Comparison Of Surface Mechanical Attrition Treatment Process Between FCC And HCP Structure Of Mental Material

In recent years, nanomaterials with excellent mechanical properties and unique microstructure have become a hot topic. Surface Mechanical Attrition Treatment(SMAT) is preparation of nano-structured surface layer with a certain thickness and overall improvement of mechanical properties by changing its organizational structure, present morphology and stress state. Being derived from SMAT, Surface Mechanical Alloying(SMA) technology belongs to another branch field of severe surface plastic deformation(SPD). When conducting surface mechanical attrition, some composition different from matrix are introduced; which forms basis for the formation of composite surface with nano-graded structure and thus meets the requirements of surface diversity. In this paper, the author compared the parameter problems of FCC and HCP structure of metals during mechanical attrition process and discussed the difference between SMAT and SMA, so as to prepare nano coating with better surface properties. The atomic diffusion problem in alloying process is also studied.(1) The difference of crystal structures and stacking fault energy between pure copper and magnesium lead to different plastic deformation mechanism. The deformation mechanism of copper includes three steps: the evolution of dislocation tangles and dislocation walls in original coarse grain and refined cell; the formation of small angle sub-boundaries; the transformation of small angle sub-boundaries into large ones and nanocrystal. The dislocation mechanism plays a major contribution to the plastic deformation process. The deformation mechanism of pure magnesium also includes three steps: the formation of twinning structure induced by stress and strain, development of interaction between twinning into dislocation entanglement and dynamic recrystallization. The twinning interaction mechanism is mainly affected in the early process of plastic deformation, while dislocation mechanism plays a major role in deformation process.(2) In the same nano-system, the increase in the size of projectile promotes nanocrystallization and the increase in the thickness of plastic deformation can lead to the reduction of nanocrystalline size. To pure magnesium matrix, in particular, the preferred orientation of the surface plane changes from the original(101) plane into(002) and(101) crystal plane under the effect of projectile with 8 mm specification.(3) In the same nano-system, the growth in the time of SMAT promotes nanocrystallization and with the increase of thickness of plastic layer, nanocrystals size decreases, the lattice distortion gets worse.(4) With the combination of SMAT and SMA technology, uniform and continuous Ni coating, Cu-Ni and Mg-Ni composite coating can be obtained. This new approach provides experimental guidance to in-situ formation of composite coating surface.(5) The alloying efficiency of Cu plate and Mg plate after SMAT pretreatment is much higher than the untreated ones. After SMA for 4 h, the thickness of Ni coating of pretreated samples reached to 40 ?m and 60 ?m respectively, which are 8 and 6 times respectively of the non-pretreated samples. The thickness, uniformity and continuity of the coating improves with extension of alloying time.(6) The surface hardness of sample can be greatly improved after SMAT treatment. The surface hardness of the copper sample increases from 98 HV up to 280 HV, almost 2.8 times; while surface hardness of pure magnesium sample increases from 45 HV to 340 HV.(7) The formation of coating mainly experiences three stages: the mechanical combination of powder and the substrate, the formation of cold welding layer and the interdiffusion(diffusion) process. After pretreatment, the substrate surface undergoes severe plastic deformation, resulting in presence of uneven surface morphology, the increase in surface area and surface hardness. These behaviors promoted the formation of alloying coating.(8) During the plastic deformation process, the original coarse grain happens severe kink and refinement due to the effects of stress and strain, a large number of grain boundaries(trigeminal grain boundaries in particular) are presented among grains; there are a lot of defects like dislocation lines in the grains.(9) In the plastic deformation process, local temperature are aroused due to the increase of defects such as grain boundaries and dislocations. Ni is diffused into pure copper and pure magnesium matrix in the form of atoms.