Phase Precipitations And Structural Stability Of β-Ti Alloys With Loe Young’s Moduli Based On A Cluster Model

Body-cenered-cubic (BCC) β-Ti alloys are expected to be much more widely used for implant materials in the biomedical fields because of their low Young’s moduli close to that of human bones, superior biocompatibility and corrosion resistance, compared with other materials. Various kinds of metastable phases, such as HCP-ω phase, orthorhombic martensite (α’and α") phases, are easy to be precipitated from β, which is stabilized at high temperature only, during the cooling process to room temperature. Since these precipitates influence the mechanical properties of P-Ti alloys, the present work aims to investigate the P structural stability and the second precipitations, as well as to establish the relationship between the Young’s modulus (E) and the P structural stability of β-Ti alloys. Series of low-E β-[(Mo,Sn)-(Ti,Zr)14]Nb1,2alloys were designed in ternary, quaternary, and quinary systems based on our cluster-plus-glue-atom model, where the occupations of alloying elements in the cluster model are determined by the mixing enthalpy between the alloying element with the base Ti.The designed [(Mo,Sn)-(Ti,Zr)14]Nb1,2alloy rods with diameters of3mm and6mm were prepared by a copper-mould suction-casting technique. These alloys were then solid-solutioned at950℃with water quenching. Alloy phase and microstructure were identified by the X-ray diffractometer (XRD) and the transmission electron microscope (TEM). Tensile properties at room temperature and corrosion-resistance in Hank’s simulated body fluid were measured by universal tensile testing machine(MTS) and electrochemical workstation, respectively.The experimental results show that the Young’s moduli of the suction-cast β-[(Mo,Sn)-(Ti,Zr)14]Nb1,2alloy series are not only related to the β structural stability that is characterized with the Mo equivalent Moeq, but also to the precipitated phase types. The Young’s moduli of β-Ti alloys increase with the Moeq values (indicating the β structural stability) monotonously. However, this tendency will befluctuated by the α" and ω precipitations, where a dramatic increase in E is generally induced from the ω phase. After solid-solution treatment, the solute atoms in alloys diffuse sufficiently, which makes the amount of precipitates decrease slightly. Thus, the Young’s moduli of P-Ti alloys are lower than the suction-cated ones. The β alloys own their low Young’s modulus and high strength to the lower Moeq values that are close to the lower limit for β formation. Among them, the quinary alloy [(Moo.5Sno.5)-(Ti13Zr1)]Nb1is located at the lower β limit with Moeq=8.1wt.%and possesses the lowest Youn’s modulus and higher strength (E=48GPa and σ=715MPa in suction-cast case,EIIT=43Gpa and σbHT=569MPa in solid-solution case). The micromorphology of low-E β-Ti alloys exhibits generally a mixed lamella and rod-shape microstructure. Furthermore, excessive amounts of Sn and Zr in the alloys will deteriorate the corrosion resistance in the electrochemical experiment, and the excellent corrosion resistance is obtained after the combined effect of the suitable addition of alloying elements.Besides, the quinary alloy [(Mo0.5Sn0.5)-(Ti13Zr1)]Nb1has excellentcorrosion resistance(Ecorr=-0.289V, Icorr=6.359μA/cm2), which is realized by the occupying of Mo0.5Sn0.5in cluster center and the low modulus elements Zr and Nb in cluster shell and glue atom sites, respectively.Therefore, it can be asa good substitute for biomedical implant materials.