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An attempt was made to seal the porous silicate-based plasma electrolytic oxidation (PEO) layer on pure magnesium (Mg) with a biodegradable polymer, poly (L-lactide) (PLLA), to delay the localized degradation of magnesium-based implants in body fluid for better inservice mechanical integrity. Firstly, a silicate-based PEO coating on pure magnesium was performed using a pulsed constant current method. In order to seal the pores in the PEO layer, PLLA was coated using a two-step spin coating method. The performance of the PEO-PLLA Mg was evaluated using electrochemical impedance spectroscopy (EIS) and potentiodynamic polarization. The EIS results showed that the polarization resistance (R p) of the PEO-PLLA Mg was close to two orders of magnitude higher than that of the PEO Mg.
b-Phase stability is a key consideration for the design of b-titanium (b-Ti) alloys and subsequent heat treatment and/or thermo-mechanical processing. The concept of the Mo equivalence (Mo-Eq), proposed by Molchanova (Phase diagrams of titanium alloys, 1965), has been commonly used as a general guideline to gauge the stability of a b-Ti alloy. A critical literature review has shown that all four existing Mo-Eq expressions deviate substantially from experimental observations and the well-established d-electron theory in predicting the b-phase stability of Ti-Nb-Zr alloys. The reasons are that existing Mo-Eq expressions either completely neglect or significantly overestimate the b-stabilizing effect of Zr. In this study, a new Mo-Eq expression, i.e., (Mo-Eq) Ti-Nb-Zr = 0.238Nb (wt.%) + 0.11Zr (wt.%) + 0.97, has been defined for Ti-Nb-Zr alloys in order to properly address the b-stabilizing effect of Zr. This new Mo-Eq expression showed proven consistency with both experimental observations and the d-electron theory in predicting the b-phase stability of various Ti-Nb-Zr alloys. With necessary modifications, the approach developed is expected to be also applicable to the assessment of the b-phase stability in other Zr-containing Ti alloys.
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