Review of studies on corrosion of magnesium alloys

This review provided some recent progress of the research on corrosion mechanisms of magnesium and its alloys and a basis for follow-on research. Galvanic corrosion, pitting corrosion, intergranular corrosion (IGC), filiform corrosion, crevice corrosion, stress corrosion cracking (SCC), and corrosion fatigue (CF) were discussed. The influence of metallurgical factors such as alloying elements, microstructure and secondary phases, processing factors such as heat treatment and weld, and environmental factors including temperature, relative humidity, solution pH values and concentration on corrosion were discussed. In particular, a mechanism of pitting corrosion caused by AlMn particles was proposed. The corrosion properties of AZ91D weld material were investigated.

Corrosion behavior of Mg-Gd-Zn based alloys in aqueous NaCl solution

The corrosion behavior of Mg-10Gd-xZn(x=2,6 wt.%)alloys in 0.5 wt.%NaCl solution was investigated.Microstructures of both the alloys consisted of(Mg,Zn)_(3) Gd phase and lamellar long period stacking ordered(LPSO)phase.The morphology of the second phase at the grain boundary differed in both alloys:it was a continuous network structure in Mg-10Gd-6Zn,whereas it was relatively discrete in Mg-10Gd-2Zn.The dendrites were finer in size and highly branched in Mg-10Gd-6Zn.The corrosion results indicated that the increase in Zn content increased the corrosion rate in Mg-10Gd-xZn alloys.Micro-galvanic corrosion occurred near the grain boundary in both alloys initially as the grain boundary phase was stable and acted as a cathode,however,filiform corrosion dominated in the later stage,which was facilitated by the LPSO phase in the matrix.Severe micro-galvanic corrosion occurred in Mg-10Gd-6Zn due to the higher volume of second phase.The stability of the second phase at the grain boundary was altered and dissolved after the long immersion times.Probably the NaCl solution chemically reacted with the grain boundary phase and de-stabilized it during the long immersion times,and was removed by the chromic acid used for the corrosion product removal.

Microstructure and mechanical properties of as-cast Mg-Sn-Ca alloys and effect of alloying elements

The effect of Sn, Ca, Al, Si and Zn addition on the compressive strength of cast Mg-Sn-Ca （TX） alloys was studied in the temperature range of 25-250 °C and correlated with the microstructure. The Sn to Ca mass ratio up to 2.5 contributes to the formation of Mg2Ca phase at the grain boundaries and CaMgSn in the matrix, while a ratio of 3 gives only CaMgSn phase mostly in the matrix. While the compressive strength decreases with the increase in temperature, for Sn/Ca up to 2.5, a plateau occurs in 100-175 °C, which is attributed to the strengthening by Mg2Ca. However, for ratio of 3, the strength is lower and decreases more gradually. Mg-3Sn-2Ca （TX32） has the highest strength and the addition of 0.4%Al increases its strength but simultaneous addition of Si lowers the strength. Likewise, the addition of Zn improves its strength but simultaneous addition of Al slightly decreases the strength. The results are correlated with the types of intermetallic phases that form in various alloys.

Microstructure evolution and tensile properties of friction-stir-welded AM50 magnesium alloy

Friction stir welding(FSW) technique was utilized to weld cast AM50 magnesium alloy plates.The microstructures in the base metal(BM) and the weld joint were observed by optical microscopy.The mechanical properties were investigated by using hardness measurement and tensile test,and the fractographs were observed by scanning electron microscopy.The results show that the microstructure of the base material was characterized by bulk primaryαphase,α-matrix and intermetallic compoundβ(or Mg_(17)Al_(12)),and the weld nugget exhibiting recrystallized microstructure consists ofα-matrix andβphase.The grain size in the weld is smaller than that in the base metal.The hardness of the weld joint is improved but the tensile strength and yield strength,as well as the elongation to failure of the base material decline.The fracture of BM has a rougher surface with more dimples,which is a characteristic of the ductile fracture,whereas the fracture on the nugget reveals a quasi-cleavage feature.The ultimate tensile strength and yield strength of the FSWed AM50 are 86.2%and 94.0%of those of the base metal,respectively.

Electrochemical behavior of magnesium alloys in simulated body fluids

Potentiodynamic electrochemical technique was utilized to study the corrosion behavior of magnesium alloys in simulated body fluids(SBFs). The influence of materials,solutions and their temperature on corrosion rate was mainly discussed. The results demonstrate that the free corrosion potential(Ecorr) of AZ31 and AZ91 alloys rises rapidly at initial stage,and then stabilizes at some value. Ecorr of WE43 alloy increases continuously. While Ecorr of AZ91 alloy with macro-arc oxidation(MAO) coating decreases drastically in 3 min,and then fluctuates between -1 607 mV and -1 503 mV. The WE43 alloy has better corrosion resistance in Hank's solution,compared with AZ31 and AZ91 alloys. Corrosion rates of the alloys are sensitive to the chemical composition and temperature of SBFs. A thin MgF2 film slightly improves corrosion resistance. An MAO coating on AZ91 alloy significantly reduces corrosion rate and enhances Ecorr. Pitting corrosion occurs on both AZ31 and WE43 alloys in Hank's solution.

Bulk and local textures of pure magnesium processed by rotary swaging

Rotary swaging processing on commercial as-cast pure Mg has been carried out. Bulk texture variation with the processing passes was investigated using large gauge volume by neutron diffraction, of which results showed a combination of different components such as {00.2} basal fibre and two weak {10.0} and {11.0} fibres. Asymmetric distribution of the basal fibre around swaging direction was observed and being related to the processing parameters. Texture gradient analysis by synchrotron radiation demonstrates a non-uniform deformation of the RS processed pure Mg from surface to the centre.

钨含量对冷挤压Cu—W复合材料性能和结构的影响

含有1,2,4,6,8和10％(体积)弥散w粉的复合Cu-w材料是通过机械合金化、压制烧结及冷挤压的方法制取的。并研究了w含量对冷挤压和在温度573K与773K退火处理后的材料硬度、电导率及耐磨性的影响。

镁合金的模锻

可锻镁合金的成形件一般使用轧制和挤压成形，其强度取决于原料的品质。作为可锻镁合金成形的辅助方法，改善原材料品质，提高了锻件强度。在细化晶粒的基础上开发系列镁合金，调整所用原始坯料的微观组织，对于进一步有效锻造是必要的。解决了这些问题后，就可用达到或超过应用技术要求的可锻镁合金来替代铝合金锻件。