300 MPa grade highly ductile biodegradable Zn-2Cu-(0.2-0.8)Li alloys with novel ternary phases

Abstract

Although a few high-strength biodegradable Zn alloys with yield strengths (YSs) over 300 MPa in rolled state have been developed, their elongations (ELs) are generally less than 30%. This study developed rolled Zn-2Cu-xLi (x = 0.2 wt.%, 0.5 wt.%, 0.8 wt.%) alloys with YSs of 316–335 MPa and ELs of 44%–61%. Three-dimensional atom probe (3DAP) and time of flight secondary ion mass spectrometry (TOF-SIMS) were employed to characterize Li distribution. Three kinds of Zn-Cu-Li ternary phases are identified, which are blocky ε′-(Cu0.5, Li0.5)Zn4, blocky β′-(Li0.9, Cu0.1)Zn4, and small round γ particles with high Li content in the annealed state. Other identified phases are Zn, β-LiZn4, and ε-CuZn4 phases. With the increase of Li content in the alloys, ε′ phase with 6.50 at.% Cu transforms into β′ phase with 2.12 at.% Cu, i.e., the average level in the alloys. Within ε′ phase, there exist nano-scale Li clusters and ε phase, resulting in ε′/ε structure. Dense Zn laths precipitate from β′ phase, resulting in β′/Zn lamellar structure. The lamellar structure is the matrix of Zn-2Cu-0.8Li and leads to near-isotropic plasticity. Electrochemistry tests show that degradation rates fall in the range of 153–196 μm/year, which decrease with Li content. All the alloys exert positive effects on the growth of MC3T3-E1 cells with 10% extract. This research reveals how microstructure evolves in Zn-2Cu-xLi alloys, which lays the foundation for their future applications.