Effect Of Boron Addition On The Microstructure And Properties Of Ti-22Nb-6Zr Shape Memory Alloys

Ti-22Nb-6Zr (at.%) shape memory alloys, which are consisted of non-toxicbiocompatible elements, are considered as ideal load-bearing implants currently due to theirrelatively low Young’s modulus, stable superelasticity and good corrosion resistance. Thebioactivity of titanium alloys, however, is unsatisfactory when being used in bone repair andtissue engineering applications. Boron is one of the essential elements in human tissues withgreat bioactivity and plays an important role in the metabolism and formation of bone tissues.In this work, therefore, various amount (0.02~1.5wt.%) of bioactive boron element was addedby sintering into Ti-22Nb-6Zr alloys with the aim of improving their bioactivity. The effect ofboron concentration on the microstructure, mechanical properties, corrosion resistance andcell compatibility of sintered Ti-22Nb-6Zr alloys has been investigated systematically.Firstly, the Ti-22Nb-6Zr alloys with boron concentration ranging from0to1.5wt.%were successfully fabricated by sintering at1400℃for10h. The effect of boron addition onthe microstructure and phase constituents of the sintered Ti-22Nb-6Zr alloys was studiedusing optical microscope, XRD and SEM. The results show that the chemical compositions ofthe alloys are close to the nominal compositions. A small amount of boron addition inducesthe precipitation of phase and TiB phase in β matrix, which facilitates a distinct refinementof microstructure.Secondly, the effect of boron on the mechanical properties and corrosion resistance ofTi-22Nb-6Zr alloys was studied and the relationship between properties and microstructurewas evaluated. Mechanical properties were investigated by cyclic loading-unloadingcompression tests at room temperature and potentiodynamic polarization measurements werecarried out to evaluate corrosion behavior. The results show that the addition of boron exertsonly little influence on the Young’s modulus (around40GPa) and superelasticity (over3%),but affects strongly the compressive strength of sintered Ti-22Nb-6Zr-B alloys.Ti-22Nb-6Zr-B alloys have a high compressive strength (over1450MPa) and yield strength(over1250MPa) with boron concentration around0.02wt.%~0.1wt.%and1.0wt.%.Polarization curve analysis shows that the corrosion resistance of these Ti-22Nb-6Zr-B alloysis very good when boron content is about0.02~1.0wt.%. Finally, in vitro studies of the MC3T3-E1osteoblast on Ti-22Nb-6Zr-B alloys wereperformed to evaluate cell growth, proliferation and adhesion of MC3T3-E1osteoblast. It isfound that the toxicity degree of all the Ti-22Nb-6Zr-B alloys is around grade0~1, meaningthat all the experimental alloys meet the requirements of clinical application. The addition ofboron to the Ti-22Nb-6Zr alloys is efficient to promote cell proliferation and adhesion after1day,3days,7days culturing. The cell amount on Ti-22Nb-6Zr+1.0wt.%B alloy is higher byimmunofluorescence assay and found to be statistically different from other alloys, indicatinga better cell compatibility while the boron concentration is1.0wt.%.