Simultaneous strength retention and controllable degradation achieved by gradient heterostructure in biomedical Mg–Zn–Mn alloys

Mg-Zn-Mn alloys are promising materials for load-bearing orthopedic implants, exhibiting excellent biocompatibility, biodegradability, and a favorable elastic modulus, which eliminates the necessity for secondary surgical intervention. However, achieving a balance between strength and corrosion resistance remains a key challenge for biodegradable load-bearing applications. Herein, a series of Mg-Zn-Mn alloys possessing a gradient heterostructure is developed via surface mechanical grinding treatment (SMGT), which ensures sustainable strength and enables graded degradation via surface nanograin (NG) structure. Specifically, the thickness of NG structure ranges from 80 to 300 μm, and the average surface grain size is refined from 12 μm to 70 nm, which significantly improved the yield strength by 37%, from 218 to 300 MPa. After immersion in modified simulated body fluid for 336 h, the strength maintained 87% of its original value, which is substantially higher than that of the sample without SMGT. Moreover, the corrosion rate of surface NG structure is about 1.01 mm/year, whereas that of core coarse-grain (CG) structure is 1.89 mm/year, indicating a gradient degradation behavior in the heterogeneous structure. The improvements in strength are attributed to grain boundary strengthening and strain hardening, resulting from heterogeneous deformation. The enhanced corrosion resistances are driven by high-density grain boundaries and Mn-supersaturated solid solution, which effectively suppress micro-galvanic corrosion and promote the formation of compact corrosion products. This work presents a novel strategy based on gradient heterostructure design to achieve controllable degradation, offering valuable theoretical insights for the development of load-bearing orthopedic implants.