Microstructure Engineering for Optimizing the Room Temperature Mechanical Properties of Fe 3 Al-Based Aluminedes
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MICROSTRUCT`URE ENGINEERING FOR OPTIMIZING THE ROOM TEMPERATURE MECHANICAL PROPERTIES OF Fe 3 Al-BASED ALUMINIDES Z. Q. SUN, Y. D. HUANG, W. Y. YANG, AND G. L. CHEN Department of Materials Science & Engineering, University of Science and Technology Beijing Beijing 100083, P. R. China. ABSTRACT A new strategy of microstructure design for improvement of the mechanical properties of Fe3A1 based aluminides was developed. This new approach emphasizes microstructure control on the basis of ordered B2 phase structure matrix instead of the conventional D03 structure. This approach is characterized by the improvement of ductility without lose of strength. The observed data from various Fe3A1 based alloys illustrated that the flattened pancake-shaped grains with ordered B2 phase structure increased both yield strength and elongation of sheet specimens compared to the equiaxed grains. The three dimensional configuration of the flattened grains and their annealing temperature dependence as well as fractography were studied. INTRODUCTION Iron aluminides based on Fe3AI have relatively low density,high specific strength,low cost, good oxidation resistance and excellent sulphidation resistance at high temperature in comparison with stainless steels. Based on these benefits iron aluminides have potential applications instead of stainless steels in many different fields, such as heat elements, automotive applications, and chemistry industry applications [1]. But the poor ductility at ambient temperature and a sharp drop in strength at 600"C greatly hinder its development for commercial use. From the 1940's to 80's, considerable work has been devoted to improvement of the room temperature ductility of iron aluminides, but no magnificent progress was made, the best elongation was about 4-6%[2,3]. Recently, C. T. Liu et al of ORNL indicated that iron aluminides were susceptible to environmental embrittlement at ambient temperature in the presence of water vapor[4,5]. This discovery is expected to help in alloy design of ductile iron aluminides as well as other aluminides for structural applications. By means of the addition of alloying elements such as chromium and the control of thermomechanical processing, the room temperature ductility of iron aluminides can be increased to 10% or more[6,7,8]. With a view toward possible development of Fe3AI based alloys for corrosion resistant structural applications, systematic work on the relationship between chemical composition, processing, structure and properties of Fe3AI based intermetallic alloys is in progress in our research group. The basic strategy is improving the ambient temperature ductility by softening first, and then increasing the strength at both ambient and elevated temperatures by optimizing the processing and chemical composition. In the present work, Fe-28AI-5Cr iron aluminide has been chosen as a softened alloy for study of the effects of complex alloying and the effects of processing. The best room temperature mechanical properties in air were yield strength of up to 520MP
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