| Surface mechanical attrition treatment（SMAT）is a simple and effective severe plastic deformation approach to obtain ultra-fine materials.Stacking fault energy（SFE）is one of main factors that effect the grain refinement process of materials during SMAT.So far,the grain refinement process for face-centered cubic（fcc）metals with different SFE is nearly systemetic.However,less research has addressed the grain refinement process for fcc metal with low-middle-SFE.Besides,most of materials applied in SMAT are pure metals or single-phase alloys.Two-phase alloys will lead to not only grain refinement but also the occurrence of instability transformation during severe plastic deformation,which is complex.Research on the two-phase alloys during severe plastic deformation is insufficient.There is still some scientific laws need to be recognized.No phase transformation takes place in plastic deformation of copper alloys（fcc low-middle-SFE）,which provide a better material base for investigation on grain refinement process of fcc low-middle-SFE metals during SMAT.To obtain different SFE metals,Ti,Zn and Ni alloying elements were added in Cu.The grain refinement process of these copper alloys during SMAT were studied.The aging process of Cu-Ti alloy involves continuous and non-continuous phase transition,which contain rich research connotation.The effect of aging treatment on the grain refinement process of Cu-Ti alloys during SMAT is also investigated in this paper.The main conclusions are as follows:（1）Cu-4Ti alloy belongs to low-SFE metal.No phase transformation takes place during SMAT.The grain refinement process involves formation of planar dislocation arrays and twins,twin-twin intersections leading to grain subdivision,formation of dense dislocation walls and microbands,microbands further divided into refined blocks and the formation of polygonal grains.（2）The stacking fault probabilities of copper alloys were determined via X-ray peak-shift method.Calculation results show that the addition of Ni element almost have no influence on SFE of copper alloys.The addition of Ti and Zn solid solution atoms decrease the SFEs of the copper alloys.Ti has a larger effect on the SFEs than Zn at the same mass fraction.According to the calculation results,the grain refinement process of Cu-2Ti,Cu-20Zn,Cu-2Zn and Cu-7Ni alloys with SFE ranging from low to high during SMAT were studied successively.SFEs is the key factor that effect the plasticity defroamtion mechanism of copper alloys during SMAT.The effect of SFE difference on the grain refinement process in copper alloys during SMAT embodies the plasticity deformation mechanism at the low strain and strain rate region.The lower the SFE,the larger the trend at the low strain rate region.The plasticity deformation mechanism at the low strain rate region are twin-twin intersection,a few of twins and dislocation cells respectively as SFE ranges from low to high in copper alloys.The main plasticity deformation mechanism is microbands at the high strain rate region.After SMAT,the hardness of all copper alloys gradually decreases with the increase in depth.The increase in hardness is due to grain refiment,large defect densities and so on.The yield strength of all copper alloys increased after SMAT.However,the elongation of these samples decreased after SMAT.The analyse of sample tensile fracture indicates that the fracture features changed from brittle to ductile from the top surface to the matrix.（3）The effect of aging treatment on the plasticity deformation mechanism of Cu-Ti alloy was studied.Cu-4Ti alloy was solid solution treated at 1203 K,and then were aged at 773 K for 0.5 h、4 h and 24 h,corresponding to the spinodal decomposition,peak-aged and over-aged stage,respectively.The samples after aging treatment were SMAT.Experimental results show that the grain refinement process of spinodal decomposition and peak-aged stage were the same as the solid solution treated Cu-4Ti alloy.For over-aged stage,deformation of two phases between precipitatesβ-Cu4Ti and the Cu matrix is non-homogenous.β-Cu4Ti phase were anew dissolved,and solid solution treated in matrix under applied stress.The dissolution preferred to occur in the interface between the precipitates and matrix. |
