Testing of soft-oriented NiAl single crystals in simple shear

Abstract

The intermetallic NiAl has been of considerable interest recently as a prospective replacement for superalloys in turbine engines because of its higher melting point, lower density and good oxidation resistance. In addition, its B2 crystal structure is considered to have good potential for ductility enhancement due to its high symmetry and simple slip directions. Unfortunately, limited room temperature ductility has hindered the development of NiAl alloys. This is primarily due to an insufficient number of independent slip systems to satisfy Von Mises' criterion for uniform deformation. However, the presence of complex dislocation core structures has also been suggested to play a role in the deformation behavior of NiAl. At room temperature, NiAl rarely exhibits slip and deforms primarily by slip on either [001] or [011] type planes. Simulations of a dislocations in NiAl suggest cores spread out of the observed slip plane, which is generally expected to give rise to Schmid's law violations. In addition, based on Schmid's law analysis, slip of a dislocations on [100] planes has been reported to occur at lower stresses than on the more closely packed [110] planes which is unexpected from Peierls stress estimates. Furthermore, Takasugi et al. observed that in compressionmore » the critical resolved shear stresses (CRSSs) were higher than in tension for a given orientation (i.e., stress asymmetry). Therefore, it is likely that Schmid's law is not observed in NiAl and simple shear tests are needed to determine the actual critical shear stress (CSS) for deformation. In the present study, CSS values were measured for slip on both [001] and [011] planes. These values are compared to CRSS values obtained from compression test using Schmid factor calculations. Prismatic punch testing was conducted to investigate the possibility of a [hk0] slip in NiAl.« less