| After surface self nano-crystallization(SSNC) for 0Cr18Ni9Ti stainless steel and TA17 titanium alloy by means of Surface Mechanical Attrition Treatment(SMAT), a gradient structure in surface layer with grain size from nano-scale to micro-scale along the depth was obtained. Then the SMAT samples were taken thermal analysis and annealed at different temperatures for different durations. The evolution of the surface nano-microstructures, grain growth behavior, mean microstrain and microhardness of the surface layer during annealing were characterize in detail by Optical microscopy(OM), scanning electronic microscopy(SEM), transmission electronic microscopy (TEM), X-ray diffraction(XRD) analysis and microhardness measurement(HV). The influencing factors of thermal stability of surface layer nanostructure was analysed , and the mechanism of thermal stability of surface nanostructure materials was discussed.About 250μm thick severe deformed layer was obtained in 0Cr18Ni9Ti stainless steel after SMAT. And almost complete content strain-induced martensite was obtained in the topest surface. After annealing ,the research results showed as follows:The gradient structure retained after annealing from 250℃to 850℃for 30 minutes. As the annealing temperature arising, the grain boundaries of the deformation region were more clear, and the dislocation,deformation twin density decreased. At 250℃, the grain size had no obvious change ; at 400℃, the average grain size grow up to 47.6nm, the mean microstrain decreased from 0.170% to 0.135%; At 600℃, it increased to about 70nm, and at 700℃, it was above 100nm, the mean microstrain sharp drop to 0.044%. Through DSC analysis, the surface nanocrystal structure recovery occurred at 200℃, the recrystallization occurred at 400~500℃, and the grain obviously grew up at 700℃. The DSC curve shows that about 660℃was the critical temperature of grain obviously growing up. The grain growth activation energy was 24.68kJ/mol calculated by Arrhenius equation. It was much less than regular grain growth activation energy, and it explained why the nanocrystal obtained by the severe plastic deformatiom had recrystallization and sharp grain growth at lower temperature. The content of the surface martensite decreased from 98.5%(when annealed at 400℃) to 68.6%, after annealing at 600℃for 30 minutes. At 700℃, the martensite almost decomposed to autensite, only remained 18.65%. When annealing at 600℃for 2～20 minutes, it did not decomposed, so the incubation time for martensite decomposing was above 20～30 minutes. The surface microhardness was a gradient structure along the depth , and it decreased as the temperature arising. But the microhardness of the topest layer was still much higher than the matrix even annealing at 850℃.About 127μm thick severe deformation layer was obtained in TA17 titanium alloy after SMAT.it contained high density deformation twin in the transition region. After annealing , the research results were as follows:At low temperature(250～400℃), the gradient structure had no obvious changes, the average grain size just reduced to 38.0nm, the mean microstrain varied from 0.15% to 0.12%; At 600℃, they changed to 50.0nm and 0.09%; At 700℃, the grain size was above 70nm, and at 850℃, it scaled out nanometer magnitude, and the mean microstrain sharp dropped to 0.037%, the gradient structure was all changed. At 250℃, the recovery occurred in top surface structure and the recrystallization occurred at 550℃, the obvious grain growth occurred at 700℃. The grain growth activation energy was 19.76kJ/mol through Arrhenius equation, it also much less than the regular grain growth activation energy. The deformation twin decreased as the temperature arising, but its thermal stability maintained to about 750℃, and it almost vanished after 850℃annealing; the microhardness decreased as the temperature arising, but at each temperature it retained the gradient structure——reducing along the depth. So the surface microhardness had good thermal stability.
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