Microstructures and mechanical properties of Ni 3 Al alloyed with iron additions
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I.
INTRODUCTION
N I C K E L aluminide, Ni3A1, is an ordered intermetallic alloy with attractive properties for high-temperature structural applications. Unlike most alloys, the yield stress of Ni3A1 actually increases with an increase in temperature. Recent efforts have alleviated the inherent grain boundary brittleness by microalloying with boron.l'2 Alloys with a substoichiometric aluminum level ( - 24 at. pct) and boron levels of 0.24 at. pct exhibit room-temperature ductilities of up to 50 pct.3 Further alloying additions can be used to increase the high-temperature yield stress. The substitutional behavior (for Ni, A1, or both) of ternary additions has been established from ternary phase diagrams 4'5 based on solubility of the ternary addition in the Lie phase. Iron was shown to substitute for both nickel and aluminum. Hardening effects as a function of lattice misfit strain for several ternary additions have also been reported. 4'6 Although some elements other than iron show a greater solidsolution hardening effect at room temperature, 4 iron has the advantage of having a wide solubility range in Ni~A1. Sufficient iron additions to Ni3AI were shown to result in partial transgranular failure instead of complete intergranular failure and an increase in ductility during room-temperature bend tests. 7 In the present investigation, tensile properties were determined as a function of temperature for various levels of iron additions. The solubility limit of iron in Ni3A1 was established, and microstructural features that develop in connection with the precipitation of iron-enriched phases were characterized by transmission electron microscopy (TEM) and by optical microscopy. Alloying with minor amounts of titanium, manganese, and carbon was used for further property improvements.
II.
EXPERIMENTAL PROCEDURE
Aluminide alloys used in this investigation were prepared by arc melting and drop casting from pure metals, pyrolytic carbon, and a master alloy of Ni-4 wt pct B. The drop cast-
J.A. HORTON, C.T. LIU, and M.L. SANTELLA are Members of the Research Staff, Oak Ridge National Laboratory, Oak Ridge, TN 37831-6376. Manuscript submitted August 26, 1986. METALLURGICAL TRANSACTIONS A
ing technique was used to refine the ingot grain structure and reduce compositional segregation during solidification. Preliminary studies indicated that the aluminides could be best fabricated by cold rolling rather than by hot fabrication. Consequently, 400-g alloy ingots were homogenized for 5 hours at 1000 ~ and then were fabricated into sheet stock by repeated rolling at room temperature with intermediate anneals at 1000 to 1050 ~ The cold work, initially a 10 to 15 pct reduction of thickness, was gradually increased to 20 to 25 pct between successive intermediate anneals. The resulting sheet stock was then annealed for 1 hour at 1050 ~ to induce recrystallization. Chemical compositions of selected aluminides were determined by volumetric and gravimetric analyses of major elements, spark source mass spectroscopy of trace impurities
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