Rezensionen
Rezensionen
Francesco Depentori
Charakterisierung der lokalen Korrosionseigenschaften von lanthan- und neodymmodifizierten (alpha-beta) Titanlegierungen
Buchbesprechung
Titanium alloys are of great value in today´s industry. The most common alloy is Ti6AI4V with a share of around 50 %. Due to its high stiffness, machining of this alloy is difficult and expensive. Recently, so called "free-machining titanium alloys" have been developed to overcome this problem. These alloys are modified by rare earth metal addition. Addition of Lanthanum and Neodymium leads to the formation of mainly grain boundary oriented precipitates. These precipitates strongly change the corrosion properties in comparison to the same alloys without rare earth metal addition. The size of the precipitates usually lies in the micrometer range. Before technical use, the influence of these precipitates has to be clarified to be able to evaluate the long term stability of these alloys. The goal of this work was to gain a fundamental understanding of the corrosion properties of four different "freemachining titanium alloys".
The alloys investigated were: - Ti6A12Fe1 Mo0.9La0.5Cu with and without the addition of 0.3Si, containing precipitates of Lanthanum and Lanthanum-Copper - Ti6AI4V2Nd, containing precipitates of Neodymium- Ti6AI2V3Nb0.9La0.7Fe0.3Si, containing precipitates of Lanthanum In this work a combination of integral techniques were used, characterizing the corrosion behavior on areas of several square centimeters, and localized techniques, characterizing the corrosion behavior on a small scale. Electrochemical methods like potentiodynamic polarization and chronoamperometry helped to gain an understanding about the stability and oxidation behavior of Lanthanum and Neodymium particles.
It could be shown, that the oxidation of precipitates took place within a few hours. After oxidation particles could not be found on the surface when using ex-situ methods. These results were confirmed by scanning electron microscopy analysis. Furthermore, the composition of precipitates was characterized by electron microprobe analysis. By the use of electrochemical atomic force microscopy (EC-AFM) the oxidation process of particles could be analyzed in-situ. EC-AFM showed a two-step process. First, the particle is oxidized, which leads to a strong increase in volume. The volume increase indicates the formation of a rare earth metal oxide or hydroxide. Second, the oxidized precipitate is removed by electrolyte flow. The remaining holes found after oxidation did not show a significant contribution to the general corrosion behavior. After the dissolution of precipitates from the surface the alloys behave passive in NaCI solution. Ti6AI2V3Nb0.9La0. 7Fe0. 3Si showed the best corrosion properties compared to the other alloys.
