OXIDATION BEHAVIOR OF 3 COMMERCIAL ODS ALLOYS AT 1200-DEGREES-C

The isothermal-oxidation behavior of three oxide-dispersion-strengthen ed (ODS) alloys, viz., MA 956, ODM 751, and PM 2000, has been examined in air at 1200 degrees C for exposure times up to 4800 hr. During exp osure all the alloys formed an external scale of alpha alumina (alpha- Al2O3). The growth rate of alumina on MA 956 was significantly faster than that formed on ODM 751 resulting in an oxide layer which was abou t twice as thick after 4800 hr. The oxide-grain morphology on MA 956 w as essentially equiaxed containing irregularly shaped, titanium-rich p articles, whereas the oxide formed on ODM 751 was slightly finer, dist inctly columnar and contained elongated yttrium-rich particles. Spalli ng of the oxide layer occurred after approximately 2400 hr on MA 956, whereas only slight spalling occurred on ODM 751 even after the longes t exposure time. Experiments revealed that the initial surface roughne ss of PM 2000 can contribute significantly to spalling by enabling the growth of highly convoluted scale layers which are mechanically unsta ble under compressive stresses (buckling). Internal porosity is also o bserved in all three alloys after exposure. The pores were generally s pherical with small Ti-, Al-, Y-rich particles distributed over their internal surfaces. The amount of porosity increases to a maximum and t hen slowly decreases.