材料期刊网

Commercially pure titanium was treated by high energy shot peening, and annealed at a series of temperatures. The surface layers are characterized by means of scan electronic microscope, X-Ray diffraction, transmission electronic microscope and micro-hardness testing machine. The results 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 50 μm, the decrease 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℃.Surface nanocrystallization is well known as one of important methods to improve surface properties. The thermal stability of nanocrystalline microstructures was related to their preparation and application. The commercial pure Ti thermal stability of nanocrystalline and deformed microstructures induced by high-energy-shot-peening (HESP) technique was investigated. The nanostructured surface and deformed sub-surface layers of specimens were prepared through HESP treatment. The thermal stability was characterized through XRD analyses of surface layers, SEM and TEM microstructure observation and microhardness measurement of specimens annealed in different temperature in the air after HESP treatments. The results showed that after HESP treatment, the microhardness of surface layers increased with treatment time, especially in the rang of about 40 micrometers from the surface, the microhardness increase was obvious. The surface microhardness decreased gradually with annealing temperature, but the sub-surface microhardness decreased suddenly over 500℃. From 550 to 650 ℃, the microhardness of surface layers almost unchanged, and is still higher than that of the undeformed microstructure. SEM observation showed that at and below 500 ℃, the microstructure changes were not obvious. At 550 ℃, the grains in surface layers grew remarkably both in SEM and TEM images at 20 micrometers deep, and the deformed twins formed in HESP treatment could be seen in the subsurface. In addition, the TEM images showed that even at 550 ℃, the nanosized grains existed within 10 micrometers from surface.