Investigation On The Preparation And Dynamic Behavior Of UFG/NC Pure Ti

Commercially pure(CP) Ti and Ti alloys possessing high specific strength,excellent properties at low and elevated temperatures, are important structure materials applied in automatics, aerospace and bio-medicine fields. However, the strength of CP Ti is not so desirable, which certainly weakens its further development.Thus, the key to widening its application is improving its strength. And grain refinement has turned out to be an effective way to improve both strength and comprehensive properties of materials. By now, much attention has been paid to the preparation of ultra-fine grained(UFG) and nanocrystalline(NC) Ti and the relation between their microstructures and properties. Nevertheless, lots of studies about the UFG/NC Ti focused on the mechanical properties under quasi-static loading conditions, while there was so limited results available documenting the dynamic behavior of CP Ti after processing by ECAP. Therefore, the investigation on the dynamic mechanical properties of UFG/NC Ti is very useful.TA2 CP Ti was employed in the study and processed by ECAP with various routings and the combination of ECAP and drawing to obtain UFG/NC Ti. We investigated the effects of temperature and strain rates on the dynamic performance of UFG/NC Ti, discussed and analyzed its structural stability and mechanism of microstructural evolution. In addition, the influence of grain size on the mechanical properties of CP Ti was also studied. The results are as followings:UFG Ti with grains in size of 300, 500 nm were obtained by ECAP processing for8 P at various temperatures. The two-step procedure of ECAP and drawing gave a nano structure with a grain size of ~90nm.There are strong temperature softening effects and strain rate hardening effect in the dynamic behavior of UFG/NC Ti, that is the flow stress decreases with increasing temperature, but increases with enhanced strain rate. Two constitutive equation were derived to relate the flow stress to the temperature, strain rate and true strain and to predict the yield strength and the peak stress of NC Ti subjected to dynamic deformation under 473~673K at different strain rates, respectively.Structural stability of CP Ti decreases after grain refinement. A thermal treatment for NC Ti at 573 K for 3min gave rise to the operation of static recrystallization(SRX),higher temperature promoted the process of SRX and subsequent grain growth.Migrate-dynamic recrystallization(DRX) took place when both NC Ti and UFG Ti subjected to deformation at room temperature with a strain rate range of 0.01~10s-1,while rotary-DRX operated when UFG/NC Ti deformed under much higher strain rates of 103s-1. DRX was also found in UFG Ti when it was compressed at 673 K with the strain rate of 4000s-1.Grain size plays an important role in the mechanical properties of CP Ti. When CP Ti with grain size in the range of 90nm~20?m were compressed at 0.01~10s-1, the result indicates the flow stresses and the strain rate sensitivity of CP Ti increase with decreasing grain size. Work hardening dominates at all strain rates in coarse grained Ti but it balances with flow softening at 0.1 and 1s-1 in UFG and NC Ti and there is obvious flow softening in these two materials at 10s-1.