Materials Science Forum Vols. 654-656

Abstract: The composition dependence of yield strength and elongation of the Mg-Zn-Y ternary alloy system were investigated. Yield strength was found to increase with the increasing Zn and Y contents, while elongation decreased in Zn- and Y- rich- regions in the system. The composition ratio of Zn to Y, both of which shows large elongation and comparatively high yield strength, was 1:1 in the Mg-rich region of the alloy system. Mg98Zn1Y1, Mg96Zn2Y2, and Mg94Zn3Y3 (at.%) cast alloys exhibited yield strength of 107, 129, and 173 MPa, and elongations of 17, 8, and 12 %, respectively. These alloys consist of Mg-, LPO-, and Mg3Zn3Y2- phases. We also prepared Mg-Zn-Y alloy sheets by hot rolling using cast alloys, and we investigated their microstructure and mechanical properties. The Mg98Zn1Y1, Mg96Zn2Y2, and Mg94Zn3Y3, alloy sheets exhibited yield strength of 276, 319, and 379 MPa, and elongations of 12, 11, and 8 %, respectively. After annealing at 673 K for 3 h, large elongations above 20 % were achieved both of the Mg98Zn1Y1 and Mg96Zn2Y2 alloy sheets.

Abstract: The coherent fine-lamellae consisting of the 2H-Mg and the 14H-type long period stacking ordered (LPSO) structure within α'-Mg matrix have been observed in an as-cast Mg–Gd–Zn–Zr alloy. During subsequent solid solution heat treatment at 773 K, in addition to the lamellae within matrix, a novel lamellar X phase [Mg–(8.37±1.0)Zn–(11.32±1.0)Gd] with the 14H-type LPSO structure was transformed from the dendritical β phase. The 14H-type LPSO structure existing in Mg–Gd–Zn–Zr alloys derives from two variant ways: formation of the 14H-type LPSO structure comes from two variant means: i.e., the formation within matrix and the phase transformation from the β phase to the X phase in grain boundaries.

Abstract: Precipitates in a Mg–Gd–Sc alloy with high Sc content (up to 13mass%) were examined by means of transmission electron microscopy. Plate-like precipitates appeared during the early stages of aging of specimens aged at 573 K. The plate-like precipitates lay on the { } magnesium matrix planes, and were similar to the β1 phase and β phase in the counterpart Mg–Gd–Zr alloy aged at 523 K. These precipitates had a high Sc content. Particle-shaped precipitate were observed alongside the plate-like precipitates, unlike the case in Mg–Gd alloys. The contrasts of the plate-like precipitates became longer and the number density of precipitate contrasts increased with increasing aging time at 573 K up to the point where the maximum hardness was attained. We suggest that the plate-like precipitates contribute to the increased hardness of Mg–Gd–Sc alloys with a high content of Sc.

Abstract: The effects of addition of Sc on the aging behavior of Mg–Gd alloy was examined by means of Vickers microhardness measurements, transmission electron microscopy (TEM), and high-resolution TEM. The maximum hardness of the specimens increased with increasing Sc content. In the early stages of aging, contrasts of precipitates that were similar to the β'' phase (D019-type structure) in binary Mg–Gd alloy were observed in the Mg–Gd–Sc alloys, whereas specimen that had started to harden again, but had not attained maximum hardness, showed predominant contrasts for precipitates that were similar to the β' phase (base-centered orthorhombic structure) in binary Mg–Gd alloy. Contrasts for precipitates with a D019-type ordered structure appeared during prolonged aging in specimen with high contents of Sc. Therefore, the addition of Sc to Mg–Gd alloys tends to slow down the transformation from the '' phase to the ' phase.

Abstract: The Mg-Sn-Ca alloys have shown superior creep properties compared to the creep resistant alloy AE42. In the present study, the effects of small amounts of Al and Si additions on the mechanical properties have been investigated on a Mg-3Sn-2Ca (TX32) alloy. The Al content in the selected alloys was 0.4 wt% and the Si content was varied from 0–0.8 wt% in steps of 0.2 wt.%. The alloys were cast in pre-heated permanent molds. Cylindrical specimens machined from the cast billets were tested in compression in the temperature range 25–250 °C at a strain rate of 0.0001 s–1. The alloy with 0.4 wt% Al shows an increased strength at all test temperatures compared with the TX32 base alloy. This is attributed to a solid solution strengthening of Al in Mg. The alloy with 0.4 wt% Al and 0.2 wt% Si has compressive strength that is closer to that of the TX32 alloy. However, increased additions of Si (from 0.4–0.8 wt%) reduce the strength, more significantly at higher temperatures.

Abstract: The microstructure and tensile properties at room temperature of as-cast, solution-treated and aged samples of a Mg-5Sn-5Zn (wt%) alloy are investigated. The microstructure of the as-cast sample consists of -Mg primary, Mg-MgZn eutectic, divorced eutectic Mg2Sn and secondary precipitation Mg2Sn. After solution treatment, all the MgZn particles and the majority of Mg2Sn phases are dissolved into the matrix and the tensile strength and the elongation of the alloy increase obviously. During aging process many fine particles precipitate which result in the improvement of the yield strength.

Abstract: The effects of rare earth addtition of Er on the microstructures and mechanical properties of Mg-9Zn-Zr magnesium alloys were examined. The results showed that the addition of Er resulted in the formation of two types of thermally stable Er-bearing compounds with distinct Zn/Er ratio. The volume fraction of the second-phase particles increased with the increase of Er addition. 2% Er could refine the as-cast microstructure effectively. The yield strength generally increased and the elongation decreased gradually. 0.5% Er-bearing alloy had optimum combined mechanical properties, with the yield strength, tensile strength and elongation being 313MPa, 366MPa and 22%, respectively. Furthermore, addition of Er brought about homogenous deformation behaviour and recrystallization behaviour to the alloy.

Abstract: Thermal stability of  grains and tensile ductilities at room and elevated temperatures were investigated and compared for Mg-3%Zn-0.4%Zr and Mg-3%Zn-0.4%Zr-1%Bi alloys in hot-rolled state. The Bi-added alloy showed slightly finer-grained microstructure with enhanced thermal stability, which is closely associated with fine Mg-Bi compounds acting as obstacles for the migration of grain boundaries. The Mg-3%Zn-0.4%Zr-1%Bi alloy exhibited better tensile strength at room temperature and tensile ductilities at elevated temperature. Finer and more homogeneous grain structure with higher thermal stability would be responsible for the enhanced tensile properties in the Bi-added alloy.

Abstract: A new composition sand-casting magnesium alloy, Mg-6Zn-2Gd-0.6Zr, was developed. It was found that the room temperature mechanical properties of the alloy were superior: b=270MPa, 0.2=175MPa, =8%. Meanwhile, the elevated-temperature strength and creep resistance were significantly improved, comparing with Mg-5Zn-0.6Zr (ZK51) commercial magnesium alloy. The as-cast microstructure of the alloy was mainly consisted of (Mg) matrix, (+Mg3Gd2Zn3+Mg3Gd) eutectic phase, which was distributed along the (Mg) grain boundary with coarse netted shape. After solid-solution treatment and subsequently aging treatment for the alloy, the secondary phases re-precipitated as fine discontinues semi-netted or short rod-like precipitations, also including some fine spherical precipitations in the  matrix, which were responsible for the properties improvement of the alloy.

Abstract: ZK60 alloy is one of the most important commercial wrought magnesium alloys. However, it suffers from several deficiencies like severe hot crack tendency and relatively low mechanical properties as compared to aluminum alloys. In this discussion, the microstructures of a ZK60-(0, 0.5 Cu, wt.%) alloy at different heat treatment states were examined by various techniques including optical microscopy (OM), X-ray diffraction (XRD) and transmission electron microscopy (TEM). The corresponding room temperature tensile properties of the alloys were also tested. The results indicate that trace Cu addition could dramatically improve the casting properties and tensile performance of the ZK60 alloy. In the peak-aged condition, for example, the ultimate tensile strength and relative elongation were 261.4 MPa and 17.51% for the current ZK60-0.5Cu alloy, in contrast to 222.9 MPa and 5.97% for the ZK60 alloy, respectively. The improvements could be mainly attributed to the elevated number density and refinement of the dominant strengthening phase β1΄, together with the presence of C15 Laves phase MgZnCu formed in the ZK60-0.5Cu alloy. In addition, no appreciable change in yield strength was observed.