Towards refining microstructures of biodegradable magnesium alloy WE43 by spark plasma sintering

Abstract

Microstructural refinement of magnesium (Mg) alloys is beneficial for mechanical and corrosion properties, both of which are critical for their successful application as temporary implant materials. One method of achieving a refined microstructure is through rapid solidification via gas-atomization-powder production. In this study we investigated spark plasma sintering (SPS) as a potential processing method for maintaining this refined microstructure while achieving a range of porosities up to full densification. We characterized the microstructural evolution as a function of sintering temperature from 250 to 450 °C for the alloy WE43 using multi-scale correlative microscopy techniques, including light microscopy and scanning and transmission electron microscopy-based methods. The spatial distribution of the two major alloying elements, neodymium (Nd) and yttrium (Y), was determined and the intermetallic phases they form identified using energy dispersive X-ray spectroscopy in conjunction with electron diffraction. The gas-atomized powder microstructure consists of Mg-rich dendrites and a percolating interdendritic Mg-Nd-Y ternary phase with structure Mg₁₄Nd₂Y, surrounded by a high Nd and Y content in solid solution. This microstructure is maintained up to a sintering temperature of 350 °C, while with higher sintering temperatures segregation of Nd and Y dominates. The percolating ternary phase breaks up into faceted globular precipitates with structure Mg₅Nd, which is isomorphous to Mg₁₄Nd₂Y. Y comes out of solution and migrates to previous powder-particle surfaces, possibly forming Y₂O₃. Sample densities ranged from 64 to 100% for sintering temperatures of 250 to 450 °C, respectively, and the grain size remained constant at about 10 µm. SPS is demonstrated to be an attractive alternative method for processing Mg alloys to a wide range of porosities and fine microstructures. The microstructural refinement achieved by SPS holds the potential for slow and homogeneous corrosion. STATEMENT OF SIGNIFICANCE: This study presents the impact spark plasma sintering (SPS) has on the microstructure of WE43, a magnesium alloy used for biodegradable implants. SPS is of great interest in this context as it is scalable, rapid, and has the potential for tuning density while maintaining a refined microstructure. The microstructure and density are explored from the gas-atomized powder to the densified material using electron microscopy and chemical mapping from the macro- to the nano-level. The insights gained reveal an original evolution of rare-earth element distribution with an isomorphous chemistry change, while the microstructure develops from the non-equilibrium state (powder) towards an equilibrium structure upon sintering. This study, including measurements of mechanical performance, sets the premises of SPS for the fabrication of Mg-based implants with tunable characteristics.

Keywords: Biodegradable alloy; Magnesium alloy; Microstructure; Rapid solidification; Sintering; Transmission electron microscopy; WE43.