High temperature strength and ductility of recrystallized Ni 3 Al-Ni 3 Mn alloys
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I.
INTRODUCTION
T H E Ll2-type intermetallic compound Ni3AI has unique mechanical properties at high temperatures. It has been shown that the yield stress increases with increasing temperature up to about 1000 K and this behavior can be explained using the Kear-Wilsdorf mechanism. 1 The increasing strength with increasing temperature is caused by the immobilization of dislocations when (ll0) screw dislocations undergo thermally activated cross slip from {111} planes to {100} planes, a process driven by the APB energy anisotropy on the two planes. In addition, Ni3A1 shows high oxidation resistance and distinctive creep properties. 2 In spite of these superior properties, its low ductility at low temperatures caused by the brittleness of the grain boundaries has prevented it from practical use as an engineering material. To improve the ductility, Aoki and Izumi added a small amount of boron 3 and increased the deformability at room temperature without lowering of the yield stress (indeed, B increased the strength). It was shown 4 that boron has a strong tendency to segregate to grain boundaries and thereby to enhance the grain boundary cohesion. Other methods have been used to ductilize polycrystalline Ni3AI: rapid solidification, 5 dynamic recrystallization, 6 and recrystallization of a single crystal. 7 Although these methods succeeded in improving the room temperature ductility, the ductility at elevated temperatures (where Ni3A1 is particularly strong) remains low. The present authors proposed, based on an atomistic analysis of the grain boundary structure, that the substitution for aluminum atoms by atoms located near nickel in the periodic table should greatly improve the grain boundary strength. This was based on the idea that such a substitution creates a homogeneous bonding environment in the grain boundary region of the Ni3A1. As a consequence, the substitution of manganese atoms (the VIIa group in a periodic table) was proposed, s'9 Figure 1 shows a portion of the Ni3A1-Ni3Mn
pseudobinary phase diagram determined by the present authors. 1~ This figure indicates that the L12 (y') structure forms a continuous solid solution at low temperatures and in addition manganese atoms appear to substitute for aluminum atoms. Ni3Mn has the same L12 structure as Ni3A1 and transforms at 783 K to fcc. ]~ The deformation behavior of Ni3Mn has been explained in terms of the degree of ordering.12'~3'14 Based on the results of a study on as-cast materials, ~s it was concluded that the Ni3AI-Ni3Mn pseudobinary alloy is ductile and yet still shows a positive temperature dependence of the strength for a wide range of manganese concentrations. In addition, simultaneous increases of elongation and strength were found with increasing temperature.
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N. MASAHASHI, formerly Graduate Student, T
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