Strength and Ductility of Fe 3 Al with Addition of Cr
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STRENGTH AND DUCTILITY OF Fe3Al WITH ADDITION OF Cr
D.G. MORRIS, M.M. DADRAS and M.A. MORRIS Institute of Structural Metallurgy, Avenue de Bellevaux 51, University of Neuchatel, NEUCHATEL 2000, SWITZERLAND ABSTRACT The development of Fe3A1 aluminides has been restricted in the past by poor ductility at ambient temperatures, and it is only recently that possible solutions to this problem have been found. It was shown a few years ago (1) that the addition of 2-6% Cr to a Fe 3AI base alloy led to good ductility and this improvement was explained by a reduction of the APB energy, increasing the separation of the superpartial dislocations and thereby allowing easier dissociation of these dislocations, easier cross slip and a reduced tendency to stress and strain concentrations. However, at a later stage, an alternative explanation was proposed (2,3) based on examinations of both FeA! and Fe3 AI alloys under different environments, and the ductility change was explained in terms of chemical attack at the tip of a crack leading to local hydrogen embrittlement. The present study re-examines the behaviour of a Fe3Al alloy both with and without the addition of Cr. Strength, work hardening behaviour and failure ductility are examined under conditions where environmental effects should not be important, and the mechanical behaviour is interpreted in terms of significant variations in the type of order, the ordered domain structure and the resulting dislocation structures. It is seen that the addition of Cr can lead to a better ordered material and the differences in ordered state between the two materials can significantly affect dislocation behaviour and mechanical properties. EXPERIMENTAL TECHNIQUES Strips of two alloys of composition Fe-28%Al and Fe-28%AI-4%Cr were supplied for study by C.T. Liu and C.G. McKamey of ORNL. These strips were produced by arc-melting, drop casting and hot rolling to a thickness of about 0.7mm. Recrystallization treatments were given to remove the worked structure remaining after rolling and to give a range of grain sizes for the study of mechanical properties, and tensile samples were cut from the sheets by spark machining. Before mechanical testing the samples were electropolished to remove any surface oxide. Based on earlier studies (2,3) it is known that such polishing largely removes any environmental protection due to iron or chromium oxides formed on the surface during heat
treatment such that mechanical properties are largely determined by intrinsic dislocation behaviour. Mechanical testing was carried out, at room temperature, under an inert atmosphere of argon in order to further ensure the absence of any different environmental effects between the two materials. For the ternary, Cr containing alloy, recrystallization was carried out at temperatures in the range 750-850'C for various times giving a range of grain sizes from about 90pm to 230gm. The binary alloy contained a small amount of TiB2 which had been added to avoid grain growth and it was difficult to obtain a large range of g
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