Fabrication Of Porous Ti-7.5Nb-4Mo-2.5Sn Shape Memory Alloys And The Effects Of The Porous Structure On Their Superelasticity

The porous titanium alloys with non-toxic elements are potential to be used ashard-tissue implantations due to the porous structure similar to bones, the low elasticmodulus, good biocompatibility and mechanical compatibility. If the alloys are givenwith the superelasticity and shape memory performance, their service life will beincreased. However, the introduction of the pore structure in the shape memory alloyusually leads to a decline in its superelasticity and shape memory performance.Therefore, the further study on the effects and mechanism of the porous structure onthe microstructure and mechanical properties of the titanium base shape memoryalloys is beneficial to optimize the pore structure for the improvement of theirsuperelasticity and shape memory performance. In this thesis, a titanium shapememory alloy, Ti-7.5Nb-4Mo-2.5Sn, was taken as the research object. The porousalloys with different porosities and pore sizes were fabricated by using powdermetallurgical method, and the microstructure, mechanical properties, including thesuperelasticity, were investigated. Moreover, the mechanical properties andsuperelasticity of the alloy samples with and without geometric defects wereevaluated, aiming at investigating the effects of the geometric defects on thesuperelasticity. The full field strain distributions of the samples were monitoredduring the uni-axis loading-unloading tensile tests by using ARAMISthree-dimensional optical deformation measurement system. The effects of thegeometric defects on the strain field, the stress induced martensite phasetransformation and superelasticity were investigated. The main contents andconclusions are as follows.(1) The effects of porosity on the microstructure and mechanical properties of theporous Ti-7.5Nb-4Mo-2.5Sn shape memory alloys were investigated. The resultsindicated that the porosity has an important effect on the pore structure andmechanical properties of the porous alloy. With the increase of porosity, the pore sizeand the connectivity increase, but the distribution of the pores becomes heterogeneous.Accordingly, the elastic modulus, yield strength and compression strength of thealloys decrease. Under the cyclic loading and unloading, the alloys show thesuperelasticity, but no obvious elastic deformation stage of the β phase appears on thestress-strain curves and the superelasticity of the alloys decreases with the increasing of the properties.(2) The effect of pore sizes on the microstructure and mechanical properties ofthe porous Ti-7.5Nb-4Mo-2.5Sn shape memory alloys was investigated. The resultsindicated that all the porous alloys yield the identical porosity and irregular shape.The increase of the pore size changes the distribution of pore size and reduces theelastic modulus, yield strength and compression strength. The porous alloys canexhibit superelasticity, but no obvious the elastic deformation stage of the β phase canbe observed on the stress-strain curves of the alloys.(3) The effects of geometric defects on the superelasticity of aTi-6.5Nb-4Mo-2.5Sn shape memory alloy were investigated. The results indicatedthat the samples with geometric defects have the stress concentration close to thedefects on loading, in which a high strain rate and strain are measured. The stressinduced the martensite phase transformation is mainly influenced by the straindistributions, and the martensite phase transformation occurs in a localized area. Thus,the samples with the geometry defects exhibit a high critical stress induced themartensite transformation and a low superelasticity in comparison with the perfectsample.