Keyou Mao | Oak Ridge National Laboratory (original) (raw)

Papers by Keyou Mao

OSTI OAI (U.S. Department of Energy Office of Scientific and Technical Information), May 31, 2021

The Journal of Physical Chemistry C

Materials Science and Engineering: A

Microscopy and Microanalysis

International Journal of Heat and Mass Transfer, 1984

Journal of Nuclear Materials, 2018

The objective of this study is to understand the influence of surface treatments, namely laser sh... more The objective of this study is to understand the influence of surface treatments, namely laser shock peening (LSP) and ultrasonic nanocrystal surface modification (UNSM), on the irradiation tolerance of a Ni-base alloy. Surface treatments such as LSP and UNSM have been demonstrated to mitigate the potential for primary water stress corrosion cracking (PWSCC). As such, there is emerging interest in the development and implementation of LSP and UNSM for nuclear reactor components. LSP process utilizes a laser-generated plasma, while UNSM utilizes mechanical contact at ultrasonic vibration speeds, to create near-surface compressive residual stresses, high dislocation densities, twins and subgrains, and nanostructuring of the workpiece. These resultant microstructural changes can substantially affect the creation and evolution of irradiation damage. Herein, we study precipitation hardened Ni-base Alloy 718+, which is utilized in reactor components exposed to PWSCC-inciting environments. Specimens are treated with LSP or UNSM, then irradiated with 2.0 MeV protons to 7 displacements per atom (dpa) at 500°C. The dislocation line density is roughly an order of magnitude larger in the unirradiated LSP and UNSM specimens than in the baseline (untreated) specimen. Irradiation-induced dislocation loop nucleation results in an increase in the areal density of dislocation-type defects. Irradiation also induces disordering of the γ' precipitates, although this disordering appears more extensive in the baseline than the LSP and UNSM specimens. These results considered in the context of overall sink strength. Finally, irradiation-induced softening is observed in all specimens through nanoindentation, and can be ascribed to the overall change in sink strength, which results from the competition between γ' disordering and dislocation loop nucleation.

Communications Materials

Characterizing oxide nuclear fuels is difficult due to complex fission products, which result fro... more Characterizing oxide nuclear fuels is difficult due to complex fission products, which result from time-evolving system chemistry and extreme operating environments. Here, we report a machine learning-enhanced approach that accelerates the characterization of spent nuclear fuels and improves the accuracy of identifying nanophase fission products and bubbles. We apply this approach to commercial, high-burnup, irradiated light-water reactor fuels, demonstrating relationships between fission product precipitates and gases. We also gain understanding of the fission versus decay pathways of precipitates across the radius of a fuel pellet. An algorithm is provided for quantifying the chemical segregation of the fission products with respect to the high-burnup structure, which enhances our ability to process large amounts of microscopy data, including approaching the atomistic-scale. This may provide a faster route for achieving physics-based fuel performance modeling.

Single crystalline microcantilevers are fabricated from the base metal and heat-affected zone (HA... more Single crystalline microcantilevers are fabricated from the base metal and heat-affected zone (HAZ) of a laser welded, neutron irradiated austenitic stainless steel, for scanning electron microscope (SEM) in-situ bending. In the HAZ, cantilevers exhibit higher yield point and lower crack tip blunting displacement than in the base metal and unirradiated archive specimen. These results suggest that radiation-induced defects harden the base metal, whereas the HAZ exhibits annealing of defects leading to mechanical softening. Dislocation nucleation ahead of the crack tip is responsible for ductile blunting behavior and provides a pathway to mitigating helium-induced cracking during weld repairs of irradiated materials.

Materials & Design, 2021

The laser welds on the lowest He/dose stainless steels are free of He bubbles and intergranular c... more The laser welds on the lowest He/dose stainless steels are free of He bubbles and intergranular cracks. The HAZ has a lower number density of cavities and loops compared to the irradiated base metal. Microchemical segregation at grain boundaries is reduced in the HAZ related to driving the Cr to precipitates. Reducing heat input and adjusting welding parameters may make laser weld repairs feasible for highly irradiated components.

ACS Polymers Au, 2021

Understanding fracture mechanics of ultrathin polymeric films is crucial for modern technologies,... more Understanding fracture mechanics of ultrathin polymeric films is crucial for modern technologies, including semiconductor and coating industries. However, up to now, the fracture behavior of sub-100 nm polymeric thin films is rarely explored due to challenges in handling samples and limited testing methods available. In this work, we report a new testing methodology that can not only visualize the evolution of the local stress distribution through wrinkling patterns and crack propagation during the deformation of ultrathin films but also directly measure their fracture energies. Using ultrathin polystyrene films as a model system, we both experimentally and computationally investigate the effect of the film thickness and molecular weight on their fracture behavior, both of which show a ductile-to-brittle transition. Furthermore, we demonstrate the broad applicability of this testing method in semicrystalline semiconducting polymers. We anticipate our methodology described here could provide new ways of studying the fracture behavior of ultrathin films under confinement.

Microscopy and Microanalysis, 2020

This is an Accepted Manuscript for the Microscopy and Microanalysis 2020 Proceedings. This versio... more This is an Accepted Manuscript for the Microscopy and Microanalysis 2020 Proceedings. This version may be subject to change during the production process.

Materials Science and Engineering: A, 2018

Materialia, 2018

We report the formation of partially amorphous Mn-Si precipitates due to laser welding of face ce... more We report the formation of partially amorphous Mn-Si precipitates due to laser welding of face centered cubic (fcc) 304 stainless steel. Transmission electron microscopy and precession electron diffraction studies in the heat affected zone (HAZ) of t he weldment indicate the formation of these precipitates in grain interiors. Precipitates have MnSi stoichiometry and the partially crystalline regions have a lattice constant of 0.45 nm. It is surmised that the rapid cooling rates during the laser weld melt pool solidification process may be sufficient to inhibit the complete crystallization of these precipitates.

International Journal of Multiphase Flow, 2017

In relation to void fraction prediction of cross-flow in horizontal tube bundle of shell-tube hea... more In relation to void fraction prediction of cross-flow in horizontal tube bundle of shell-tube heat exchangers, a drift-flux correlation has been developed to meet the demand on the study of two-phase flow gas and liquid velocities, two-phase pressure drop, heat transfer, flow patterns and flow induced vibrations in the shell side. Two critical parameters such as distribution parameter and drift velocity have been modeled. The distribution parameter is obtained by constant asymptotic values and taking into account the differences in channel geometry. The drift velocity is modelled depending on the density ratio and the non-dimensional viscosity number. The relationship between the channel averaged and gap mass velocity has been discussed in order to obtain the superficial gas and liquid velocities in the drift-flux correlation. The newly developed drift-flux correlation agrees well with cross-flow experimental databases of air-water, R-11 and R-113 in parallel triangular, normal square and normal triangular arrays with the mean absolute error of 1.06% and the standard deviation of 4.47%. In comparison with other existing correlations, the newly developed drift-flux correlation is superior to other studies due to the improved accuracy. In order to extend the applicability of the newly developed drift-flux correlation to void fraction of unity, an interpolation scheme has been developed. The newly developed drift-flux correlation is able to calculate the void fraction of cross-flow over a full range with different sub-channel configurations in shell-tube type heat exchangers.

2016 7th International Conference on Information, Intelligence, Systems & Applications (IISA), 2016

Dealing with issues related to safety of Nuclear Power Plants (NPPs) is of high importance and pr... more Dealing with issues related to safety of Nuclear Power Plants (NPPs) is of high importance and priority for assuring nonstop energy production. To enhance safety, modernization and upgrade of the aging installations by incorporating automation processes is unavoidable for many reasons, among them, environmental protection and lifetime extension of currently operating NPP. In that context, Human Machine Interface (HMI) applications are a subject of thorough study. The aim of this work is to develop a mechanism to evaluate the efficiency of HMI in nuclear power plant safety. To that end, HMI applications are addressed as a single joint system and they are not seen separately as a human and machine part. The proposed evaluator was implemented by utilizing Fuzzy Logic and holistic approaches. The implementation and all the experiments were conducted in Matlab by using the fuzzy toolbox.

International Journal of Multiphase Flow, 2017

In relation to void fraction prediction of cross-flow in horizontal tube bundle of shell-tube hea... more In relation to void fraction prediction of cross-flow in horizontal tube bundle of shell-tube heat exchangers, a drift-flux correlation has been developed to meet the demand on the study of two-phase flow gas and liquid velocities, two-phase pressure drop, heat transfer, flow patterns and flow induced vibrations in the shell side. Two critical parameters such as distribution parameter and drift velocity have been modeled. The distribution parameter is obtained by constant asymptotic values and taking into account the differences in channel geometry. The drift velocity is modelled depending on the density ratio and the non-dimensional viscosity number. The relationship between the channel averaged and gap mass velocity has been discussed in order to obtain the superficial gas and liquid velocities in the drift-flux correlation. The newly developed drift-flux correlation agrees well with cross-flow experimental databases of air-water, R-11 and R-113 in parallel triangular, normal square and normal triangular arrays with the mean absolute error of 1.06% and the standard deviation of 4.47%. In comparison with other existing correlations, the newly developed drift-flux correlation is superior to other studies due to the improved accuracy. In order to extend the applicability of the newly developed drift-flux correlation to void fraction of unity, an interpolation scheme has been developed. The newly developed drift-flux correlation is able to calculate the void fraction of cross-flow over a full range with different sub-channel configurations in shell-tube type heat exchangers.

OSTI OAI (U.S. Department of Energy Office of Scientific and Technical Information), May 31, 2021

The Journal of Physical Chemistry C

Materials Science and Engineering: A

Microscopy and Microanalysis

International Journal of Heat and Mass Transfer, 1984

Journal of Nuclear Materials, 2018

The objective of this study is to understand the influence of surface treatments, namely laser sh... more The objective of this study is to understand the influence of surface treatments, namely laser shock peening (LSP) and ultrasonic nanocrystal surface modification (UNSM), on the irradiation tolerance of a Ni-base alloy. Surface treatments such as LSP and UNSM have been demonstrated to mitigate the potential for primary water stress corrosion cracking (PWSCC). As such, there is emerging interest in the development and implementation of LSP and UNSM for nuclear reactor components. LSP process utilizes a laser-generated plasma, while UNSM utilizes mechanical contact at ultrasonic vibration speeds, to create near-surface compressive residual stresses, high dislocation densities, twins and subgrains, and nanostructuring of the workpiece. These resultant microstructural changes can substantially affect the creation and evolution of irradiation damage. Herein, we study precipitation hardened Ni-base Alloy 718+, which is utilized in reactor components exposed to PWSCC-inciting environments. Specimens are treated with LSP or UNSM, then irradiated with 2.0 MeV protons to 7 displacements per atom (dpa) at 500°C. The dislocation line density is roughly an order of magnitude larger in the unirradiated LSP and UNSM specimens than in the baseline (untreated) specimen. Irradiation-induced dislocation loop nucleation results in an increase in the areal density of dislocation-type defects. Irradiation also induces disordering of the γ' precipitates, although this disordering appears more extensive in the baseline than the LSP and UNSM specimens. These results considered in the context of overall sink strength. Finally, irradiation-induced softening is observed in all specimens through nanoindentation, and can be ascribed to the overall change in sink strength, which results from the competition between γ' disordering and dislocation loop nucleation.

Communications Materials

Characterizing oxide nuclear fuels is difficult due to complex fission products, which result fro... more Characterizing oxide nuclear fuels is difficult due to complex fission products, which result from time-evolving system chemistry and extreme operating environments. Here, we report a machine learning-enhanced approach that accelerates the characterization of spent nuclear fuels and improves the accuracy of identifying nanophase fission products and bubbles. We apply this approach to commercial, high-burnup, irradiated light-water reactor fuels, demonstrating relationships between fission product precipitates and gases. We also gain understanding of the fission versus decay pathways of precipitates across the radius of a fuel pellet. An algorithm is provided for quantifying the chemical segregation of the fission products with respect to the high-burnup structure, which enhances our ability to process large amounts of microscopy data, including approaching the atomistic-scale. This may provide a faster route for achieving physics-based fuel performance modeling.

Single crystalline microcantilevers are fabricated from the base metal and heat-affected zone (HA... more Single crystalline microcantilevers are fabricated from the base metal and heat-affected zone (HAZ) of a laser welded, neutron irradiated austenitic stainless steel, for scanning electron microscope (SEM) in-situ bending. In the HAZ, cantilevers exhibit higher yield point and lower crack tip blunting displacement than in the base metal and unirradiated archive specimen. These results suggest that radiation-induced defects harden the base metal, whereas the HAZ exhibits annealing of defects leading to mechanical softening. Dislocation nucleation ahead of the crack tip is responsible for ductile blunting behavior and provides a pathway to mitigating helium-induced cracking during weld repairs of irradiated materials.

Materials & Design, 2021

The laser welds on the lowest He/dose stainless steels are free of He bubbles and intergranular c... more The laser welds on the lowest He/dose stainless steels are free of He bubbles and intergranular cracks. The HAZ has a lower number density of cavities and loops compared to the irradiated base metal. Microchemical segregation at grain boundaries is reduced in the HAZ related to driving the Cr to precipitates. Reducing heat input and adjusting welding parameters may make laser weld repairs feasible for highly irradiated components.

ACS Polymers Au, 2021

Understanding fracture mechanics of ultrathin polymeric films is crucial for modern technologies,... more Understanding fracture mechanics of ultrathin polymeric films is crucial for modern technologies, including semiconductor and coating industries. However, up to now, the fracture behavior of sub-100 nm polymeric thin films is rarely explored due to challenges in handling samples and limited testing methods available. In this work, we report a new testing methodology that can not only visualize the evolution of the local stress distribution through wrinkling patterns and crack propagation during the deformation of ultrathin films but also directly measure their fracture energies. Using ultrathin polystyrene films as a model system, we both experimentally and computationally investigate the effect of the film thickness and molecular weight on their fracture behavior, both of which show a ductile-to-brittle transition. Furthermore, we demonstrate the broad applicability of this testing method in semicrystalline semiconducting polymers. We anticipate our methodology described here could provide new ways of studying the fracture behavior of ultrathin films under confinement.

Microscopy and Microanalysis, 2020

This is an Accepted Manuscript for the Microscopy and Microanalysis 2020 Proceedings. This versio... more This is an Accepted Manuscript for the Microscopy and Microanalysis 2020 Proceedings. This version may be subject to change during the production process.

Materials Science and Engineering: A, 2018

Materialia, 2018

We report the formation of partially amorphous Mn-Si precipitates due to laser welding of face ce... more We report the formation of partially amorphous Mn-Si precipitates due to laser welding of face centered cubic (fcc) 304 stainless steel. Transmission electron microscopy and precession electron diffraction studies in the heat affected zone (HAZ) of t he weldment indicate the formation of these precipitates in grain interiors. Precipitates have MnSi stoichiometry and the partially crystalline regions have a lattice constant of 0.45 nm. It is surmised that the rapid cooling rates during the laser weld melt pool solidification process may be sufficient to inhibit the complete crystallization of these precipitates.

International Journal of Multiphase Flow, 2017

In relation to void fraction prediction of cross-flow in horizontal tube bundle of shell-tube hea... more In relation to void fraction prediction of cross-flow in horizontal tube bundle of shell-tube heat exchangers, a drift-flux correlation has been developed to meet the demand on the study of two-phase flow gas and liquid velocities, two-phase pressure drop, heat transfer, flow patterns and flow induced vibrations in the shell side. Two critical parameters such as distribution parameter and drift velocity have been modeled. The distribution parameter is obtained by constant asymptotic values and taking into account the differences in channel geometry. The drift velocity is modelled depending on the density ratio and the non-dimensional viscosity number. The relationship between the channel averaged and gap mass velocity has been discussed in order to obtain the superficial gas and liquid velocities in the drift-flux correlation. The newly developed drift-flux correlation agrees well with cross-flow experimental databases of air-water, R-11 and R-113 in parallel triangular, normal square and normal triangular arrays with the mean absolute error of 1.06% and the standard deviation of 4.47%. In comparison with other existing correlations, the newly developed drift-flux correlation is superior to other studies due to the improved accuracy. In order to extend the applicability of the newly developed drift-flux correlation to void fraction of unity, an interpolation scheme has been developed. The newly developed drift-flux correlation is able to calculate the void fraction of cross-flow over a full range with different sub-channel configurations in shell-tube type heat exchangers.

2016 7th International Conference on Information, Intelligence, Systems & Applications (IISA), 2016

Dealing with issues related to safety of Nuclear Power Plants (NPPs) is of high importance and pr... more Dealing with issues related to safety of Nuclear Power Plants (NPPs) is of high importance and priority for assuring nonstop energy production. To enhance safety, modernization and upgrade of the aging installations by incorporating automation processes is unavoidable for many reasons, among them, environmental protection and lifetime extension of currently operating NPP. In that context, Human Machine Interface (HMI) applications are a subject of thorough study. The aim of this work is to develop a mechanism to evaluate the efficiency of HMI in nuclear power plant safety. To that end, HMI applications are addressed as a single joint system and they are not seen separately as a human and machine part. The proposed evaluator was implemented by utilizing Fuzzy Logic and holistic approaches. The implementation and all the experiments were conducted in Matlab by using the fuzzy toolbox.

International Journal of Multiphase Flow, 2017

In relation to void fraction prediction of cross-flow in horizontal tube bundle of shell-tube hea... more In relation to void fraction prediction of cross-flow in horizontal tube bundle of shell-tube heat exchangers, a drift-flux correlation has been developed to meet the demand on the study of two-phase flow gas and liquid velocities, two-phase pressure drop, heat transfer, flow patterns and flow induced vibrations in the shell side. Two critical parameters such as distribution parameter and drift velocity have been modeled. The distribution parameter is obtained by constant asymptotic values and taking into account the differences in channel geometry. The drift velocity is modelled depending on the density ratio and the non-dimensional viscosity number. The relationship between the channel averaged and gap mass velocity has been discussed in order to obtain the superficial gas and liquid velocities in the drift-flux correlation. The newly developed drift-flux correlation agrees well with cross-flow experimental databases of air-water, R-11 and R-113 in parallel triangular, normal square and normal triangular arrays with the mean absolute error of 1.06% and the standard deviation of 4.47%. In comparison with other existing correlations, the newly developed drift-flux correlation is superior to other studies due to the improved accuracy. In order to extend the applicability of the newly developed drift-flux correlation to void fraction of unity, an interpolation scheme has been developed. The newly developed drift-flux correlation is able to calculate the void fraction of cross-flow over a full range with different sub-channel configurations in shell-tube type heat exchangers.

—Dealing with issues related to safety of Nuclear Power Plants (NPPs) is of high importance and p... more —Dealing with issues related to safety of Nuclear Power Plants (NPPs) is of high importance and priority for assuring nonstop energy production. To enhance safety, modernization and upgrade of the aging installations by incorporating automation processes is unavoidable for many reasons, among them, environmental protection and lifetime extension of currently operating NPP. In that context, Human Machine Interface (HMI) applications are a subject of thorough study. The aim of this work is to develop a mechanism to evaluate the efficiency of HMI in nuclear power plant safety. To that end, HMI applications are addressed as a single joint system and they are not seen separately as a human and machine part. The proposed evaluator was implemented by utilizing Fuzzy Logic and holistic approaches. The implementation and all the experiments were conducted in Matlab by using the fuzzy toolbox.