The irradiation-softening phenomenon in high-purity tungsten single crystals with [110] and [100] tensile axes observed by high-voltage electron microscopy

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Abstract
The dynamic behaviour of screw dislocations in high-purity tungsten single crystals with [101] and [100] tensile axes has been observed by in situ deformation with a high-voltage electron microscope operating at 2 MV as a function of irradiation time and compared with those observed at 1 MV. Screw dislocations in the [110] specimen move intermittently on the soft {112} slip planes when observed at 1 MV, but their motion becomes very smooth with increasing irradiation time at 2 MV by the aid of double cross-slip between soft and hard {ll2} which is induced by the irradiation. The mean velocity of screw dislocations increases with increasing irradiation time at 2 MV in the [110] specimen and decreases again after reaching a maximum. The multiplication of dislocations in the [110] specimens is increased remarkably by irradiation at 2 MV because double cross-slip of screw dislocations produces many dislocations. On the other hand, interaction between screw dislocations moving on different slip planes of the soft {112} occurs frequently in the [100] specimen both at 1 MV and 2 MV and the mean velocity of screw dislocations at 2 MV is not increased so much by irradiation compared with that at 1 MV. On the basis of the observations, the causes of irradiation softening and its orientation dependence in b.c.c. metals are discussed.