Deformation Behavior of NiAl-Based Alloys Containing Iron, Cobalt, and Hafnium
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DEFORMATION BEHAVIOR OF NiAI-BASED ALLOYS CONTAINING IRON, COBALT, AND HAFNIUM. D.R. PANK+, M.V. NATHAL*, AND D.A. KOSS+ +Dept. Mat. Sci. & Eng., Penn State, University Park, PA 16802 *NASA Lewis Research Center, Cleveland, OH 44135
ABSTRACT The effects of alloying additions on the mechanical properties of the B2 intermetallic NiAl have been investigated in both the melt-spun ribbon and consolidated, bulk form. The study is based on a matrix of NiAl-based alloys with up to 20 a/o Co and Fe additions and with reduced Al levels in the range of 30 - 40 a/o. Characterization of the melt-spun ribbon by optical and scanning electron microscopy indicates a range of microstructures: single phase P3,Y necklace phase surrounding either martensitic or 13 grains, and a mixture of equiaxed martensitic and y grains. Bend ductility is present in melt-spun and annealed ribbons exhibiting the Y necklace structure and in a single phase P3 material containing 20 a/o Fe. The analysis of compressive flow behavior on consolidated, bulk specimens indicates that the single phase P3 alloys exhibit a continuous decrease in yield stress with increasing temperature and profuse microcracking at grain boundaries. In contrast, multiphase (y + either martensite or 13) alloys tend to display a peak in flow stress between 600 and 800K with little or no signs of microcracking. In general, heat treatments which convert the martensitic grains to !5 + Y result in improved strength at temperatures above 600K and better resistance to crack initiation. These results are discussed in terms of the effects of 13, martensite and Y/on the yield stress and flow behavior of NiAl-based alloys. INTRODUCTION Ordered intermetallic compounds are of interest for high temperature applications because of their potential for high temperature stability, high creep resistance, high melting point, and (in many cases) low density. Among the intermetallics, compounds based on the aluminides are of particular interest because many possess oxidation resistance due to their ability to form protective oxide films on surfaces. Of the aluminides, alloys based on NiAl offer considerable potential because they exhibit an attractive combination of the properties listed above. However, a major problem with most intermetallic aluminides, and NiAl-based alloys in particular, is their low ductility and tendency to fracture in a brittle manner at low temperatures. Therefore, the general goal of this work is to utilize processing and compositional control to obtain NiAl-based alloys which exhibit both low temperature fracture resistance as well as good strength at high temperatures. Utilizing rapid solidification techniques, previous investigators have added Fe and Co to NiAl-based alloys and have observed increases in ductility in rapidly solidified wire specimens [1]. The ductility increases have been attributed to grain size refinements, elimination of grain boundary segregation, and also increases in alloy homogeneity [1]. However, it is known that Cr and Mn additions to NiAl alloys ac
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