Effects of Pressure on the Flow and Fracture of Polycrystalline NiAl
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EFFECTS OF PRESSURE ON THE FLOW AND FRACTURE OF POLYCRYSTALLINE NiA1 R. W. MARGEVICIUS, J. J. LEWANDOWSKI, I. E. LOCCI, AND G. M. MICHAL, Department of Materials Science and Engineering, Case Western Reserve University, Cleveland, Ohio 44106
ABSTRACT The effects of testing NiAl in tension under a superimposed pressure are described. The flow stress decreases when subjected to pressure due to the generation of mobile dislocations. These dislocations can become pinned when subjected to aging at moderate temperatures and times. The ductility increases substantially when tested under a superimposed hydrostatic pressure. A new method for performing fracture toughness tests under superimposed pressure is also described. INTRODUCTION Nickel aluminide, NiAl, is a candidate for high temperature structural applications because of its low density, high melting temperature, and good oxidation resistance. However, inadequate low temperature ductility and toughness may limit its use in structural applications. Several attempts have been designed to look at this low ductility including examination of slip systems'- 3 , grain size effects 4, 5 , grain 6oundary fracture 6 , and alloying additions 7 ,8 . The purpose of this study was to carefully modify the stress state to examine how ductility changes with altering the applied stress. These experiments were done by uniaxially loading tension and compression samples under a superimposed hydrostatic pressure. Recently published work has given the details of how hydrostatic pressure influences the flow properties of NiAI 9- 12 . Both pressurization (i.e., the application of a hydrostatic pressure to a sample, its removal and subsequent testing at atmospheric pressure) and axial loading under an applied pressure decreased the flow stress by up to 40% below that of samples not subjected to pressure. Detailed TEM analyses indicated that dislocations, presumably mobile, were injected into the material. They were seen to be generated at second phase particles (viz., 10 11 inclusions), as well as grain boundaries. . The mechanism of dislocation generation at second phases is well known. These dislocations are generated because of the shear stresses which arise from the differences in the bulk moduli between the matrix and the inclusion when the system is subjected to pressure. Another manifestation of this mechanism also became apparent in the grain boundary regions. The dislocations generated at grain boundaries were done so in the absence of any second phases. Energy dispersive spectroscopy showed that slight variations in composition existed from region to region in the cast material. (Similar tests conducted on a powder metallurgy Mat. Res. Soc. Symp. Proc. Vol. 288. ©1993 Materials Research Society
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material, which showed no decrease in the yield stress and no increase in dislocation density, revealed no such compositional variations.)' 1 These results suggested that processing of the cast material (i.e., extrusion and annealing) did not completely homogenize the initially compo
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