NICKEL AND IRON ALUMINIDES - AN OVERVIEW ON PROPERTIES, PROCESSING, AND APPLICATIONS

Intermetallics have long been recognized as potential candidates for a variety of high-temperature structural applications to operate well b eyond the operating temperatures of conventional materials due to thei r excellent oxidation and corrosion resistances. In this paper, we com pare and contrast the mechanical properties such as yield strength, ul timate tensile strength, and tensile elongations of Ni3Al-based alloys , Fe3Al-based alloys, and FeAl alloys with several of the commercially available superalloys such as Haynes 214 (NiCrAlY), MA-956 (yttria-di spersed FeCrAlY), and a FeNiCr alloy (HU steel) used in carburizing ap plications. Our comparisons clearly show that cast and wrought Ni3Al-b ased alloys exhibit superior mechanical properties over the commercial ly available alloys such as the FeNiCr HU steel and Haynes 214. Electr ical resistivity of iron aluminides increases with the increase of alu minum content, and the electrical resistivities of Fe3Al- and FeAl-bas ed alloys are 50-100% higher than those of commercially available heat ing-element materials. Processing problems associated with the melting and casting of intermetallics are discussed in light of their large, negative heats of formation; high-aluminum content of intermetallics; and the safe operating temperatures of crucible materials for melting them. A furnace-loading sequence enabled us to properly utilize the he at of reaction of intermetallics resulting in the development of the E xo-Melt(TM) process for melting and casting of intermetallics for a va riety of structural applications. The Exo-Melt(TM) process allowed us to cast a wide variety of structural intermetallic parts using sand, c entrifugal, and investment casting techniques, and a total of 15 000 k g of intermetallic parts were cast by the Exo-Melt(TM) process during 1995. Copyright (C) 1996 Elsevier Science Ltd