Environment-sensitive fracture of iron aluminides under monotonic tensile loading

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I. INTRODUCTION

DURING the past decades, ordered intermetallics have been receiving increasing interest due to their attractive properties. Indeed, thanks to reasonable to good mechanical properties at elevated temperatures associated with a low density, they may replace conventional engineering alloys in many hot components where there is a need for stiff and light materials. However, a broad introduction of these materials into engineering components has been retarded by the poor ductility and fracture toughness they exhibit at room temperature. In the case of iron aluminides, it has been shown that, in addition to the various metallurgical parameters that may affect tensile ductility,[1] environmental effects could also partly account for the limited room-temperature ductility.[2,3] Indeed, ductilities measured in a moist environment are generally lower than those obtained in an environment with a lower humidity. The loss of ductility is related to a kind of hydrogen embrittlement that results from the dissociation of adsorbed water vapor molecules. In addition, Liu and coworkers[2–7] reported improved ductility of FeAl when tested in an oxygen atmosphere as compared to vacuum, despite a much higher moisture content in the oxygen environment. The authors proposed a competitive adsorption mechanism to account for this discrepancy: Oxygen would adsorb on surfaces with comparable kinetics and limit the number of available adsorption sites for water vapor. This would reduce the hydrogen production on surfaces and limit the severity of the observed embrittlement.[4,7] This mechanism is further substantiated by kinetics analysis, which indicates that the reactions of water vapor and oxygen are, from a kinetical point of view, of the same order.[8,9] The purpose of the present investigations was to examine the relevance of this moisture-induced embrittlement in the case of an iron aluminide alloy obtained by mechanical GILBERT HE´NAFF, Assistant Professor, is with the Laboratoire de Mecanique et de Physique des Materiaux, ENSMA, F-86961 FuturoscopeChasseneuil, France. ANNE TONNEAU, formerly Postdoctoral Student, Laboratoire de Mecanique et de Physiques des Materiaux, is with ENSAM, F-49035 Angers, Cedex, France. E-mail: [email protected] Manuscript submitted April 12, 2000. METALLURGICAL AND MATERIALS TRANSACTIONS A

alloying with a superior tensile ductility. (In a companion article, these issues will be addressed in the case of crack growth under cyclic loading for the same alloy.) A key issue was to determine whether this enhanced ductility is due solely to the fabrication process or, to some extent, to a reduced sensibility to environmental attack. The respective role of the various processes was investigated, i.e., competitive adsorption between water vapor and oxygen, hydrogen transport, and the subsequent embrittlement. Toward this end, tensile tests were conducted under various environmental conditions and different strain rates, in order to obtain insights about the kinetical aspects of the embrittling proc