arXiv subjects

Thaddeus Song En Low

Publications and source records attributed to Thaddeus Song En Low.

3 recordsLinked to original sources

arXiv2018

Modeling the Thermal Stability of the $α/ω$ Microstructure in Shocked Zr: Coupling between defect state and phase transformation

Under high pressure, Zr undergoes a transformation from its ambient equilibrium hexagonal close packed $α$ phase to a simple hexagonal $ω$ phase. Subsequent unloading to ambient conditions does not see a full reversal to the $α$ phase, but rather a retainment of significant $ω$. Previously, the thermal stability of the $ω$ phase was investigated via in-situ synchrotron X-ray diffraction analysis of the isothermal annealing of Zr samples shocked to 8 and 10.5 GPa at temperatures 443, 463, 483, and 503 K. The phase volume fractions were tracked quantitatively and the dislocation densities were tracked semi-quantitatively. Trends included a rapid initial (transient) transformation rate from $ω\toα$ followed by a plateau to a new metastable state with lesser retained $ω$ (asymptotic). A significant reduction in dislocation densities in the $ω$ phase was observed prior to initiation of an earnest reverse transformation, leading to the hypothesis that the $ω\toα$ transformation from is being hindered by defects in the $ω$ phase. As a continuation of this work, we present a temperature dependent model that couples the removal of dislocations in the $ω$ phase and the reverse transformation via a barrier energy that is associated with the free energy of remaining dislocations. The reduction of dislocations in the $ω$ phase occur as a sum of glide and climb controlled processes, both of which dictate the transient and asymptotic behavior of the annealing process respectively.

cond-mat.mtrl-sci↗

arXiv2015

An integrated full-field model of concurrent plastic deformation and microstructure evolution: Application to 3D simulation of dynamic recrystallization in polycrystalline copper

Many time-dependent deformation processes at elevated temperatures produce significant concurrent microstructure changes that can alter the mechanical properties in a profound manner. Such microstructure evolution is usually absent in mesoscale deformation models and simulations. Here we present an integrated full-field modeling scheme that couples the mechanical response with the underlying microstructure evolution. As a first demonstration, we integrate a fast Fourier transform-based elasto-viscoplastic (FFT-EVP) model with a phase-field (PF) recrystallization model, and carry out three-dimensional (3D) simulations of dynamic recrystallization (DRX) in polycrystalline copper. A physics-based coupling between FFT-EVP and PF is achieved by (1) adopting a dislocation-based constitutive model in FFT-EVP, which allows the predicted dislocation density distribution to be converted to a stored energy distribution and passed to PF, and (2) implementing a stochastic nucleation model for DRX. Calibrated with the experimental DRX stress-strain curves, the integrated model is able to deliver full-field mechanical and microstructural information, from which quantitative description and analysis of DRX can be achieved. It is suggested that the initiation of DRX occurs significantly earlier than previous predictions, due to heterogeneous deformation. DRX grains are revealed to form at both grain boundaries and junctions (e.g., quadruple junctions) and tend to maintain a triple line with old grains during the growth. The resulted stress redistribution is found to have a profound influence on the subsequent dislocation evolution and the softening.

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↗