High-pressure strengths of Ni3Al and Ni-Al-Cr

High-pressure x-ray diffraction on Ni3Al, non-hydrostatically compressed at room temperature to 30 GPa in radial geometry can be interpreted in terms of a hydrostatic pressure–volume equation of state. We found the yield strength of Ni3Al to increase from about 0.2 to 2 GPa as pressure increases from 0 to 30 GPa. Yield-strength values determined from x-ray diffraction in axial geometry by including a Poisson ratio effect and pressure gradient methods are in good accord with each other. Our results indicate that the strengths of both pure and Cr-doped (7 at. %) Ni3Al increase with pressure, with the yield strength of Ni3Al slightly higher than Ni-Al-Cr alloy

Yield strength of Ni-Al-Cr superalloy under pressure

Ni based superalloy Ni-Al-Cr with γ and γ′ phase was studied under high pressure up to 30 GPa using diamond anvil cell technique. In-situ X-ray diffraction data was collected on these alloys under hydrostatic and non-hydrostatic conditions. Cubic phase remains stable up to the highest pressure of about 30 GPa. Bulk modulus and its pressure derivative obtained from the volume compression of pressure data are K = 166.6 ± 5.8 GPa with K′ set to 4 under hydrostatic conditions and K = 211.3 ± 4.7 GPa with K′ set to 4 for non-hydrostatic conditions. Using lattice strain theory, maximum shear stress 't' was determined from the difference between the axial and radial stress components in the sample. The magnitude of shear stress suggests that the lower limit of compressive strength increases with pressure and shows maximum yield strength of 1.8 ± 0.3 GPa at 20 GPa. Further, we have also determined yield strength using pressure gradient method. In both the methods, yield strength increases linearly with applied pressure. The results are found to be in good agreement with each other and the literature values at ambient conditions

High-Pressure Study of Adamantane: Variable Shape Simulations up to 26 GPa

We report simulations of adamantane by carefully combining ab initio and empirical approaches to enable simulations with internal degrees of freedom on crystalline adamantane up to a pressure of 26 GPa. Two sets of simulations, assuming the adamantane molecule as a rigid (RB) and flexible body (FB), have been carried out as a function of pressure up to 26 GPa to understand changes in the crystal structure as well as molecular structure. The flexible body simulations have been performed by including 6 lowest frequency internal modes (obtained from DFT calculations performed with Gaussian98) out of the total of 72. The calculated variation in c/a and VIV0 from the RB and FB calculations as a function of pressure have been compared with the experimental curve. Other relevant thermodynamic and structural properties reported are the radial distribution functions, deviation in the position of a given type of atom with respect to its position at standard pressure, \delta, cell parameters, volume, and energy. With an increase in pressure, three additional peaks are seen to develop gradually at three different pressures in the center of mass (com)-com radial distribution function (rdf). We attribute these changes to structural transformations (probably second-order phase transitions) which is consistent with the three phase transitions reported by Vijayakumar et al. for adamantane in the pressure range of 1 atm-15 GPa. Our simulations also show that these additional peaks in the rdf's are associated with the differences between opposite and parallel spin neighbors of Greig and Pawley as well as the crystallographic directional dependence of intermolecular distances in the first three shells of the neighbors. Also, the structural quantities from the RB calculation show considerable deviation from the FB calculation for pressures greater than 5 GPa, which suggests that the rigid body assumption for molecules may not be valid above this pressure

Equation of state, phase transitions, and band-gap closure in PbCl2 and SnCl2

The equations of state and band-gap closures for PbCl2 and SnCl2 were studied using both experimental and theoretical methods. We measured the volume of both materials to a maximum pressure of 70 GPa using synchrotron-based angle-dispersive powder x-ray diffraction. The lattice parameters for both compounds showed anomalous changes between 16-32 GPa, providing evidence of a phase transition from the cotunnite structure to the related Co2Si structure, in contrast to the postcotunnite structure as previously suggested. First-principles calculations confirm this finding and predict a second phase transition to a Co2Si-like structure between 75- 110 GPa in PbCl2 and 60-75 GPa in SnCl2. Band gaps were measured under compression to ∼70 GPa for PbCl2 and ∼66 GPa for SnCl2 and calculated up to 200 GPa for PbCl2 and 120 GPa for SnCl2. We find an excellent agreement between our experimental and theoretical results when using the Heyd-Scuseria-Ernzerhof (HSE06) hybrid functional, which suggests that this functional could reliably be used to calculate the band gap of similar AX2 compounds. Experimental and calculated band-gap results show discontinuous decreases in the band gap corresponding to phase changes to higher-coordinated crystal structures, giving insight into the relationship between interatomic geometry and metallicity

High pressure phase transition in metallic LaB6: Raman and X-ray diffraction studies

High pressure Raman and angle dispersive X-ray diffraction (ADXRD) measurements on the metallic hexaboride LaB6 have been carried out upto the pressures of about 20 GPa. The subtle phase transition around 10 GPa indicated in Raman measurements is confirmed by ADXRD experiments to be a structural change from cubic to orthorhombic phase. Ab-initio electronic band structure calculations using full potential linear augmented plane wave method carried out as a function of pressure show that this transition is driven by the interception of Fermi level by electronic band minimum around the transition pressure

Stabilization of body-centred cubic iron under inner-core conditions

The Earths solid core is mostly composed of iron. However, despite being central to our understanding of core properties, the stable phase of iron under inner-core conditions remains uncertain. The two leading candidates are hexagonal close-packed and body-centred cubic (bcc) crystal structures, but the dynamic and thermodynamic stability of bcc iron under inner-core conditions has been challenged. Here we demonstrate the stability of the bcc phase of iron under conditions consistent with the centre of the core using ab initio molecular dynamics simulations. We find that the bcc phase is stabilized at high temperatures by a diffusion mechanism that arises due to the dynamical instability of the phase at lower temperatures. On the basis of our simulations, we reinterpret experimental data as support for the stability of bcc iron under inner-core conditions. We suggest that the diffusion of iron atoms in solid state may explain both the anisotropy and the low shear modulus of the inner core.Funding Agencies|Swedish Research Council (VR) [2013-5767, 2014-4750]; National Magnetic Confinement Fusion Program of China [2015GB118000]; China Scholarship Council; Swedish Government Strategic Research Area in Materials Science on Functional Materials at Linkoping University (Faculty Grant SFO-Mat-LiU) [2009 00971]</p