arXiv subjects

Paul Chao

Publications and source records attributed to Paul Chao.

5 recordsLinked to original sources

arXiv2026

Origin of the reaction temperature in solid-state materials synthesis

Temperature plays a crucial role in solid-state materials synthesis, but there is currently no mechanistic theory to explain or predict which temperature is best to conduct a solid-state reaction. Reactions between powder precursors are conventionally assumed to be slow diffusion-limited processes; however, recent in situ experiments show that solid-state reactions can complete in minutes above a critical onset temperature. Here, we present evidence that a transient liquid phase forms above the metastable eutectic temperature, and that this non-equilibrium liquid serves as a fast diffusion medium to intermix precursors and initiate a solid-state reaction. This thermodynamic principle is agnostic to the structure or chemistry of the reactants, and can be applied towards the synthesis and manufacturing of a wide range of complex materials.

cond-mat.mtrl-sci↗

arXiv2024

From irregular to regular eutectic growth in the Al-Al3Ni system: in situ observations during directional solidification

We investigate the irregular eutectic growth dynamics of the Al-Al3Ni alloy, in which one of the solid phases (Al3Ni) grows faceted from the liquid. Leveraging in situ optical microscopy and synchrotron transmission x-ray microscopy, we address the question of the degree of coupling between Al and Al3Ni at the growth front and that of the shape of the microstructures left behind in the bulk solid during directional solidification. Real-time optical observations bring evidence for a morphological transition from a eutectic-grain dependent, irregular eutectic growth at low solidification velocity V (typically 1 $μms^{-1}$), to a weakly anisotropic, regular growth at higher V (reaching 10 $μms^{-1}$). Unprecedented x-ray nano-imaging of the solid-liquid interface, and 3D characterization of the growth patterns, were made possible by a new Directional Solidification (DS) setup at Brookhaven National Laboratory's NSLS-II. At low V, the leading tips of partly faceted Al3Ni crystals are observed to grow not far ahead of the Al growth front. Correlating in situ images and postmortem 3D tomographic reconstructions reveals that the presence of faceted and non-faceted regions of Al3Ni crystals in the solid is a direct consequence of coupling and decoupling during DS, respectively. Upon increasing V , the lead distance of Al3Ni vanishes, and the shape of Al3Ni ceases to be governed by faceted growth. These observations shed light on the basic mechanisms (faceted growth, diffusive coupling, and the dynamics of trijunctions) governing the transition from faceted to rod-like growth upon increasing V in the Al3Ni system, with broad implications for a large class of irregular eutectics.

cond-mat.mtrl-sci↗

arXiv2023

Three-dimensional morphology of an ultrafine Al-Si eutectic produced via laser rapid solidification

Al-Si alloys processed by laser rapid solidification yield eutectic microstructures with ultrafine and interconnected fibers. Such fibrous structures have long been thought to bear resemblance to those formed in impurity-doped alloys upon conventional casting. Here, we show that any similarity is purely superficial. By harnessing high-throughput characterization and computer vision techniques, we perform a three-dimensional analysis of the branching behavior of the ultrafine eutectic and compare it against an impurity-modified eutectic as well as a random fractal (as a benchmark). Differences in the branching statistics point to different microstructural origins of the impurity- and quench-modified eutectic. Our quantitative approach is not limited to the data presented here but can be used to extract abstract information from other volumetric datasets, without customization.

cond-mat.mtrl-sci↗

arXiv2022

Pseudo-4D view of the growth and form of locked eutectic colonies

We investigate solidification of an Al-Al2Cu as a model system to understand the emergence of patterns (such as lamellar, rod and maze-like) within eutectic colonies. To uncover the morphological transitions in-situ and in 3D, we introduce here a new synchrotron-based, X-ray imaging procedure. Our method simultaneously maximizes the temporal (200 ms) and spatial resolution ($0.69^2 μm^2/pixel$) over that of traditional imaging approaches. The wealth of information obtained from this procedure enables us to visualize the development of a crystallographically `locked' eutectic microstructure in the presence of thermosolutal convection. This data provides direct insight into the mechanism of the lamella-to-rod transition as the eutectic accommodates fluctuations in interfacial composition and growth velocity. We find that this transition is brought about by impurity-driven forces acting on the solid-solid-liquid trijunction that must overcome the stiffness of the solid-solid interfaces. Our pseudo-4D imaging strategy holds broad appeal to the solidification science community, as it can overcome the space-time trade-off in conventional in situ X-ray microtomography.

cond-mat.mtrl-sci↗

arXiv2016

Hydrogen Isotope Trapping in Al-Cu Binary Alloys

The trapping mechanisms for hydrogen isotopes in Al-X Cu (0.0 at.% < X < 3.5 at.%) alloys were investigated using thermal desorption spectroscopy (TDS), electrical conductivity, and differential scanning calorimetry. Constant heating rate TDS was used to determine microstructural trap energies and occupancies. In addition to the trapping states in pure Al reported in the literature (interstitial lattice sites, dislocations, and vacancies), a trap site due to Al-Cu intermetallic precipitates is observed. The binding energy of this precipitate trap is ($18\pm3$) kJ$\cdot$mol$^{-1}$ ($0.19 \pm 0.03$ eV). Typical occupancy of this trap is high; for Al-2.6 at.% Cu (a Cu composition comparable to that in AA2219) charged at 200°C with 130 MPa D$_2$ for 68 days, there is ca. there is $3.15\times10^{-7}$ mol D bound to the precipitate trap per mol of Al, accounting for a third of the D in the charged sample.

cond-mat.mtrl-sci↗