Carbon effects in rapidly solidified Ni 3 Al
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I. INTRODUCTION The interstitial element boron has been shown to have a dramatic effect on the room temperature tensile properties of polycrystalline Ni 3 Al. Aoki and Izumi 1 first found that small additions of boron substantially eliminated the intergranular brittleness problem encountered in the pure intermetallic. For example, a doping of only 0.5 at. % boron resulted in more than 30% tensile strain in arc-melted, homogenized Ni3 Al. Similar results were later obtained by other researchers using specimens prepared by thermomechanical processing2 and rapid solidification.3 The boron ductilization effect was attributed to boron increasing the grain boundary cohesive strength and allowing the deformation of grain interiors without premature intergranular failure.4'5 Subsequently, Huang, Chang, and Taub 6 found that boron also exhibited a large solid solution strengthening effect in rapidly solidified Ni 3 Al. Within the solubility limit of ~ 1.5 at. %, boron showed a strengthening potency of ~ 300 MPa/at. % of addition, which was significantly greater than that of substitutional elements.7 The large boron strengthening potency could be associated with the large lattice strain that it produced by occupying the interstitial lattice positions,6 in accordance with the Mott and Nabarro theory.8 The interstitial element carbon might also affect the Ni3 Al properties, but the available data in the technical literature are very limited. The effect of carbon on the mechanical properties of Ni 3 Al was first studied by Guard and Westbrook,9 who measured the same room temperature hardness of ~200 kg/mm 2 for this aluminide containing 0, 0.2, and 2.0 at. % carbon. The result indicated that carbon had little effect on the mechanical behavior of Ni 3 Al. The solubility of carbon in Ni3 Al was determined to be 5.8 at. %, 10 and it was extended to 7.8 at. % by rapid solidification.11 The above two papers did not deal with mechanical properties. The present paper reports the effects of carbon on the mechanical behavior of polycrystalline Ni 3 Al prepared by the melt-spinning technique. This rapid solidi60
J. Mater. Res. 1(1), Jan/Feb 1986
http://journals.cambridge.org
fication technique produces smooth ribbons suitable for bending and tensile tests at room temperature. The results show that carbon provides no ductilization to the Ni 3 Al, but exerts a large solid solution strengthening effect similar to boron.
II. EXPERIMENTAL Ingots of Ni 3 Al with various stoichiometric ratios and carbon additions were prepared by vacuum induction melting using high purity Ni, Al, and C. Additional ingots of Ni 3 Al containing both boron and carbon were also prepared using Ni, Al, NiB, and C. The ingots were processed into ribbon form (approximately 6 mm wide by 35 /urn thick) via melt spinning in vacuun. The solidliquid interface velocity during the ribbon solidification was estimated to be greater than 10 cm/s (see Ref. 12), which is equivalent to a ribbon cooling rate greater than 5 X 105 K/s. The resultant ribbon microstructure was studi
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