Yong Chun Guo

Phase Diagram of Mg-Zn-Gd System Alloy at Mg Rich Corner and its Application in the Development of Two New Alloys

The Mg-rich corner of the equilibrium phase diagram of the Mg-Zn-Gd system has been calculated in detail using the phase diagram calculation software PANDAT and the thermodynamic database for Mg alloys. The calculated phase diagram includes the liquidus projection, isothermal sections and vertical sections. It is found that an increase of Zn content in the Mg-Gd alloy reduces the phase field of α-Mg + GdMg5. Based on the calculated phase diagrams, two alloys, Mg-5.5Zn-2Gd-0.5Zr and Mg-1.6Gd-5.5Zn-0.5Zr (wt.%), denoted as ZGK620 and ZGK616, were developed and their solidification and precipitation processes were analyzed in detail. The optimized thermal mechanical processing and heat-treatment processes were defined by referring to the calculated phase diagrams of the Mg-Zn-Gd system.

Microstructure and microgalvanic corrosion of an extruded Mg-10Gd-2Y-0.5Zr magnesium alloy

In the present study, the microstructure and corrosion behaviour of an extruded Mg-10Gd-2Y-0.5Zr alloy (noted as GW102) in 3.5 wt.% NaCl for different times have been investigated using a combination of computed phase diagrams, optical microscopy, high resolution scanning electron microscopy (SEM) and scanning Kelvin probe force microscopy (SKPFM). It was revealed that the extruded alloy is composed of fine recrystallized, equiaxed Mg solid solution grains. Additionally different second phases of varying sizes and shapes are present, including a large square-shaped Mg5(Gd,Y) particles which solidified from the melt and are located within the Mg grains and/or at grain boundaries, fine spherical zirconium-rich particles which are located in the Mg grains and fine needle-like precipitates of Mg5(Gd,Y) and Mg24(Gd,Y)5 which are uniformly distributed within the Mg grain interior. The latter was formed during thermal extrusion processing. Further, a number of deformation bands or twins were found existing in the magnesium grains. SKPFM potential maps showed that Zr-rich particles and Gd(Y)-rich particles have positive potentials relative to the Mg matrix and the highest potential was recorded on Zr-rich particles. Immersion testing revealed that microgalvanic corrosion initiated at the periphery of Zr-rich particles due to their strong cathodic activity, thereby forming microgalvanic couples with the adjacent Mg grains.

Effect of Tin on Microstructure and Electrochemistrical Properties of Mg-Al-Sn-Zn Magnesium Alloys Anodic Materials

Morphologies, microstructure and composition distribution of the magnesium anodic materials were studied by metallographic microscopy, x-ray diffraction (XRD), scanning electron microscope (SEM) and energy dispersive spectroscopy (EDS). The corrosion behavior and electrochemical properties of Mg alloy were also investigated by constant current method, potential polarization, collecting gas through drainage. The results show that tin restrained β-Mg17Al12 phase precipitation along the grain boundary. With the content of tin increasing, granular Mg2Sn phase was improved. After uniform heat treatment, most of β-Mg17Al12 phase was dissolved, but most of Mg2Sn was not dissolved. Tin could improve self-corrosion potential and release hydrogen rate. Magnesium alloy anode with 1% tin content had high discharge potential and current efficiency. With the current density increasing, the release hydrogen rate augmented. The current efficiency reached 82 % at 20mA/cm2. The main composition of the corrosion products were MgO and Al2O3 which were easily peeled off. As a result, more negative and stable work potential was produced and the reaction was accelerated continuously.

Microstructure Evolution and Mechanical Properties of an Al-Si-Cu-Mg-Ni Aluminium Alloy after Thermal Exposure

The microstructures and mechanical properties of an Al-Si-Cu-Mg-Ni aluminium alloy have been investigated after thermal exposure at 350 °C for time intervals up to 1000 h. Experimental results showed that, with increasing the thermal exposure time, room temperature ultimate tensile strength, elevated temperature ultimate tensile strength, and Brinell hardness firstly decreased remarkably (up to 100 h) and then decreased slightly to a certain constant value (100-1000 h). Before thermal exposure, room temperature ultimate tensile strength, elevated temperature ultimate tensile strength, elevated temperature elongation percentage, and Brinell hardness of the alloys are 203.5 MPa, 48.7 MPa, 9.2%, and 82.3, respectively. With increasing the thermal exposure time, eutectic silicon grows up steadily, and the amount of Q phase with a flower shape increases. Transmission electron microscopy analysis showed that the formation of stable θ precipitates was found in the microstructure.

Corrosion Resistance of Al-12Si Coatings on AZ91 Magnesium Alloy Prepared through Flame Spray

In this study, Al-12Si alloy coatings with different thickness were prepared through flame spray on the surface of the AZ91 magnesium alloy to improve its corrosion resistance. The corrosion resistance was characterized through corrosion potential using electrochemical methods. The Al-12Si alloy coatings were heat treated at 100 °C, 200 °C and 300 °C for 6, 12, 18 and 24 hours. The effects of heat treatment temperature and time on the coatings’ corrosion resistance were discussed. It was found that there were no phase changes during the deposition of Al-12Si coatings through flame spray and heat treatment. The greater the coating thickness was, the higher the corrosion potential was. After annealing, the inner microstructure of the Al-12Si coating was densified furtherly and the annealed coatings had higher corrosion potential and better corrosion resistance. The coating annealed at 100 °C for 18 hours had the highest corrosion potential and the best corrosion resistance in the same coating thickness.

Effect of Yttrium Ion on YSZ-Al2O3 Composite Coating Formed by PEO on a Cast Al-Si-Cu-Ni Alloy

Y2O3-ZrO2-Al2O3 (YSZ-Al2O3) composite ceramic coating was formed on a cast Al‑12Si-3Cu-2Ni alloy by plus AC plasma electrolytic oxidation (PEO) process, with yttrium ion concentrations varying from 0 to 0.5 g/l and process time from 0 to 45 min. The microstructures of the coatings were investigated by SEM and XRD. The results show that the PEO coating growth behaviour and microstructure were affected greatly by Y ion additions. At a given oxidation time, the coating growth rate accelerated and thickness increased in the PEO process when adding yttrium ions. When the content of Y ions is greater than the optimum value of 0.05 g/l, coating growth gradually decreases. In comparison with a ZrO2-Al2O3 coating, the amount of micro particle in YSZ-Al2O3 coating is less and the ceramic layer is repeatedly deposited by ceramic particles and good binding was achieved between the ceramic coating and the base alloy. The uniform and compact YSZ-Al2O3 composite coating is mainly composed of c-Y0.15Zr0.85O1.93 solid solution, m‑ZrO2, α-Al2O3, γ-Al2O3 and Y2O3 phases. The ZrO2 phase was well stabilized by solid solution containing yttrium element and Al2O3.

Effect of Zn Content on the Microstructure and Corrosion Behaviour of Mg-7Y-0.6Zr Alloy

Mg-7Y-0.6Zr-xZn (x = 0, 0.5, 1.0, 1.5, 2.0, 2.5, wt.%) alloys were prepared by the metal mould casting method. Effect of Zn content on the microstructures and corrosion behaviour were investigated. Results showed that microstructures were refined and volume fraction of secondary phase Mg24(YZn)5 was increased with increasing addition of Zn element. Results of electrochemical tests demonstrated that the corrosion potential of Mg-7Y-0.6Zr alloy was about -1.77 V, and, with addition of 0.5~2.0 wt.% Zn element, corrosion potential moved to more positive values than that of Mg-7Y-0.6Zr alloy. Mg-7Y-0.6Zr-0.5Zn alloy possessed the most positive corrosion potential of -1.53 V. The results of immersion test with different time also indicated that corrosion rate could be decreased by addition of 0.5~2.0 wt.% Zn, and Mg-7Y-0.6Zr-0.5Zn alloy exhibited the lowest corrosion rate.

Calculation of the Mg-Gd-Y Alloy Equilibrium Phase Diagram and its Verification at 450°C

The Mg-Gd alloy, Mg-Y alloy equilibrium phase diagram has been characterized using the multiple phase equilibrium calculation software (Pandat) and the magnesium alloy thermodynamic database. The Mg-Gd and Mg-Y diffusion couples were made by the rivet method. According to the local balance principle, these diffusion couples were processed using an equalization treatment at 450 °C, followed by EDS analysis with a scanning electron microscope. The results show that a concentration gradient resulting from atomic diffusion is apparent in the Mg-Y and Mg-Gd diffusion layer, showing that the diffusion layers belong to different phases. There are 5 two-phase regions and 2 single phase regions in the Mg-Gd diffusion layer and 4 two-phase regions and 2 single phase regions in the Mg-Y diffusion layer. These results are consistent with the data from the phase equilibrium calculation. This research can provide experimental support for the Mg-Gd-Y three element alloy phase diagram calculation.

Development of Thermal Barrier Coatings on AZ91 Magnesium Alloy Surface

In this paper, MCrAlY bonding layer was prepared through high velocity-oxygen fuel spray on the AZ91 magnesium alloy surface. 8wt%YSZ ceramic layer was deposited on the surface of bonding layer through atmospheric plasma spraying. The coatings were characterized by XRD and SEM. The hardness of the coating was tested. The coefficients of thermal expansion of the coating were measured through TMA. It was found that there were un-melted 8YSZ particles surrounded by melted particles layer by layer. Parts of ZrO2 transited from cubic to tetragonal. The microhardness of the YSZ coating and bonding layer were 832.1Hv0.3 and 700.3Hv0.3, respectively. The coefficients of thermal expansion of the magnesium alloy, MCrAlY layer and YSZ coating were 42.46 ×10-6·k-1, 15.26 ×10-6·k-1, 9.28 ×10-6·k-1, respectively. The bonding coat matched the magnesium alloy substrate and the YSZ coating well. The dimension of AZ91 magnesium alloy sample was very stabilized before and after coating deposition.

The Mechanism of PEO Process on Al–Si Alloys in Zirconate Solution

Zirconia coating was produced on Al-Si alloys by plasma electrolytic oxidation (PEO). The alkaline electrolyte containing Zr(OH)4 powders was used. The composition and structure of the coating were investigated by SEM and XRD. The results show that in the initial stages of oxidation the growth of coating belongs to the stage of anodic oxidation controlled by electrochemical polarization. The growth of coating is mainly outward growth and the growth rate is faster. With elongated treated time and increased thickness of the coating, the growth of coating is mainly ingrowth. In contrast with the stage of anodization, the growth rate of plasma electrolytic oxidation is slower than anodization. The coating consists of t-ZrO2, m-ZrO2, α-Al2O3 and γ-Al2O3.T-ZrO2 is the main phase and distributes in outer layer of the coating, however, α-Al2O3 appears in inner layer of the coating. Many micro-particles appear on the coating surface with dimension of 1-2μm.. In the process of plasma electrolytic oxidation, Zr(OH)4 powders move and deposit on the mouth of plasma discharge channel under the effect of electric field force, then it is transformed to ZrO2 by the high temperature of plasma discharge.