Materials Science Forum Vols. 747-748

Abstract: Secondary extrusion at 350 °C with the extrusion ratio of 12.25, 25 and 44 was carried out on the Mg-12Gd-3Y-0.5Zr (GW123K) alloy, and the cooling method of the secondary extruded alloys was air cooling and water quenching. Quantitative metallographic analysis method was also employed to study the distribution and volume fraction of dynamic precipitates during the extrusion process. The results showed that secondary extrusion could result in significant grain refinement and the grain size increased with extrusion ratio, which the minimum average grain size was about 5.4μm in the alloy under λ=12.25. The volume fraction of dynamic precipitates decreased with increasing extrusion ratio and the maximum volume fraction was measured to be about 49.2% in the alloy under λ =12.25. Water quenching after extrusion can effectively inhibit dynamic precipitation and the volume fraction of the precipitates ratio decreased from 41.1% after air cooling to 19.6% after water quenching in the same extrusion condition. Tensile properties results showed that the age hardening response of the alloys was decreased by dynamic precipitation and water quenching was an efficient method which is able to avoid this behavior in some extent.

Abstract: A cylinder block with δ-Al₂O₃/AE44 (Mg4.0Al4.1RE0.3Mn) magnesium matrix composite was manufactured by Slow Shot High Pressure Die Casting (SS-HPDC) technique. The temperature and wall thickness of the fiber added affect its formation. The block is not infiltrated well when the temperature of the preform is below 450°C. Micro-cracks occur if the thickness of the preform is larger than 8.625mm. Microstructures in the AE44 alloy with and without δ-Al₂O₃ fibers (Saffil) were both investigated by means of X-ray diffraction (XRD), optical microscopy (OM) techniques and scanning electron microscopy combined with energy dispersive spectrometry (SEM-EDS) equipment. The results show that many Al-RE intermetallic compounds present not only in the matrix but also in the composite, although the grain sizes, the concrete types and their distribution differ. Al-RE second phases in AE44 appear around the grain boundaries in large quantities mainly in globular, lamellar and acicular shape but few in particulate shape, while most of them grow on the fibers in small whiskers and only few scatter in the matrix. Meanwhile, the appearance of Mg₂Si in the composite may account for the interfacial reaction between the binder in the preform and the matrix alloy.

Abstract: Gleeble 3500 thermo mechanical simulator was used to perform hot compression tests of GW103K (Mg-10Gd-3Y-0.6Zr) magnesium alloy at a temperature range of 573K-723K and strain rates of 0.001-1. The workability of the alloy can be evaluated by means of processing maps on the basis of dynamic materials model (DMM) and the superior processing condition is selected. Combining true compression stress-strain curves with the results of microstructure observation, it was found that the peak stress decreased observably as the decrease of strain rate and the increase of deformation temperature. Constitutive equation is built to reveal the accurate relationship among flow stress, temperature and strain rate.

Abstract: The grain size evolvement of AZ31 magnesium alloy during high ratio extrusion (HRE) was investigated by experiment and numerical simulation. A Yada model was established according to the results of experimental study and trial calculation. Effects of the deformation temperature, extrusion ratio and extrusion velocity on the grain size were analyzed. The results showed that the microstructure of the AZ31 alloy was dramatically refined by high ratio extrusion. The deformation conditions exhibited obvious effects on the grain size, and the deformation conditions were interplaying and interacting with each other during the HRE process. Compared to the deformation temperature and extrusion velocity, the severe plastic deformation induced by the high ratio extrusion is a major cause of grain refinement. With the increasing of the extrusion velocity, the grains could be refined due to a shorter recrystallized grain growth time. At the same time, the deformation temperature increased with the increasing of the plastic strain rate, which results in faster recrystallized grain growth. Finally, recommended method is advised to further refine the grain size.

Abstract: A new Mg₆₅Cu₁₈Ni₆Gd₁₁ bulk metallic glass (BMG) with high level of Ni content, exhibiting glass-forming ability (GFA) of 6 mm, was fabricated by copper mold casting. The compressive strength and plasticity of the Mg₆₅Cu₁₈Ni₆Gd₁₁ BMG were 938 MPa and 0.14%, respectively. The composition design of the Mg₆₅Cu₁₈Ni₆Gd₁₁ BMG with enhanced GFA had succeeded simply using Gd substituting Y in the Mg₆₅Cu₁₈Ni₆Y₁₁ BMG, which was a result of a comparison on the GFA between the Mg-TM-Gd and Mg-TM-Y BMGs (TM: transition metals). Compared to the Mg-TM-Y BMGs, the Mg-TM-Gd BMGs exhibited improved GFA, which was demonstrated to be mainly attributed to the more negative amorphous formation enthalpy resulting from Gd substituting Y. The effects of rare earth (RE) elements on the GFA of Mg-TM-RE were discussed. This work may suggest an easy way for developing new Mg-based BMGs with improved GFA by rare earth (RE) design.

Abstract: Rolling processing has been carried out on samples cut from the extruded Mg-9Gd-3Y-0.5Zr seamless tubes. Effects of rolling reduction, 5%, 20% and 70% per pass, on its microstructure and mechanical properties were investigated. The results showed that the strength and ductility varied with increasing rolling passes at different rolling reduction. The strength of the alloy rolled by 5% reduction per pass gradually improved with increasing rolling passes, and its ductility remained basically constant. However, when 20% reduction per pass was applied, the strength and ductility of the alloy after rolling increased at first and then decreased a little after the accumulative strain higher than 52%. Moreover, as reduction reached 70% per pass, macro-cracks were induced when rolling at 420°C, while the samples were rolled at a high temperature of 500°C and a larger reduction of 70% per pass exhibited the mechanical properties comparable to those fabricated by 5% and 20% reduction. This indicated that a relatively higher productivity via rolling as well adequate mechanical properties can reach for the large scale of industrial products.

Abstract: The effects of heat treatments, including T4, T5, T6 treatments, on corrosion behaviors of Mg-7Al-2Sn (AT72) alloy processed by high vacuum die casting have been investigated. The optical microscope, scanning electron microscope observations were used to analyze the microstructures, especially the distribution of second phase, phase contents and grain size. The hydrogen evolution tests, electrochemical methods including open circuit potential curves and polarization curves were used to describe the corrosion properties as well. The results indicated that different heat treatments had influence on microstructures, especially distribution of second phase, phase contents and grain sizes. The existence of second phase had a relationship with corrosion properties, in which the as-cast samples showed low corrosion rate in hydrogen evolution tests while the heat treatment methods had little impact on improving corrosion resistance. The electrochemical analyses also agreed with this observation. Based on these results, the optimized heat treatment method for corrosion resistant AT72 magnesium alloy has been established.

Abstract: The effect of Sm content (2, 4, 6, 8wt.%) on the microstructures and mechanic properties of gravity casting Mg-xSm-0.4Zn-0.3Zr alloys were investigated. The results showed that the cast alloy mainly consists of α-Mg matrix and Mg41Sm5 phase distributed at the grain boundaries, and the amount of the second phase increased with the rise of Sm content. After the solution treatment (T4), the second phase disappeared, however small amount of cuboid-like phase were found inside the grain, and its volume fraction also increases with the rise of Sm content. It is found in the age hardening curves of the alloys at 175, 200, and 225 °C that the alloys with various components almost showed the same trend of age hardening. While the higher the Sm content, the harder the alloy. With the Sm content increases, the yield strength (TYS) of cast alloys (F state) were obviously improved, however the tensile strength (UTS) became lower because of the quick drop of the elongation. The elongations of solid solution (T4) and aging (T6) alloy became lower, while the yield strength and tensile strength increased at first and then decreased. The optimized component is Mg-4Sm-0.4Zn-0.3Zr, which the mechanic properties at T6 state are: TYS=131MPa; UTS=261MPa; δ=6.8%.

Abstract: The microstructure and mechanical properties of as-cast Mg-3Sn-xGd (x=0, 0.2, 1 wt.%) alloys were studied by using OM, SEM, EDX, XRD etc. With the increase of Gd, the formation of Mg₂Sn phase was impeded and the MgSnGd phase formed and the volume fraction of this new phase obviously increased. The ultimate tensile strength and elongation to failure increased with dilute Gd addition but sharply decreased when the Gd addition comes to 1.34 wt.%. The possible reasons for the variation in microstructure and mechanical properties were discussed.

Abstract: In this paper, a new biomedical Mg-Li alloy for the improvement of the comprehensive mechanical properties by micro-alloying and processing to meet the need of mechanical properties of biomedical materials. And the Mg-Li (Mg-Li-Al-Zn-Ca-Sr) alloy's processing and heat treatment were investigated in detail. The crystal texture of cast state, forged state and rolled state were observed and analyzed by OM, XRD and SEM. The mechanical properties of every stage were tested as well. The results showed that the grain size was refined obviously by the concentrating of Ca and Sr in the grain boundary. With the increase of rolling lane, the second phase's distribution was changed to a scattered state gradually from the reunion state. The tensile strength of the forged alloy was improved as well as its elongation after cold rolling and with rolled heat treatment process. The tensile strength reached 220MPa and the elongation reach 22%, which might meet the demand of cardiovascular stents mechanics.