Bulletin of the American Physical Society
2005 APS March Meeting
Monday–Friday, March 21–25, 2005; Los Angeles, CA
Session L2: Martensitic Phase Transformations Under Pressure |
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Sponsoring Units: DCMP Chair: Robert Albers, Los Alamos National Lab Room: LACC 151 |
Tuesday, March 22, 2005 2:30PM - 3:06PM |
L2.00001: Synchrotron X-Ray and Magnetic Susceptibility Probes in Diamond-Anvil Cell Invited Speaker: Multiple x-ray and allied techniques have been developed and integrated at synchrotron facilities focusing on a unified scientific goal -- exploring the rich behavior of materials under extreme pressures and temperatures. A plethora of synchrotron x-ray inelastic spectroscopic techniques has been introduced and applied, many of them for the first time, for high-pressure (HP) applications. These include \textit{HP x-ray emission spectroscope} which analyzes energies of the x-ray fluorescent photons with sub-eV energy resolution of the emission spectral lineshape to provide valuable information on the filled electronic states of the HP samples, \textit{HP x-ray inelastic near-edge spectroscopy} which opens a wide new field of HP chemical bonding studies of the light elements, \textit{HP electronic inelastic x-ray scattering spectroscopy }which provides unlimited access to high energy electronic phenomena, including electronic band structure, Fermi surface, excitons, plasmons, and their dispersions at HP, \textit{HP resonant inelastic x-ray scattering spectroscopy }which probes shallow core excitations and multiplet structures for highly correlated electronic systems as well as spin-resolved electronic structures for magnetic samples, and \textit{HP nuclear resonant x-ray spectroscopy }which reveals phonon densities of state and time-resolved M\"{o}ssbauer information. These new tools integrated with the existing magnetic susceptibility and electrical conductivity probes are unleashing the full power of high pressure in numerous scientific disciplines. Fundamental understanding in electronic structure, from simple electron gas to strongly-correlated systems, will be manifested through tuning of the pressure variable. New rules of crystal structure and superconductivity, for example, will be established across the Periodic Table in each pressure regime. [Preview Abstract] |
Tuesday, March 22, 2005 3:06PM - 3:42PM |
L2.00002: Time-resolved measurements of structural changes in shocked crystals Invited Speaker: Plane shock wave experiments provide a unique approach to examine compression induced structural changes in real time (sub-ns and ns resolution). Past studies have focused on time-resolved continuum measurements and the propagating wave profiles have been related to material thermodynamic states. Following a brief review of past continuum studies, recent experimental developments related to fast time-resolved optical spectroscopy and x-ray diffraction to examine structural changes at the microscopic level will be presented. Challenges associated with the use of these methods in shock wave experiments will be indicated. Specific examples will be described to demonstrate how continuum and microscopic results can be combined to gain detailed insight into shock wave induced structural changes in condensed matter. Directions for future work will be discussed. Work supported by DOE/NNSA. [Preview Abstract] |
Tuesday, March 22, 2005 3:42PM - 4:18PM |
L2.00003: Martensitic phase transitions at the atomic length scale: Titanium Alpha to Omega Invited Speaker: Martensitic phase transitions---diffusionless, first-order structural transitions occurring near the speed of sound---are abundant in both nature and technological applications from the earth's core to steels to shape memory alloys. Of particular interest to the aerospace industry are titanium alloys. However, pure titanium transforms under pressure to the brittle omega phase, a transformation that must be suppressed for technological applications. The theoretical understanding of this transformation involves finding the atomic pathway of the martensitic transformation. A systematic approach generates all possible pathways; they are successively pruned by energy estimates using elastic theory, tight-binding and {\it ab initio} methods. This general method reduces one thousand possibilities down to seven, and finally to the lowest energy pathway\footnote{D.R. Trinkle {\it et al.}, Phys. Rev. Lett. {\bf 91}, 025701 (2003).}. The lowest energy barrier pathway has a barrier four times lower than all others, and remains the lowest even when nucleation effects are considered. Molecular dynamics simulates the mobile interfacial boundary, and shows the transformation occuring at a fraction of the speed of sound. The resulting microscopic picture provides the starting point for understanding the effect of impurities and for the alloy transformations. [Preview Abstract] |
Tuesday, March 22, 2005 4:18PM - 4:54PM |
L2.00004: Impurities Block the Alpha to Omega Martensitic Transformation in Titanium Invited Speaker: Impurities control phase stability and phase transformations in nature: from shape memory alloys to steel to planetary cores. Experiments and empirical databases are still central to tuning the impurity effects. Missing is a broad theoretical underpinning. Consider, for example, the titanium martensitic transformations: diffusionless structural transformations proceeding near the speed of sound. Pure Ti transforms from ductile $\alpha$ to brittle $\omega$ at 9~GPa creating serious technological problems for $\beta$-stabilized Ti alloys. Impurities in the Ti alloys A-70 and Ti-6Al-4V suppress the transformation up to at least 35 GPa enhancing their technological utility as lightweight material in aerospace applications. These and other empirical breakthroughs in technological materials call for broad theoretical understanding. Impurities pose two theoretical challenges: The effect on \emph{the relative phase stability} and \emph{the energy barrier} of the transformation. {\it Ab initio} nudged-elastic band methods calculate both changes due to impurities. We show that interstitial O, N, and C retard the transformation while substitutional Al and V influence the transformation by changing the d-electron concentration. 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. [Preview Abstract] |
Tuesday, March 22, 2005 4:54PM - 5:30PM |
L2.00005: Fermi Surface as a Driver for the Shape-Memory Effect in AuZn Invited Speaker: Martensites are materials that undergo diffusionless, solid-state transitions. The martensitic transition yields properties that depend on the history of the material and if reversible can allow it to recover its previous shape after plastic deformation. This is known as the shape-memory effect (SME). We have succeeded in identifying the operative electronic mechanism responsible for the martensitic transition in the shape-memory alloy AuZn by using Fermi-surface measurements (de Haas-van Alphen oscillations) and band-structure calculations. Our findings suggest that electronic band structure gives rise to special features on the Fermi surface that is important to consider in the design of SME alloys. [Preview Abstract] |
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