Effect of Ni on vacancy concentrations and hardness in FeAl alloys
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I.
INTRODUCTION
The ordered intermetallic compound FeAl is being investigated as a potential structural material to be used in oxidizing and/or corrosive atmospheres at elevated temperatures. Unfortunately, this compound suffers from limited room-temperature ductility. This lack of ductility is due, in part, to an environmental embrittlement effect which occurs even in an ambient air atmosphere.[1] In addition, this compound may suffer from intrinsic grain boundary brittleness. However, grain boundary cohesion may be improved by small additions of boron.[2] A third factor which can limit the ductility of FeAl alloys is the presence of quenched-in thermal vacancies. It has been well established that the presence of these nonequilibrium defects can contribute to extensive hardening and a corresponding decrease in the ductility of FeAl alloys.[3,4,5] The crystal structure of FeAl is that of B2 in the Strukturbericht designation. This crystal structure can be thought of as an ordered bcc lattice with one type of atom on the cube corners and the other type of atom at the cube centers. In a study by Chang et al.,[6] the hardness of FeAl was found to linearly relate to the square root of the vacancy concentration as calculated from a thermodynamic model. This effect was seen at Fe-rich, stoichiometric, and Al-rich compositions. The vacancy concentrations were later confirmed experimentally.[7,8,9] The square-root relationship agrees with the classical theory of solid-solution hardening in metals, developed by Fleischer for the interaction between a moving dislocation and a point obstacle.[10] This relationship is sometimes referred to as the Fleischer–Orowan equation.[11] The fact that the vacancy concentration controls the hardness of Fe-rich compositions (where vacancies are outnumbered by Fe antisite defects), as well as L.M. PIKE, Postdoctoral Research Associate, formerly with the Department of Materials Science and Engineering, University of Wisconsin, is with the Metals and Ceramics Division, Oak Ridge National Laboratory. C.T. LIU, Senior Corporate Fellow, is with the Metals and Ceramics Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831-6115. Y.A. CHANG, Professor, is with the Department of Materials Science and Engineering, University of Wisconsin, Madison, WI 53706-1595. Manuscript submitted November 7, 1997. METALLURGICAL AND MATERIALS TRANSACTIONS A
stoichiometric and Al-rich compositions (where vacancies are the most prevalent defect), suggests that vacancies are considerably more potent hardeners than the Fe antisite defects. This subject will be discussed further later in Section III. Attempts to improve the ductility of intermetallic alloys often involve the use of ternary or higher-order alloy additions. The effects of several substitutional-type alloy additions to FeAl have been the subject of several recent investigations. In a study by Munroe, the effects of Ni additions in the range of 0 to 10 at. pct to (Fe55xNixAl45 were investigated.[12] The hardness of samples water quenched (WQ) at 9
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