| More and more attention has been paid to metastable ?-type titanium alloys owing to its low elastic modulus,high corrosion resistance and excellent biocompatibility.However,the strength of metastable ?-type titanium alloys is too low to satisfy the requirement of engineer application due to the stress-induced martensitic transformation resulted from insufficient ?-stabilizer elements.By adjusting the content of ?-stabilizer element and thermos-mechanical treatment,our team developed the Ti-33Nb-4Sn alloy with ultralow modulus and high strength.However,the martensitic transformation in the Ti-33Nb-4Sn alloy has not been clarified in detail.In this dissertation,the influence of martensitic transformation on mechanical behavior of Ti-Nb-Sn alloys was systematically investigated by combining results from optical microscope,X-ray diffraction,transmission electron microscopy,synchrotron X-ray diffraction and tensile test.The main researches are summarized as follows:This dissertation studies the influence of SIM transformation on mechanical behavior of Ti-33Nb-4Sn alloy with different thermos-mechanical treatment during the single loading.The main results are summarized as follows,solution-treated Ti-33Nb-4Sn alloy undergo martensitic transformation after a solution treatment at 800? for 1h followed by water quenching to room temperature.“Double yield” can be observed in the tensile stress-strain curve.Upon a cold rolling deformation,Ti-33Nb-4Sn alloy performs “non-linear elastic” deformation.The experimental result of synchrotron radiation analysis shows that the “non-linear elastic” deformation can be mainly attributed to stress-induced ? " martensitic transformation suppressed by dislocations and grains introduced by cold rolling.Upon aging at 425? for 30 min,?" martensite disappears in the Ti-33Nb-4Sn alloy.In such a case,the Ti-33Nb-4Sn alloy exhibits a microstructure of small-sized ? phase whose stable improved significantly,resulting in linear elastic deformation.It was shown that martensitic transformation has a significantly influence on the mechanical properties of Ti-33Nb-4Sn alloy during the single loading in the previous study.However,the respone of stress-induced ?" martensite and residual ? phase to stress during the cyclic loading and unloading is still ambiguous.Therefore,this dissertation studies the character of martensitic transformation occurred in the cycle loading and unloading.The main results are summarized as follows: the mechanical behavior of solution-treated Ti-33Nb-4Sn alloy is different during the cyclic loading.“Double yield” can be observed in the first cycle stress-strain curve and part of strain is remained after unloading.This is attributed to intense SIM occurred during loading and most of SIM would not reverse.In the second cycle,the stress-strain curves of loading and unloading are linear.Linear elastic deformation of ST Ti-33Nb-4Sn alloy can be mainly attributed to slightly martensitic transformation owing to high stability of residual ? phase after the first cycle of loading-unloading.Cold-rolled Ti-33Nb-4Sn with 2% predeformation shows linear elastic deformation due to the slight SIM transformation occurred in a wide strain.Based on the above experimental results,we proposed that by the retard of martensitic transformation achieved by grain refinement and high-density dislocations,a high strength and low elastic modulus can be obtained in the(?+?")titanium alloys which was not considered that have a good engineering application.Therefore,we try on binary Ti-36 Nb alloy which is convenient for production and application in industry.Cold-drawn Ti-36 Nb after the first cycle loading and unloading performs linear elastic deformation.The experimental result of tensile test shows a value of ~890 MPa for strength and a value of ~2.4% for elastic strain which prior to other titanium alloy. |
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