Environment-sensitive fracture of iron aluminides during cyclic crack growth
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I. INTRODUCTION
SINCE iron aluminide–based alloys now appear to be potential structural materials, their resistance under cyclic loading needs to be assessed. As a matter of fact, the data pertaining to the fatigue resistance of these materials are very scarce, especially for the B2 compounds. The only work published on the fatigue-crack growth (FCG) behavior of iron aluminides has been carried out by Stoloff and coworkers, mostly on Fe3Al compounds.[1–5] This lack of data, in particular as regards environmental effects, constitutes a major obstacle to a thorough understanding of the mechanisms controlling the fatigue resistance. Indeed, from what is observed in conventional engineering alloys, the fatigue resistance and, especially, the FCG resistance might be expected to be strongly influenced by several parameters of both an intrinsic and extrinsic nature. The intrinsic parameters pertains to the chemical composition, processing, and heat treatment of the material. The extrinsic parameters represent the various factors related to the loading conditions, either mechanical or physico-chemical. In particular, it is now well established that the environment severely alters the FCG resistance, especially in the slow fatigue-crack growth-rate (FCGR) regime. As a matter of fact, Stoloff and co-workers did not explore FCGRs lower than 10⫺9 m/cycle, although they make claims about threshold values. Meanwhile, as stressed in a previous article,[6] the roomtemperature ductility of iron aluminides is highly sensitive to parameters such as strain rate and environmental attack. Several questions are then raised. First, due to this pronounced environmental sensitivity under monotonic loading, a significant effect might also be expected under cyclic crack growth. On the other hand, one must also wonder whether ANNE TONNEAU, Assistant Professor, is with Laboratoire Proce´de´sMate´riaux-Instrumentations, ENSAM, BP325, 49000 Angers. MICHEL GERLAND, Researcher, and G. HE´NAFF, Assistant Professor, are with the Laboratoire de Mecanique et de Physiques des Materiaux, 86961 Futuroscope-Chasseneuil, France. Manuscript submitted July 19, 2000. METALLURGICAL AND MATERIALS TRANSACTIONS A
the mechanisms proposed to account for the loss of FCG resistance under active environments in engineering alloys still apply in the case of iron aluminides. The present article precisely tackles these issues. The FCG behavior of an Fe-40Al alloy prepared by mechanical alloying has been investigated under various environmental conditions. Special emphasis is placed on the identification of the respective contribution of the sequential steps involved in environmentally assisted fatigue-crack propagation and, possibly, rate control,[7,8] namely, (1) transport of active species to the crack tip, (2) adsorption and dissociation of active species on cracktip surfaces, and (3) hydrogen entry and subsequent embrittlement in the strained region at the crack tip. Transmission electron microscopy (TEM) observations of the dislocation structures within plastic zone
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