Mechanical Properties And Deformation Mechanism Of Bi-modal Cu

Ultrafine grained metallic materials have attracted great interests because of its high strength in the last decades. However, their low tensile ductility(less than 5%) comes to be an obstacle in industrial applications. Previous investigations indicate that bimodal structural materials, which introduce coarse grains in ultrafine or nano grained matrix, obtained both high strength and ductility. Based on these discoveries, important questions concerning fundamental knowledge remain to be answered. For instance, controversy of the relationship between the volume fraction of the coarse grains and the overall ductility still exists, what’s more, how the coarse grains and ultrafine grains coordinate to improve tensile ductility is still unclear, and etc. Therefore, the research on deformation mechanism of bimodal metallic materials becomes an important issue. In this work, ultrafine grained Cu with homogeneous microstructures was processed by equal channel angular pressing (ECAP). To obtain the exact annealing parameters, a series of annealing experiments with different time were carried out. Finally ultrafine grained Cu was oil-bath annealed at 200℃ to form a bimodal grain size distributed Cu sample. Mechanical properties testing and fracture analysis were employed on ultrafine grained, bimodal and coarse grained Cu. To study the deformation mechanisms, in situ electron backscattered diffraction (EBSD) technique was applied to characterize microstructural evolutions during tensile strain. At the area away from necked region, in situ EBSD scanning and analysis were employed at the strain of O%、3%、8%、14%. At the necked region, in situ EBSD technique was employed at the stain of 0% and 11%. The detailed results are as follows:i. Ultrafine grained Cu prepared.by equal channel angular pressing at ambient temperature has a high strength (a yield strength of over 400 MPa) but a low tensile ductility (elongation to failure of 8%). The bimodal Cu annealed at 200℃ was composed of coarse grains from the secondary recrystallization of partial ultrafine grains, has a good combination of strength and tensile ductility (a yield strength of 225 MPa and elongation to failure of 20%).ii. At the area away from necked region, in situ electron backscattered diffraction technique found that some of the ultrafine grains and recrystallized grains rotate gradually with increasing tensile strain. Moreover, tensile deformation results in local strain accumulation and de-twinning within individual coarse grains. Local strain only exists in the ultrafine grains before deformation, and gradually appears in coarse grains. The large local strain further causes the formation of sub-grain boundaries and the increase of low angle boundaries.iii. At the necked region, SEM-EBSD observation revealed that neither coarse grain regions nor the UFG regions have an obvious misorientation variation, probably because of a large driving force for the CG rotation, and the UFG mixed area tend to rotate simultaneously. On the contrary, in CG and UFG mixed area, the movement of CG easily drives the rotation of UFG, resulting in abrupt misorientation variations.iv. At fracture area, Local strain in individual grains is perpendicular to tensile direction, and increases gradually during deformation, resulting in the formation of subgrain boundary, further leading to the decrease of average grain size.v. The length and width of annealing twins at strain of 0% and 11% were calculated. We found that for twins paralleling to tensile direction, their length increases and width decreases; inversely, for the twins normal to tensile direction, their length decreases and width increases.