在透射电镜中原位观察了ＰＤ_３重轨钢氢气泡及氢压裂纹的形核过程．结果表明，充氢时先形成氢气泡，随氢压增大，周围局部屈服，但在气泡壁附近存在无位错区，其中的应力可能很高．当气泡中的氢压升高到等于被富集氢降低了的原子键合力时，氢压裂纹就在气泡壁上形核． The nucleation process of hydrogen gas bubbles and hydrogen fractures in PD_3 heavy rail steel was observed in situ by TEM. The results show that when hydrogen is charged, first hydrogen bubbles are formed. With the increase of hydrogen pressure, the surrounding areas yield locally. However, there is a dislocation-free zone in the vicinity of the bubble wall, of which the stress may be high. When the pressure of hydrogen in the bubble rises to equal to the atomic bonding force reduced by the hydrogen-enrichment, the hydrogen pressure crack nucleates on the bubble wall.