Materials Science Forum Vols. 546-549

Abstract: Ion implantation was applied to modify the surface properties of magnesium alloy. About 75 keV titanium ions with a dose of 5×1017 ions/ cm-2 and about 35 keV nitrogen ions with a dose of 3×1017 ions/ cm-2 were implanted into AZ31 magnesium alloy, respectively. AFM and FESEM were used to examine the surface morphology of the treated samples. In contrast to the smooth surface treated by N ion implantation, the Ti-treated surface was severely damaged. AES analysis reveals that both kinds of ions implanted in AZ31 magnesium alloy took on a quasi-Gaussian distribution in the near surface region. In the corrosion test, the N-implanted sample showed a better corrosion resistance in 3.5wt.% NaCl solution than bare AZ31 sample, whereas the Ti-implanted sample accelerated the corrosion behavior of AZ31 in this solution

Abstract: Rare earth chemical conversion coating of Mg alloys was studied. Corrosion and electrochemical behavior in chloride environment were investigated with tests of evolution of hydrogen and electrochemical measurements. The surface morphologies and composition of rare earth conversion coating were studied through SEM, EDAX and XPS. The results showed that rare earth conversion coatings could improve corrosion resistance and their corrosion resistance was comparable with that of chromate coatings (HB/Z5078278). This result was further proved by Polarization and EIS.

Abstract: The effects of CeCl3-containing fluxes on RE element loss, the high temperature mechanical properties, microstructure and corrosion properties of magnesium alloy containing RE have been studied. The results showed that a certain amount of CeCl3 in purification fluxes restrained the reaction between the Ce element and fluxes to decrease the loss of Ce in alloys. With the flux containing 9% CeCl3, the loss rate of alloy element Ce can be reduced from 26.9% to 3.4%, and the σb and δ of the alloys at 150°C counld be improved from 160.9MPa and 6.2% to 176.5MPa and 7.9% compared with that of conventional MgCl2-containing flux , respectively. Besides, the coarse β phase and rod-like RE phase were refined effectively by CeCl3-containing fluxes.

Abstract: With adding 2% strontium in AZ91D, the ultimate tensile strength and the elongation increased by 10.3% and 55.3%, respectively. This is mainly caused by the refinement of the β phase and the formation of Al4Sr strengthening phase. Furthermore, with adding 2% strontium in AZ91D alloy, the weight loss corrosion rate in 5wt.% NaCl solution decreases to 0.048 mg·cm-2·d-1, which was 33.8 % of the AZ91D corrosion rate. Therefore, the mechanical properties and corrosion resistance of AZ91D could be improved by the addition of 2% strontium, which is due to the refinement of Mg17Al12 phase and the formation of Al4Sr phase.

Abstract: The corrosion behavior of GW63 (Mg-6wt.%Gd-3wt.%Y-0.4wt.%Zr) alloys in 5% NaCl aqueous solution has been investigated by PARSTAT 2273 instrument. The Open Circuit Potential (ECORR) vs. time curve, cyclic polarization (Pitting Scans) curve and Electrochemical Impedance Spectroscopy (EIS) was measured for the GW63 alloys in as-cast and T6 heat treatment conditions. The EIS results indicated that the tendency of impedance variation for as-cast condition was monotonic decreasing, however, the tendency of variation for T6 condition was not completely monotonic but the total tendency was decreasing. The values of impedance of GW63 alloy at 0.1 Hz are about 103 ohm-cm2 for as-cast and T6 condition.

Abstract: Characteristics of ceramic coatings on AM50 magnesium alloy by micro-arc oxidation in silicate and phosphate electrolytes have been investigated in this study. This study reveals that the thickness of the ceramic coatings increases with the treated time in both electrolytes, the growth rate of ceramic layer in phosphate is faster than that in silicate electrolyte. The surface roughness of the ceramic coating formed in phosphate electrolyte is higher than that formed in silicate electrolyte. The coatings formed in silicate, containing a thicker inner barrier layer and a thinner outer porous layer, consist of MgO, Mg2SiO4 and MgSiO3 phases. For the coatings formed in phosphate, the outer porous layer is thicker than the inner layer, it consist mainly of MgO and MgAlO4 phases.

Abstract: The acoustic emission (AE) detected during the fatigue process in an as-rolled magnesium alloy AZ31B was analyzed. Measurements were made during fatigue in air as well as in aqueous 0.1% NaCl solution. Three stages of fatigue were detected with AE. It is concluded that plasticity, crack extension or friction was the main AE sources during fatigue.

Abstract: Micro arc oxidation is an effective method to improve corrosion resistance of magnesium alloys and the selected electric parameters have great effects on coating properties. In this paper, the effects of the unipolar pulse and the bipolar pulse on surface morphology, chemical compositions and corrosion resistance were studied. When working voltage was higher than breakdown voltage, sparks appeared on sample surface whether the unipolar pulse or the bipolar pulse was used. However, under the same positive working voltage, the spark size and lifetime obtained by the bipolar pulse were evidently larger and longer than those by the unipolar pulse, which resulted in rougher anodic coatings and worse corrosion resistance by the former than the latter.

Abstract: The kinetics of film-forming process of anodizing on AZ91D Mg alloy have been studied through analyses of voltage-time and thickness–time curves. The surface morphology, structure, composition and valence of element, phase constituent of anodic films have been analyzed by SEM, EDS, XPS and XRD respectively. The results show that the film-forming process can be divided into four stages: I - formation of a dense layer; II - formation of a porous layer; III - fast growth of the porous layer; IV - slow growth of the porous layer. The growing process of the dense film is characterized by the rapidly formed intact and imperforate oxide film; there are few micro-cracks in partial film formed on α phase, on the contrary, intact film is formed on β phase; the forming of porous film is based on the particulate sparking product with micro-porous structure. There are two kinds of pores in the anodic film; the first one is circular or ellipse, while the second one is the irregular gaps. The dense layer is composed mainly of MgO and small amount of Al2O3, while the porous layer mainly consists of MgSiO3 and Mg3B2O6. We propose a possible model about growing anodizing film on AZ91D Mg alloy based on the experimental results.