Phase Structure and High-Temperature Mechanical Properties of Two-Phase Fe-25Al- x Zr Alloys Compared to Three-Phase Fe-
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RECENTLY, considerable effort has been devoted to the research and development of Fe-Al-based alloys. While basic research has been focused on understanding the deformation mechanisms and brittleness, applied research has concentrated on improving the mechanical and metallurgical properties of these alloys through the control of composition and microstructure and materials processing optimization.[1–3] The improvement of high-temperature (H.T.) mechanical properties of Fe3Al iron aluminides remains the main task of the present research. The interval between 873 K and 1173 K (600 C and 900 C), i.e., above the stability limit of DO3, is the region of interest. The corrosion resistance is often crucial. Different methods may be used to enhance the H.T. mechanical properties (see, e.g., in References 4, 5): a) Strengthening by solid solution hardening (e.g., with Cr, Ti, Mn, Mo, Si, V, or Ni), which is achieved through the small ternary addition of elements soluble in Fe3Al. b) Strengthening by incoherent precipitates of phases (e.g., using Nb, Cu, Ta, Zr, B, or C additions) formed mainly with higher concentrations of the ternary addition. If, in this case, the amount of the PAVEL KEJZLAR, Junior Researcher, and VEˇRA VODICˇKOVA´, Senior Assistant Professor, are with the Department of Material Science, Faculty of Mechanical Engineering, Technical University in Liberec, Liberec, Czech Republic. PETR KRATOCHVI´L, Professor, and ROBERT KRA´L, Associate Professor, are with the Department of Physics of Materials, Faculty of Mathematics and Physics, Charles University in Prague, Prague, Czech Republic. Contact e-mail: [email protected]ff.cuni.cz Manuscript submitted May 13, 2013. Article published online September 7, 2013 METALLURGICAL AND MATERIALS TRANSACTIONS A
additive and the volume fraction of phases formed in the alloy increase, the strengthening exhibits a composite character, which is highly dependent on the geometry of the phase that originates in the as-cast or heat-treated alloy. In alloys with carbideor boride-forming elements, their role is then very important.[6,7] c) Strengthening by coherent precipitates, which is a very effective method. d) Strengthening by increased crystallographic order, which stabilizes the DO3 structure to higher temperatures, which is a method that is used very often. The microstructure necessary to enhance the H.T. mechanical properties is produced mainly using a technological procedure in which the heat treatment is often the determining process. The determining factor for the application of the material is the stability of the microstructure at high temperatures. The beneficial effect of Zr and Nb addition on H.T. strength was first reported by McKamey and Maziasz, see, e.g., in Reference 8, summarizing the results obtained for a complex alloy containing 28 pct Al, 5 pct Nb, 0.8 pct Mo, 0.025 pct Zr, 0.05 pct C, and 0.005 pct B. Zirconium was observed to be a useful alloying element for the Fe3Al matrix. Zr solid solubility in all phases in Fe-Al alloys is known to be nearly zero (in
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