An Ultra-Low Modulus of Ductile TiZrHfTa Biomedical High-Entropy Alloys through Deformation Induced Martensitic Transformation/Twinning/Amorphization.

Biomedical alloys are paramount materials in biomedical applications, particularly in crafting biological artificial replacements. In traditional biomedical alloys, a significant challenge is simultaneously achieving an ultra-low Young's modulus, excellent biocompatibility, and acceptable ductility. A multi-component body-centered cubic (BCC) biomedical high-entropy alloy (Bio-HEA), which is composed of non-toxic elements, is noteworthy for its outstanding biocompatibility and compositional tuning capabilities. Nevertheless, the aforementioned challenges still remain. Here, a method to achieve a single phase with the lowest Young's modulus among the constituent phases by precisely tuning the stability of the BCC phase in the Bio-HEA, is proposed. The subtle tuning of the BCC phase stability also enables the induction of stress-induced martensite transformation with extremely low trigger stress. The transformation-induced plasticity and work hardening capacity are achieved via the stress-induced martensite transformation. Additionally, the hierarchical stress-induced martensite twin structure and crystalline-to-amorphous phase transformation provide robust toughening mechanisms in the Bio-HEA. The cytotoxicity test confirms that this Bio-HEA exhibits excellent biocompatibility without cytotoxicity. In conclusion, this study provides new insights into the development of biomedical alloys with a combination of ultra-low Young's modulus, excellent biocompatibility, and decent ductility.

High stress twinning in a compositionally complex steel of very high stacking fault energy.

Deformation twinning is rarely found in bulk face-centered cubic (FCC) alloys with very high stacking fault energy (SFE) under standard loading conditions. Here, based on results from bulk quasi-static tensile experiments, we report deformation twinning in a micrometer grain-sized compositionally complex steel (CCS) with a very high SFE of ~79 mJ/m2, far above the SFE regime for twinning (<~50 mJ/m2) reported for FCC steels. The dual-nanoprecipitation, enabled by the compositional degrees of freedom, contributes to an ultrahigh true tensile stress up to 1.9 GPa in our CCS. The strengthening effect enhances the flow stress to reach the high critical value for the onset of mechanical twinning. The formation of nanotwins in turn enables further strain hardening and toughening mechanisms that enhance the mechanical performance. The high stress twinning effect introduces a so far untapped strengthening and toughening mechanism, for enabling the design of high SFEs alloys with improved mechanical properties.