Synthesis And Properties Optimization Of Porous Ti-Nb Based Memory Alloys For Biomedical Application

Nickel-free Ti-based shape memory alloys(SMAs)have drawn increasing attention as biomedical materials in recent years due to their great biocompatibility and high recoverable strain.The porous Ti-based shape memory alloys inherit the excellent biocompatibility of the compact conterpart,and the porous structure also brings the advantages of lower elastic modulus(more matched with the elastic modulus of human bone tissue)and better bone tissue growth.However,the high porosity of Ti-based SMAs will decrease its strength and recovery strain.So far,the porosity of Ni-free Ti-based SMAs which can satisfy the requirements of recoverable strain and strength for human bone replacement is less than 40%(far below the porosity of the human bone).Therefore,it is of significance to explore Ti-based alloy system with higher recoverable strain and strength so as to compensate the recoverable strain and strength loss in higher porosity Ti-based SMAs.In this paper,the composition of sintered Nickel-free Ti-based shape memory alloys was adjusted and regulated with the attempt to slove the problem of low recoverable strain of the high porosity Ti-based SMAs.Based on Ti-13 Nb with high recoverable strain,Zr was added to explore Ti-Nb-Zr alloy system for better recoverable strain.Ti-Nb-Zr alloys(nearly dense)were fabricated by conventional powder metallurgy sintering method.Their microstructure,phase composition,phase transformation temperature,mechanical properties and recoverable strain were investigated by optical microscopy,scanning electron microscope,X-ray diffraction,differential scanning calorimetry and compression test.For another aim of improving the low strength of Ti-based memory alloy with high porosity,the idea of using SiC fiber to enhance Ti-based memory alloy was put forward.The primary dispersion process of SiC fibers in the metal powders was obtained,and Ti-13 Nb alloy reinforced with SiC fibers coated by BN was prepared by powder metallurgy sintering method.The phase composition,transformation temperature,interfacial binding and mechanical properties were investigated by X-ray diffraction,scanning electron microscopy,DSC and compression test.The effect of SiC fiber content on the compressive strength and recoverable strain of Ti-13 Nb memory alloy was primarily obtained.The results show that the thermoelastic martensitic transformation behavior was observed in the Ti-13Nb-(0~6)Zr alloys by DSC.With 1 at.% Zr increase,the Ms of Ti-13Nb-(0~6)Zr alloys decreased linearly by around 10°C.The Ms point of the sintered Ti-13Nb-2Zr alloy(18°C)was below and closest to room temperature and the recoverable strain of as high as 3.7% was obtained at room temperature.The recoverable strain of Ti-13Nb-4Zr and Ti-13Nb-6Zr alloys with near ? parent phase but much lower Ms temperature was only 3.1% and 2.8% at room temperature,respectively.The compression tests of Ti-13Nb-(4~6)Zr alloys at their Ms points were carried out.It was found that the sintered Ti-13Nb-(4~6)Zr alloys exhibited a remarkable recoverable strain of 4.4% at their Ms temperature,indicating that they have a intrinsic high recoverable strain.On the basis of the sintered Ti-13Nb-2Zr alloy with Ms point near room temperature,it was found that the near-? phase microstructure could not be obtained by adjusting Zr and Nb content.Therefore,the recoverable strain of the proposed Ti-(13-X)Nb-YZr alloys at room temperature did not improve as expected.The Ms point of the sintered Ti-12Nb-6Zr alloy(11°C)was below and closest to room temperature and a recoverable strain of 4.0% at room temperature was obtained in this alloy.The oxygen decreases greatly the Ms of Ti-based SMAs(around 160K/1 at.% O).In order to explore a higher recoverable strain,the oxygen content of the sintered Ti-13Nb-4Zr alloy was further controlled to regulate its Ms point.After oxygen regulation,the Ms point was slightly increased due to the slightly decreased oxygen content and its recoverable strain at room temperature was improved a little bit.It proves at least the feasibility of adjusting the Ms point of alloys to room temperature to obtain a higher recoverable strain at room temperature.The SiC fibers with mass fraction of 0~0.4% were dispersed in the alloy powder by pre hand-dispersion and powder mixing was done without steel balls.The Ti-13 Nb alloys with different SiC fiber content were fabricated by sintering.The results show that the SiC fiber has no visible effect on the phase transformation behavior of sintered SiC fiber reinforced Ti-13 Nb alloys.Due to the BN coating on the surface of SiC fibers,the interfacial reaction between SiC fiber and Ti-13 Nb matrix was effectively alleviated,and the mismatch of thermal expansion coefficient between SiC fibers and matrix was effectively mitigated.The addition of 0~0.4 at.% SiC fibers in the Ti-13 Nb alloys resulted in a reduction in the recoverable strain of the sintered Ti-13 Nb alloy by about 10%,the compressive strength of the alloys was increased by about 10% while the compressive strain of the alloys was obviously reduced.