Dislocation structures introduced by high-speed deformation in bcc metals

Systematic experiments were carried out over a wide range of strain rate, 100–106 s−1, so as to reveal the deformation mode in bcc crystals, especially at high strain rate. Dislocation structure showed heterogeneous distribution at low strain rates in all three bcc metals examined. At higher strain rates exceeding ∼103 s−1, distribution of dislocations was random, and the formation of small dislocation loops was observed in V and Nb. In Mo, small dislocation loops were not formed by deformation, even at high strain rates. However, post-deformation annealing of an Mo specimen that had been deformed by 20% at 5×105 s−1 produced dislocation loops. The inside–outside contrast method identified these loops to be of vacancy type. These results reveal that in Mo vacancy clusters are not formed directly from the interaction of dislocations, but by the aggregation of vacancies. In V and Nb, the same formation process is believed to occur at high strain rates. These results suggest that the different mode of plastic deformation at high strain rates accompanied by production of vacancies also occurred in bcc metals.