Effect of Extrusion on Mechanical Property, Corrosion Behavior, and In Vitro Biocompatibility of the As-Cast Mg-Zn-Y-Sr Alloy

Abstract

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.

Keywords: MTT; Mg; extrusion; galvanic corrosion; grain refinement.

Figure 1

OM images of the (a) as-cast and (b) as-extruded alloys. The inset in (b) shows the longitudinal section of the as-extruded alloy.

Figure 2

Figure (a) and (b) represent the SEM images of the as-cast and as-extruded, respectively. Figure (c) shows the longitudinal section of the as-extruded alloy.

Figure 3

(a) XRD patterns, (b) DSC curve of the as-cast alloy, TEM images of the (c) as-cast and (d) as-extruded alloys, (e) HRTEM of W-phase in the as-extruded alloy, (f) EDS mapping of the as-extruded alloy. The insets in (c,d) represent the SAED patterns.

Figure 4

(a) Stress–strain curves and (b) Vickers hardness of the as-cast and as-extruded alloys. **** p < 0.0001.

Figure 5

Fracture morphology of the (a,c) as-cast and (b,d) as-extruded alloys. Red arrows in (c) represent the microcracks at the grain boundaries. Yellow arrows in (c,d) represents the retained W-phase.

Figure 6

(a) pH variations during 240 h immersion in SBF, (b) Mg²⁺ ion concentration, and (c) corrosion rates after immersion in SBF for 240 h. **** p < 0.0001.

Figure 7

Curves of (a) OCP, (b) PDP, and (c) Nyquist plot measured in SBF. The inset in (c) represents equivalent circuits.

Figure 8

Corrosion morphologies, surface corrosion products, and longitudinal section images of the (a–c) as-cast and (d–f) as-extruded alloys after 240 h immersion. The arrows in (d) represent small pits left by the detachment of the W-phase. The arrows in (e) show a local rupture caused by pitting corrosion.

Figure 9

(a) XRD patterns, (b) FTIR spectra, and (c–f) XPS spectra with high-resolution scanning of Mg 2p, O 1s, Ca 2p, and P 2p of the corrosion products after immersion in SBF for 240 h.

Figure 10

RGRs of L-929 cells after (a) 24 h, (b) 48 h, and (c) 72 h incubation at different extract concentrations. * p < 0.05.

Figure 11

Cell morphology after culturing for (a–c) 24 h, (d–f) 48 h, and (g–i) 72 h in DMEM (control group) and 100% extracts of the as-cast and as-extruded alloys.

Figure 12

High-content analysis of L-929 cells after culturing for (a,d) 24 h, (b,e) 48 h, and (c,f) 72 h in 100% extracts. * p < 0.05, ** p < 0.01, *** p < 0.001, **** p < 0.0001.

Figure 13

Cell adhesion on the (a–c) as-cast and (b–f) as-extruded alloys after culturing for 24 h.

Figure 14

Schematic diagram of microvoid coalescence fracture in the as-extruded alloy.

Figure 15

(a) Nanoscale precipitates and dislocations in the as-extruded alloy; (b) HRTEM of nanoscale precipitates in the as-extruded alloy. The inset in (a) represents the EDS results of the white box.

Figure 16

Corrosion mechanism of the as-cast and as-extruded Mg-1.5Zn-1.2Y-0.1Sr alloys.

Figure 17

(a) pH and (b) ion concentrations of the extracts after 72 h immersion. * p < 0.05, ** p < 0.01, *** p < 0.001.