Enhancing high temperature mechanical properties via modulating B2 phase with Al contents in FeCrNiAlx(x = 0.63,0.71,0.77) high entropy alloys

High entropy alloy has attracted extensive attention due to the increased requirement for future high temperature structural superalloys. Quaternary Al-Cr-Fe-Ni alloys have been developed in recent years and have the potential to be employed as superalloy materials. How to improve the mechanical properties with strength and ductility balancing at room temperature and further improve the high temperature performance is an urgent problem to be solved. Thus, phase regulations were achieved by designing quaternary Al-Cr-Fe-Ni high-entropy alloys with different Al content to achieve excellent room temperature and high temperature mechanical properties to lay the foundation for its application. Here, a Co-free FeCrNiAl0.77 alloy (Al0.77 alloy) with low density and low cost was proposed and successfully manufactured by arc-melting. With the variation of Al element, the alloys were identified to be eutectic-like spinodal decomposition microstructures with dual BCC phase, consisting of the ordered B2 dendritic regions (enriched in Ni and Al), and disordered BCC inter-dendritic regions (enriched in Fe and Cr elements). The addition of Al can effectively decrease the density and slightly reduce the hardness. The Al0.77 alloy has a minimum density with 6.89 g/cm3 and a high hardness above 470 kgf/mm2. The light-weight alloys exhibit promising comprehensive mechanical properties by virtue to balance the strength and ductility, and have a compressive strength of above 3000 MPa and a compressive strain more than 44%. Furthermore, the Al0.77 alloy still remains high yield strength of 219 MPa at 900 ℃ with the volume fraction of B2 phase of 50.92% via the modulation of B2 phase. The quantitative strengthening mechanisms of Al0.77 alloy were presented, and the competitive strength increments of about 1148 MPa could be mainly attributed to the ordered and coherent microstructures and the nanosized precipitates embedded in the B2 phase regions. The proposed Al0.77 alloy with well balancing of specific strength and strain shows a great potential in the application in aerospace fields.