Formation of dendrites and strengthening mechanism of dual-phase Ni36Co30Fe11Cr11Al6Ti6 HEA by directional solidification

Abstract

In order to reveal the effect of solidification rate on microstructure and improve room and high temperature tensile properties, Ni36Co30Fe11Cr11Al6Ti6 high entropy alloys (HEAs) were prepared by directional solidification at different solidification rates (5, 10, 20, 50 and100 μm/s). Results show that two kinds of dendrite form in the directionally solidified process. One is primary solidified dendrite, which is composed of FCC1 phase enriched in Co, Cr and Fe. Another is secondary solidified dendrite, forming between the primary solidified dendrites and consisting of the FCC2 phase enriched in Ni, Al and Ti. As the solidification rate increases, the primary solidified dendrite spacing decreases, the solute enrichment layer overlaps, and the constitutional undercooling in the inter-dendrite region decreases, increasing the difficulty of the secondary solidified dendrite formation. Tensile results show that the strength of Ni36Co30Fe11Cr11Al6Ti6 HEAs gradually increases with the increase of solidification rate. The alloy with a solidification rate of 100 µm/s exhibits excellent comprehensive room temperature properties, the yield strength, fracture strength and tensile strain are 732 MPa, 1020 MPa and 34.9 %, respectively. Furthermore, the directionally solidified Ni36Co30Fe11Cr11Al6Ti6 HEA still remains high yield strength of 533 MPa at 900 ℃. Fine grain strengthening, solid solution strengthening, second phase strengthening and hysteresis diffusion effects play an important role in enhancing the mechanical properties. By comparing the mechanical properties of other HEAs at room and high temperature, the proposed directionally solidified Ni36Co30Fe11Cr11Al6Ti6 HEAs with excellent mechanical properties show great potential in industrial applications.