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Biocompatibility of HEAs in the TiZrHfNbTa system in which all the constituents are non-toxic and allergy-free was scrutinized systematically, and novel biomechanical materials with a unique combination of low modulus (57 GPa, almost half that of conventional biomedical titanium alloys), good mechanical biocompatibility and low magnetic susceptibility (1.71 × 10 −6 cm 3 g −1 , similar to that of pure Zr) were successfully developed. Moreover, the underlying mechanisms responsible for phase formation and promising properties were explored. This work not only offers a series of novel bio-metallic materials with prominent properties for practical applications, but also shed light on understanding of phase formation and strengthening of HEAs in general. IMPACT STATEMENT Several biocompatible TiZrHfNbTa HEAs with prominent properties for practical applications were developed and the relevant alloy design principles were revealed.

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We conducted systematically tensile tests with a series of Nb-based equiatomic alloys with singlephase, body-centered-cubic (bcc) structure. By subtracting all possible strengthening contributions, the intrinsic strength (or lattice friction stress) of each alloy was extracted. It was found that lattice friction stress scaled linearly with the lattice distortion in these Nb-based alloys. A simple model was developed to interpret this result. Strengthening effects of lattice distortion and dislocation core width between bcc Nb-based and face-centered-cubic (fcc) Ni-based equiatomic alloys were also compared. It was demonstrated that the enhanced strength in high-entropy alloys was attributable to the lattice distortion. IMPACT STATEMENTLattice friction stress in equiatomic alloys correlates linearly with the lattice distortion, quantitatively defined as the average lattice strain. A simple physical model was developed to interpret this result.