Evidence of Inherent Ductility in Single Crystal NiAl
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EVIDENCE OF INHERENT DUCTILITY IN SINGLE CRYSTAL NiAI J. E. HACK', J. M. BRZESKI', R. DAROLIA" and R. D. FIELD" "Department of Mechanical Engineering, Yale University, New Haven, CT 06520 "-GE Aircraft Engines, Cincinnati, OH 45215 ABSTRACT The ductility and fracture toughness of single crystal NiAl have been studied as functions of thermal treatments at moderate and high temperatures. The data indicate that fast cooling through the temperature range 400*C - 20'C results in a material with a tensile elongation of 7% and a fracture toughness in the range of 13 -17 MPam'12. It is concluded that prior reports of brittle behavior in single crystal NiAl may be a result of strain-age embrittlement, similar to that observed in mild steels. The data strongly suggest that ductility and toughness in NiAI are more strongly dependent upon mobile dislocation density rather than on the inherent mobility of dislocations in the ordered lattice. Similar behavior may also be possible in other intermetallic compounds. INTRODUCTION The relatively low densities of intermetallic compounds, combined with their ability to retain strength and stiffness at elevated temperatures, have made these materials attractive for aerospace applications for over three decades" 2 . Most of the compounds which possess excellent high temperature properties have not achieved their potential in structural applications because of prohibitively low room temperature ductility and fracture toughness values. It has been believed that the high melting temperatures and high ordering energies found for these compounds are indicative of an inherently poor dislocation mobility at ambient temperatures. However, several aspects of the mechanical behavior of intermetallic compounds suggest that, while perhaps limited, the room temperature dislocation mobility in these materials is sufficient to provide significant ductility. As one example, recent studies have shown that several ordered intermetallics, primarily aluminides, possess significant room temperature ductility if protected from moisture in the environment35 . In addition, some compounds which appear to exhibit inherently low room temperature ductility show tensile behavior reminiscent of the brittle body-centered-cubic refractory metals. Such phenomena as yield drops and regions of Luder's deformation have been observed in NiAl6. These phenomena are generally attributed to difficulty in the generation of fresh dislocations rather than a low inherent dislocation mobility"8 . When dislocation generation is difficult, ductility can suffer if the initial mobile dislocation density cannot provide sufficient plastic deformation for a given loading state 9 "-. The situation is exacerbated at the tip of a loaded crack in a fracture toughness test where the steep gradients in stress severely restrict the volume of material which experiences stresses above the level necessary to cause dislocation motion". The population of mobile dislocations can be further reduced by strain aging processes which pin potentially mobile
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