Fatigue crack propagation resistance of a FeAl-based alloy

The fatigue crack growth behavior of a powder metallurgically produced iron aluminide of composition Fe-40 at.%Al is investigated. The material is processed by mechanical alloying followed either by extrusion or by hot isostatic pressing. The resulting microstructures are examined by transmission electron microscopy. The test results indicate that the processing route has almost no influence on the resistance to fatigue crack propagation. Special emphasis is given to the roles of crack closure and environment. The fatigue crack growth resistance is shown to be significantly reduced in ambient air. The results, including closure measurements, indicate that this is due to an environmental enhancement of the growth process in conjunction with a lack of complementary shielding from oxide-induced closure. This enhancement appears to be mainly governed by the moisture content of the test atmosphere. Oxygen adsorption is proved to be efficient in preventing this embrittling effect. The mechanisms of embrittlement are discussed and a limited influence of hydrogen on the fracture process is suggested.