The effect of various metallurgical parameters on the flow and fracture behavior of polycrystalline nickel-aluminum near the brittle-to-ductile transition

This dissertation represents an investigation of the effect of various metallurgical parameters such as interfaces, alloying additions, test temperature and strain rate on the flow and fracture behavior of polycrystalline NiAl.;As a result of this work a detailed description of the room temperature flow and fracture behavior of both single and two phase NiAl alloys has been produced. The primary objective was to determine the reason that NiAl is brittle below the BDTT and to develop a suitable response to this problem, i.e., ductile phase toughening. While a number of different explanations have been proposed in the literature for the brittle behavior of polycrystalline NiAl it was found that the overriding problem is an insufficient number of independent slip systems to accommodate generalized plastic flow. Ductile phase toughening is the most likely method for overcoming this deficiency. Finally, the flow and fracture of polycrystalline NiAl at room temperature serves as an example of the general behavior of this intermetallic below the brittle-to-ductile transition temperature (BDTT).;Another purpose of this research was to define the brittle-to-ductile transition temperature in terms of consistently measurable properties. It was observed that the BDTT for NiAl alloys always corresponded to a dramatic change in tensile ductility, which is normally how the BDTT is defined. But the BDTT also corresponded to a significant increase in fracture strength (fracture toughness) as well as a change in deformation mechanism as determined by a change in slope in an Arrhenius representation of the yield strength.;The third purpose of this research was to qualitatively determine the mechanism for the brittle-to-ductile transition in NiAl alloys. The proposed mechanism is based on the operation of localized dislocation climb processes that operate within the vicinity of the grain boundaries and that provide the additional deformation mechanisms necessary for grain-to-grain compatibility during plastic deformation. These localized dislocation climb processes operate at much lower temperatures than for bulk climb processes, which are not dominant until temperatures greater than about 1000 K. This is the reason for the low BDTT, approximately 550 K, for binary NiAl.;Another goal was to determine the effect of various metallurgical parameters such as strain rate and alloying additions on the BDTT and to ascertain whether all the observations are consistent with the proposed mechanism for the brittle-to-ductile transition. Finally, methods for improving the low temperature mechanical behavior of NiAl were considered and reviewed within the context of the present knowledge of NiAl-based materials and the operative deformation and fracture mechanisms determined in this study. Special emphasis was placed on the use of second phases for improving low temperature properties. (Abstract shortened by UMI.).