材料期刊网

The oxidation of four Ni-10Cr-ySi-4Al alloys has been studied in 1 atm O(2) at 900 and 1000 degrees C to examine the effects of various Si additions on the behavior of the ternary alloy Ni-10Cr-4Al, which during an initial stage formed external NiO scales associated with an internal oxidation of Cr + Al, later replaced by the growth of a chromia layer at the base of the scale plus an internal oxidation of Al. The addition of 2 at.% Si was able to prevent the oxidation of nickel already from the start of the test, but was insufficient to form external alumina scales at 1000 degrees C, while at 900 degrees C alumina formed only over a fraction of the alloy surface. At 1000 degrees C the addition of 4 at.% Si produced external chromia scales plus a region of internal oxidation of Al and Si, a scaling mode which formed over a fraction of the alloy surface in combination with alumina scales also by oxidation at 900 degrees C. Conversely, the presence of about 6 at.% Si produced external alumina scales over the whole sample surface at 900 degrees C, but only over about 60% of the alloy surface at 1000 degrees C. The changes in the oxidation modes of the ternary Ni-10Cr-4Al alloy produced by Si additions have been interpreted by extending to these quaternary alloys the mechanism of the third-element effect based on the attainment of the critical volume fraction of internal oxides needed for the transition to the external oxidation of the most-reactive-alloy component, already proposed for ternary alloys.