The Effect of Mn Addition on Oxidation Resistance of Ti-48Al Synthesized by Mechanical Alloying

Titanium aluminide and its alloys have been utilized in automotive, aerospace and power industries because of their specific properties such as low density (~ 4 g/cm3), high melting temperature (~ 1460 °C) and excellent high-temperature strength. Ti-48Al intermetallic compound and its alloys with 1.5, 3 and 5 at.%Mn were produced with mechanical alloying of elemental powders for 50 hours. The process of mechanical alloying was carried out in a planetary ball mill machine with WC cups and balls under inert atmosphere of argon. To obtain bulk samples, the mechanically alloyed powders were cold pressed with the load of 40 ton and then heat treated at 1050 °C under argon atmosphere in an electrical furnace with quartz tubes. The X-ray diffraction analysis showed that the addition of manganese to Ti-48Al during mechanical alloying causes no formation of a new phase in titanium aluminide system. Scanning electron microscopy observations on the surface of sintered bulk samples revealed that boundaries between powder particles have been well formed at 1050 °C. The results of hardness tests showed that the addition of Mn up to 5 at.% increases the hardness value of Ti-48Al compound (~22.1 HRC) to ~ 40.7 HRC. Evaluation of oxidation behavior showed that oxidation resistance of Ti-48Al-5Mn alloy is slightly smaller (~0.015 g) than that of Ti-48Al. On the contrary to Ti-48Al-5Mn alloy, Ti-48Al compound had a steady-state oxidation behavior for oxidation periods longer than 14000 seconds. Scanning electron microscopy images and the results of energy dispersive spectroscopy showed that the lower oxidation resistance of Ti-48Al-5Mn compared to that of Ti-48Al is related to the formation of rutile at 900 °C on the oxidized surfaces of Ti-48Al-5Mn.