51 Introduction

Intermetallics are a broad class of metals resulting from the combination of various elements including nickel aluminide, titanium aluminide, niobium aluminide, iron aluminide, iron silicide, and various other silicides. Each has a unique set of properties. Titanium aluminide is valued for light weight (lower than nickel-based superalloys), oxidation resistance, and stiffness. Niobium aluminide is light weight and, with a melting point of 2060°C (3740°F), operates at higher temperatures than nickel-based superalloys but has low fracture toughness and poor oxidation resistance at elevated temperatures.

Various silicides have been commercially available for many years, particularly molybdenum disilicide, used in heating elements. Iron silicide (FeSi) is sold under

Handbook of Advanced Materials Edited by James K. Wessel ISBN 0-471-45475-3 Copyright © 2004 John Wiley & Sons, Inc.

the trade name Hastalloy D and used in high-temperature castings. Another iron silicide (Fe3Si) is available as Duriron™. Other silicides are used for their oxidation resistance. All have attractive melting temperatures with some reaching 2400°C (4352°F). They are also used as coatings to protect other materials such as niobium aluminide from oxidation.

The nickel aluminide composition Ni3Al has been known for years as an intermetallic material that, due to its ordered crystal structure and high melting temperature, is strong, hard, and thermally stable. It is particularly attractive because it combines lower density (25% less than superalloys) and resistance to wear, deformation, fatigue, oxidation, carburization, and coking. Particularly attractive is the unusual characteristic of increasing strength with increasing temperature. Despite such attractive properties, Nickel aluminide did not find wide use because it was too brittle to fabricate and too expensive. In 1982, Oak Ridge National Laboratory scientists, led by C. T. Liu, discovered a way to make this desirable material ductile. The result is a material (IC-221M) lighter and five times stronger than stainless steel that becomes stronger as the temperature approaches 800°C (1472°F), as shown in Fig. 5.1.

The research, development, and commercialization of Ni3Al included a lower cost, safer process developed by a team of Oak Ridge National Laboratory scientists led by Vinod Sikka. All of this effort combined to produce a useful structural material of value in many industrial structural applications ranging from furnace furniture and steel processing rollers to making dies for forming beverage containers.

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