Role of boron in yield strength softening and plastic deformation mechanisms in an equiatomic TiNbZrHfTa refractory high-entropy alloy

Grain boundary (GB) engineering is an effective tool for improving the mechanical properties of metallic materials. In this study, we added 30 appm boron (B) to an equiatomic TiNbZrHfTa refractory high-entropy alloy (HEA), and found that the GB chemistry has been altered. B preferentially segregates to GBs with the co-segregation of Zr and depletion of Nb and Ta, as revealed by atom probe tomography probing. This change in GB chemistry affects the macroscopic load-bearing performance of the HEA, where a reduction in the yield strength by ∼6.2 ± 0.7% is observed, i.e. , 869.8 ± 10.4 MPa of the B-doped HEA vs. 920.7 ± 3.7 MPa of the B-free HEA. Additionally, a correlation between GB chemistry and the plastic deformation at GBs is found, indicated by changes in the crystallographic slip traces and the Luster-Morris compatibility factor among adjacent crystals. More specifically, we find that (i) B-decorated GBs accommodate higher adjacent plastic strain levels, which is manifested as an out-of-plane offset (via GB shear localization) between two neighboring grains; and (ii) in the B-doped case, slip transfer across GBs requires a lower misalignment between slip planes or slip directions as compared with the B-free sample. This study thus enhances our understanding of the role of GB chemistry in the mechanical properties of polycrystalline body-centered cubic HEAs, leveraging opportunities for GB segregation engineering in this field.