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R C Albers

Publications and source records attributed to R C Albers.

6 recordsLinked to original sources
PubMed2004

Extended X-ray absorption fine structure measurements of laser-shocked V and Ti and crystal phase transformation in Ti.

Extended x-ray absorption fine structure (EXAFS), using a laser-imploded target as a source, can yield the properties of laser-shocked metals on a nanosecond time scale. EXAFS measurements of vanadium shocked to approximately 0.4 Mbar yield the compression and temperature in good agreement with hydrodynamic simulations and shock-speed measurements. In laser-shocked titanium at the same pressure, the EXAPS modulation damping is much higher than is warranted by the predicted temperature increase. This is shown to be due to the alpha-Ti to omega-Ti crystal phase transformation, known to occur below approximately 0.1 Mbar for slower shock waves.

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PubMed2004

New pseudophase structure for alpha-Pu.

We propose a new pseudophase crystal structure, based on an orthorhombic distortion of the diamond structure, for the ground-state alpha phase of plutonium. Electronic-structure calculations in the generalized-gradient approximation give approximately the same total energy for the two structures. Interestingly, our new pseudophase structure is the same as the gamma-Pu structure except with very different b/a and c/a ratios. We show how the contraction relative to the gamma phase, principally in the z direction, leads to an alpha-like structure in the [0,1,1] plane and reproduces the short range bonds of the alpha phase. This is an important link between two complex structures of Pu and opens new possibilities for exploring the very rich phase diagram of Pu through theoretical calculations.

Journal Article↗
PubMed2003

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.

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PubMed2001

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.

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