Investigation of a directionally solidified NiAl/Cr(Mo) composite alloyed with Hf
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9/27/03
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Investigation of a Directionally Solidified NiAl/Cr(Mo) Composite Alloyed with Hf C.Y. CUI, J.T. GUO, Y.H. QI, and H.Q. YE A NiAl/Cr(Mo) composite alloyed with Hf was directionally solidified under a Ar atmosphere in Al2O3-SiO2 mold by the standard Bridgman technique. The as-grown composite was composed of NiAl matrix, lamellar Cr(Mo), and semicontinuously distributed Ni2AlHf. The G phase (Ni16Hf6Si7) formed in the presence of Si, which was a contamination resulting from the ceramic mold. The interfaces were atomically flat with no transition layer. After hot isostatic pressing (hipping) and aging treatment, the G phase completely disappeared and the amount of intercellular Ni2AlHf phase was partially reduced. The compressive yield stress and tensile strength of the composite were higher than those of many NiAl-based alloys. Embrittlement existed at high temperatures, induced by hipping and aging. The effect of the G phase and Heusler phase on the mechanical property of the composite was also discussed.
I. INTRODUCTION
IN polycrystalline NiAl form, conventional NiAl alloys do not possess adequate strength to be useful at elevated temperature which is necessary to compete with Ni-based superalloys,[1,2] and there is little interest today in continuous fiberreinforced NiAl-based composites due to a number of presently irresolvable technique barriers.[3] However, there is a NiAlbased eutectic alloy system that has been actively investigated for potential use in niche high-temperature applications.[4] The idea of reinforcing intermetallics with in-situ refractory metal by directional solidification (DS) of eutectic alloys was actively pursued in the early 1970s.[4] Recently, there has been renewed interest in developing intermetallic-based DS eutectics for high-temperature structural applications.[5,6,7] This is primarily motivated by the advancement of processing technology, which has led to better control of processing parameters. The well-aligned structure can provide significant improvement in strength and toughness than other NiAl-based alloys. Another advantage of the DS eutectics is the natural chemical compatibility between the matrix and reinforcement. A small addition of refractory metal from IVB, in particular, Hf, was found to be very effective in improving hightemperature creep strength of single crystals,[8] and there are considerable data that suggest that the desired creep strength be achieved.[9] These elements may provide strengthening by solid-solution effects and precipitate hardening. The latter is believed to due to the nucleation of G phase from Si contamination, which, after aging at elevated temperature, may be replaced by the Heusler phase.[10] Because Si was not an intentional alloying addition, there is little literature on the effect of Si on the mechanical property. Although Hf was a very effective solid solution strengthening element for NiAl single crystal at elevated temperature, little effort has been made to study its strengthening effect on the promisin
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