Solidification microporosity in directionally solidified multicomponent nickel aluminide
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INTRODUCTION
NICKEL-aluminide intermetallic compositions (Ni3AI type) containing boron are being conside~ed for use. as new structural materials because of their outstandmg properties related to cavitation, erosion, high-temperature oxidation, wear, fatigue resistance, and low density.[l] Optimizing properties in these alloys have led to the binary composition of nickel aluminide being assayed with several elements, e.g., Zr, Cr, Fe, Mo, Ce, and B. Zirconium is a solid solution strengthener and increases hightemperature strength and ductility; Cr alleviates the lack of ductility at intermediate temperatures; Fe helps sulfidation resistance; Mo is a solid solution strengthener especially helpful at high temperatures; Ce reduces t?e hot tearing tendency; and B ductilizes the alloy by diSordering the grain boundaries. Oxygen is detrimental, as it is known to give rise to dynamic embrittlement and medium temperature loss of ductility. . . For the binary Ni-AI alloys, two types of phase diagrams have been proposed, as show? ~". Fi!?ures l(a) and (b). [2,3] As discussed below, our sohdlflcatIOn results, as well as the results of Verhoeven and Bremer et ai.,[4] indicate that the diagram in Figure 1(b) is more representative of the eutectic region. We will follow this diagram in all of the following discussions. Compositions of commercial nickel aluminide alloys that are currently being introduced are listed in Reference 1. Fabrication of these alloys has normally followed a route ~n~olv~ng deformation processing. Increasingly, however, It IS .be~g recognized that economical fabrication meth.o?s WIll mvolve net-shaped castings. The alloy composItIons, however have been developed for wrought materials, and usin~ the same compositions for castings is. causi".g problems associated with porosity .a~~ h~t teanng .. ThiS paper is aimed at detailing the sohdlflcatIOn path m the alloy IC-396M (Table I) and studying the ~ffect of .solidification condition on the formation of mlcroporoslty.
c.T. HO and c.J. CHENG, Graduate Research Assistants, and J.A. SEKHAR, Associate Professor, are with the Department of Materials Science and Engineering, University of Cincinnati, Cincinnati, OH 45221-0012.
Manuscript submitted June 6, 1990. METALLURGICAL TRANSACTIONS A
From an examination of shrinkage of various metals, we note that both aluminum and nickel are high shrinkage metals. There is reason to expect that ordered Ni3AI will be prone to high volume shrinkage. Tensile and fatigue fracture surfaces of IC-396M invariably indicate that porosity plays a major role in crack initiation and propagation. A clear evidence of tensile te.aring at t?e microporosity and fatigue crack deflectIOn (porosity toughening) by the microporosity has been noted and previously documented by us. This paper is aimed at understanding the origin and microstructural distribution of the microporosity. The basic approach to be employed in this study involves directional solidification (OS). Directional solidification is a powerful technique in which different e
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