Jinshan Zhang

The effect of extrusion on the microstructure, mechanical property, corrosion behavior, and in vitro biocompatibility of as-cast Mg-1.5Zn-1.2Y-0.1Sr (wt.%) alloy was investigated via tensile tests, electrochemical methods, immersion tests, methylthiazolyl diphenyltetrazolium bromide (MTT), and analytical techniques. Results showed that the as-cast and as-extruded Mg-1.5Zn-1.2Y-0.1Sr alloys comprised an α-Mg matrix and Mg₃Y₂Zn₃ phase (W-phase). In the as-cast alloy, the W-phase was mainly distributed at the grain boundaries, with a small amount of W-phase in the grains. After hot extrusion, the W-phase was broken down into small particles that were dispersed in the alloy, and the grains were refined considerably. The as-extruded alloy exhibited appropriate mechanical properties that were attributed to refinement strengthening, dispersion strengthening, dislocation strengthening, and precipitation strengthening. The as-cast and as-extruded alloys exhibited galvanic corrosion between the W-phase and α-Mg matrix as the main corrosion mechanism. The coarse W-phase directly caused the poor corrosion resistance of the as-cast alloy. The as-extruded alloy obtained via hydrogen evolution and mass loss had corrosion rates of less than 0.5 mm/year. MTT, high-content screening (HCS) analysis, and cell adhesion tests revealed that the as-extruded alloy can improve L929 cell viability and has great potential in the field of biomedical biodegradable implant materials.

To develop a biomedical magnesium alloy with desirable mechanical properties and corrosion resistance, Mg–6Zn–8.16Y–2.02Mn and Mg–6Zn–8.16Y–2.02Mn–0.3Mo alloy were prepared. Then the alloys were solution-treated at 500°C for 40 h and cooled in the furnace. The results showed that partly 18R-long-period stacking ordered (LPSO) phase transformed to 14H-LPSO phase during solid-solution treatment. The hollowed-out W phase vanished and spherical W phase was formed. And the volume fraction of 18R-LPSO and the average size of the spherical W phase particles were reduced. Corrosion behaviour of the alloy in Hank’s solution under each condition was studied by immersion test and electrochemical test. The solution-treated Mg–6Zn–8.16Y–2.02Mn–0.3Mo alloy exhibited the best mechanical property (ultimate tensile strength, UTS 270 MPa and elongation, EL 23%) and corrosion resistance (P_W 1.03 mm/year and I_corr 0.20 mm/year) due to fine and homogeneous microstructure.

The effect of Al microalloying on microstructure and corrosion behaviors of as‐cast Mg–6Zn–8.16Y–2.02Mn is investigated. Microstructural characterization reveals that Al addition can refine grain and lead to the formation of Al(Y, Zn) phase. When a 0.3 wt% Al is added, the alloy exhibits the lowest weight loss rate of 0.99 mm/Y and displays relatively uniform corrosion morphology. However, alloys with more Al addition present localized corrosion on the surface. It results from more W phase (Mg₃Zn₃Y₂) and a little long‐period stacking order (LPSO) phase along the grain boundary. Electrochemical tests are also used to rationalize the corrosion behaviors observed in weight loss. In conclusion, the decrease in corrosion rate is due to grain refinement, optimal quantification of Al(Y, Zn) with short‐rod shape and LPSO phase.

The previous study conducted on the as-cast Mg-2Y-1Zn-0.6Zr alloy showed that the tensile strength, yield strength and elongation of the as-cast alloy were 245 MPa, 135 MPa and 14.4%, respectively. In order to further explore the potential of the material, the hot extrusion process of variable temperature (250 °C, 300 °C and 350 °C) was carried out on the basis of the as-cast alloy. After hot extrusion, the mechanical properties of the material have been greatly improved compared with as-cast alloy. The tensile strength, yield strength and elongation of the extruded alloy reached 327 MPa, 322 MPa and 24.9%, respectively. The reason for the significant improvement of material properties is mainly due to the dynamic recrystallization during thermal processing, which greatly fines the grains of as-cast alloy. Moreover, the experimental results shown that the corrosion performance of the alloy after hot extrusion at 300 °C is also optimal.

Herein, the effect of calcium (Ca) microalloying on the microstructure and mechanical properties of Mg₉₄Zn_2.5Y_2.5Mn₁ alloy is identified. The long‐period stacking‐ordered (LPSO) phase transformation mechanism during solid‐solution treatment and hot deformation behavior during extrusion process are analyzed. The role of minor (0.34 at%) Ca addition in the as‐cast alloy is related to the fine microstructure and abundant 18R‐LPSO phase. Furthermore, the precipitation of 14H‐LPSO phase is greatly induced by Ca addition in subsequent solid‐solution treatment. As for as‐extruded alloys, the dynamic precipitation behavior of W phase nanoparticles is found. The high‐density W phase nanoparticles suppress the dynamic recrystallization (DRX) behavior and inhibit the growth of dynamical recrystallized (DRXed) grains. The as‐extruded Ca‐modified alloy shows a superior mechanical property with yield strength (YS), ultimate tensile strength (UTS), and elongation of 400, 434 MPa, and 19.5%, respectively. The tiny microstructure, W phase nanoparticles, and kinky LPSO (both 18R and 14H type) phase are the reasons for the outstanding mechanical properties.

Ni alloying is first adopted to explore the precipitation of quasicrystal phase (I phase) in Mg_96.5Zn₁Y_1.5Mn₁ alloy containing LPSO structure and W phase during solidification. Results show the component of I phase is Mg₃(Zn, Ni)₆Y₁ which mainly distributes near the W phase. Besides, during the formation of I phase, Ni acts as Zn to participate in the precipitation process of I phase, making up for the deficiency of Zn. Meanwhile, an excellent strength‐ductility balance with ultimate tensile strength (210 MPa) and elongation (9.5%) is greatly optimized.