arXiv subjects

Donald W. Brown

Publications and source records attributed to Donald W. Brown.

3 recordsLinked to original sources

arXiv2021

Deformation and dislocation evolution in body-centered-cubic single- and polycrystal tantalum

A physically-informed continuum crystal plasticity model is presented to elucidate the deformation mechanisms and dislocation evolution in body-centered-cubic (bcc) tantalum widely used as a key structural material for mechanical and thermal extremes. We show our unified structural modeling framework informed by mesoscopic dislocation dynamics simulations is capable of capturing salient features of the large inelastic behavior of tantalum at quasi-static (10$^{-3}$ s$^{-1}$) to extreme strain rates (5000 s$^{-1}$) and at room temperature and higher (873K) at both single- and polycrystal levels. We also present predictive capabilities of our model for microstructural evolution in the material. To this end, we investigate the effects of dislocation interactions on slip activities, instability and strain-hardening behavior at the single crystal level. Furthermore, ex situ measurements on crystallographic texture evolution and dislocation density growth are carried out for the polycrystal tantalum specimens at increasing strains. Numerical simulation results also support that our modeling framework is capable of capturing the main features of the polycrystal behavior over a wide range of strains, strain rates and temperatures. The theoretical, experimental and numerical results at both single- and polycrystal levels provide critical insight into the underlying physical pictures for micro- and macroscopic responses and their relations in this important class of refractory bcc materials undergoing severe inelastic deformations.

cond-mat.mtrl-sci↗

arXiv2015

Long-time behavior of the $ω\to α$ transition in shocked Zirconium: Interplay of nucleation and plastic deformation

We study the thermally activated, slow conversion of the hysteretically retained $ω$ phase into stable $α$ phase in recovered samples of shocked zirconium. The $ω$-phase decays in time following an algebraic law, unlike the predictions of the nucleation-growth framework for first order transitions, and residual volume fractions of phases and dislocation densities are related by a power law. We propose an explanation for the annealing mechanism through coupled dynamics of dislocations and phase change. We find that the long-time behavior is controlled by the interplay of dislocations, shear fluctuations, and remnant volume fractions of phases, which lead to an algebraic decay in time. For late time, thermally activated quantities such as the dislocation mobility and nucleation rate set the timescale and control the algebraic behavior, respectively. At high enough temperatures this behavior is effectively indistinguishable from standard Avrami kinetics.

cond-mat.mtrl-sci↗

arXiv2015

Isothermal Annealing of Shocked Zirconium: Stability of the Two-phase $α/ω$ Microstructure

Under high pressure conditions, Zr undergoes a phase transformation from its ambient equilibrium hexagonal close packed $α$ phase to hexagonal $ω$ phase. Upon returning to ambient conditions, the material displays hysteretic behavior, retaining a significant amount of metastable $ω$ phase. This study presents an in-situ synchrotron X-ray diffraction analysis of Zr samples shock-loaded to compressive peak stresses of 8 and 10.5 GPa and then annealed at temperatures of 443, 463, 483, and 503K. The evolution of the $α$ phase volume fraction was tracked quantitatively, and the dislocation densities in both phases were tracked qualitatively during annealing. Upon heating, the reverse transformation of $ω\toα$ does not go to completion, but instead reaches a new metastable state. The initial rate of transformation is faster at higher temperatures. Samples shock-loaded to higher peak pressures experienced higher initial transformation rates and more extensive transformation. Dislocation content in both phases was observed to be high in the as-shocked samples. Annealing the samples reduces the dislocation content in both phases, with the reduction being lesser in the $ω$ phase, leading to the postulation that transformation from $ω\toα$ is restricted by the pinning effect of dislocation structures within the $ω$ phase. Electron backscatter diffraction analysis affirmed that the expected $(0\;0\;0\;1)_α\parallel(1\;0\;\overline{1}\;1)_ω$ and $[1\;0\;\overline{1}\;0]_α\parallel[1\;1\;\overline{2}\;\overline{3}]_ω$ orientation relationship is maintained during nucleation and growth of the $α$ phase during the annealing. \end{abstract}

cond-mat.mtrl-sci↗