Formation, thermal stability and mechanical properties of high entropy (Fe,Co,Ni,Cr,Mo)-B amorphous alloys

The formation, thermal stability and mechanical properties of high entropy (HE) (Fe0.25Co0.25Ni0.25Cr0.125Mo0.125)100-xBx (x = 11–31 at%) amorphous alloys were examined with the aim of clarifying the partial replacement effect of Cr by Mo in comparison with previously reported HE (Fe0.25Co0.25Ni0.25Cr0.25)100-xBx amorphous alloys. The amorphous phase was formed in a very wide B range of 11–31 at%, and crystallization temperature (Tx) and Vickers hardness (Hv) increased with increasing B content. The peak position of the X-ray diffraction patterns shifts to the higher diffraction angle side with increasing B content, implying the development of denser coordinated atomic configuration. There is a good linear relation between Hv and the angle shift of the peak position in the broad diffraction pattern. The crystallization mode changes from Am → fcc + Am for the 15–20 at% B alloys to Am → glass transition (GT) →M23B6 for the 25 and 27 at% B alloys. It is noticed that the GT is observed for the alloys with 22–31 at% B. The good bending ductility is obtained for the alloys up to 25 at% B and the ductile 25 at% B alloy exhibits high Tx and high Hv. Considering the previous data that the Mo-free amorphous alloys are formed in the limited B range of 18–22 at% B and do not show the GT, the partial replacement of Cr by Mo is effective for the increase in glass-forming ability and Hv through the development of more coordinated atomic configuration resulting from the increase in the level of satisfaction of the three component rules for stabilization of supercooled liquid. The interpretation is also supported from the differences in the calculated entropy parameters between the HE amorphous alloys and HE crystalline solid solution alloys. The first formation of an amorphous phase for the lower and higher B content alloys of 11–14 and 26–31 at% is also promising for future application to structural materials.