Nucleation Process of Voids Studied by Positron Lifetime Spectroscopy

为了阐明氢在铝中形成空穴的机理，对纯铝、稀Al- mg、Al- si和Al- ag合金进行了正电子寿命研究。主要研究结果总结如下：采用四种不同的方法将氢引入纯铝（99.9999%）试样中：(1)在氢气气氛中淬火，(2)在盐酸中淬火，(3)在盐水中淬火后在氢气中辉光放电，(4)在盐水中淬火后电解。在所有情况下，在随后的等时退火过程中都观察到空洞的形成。研究发现，在淬火前或淬火中引入氢对空穴形成的影响远大于淬火后。在充氢淬火的情况下，随着淬火温度的降低，空穴形成与断层位错形成的比例增加。这些结果有力地表明，氢仅在空位聚集的早期阶段才对空穴成核产生影响。通过对淬火过程中空位动力学的计算机模拟，提出了二次缺陷的成核模型：在没有氢原子的情况下，三空位和四空位伴随着极大的原子弛豫，成长为有缺陷的位错环。另一方面，对于氢原子，三空位和四空位不能有松弛的构型，而起空洞核的作用。未观察到充氢的稀Al-Mg、Al-Si和Al-Ag合金形成空洞的迹象。这一结果表明，这些溶质原子捕获氢并抑制上述空洞的成核。研究发现，在5N Al中形成的空洞密度始终低于6N Al。这清楚地表明，即使是10ppm这样少量的杂质也会抑制氢形成空洞。

In order to clarify the void formation mechanism by hydrogen in aluminium, positron lifetime study has been performed on pure Al, and on dilute Al-Mg, Al-Si and Al-Ag alloys. The main results are summarized as follows.1. Hydrogen were introduced into pure Al(99.9999%) specimens by four different methods (1)quenching from hydrogen atmosphere,(2)quenching into hydrochloric acid,(3)glow discharge in hygrogen gas after quenching into brine and (4)electrolysis after quenching into brine. In all cases void formation were observed during subsequent isochronal annealing.2. It has been found that the hydrogen introduction before or during quenching is much more effective than after quenching on void formation. Furthermore, in the case of hydrogen charge arter quenching, the ratio of the void formation to faulted dislocation fromation increases with decreasing quenching temperature. These results strongly suggest that hydrogen has the effect on void nucleation only at the very early stage in vacancy clustering.3. With the aid of computer simulations of vacancy kinetics during quenching, a nucleation model for the secondary defects is proposed: Without a hydrogen atom trivacancies and tetravacancies are attended with great atomic relaxations and grow into faulted dislocation loops. On the other hand, with hydrogen atoms trivacancies and tetravacancies cannot have the relaxed configulations and act as void nuclei.4. No indication of void formation has been observed for dilute Al-Mg,Al-Si and Al-Ag alloys hydrogen charged. This result indicates that these solute atoms trap hydrogen and inhibit the nucleation of voids mentioned above.5. It has been found that the density of voids formed in 5N Al is always lower than that in 6N Al. This clearly shows that even such a small amount of impurities as 10ppm suppress the void formation by hydrogen.