The effect of dispersed oxides on the oxidation behavior of Al-and Ti-Containing Ni-20 pct Cr alloys
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IT has
been established~ that the incorporation of certain dispersed oxides in a Ni-20Cr* m a t r i x ira*All compositions are expressed in terms ofweight percent.
proves both high temperature (ll00°C) c r e e p r e s i s t ance and oxidation resistance. However, such alloys exhibit relatively low strength at intermediate temperatures (e.g. 760°C). The addition of aluminum and titanium g i v e s rise t o the formation of the ~/ precipitate [Ni3 (A1, Ti)] which improves the intermediate temperature strength.2 The present investiTation was undertaken to examine the effect of dispersed oxide additions on the oxidation behavior of Ni-20Cr alloys containing sufficient Al and Ti to form
EXPERIMENTAL The dispersion strengthened alloys were prepared u s i n g the mechanical alloying technique as described elsewhere. 1'3 A n u m b e r of Ni-20Cr alloys were prepared containing different amounts of aluminum and titanium and also different types and amounts of dispersoid. The compositions of the alloys prepared are listed in Table I. Included in this table is a conventionally cast and wrought alloy (Ni-20Cr-I.6AI2.4Ti) which was laboratory melted and used as an experimental control material. As shown in T a b l e I, the alloys falt into two groups, those with lower aluminum plus titanium contents (i.e., ~-'0.5A1 + 0.5Ti) and those with h i g h e r aluminum plus titanium contents (i.e., ~l.6A1 + 2.4Ti). Typical g r a i n dimensions of the alloys a r e listed in Table II. The alloys prepared by the mechanical alloying technique had elongated g r a i n structures. A typical example is shown in Fig. 1. The cast and wrought control m a t e r i a l had equiaxed g r a i n s . The dispersoids were uniformly distributed in the mechanically alloyed materials. A thin foil electron micrograph illustrating the dispersoid distribution and size which r a n g e d from 10 to 50 nm in the 0.6 pet Y203 alloy is given in Fig. 2. Alloys having h i g h e r dispersoid contents also displayed a uniform but h i g h e r density distribution of s i m i l a r sized particles. HAROLD T. MICHELS is with The InternationalNickel Company, Inc., One New York Plaza, New York,NY 10004. Manuscript submitted February 2, 1976. METALLURGICAL TRANSACTIONS A
Isothermal oxidation tests were performed in a Cahn RH microbalance and the cyclic oxidation tests were c a r r i e d out in a resistance heated tube furnace. In the latter t e s t , specimens were removed from the tube furnace e v e r y 24 h, cooled, weighed, and r e turned t o temperature. The specimen preparation and test procedures used in this study were the same as those described previously.1 Dry air was utilized as an experimental atmosphere in both the isothermal and cyclic oxidation tests. RESULTS A N D D I S C U S S I O N In the 100 h isothermal exposures, the alloys exhibited parabolic oxide growth. This tended t o shift t o a l i n e a r relationship a f t e r longer t i m e s , especially at higher temperatures and in the h i g h e r A1- and Ti-containing alloys. This is shown in F i g s . 3(a),
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