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1. chinaXiv:201703.00587 [pdf]

Modification of source contribution in PALS by simulation using Geant4 code

Xia Ning; Xingzhong Cao; Chong Li; Demin Li; Peng Zhang; Yihao Gong; Rui Xia; Baoyi Wang; Long Wei
Subjects: Physics >> Condensed Matter: Structural, Mechanical and Thermal Properties

The contribution of positron source for the results of a positron annihilation lifetime spectrum (PALS) is simulated using Geant4 code. The geometrical structure of PALS measurement system is a sandwich structure: the 22Na radiation source is encapsulated by Kapton films, and the specimens are attached on the outside of the films. The probabilities of a positron being annihilated in the films, annihilated in the targets, and the effect of positrons reflected back from the specimen surface, are simulated. The probability of a positron annihilated in the film is related to the species of targets and the source film thickness. The simulation result is in reasonable agreement with the available experimental data. Thus, modification of the source contribution calculated by Geant4 is viable, and it beneficial for the analysis of the results of PALS.

submitted time 2017-03-26 Hits1943Downloads1083 Comment 0

2. chinaXiv:201703.00586 [pdf]

Estimation of dislocation concentration in plastically deformed FeCrNi alloy by positron annihilation lifetime

Eryang Lu; Xingzhong Cao; Shuoxue Jin; Yihao Gong; Peng Zhang; Runsheng Yu; Baoyi Wang
Subjects: Physics >> Condensed Matter: Structural, Mechanical and Thermal Properties

Dislocations would be induced after plastic deformation, which might change the mechanical properties of solids. FeCrNi austenitic model alloy and its Mo-diluted alloy were cold rolled with different degree of thickness reduction. Positrons are sensitive to point defects, which are easily trapped and annihilated around the trapping sites. The mean positron lifetimes have been used to estimate the average dislocation concentration in solids. Meanwhile, the trapping efficiency μ was calculated from the lifetime results. The trapping efficiency value is estimated about 3.31×10-7 cm3s-1 for FeCrNi alloy and 3.31×10-7 cm3s-1 for Mo-diluted alloy, respectively. The increment of the hardness value during plastic deformation is related to the increase of the dislocation density and dislocation pile up in solids.

submitted time 2017-03-26 Hits1954Downloads1107 Comment 0

3. chinaXiv:201703.00585 [pdf]

Irradiation induced defects by Fe-ion in 316L stainless steel studied by Positron Annihilation Spectroscopy

Xingzhong Cao; Eryang Lu; Shuoxue Jin; Yihao Gong; Yuanchao Hu; Te Zhu; Peng Zhang; Long Wei; Baoyi Wang
Subjects: Physics >> Condensed Matter: Structural, Mechanical and Thermal Properties

Solution annealed type 316L austenitic stainless steels were irradiated using 2 MeV Fe ions at room temperature. The implanted fluences were 2012 ions/cm2 and 1013 ions/cm2, respectively. Variable mono-energetic positron beam was performed to characterize the evolution of microstructure and irradiation induced defects. Results show that large amount of vacancy defects formed after heavy ion irradiation. In which, some of mono-vacancies might migrate to form small-sized clusters at room temperature. After irradiation, implanted Fe atoms mainly be interstitials atoms, but some Fe atoms might recombine with vacancies due to their high mobility, which could decrease the defect concentration, effectively.

submitted time 2017-03-26 Hits1907Downloads1061 Comment 0

4. chinaXiv:201703.00583 [pdf]

Detection of helium in irradiated Fe9Cr alloys by coincidence Doppler broadening of slow positron annihilation

Xingzhong Cao; Te Zhu; Shuoxue Jin; Peng Kuang; Peng Zhang; Eryang Lu; Yihao Gong; Liping Guo; Baoyi Wang
Subjects: Physics >> Condensed Matter: Structural, Mechanical and Thermal Properties

An element analysis method, coincidence Doppler broadening spectroscopy of slow positron annihilation, was employed to detect helium in ion irradiated Fe9Cr alloys. Spectra with higher peak to background ratio were recorded using a two-HPGe-detector coincidence measuring system. It means that information in the high-momentum area of the spectra can be used to identify helium in metals. This identification is not entirely dependent on the helium concentration in the specimens, but is related to the structure and microscopic arrangement of atoms surrounding the positron annihilation site. The results of Doppler broadening spectroscopy and transmission electron microscopy show that vacancies and dislocations were formed in ion irradiated specimens. Thermal helium desorption spectrometry was performed to obtain the types of He traps.

submitted time 2017-03-26 Hits1744Downloads982 Comment 0

5. chinaXiv:201703.00529 [pdf]

The evolution of micro defects in He+ irradiated FeCrNi alloy during isochronal annealing

Eryang Lu; Xingzhong Cao; Shuoxue Jin; Chunxiong Zhang; Peng Zhang; Liping Guo; Te Zhu; Yihao Gong; Baoyi Wang; ?
Subjects: Physics >> Condensed Matter: Structural, Mechanical and Thermal Properties

In order to study the microstructural evolution of the He+ irradiated FeCrNi model alloy, 140 keV He+ ions were implanted into the specimen with the fluence of 1015 ions/cm2 at room temperature. Positron Annihilation Spectroscopy was used to characterize the evolution of micro defects during isochronal annealing between 423 K and 873 K. The decrease of the S parameter between 573 K and 623 K has been associated to the annihilation of vacancy clusters, which were not trapped by helium atom. While the decline of the S parameter between 773 K and 823 K in the damage region might be caused by the dissociation of HenVm clusters. The dissociated vacancy clusters were unstable and annihilated rapidly, which decreases the concentration of vacancy defects.

submitted time 2017-03-24 Hits1240Downloads697 Comment 0

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