放電プラズマ焼結チタン合金の耐食性に及ぼす焼結温度の影響

Abstract

The present study was conducted in order to analyze the effects of sintering temperature on the microstructure and corrosion resistance of Ti-M (M=Ru, Rh, Pd, Ir, Pt) alloys prepared from titanium and M powders using spark plasma sintering (SPS). In addition, the practicality of the Ti-Ru alloy as a low-cost Ti alloy was discussed. The specimens were prepared at SPS temperatures of 800, 1100 and 1500°C. The microstructure of specimens were investigated using scanning electron microscopy (SEM) and X-ray diffraction. The corrosion resistances of specimens were determined by potentiodynamic polarization measurement at a scanning speed of 0.124 V/min using a 5% HCl solution open to air at 25°C. Measurement was performed at a specimen potential of from −1.0 to 2.0 V and the potential approached noble.At a low SPS temperature, SPS alloys show a particle dispersion microstructure, in which the added element phases and intermetallic compound phases are dispersed in the Ti matrix. These phases affect the corrosion resistance of the alloys. At a high SPS temperature, the added elements and compounds are diffused in the Ti matrix, causing the alloy to approach a solid solution microstructure and increasing the formation of metastable βTi. In particular, at a high SPS temperature, in the Ti-Pt alloy increases slightly the amount of βTi, whereas increases greatly the amount of Ti3Pt. High SPS temperatures reduce the added element phases that serve as the cathodic pole, thus diminishing the cathodic current. In the anodic area, the reduction or dispersion of compound phases reduces the compound active region, thus diminishing the anode current.Furthermore, high SPS temperatures produce similar polarization curves among Ti-M alloys, and provide SPS alloys having the same corrosion resistance equivalent to that of arc-melted alloys. Therefore, no problem is associated with the replacement of Pd with Ru.