An Experimental Study of Mg Aggregation in AA5754 Alloys by Positron Annihilation Spectroscopy

Defects in an AA5754 （Al-3.0%Mg） alloy are investigated by coincidence Doppler broadening spectroscopy and positron lifetime spectroscopy. The results indicate enhancement of positron trapping by Mg atoms in this Al-Mg alloy after quenching treatment at 623K, which may be due to the formation of vacancy-Mg complexes or the aggregation of Mg near the vacancy sites. It is speculated that the aggregation of Mg atoms in the moderate temperature range is responsible for cracking in spot welding of AA5754 alloys.     

符合多普勒展宽谱的源修正研究

测量了Ti,Ni,Cu,Al以及Si的符合多普勒展宽谱。对于Ni的多普勒展宽谱,采用最小二乘法拟合得出其中的源强度。给出了源修正前后湮没量子在Si中的多普勒展宽谱,讨论了源成分的影响。用高斯-抛物线模型拟合多普勒展宽谱,将多普勒展宽谱中自由电子的湮没贡献和束缚电子的湮没贡献分开,进而探讨了只对束缚电子的湮没贡献做源修正的方法。In the present work, Coincidence Doppler Broadening （CDB） measurements have been performed for five elements i.e. Ti, Ni, Cu, Al and Si. As to the CDB spectra of Ni, we obtained the annihilation fraction of positron-electron pairs in the source by least square fitting. After source correction, spectra for Si are also given to indicate the influence of source components. CDB spectra were simulated with Gauss-Parabola model to separate annihilation contribution of core electrons from outermost electrons. Furthermore, a new source correction method, i.e. source correction will be done only in the contribution of core electron, has been presented     

慢正电子束注入多孔硅材料的Monte Carlo模拟

建立了慢正电子束注入多孔硅材料的Monte Carlo模型，研究了在该材料中不同孔洞的直径、孔洞体积百分比对正电子几率分布的影响，得到了相应的正电子深度分布曲线。同时分析了不同能量正电子平均注入深度的分布，通过适当的标度，得到了统一描述正电子分布的公式，为慢正电子束对多孔材料的实验和理论研究提供有用的数据和计算模型。