Hydrogen Embrittlement of Ni 3 Al at Various Hydrogen Contents and Strain Rates
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HYDROGEN EMBRITTLEMENT OF Ni3 Al AT VARIOUS HYDROGEN CONTENTS AND STRAIN RATES HUAXIN LI AND T. K. CHAKI State University of New York, Department of Mechanical and Aerospace Engineering, Buffalo, NY 14260 ABSTRACT Hydrogen embrittlement has been studied in continuous cast sheet of a Ni 3 AI alloy (Ni 77.83Al21. 73Zro. 34Bo.1), known as IC-50, after introducing various amounts of hydrogen cathodically. The elongation and UTS decreased with the increasing content of hydrogen. When tensile-tested at a strain rate of 5.8 x 10-' s-1, the elongation decreased from 32.7% for no charging to 1.9% for 330 min of charging with 50 mA cm- 2 current. The yield stress, however, did not change. When tested at a higher strain rate of 5.8 x 10-3 S-1, the embrittlement was less, but the yield stress increased with the hydrogen content. With increasing hydrogen content the fracture mode changed from dimpled to intergranular and cleavage modes. INTRODUCTION Nickel aluminide, Ni 3Al, an L1 2 compound, has attractive mechanical properties. The yield strength of single crystals of Ni 3AI increases with temperature [1], but polycrystalline material is severely brittle and fractures intergranularly [2]. In 1979 Aoki and Izumi [3] showed that alloying with a small amount of boron (a few hundred p.p.m.) increases room temperature ductility in Ni-rich Ni 3AI dramatically. By Auger electron microscopic study, it has been shown [4] that B segregates at the grain boundaries of Ni3 Al. Liu et al. [4] argued that B atoms at the grain boundaries increase the cohesive strength of the boundaries. Chaki [5,6] has proposed that interstitial B atoms distort the directionality of Ni-Al bonds in Ni 3Al lattice and, as a result, the atoms can relax easily and fill up the cavities at the grain boundaries. This makes movement of dislocations easier within the grains as well as across the grain boundaries. However, boron-doped Ni 3AI again becomes brittle upon cathodic charging of hydrogen. Kuruvilla and Stoloff [7] reported a 70% decrease in tensile ductility in cathodically charged Ni-24 at.% Al-0.2% B specimens and fracture was intergranular. Bond et al. [8] claimed from their transmission electron microscope (TEM) observations that the dislocation pile-ups at the grain boundaries of Ni 3Al were similar with and without Mat. Res. Soc. Symp. Proc. Vol. 288. Q1993 Materials Research Society
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hydrogen, and argued that hydrogen embrittles the grain boundaries. Wan et al. [9] elongated cathodically charged Ni-23 at.% Al-120 wt. ppm B specimens inside a scanning electron microscope (SEM) and observed that the slip bands had difficulties in crossing the grain boundaries and that the cracks developed along boundaries. Here we present the results of hydrogen embrittlement in a ductile Ni 3Al alloy at various hydrogen contents and strain rates, and make a detailed fractographic analysis. The results will shed light on the mechanism of hydrogen embrittlement in Ni3 Al. EXPERIMENTAL PROCEDURE Ni 3 Al alloy used in this study contained by weight 11.30% Al, 0.60% Z
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