Microstructural evolution and room-temperature mechanical properties of as-cast and heat-treated Fe50Al50−nNbn alloys (n=1, 3, 5, 7, and 9 at%)

The microstructural evolution and room-temperature mechanical properties of Fe50Al50−nNbn alloys (n=1, 3, 5, 7, and 9 at%) were investigated after solidification and subsequent heat treatment. For all the compositions, the (Fe, Al)2Nb Laves phase formed because of the incomplete solid solubility of Nb in the Fe-Al-based phases and tended to develop an eutectic mixture with the Fe-Al-based phase. According to the results of EDS analysis and microstructural investigations, the Nb concentration of the eutectic composition was 9 at%, and the solid solubility of Nb in the B2-type Fe-Al-based phase was 3 at%. In addition, the eutectic phase transition temperature was approximately 1265 °C. Compared with the as-cast state, all the heat-treated alloys exhibited ultrahigh compressive strength and considerably increased compressive fracture strains. The heat-treated hypoeutectic Fe50Al47Nb3 alloy exhibited the highest compressive strength and fracture strain of 3.02 GPa and 33.1%, respectively, and the eutectic Fe50Al41Nb9 alloy exhibited the lowest compressive strength and fracture strain of 2.66 GPa and 21.8%, respectively, because of the absence of the comparably softer Fe-Al-based primary dendrites. The superior mechanical properties of the heat-treated alloys were attributed to the bimodal distribution of the microstructure, structural incoherency between the crystalline phases, and elimination of solidification artifacts and lattice defects.