Corrosion Behavior Of Mg-RE-Zn-Zr Alloys Containing Stacking Faults/Long Period Stacking Ordered Structure

Magnesium（Mg）alloys,as a"green engineering material of the 21st century",have attracted great attention due to its unique advantages.However,low absolute strength and poor corrosion resistance,especially it is difficult to obtain both good indexes in a same alloy,have become key issues limiting the widespread application of Mg alloys.Rare earth（RE）elements can regulate the microstructure of Mg alloys and improve their comprehensive properties.Stacking faults（SFs）and long period stacking order（LPSO）structures can significantly improve the strength of Mg alloys without sacrificing plasticity,and show excellent strengthening effects.If the research on its anti-corrosion mechanism is deepened,it is expected to become a candidate material for high-strength and high corrosion resistant Mg alloys.However,most studies on Mg alloys containing SFs/LPSO have focused on their microstructure and mechanical properties,and the impact of SFs/LPSO on alloy corrosion behavior has not been thoroughly studied.In response to this issue,this work introduces SFs/LPSO structure into Mg-RE-Zn-Zr alloys,adjusts its morphology and distribution using specific processing techniques,and analyzes the galvanic corrosion tendency and corrosion film formation of SFs/LPSO structure.Focusing on the relationship of"preparation-microstructure-performance",this work delves into the corrosion behavior of SFs/LPSO-strengthened Mg alloys and establishes a low-speed uniform corrosion mechanism centered on SFs.The Mg_96.9Er_2.4Zn_0.6Zr_0.1alloy containing SFs/LPSO structure were prepared based on a Er and Zn atomic ratio of 4:1 and Er atomic ratio of 2.4 at.%.The main strengthening structure of the as-cast Mg_96.9Er_2.4Zn_0.6Zr_0.1alloy is 18R-LPSO phase distributed along grain boundaries.After heat treatment,the block 18R-LPSO phases decrease.Both as-cast and heat-treated alloys exhibit a corrosion behavior dominated by LPSO phases.The reduction of size/volume fraction/potential difference after heat treatment can improve their corrosion resistance(heat-treated alloy:4.46 mm y^-1<as-cast alloy:10.18 mm y^-1).Subsequently,the hot-extrusion process can effectively introduce nano-spacing solute-enriched SFs（SESFs）structures into fine-grained structures.As-extruded alloys exhibit excellent anti-corrosion properties(1.11 mm y^-1),which are related to factors such as grain refinement and intragranular lamellar structure.To further optimize the corrosion performance of Mg alloys with SFs structure and explore the influence of intragranular lamellar structure on the corrosion behavior of Mg alloys.Through hot extrusion and subsequent heat treatment,nano-spacing SESFs structure was introduced into the fine-grained Mg_96.9Er_2.4Zn_0.6Zr_0.1alloy,and the transition from lamellar SESFs to lamellar LPSO structure was achieved within the the same alloy.The Mg alloys with uniform SESFs structure exhibit the lowest corrosion rate(0.96 mm y^-1).On the one hand,the SKPFM method is used to accurately measure weak anode SESFs at the nanoscale,with a potential difference of approximately 19-42 m V from the matrix.The cathode LPSO structure has a potential difference of approximately 48-80 m V from the matrix.The EDS results directly confirmed the local micro galvanic corrosion of SESFs as weak anodes and LPSO as cathodes,providing strong support for the scanning Kelvin probe force microscopy（SKPFM）results.Nano-spacing SESFs,as a special weak anode structure,would improve overall electrochemical uniformity and reduce the tendency for micro galvanic corrosion.On the other hand,electrochemistry,corrosion product film analysis confirmed that SESFs,as weak anode preferential corrosion,release a large amount of Er³⁺,which is conducive to the formation of an effective passivation film.This makes the corrosion resistance of Mg alloy with lamellar SESFs structure better than that of Mg alloy with lamellar LPSO structure.To reveal the effect of potential fluctuations on micro galvanic corrosion and subsequent film formation,the corrosion mechanism of Mg alloys containing multiple strengthening structures has been further studied.The complex structure composed by lamellar SFs/LPSO and elongated LPSO structure was directly extruded to form.The SKPFM results indicate that the LPSO phase serves as the cathode structure（67-87 m V）,and SFs structure serves as the anode structure（30 m V）.The overall potential fluctuation of the Mg alloys containing intragranular SFs and elongated LPSO structure is relatively large,while the overall potential fluctuation of the Mg alloys containing intragranular and elongated LPSO structure is relatively small.Quasi in-situ atomic force microscope analysis found that relatively small potential fluctuations are more conducive to weakening micro galvanic corrosion and rapid formation of protective films.In order to broaden the application field of Mg alloys with SFs structure,further research was conducted on the degradation behavior of alloys in simulated body fluids.The Mg_96.9Y_1.2Ho_1.2Zn_0.6Zr_0.1alloys with SFs structure meet the basic mechanical properties（yield strength:370 MPa>300 MPa,elongation:17%>10%）and degradation rate(degradation rate:0.3 mm y^-1<0.5 mm y^-1)requirements for orthopedic applications.More importantly,the alloy can spontaneously produce a composite corrosion film with high content of RE₂O₃and hydroxyapatite,which improves the corrosion resistance and biocompatibility of the alloy.Specifically,the preferential corrosion of solute-enriched SFs in simulated body fluid will release a large amount of RE³⁺（RE=Y,Ho）,and high content of RE³⁺is conducive to promoting the production of hydroxyapatite.Meanwhile,the formation of hydroxyapatite also hinders the ion exchange in the simulated body fluid to a certain extent,which is conducive to the deposition of RE³⁺ and the production of RE₂O₃.