Strengthening of Fe 3 Al Aluminides by One or Two Solute Elements PETR KRATOCHVI ´ L, STANISLAV DANIS ˇ , PETER MINA ´ RIK, JOSEF PES ˇ IC ˇ KA, and ROBERT KRA ´ L The compressive yield stress of Fe-26Al with additives Ti (0.5 to 4 at. pct), Cr (0.5 to 8 at. pct), Mo (0.5 to 4 at. pct), and V (0.5 to 8 at. pct) at 1073 K (800 °C) has been determined. The effect of the concentration of diverse solutes on the yield stress at 1073 K (800 °C) was compared, and the additivity of the effects of solutes was tested. The effects in iron aluminides with two solutes (V and Ti, Ti and Cr, V and Cr) are compared with those of a single solute V, Ti, and Cr. It is found that the additivity of yield stress increments is valid only for lower solute concentrations. When the amount of the solute atoms increases, the yield stress increment is substantially higher than the sum of the yield stress increments of single solutes. This behavior is related to the high-temperature order in iron aluminides. DOI: 10.1007/s11661-017-4211-x Ó The Minerals, Metals & Materials Society and ASM International 2017 I. INTRODUCTION ALLOYS based on iron aluminides Fe 3 Al show potential for structural applications at high tempera- tures owing to their excellent oxidation and sulfidation resistance. They display lower density compared to other iron-based materials, and their low cost is advan- tageous as well. Unfortunately, they also show unfa- vorable lack of room temperature ductility and low high-temperature strength. [1–5] During the last decades, research efforts have focused on enhancing the ductility, strength, and creep resistance of iron aluminides by alloying. Several approaches have been explored to improve the high-temperature mechan- ical properties of these alloys. Basically, solid solution hardening (SSH), strengthening by coherent and inco- herent precipitates, and increasing the crystallographic order were considered for the strengthening of iron aluminides. [1,6–9] Elements such as Cr, Ti, Mn, Si, Mo, V, and Ni were added for SSH [2,10–12] and Palm [8] compared the SSH by Ti, V, Cr, and Mo at 873 K, 973 K, and 1073 K (600°C, 700°C, and 800 °C). Recently, Kratochvil et al. [12] described the SSH effect of vanadium in Fe 3 Al using the SSH theories for binary alloys of Fleischer [13] and Labusch. [14] It was reported that the SSH of Fe 3 Al by vanadium depends mostly on the elastic modulus misfit, and the atom size misfit plays a minor role. In this work, we first describe the SSH of Fe 3 Al by Ti, Cr, V, and Mo solutes. This is followed by the study of the combined effect of two solutes in Fe 3 Al. II. EXPERIMENTAL Iron aluminide alloys were produced by vacuum induction melting. The cooling took place under argon. Samples were prepared from the alloys by electrical discharge machining (EDM). The compressive yield stress was evaluated using a digitally controlled testing machine (INSTRON 1186R). Parallelepipeds with dimensions of 6 9 6 9 10 mm 3 were cut by EDM. The deformation rate was 8 9 10 5 s 1 . The tempera- ture of 1073 K (800 °C) was chosen because this temperature is well above the yield strength anomaly, which is an unusual increase of the strength with increasing temperature, typically observed for Fe-Al- based alloys with the maximum in the 773 to 873 K (500 to 600 °C) range. [15] Additionally, 1073 K (800 °C) is high enough for the vacancy concentration to be in thermal equilibrium. This is noteworthy because at lower temperatures the strength of Fe-Al-based alloys is markedly influenced by quenched-in thermal vacan- cies, [16] i.e., the strength depends on the processing of the alloys. The investigated alloys are B2-ordered at 1073 K (800 °C), allowing direct comparison of their yield strengths. PETR KRATOCHVI ´ L, PETER MINA ´ RIK, JOSEF PES ˇ IC ˇ KA, and ROBERT KRA ´ L are with the Charles University, Faculty of Mathematics and Physics, Department of Physics of Materials, Ke Karlovu 5, Prague 2, CZ-12116, Czech Republic. Contact e-mail: pekrat@met.mff.cuni.cz STANISLAV DANIS ˇ is with the Charles University, Faculty of Mathematics and Physics, Department of Condensed Matter Physics, Ke Karlovu 5, Prague 2, CZ-12116, Czech Republic. Manuscript submitted June 3, 2017. METALLURGICAL AND MATERIALS TRANSACTIONS A