| Metastable β-type titanium alloys consisting of non-cytotoxic elements have become the most attractive biomedical metal materials due to its excellent biocompatibility,good superelasticity and shape memory effect and low elastic modulus.By adjusting the content of β-stabilizers,the metastable β-type Ti alloys can exhibit single α′′ martensite or single β phase upon quenching from high temperatureβ-phase field.The former is expected to possess shape memory effect,while the latter is expected to exhibit superelasticity.When the content of β-stabilizers is between the two cases mentioned above,a dual-phase structure with β and α" phases is obtained in the metastable β-type Ti alloys,where both shape memory effect and superelasticity are imperfect.However,the metastable β-type Ti alloys containing a mixture of β and α′′phases has the potential to achieve low elastic modules and large linear elastic deformation.In this dissertation,the microstructure and mechanical behavior of metastable β-type Ti-41 Nb alloy containing(β+α′′)dual phases were investigated by combining results from X-ray diffraction,optical microscope,tensile test and in situ synchrotron X-ray diffraction.The main researches are summarized as follows:After solution treated at 800℃ plus water quenching,Ti-41 Nb alloy(referred to as ST Ti-41 Nb alloy henceforth)presents a typical dual-phase structure,where a large amount of β parent phase accompanied by a small amount of α" martensite was formed.EBSD results shows that a large amount of {001}<110> recrystallized texture is formed in the ST Ti-41 Nb alloy.The ST Ti-41 Nb alloy exhibits typical “double yielding”deformation behavior during single loading.The tensile test and SXRD test results indicate that the stress induced martensitic transformation,together with elastic deformation,takes place concurrently in the ST Ti-41 Nb alloy during 0.0-4.0% strain range.The stress induced martensitic transformation from β to α″ takes place intensively in the strain range from 0.0% to 3.3% but slightly from 3.3% to 4.0%.In the subsequent unloading process,the ST Ti-41 Nb alloy undergoes elastic recovery,accompanied by the slight reverse martensitic transformation from α″ to β.After unloading,a large amount of α″ martensite has not been transformed into β parent phase,which results in a residual strain of 2.9%.The cyclic loading-unloading deformation results of the ST Ti-41 Nb alloy indicate that with the increase of the number of cycles,the ST Ti-41 Nb alloy gradually exhibits near linear elastic deformation,which may be ascribed to the auxiliary effect of residual stress on stress-induced martensitic transformation.Previous studies have shown that cold deformation treatment can significantly affect the microstructure and mechanical behavior of β-type titanium alloy.Therefore,the cold rolling and cold drawing treatments were performed on the ST Ti-41 Nb alloy(referred to as CR and CD Ti-41 Nb alloy henceforth).Compared with ST Ti-41 Nb alloy,the volume fraction of the β parent phase in the CR Ti-41 Nb alloy has greatly reduced,which indicates that the stress-induced martensitic transformation has occurred during the cold rolling process.The microstructure observations of the CR alloy shows that dense shear bands and high-density dislocations was formed.The CR Ti–41Nb alloy exhibits nonlinear elastic deformation rather than double yielding during single loading,which is attributed to stress-induced martensitic transformation suppressed by dislocations introduced by cold rolling.The cyclic loading-unloading deformation results of the CR Ti-41 Nb alloy indicate that the CR Ti-41 Nb alloy gradually exhibits near linear elastic deformation.Compared with the CR Ti-41 Nb alloy,the volume fraction of β parent phase in the CD Ti-41 Nb alloy has been further reduced.The metallographic images of CD Ti-41 Nb alloy show a slender fiber structure is formed in the longitudinal section,and curled grains is formed in the cross section.The cyclic loading-unloading deformation results of the CD Ti-41 Nb alloy indicate that the CD Ti-41 Nb alloy gradually exhibits near linear elastic deformation with the increase of the number of cycles,which may be attributed to the combined effect of elastic deformation and slight stress-induced martensitic transformation occurring in a wide stress range. |
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