Microstructure-properties Tailoring Mechanisms For Si-alloyed Biomedical β-type TiNbZrTa Alloy Manufactured By Selective Laser Melting

As a medical implant material,β-type titanium alloy has the advantages of low elastic modulus,no toxic elements,excellent corrosion resistance and biocompatibility.However,the yield strength is relatively low due to stress-induced phase transformation and stress-induced twinning.Therefore,it is easy to yield and cause displacement or even tearing between implant and bone tissue.High-performance medical titanium alloys processed by traditional manufacturing methods often sacrifice plasticity or elastic modulus,and it is impossible to prepare implants that can adapt to the regulation of the precise modulus under the complex stress environment of the human body.As a bioactive element,Si can improve the biocompatibility and strengthening alloys.In this work,a high-strength,Si-alloyed,medicalβ-Ti alloy with controllable low-modulus was prepared by selective laser melting（SLM）,and its strengthening strategy and elastic modulus regulation strategy were explored.First,taking(Ti_69.71Nb_23.72Zr_4.83Ta_1.74)_100-xSi_x（x=0,3,5 at%）（named as TNZT,TNZT-Si₃and TNZT-Si₅respectively）alloys as the research objects,the influence of SLM parameters on the track morphology and cross-section geometry of melt pool in three alloys was studied through single-track experiments,then predicting the forming-process-windows together with the keyhole threshold.The results show that the mode of melt pool changes from the conduction mode to the keyhole mode with the increase of Si content,together with the decrease of the keyhole threshold,increase of the keyhole formation trend,and decrease of the range of the optimal process parameter.Based on the optimized process parameter range,the TNZT alloy was strengthened by two strategies:defect engineering and Si alloying.The results show that the defect engineering strategy of introducing high-density dislocations and{112}?111?twins through both high power and high speed can strengthens TNZT alloy,exhibiting yield strength（816±26 MPa）and fracture strain（16.5±1.8%）,superior to most conventionally fabricated Ti Nb Zr Ta alloys.Quantitative analysis shows that the contribution of dislocation strengthening and twinning strengthening to yield strength is 58%and 26.1%,respectively.Another way to strengthen TNZT alloy is introducing Si element.A high compressive yield strength of 1100-1300MPa and a fracture strain of 40%-50%could be obtained,which is better than mostβ-type titanium alloys produced by traditional process and selective electron beam melting.The strengthening mechanism analysis shows that the high strength is mainly attributable to fine-grain strengthening,second-phase strengthening and solid-solution strengthening.Second,taking optimized TNZT-Si₃ alloy as the research objects,the metastable（Ti,Nb,Zr）₅Si₃（S1）phase in TNZT-Si₃ alloy was trapped and precipitated by SLM,and then the granular stable（Ti,Nb,Zr）₂Si（S2）phase was obtained by spheroidizing heat treatment,thereby excellent performances.The results show that the thin-shell intergranular metastable S1 phase gradually becomes granular with the increase of solution temperature.As a result,the TNZT-Si₃ alloy exhibits the best mechanical properties（σ_s=978MPa,σ_f=1010MPa,σ_f=1010MPa,ε_f=10.2%,E=64.9 GPa）,which is better than that of the SLM-fabricated TNZT-Si₃ alloy and most 3D-printingβ-type titanium alloys.The analysis of strengthening mechanism shows that its high strength comes from fine grain strengthening,dislocation strengthening and precipitation strengthening.Subsequent aging treatment of the TNZT-Si₃ alloy revealed two new silicide precipitates morphologies:（1）A shell-like S2 phase transformed by the reticular supersaturated silicon-containingβ-Ti;（2）neonatal short rod-like S2 phase precipitate on the preexisting dot-shaped S2 phase.The former originates from the segregation of solute atoms to dislocation walls or microscopic bands,and the latter is due to the defect driving force provided by the large microscopic strain at the phase boundary betweenβ-Ti and dot-shaped S2 phase.Third,taking TNZT alloy as the research object,the customization of<001>orientations in different three-dimensional（3D）directions are realized based on the strategy of multi-tracks coupled directional solidification（MTCDS,the regular stacking of the melt pool promotes continuous epitaxial growth to achieve<001>orientation directional solidification）.The results show that the melt pool angle and the growth angle of columnar grains are approximately linearly related to the scanning speed.With the increase of the scanning speed,the<001>orientation of TNZT alloy gradually deflects toward the building direction,achieving the<001>orientation along arbitrary 3D direction.The stable<001>orientation is derived from the parallel and stable stacking of grains at the boundary of the melt pool with similar temperature gradients.The elastic modulus of the TNZT alloy along the growth direction of the columnar grains is basically same（about 60 GPa）via impulse excitation of vibration.On the contrast,the elastic modulus along the building direction gradually increased with the decrease of the scanning speed,which were 63.2±1.2 GPa（1400 mm/s）,65.7±2.1 GPa（1000 mm/s）,68±1.5 GPa（800 mm/s）,and 74±2.3 GPa（600 mm/s）,respectively.It demonstrates the feasibility of tailoring the elastic modulus by the MTCDS method.Finally,the MTCDS method was applied to the TNZT-Si₃ alloy.Since Si can refine grains and weaken texture,it is impossible to strictly tailor<001>orientation by a single variable（such as scanning speed）.Fortunately,with appropriate laser power and scanning speed,such as 250 W-800 mm/s（E_c=55.2±1.2 GPa）,250 W-1000 mm/s（E_c=49.7±2.0 GPa）,250W-1200 mm/s（E_c=52.7±1.9 GPa）and 100 W-100 mm/s（E_c=46.3±1.5GPa）,similar tailoring<001>orientation can be achieved.Finally,the biocompatibility of the TNZT-Si₃ alloy was evaluated.Taking the SLM-fabricated Ti-6Al-4V,TNZT and solution-treatment TNZT-Si₃ alloy as research object,the electrochemical corrosion results in phosphate buffer saline solution show that the corrosion current density i_corr and passivation current density i _P of TNZT-Si₃ alloy are lower than those of Ti-6Al-4V and TNZT alloy,indicating that the corrosion resistance of TNZT-Si₃ alloy is slightly better.Nyquist diagram shows that TNZT-Si₃ and Ti-6Al-4V passivation film impedance value is close,slightly better than TNZT alloy.The results of in vitro cell experiments showed that three alloys had low cytotoxicity.In addition,the cell adhesion of TNZT and TNZT-Si₃ alloys was better than that of Ti-6Al-4V.The cell proliferation of TNZT and TNZT-Si₃ alloy was significantly better than that of Ti-6Al-4V in 5th day of cell seeding.In summary,the strategies to predict the SLM-parameter window based on keyhole threshold and melt pool geometry,strengthening TNZT alloy based on defects engineering and Si alloying,strengthening and toughening of TNZT-Si₃ alloy based on the trap of metastable silicon-containing phase,tailoring<001>orientation along arbitrary 3D direction based on the MTCDS method were proposed.This provides valuable theory and reference significance for theβ-type alloys design and the tailoring of microstructure,mechanical properties and elastic modulus,which are expected to achieve a new generation of medicalβ-type Ti alloys with high strength,controllable low-elastic modulus,excellent corrosion resistance and biocompatibility.