Research on oxidation and embrittlement of Intermetallic Compounds in the U.S.

Abstract

Research on the oxidation behavior of intermetallic compounds has been conducted in the U.S. for many years. However, until about ten years ago, this work focusses on the compounds which are important in Ni-base superalloys and their coatings: mainly Ni3AI and NiAI. More recent work has been directed at systems which may be used in monolithic form or as the base for composites. Work has concentrated on three types of systems: Ni-and Fealuminides, refractory metal compounds, and titanium aluminides. Work on the Ni-and Fe-aluminides has concentrated mainly on adherence problems and some anomolous behavior. Work on the refractory metal compounds, particularly MoSi2 and NbAI3, has dealt with the problem of selectively oxidizing Al or Si from a refractory metal base and various intermediate-temperature forms of degradation, such as "pesting". It has become increasingly more clear that, for a number of reasons, the titanium aluminides will be the first "new" metallic materials introduced into commercial hightemperature applications (probably aircraft gas turbines and automobiles engines) in many years. As a result a very large amount of work is being done on the oxidation behaviour of these compounds. Initial work dealt with oxidation mechanisms at temperatures on the order of 1000°C. However, both oxidation and mechanical property considerations dictate that the alloys will not be used at temperatures much above 750°C. Therefore, current work is being focussed on oxidation mechanisms at lower temperature and on what may be the "Achilles heel" of these materials, environmental embrittlement.This paper summarizes the work being done in the US. and highlights work on what the author believes are the more important problems.Forschung auf dem Gebiet des Oxidationsverhaltens von intermetallischen Verbindungen wird in den USA seit vielen Jahren durchgefiihrt. Bis vor etwa zehn Jahren beschrankten sich die Arbeiten jedoch auf Verbindungen, die in Nickelbasis-Superlegierungen und ihren Beschichtungen wichtig sind: im wesentlichen Ni,A1 und NiAI. Neuere Arbeiten haben sich mit Systemen beschaftigt, die in monolithischer Form oder als Basis fur Verbundwerkstoffe genutzt werden konnen. Die Arbeiten konzentrieren sich auf drei Typen von Systemen: Ni-und Fe-Aluminide, Verbindungen der refraktaren Metalle und Titanaluminide. Arbeiten an Ni-und Fe-Aluminiden haben sich hauptsachlich mit Haftungsproblemen und einigen anomalen Verhaltensweisen beschaftigt. Arbeiten an Verbindungen der refraktaren Metalle, im wesentlichen MoSi2 und NbAI,, befassen sich mit dem Problem der selektiven Oxidation von A1 oder Si aus dem refraktaren Basismetall und verschiedenen Formen der Schadigung im mittleren Temperaturbereich, wie z. B. dem Pest-Effekt. Aus einer Reihe von Griinden wurde zunehmend klarer, daB die Titanaluminide die ersten ,,neuen" Werkstoffe sein werden, die in mehreren Jahren in kommerziellen Hochtemperaturanwendungen (wahrscheinlich Flugzeuggasturbinen und Kraftfahrzeugmotoren) eingefiihrt werden. Deshalb sind zahlreiche Untersuchungen...

Cyclic oxidation response of multiphase niobium-based alloys

“…The early efforts, which were reviewed extensively by Stringer [10] and others, [11,12,13] were largely unsuccessful because of the formation of Nb 2 O 5 , a nonprotective oxide, which was too dominant to be altered by alloying addition. Subsequent studies [14][15][16][17][18][19] were concentrated on an aluminidebased system and the use of Al addition to improve oxidation resistance of Nb-based alloys through the formation of a protective alumina layer. While the formation of a protective alumina was feasible, the resulting alloys had low melting points and were too brittle to be used as structural materials.…”

High-temperature characterization of reactively processed nanostructure nickel aluminide intermetallics

“…4(a) and (b)). In addition, microalloying free Ni 3 Al was severely oxidized in sulphur-containing environment at 800 • C compared to nanostructure NiAl, which, is considered to be a typical alumina-forming alloy due to the high level of Al-contents compared to Ni 3 Al [20]. However, considering the level of porosity initially present in the reactively fabricated nanostructure nickel aluminides before the high temperature characterization as shown in Fig.…”

“…Because of the potential use of nickel aluminides at high temperatures, it is imperative to understand and study their oxidation behavior. It was reported that the types of scale (oxide or mixture of oxides) and scale morphology formed on the surface of the alloy during high-temperature oxidation/or sulphidation were influenced by both the compositions of the gas and the alloy as well as the reaction temperature [13][14][15][16][17][18][19][20][21][22][23][24][25][26][27]. Both thermodynamic and kinetic factors should be considered to understand this complex corrosion process.…”

Effects of W on high-temperature antioxidative properties of Nb/Nb5Si3 composite

“…Germanium and B additions, as a partial replacement of Si, also improve the oxidation behaviour of Nb/Nb 5 Si 3 in-situ composites [3]. However, refractory metals and their alloys can suffer from pest damage at intermediate temperatures (< 850°C) [11][12][13]. Recent works [3,[14][15][16][17][18] reported beneficial effects of adding tin to Nb/Nb 5 Si 3 composites, which delayed the pest phenomena at 800°C.…”