Surface Nanocrystallization Of Titanium And Titanium Alloy And The Texture Properties Of Their Surface Layers

The preparation technique of nanostructured surface layer in metals and alloys is with most important significance for the industry application. For its simplicity of craft and the uniform change of the structure from surface to inner, surface nanocrystallization has been regarded as one of the most prosperous surface technologies.The microstructural evolution of nanocrystal and the texture properties of microstructures of surface layers was investigated in commercially pure Ti and titanium alloy. The surface nanocrystallization of commercially pure titanium and titanium alloy 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) and the transmission electron microscope(TEM);Furthermore, the microhardness of the surface layers were examined by microhardness testing machine.A grain refinement mechanism of commercial pure titanium and titanium alloy was summed up as follows: at the beginning of deformation, lots of dislocation intersect formed stress field, when the stress reach the resolved shear stress inducing mechanical twinning, one-system twin form and develop gradually to multi-system twins, twin-twin intersections reduce the grain size. With increase of the twin systems and enhancement of the twin intersection activities, grains are further refined. Finally nano-scale grain with equaixed shape and random crystallographic orientation forms upon the much larger strain, high strain rate and multidirectional repeated loads.Experimental results showed that the layers of severe plastic deformation with nanocrystalline structures were obtained. On commercial pure titanium and titanium alloy after the high-energy shot peening treatment. The longer the peening time, the thicker thedeformation layers. The surface-hardness of the samples increased greatly after the treatment, and decreased gradually from the surface to the centre. The increasing of surface-hardness will improve friction wear property and mechanical property of material. This kind of surface technique will be extensively applied in prolonging service life of machinery parts and improving other suface properties of them.