The Influence of some Microstructural and Test Parameters on the Tensile Stress and Ductility Behaviour of a MA FeAl Int
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The Influence of some Microstructural and Test Parameters on the Tensile Stress and Ductility Behaviour of a MA FeAl Intermetallic J. Chao, M.A. Muñoz-Morris, J.L. Gonzalez-Carrasco and D.G. Morris, Department of Physical Metallurgy, CENIM, CSIC, Avenida Gregorio del Amo 8, 28040 Madrid, SPAIN ABSTRACT This study examines the influence of microstructural parameters (grain and dispersoid size, vacancy content) and some test parameters (strain rate, protective oxide coatings, air and water vapour excluding films, and surface geometrical quality) on the tensile behaviour (yield stress, work hardening rate, tensile stress, ductility) of a mechanicallyalloyed, fine-grained Fe-40Al intermetallic. Major changes of strength and ductility are obtained by changing grain size (1% and 10% for grain sizes of 100µm and 1µm) and by avoiding premature stress/strain concentrators (ductility increased from 5% to 10% for imperfectly machined to prepolished samples). Ductility variations are interpreted using a slow-crack-propagation-to-instability model, where the roles of environment, surface state, deformation processes, and fracture mechanisms can be distinguished. INTRODUCTION The intermetallic FeAl has many attractive properties for use as a structural material but suffers from a relatively low ductility [1-3]. Some of the factors involved in producing this low ductility are environmental embrittlement [2,3], brought about by the reaction of atmospheric water vapour with fresh Al near a crack tip and leading to hydrogen injection below the surface, weak grain boundaries tending to lead to intergranular failure, particularly for materials of high Al content, and hardening-embrittlement by quenched-in vacancies [4-6]. Many studies have examined various aspects of failure in Fe3Al and FeAl alloys and show how, for example, higher strain rates lead to increased flow stress and ductility [7-9] and Boron additions improve grain boundary cohesion leading to a suppression of intergranular failure and increased ductility [3,4,10,11]. These aspects have been described in a recent review of iron aluminides [12]. The present study re-examines the influence of many such test and material variables on the behaviour of a mechanically alloyed (MA) Fe-40Al material which is known to show high strength, because of the fine grain size and the presence of fine Y2O3 particles, as well as reasonably good, for FeAl, ductility, presumably because of its fine grain size [13-15]. In addition, special attention is given to a careful examination of the fracture surfaces obtained by failure and the evolution of fracture surface morphology throughout failure. Little attention has in fact been given to details of fracture surfaces apart from the common distinction as transgranular cleavage or as intergranular failure. Attention is also given to the influence of pre-oxidation treatments on the mechanical properties [16], since such oxide coatings may be expected to diminish the environmental sensitivity of the material and hence improve ductility. EXPERIMENTAL
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