Effects Of Intermediate And Post-annealing On The Mechanical Properties And Microstructure Of Constrained Groove Pressed Pure-Ti

Pure titanium gets the favor of the medical profession,because it is non-toxic,no sensitization effect,no irritant,no genetic toxicity and no carcinogenicity to human body and it does not produce adverse reactions to human tissue,blood and immune system.However,the strength of pure titanium is low,which limits its application as human body parts under high load.As a new kind of severe plastic deformation process,constrained groove pressing(CGP)has attracted much attention,because it can make big size ultrafine crystal plate.At present,the method has been successfully used to refine the microstructure of the pure aluminum and copper,but its effects on the microstructure and mechanical properties of pure titanium have not been reported.Therefore,it is important to study the microstructure and tensile properties of the pure titanium processed by the CGP,to further improve the performance and expand the application of pure titanium.In this study,TA1 was used as the testing material treated by CGP.Changing the pressing rate,increasing the pressing pass,annealing and other ways was used to study the change of microstructure and mechanical properties of pure titanium.The microstructure refinement and deformation mechanism of pure titanium during the process of compression deformation were explored by transmission electron microscope(TEM)and electron backscattered diffraction(EBSD).The results show that after CGP,the strength of pure titanium increases with the increase of pressing pass number.After CGP,the microstructure of the pure titanium is refined and there exists a lot of twin and parallel slabs structure.With the increase of pressing number,the number of twin increases.The pressing rate has little effect on the strength and elongation of pure titanium.The elongation of the pure titanium decreases when the deformation rate exceeds 100mm/min.In the present study,the pressing deformation rate is l00mm/min.It is found that the intermediate annealing can release deformation storage to increase the pressing pass number.The strength can be improved further by improving the molding process.The highest tensile strength(488MPa)was get by the PAPP550? process,a 67%increase over the original pure titanium.During the CGP,the dislocation slip and twinning deformation mechanism work together.There are two kinds of twins 65°<10-10>and 85°<11-20>during the deformation process,and with the increase pressing number,the amount of twin increases.The intermediate annealing is good for activating the 85°<11-20>twin,and continuing to press after annealing is good for activating the 29°<11-20>twin.Under the action of shear force,a large number of banded structure is generated in pure titanium.With the increase of pressing passes number,the dislocation tangles and piles up around these banded structure gradually,thus the strength increases.In the process of PAPP550?,the intermediate annealing process leads to the dislocation polygonization;in the next two pressing passes,the dislocation piles up in the trigeminal grain boundary deformation,causing the material hardening,then the strength is further improved.