| In this paper, surface mechanical attrition treatment (SMAT) was performed to 0Cr18Ni9Ti stainless steel, TA17 titanium alloy, HRB400 low carbon steel and AZ31 magnesium alloy respectively by means of shot-peening. The microstructures of the surface layer of SMATed samples were characterize in detail by using Optical microscopy (OM), scanning electronic microscopy (SEM), transmission electronic microscopy (TEM), X-ray diffraction (XRD) analysis and microhardness measurement. Research results showed that a nanostructured surface layer was formed on 0Cr18Ni9Ti stainless steel, TA17 titanium alloy and HRB400 low carbon steel and that surface self nano-crystallization (SSNC) of these three metals was successfully realized through shot-peening. However, no nanostructured surface layer was obtained on SMATed AZ31 magnesium alloy samples, but only superfine submicron grains.The surface layer of 0Cr18Ni9Ti stainless steel was severely deformed after SMAT, and such a gradient structure was formed from the topmost surface to the matrix: the severely deformed layer, transition layer and matrix. Particularly, the severely deformed layer can be further divided to nanostructured layer and layer with superfine structure. The depth of the severely deformed layer increased with the prolonging of treatment time and finally reached a plateau at about 250μm, while the depth of the whole strained layer could be as deep as 700μm. 700μm was the depth that can be influenced by stress and strain induced by impacting of shots during shot-peening in the interior of 0Cr18Ni9Ti. Nano-grains can be obtained after 1min's SMAT and grain sizes in the topmost surface of samples decreased with the prolonging of treatment time. Grain size in the topmost surface of the sample treated for 13min was 52nm. Grain size increased when away from the topmost surface, and the depth of nanostructured layer for the sample treated for 13min was about 70μm. There was strain-inducedε-martensite with close-packed hexagonal structure in the surface layer during SMAT. Martensite transition in the topmost surface could be completed within a short time. Meanwhile, martensite volume percent decreased exponentially with the increment of the depth from the topmost surface. Microhardness of the surface layer was obviously elevated after SMAT and it decreases gradually to the level of the matrix along the direction away from the topmost surface. The microhardness of the topmost surface of 0Cr18Ni9Ti could be 2.5 times as high as that of the matrix. Similar to 0Cr18Ni9Ti, a surface with gradient structure was formed on TA17 titanium alloy after SMAT. Microstructures from the topmost surface to the matrix of TA17 were just as follows: nanostructure, superfine submicron structure, lamellar twin-matrix alternate blocks (LTMABs) together with dislocation tangles (DTs) and dense dislocation walls (DDWs), dense twins and LTMABs formed by twin-twin intersection, single twins and finally strain-free matrix. The depth of the severely deformed layer could reach 230μm under 13min's SMAT. Grain sizes in the topmost surface of samples decreased with the prolonging of treatment time and it would be 30nm after 13min's treatment. The depth of the nanostructured layer was about 50μm after 5min's SMAT, and it would be a little deeper for the sample under 13min's treatment, which had a grain size of about 90~95nm 50μm beneath the topmost surface. The corrosive property of the surface layer of TA17 deteriorated after the achievement of SSNC through SMAT, and therefore nanograins in the surface layer could be easily deprived after corrosion by acids. Microhardness of the surface layer of TA17 had got evident enhancement after SMAT, and the microhardness of the topmost surface of TA17 could be 2 times as high as that of the matrix. The microhardness would decrease gradually to the level of the matrix, which was about 500μm beneath the topmost surface.As for HRB400 low carbon steel, a fiber-like microstructure was formed on the surface after SMAT. For the sample under 13min's SMAT, the width of these'fibers'was under 100nm and these'fibers'comprised grains with sizes under 100nm. These showed that these'fibers'were of nanostructure. The depth of the severely deformed layer was about 200μm and the whole strained layer was about 500μm after SMAT. Microhardness of the topmost surface could be 1.7 times higher than that of the matrix of HRB400, and similar to those metals mentioned above, microhardness distribution along the depth from tne topmost surface was gradient.It was observed that much magnesium chips flaked away from the topmost surface of AZ31 during SMAT, and therefore there was no severely deformed layer in the surface layer of SMATed AZ31 samples. The grain size of the topmost surface of AZ31 after SMAT was in the range of submicron. Enhancement of the microhardness of the topmost surface of AZ31 after SMAT was not so evident that it basically kept at about 90HV0.025, in other words, microhardness of the topmost surface had no prominent dependence on treatment time.Investigation results in this paper also showed that diffusion coefficient of elements during diffusive bonding and strength of the bond could be elevated when the SSNC technology was applied in the diffusive boning of TA17 titanium alloy to 0Cr18Ni9Ti stainless steel.
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