurements, X-ray diffraction and SEM methods.

The starting and sintered nanocrystalline grain compacts were found to have strongly one-dimensionally disordered cubic modification. The nanocrystalline powder had a bimodal grain size distribution function as determined from X-ray diffraction data and ab initio intensity calculations performed with the use of Debye functions. It was found that neither the grain size nor one-dimensional disordering change under high-pressure high-temperature conditions. There is a general tendency in a decrease of density of compacts with increase in the sintering temperature what resulting partly from graphitization above 1000–1200°C. The main factor which determines the density of the diamond compacts is closed porosity. Typically, the nanocrystalline diamond compacts sintered from 30 sec. to 6 min. have densities around 90% of the theoretical value. Their Vickers microhardness is 24 GPa and less.

Experimental investigations of the electrical conductivities of sulphur, iodine, bromine, and water under shock compression are reviewed. The value of sulphur conductivity is approximately 105 Ω−1 m−1 at 30 GPa and increases slowly with pressure up to 110 GPa. The conductivity of iodine is 105 Ω−1 m−1 at approximately 15 GPa and decreases moderately up to 110 GPa. The conductivity of liquid bromine is 12.5 Ω−1 m−1 at 9 GPa and approximately 104 Ω−1 m−1 at 30 GPa. In electrochemical experiments galvanic cells with copper and aluminium electrodes and bromine as an electrolyte were subjected to shock loading and their electrical response was measured. We discovered that the cells produce open circuit voltages of about 0.5 V under shock pressures of 7.4 to 30 GPa. This voltage level clearly shows that the observed electrical signals are electrochemical in nature and connected with an ionic character of bromine conductivity. Analogous experiments were made with water, whose conductivity is ionic at 75 GPa with a value of 2000 Ω−1 m−1.

Statistical mechanical studies predict that a chemically reactive system containing species composed of C, H, N, O atoms can exhibit a phase separation into a N2-rich and a N2-poor phase. The present work is concerned with the effect of the fluid phase separation upon addition of F atoms to the system. Our study shows that F atoms mainly appear as a constituent of HF in a N2-poor fluid phase up to a certain pressure beyond which they occur as CF4 in a N2-rich phase and that the phase separation may be abrupt in a thermodynamic sense. The pressure at the phase boundary can occur at about 30 GPa at 3000 K and about 10 GPa to 20 GPa at 1000 K. Some of these ranges may be accessible by present-day experimental high-pressure techniques. We discuss implications of this study to detonation physics.

The equation of state (EOS) of Li3N has been determined by energy-dispersive x-ray diffraction (EDXD) using synchrotron radiation up to 35 GPa at ambient temperature. Both the hexagonal D6h4(P63/mmc) and the hexagonal D6h1(P6/mmm) phases were present at ambient pressure. The D6h1 -structure completely transforms into the D6h4 -structure at modest pressure. The change in Gibb's free energy as a function of pressure for Li3N was calculated using the experimental EOS.

A new high-pressure polymorph of CrSb2 was synthesized under high-pressure/ temperature conditions of 7–7.7 GPa and 600–650°C. The crystal structure is body-centered tetragonal with the space group 14/mcm, which is assigned to CuAl2-type structure. The experiments under various pressure conditions revealed that the high-pressure polymorph was formed above 5.5 GPa, and the compound crystallized into the low-pressure marcasite-type structure below 5 GPa. The characteristic of the high-pressure phase is the metallic bond nature including the formation of Cr-Cr-Cr linear chain along the c-axis. The compound shows metallic conductivity and itinerant-electron ferromagnetic behavior with the Curie temperature of ca. 160 K.

Lattice strains have been measured as a function of the angle, ψ, from the diamond cell stress axis in a sample of gold and rhenium at pressures of 15–37 GPa. Experiments were conducted using X-ray transparent gaskets made from beryllium. The differential stresses supported by gold and rhenium have been characterized to 37 GPa. It is also shown that proper choice of the diffraction geometry allows recovery of a quasi-hydrostatic compression curve under these highly non-hydrostatic conditions. X-ray elastic moduli have also been determined, and while good agreement with previous data is achieved for gold, there is a large discrepancy between the present results and extrapolated ultrasonic data for rhenium.

The purpose of this study is to explore the potential of high pressure methods for preparation of new series of compounds in the A3(VO4)2 systems (A: Ca, Sr, Ba). In this study, we present our in situ vibrational and X-ray diffraction data on the behavior of the A3(VO4)2 compounds at high pressure and room temperature. Upon compression up to 290 kbar, there is no phase change in Ba3(VO4)2. Sr3(VO4)2 undergoes a first order phase transition to an olivine-like structure at about 150 kbar. In both the ambient pressure and olivine structures of Sr3(VO4)2, oxygen atoms form a hexagonal close packing. The packing in the olivine structure is distorted from this due to loss of 3-fold axis. Ca3(VO4)2 amorphizes at about 100 kbar. The high pressure behavior of the compounds studied here is related to the size of the A(2)2+ cations. Small Ca(2)2+ cations hinder the completion of crystal-to-crystal transformations in Ca3(VO4)2.

In the general framework of high pressure Hydrogen studies[l][2], it has been recently suggested[3] that hydrogen could metallize at much lower pressures, within experimentally accessible values, in a new compound, Ar(H2)2. Subsequent Raman measurements showed a transition around Pobs=175 GPa, but have not been interpreted satisfactorily up to now. Moreover, the experimental situation is quite confusing. Infra-red measurements on Ar(H2)2 up to 220 GPa[4] did not confirm the Raman results. We have thus carefully investigated the physical properties of Ar(H2)2 in the multimegabar range, especially as the experiments are difficult, and the diagnostics can suffer from spurious effects (such as the influence of the uniaxial component of pressure on the transition) [5].

Ductile materials are difficult to powderize for use in high pressure work. The potential of different techniques (gas-atomization, mechanical alloying, ball milling and subsequent annealing) for preparing suitable powders of some aluminides is investigated. Compression of Ti46Al54 and NiAl prepared by these methods yields equation of state parameters in good agreement with determinations by other methods.

We present the results of an exploratory theoretical study of Na3ClO in the anti-perovskite structure. The FLAPW method is used to calculate the static lattice properties, pressure equation of state, and the ferroic Flu phonon frequencies in the cubic PmSm phase. We also compute the compression behavior of NaCl and Na2O and find that the lattice energy of Na3ClO at ambient pressure (static lattice) is only marginally larger than of products, in agreement with experiment. However, detailed calculations reveal M- and R-point phonon instabilities in the ideal cubic phase and suggest the existence of lower symmetry structure involving slight rotations of ONa6 octahedra.

In this work we address the behaviour of electronic structure under uniaxial stress, by first-principles calculations and experiments of conductance in nanometer-sized metallic contactes of Au and Al. These contacts are shown to be specially suitable for this purpose. The conductance behaviour is related to the change with strain of Fermi surface. Both experimental and theoretically Au behaves like the free electron gas but Al has the opposite behaviour.