The effects of tantalum on the microstructure of two polycrystalline nickel-base superalloys: B-1900 + Hf and MAR-M247
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I.
INTRODUCTION
T A N T A L U M is an alloying element used to improve the high temperature mechanical properties of nickel-base superalloys. For example, the addition of approximately 1 at. pct Ta to both directionally solidified and single crystal MAR-M247-type alloys increased their rupture life by a factor of 3 to 4 at 982 ~ at various stress levels. 1,2 Similar improvements in rupture strength as well as hot tensile properties have also been reported in other alloys as a result of Ta additions. 3'4 The strengthening increment that tantalum imparts to nickel-base superalloys may result from several "Ta effects" on the microstructure. Tantalum is a strong MC carbide forming element and its presence can improve carbide stability. 5 Tantalum has also been observed to partition to ~tl,3,6,7 which should result in an increase in the ~' volume fraction. Partitioning of tantalum to 3,' may also lead to improved strength through solid solution hardening 8 and, it has been suggested, an increase in the magnitude of the anti-phase boundary energy. 5 However, other indirect effects, which may be beneficial to mechanical behavior, can result from tantalum additions. For example, those elements replaced in the MC carbides on adding tantalum are released to the y and ~/' phases. Changes in the partitioning of various elements may also occur when tantalum is present in the alloy. Because of these direct and indirect effects on microstructure when tantalum is added, studies are needed in which tantalum is systematically replaced with nickel in *MAR-M is a trademark of Martin Marietta Company.
G. M. JANOWSKI, Graduate Student, R. W. HECKEL, Professor, and B.J. PLETKA, Associate Professor, are with the Department of Metallurgical Engineering, Michigan Technological University, Houghton, MI 49931. Manuscript submitted June 20, 1985.
METALLURGICALTRANSACTIONS A
order to understand the effects that tantalum has on the mechanical properties of nickel-base superalloys. In the present study, the effects of tantalum on the microstructure of two commercially-important, polycrystalline, nickel-base superalloys, B-1900 + Hf and MAR-M247, were examined. The study was conducted by replacing tantalum with nickel and monitoring the changes that occurred in the microstructure, including changes in phase compositions, as the tantalum level was varied. By examining the effect of tantalum variations in these two alloy series, which differ significantly only in their refractory metal additions (see Table I for nominal compositions), a more general understanding of the beneficial "tantalum effect" is possible. Since tantalum is also designated a strategic element (defined as an element obtained primarily from sources outside the United States), 9'1~this information also helps provide data that can be used in identifying potential alternate alloying elements for tantalum.
II.
EXPERIMENTAL PROCEDURE
Five conventionally cast (CC) and two directionally solidified (DS) MAR-M247-type alloys were vacuum induction melted and cast by Howmet Turbine
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