The Nb single crystals of both[321]and[110]orientations have been cyclicly deformed in tension-compression at constant strain rate 8×10~(-4)s~(-1)over a range of plastic strain amplitudes between 10~(-3)and 10~(-4).The cyclic hardening,the changes in shape of crystals and the asymmetry of stress have been studied.The hardening curve can be divided into three stages,i.e.first,rapid-hardening and saturated stage. In the first stage of cyclic hardening curve dominant features of dislocation configura- tions are high density networks and debris loops.In the rapid-hardening stage the main fea- ture is the formation of dislocation bundles.In the saturated a well defined bundle structure fully develops and between them it is filled with only screw dislocations and the imposed strain is accommodated mainly by the motion of screw dislocations travelling to and from between the bundles.Three-dimension cell,two-dimension cell or bundle structures are summarized as the saturated structures of bcc metals.
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