The significant size effect on nucleation and propagation of crack during tensile deformation of copper foil: Free surface roughening and crystallography study

摘要

Tensile tests were conducted on copper foils with 80 pm in thickness to elucidate the effect of grain size on nucleation and growth of crack. Experimental results indicated that samples with larger grain size had smaller fracture stress and strain when the ratio of sample thickness to grain size (T/D) was below 10. This is attributed to strain localization during the early stage of tensile deformation. Confocal laser scanning microscopy (CLSM) indicated an increase in free surface roughening with increase in grain size during tensile straining. The local depression trough and thinning of sample caused by surface roughening were precursors to crack propagation. (EBSD) studies of crack in sample with T/D value of 1.8, it was observed that in the vicinity of the crack tip, a high density of dense and deep slip traces were present. The slip process gradually changed from single slip to multiple slip and then to cross slip. Both grain boundary and annealed twin boundary acted as obstacles to the movement of dislocations and led to strain localization. Furthermore, during tensile deformation, the crack propagation direction was not always perpendicular to the loading direction. A change in the direction of crack propagation occurred at the junction of grain boundaries and entered into the adjacent grain. Large angle grain boundaries provided greater resistance to the propagation of crack.