Achieving ambient temperature quasi-superplasticity in a high strength Zn-2Cu-0.15Mg alloy with ultrafine/fine grained structure

Abstract

Zinc (Zn) alloys are regarded as one of the most promising candidates to replace traditional implant metals due to their moderate degradation rate and good biocompatibility. Superplastic Zn alloys are favorable for the forming of complex medical devices, however, superplastic alloys usually exhibit relatively low strength. In this work, the alloy design concept for biodegradable Zn is employed to break the trade-off between strength and ductility. Quasi-superplasticity was achieved in a high-strength Zn-2Cu-0.15Mg alloy with a bimodal grain structure (ultrafine and fine grains) through a combined process of hot extrusion and room-temperature (RT) rolling. RT tensile tests were subsequently conducted under various strain rates. Notably, the processed alloys demonstrated an outstanding combination of properties: a quasi-superplastic strain of approximately 138.2%, a yield strength (YS) of ∼219.5 MPa, and ultimate tensile strength (UTS) of ∼301.5 MPa at a strain rate of 1 × 10⁻⁴ s⁻¹. Using quasi-in-situ electron backscatter diffraction (EBSD) analyses, we systematically investigated the microstructure and texture evolution of the rolled alloy during quasi-superplastic deformation at different strains. The findings indicated that the ultrafine grains experienced grain rotation and grain boundary sliding (GBS), whereas dislocation creep was predominant in fine grains. Dynamic recrystallization (DRX) and GBS significantly contributed to the quasi-superplastic strain during tensile deformation. Additionally, numerous spherical submicron-sized CuZn₄ phases created abundant phase interfaces, which facilitate quasi-superplastic deformation through phase boundary sliding (PBS).