Materials Science Forum Vols. 546-549

Abstract: In accordance with the ignition-prevention mechanism of F-containing shielding gas, HFC125 (pentafluoroethane) possesses the potential for forming a F-containing dense film on the surface of a Mg melt to protect it from oxidation and ignition. Importantly, HFC125 has a GWP (global warming potential) value around one eighth that of SF6, and is one-third the price (in China). The effect of HFC125, diluted in a nitrogen based shielding gas was examined via systematic experiments under variable temperature, concentration, and flux. The results showed that a shiny metallic film instantly formed on the surface of an Mg melt similar to what was observed with SF6. The protective effect varied with temperature, concentration and flux. The chemistry and structure of the protective film was characterized by SEM, EDS and XRD, and showed that the film was a dense, continuous layer of Mg oxide and fluoride. The results obtained lead to a conclusion that HFC125 is capable of providing effective protection and is, therefore, an ideal alternative to SF6 in terms of protective effectiveness, reduced GWP and cost.

Abstract: The practical phase constituent diagram has been used to determine the composition of a low alloying τ-type Mg-Zn-Al alloy, which has a nominal element content of 7wt%Zn and 3wt%Al (ZA73), as a basis for further improvement of τ-type alloy through appropriate thermal processing and micro-alloying. The microstructure and solidification characteristics of the alloy have been experimentally examined using optical microscopy, scanning electron microscopy, X-ray diffraction, and differential scanning calorimetry analysis. The results confirm that the as-microstructure of ZA73 alloy consists of globular equiaxed dendrite with τ phase as the only secondary phase evenly distributed in interdendritic spacing. Solidification sequence has been proposed with the help of DSC analysis and Mg-Zn-Al ternary liquidus projection phase diagram. Consistent with previous observed results for τ-type Mg-Zn-Al alloy, due to the decrease of element contents, ZA73 alloy has higher liquidus temperature (~627°C) and wider solidification range (~283°C), while at the mean time lower starting temperature of the second phase transformation (~354°C), compared to higher element containing ZA104 alloy. The phase constituent diagram has been shown to be a practical and effective tool for predicting the as-cast microstructural constituent of high zinc Mg-Zn-Al alloys under normal permanent mould solidification condition.

Abstract: The effects of high intensity ultrasonic melt treatment on the microstructure of magnesium alloys were investigated in this paper. Magnesium melts were treated with power ultrasonic wave and then cooled to a predetermined temperature. With the increase in ultrasonic power, the structure exhibited refined and spheroidzed crystal grains. After further increasing the ultrasonic power, the grains tended to somewhat coarsened. And increasing the ultrasonic processing time led to a grain refinement of magnesium alloy.

Abstract: A cellular automaton (CA)-based model for two-dimensional simulation of the dendritic morphology of magnesium alloys was developed. The model considers solving the solute and heat conservation equations in the modeling domain, including calculation of the solid fraction, the tip velocity, and the solute diffusion process, all of which have significant influence on the dendrite evolution. The microstructure of a step-shape die cast part of AZ91D magnesium alloys was investigated utilizing SEM-EBSD analysis. The microstructure simulation results were compared with the experimental results and they were in good agreement on grain size.

Abstract: The microstructures and corrosion properties of AZ91 magnesium alloy with trace additions of Sc were investigated. The microstructures were observed by OM and SEM, and the corrosion properties were studied by electrochemical technique. The results show that the alloys with Sc form a second-phase Al3Sc, the grains become finer with the increasing content of Sc and the microstructure is the finest when Sc content up to 0.6%, the corrosion resistant of alloy combining with Sc is improved.

Abstract: The microstructures of magnesium alloy AZ91D in as-cast, solid solution and aging conditions were studied by optical, scanning electron and transmission electronic microscopes. The relationship between microstructure and the microhardness was analyzed. It has been found that the microhardness in the matrix varied slightly with the increasing solution time, but increased quickly with the aging time. The difference of the microhardness between the matrix and the second phase in as-cast condition is larger than that in T6 aging condition, because the aluminum-zinc-magnesium (poor) phase existed in the as-cast condition.

Abstract: Totally strain-controlled low cycle fatigue tests with a strain ratio Rε= -1 were carried out on die-cast AZ91HP, AM50HP and AE 42 alloys at a constant strain rate of 2.5×10-3 s-1 and room temperature. The cyclic deformation behavior of the three alloys was investigated through the characteristics of representative hysteresis loops at various total strain amplitudes. Cyclic deformation hardening was observed and the low-cycle fatigue life data were analyzed using the well known Basquin and Coffin-Manson equations. The transition life of the three alloys is considerably low, which can be attributed to the low ductility of these die cast Mg alloys.

Abstract: The semi-solid compression deformation behavior of the AZ91D alloy with non-dendritic structure, which was obtained under the semi-solid isothermal treatment condition of 570°C×60min, was studied by means of Gleeble-1500 thermal-mechanical simulator. When the compression strain was lower than 0.7, along with the compression strain increasing, the compression stress firstly increased rapidly, then decreased rapidly, and finally kept a constant stress level gradually. Under the condition of different deformation temperatures and deformation rates, the maximum compression stress was obtained simultaneously when the compression strain value was 0.025 approximatively. Furthermore, when the deformation rate kept a constant, the compression stress decreased along with the deformation temperature increasing, and when the deformation temperature kept a constant, the compression stress increased along with the deformation rate increasing.

Abstract: Effects of CaCO3 modificator on microstructure and mechanical properties of cast AZ91 Magnesium alloy have been investigated. Tensile fracture behavior of AZ91 alloys modified by CaCO3 has also been studied. Results show that CaCO3 modificator can obviously refine the grain of AZ91 magnesium alloy and Mg17Al12. Mg17Al12 in grain boundary of AZ91 alloy after modified by CaCO3 changes from continuous reticular structure to discontinuous reticular structure, even so much as granular structure and rod structure. After modified by 0.5wt% CaCO3 modificator, ultimate tensile strength, yield strength, impact toughness and elongation of AZ91 alloy increase from 186MPa to 200MPa, from 147MPa to 160MPa, from 4J to 9J and from 2.6% to 5%, respectively. And 0.5wt% CaCO3 modificator brings about an optimal refining effect. The study also shows that the fracture mechanism of modified AZ91 alloy is between cleavage fracture and quasi-cleavage fracture, which is as same as that of unmodified AZ91 alloy.

Abstract: Microstructure and mechanical properties of three kinds of Mg-Gd-Sm-Zr alloys have been analyzed in this paper. Results exhibit that the microstructure of as-cast Mg-Gd-Sm-Zr alloy contains α-Mg and eutectic compounds which are mainly comprised of most Mg5Gd-base phases and a few Mg41Sm5-base phases by EDX and XRD analysis. Ultimate tensile strength and yield strength of the alloys can be significantly improved after T6 treatment. Mechanical properties of studied alloys in T6 condition are better than that of WE54-T6 alloy.