INTRODUCTION

ABSTRACT

Many intermetallic compounds are brittle at low temperatures. Some compounds, including NiAl and MoSi, seem to be intrinsically brittle. Other compounds, including N&Al, FeAl and FqAl, are reasonably ductile when tested in inert environments. Water and water vapor can severely embrittle these and other compounds because they contain an active element such as aluminum or silicon. The release of atomic hydrogen from water leads, at room temperature, to hydrogen embrittlement. Another source of embrittlement, at elevated temperatures, is oxygen from the atmosphere. The effects of aggressive environments on the crack growth behavior of several intermetallics under monotonic or cyclic loading are described. Alleviation of embrittlement by means of alloying, the use of coatings or by prestrain are described, although no single method is effective for all intermetallics. A brief survey of environmentally induced crack growth in superalloys is included.

Intermetallic compounds constitute an important new class of high temperature materials. Although there are numerous publications available dealing with monotonic mechanical properties, there has been considerably less attention directed to their properties under cyclic loading conditions. Nevertheless, there now is a significant body of data on the cyclic crack growth resistance of alumin- ides and silicides. For the most part these are brittle materials at low to intermediate temperature, but a brittle to ductile transition usually is noted well below the melting point of a given alloy. Thus, the intermetallics fall somewhere between metals and ceramics in both their monotonic and cyclic fracture behavior. In this paper we will restrict our review to the following high temperature intermetallics: TiAl, NiAl, N&Al, FqAl, FeAl and MoSi,, as well as to nickel-base superalloys.

ALLOY SYSTEMS

TiAl

Numerous studies have been reported of the fracture and fatigue crack growth resistance of TiAl alloys (? n - . These have shown that while toughness and ductility are highest with a duplex, two phase structure, better resistance to fatigue crack growth is shown by lamellar microstructures, provided that fatigue cracks are large o). It has been reported elsewhere that the main crack propagates by the initiation and coalescence of small microcracks ahead of the crack tip, which then link up with the main crack, similar to the behavior of ceramics @). The effect of temperature is complex, in that the crack growth rate first increases, and then decreases, as temperature is increased to EOO???. It has been shown that crack growth resistance is reduced in ambient air, see Fig. lo. The effect is due to moisture in the air, which is assumed to produce hydrogen on contact with the specimen surface.

NiAl

NiAl has been under intensive study as a potential replacement for superalloys in advanced aircraft gas turbines. Hindering development is the lack of sufficient ductility in either polycrystals or single crystals at low temperatures. Only recently has the first information been published on fatigue crack growth in this material, see Fig. 2 (@ The data were obtained at room temperature. Note the . high slope, m = 34, and low threshold of the crack growth curves for the duplicate spe

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