Continuous ECAP refines grain in LZXM2100 and WE22 magnesium alloys for biodegradable implants

An international team of materials researchers applied severe plastic deformation by continuous equal-channel angular pressing (C-ECAP) to two biomedical magnesium alloys, LZXM2100 and WE22. The microstructure analysis is described in a paper available via doi.org. The processing temperatures were 250–350 °C, and eight passes gave a total effective strain above 5.3 springer.comdoi.orgspringer.comOpen source ↗.

Magnesium alloys are considered for biodegradable orthopedic and vascular implants because their density and elastic modulus are close to those of bone. Their hexagonal close-packed lattice, however, gives low strength and poor ductility at room temperature. Continuous pressing, unlike conventional equal-channel pressing, can process long workpieces, which the authors present as a route to scalable manufacturing.

Starting material and alloys

LZXM2100, developed by Fort Wayne Metals, contains lithium, zinc, calcium and manganese. WE22 is a reduced rare-earth alloy with lower yttrium and neodymium content than WE43. Both were supplied as extruded rods with an extrusion ratio of 16:1 and a ram speed of 4 mm/s. LZXM2100 was extruded at 300 °C and had an area-weighted average grain size of 12.0 ± 5.79 µm; WE22 was extruded at 400 °C and had 7.50 ± 4.35 µm.

Recrystallization during pressing

Continuous dynamic recrystallization dominated in both alloys. In LZXM2100 the fraction of low-angle grain boundaries was high and the process remained incomplete. Coarse Mg2Ca particles larger than 1 µm promoted particle-stimulated nucleation, but because of their low volume fraction and uneven distribution the effect stayed local. The result was a refined but relatively heterogeneous microstructure springer.comdoi.orgspringer.comOpen source ↗.

WE22 showed a much more uniform ultrafine-grained structure with a higher fraction of high-angle boundaries. Most grains were fully recrystallized, and internal misorientation was generally low. The authors link this to yttrium and neodymium, which segregate to grain boundaries and weaken discontinuous recrystallization through solute drag. They also observed a high density of nanoscale rare-earth-rich particles, likely Mg–Nd and Mg–Y phases according to XRD, which restrict grain boundary migration.

At a total effective strain above 5.3 the grain size became small enough that twinning, and with it twinning-induced recrystallization, was eliminated. TEM of WE22 revealed stacking faults or slip lines, which the authors attribute to reduced stacking fault energy from zinc and rare-earth additions.