Effects of C and Hf concentration on phase relations and microstructure of a wrought powder-metallurgy superalloy
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ALLOY
NASA IIB-11 was identified by Kent in a study of compositions intended t o provide advanced wrought nickel-base superalloys.1 It is a heavily alloyed composition containing about 55 wt pet of the ~' phase and high concentrations of refractory elements. In the form of 1.3 cm diam wrought bar produced from a cast ingot, IIB-11 exhibited excellent high-temperature strength and was considered a promising candidate for further development for gas turbine disks that could operate with rim temperat u r e s as high as 750°C. The study also indicated that the interaction between C and Hf concentrations was very important in determining strength in the alloys of that statistically designed experiment, suggesting that s t i l l g r e a t e r strength might be obtained from the IIB-11 composition by proper modification of its C and Hf concentrations. The effects of these variables on microstructure were also of interest since Hf was known to have a strong effect on y ' and carbide morphologies in cast alloys.2-~ Effects on the types and stabilities of the carbides f o r m e d were of particular interest. It was anticipated that any optimized alloy based on the IIB-11 composition would be produced by powder-metallurgy methods s i n c e l a r g e cast ingots of such a heavily alloyed composition would likely be difficult t o work. Carbide stability during powder consolidation at high temperat u r e s is necessary t o prevent formation of a carbide network at the p r i o r particle surfaces. Thus, a study was conducted of effects of C and Hf concentrations on the mechanical properties, m i c r o structures, and phase compositions in alloys b a s e d on the IIB-11 composition and produced from prealloyed powders.6 Only microstructures and phase compositions will be discussed h e r e , however.
was that of the low-C and tow-Hf modification (Alloy 1), which is otherwise the same as that of IIB-11 (Ref. 1). The aim composition and the vendor's analysis of the r e m e l t stock are g i v e n in Table I. The actual composition was very c l o s e t o the aim. Prealloyed powders of the five alloys were produced by the hydrogen atomization method.7 Alloy 1 powder was produced by d i r e c t atomization of the remelt stock. The other alloys were produced by adding C a n d / o r Hf t o the r e m e l t stock during melting for atomization. Analyses by atomic absorption spectroscopy on the hot isostaticatly compacted and c r o s s r o l l e d disks produced from the powders a r e presented in T a b l e I. Note that the oxygen concentrations of the alloys with Hf added (97, 195, and 136 ppm for Alloys 2, 4, and 5, respectively) were considerably h i g h e r than those of the essentially Hf-free alloys (~50 ppm). The additional oxygen in the alloys w i t h Hf added was most likely due t o an increased reaction with the oxide crucible m a t e r i a l d u r i n g melting for atomization. The alloy powders were compacted by hot isostatic pressing at 100 M P a and 1195 + 5°C for 2 h. The compacts were then reduced about 55 pct in thickness
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