CONFIGURATION OF LOOP PATCH AT LOW STRAIN AMPLITUDE IN POLYCRYSTALLINE COPPER

The loop patch structure produced in polycrystalline copper, fatigued at low strain amplitudes below the quasiplateau region of the cyclic s tress strain curves, reveals three types of configuration: cylindrical , irregular and cellular shapes. It is identified that the cylindrical loop patches consist of edge dipolar debris with primary Burgers vect or and are formed from the deposition of edge segments by sweeping scr ew dislocations. The irregular loop patches are developed predominantl y from stacking of two kinds of dislocation dipolar debris and loops i n the grains oriented with double slip. The cellular loop patches are formed from the interaction of four types of dislocation in the grain oriented with multislip. In addition, cubic sessile dislocations are f ound to coexist with the irregular loop patches and the cellular loop patches. These cubic sessile dislocations are formed from the reaction between edge dislocations and screw dislocations which are perpendicu lar to each other. It is explained that all three types of structures are low-energy dislocation configurations and can be modelled by the T aylor lattice, the double Taylor lattice and the complex Taylor lattic e respectively.