Microstructure and Corrosion Resistance of Mg-xGd-0.8Mn Magnesium Alloy by Gd Addition

Magnesium alloy has a series of advantages such as light weight，high specific stiffness，high casting productivity，and easy recycling，making it especially suitable for transportation，national defense and military industry，3C products，and many other fields. However，the corrosion resistance of magnesium alloy is relatively poor due to the low standard electrochemical potential，poor metallurgical quality，unstable hydrogen oxide film formed on the surface and easy to form galvanic couple with the second phase，which will seriously limit its application. Therefore，improving the corrosion resistance of magnesium alloy has become an urgent problem in the research and engineering application of high performance magnesium alloy. Generally，proper addition of alloying elements （such as Gd element）can not only effectively remove Fe impurities in the alloy and improve its purity，but also improve the structure and properties of the alloy. In fact，the content of rare earth elements in most high-performance Mg-Gd alloys is relatively high（mass fraction ≥ 10%），which greatly increases the manufacturing cost of Mg-Gd alloys，and seriously weakens the effect of weight reduction and lightweight. Thus，it is of great significance to develop and design high-performance Mg-Gd alloys on the premise of reducing Gd addition. The effect of the rare earth element Gd on the microstructure and corrosion resistance of Mg-xGd-0.8Mn（x=0，2，4，6；%，mass fraction）alloy was studied，and the corrosion mechanism of the extruded Mg-xGd-0.8Mn alloy was explained in this paper. The alloy ingots were prepared from high-purity Mg（99.99%），Mg-25Gd（%，mass fraction）master alloy and Mg-3.25Mn（%，mass fraction）master alloy by fabricating in a low carbon steel crucible under a mixed protective gas consisting of SF6 and CO2（volume ratio=1∶ 100）. After stirring at 760 ℃ for 5 min，the melt was keeping at 720 ℃ for 30 min，then quenched the crucible into cold water to get a solidified bar. The actual chemical compositions of Mg-xGd-0.8Mn alloy were determined by inductively-coupled plasma/atomic emission spectroscopy（ICP-AES）. The machined ingots were subsequently homogenized at 350 ℃ for 6 h and followed by 510 ℃ for 18 h，and cooled in air. After that，the ingots were extruded at 430 ℃ to produce a final sheet with a size of 5 mm in thickness and 60 mm in width，respectively. The microstructures of the alloy were characterized by optical microscopy（OM），scanning electron microscopy （SEM）equipped with energy-dispersive X-ray spectroscopy（EDS），and the phase constituent were determined employing X-ray diffraction（XRD）. The corrosion resistance of alloy samples was evaluated through corrosion weight loss test and corrosion hydrogen evolution test. With Gd content increasing from 0 to 6%，the average grain size was significantly refined from about 25 μm to about 7 μm，showing that Gd element had a strong grain refinement effect on the as-extruded Mg-0.8Mn alloy. According to the analysis results of XRD and EDS，Mn could not form any new phase with Gd，and Mg-xGd-0.8Mn alloys with Gd addition was mainly composed of α-Mg，Mn and Mg5Gd phase. After hot extrusion，the second phase of the alloy was mainly distributed at the grain boundaries. With the increase of Gd content，the volume fraction of the second phase of the as-extruded Mg-xGd-0.8Mn alloy increased，while the average size of the second phase decreased first，then increased and tended to be a stable value. With the increase of Gd content，the maximum intensity poles of the extruded alloys were tilted from normal direction（ND）to extrusion direction（ED），and the maximum texture intensity decreased from 14.84 to 6.81. With the addition of Gd content，the corrosion rate of the as-extruded Mg-0.8Mn alloy generally shown a downward trend. The corrosion resistance of the as-extruded Mg-xGd-0.8Mn alloys mainly depended on the grain size and the second phase. The microstructure refinement caused by Gd addition significantly enhanced the corrosion resistance of the alloy，especially when Gd content increased from 0 to 2%. However，when Gd content increased to 4%，the second phase was coarse and unevenly distributed. Namely，in the region where the phases were relatively concentrated，the micro galvanic corrosion occurred first，and the micro galvanic corrosion expanded rapidly to the interior of the alloy along the second phase，which made the second phase fall off during the corrosion process and accelerates the corrosion weight loss and hydrogen evolution. Therefore，when Gd content was 4%，the corrosion resistance of the alloy decreased abnormally. When the content of Gd was 6%，the grain size of the alloy was the smallest，the volume fraction of the second phase increased obviously，and the relatively uniform and dense corrosion product layer was formed，which continued to improve the corrosion resistance of the alloy，the corrosion weight loss rate and hydrogen evolution rate decreased to（1.79±0.26）mg·cm-2·d-1 and（1.39±0.18）ml·cm-2·d-1，respectively. Based on the alloying cost and corrosion resistance improvement effect of Gd addition，the extruded Mg-2Gd-0.8Mn alloy had a better application prospect. © 2024 Editorial Office of Chinese Journal of Rare Metals. All rights reserved.