An Explanation for the Environmental Sensitivity of Ni 3 Al
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ABSTRACT Secondary Ion Mass Spectrometry has been used to study the distribution of elements in and near grain boundaries in boron-free and boron-doped Ni 76A124 alloys with and without -220 wt. ppm of deuterium. In boron-free alloys, sulfur was distributed about the grain boundaries in both deuterium-free and deuterium-charged samples. The distribution of deuterium followed that of sulfur and was segregated to grain boundaries. In the boron-doped material, sulfur was not found at most grain boundaries in the uncharged material, but was in the charged material. No deuterium was found at the grain boundaries in the boron-doped material. It is proposed that in the boron-free material it is the synergistic effect of sulfur and hydrogen that is responsible for the environmental sensitivity of this alloy. In boron-doped material, boron segregation to the grain boundary prevents sulfur, and to some extent hydrogen, segregating to the grain boundary. INTRODUCTION The effects of boron and hydrogen on Ni 3A1 have been well documented (1,2). Polycrystalline Ni 3A1 produced by conventional metallurgical techniques fails by intergranular separation when tested in air at room temperature after little elongation. The addition of boron to nickel-rich Ni3A1 significantly increases the elongation to failure and alters the failure mode to ductile transgranular. Hydrogen in the material or test environment has been shown to reduce the elongation to failure and promote intergranular separation in all Ni3AI alloys. Although the effect of boron on the mechanical properties has been well documented, the mechanism(s) still remains unclear. As explanations, boron has been attributed with improving boundary cohesion (2), enhancing slip transfer through boundaries (3), and reducing the environmental effect (4,5). Increased boundary cohesion in the presence of boron is supported by computational studies (68), and by electron energy loss studies of nickel bonding at grain boundaries (9). Differences in slip transfer or in dislocation mobility in the grain boundary was not been observed in in-situ TEM deformation experiments, showing that boron enhancement of slip transfer does not occur at least in wellordered alloys (10-12). The presence of boron in nickel-rich Ni3Al alloys suppresses, but does not eliminate, the effect of hydrogen from the test environment on the total elongation to failure (13-15). It has been suggested that boron is only effective at eliminating the effect of water from the test environment, but actually increases embrittlement by hydrogen gas (15). In this regard, boron is thought to either promote the dissociation of H2 by facilitating charge transfer between Ni and H2 (15) or strengthening grain boundaries allowing them to become active H2 dissociation sites (16). These ideas are in conflict with experimental results which indicate the measured solubility of hydrogen in Ni3AI is independent of the presence of grain boundaries and boron (17). In addition, the diffusivity of hydrogen is reduced at grain boundaries contain
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