Microstructure And Properties Of Corrosion Resistant TiAl Based Alloys

TiAl-based alloys have been widely studied due to its low density(less than half of that of Ni-based alloys), high strength, high hardness, high creep strength at evaluated temperature, higher temperature oxidation resistance than titanium alloys, high resistance to corrosion. To date, less attention has been paid to the corrosion resistance of TiAl-based alloys in environment of seawater at room temperature. While if the use of TiAl-based alloys is extended to seawater environment, the corrosion resistance of TiAl-based alloys to seawater will then become very important. Thus, it's necessary to evaluate the corrosion resistance of TiAl-based alloys to eawater at room temperature.Based on the classical alloy Ti-47Al-2Cr-2Nb, this paper studies twelve different TiAl-based alloys by changing alloying elements of the former alloy. The microstructure of the prepared alloys is studied, so are the effects of alloying elements on the microstructure. EDX has been applied to study the distribution of the alloying elements. By using XR-D method, phase composition of the prepared alloys has been studied, mainly composited ofγ-TiAl andα2-Ti3Al. But when high content of alloying elements is added, new phases will emerge. Compression tests have been taken to study the mechanical properties of the prepared alloys and effects of different alloying elements on mechanical properties are summarized. Potential polarization curve method is used to obtain polarization curve of the prepared alloys, from which free-corrosion potential, passivity properties can be seen. Due to the passivation nature of the alloys, break potential and maintaining passivity current density are considered to evaluate the alloys'corrosion resistance and impact of alloying elements on their corrosion resistance. Electrochemical Impedance Spectroscopy (EIS) method has been taken, from which Bode plots and Nyquist plots have been obtained. Corrosion resistance and electrochemical properties of corrosion process are investigated. Equivalent circuit has been fitted by Zsimpwin soft-ware. And according to the circuit, electrochemical properties of corrosion process are studied. Surface morphology of the alloys after corrosion has been observed and pitting corrosion, the classical corrosion type of passivation metal, has occurred. Based on the experiments above, one alloy composition is chosen on the basis of its excellent mechanical and corrosion properties. Two different surface oxidation treating processes have been applied to the chosen alloy. Morphology and composition of alloys'surface are investigated. Electrochemical Impedance Spectroscopy (EIS) test has been taken. Bode plots and Nyquist plots have been studied to analyze the effect of treating process on alloys'corrosion resistance.