Research On Microstructural Evolution And Thermal Stability In Ultrafine-grained Pure Titanium Processed By High-pressure Torsion

The demand on micro-parts has been increased significantly due to product miniaturization. Micro-forming technology inherits the traditional advantages of plastic processing, to realize product miniaturization, micro-forming is one of the promising micro-manufacturing approaches to fabricating micro-parts for its high productivity, low production cost and good mechanical properties. However, the grain size of common metal materials is the same magnitude of the size of microparts, as a result the size effect occurs obviously, affected the filling quality and dimensional accuracy of micro-parts. Ultrafine-grained material could reduce the material deformation inhomogeneity and enhanced dimensional accuracy of micro-parts, are especially suitable for micro-forming.The paper is studying on ultra-fine grained pure titanium that processed by high-pressure torsion(HPT) at room temperature, the disks were processed through selected numbers of turns, N, ranging from 1/4 to 10 revolutions. Microstructural evolution and microhardness evolution were analysised. Stored energy, recrystallization temperature and thermal stability of ultrafine grained titanium were studied. The mechanical properties and microstructure during micro compression test under different strain rate and grain size were observed.Microstructural and microhardness analysis was performed to provide detailed information on the effect of shear strain. Analysis by X-ray diffraction(XRD) revealed an allotropic phase transformation from the α phase(hexagonal closed-packed) to the ω-phase(hexagonal) during HPT processing, the volume fractions of ω-phase rised with the increase of turns. Measurements of the Vickers microhardness show that the microhardness rised with the increase of turns, but the lower hardness zone existed at the centre of specimen. The microhardness were saturation after 5 or more turns of torsional straining. Large quantity of dislocations and twinning existed in grains after 1/4 turns of torsional straining. The grain size of the centre of specimen is large after 1 turns, but the grain size of the edge of specimen is below 300 nm. Fine grains and large grains are existed at the centre of specimen after 5 or more turns,but the grain size of the edge of specimen is below 150 nm. The results demonstrate that significant grain refinement is achieved through HPT processing.The evolution of stored energy and recrystallization temperature was studied by differential scanning calorimetry. The activation energy involved in the recrystallization process was also determined. The results show that the recrystallization temperature of HPT 5 turns is between 703.8℃ and 748.8℃, the activation energy of recrystallization is between 94.4 KJ/mol and 112.2 KJ/mol, the activation energy of grain growth is about 109 KJ/mol. The microstructural evolution evolution of HPT Ti during annealing can divide into several stages. During the first stage,the dislocations in grains of HPT Ti rearranged and the amount of dislocations decreased. During the second stage, the grain boundaries with high-angles of misorientation are forming. During the third stage,the microstructure of non-equilibrium grain boundaries occurs recovery. During the forth stage, the microstructure of non-equilibrium grain boundaries occurs recrystallization. During the fifth stage, the grains are growing during annealing.The results of micro-compression tests shows that: the flow stress of ultrafine grain pure titanium is highly influenced by strain rate, the flow stress gradually increases with the increases of strain rate. When grain size is below 200 nm, the flow stress of the material is very steady, because there are a amount of dislocations in grains of ultrafine grain pure titanium. With decrease of grain size, the flow stress of ultrafine grained material increases gradually, but the flow stress of the coarse grain material appeared abn ormal. The results of surface topography of micro-compression samples shows that the surface of sample is smooth and high surface quality when grain size is very small. The results of the observation of microstructure of compression samples by EBSD technique shows that the substructure in the coarse grain interior increases gradually, and the misorientation of small angle grain boundary has the gradually increasing tendency, the important deformation modes of coarse grain pure titanium is dislocation glide..