R. C. Albers

Impurities block the α to ω martensitic transformation in titanium

Impurities control phase stability and phase transformations in natural and man-made materials, from shape-memory alloys1 to steel2 to planetary cores3. Experiments and empirical databases are still central to tuning the impurity effects. What is missing is a broad theoretical underpinning. Consider, for example, the titanium martensitic transformations: diffusionless structural transformations proceeding near the speed of sound2. Pure titanium transforms from ductile α to brittle ω at 9 GPa, creating serious technological problems for β-stabilized titanium alloys. Impurities in the titanium alloys A-70 and Ti–6Al–4V (wt%) suppress the transformation up to at least 35 GPa, increasing their technological utility as lightweight materials in aerospace applications. These and other empirical discoveries in technological materials call for broad theoretical understanding. Impurities pose two theoretical challenges: the effect on the relative phase stability, and the energy barrier of the transformation. Ab initio methods4,5 calculate both changes due to impurities. We show that interstitial oxygen, nitrogen and carbon retard the transformation whereas substitutional aluminium and vanadium influence the transformation by changing the d-electron concentration6. The resulting microscopic picture explains the suppression of the transformation in commercial A-70 and Ti–6Al–4V alloys. In general, the effect of impurities on relative energies and energy barriers is central to understanding structural phase transformations.

Shock-induced α–ω transition in titanium

Equilibrium free energies for the α and ω phases of Ti are constructed. The result is a consistent picture of the ambient pressure, static high pressure, and shock data, as well as first-principles electronic structure calculations. The Hugoniot consists of three segments: a metastable α-phase region, a transition region, and an ω-phase branch. All the Hugoniot data are consistent with a transition occurring at ∼12 GPa. An early identification [R. G. McQueen et al., in High Velocity Impact Phenomena, edited by R. Kinslow (Academic, New York, 1970)] of a phase transition at 17.5 GPa appears to have been an artifact. The shock Hugoniot extends further into the metastable region than static data, indicating the existence of a relaxation process occurring on a time scale intermediate between those of the static and dynamic measurements.

New mechanism for the alpha to omega martensitic transformation in pure titanium.

We propose a new direct mechanism for the pressure driven alpha-->omega martensitic transformation in pure titanium. A systematic algorithm enumerates all possible pathways whose energy barriers are evaluated. A new, homogeneous pathway emerges with a barrier at least 4 times lower than other pathways. The pathway is shown to be favorable in any nucleation model.

Three-Dimensional Elastic Compatibility and Varieties of Twins in Martensites

We model a cubic-to-tetragonal martensitic transition by a Ginzburg-Landau free energy in the symmetric strain tensor. We show in three dimensions (3D) that solving the St. Venant compatibility relations for strain, treated as independent field equations, generates three anisotropic long-range potentials between the two order parameter components. These potentials encode 3D discrete symmetries, express the energetics of lattice integrity, and determine 3D textures. Simulation predictions include twins with temperature-varying orientation, helical twins, competing metastable states, and compatibility-induced elastic frustration. Our approach also applies to improper ferroelastics.

Condensed-matter physics: An expanding view of plutonium

Interactions between electrons make it hard to predict the properties of exotic metals, such as plutonium. Better calculations that include a thorough treatment of electronic structure are the answer.

Theoretical studies of grain boundaries in Ni3Al with boron or sulfur

It is well known that grain boundaries (GB) can have pronounced effects on the physical properties of materials (mechanical properties, corrosion resistance, fracture path, resistivity, etc.). Accordingly, a great deal of effort has been devoted to trying to understand the structure, energetics, and properties of grain boundaries. Significant experimental and theoretical progress has been made in understanding grain boundaries in pure systems, while the understanding of grain boundaries in alloy systems is much less developed. Also the mechanical properties of the grain boundaries are not well understood. In the present report, the authors summarize recent results on atomistic simulations of grain boundaries in the Ll/sub 2/ ordered alloy Ni/sub 3/Al. Understanding grain boundaries in this material is of particular importance since intergranular fracture limits the applicability of this otherwise promising material. To put these results into perspective, additional simulations were performed on grain boundaries in pure Ni and Al. Many features of grain boundaries in the ordered alloy may be understood in terms of the results on pure Ni and Al grain boundaries. The authors also consider the effect of boron, sulfur, and nickel segregation on the strength of grain boundaries in Ni and Ni/sub 3/Al.

Fermi surface and effective masses for the heavy-electron superconductors UPt3

Local-density-approximation (LDA) calculations for the Fermi-surface extremal cross-sectional areas of UPt{sub 3} are presented and compared to deHaas-van Alphen experiments of Taillefer et al. The topology of the calculated surfaces is in excellent agreement with experiment and allows a determination of the directional dependence of the anisotropic mass-renormalization factor. The source of this renormalization is briefly discussed. 12 refs., 4 figs., 2 tabs.

Reversal of spin polarization in Fe/GaAs (001) driven by resonant surface states: first-principles calculations.

A minority-spin resonant state at the Fe/GaAs(001) interface is predicted to reverse the spin polarization with the voltage bias of electrons transmitted across this interface. Using a Greens function approach within the local spin-density approximation, we calculate the spin-dependent current in a Fe/GaAs/Cu tunnel junction as a function of the applied bias voltage. We find a change in sign of the spin polarization of tunneling electrons with bias voltage due to the interface minority-spin resonance. This result explains recent experimental data on spin injection in Fe/GaAs contacts and on tunneling magnetoresistance in Fe/GaAs/Fe magnetic tunnel junctions.

Hubbard- U calculations for Cu from first-principle Wannier functions

We present first-principles calculations of optimally localized Wannier functions for Cu and use these for an ab initio determination of Hubbard ~Coulomb! matrix elements. We use a standard linearized muffin-tin orbital calculation in the atomic-sphere approximation to calculate Bloch functions, and from these determine maximally localized Wannier functions using a method proposed by Marzari and Vanderbilt. The resulting functions were highly localized, with greater than 89% of the norm of the function within the central site for the occupied Wannier states. Two methods for calculating Coulomb matrix elements from Wannier functions are presented and applied to fcc Cu. For the unscreened on-site Hubbard U for the Cu 3d bands, we have obtained about 25 eV. These results are also compared with results obtained from a constrained local-density approximation calculation.

Zirconium under pressure: phase transitions and thermodynamics

In this paper, the full-potential linearized augmented plane-wave (LAPW) method within the generalized gradient approximation (GGA) was used to calculate the effect of hydrostatic pressure at zero temperature on the 4d transition metal zirconium. For the hexagonal close-packed (hcp), omega (ω), and body-centred cubic (bcc) structures the enthalpy, H = E+pV, was calculated as a function of pressure p. We obtained an ω to bcc transition pressure of 28.2 GPa. Temperature-dependent contributions were obtained from tight-binding calculations of the phonons in the quasiharmonic approximation and were used to calculate the Gibbs free energy as a function of both temperature T and pressure for these three structures. From the comparison of these free energies the phase boundaries were calculated in the T–p phase diagram, and compared to the experimentally determined boundaries. We discuss the importance of anharmonicity for understanding the material properties of zirconium, and limitations of quasiharmonic phonon theories for predicting phase transformations.