Fig. 1. XRD patterns of as-cast Mg-2.5Cu and Mg-2.5Cu-xAl (x = 4.5, 6.0 wt.%) alloys. Crystalline Mg peaks were identified in three alloys. The Mg2Cu, β-Mg17Al12 and MgAlCu phases were clearly identified.

Fig. 2. Microstructural images and corresponding EDS maps of as-cast (a-c) Mg-2.5Cu, (d-g) Mg-2.5Cu-4.5Al and (h-k) Mg-2.5Cu-6.0Al alloys. (a), (d) and (h) show the etched optical micrographs. (b), (e) and (i) show the backscattered electron images with the bright contrast Cu-rich regions (as indicated by green arrow), Al/Cu-rich regions (as indicated by yellow arrow) and grey contrast Al-rich regions (as indicated by blue arrow). (c), (f) and (j) show the EDS maps with Cu elements. (g) and (k) show the EDS maps with Al elements.

Fig. 3. TEM images and corresponding EDS mapping of as-cast Mg-2.5Cu-6.0Al alloy. (a) TEM image of a coarse shaped particle embedded in the Mg matrix. A selected-area electron diffraction pattern taken along (b) [-113] and (c) [011] from the area of point A. Point B represents the dark needle-shaped particles. The EDS analyses of grey contrast particles with body-centered cubic structure are rich in Mg and Al. The Al and Cu atoms are rich in the needle-shaped precipitates with dark contrast. (d) TEM image of a rod-shaped particle embedded in the Mg matrix. A SAED pattern taken along (e) [01-11] and (f) [11], [12], [13], [14], [15], [16], [17], [18], [19], [20] at the area of point C. The EDS analyses of dark contrast particles with hexagonal closed packed structure are rich in Mg, Al and Cu.

Fig. 4. Engineering stress-strain curves obtained from uniaxial (a) tension and (b) compression tests with a strain rate of 1.0 × 10-4 s-1 at room temperature. The samples with Al addition exhibit enhanced strength and ductility.

Fig. 5. Strain hardening rates for (a) tension and (b) compression tests in as-cast Mg-2.5Cu and Mg-2.5Cu-xAl (x = 4.5, 6.0 wt.%) alloys.

Fig. 6. Potentiodynamic polarization curves for as-cast Mg-2.5Cu and Mg-2.5Cu-xAl (x = 4.5, 6.0 wt.%) alloys in a 3 wt.% KCl solution at 25±1 °C.

Fig. 7. Corrosion behaviors of as-cast Mg-2.5Cu and Mg-2.5Cu-xAl (x = 4.5, 6.0 wt.%) alloys exposed to a 3 wt.% KCl at 25±1 °C. The variation of (a) H2 evolution volume and (b) H2 evolution rate as a function of immersion test.

Fig. 8. The comparison of three alloys with mass loss rate, ultimate compressive strength, and compressive strain. The as-cast Mg-2.5Cu-6.0Al exhibits the combination of good mechanical properties (ultimate compressive strength = 378.8 MPa, compressive strain = 27.3%) and rapid degradation rate (Pw = 383.1 mm/y).

Fig. 9. Scanning electron micrographs of (a)-(c) surfaces after immersion 20 s and (d)-(i) cross-sections after immersion 2 h in a 3 wt.% KCl solution at 25±1 °C for as-cast Mg-2.5Cu and Mg-2.5Cu-xAl (x = 4.5, 6.0 wt.%) alloys. (a)-(c) The corrosion initiates from Mg matrix (green arrows). (d)-(f) The residual second phases (dashed pink, yellow, blue arrows) contact with solution surrounded by the corrosion products. (g)-(i) The magnified pictures of selected areas in (d)-(f) are presented.

Fig. 10. EDS analysis of surface morphologies after immersion 2 hours in a 3 wt.% KCl solution at 25±1 °C for Mg-2.5Cu-6.0Al alloys in Fig. 9(i). (a)-(d) EDS maps for Mg, O, Al and Cu.

Fig. 11. Surface Volta potential maps and linear scan profiles of as-cast (a, c) Mg-2.5Cu, (b, d) Mg-2.5Cu-6.0Al alloys. (a) SKPM surface potential image with Mg2Cu phase (dark region). (b) SKPM surface potential images with β-Mg17Al12+Al2Cu phase (grey region) and MgAlCu phase (dark region). Corresponding surface potential profiles along (c) the red line A and (d) the red line B.

Fig. 12. Schematic illustration of corrosion process of as-cast Mg-Cu-Al alloys during exposure to a 3 wt.% KCl solution. The incomplete surface films and particles form micro-galvanic couples at stage I. Mg matrix corrodes around the second phases preferentially at stage II. The particles are surrounded by the corrosion products at stage III. Finally, both of them split off simultaneously at stage IV, leading to deteriorated corrosion.

Fig. 13. Bright-field TEM images of microstructure of Mg-2.5Cu-6.0Al alloy after uniaxial tension. (a) A selected area with g = 01-11 near the [2-1-10] zone axis. Both basal (as indicated by white arrows) and non-basal (as indicated by yellow arrows) <a> dislocations are activated. (b) Stacking faults were observed as fringes with bright/dark features (as indicated by blue arrows).

Fig. 14. The variation of ultimate compressive strength as a function of (a) degradation rate and (b) compressive strain for various degradable Mg alloys [28, 51-53]. Our developed alloys show the combined high strength and improved compressive strain, while keep a high corrosion rate compared with other degradable Mg alloys.