Researches On Fabrication, Microstructure And Properties Of Biodegradable Mg-Gd Alloy

Magnesium alloy, working as an exceptionally biodegradable implant due to its low corrosion potential, has been attracted extensive attention in recent years. Compared to commercial biodegradable implants made of polymeric materials, magnesium and its alloys provide desirable mechanical properties:the density, compressive yield strength and elastic modulus are closer to natural bone. Therefore, magnesium alloys have become a potential candidate for temporary implants, such as cardiovascular stents and bone fixtures. However, corrosion behavior of Mg and its alloys are susceptible in physiological systems. Thus, adding effective alloying elements is considered to be an efficient approach to enhance mechanical and corrosion properties simultaneously. We have designed the biodegradable Mg-4.75Gd-1.42Nd-0.59Zn-0.37Zr magnesium alloy with favorable mechanical properties and corrosion resistance.The as-cast alloy is consisting of a-Mg and some secondary phases (Mg5RE) in grains boundaries. Moreover, cuboidal phases (Mg3Gd) are found in the grains. Research results show the best heat treatment is 540℃ for 10 h, the strength is improved slightly, but elongation is almost 42% higher than as-cast alloy. After aging treatment at 200℃, the strength reaches to 258 MPa, which is the result of precipitation.With the combination of solution treatment and extrusion, we investigate the influence of temperature and extrusion ratio on mechanical and corrosion properties. The grains grow with the increase of extrusion temperature. And the elongation is 36.6% when extruded at 400℃. Grain boundaries decrease while extrusion temperature rises, as a result, improving the corrosion resistance of Mg-Gd alloy. When extrusion ratio is 8:1, the alloy possesses minimum grain size and maximum tensile strength of 268 MPa. With the increase of extrusion ratio, the grain grows and dislocation density decreases, which is beneficial for dislocation accumulation, indicating obvious work hardening.Billets of as-cast and solution treated Mg-Gd alloy were successfully processed by ECAP to investigate influence of the pre-solution treatment on the microstructure and mechanical properties of the alloy. The average grain size was refined from 80μm to 1.5μm after 8 passes at 375℃. It was observed that both yield stress and ultimate tensile strength of the alloy in two conditions were improved after multi-pass ECAP. The ductility decreased just after one pass but increased with further pressing. The maximum UTS and YS were reached to 308 MPa and 258 MPa in the case of as cast alloy after 8 passes ECAP, respectively. In the case of as-cast alloy, while the solution treated alloy possesses higher elongation after ECAP. The variation in strength and ductility can be explained by the texture modification and microstructure evolution. The ECAPed alloy displayed a lower corrosion resistance immersed in Hank’s solution due to the crystalline defects as well as the galvanic corrosion induced by precipitation of ultra-fine β phase particles.