Effects of B, C, and Zr on the structure and properties of a P/M nickel base superalloy
- PDF / 2,738,586 Bytes
- 9 Pages / 594 x 774 pts Page_size
- 67 Downloads / 246 Views
I.
INTRODUCTION
THE importance
of minor element additions to the mechanical properties of polycrystalline nickel base superalloys is well established. Certain elements, such as carbon, boron, and zirconium, are reported to significantly improve creep life, rupture strength, and tensile ductility at elevated temperatures, t'2'3 Carbon additions are reported to form grain boundary precipitates, which when present as discrete particles, are believed to pin grain boundaries and thereby inhibit grain boundary sliding. 4 Boron and zirconium additions are cited for a variety of strengthening mechanisms including (1) decreased grain boundary diffusivity, (2) increased grain boundary interfacial strength, (3) lowering of grain boundary surface energy, (4) removal of tramp elements by precipitating them as stable compounds (e.g., Zr2S), and (5) by creating changes in fine y' or M23C6 carbide morphologies. 1,5,7 Despite the obvious importance of minor element additions, the actual mechanisms by which these elements improve mechanical properties remain unclear. With the advent of P/M processing and the advanced alloy compositions used for gas turbine disk applications, the role of minor element additions and their effect on grain boundary strength becomes increasingly important. This is due to the stringent strength requirements of disk alloys which require fine grain sizes for Hall-Petch strengthening, combined with time dependent mechanical properties such as creep, fatigue crack growth, and stress-rupture strength, which often exhibit intergranular failure modes. Although the advent of single crystal superalloys for turbine blade applications has lessened the impetus for understanding grain boundary structure and chemistry, grain boundaries remain crucial to T.J. GAROSSHEN, Research Metallurgist, and G.P. McCARTHY, Electron Microscopist, are with Materials Technology Department, United Technologies Research Center, East Hartford, CT 06108. T. D. TILLMAN is Materials Engineer with Pratt and Whitney Aircraft Company, West Palm Bea~ch, FL. Manuscript submitted February 19, 1986.
METALLURGICALTRANSACTIONSA
the strength and performance of P/M nickel base superalloys. Further understanding and optimization of grain boundary chemistry in these alloys is therefore warranted. The studies reported in this paper compare the properties and structure of a P/M superalloy in which the boron and carbon levels were systematically varied. The roles of the various minor chemistry modifications are discussed in terms of observed microstructural changes and associated strengthening mechanisms.
II.
EXPERIMENTAL PROCEDURE
The compositions of the alloys analyzed in this study are listed in Table I. The alloys were prepared from the same master heat with the exception of alloy number 4. Atomization was performed by Homogeneous Metals, Inc., Clayville, New York, and the minor elements were added as late additions to the melt prior to atomization. The powder was processed under inert atmospheres, hot compacted at 1010 ~ and extruded at 1080 ~
Data Loading...
