Influence of Ti on enhancing the strength and ductility of Fe-rich medium-entropy alloys: A combined theoretical and experimental study

Abstract

Developing body-centered-cubic (BCC) typed high/medium-entropy alloys (H/MEAs) with an exceptional combination of high strength and ductility remains a challenge for structural applications. Herein, by adjusting phase composition, an ambient ductile TixNi0.6CoFe1.4 (x = 0, 0.05 and 0.1) MEAs were prepared. The effects of Ti on the mechanical properties of TixNi0.6CoFe1.4 MEAs are revealed by combination of experiments and first principles calculations. The addition of Ti not only promotes the formation of (Co, Ti)-rich face-centered cubic (FCC) and (Ni, Ti)-rich R phases in BCC matrix, but also refines the microstructure of the TixNi0.6CoFe1.4 alloys. The mechanical results show that a proper Ti content can significantly improve the ductility of the alloys with sustaining a high strength. Specifically, the yield strength, tensile strength and fracture elongation of Ti0.1Ni0.6CoFe1.4 alloy are 733.0 ± 2.5 MPa, 1003.9 ± 14.9 MPa and 20.1 ± 0.4 %, respectively, which are increased by 22.4 %, 33.3 % and 286.5 % compared to single-phase Ni0.6CoFe1.4 alloy. The impressive mechanical properties are mainly attributed to the synergetic effect of the grain refinement, precipitation strengthening and the coherent boundaries between the precipitate-matrix. Furthermore, the first principles calculation results suggest that the charge enrichment of the Ti site and its induced sparseness of the surrounding charge are responsible for the strength-ductility equilibrium.