Yoshihito Kawamura

The Mg-Y-Zn alloy system is well known for its outstanding combination of high strength and ductility, even at relatively low concentrations of alloying elements. This exceptional performance is primarily at-tributed to its characteristic micr...

The Mg-Y-Zn alloy system is well known for its outstanding combination of high strength and ductility, even at relatively low concentrations of alloying elements. This exceptional performance is primarily at-tributed to its characteristic microstructure, which features Long-Period Stacking Ordered (LPSO) phases and the distinctive Mille-Feuille Structure (MFS). Kink-induced strengthening, developed during thermomechanical processing, has emerged as a promising strategy to simultaneously enhance strength and ductility. In this study, the beneficial effect of pre-deformation aimed at introducing additional kinks into the microstructure prior to extrusion is demonstrated. The subsequent extrusion process promotes dynamic recrystallization (DRX), generating fine DRX grains while preserving kink structures in the non-DRX regions. As a result, the yield strength is enhanced by approximately 80 MPa, accompanied by a slight improvement in ductility.

The Mg-Y-Zn magnesium alloy system is known for the presence of Long-Period Stacking Ordered (LPSO) phases that improves strength and ductility with minimal amounts of alloying elements. Even better improvements are associated with the specific microstructure known as the Mille-Feuille structure (MFS) that can occur in this alloy as well after proper heat treatment. This study systematically compares the traditional ingot metallurgy method with the Bridgman method (slow cooling), coupled with diverse heat treatments and extrusion process. Microscopic analyses reveal variations in the presence of LPSO phases, Mille-Feuille structure, and especially grain size, leading to divergent mechanical and corrosion properties. The Bridgman approach surprisingly stands out, ensuring superior mechanical properties due to kink and texture strengthening.

Background: Bioresorbable flow diverters (BRFDs) have the potential to solve several problems associated with conventional permanent flow diverters. We have constructed bare and poly-L-lactic acid (PLLA)-coated magnesium BRFDs (MgBRFDs) using a high-strength corrosion-resistant magnesium alloy. This study aimed to compare bioresorption and biocompatibility between the two types in a rabbit vascular model to determine which is more clinically feasible in humans. Methods: Bare and PLLA-coated MgBRFDs were fabricated by braiding 48 thin magnesium alloy wires. Mechanical testing was conducted. Bare (n=13) and PLLA-coated (n=13) MgBRFDs were implanted into rabbit aortas and harvested 14, 30, and 90 days after implantation. The physical structure of the resolution process was examined using optical coherence tomography (OCT), micro-computed tomography, and scanning electron microscopy (SEM). The biological response of the vascular tissue was examined using SEM and histopathological analysis. Results: The porosity and pore density of the bare MgBRFD were 64% and 16 pores/mm ² , respectively; corresponding values for the PLLA-coated MgBRFD were 63% and 12 pores/mm ² , respectively. The OCT attenuation score was significantly higher for the PLLA-coated MgBRFD at all time points (14 days, P=0.01; 30 days, P=0.02; 90 days, P=0.004). OCT, micro-computed tomography, and SEM demonstrated better stent structure preservation with the PLLA-coated MgBRFD. Neointimal thickness did not significantly change over time in either type of MgBRFD (bare, P=0.93; PLLA-coated, P=0.34); however, the number of inflammatory and proliferative cells peaked at 14 days and then decreased. Conclusions: Both bare and PLLA-coated MgBRFDs had excellent biocompatibility. The PLLA-coated MgBRFD has greater clinical feasibility because of its delayed bioresorption.

High thermal conductivity ternary Mg alloys composed of Zn and Y pair with a negatively high mixing enthalpy was developed by optimization of alloy composition and heat-treatment conditions. Optimal alloy composition was Mg-1.88Zn-0.75Y (at.%) alloy, in which the ratio of Zn content to Y content was 2.5 and the Y content was 0.75 at%. The alloy was composed of α-Mg+W phase (Mg3Zn3Y2)+I phase (icosahedral Mg3Zn6Y). Heat treatment under the optimal heat treatment conditions, where temperature, time and cooling rate were 633 K, 15 h and air cooling, respectively, improved the thermal conductivity from 114 to 141 Wm−1K−1 that corresponds to 90% of the pure Mg thermal conductivity. Fine W phase precipitation in α-Mg matrix by the heat-treatment caused a reduction of solute Y element in α-Mg matrix, resulting in improvement of the thermal conductivity.

Mg-Y-Zn-Al alloys processed by the rapidly solidified ribbon consolidation (RSRC) technique are candidate materials for structural applications due to their improved mechanical performance. Their outstanding mechanical strength is attributed to solute-enriched stacking faults (SESFs), which can form cluster-arranged layers (CALs) and cluster-arranged nanoplates (CANaPs) or complete the long-period stacking ordered (LPSO) phase. The thermal stability of these solute arrangements strongly influences mechanical performance at elevated temperatures. In this study, an RSRC-processed Mg—0.9%, Zn—2.05%, Y—0.15% Al (at%) alloy was heated at a rate of 0.666 K/s up to 833 K, a temperature very close to melting point. During annealing, in situ X-ray diffraction (XRD) measurements were performed using synchrotron radiation in order to monitor changes in the structure. These in situ XRD experiments were completed with ex situ electron microscopy investigations before and after annealing. At 753 K and above, the ratio of the matrix lattice constants, c/a, decreased considerably, which was restored during cooling. This decrease in c/a could be attributed to partial melting in the volumes with high solute contents, causing a change in the chemical composition of the remaining solid material. In addition, the XRD intensity of the secondary phase increased at the beginning of cooling and then remained unchanged, which was attributed to a long-range ordering of the solute-enriched phase. Both the matrix grains and the solute-enriched particles were coarsened during the heat treatment, as revealed by electron microscopy.

High thermal conductivity ternary Mg alloys composed of Zn and Y pair with a negatively high mixing enthalpy was developed by optimization of alloy composition and heat-treatment conditions. Optimal alloy composition was Mg-1.88Zn-0.75Y (at%) alloy, in which the ratio of Zn content to Y content was 2.5 and the Y content was 0.75 at%. The alloy was composed of α-Mg＋W phase (Mg₃Zn₃Y₂）＋I phase (icosahedral Mg₃Zn₆Y). Heat treatment under the optimal heat treatment conditions, where temperature, time and cooling rate were 633 K, 15 h and air cooling, respectively, improved the thermal conductivity from 114 to 141 Wm^‒1K^‒1 that corresponds to 90% of the pure Mg thermal conductivity. Fine W phase precipitation in α-Mg matrix by the heat-treatment caused a reduction of solute Y element in α-Mg matrix, resulting in improvement of the thermal conductivity.

We investigated the alloy composition of Mg-Ca-Zn alloys suitable for rapidly solidified powder metallurgy (RS PM) processing and tried to develop biomedical Mg-Ca-Zn RS PM alloys with high strength, high ductility, and high corrosion resistance. The alloy composition suitable for RS PM processing was Mg-1.0Ca-0.5Zn (at%) in ternary Mg-Ca-Zn alloys. An Mg-1.0Ca-0.5Zn RS PM alloy with combined additions of 0.1 at% Y and 0.03 at% Mn exhibited excellent performance with a tensile yield strength of 376±17 MPa, an elongation of 14.1±3.1%, and a corrosion rate of 0.46±0.04 mm·year-1 when immersed in HBSS, resulting in the achievement of our target properties. Especially the yield strength of the Mg-1.0Ca-0.5Zn-0.1Y-0.03Mn RS PM alloy developed in this study was higher than that of previously reported Mg-Zn-Ca(-Mn-Zr) or Mg-Ca-Zn(-Mn-Zr) ingot metallurgy (IM) alloys, which were produced by single extrusion, double extrusion or severe plastic deformation (SPD) of cast ingots.