Surface Nanocrystallization Of Commercially Pure Ti And The Thermal Stability Of Microstructures Of Surface Layers

For its simplicity of craft and the uniform change of the structure from surface to inner, the self surface nanocrystallization is regarded as one of new important surface modification methods which has prospective applied future. But all the related investigations are concentrate on Body-Centered Cubic(bcc) and Face Centered Cubic(fcc) metals, hexagonal close-packed(hcp) metals have not been involved yet.The microstructural evolution of nanocrystal and the thermal stability of microstructures of surface layers was investigated in commercially pure Ti. The surface nanocrystallization of commercially pure titanium was realized by using high-energy shot peening (HESP) technique. The microstructural evolution of nanocrystal was examined by using the optical microscope (OM), the scanning electron microscope (SEM), X-Ray diffraction (XRD) and the transmission electron microscope (TEM); Furthermore, the microhardness of the surface layers were examined by microhardness testing machine.The results showed that severe plastic deformation was occurred in surface layers of commercially pure titanium after HESP treatments, At first, certain density dislocation was formed, then deformation twins became prevailing, one-system twins develop gradually to multi-system ones, twin-twin intersections reduce the grain size, with the increment of the twin systems and the enhancement of the twin intersections activities, grains were further defined, finally, equaixed shape and random crystallographic orientations nano-scaled grains formed upon the much larger strain, high strain rate and multidirectional repeated loads.The results also showed that microhardness of surface layers decreases with anneal temperature, the tendency of microhardness is similar to unannealed one, in other words, the more close to the surface, the more rapidly the hardness decreases, after reaches the depth of 50m, thedecrease becomes steadily. But the sub-surface microhardness decreased suddenly over 500, From 550 to 650, the microhardness of surface layers almost unchanged. Observing by TEM and SEM, the grain sizes of pure titanium surface layers have increased below 500 ; Deformation twins begin disappearing obviously at 550; The nano-scaled grains within about 10 micrometers from surface existed even at 550 .