Gradient Ultrafine-grained Structure By Severe Plastic Burnishing

The effects of traditional burnishing normally focus on the improvement of surface quality and fatigue strength, but the effect of microstructural strengthening is always unsatisfactory. Surface nanocrystallization(SNC) technology is a kind of new method that can dramatically improve the surface performances of materials by the unique mechanical, physical and chemical properties of nano-materials, but it leads to the increase of surface roughness of the treated surface in most cases. In fact, combining the advantages of both two surface strengthening technologies, the gradient ultrafine grained(GUG) microstructures have been fabricated on the surface layer successfully. Gradient ultrafine grained structure refers to the gradient distribution of grain size, which means the grain size consecutively increases from nanometer or submicron crystal size to coarse grained size. The essence of GUG structure is the gradient change of grain boundary density in space, which leads the gradient variation of physical, chemical and mechanical properties. This unique microstructure is different from the simple composite of nanocrystalline and matrix materials, which effectively avoids the sudden change of functional performance caused by the mutation of grain size. Meanwhile, the functional mechanism by mutual-coordination of different grain size structure can optimize and improve the integral performance and behavior. So far, the mechanisms of microstructure evolution, surface characteristics and so forth are lack of the uniform learning and understanding for gradient nano/ultrafine grained microstructures.In our paper, we use a new SNC method called Severe Plastic Burnishing(SPB) to induce the gradient nano/ultrafine grained surface layers on face-centered cubic pure copper with medium stacking fault energy(SFC) and body-centered cubic pure iron with high SFC. Then the grain refinement mechanisms and the surface performance of the samples have been comprehensive studied. The selection of pure metals can exclude the influences of solid solution strengthening, precipitation strengthening and phase transformation, and is also beneficial of understanding the effects of grain refinement on mechanical properties and functional performance of materials. Meanwhile, choose the metals with different SFCs and crystal structures just because the magnitude of SFC and crystal structure are closely related to grain refinement mechanisms. The obtained results and conclusion are as follows: Use SPB to treat the pure copper and iron with different burnishing forces and times, then the pre-and post-treatment grain size changes were researched. The grains are refined to sub-micron scale after two times of burnishing on pure copper, and the average size of which is 643 nm. The grains are continuously refined to nano scale after eight times of burnishing, with 12 nm average size. The grains are found to be refined obviously on pure iron with the same burnishing times, and the smallest grain size reaches 300 nm.The microstructure characteristics of samples at cross section, surface, various depths from the topmost surface were investigated by TEM, SEM, XRD, OM and so on. The grain refinement mechanism of GUG metals with different SFCs and crystal structures were revealed. For face-centered cubic metals with medium SFC,the mechanism as follows: In the condition of low plastic strain and strain rate, the coarse grains were refined into the few micro-sized/submicro-sized grains by combining the dissections of shear bands and dislocation activities; In the condition of high plastic strain and strain rate, the interaction of high density dislocations and mechanical twinning further refined the grains into submicron or nano size. The formation of random orientations of equiaxial nano-grains was dominated by the grain rotation or the slip of grain boundary. For body-centered cubic metals with high SFC, the development of dense dislocation walls(DDWs), dislocation tangles and cellular structure were formed in coarse grains by the dislocation effects of multiplication, accumulation, interaction at first. With the increase of plastic deformation, high density dislocations nearby the DDWs and dislocation tangles constantly generated, annihilated and rearranged, which makes these dislocations gradually evolve into the subgrain boundaries and then the dislocation cells gradually formed into nano-size subgrains. The further dislocation activities occurred in the internal of these subgrains and dislocation cells finally result in the grain refinement to nano-size or submicro-size.Study the influence of process parameters on mechanical properties and surface performance on the GUG copper. The results indicate that the surface roughness of GUG copper substantially reduced to 1/10 than that of the original roughness; SPB can dramatically improve the hardness of surface layers of samples up to 1.7 times than that of base hardness, and the relation of grain size and hardness is consistent with the equation of Hall-Petch. Under the dry and oil lubrication fretting condition, the GUG copper both has a better friction and wear performance in the whole load range, but under the oil lubrication condition, the GUG copper has higher friction coefficient than that of coarse grained copper. The GUG copper has negative corrosion potential and obviously higher corrosion current density in 3.5% NaCl solution, which reveals the corrosion resistance decreases. But simultaneously we found the pitting corrosion resistance were remarkably improved in 6%FeCl3 solution. The activation energy of grain growth for the GUG copper is 110.9kJ/mol, which is far less than that of coarse grained copper. When the annealing temperature was over 200 ℃, the growth of the GUG copper was observed, and at the same time the micro strain sharply reduced, which indicates the decline of thermal stability of GUG copper.By researching the surface roughness, microhardness, friction and wear performance, corrosion resistance of GUG iron by SPB treatment, some conclusions can be listed as follows: the roughness of GUG iron reduced from 3.67 μm to 0.41 μm. The microhardness of the treated surface is 1.6 times harder than that of original coarse grained iron, which show the remarkable work hardening effect. Under the dry fretting condition, the friction and wear resistance of GUG iron significantly reduce. Under the oil lubrication fretting condition, the maximum wear volume ratio is less than 1%. The wear resistance increases significantly, but the steady state friction coefficient turns larger. The corrosion potential of pure iron have a anodic shift and the corrosion current density also reduced after SPB treatment. The pitting corrosion velocity is only half of the original samples, which shows an obvious enhancement of corrosion resistance of SPB iron.