Effect of Preoxidation and Grain Size on Ductility of a Boron-Doped Ni3 Al at Elevated Temperatures.

摘要

The ductility of preoxidized Ni3 Al(Ni-23Al-0.5Hf-0.2B, at.%) specimens with various grain sizes (17 /approximately/ 193 mu m) was evaluated by means of tensile tests at 600 and 760/degree/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/degree/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 nickel-rich 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 Ni3 Al surfaces. 11 refs., 5 figs., 1 tab. (ERA citation 14:013101)