Mechanical Properties And Deformation Mechanisms Of Gradient Nanotwinned Cu

Gradient nano-structured metals have attracted extensive attention in the field of materials science and engineering,owing to their superior mechanical properties and unique deformation mechanisms.For instance,gradient nano-structured metals with increasing microstructural sizes from the surface to the core,prepared by using surface mechanical deformation,exhibit higher strength,considerable ductility and work hardening with respect to metals with homogeneous microstructure.However,the volume fraction of gradient nano-gained layers on the surface of these gradient metals is very limited;meanwhile nano-grains trend to be unstable during the deformation due to the presence of high density of high-energy defects such as grain boundaries(GBs).These factors greatly limit the understanding of the intrinsic strength ening mechanism of gradient nano-structured metals.Recently studies showed that the preferentially oriented nanotwinned(NT)metals have superior mechanical properties with stable and controllable microstructures,and are therefore potential to become an ideal mode for designing gradient nano-structured metals.In this work,we fabricated a series of gradient nanotwinned Cu(GNT)samples with different structural gradient or gradient order by means of a direct-current electro-deposition technique.It is revealed that unprecedented extra strengthening mechanism and unique deformation behaviors are closely related to structural gradient or gradient order.The following results are obtained:To control accurately the microsture of preferentially oriented NT Cu using the direct-current electrodeposition,the electrolyte temperature effect is explored.When the temperature is kept at 20,25,30 and 35 0C,a series of homogenous NT samples NT-(?),NT-(?),NT-(?)and NT-(?)with increasing grain size and twin thickness were fabricated,respectively.These GNT Cu samples with controllable structural gradient are fabricated by means of direct-current electro-deposition through changing the electrolyte temperature.The GNT Cu samples are composed of homogenous NT structures(?),(?),(?)and(?).GNT-1,GNT-2,GNT-3 and GNT-4 are designed with various spatial distributions of(?),(?),2×(?)and 4×(?)(?),respectively.From GNT-1 to GNT-4,the structural gradient,defined as the variation of hardness per unite sample thickness along the gradient direction,increases from 1.8,3.2,6.0 to 11.6 GPa/mm.GNT Cu samples exhibit unprecedented extra strengthening and work hardening relative to the homogeneous NT samples.As the structural gradient increases,both the strength and the work hardening increase simultaneously with comparable ductility.A large structural gradient allows for improved strength that can exceed that of the strongest component of GNT Cu samples.The unique extra strengthening in GNT Cu is attributed to the presence of the geometrically necessary dislocations.Geometrically necessary dislocations are spontaneously produced to accommodate the strain gradient at small strains when the progressive yielding from(?)to(?)happens.These geometrically necessary dislocations are in the form of bundles of concentrated dislocations along the gradient direction in the grain interior,and act as strong barriers for dislocations to move and to alleviate strain localization at GBs,both of which contribute to the improved strength and work hardening.The statistics results show the dislocation density of GNT Cu deformed at ?=1%increases with increasing structural gradient,consistent with the more compatible deformation and higher strength and work hardening.The compatible deformation behaviors of GNT Cu are closely related to the structural gradient.When the applied strain is kept constant,the lateral strain difference between adjacent components decreases with increasing structural gradient,indicating the character of more compatible plastic deformation,which is consistent with increasing dislocation density.At larger strains,the microstructures of GNT Cu keep stable with two deformation mechanisms.As the tensile strain increases,the hardness of component(?)and(?)increases while the hardness of component(?)and(?)increases firstly and then decreases or saturates with increasing strains,but still higher than those before deformation.It is revealed by TEM observations that the plastic deformation of GNT Cu deformed at ?=9%is dominated by two typical deformation mechanisms:1)Bundles of concentrated dislocations are formed in component(?)and(?);2)A few detwinning regions appear in component(?)and(?).The stable microstructures of GNT Cu keep the structural gradient or strain gradient steadily during deformations and accordingly the geometrically necessary dislocations are persistently produced,which is critical for the improved work hardening.The gradient order plays a critical role in the mechanical behaviors of GNT Cu samples.The two gradient orders are designed.The normal gradient order GNT Cu has hard surface and soft core((?))but the reverse gradient order GNT Cu has soft surface and hard core((?)).Tensile tests showed that the normal gradient order GNT Cu exhibits higher strength and lower surface roughness.In the normal gradient order GNT Cu,the progressive plastic deformation from the core to the surface offsets the surface effect,indicating the stronger constraint between adjacent components,which benefits for enhancing the strength.The back stress and effective stress of GNT Cu samples was measured and compared to the homogeneous NT Cu samples.For homogeneous NT samples from NT-(?)to NT-(?),back stress increases substantially but effective stress increases slightly.The back stress is caused by threading dislocations bowing out within twin lamellae and pile-up at GBs.Both back stress and effective stress of GNT Cu samples are higher than the average values in terms of homogeneous NT samples.As the structural gradient increases,the back stress increases substantially but the effective stress increases slightly.The geometrically necessary dislocations not only improve back stress substantially but also are in favor of enhancing effective stress.