Chi Xu

Profile Information
Dr Chi Xu
Beijing Normal University
Assistant Professor
Beijing Normal University
"Effects of neutron irradiation and post-irradiation annealing on the microstructure of HT-UPS stainless steel" Chi Xu, Wei-Ying Chen, Xuan Zhang, Meimei Li, Yong Yang, Yaqiao Wu, Journal of Nuclear Materials Vol. 507 2018 188-197 Link
Microstructural changes resulted from neutron irradiation and post-irradiation annealing in a high-temperature ultra-fine precipitate strengthened (HT-UPS) stainless steel were characterized using transmission electron microscopy (TEM) and atom probe tomography (APT). Three HT-UPS samples were neutron-irradiated to 3 dpa at 500?°C, and after irradiation, two of them were annealed for 1?h?at 600?°C and 700?°C, respectively. Frank dislocation loops were the dominant defect structure in both the as-irradiated and 600?°C post-irradiation-annealed (PIAed) samples, and the loop sizes and densities were similar in these two samples. Unfaulted dislocation loops were observed in the 700?°C PIAed sample, and the loop density was greatly reduced in comparison with that in the as-irradiated sample. Nano-sized MX precipitates were observed under TEM in the 700?°C PIAed sample, but not in the 600?°C PIAed or the as-irradiated samples. The titanium-rich clusters were identified in all three samples using APT. The post-irradiation annealing (PIA) caused the growth of the Ti-rich clusters with a stronger effect at 700?°C than at 600?°C. The irradiation caused elemental segregations at the grain boundary and the grain interior, and the grain boundary segregation behavior is consistent with observations in other irradiated austenitic steels. APT results showed that PIA reduced the magnitude of irradiation induced segregations.
"In situ high-energy X-ray diffraction study of tensile deformation of neutron-irradiated polycrystalline Fe-9%Cr alloy" Meimei Li, James Stubbins, Chi Xu, Xuan Zhang, Jun-Sang Park, Peter Kenesei, Jonathan Almer, Acta Materialia Vol. 126 2017 67-76 Link
The effect of neutron irradiation on tensile deformation of a Fe-9wt%Cr alloy was investigated using in situ high-energy synchrotron X-ray diffraction during room-temperature uniaxial tension tests. New insights into the deformation mechanisms were obtained through the measurements of lattice strain evolution and the analysis of diffraction peak broadening using the modified Williamson-Hall method. Two neutron-irradiated specimens, one irradiated at 300 °C to 0.01 dpa and the other at 450 °C to 0.01 dpa, were tested along with an unirradiated specimen. The macroscopic stress–strain curves of the irradiated specimens showed increased strength, reduced ductility and reduced work-hardening exponent compared to the unirradiated specimen. The evolutions of the lattice strain, the dislocation density and the coherent scattering domain size in the deformation process revealed different roles of the submicroscopic defects in the 300 °C/0.01 dpa specimen and the nanometer-sized dislocation loops in the 450 °C/0.01 dpa specimen; the dislocation loops were more effective in dislocation pinning. While the work hardening rate of stage II was unaffected by irradiation, significant dynamic recovery in stage III in the irradiated specimens led to the early onset of necking without stage IV as observed in the unirradiated specimen.
"In-situ high-energy X-ray characterization of neutron irradiated HT-UPS stainless steel under tensile deformation" Chi Xu, Xuan Zhang, Yiren Chen, Meimei Li, Jun-Sang Park, Peter Kenesei, Jason Almer, Yong Yang, Acta Materialia Vol. 156 2018 330-341 Link
The tensile deformation behavior of a high-temperature, ultrafine-precipitate strengthened (HT-UPS) stainless steel was characterized in-situ with high-energy X-ray diffraction at 20 and 400?°C. The HT-UPS samples were neutron irradiated to 3 dpa at 400?°C. Significant irradiation hardening and ductility loss were observed at both temperatures. Lattice strain evolutions of the irradiated samples showed a strong linear response up to near the onset of the macroscopic yield, in contrast to the unirradiated HT-UPS which showed a pronounced non-linear behavior well below the macroscopic yield. While the room-temperature diffraction elastic moduli in the longitudinal direction increased after irradiation, the 400?°C moduli were similar before and after irradiation. The evolution of the {200} lattice strain parallel to the loading axis () showed unique characteristics: in the plastic regime, the evolution of after yield is temperature-dependent in the unirradiated specimens but temperature-independent in the irradiated specimens; and the value of at the yield is an irradiation-sensitive, temperature-independent parameter. The evolution of corresponds well with the dislocation density evolution, and is an effective probe of the deformation-induced long-range internal stresses in the HT-UPS steel.
"In-situ TEM characterization of the tensile deformation of ion-irradiated HT-UPS steel at RT and 400°C" Chi Xu, Journal of Nuclear Materials Vol. 529 2020 151911 Link
High-temperature ultra-fine precipitate strengthened (HT-UPS) steel samples were irradiated with 1 MeV Kr ions, and in-situ tensile deformed at room temperature (RT) and 400 degrees C. The dislocation flow in irradiated samples was significantly restricted by the irradiation defects, and the deformation took place in localized deformation bands. While in unirradiated specimens, the deformation happened evenly across the grains. As a result of the localized deformation, intricate deformation band structures were observed in the irradiated specimens, which are composed of a central band and two defect reduced zones on the two sides. The irradiation defects were in-situ observed to interact with deformation induced dislocations, and were removed by the latter. Despite of the differences, the irradiated samples reserved similar work hardening behaviors as the unirradiated samples. The deformation inside the defect reduced bands in irradiated samples showed similar characteristics as in the unirradiated sample. The last stage deformation also showed similar deformation characteristics, regardless of the irradiation history. (C) 2019 Elsevier B.V. All rights reserved.
"Microstructural evolution of NF709 austenitic stainless steel under in-situ ion irradiations at room temperature, 300, 400, 500 and 600 °C" Chi Xu, Wei-Ying Chen, Yiren Chen, Yong Yang, Journal of Nuclear Materials Vol. 509 2018 644-653 Link
"In-Situ Synchrotron X-Ray Scattering Study on the Tensile Properties of Neutron Irradiated Ferritic/ Martensitic Alloys" Xiang Liu, Kuan-Che Lan, Meimei Li, Xuan Zhang, Chi Xu, James Stubbins, ANS Annual Meeting 2018 June 11-22, (2018)
NSUF Articles:
U.S. DOE Nuclear Science User Facilities Awards 35 Rapid Turnaround Experiment Research Proposals - Awards total approximately $1.3 million These projects will continue to advance the understanding of irradiation effects in nuclear fuels and materials in support of the mission of the DOE Office of Nuclear Energy. Wednesday, September 20, 2017 - Calls and Awards