Effect of Preliminary Firing on Oxidation of Mo–Mo₃Si Alloyed with Sc or Nd

The main tasks of studying molybdenum silicides for use at elevated temperature include development of materials with a melting point T_me ≥ 2000°C, possibly with a low brittle-ductile transition temperature and resistance to high temperature oxidation. The possibility is considered of increasing corrosion resistance of a model alloy consisting of molybdenum solid solution and Mo₃Si. The effect of Sc or Nd alloying additives, as well as preliminary high-temperature annealing. i.e., pre-oxidation in order to create a protective oxide layer on the alloy surface of a model alloy, is evaluated. Non-isothermal oxidation of alloys is performed using combined thermogravimetric and differential thermal analysis methods. The phase composition of the scale formed is determined by an X-ray diffraction method. As a result of these studies it is established that preliminary oxidation firing of Mo–Mo₃Si–REE (about 3 at.%) alloys at 1100°C for 5–10 min provides conditions for forming a layer of SiO₂ and REE molybdates Sc₂(MoO₄)₃ and Nd₂Si₂O₇ at the alloy surface. Oxidation in a dynamic regime for treated alloys in the range 500–850°C proceeds much more slowly than for the initial samples. This can be explained by the difficult access of oxygen to the reaction zone through a protective layer of non-volatile oxides. In addition, preliminary oxidation of Mo–Mo₃Si alloyed with Sc or Nd during subsequent oxidation leads to an increase in the transition temperature for the process from the stage of oxide formation to intense MoO₃ sublimation.