Bioerodible magnesium alloy microstructures for endoprostheses

A method of processing a bioerodible magnesium alloy for a stent comprising:

• forming an ingot or billet comprising a molten magnesium alloy comprising from 50 weight percent to 92 weight percent magnesium, from 5.5 weight percent in sum to 50 weight percent in sum of one or more elements selected from the group consisting of Ho, Er, Lu, Tb and Tm, and from 2.0 weight percent in sum to 50 weight percent in sum of one or more elements selected from the group consisting of Y, Nd and Gd;
• cooling the molten magnesium alloy to form an ingot or billet;
• and
• performing at least one process, comprising Equal-Channel Angular Extrusion (ECAE) or Equal-Channel Angular Pressing (ECAP), on the ingot or billet to form a microstructure comprising equiaxed Mg-rich solid solution-phase grains having an average grain diameter of less than or equal to 15 microns and continuous or discontinuous second-phase precipitates or ceramic nanoparticles in grain boundaries between the equiaxed Mg-rich solid solution-phase grains, the second-phase precipitates or ceramic nanoparticles having an average longest dimension of 2.0 micron or less;
• and
• performing an intermediate annealing of the ingot or billet after performing the at least one ECAE or ECAP process on the ingot or billet at a temperature of from 150° C. and 250° C.