Study On Mechanical Properties Of Pure Titanium In Large Plastic Deformation

In recent years, Ti and Ti alloys have been widely applied in aerospace, shipping, aviation industry, chemical industry, auto industry and biomedical science field for their superior heat resistance, corrosion resistance, non-toxicity, biocompatibility and high strength properties. Optimizing the strength and ductility of Ti and Ti alloys has always been a hotspot in materials researches. At present, there produces bulk ultra-fine grained (UFG)/nanocrystalline materials by severe plastic deformation (SPD) methods to improve the strength and ductility of material.This paper regards TA1 commercially pure titaniumas the research object, and mainly discusses the influence of SPD methods on the microstructure and mechanical properties of pure Ti, such as low temperature rolling (LNR), room temperature rolling (RR), surface mechanical attrition treatment (SMAT), ball milling surface mechanical attrition treatment (BM-SMAT) deformation and surface nitriding process after SMAT, low temperature annealing process. And the main deformation mechanism of materials under different deformation methods is also studied. The parameters such as strain, strain rate and deformation temperature during the deformation process, have impact with great degrees on the microstructure, mechanical properties and deformation mechanism of pure Ti. A control variable and comparative law were used to study mechanical properties of 2mm thick after vacuum annealed pure Ti sheet under different processes. The microstructure and deformation mechanism characteristics were also studied and discussed.In this work, mechanical properties of samples processed using different deformation methods were characterized by Viker microhardness tester and Shimadzu universal mechanical tester. Optical microscopy (OM), X-ray diffraction (XRD), scanning electron microscopy (SEM) and transmission electron microscopy (TEM) were used to test the microstructure. Experimental results show that the plastic deformation temperature has an important influence on the microstructure and macroscopic mechanical properties of titanium, with the same strain, LNR samples have a higher strength and quite same plastic than RR samples. LNR samples have more small grain size, more dense dislocations and tiny twins, by TEM observation. The yield strength of LNR-75%, with the highest strength, is 720 MPa. Increasing the amount of plastic strain is also conducive to enhancing the strength of pure titanium. the strength of annealed at low temperatures samples after LNR there show a certain degree of reduction, but the plastic of them has not been greatly improved.Tensile tests show that compared with the SMAT samples were treated under the same amount of time at room temperature, at low temperatures (close to the temperature of liquid nitrogen) LN-SMAT samples has a high strength and good ductility. With the extension of treatment time, a higher strength can be gain in SMAT samples. And the range of the increased strength in LN-SMAT samples is greater. BM-10h sample has the highest yield strength. The yield strength of BM-10h improved 68% relative to the original sample, and still retain the good plasticity, with good mechanical properties. The overall SMAT samples show a good ductility and a limited increment in strength relative to the original annealed sample. SMAT samples exhibits a gradient structure features with grain size become larger and hardness become soft from the surface to the matrix structure. The grains in the outermost layer of SMAT samples can be refined to 100 nm or less, This means that they are nanocrystalline. surface nitriding samples at temperatures of below 560 ℃ after LN-SMAT obtain a nitrided layer with a depth of about 40 μm. but the strength of those nitrided samples did not improve, corresponding to those of the LN-SMAT samples.