Some twins tissue are produced in the deformation region of the grain, as seen in Fig. 3(b). It is well known that the W-W
boundaries are the weak parts in the tungsten-copper alloy and are harder to deform than copper, so, the severe plastic deformation occurred in Cu, while the W particles exhibit very slight deformation, which resulting in the concentration of strain on the grain boundaries or within the grain. With further straining, dislocations pileup in grain boundaries and when it reaches the requiring critical shear strain of mechanical twinning deformation, the twins occurred.