Microstructure and Mechanical Properties of Hot-Extruded Mg-Zn-Ga-(Y) Biodegradable Alloys

Abstract

Magnesium alloys are attractive candidates for use as temporary fixation devices in osteosynthesis because they have a density and Young's modulus similar to those of cortical bone. One of the main requirements for biodegradable implants is its substitution by tissues during the healing process. In this article, the Mg-Zn-Ga-(Y) alloys were investigated that potentially can increase the bone growth rate by release of Ga ions during the degradation process. Previously, the effectiveness of Ga ions on bone tissue regeneration has been proved by clinical tests. This work is the first systematic study on the microstructure and mechanical properties of Mg-Zn-Y alloys containing Ga as an additional major alloying element prepared by the hot-extrusion process. The microstructure and phase composition of the Mg-Zn-Ga-(Y) alloys in as-cast, heat-treated, and extruded conditions were analyzed. In addition, it was shown that the use of hot extrusion produces Mg-Zn-Ga-(Y) alloys with favorable mechanical properties. The tensile yield strength, ultimate tensile strength, and elongation at fracture of the MgZn4Ga4 alloy extruded at 150 °C were 256 MPa, 343 MPa, and 14.2%, respectively. Overall, MgZn4Ga4 alloy is a perspective for applications in implants for osteosynthesis with improved bone regeneration ability.

Keywords: biomaterials; gallium; hot extrusion; magnesium; mechanical properties; microstructure.

Figure 1

The scheme of extruded bars processing from casting to hot extrusion.

Figure 2

(a) Graph defining the quality of the Mg–Zn–Ga–(Y) alloy bars as a function of alloy composition and extrusion temperature. The quality examples are shown in: (b) bar with large radial cracks (e.g., MgZn4Ga2 alloy extruded at 250 °C), (c) bar with small (~1−2 mm) surface cracks (e.g., MgZn4Ga4 alloy extruded at 150 °C), and (d) bar without cracks (e.g., MgZn2Ga2 alloy extruded at 150 °C).

Figure 3

Microstructure of the (a–d) MgZn4Ga4, (e–h) MgZn4Ga4Y0.5, (i–l) MgZn6.5Ga2, (m–p) MgZn4Ga2, and (q–t) MgZn2Ga2 alloys in the (a,e,i,m,q) as-cast condition, (b,f,j,n,r) after HT for 15 h at 300 °C + 30 h at 400 °C, and after hot extrusion at (c,g,k,o,s) 150 or (d,h,l,p,t) 200 °C. The insets with pink boxes show higher-magnification images, while those in blue boxes show the microstructure after etching (only for extruded alloys).

Figure 3

Microstructure of the (a–d) MgZn4Ga4, (e–h) MgZn4Ga4Y0.5, (i–l) MgZn6.5Ga2, (m–p) MgZn4Ga2, and (q–t) MgZn2Ga2 alloys in the (a,e,i,m,q) as-cast condition, (b,f,j,n,r) after HT for 15 h at 300 °C + 30 h at 400 °C, and after hot extrusion at (c,g,k,o,s) 150 or (d,h,l,p,t) 200 °C. The insets with pink boxes show higher-magnification images, while those in blue boxes show the microstructure after etching (only for extruded alloys).

Figure 4

Microstructure and EDS maps of the (a–c) MgZn4Ga4 and (d–g) MgZn4Ga4Y0.5 alloys in the as-cast condition. The EDS maps showing the distribution of (b,e) Zn, (c,f) Ga, and (g) Y.

Figure 5

(a) Total fraction of intermetallic phases Mg₅₁Zn₂₀ (or Mg₁₂Zn₁₃ after HT), Mg₅Ga₂, and Ga–Y, and (b) content of Zn and Ga in the α-Mg solid solution for Mg–Zn–Ga–(Y) alloys in as-cast and HT conditions.

Figure 6

DSC heating curves for Mg–Zn–Ga–(Y) alloys in the as-cast condition and after HT.

Figure 7

TEM images of (a) MgZn4Ga4 alloy extruded at 200 °C and (b) MgZn2Ga2 alloy extruded at 150 °C.

Figure 8

Average grain size vs. extrusion temperature of the Mg–Zn–Ga–(Y) alloys.

Figure 9

XRD patterns of (a) MgZn4Ga4, (b) MgZn4Ga4Y0.5, (c) MgZn6.5Ga2, (d) MgZn4Ga2, (e) MgZn2Ga2 after hot extrusion at 150 °C.

Figure 10

Engineering tensile stress–strain curves for the Mg–Zn–Ga–(Y) alloys extruded at 200 or 150 °C.

Figure 11

Mechanical properties as a function of the extrusion temperature of the Mg–Zn–Ga–(Y) alloys evaluated using (a–c) large standard cylindrical tensile-test specimens, (d–f) small flat-plate tensile-test specimens, or (g,h) small compression-test specimens cut along the directions parallel and perpendicular to the ED. (a,d) TYS; (b,e) UTS; (c,f) El; (g) CYS; and (h) CS; (i) legend.

Figure 11

Mechanical properties as a function of the extrusion temperature of the Mg–Zn–Ga–(Y) alloys evaluated using (a–c) large standard cylindrical tensile-test specimens, (d–f) small flat-plate tensile-test specimens, or (g,h) small compression-test specimens cut along the directions parallel and perpendicular to the ED. (a,d) TYS; (b,e) UTS; (c,f) El; (g) CYS; and (h) CS; (i) legend.