Nucleation Process of Voids Studied by Positron Lifetime Spectroscopy

为了阐明氢在铝中形成空洞的机理，对纯铝、稀铝-镁、铝-硅和铝-银合金进行了正电子寿命研究。主要研究结果如下：1.用氢气氛淬火、盐酸淬火、盐水淬火后在氢气中辉光放电和盐水淬火后电解四种不同的方法对纯铝（99.9999%）进行了充氢实验。在所有情况下，在随后的等时退火过程中观察到空隙形成。已经发现，在淬火之前或期间引入氢比在淬火之后对空隙形成更有效。此外，在充氢淬火的情况下，随着淬火温度的降低，空洞形成与位错形成的比例增加。这些结果有力地表明，氢只在空位聚集的早期阶段对空位形核有影响.借助于淬火过程中空位动力学的计算机模拟，提出了二次缺陷的形核模型：在没有氢原子的情况下，三空位和四空位伴随着强烈的原子弛豫，长大为位错环。另一方面，三空位和四空位在氢原子存在时不能产生弛豫的晶格结构，而起空核的作用.对于充氢的稀Al-Mg、Al-Si和Al-Ag合金，没有观察到空洞形成的迹象。这一结果表明，这些溶质原子捕获氢，抑制上述空洞的形核.发现5 N Al中形成的空穴密度总是低于6 N Al中的空穴密度，这清楚地表明，即使是10 ppm这样少量的杂质也抑制氢形成空穴。

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.