Room Temperature Tensile Ductility in Polycrystalline B2 NiAl
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ROOM TEMPERATURE TENSILE DUCTILITY IN POLYCRYSTALLINE B2 NiAl K. VEDULA, K. H. HAHN AND B. BOULOGNE Associate Professor, Graduate Student and Research Associate, Department of Materials Science and Engineering Case Western Reserve University, Cleveland, Ohio 44106
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ABSTRACT Binary polycrystalline alloys of B2 NiAl, ranging from 47 to 51.5 at% Al, were prepared by casting and extrusion. These were tested in tension at temperatures ranging from 300 K to 873 K. Significant tensile ductility is observed in the near-stoichiometric alloy at room temperature as well as at 473 K in spite of the presence of only dislocations. This anomaly appears to be related to the recrystallized texture, to the lower yield strength and to the presence of residual dislocations at grain boundaries in this alloy. INTRODUCTION Polycrystalline NiAl with the B2 crystal structure exhibits no tensile ductility at room temperature according to almost all of the studies reported in the literature. There is probably only one intriguing report, that of Rozner and Wasilewski [1], which shows evidence of significant (62%) tensile ductility at room temperature in cast and extruded stoichiometric NiAl. A few other reports including those of Pascoe and Newey [2] and Ball and Smallman [3] have, however, shown evidence of substantial compressive ductility in stoichiometric NiAl at room temperature. These investigations have also reported that deviations from the stoichiometric composition make the alloys more brittle at low temperatures. This lack of tensile ductility in polycrystalline B2 NiAl has been attributed to the lack of a sufficient number of independent slip systems to satisfy von Mises' criterion for compatability in polycrystalline deformation. Single crystal deformation studies [4-10] show that this alloy is highly anisotropic. The non- oriented crystals (soft orientations) are much more ductile than the oriented crystals (hard orientations) at low temperatures. This is because the critical resolved shear stress required to activate primary slip of the type is much less than that required to activate other slip systems such as the type. The number of slip systems which operate if only the {110} slip occurs are insufficient for von Mises' criterion for polycrystalline deformation. In compression, ductility is observed since there is no driving force to open cracks that must form at grain boundaries due to the lack of sufficient slip systems. In tension, however, the applied stress acts directly to open the crack and failure may occur before further deformation can take place. On the other hand, if slip is possible (as in the case of disordered BCC alloys as well as some other B2 compounds), there are enough number of slip systems of the type ll0} to satisfy von Mises' criterion and lead to polycrystalline tensile ductility. Based on this premise, many of the current efforts at obtaining room temperature ductility in B2 NiAl are concentrating on encouraging slip of the type in these alloys through alloying additions. This approach
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