Powder Processing of High Temperature Aluminides
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POWDER PROCESSING OF HIGH TEMPERATURE ALUMINIDES R. M. GERMAN, A. BOSE and N. S. STOLOFF Materials Engineering Department Rensselaer Polytechnic Institute Troy, New York 12180-3590 ABSTRACT
This paper reviews the several powder processing techniques that have been applied to the fabrication of both monolithic aluminides and their composites. Attention is given to the active research at Rensselaer on reactive sintering and reactive hot isostatic pressing of elemental powders, hot pressing of prealloyed powders, and molding of powder-fiber-binder slurries. The application of one or more of these techniques to the following monolithic intermetallics is described: NiAl, N13A, TiAl, NbAI 3 , Ta 2 AI, and TaAI 3 . Progress has occurred in fabrication of dense compacts, including some composites, from these aluminides. INTRODUCTION Among the intermetallic compounds considered for high temperature structural applications, most attention has been given to the aluminides, especially those based on nickel, titanium, and iron [1]. These compounds have a low density, retain their elastic modulus to high temperatures, and form alumina scales that provide inherent oxidation resistance. Beside aerospace applications, considerable interest in intermetallics exists in some common industrial products ranging from heating elements, pollution control equipment, furnace hardware, and automotive engine components. For these applications to mature, progress is needed with the fabrication of these compounds into engineered shapes. There is a strong interest in powder processing for the fabrication of aluminide intermetallic compounds and composites. Powder processing provides a means of controlling microstructure in a high productivity setting. Much of the early developmental work on intermetallic compounds relied on casting and forging technology, in spite of segregation problems. Subsequently, attention shifted to rapid solidification technologies [2-5]. From this viewpoint it is natural to consider the use of powder techniques for several reasons. First is the improved microstructural homogeneity and minimized segregation inherent with small particles. Second, inexpensive net shape forming approaches are possible using powders. Third, there is an emerging interest in high temperature intermetallic matrix composites which can be fabricated using powders. The initial powder processing efforts have focused on Ni 3 AI, Ti 3 AI, and FeAl. From this base several other compositions are being fibricated on a laboratory scale using powders. Many powder processing techniques have been used to fabricate these aluminides and their composites [1-18]. The list includes reactive sintering, reactive hot isostatic pressing (RHIP), hot isostatic pressing (HIP), vacuum hot pressing, extrusion, dynamic compaction, plasma spraying, and molding of powder-binder slurries. In many cases combinations of these processing approaches are used to deliver the desired chemistry, homogeneity, and microstructure. With the titanium and nickel aluminides there are suppl
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