1000 to 1200 K time-dependent compressive deformation of single-crystalline and polycrystalline B2 Ni-40Al

Abstract

Single-crystalline and polycrystalline Ni-40A1 have been compression tested at rates ranging from 0.1 to 10-7 s-1 in air between 1000 and 1200 K. The [100] and [013] crystals were grown by a modified Bridgman technique, while the polycrystalline material was made by hot pressing XD synthesized powder. The as-pressed polycrystalline Ni-40A1 had approximately 10 μm grains which were decorated with A12O3 particles. This form also contained a small amount of γ′ (Ni3Al), whereas neither single crystal showed evidence of this phase. In general, the stress-strain diagrams for the single crystals showed strain hardening over the first 1 pct of deformation, leading to diffuse yield points which existed over 0.5 to 1 pct strain. Following yielding, the single crystals strain soften and then, after about 3 pct deformation, begin to strain harden. As the test temperature increased or the strain rate decreased, the yield points tended to disappear, and the single crystals deformed under a more or less constant flow stress. Irrespective of the test temperature or strain rate, the polycrystalline Ni-40A1 usually exhibited continued flow under a constant stress. While the [013] single crystals had higher yield strengths than the [100] variation, evaluation of the flow stress-strain rate data at 3 pct strain indicated that both forms had equivalent strength, but they were slightly weaker than the polycrystalline version.