Effect of Preoxidation and Grain Size on Ductility of a Boron-Doped Ni 3 Al at Elevated Temperatures
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EFFECT OF PREOXIDATION AND GRAIN SIZE ON DUCTILITY OF A BORON-DOPED Ni 3A1 AT ELEVATED TEMPERATURES M. TAKEYAMA AND C. T. LIU Metals and Ceramics Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831-6115, U.S.A. ABSTRACT The ductility of preoxidized Ni3 Al (Ni-23A1-0.5Hf-0.2B, at.%) specimens with various grain sizes (17-193 pim) was evaluated by means of tensile tests at 600 and 760'C in vacuum. It was found that the preoxidation does not affect the ductility of the finest-grained material at either temperature, whereas it causes severe embrittlement in the largest-grained material, especially at 760'C. A continuous, thin Al-rich oxide layer, which forms on the fine-grained samples, protects the underlying alloy from oxygen penetration, preventing any loss of ductility, whereas the nickelrich oxide which forms on the large-grained samples allows oxygen to penetrate along grain boundaries, causing severe embrittlement. The grain boundaries act as short-circuit paths for rapid diffusion of aluminum atoms from the bulk to the surfaces, and this is responsible for the difference in oxidation behavior between fine- and large-grained materials. The embrittlement of large-grained samples can be eliminated through control of oxide formation on Ni 3A1 surfaces. INTRODUCTION The ductility of nickel aluminides at elevated temperatures is affected by both test environment and grain size [1-5]. Liu et al. [2] studied the effect of oxidizing environment on the ductility of Ni 3A1 alloys at 600'C and revealed a sharp drop in ductility from 50 to 3% when the test environment was changed from vacuum to air. They also found that preoxidation did not affect the ductility of those samples that were tested in vacuum. Based on these findings, they attributed the loss in ductility to dynamic embrittlement. The grain size of specimens used in their study, however, was relatively small (about 20 lim) and a grain size effect was not specifically investigated. Takeyama et al. [4], on the other hand, systematically investigated the grain size dependence of ductility of a boron-doped Ni3Al from room temperature to 1000 0C in a carefully controlled vacuum (
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