High-strength magnesium alloys for degradable implant applications

▶ Successful development of high-strength–high-ductility Mg alloys. ▶ Tailorable mechanical properties by means of thermal treatment. ▶ Explanation of mechanical properties on basis of accurate microstructural analysis, in particular TEM. This article describes the design principles deployed in developing high-strength and ductile Mg–Zn–Zr–Ca–Mn(–Yb) alloys based on a concept, which aims to restrict grain growth considerably during alloy casting and forming. The efficiency of the development approach is discussed. Moreover, the microstructure and phase analysis of the alloys subjected to different thermal treatments are presented and the influence of the alloy composition, particularly the addition of Yb, on the evolution of the microstructure is discussed in connection with the mechanical properties of the materials. The newly developed alloys exhibit high strength (yield stress of up to 350MPa) at considerable ductility (elongation to fracture of up to 19%) in the as-extruded state and reveal age hardening potential (increase in hardness of 10–15% compared to that in the recrystallization heat-treated state). Appropriate heat treatments enable tailoring of the strength–ductility relation. Thermal annealing of the material resulted in a remarkable increase in ductility (elongation to fracture of more than 20% for all heat-treated samples) while high strength is retained (yield stress ranging from 210 to 315MPa). We attribute the attractive mechanical properties of the developed alloys to their fine-grained microstructure, where the grain boundaries and lattice defects are stabilized by second phase particles formed during casting and thermal treatments.