Materials Science Forum Vols. 706-709

Abstract: The formability of the magnesium-lithium alloy was examined by multi-stage deep drawing. Long cups of magnesium alloy were formed at ambient temperatures by multi-stage deep drawing processes. In multi-stage deep drawing, the magnesium-lithium alloy sheets were employed and a flat sheet blank is formed into a cylindrical by a punch. Various cups were drawn by exchanging the punch and ringed die. The die was flat in the first stage, and was taper without a blankholder in the subsequent stages. The effects of the ratio of blank diameter to punch diameter and blank thickness on the deep drawability were examined. It was confirmed that the Mg-Li alloy long-cups were successfully formed by a multi-stage deep drawing operation in cold.

Abstract: To elucidate the phase equilibria in the vicinity of the X or W phase in the Mg-Zn-Y ternary system, five Mg-Zn-Y ternary alloys containing the X or W phase were prepared and isothermally heat-treated to reach thermodynamic equilibrium. The microstructure and chemical compositions of the equilibrium phases of the alloys were analyzed. Furthermore, the crystal structure of the W phase was investigated, and the liquidus temperatures of the alloys were examined using differential thermal analysis. The results showed that the X phase can be crystallized not by a eutectic reaction, as reported previously, but by a peritectic reaction. They also showed that the W phase has the Heusler-type crystal structure with a stoichiometric composition of Mg₁Zn₂Y₁, and that Mg and Zn in the W phase can be substituted for each other. The Mg-Zn-Y ternary phase diagram was calculated using an assessed CALPHAD thermodynamic database based on the experimental results of the present study, and compared with a recently reported phase diagram.

Abstract: Microstructures of the long period stacking ordered (LPSO) phase deformed by compression test or rolling at room temperature were investigated. The Mg₈₅Ni₆Y₉ (at.%) alloy was composed mostly of plate-type phase with 10H-type LPSO structure. The Mg₈₅Ni₆Y₉ alloy exhibited compression yield stress of 365 MPa and fracture strain of 30 % at room temperature. After compression test with applied stress of 25 %, a bend, delamination between basal planes and crack initiation at boundary of the LPSO phase were observed. From these microstructural features, it was considered that the deformation of the LPSO phase significantly influenced by a kink deformation. The Mg₈₅Ni₆Y₉ alloy could be rolled with 30 % reduction at room temperature. The kink deformation was frequently observed in the LPSO phase of the alloy sheet. The basal plane texture was formed in plane sheet of the alloy sheet. However, it was more difficult to form basal plane texture in the LPSO phase than in the pure-Mg due to introduce the kink deformation. Therefore, it was considered that better rollability of the Mg₈₅Ni₆Y₉ alloy compared with the pure-Mg was brought for the kink deformation.

Abstract: Recently, Mg₉₆Zn₂Y₂ alloy was widely studied due to its excellent mechanical properties. These properties were considered to relate to the nanostructure of the alloy such as α-Mg matrix and bent long period stacking ordered (LPSO) structure. In this study, as the first step to the mechanical property control, we applied various spot welding methods to the Mg alloy to control the structure of the joints. The specimen used was an extruded Mg₉₆Zn₂Y₂ alloy. The sheets were cut from this alloy rod for resistance spot welding. Three kinds of spot welding methods were applied to join the Mg alloy, such as spot welding with cover plates, long-time spot welding, and water-cooled spot welding. In the base metal, secondary phases extended along extruded direction at approximately 30μm intervals were produced in the α-Mg matrix. The secondary phase contained 14H-type LPSO structure. In the nugget of the joint welded with cover plates, a secondary phase was produced like a net in the α-Mg grain which was approximately 30μm in diameter. Both 14H-type and 18R-type LPSO were observed in the secondary phase. In the nugget produced by the long-time spot welding, a secondary phase tended to segregate to the grain boundary of the α-Mg grains. The structure of LPSO was 18R-type. In the water-cooled welding, strong directionality of the secondary phase texture was observed around the edge of the nugget. These results showed that the nugget nanostructure of Mg₉₆Zn₂Y₂ dramatically varied depending on the above adopted methods of spot welding.

Abstract: Mg₉₆Zn₂Y₂ is widely studied due to its high tensile yield strength and elongation. These excellent properties were considered to relate to the nanostructure of the alloy such as fine-grained α-Mg matrix and bent long period stacking ordered (LPSO) structure. In order to extend the application of this Mg alloy, development of the welding technique is indispensable. In the light of the reliability and productivity, resistance spot welding is an attractive method to join this Mg alloy. In this study, we joined the Mg alloys by the resistance spot welding with cover plates. Mechanical properties and structures of the nugget of the joints were investigated. In the case of the joints welded with cover plates, the welded region of the Mg alloy sheets were covered with steel plates to supply adequate heat for welding. Joints of extruded Mg₉₆Zn₂Y₂ were obtained at relatively low welding current. The tensile strength of Mg₉₆Zn₂Y₂ alloy was higher than that of the commercial Mg alloy AZ31B joints. Mg₉₆Zn₂Y₂ base alloy had extended LPSO phase along extruded direction in α-Mg grains. At the nugget, net-like secondary phase was produced in the Mg grains. The relationship between the microstructure in the joints and mechanical properties were discussed.

Abstract: Compression tests were employed to characterize the DSA behaviour of Mg-Ce alloys. Samples were taken from cast billets and extruded bars of Mg-0.5 wt.% Ce. The DSA behavior was examined at temperatures from 150°C to 400°C at strain rates of 0.001/s to 1.5/s. A rate equation was fitted to the experimental results, which is employed to predict whether or not DSA will occur at the strain rates and temperatures involved in the formation of the RE texture component during extrusion.

Abstract: Mg-Ce based alloy is a significant extrusion alloy for high ductility and Zn is a promising element for the increase of strength in the alloys. The effects of Zn addition to the Mg-0.5%Ce-0.5%Zr alloy and the extrusion parameters on the microstructures and mechanical properties of Mg-Zn-Ce-Zr alloy have been investigated. The results suggest that Zn element addition will enhance alloy strength and save accepted ductility. The extrusion parameters have been optimized to get the fine grain size, random texture, and therefore to develop a higher ductility and lower yield asymmetry alloy.

Abstract: High resolution transmission electron microscopy (HRTEM) observation was performed to clarify the early stage of precipitation in Mg-Gd (-Zr) alloy and Mg-Y(-Zr) alloy aged at 423 K. At the early stage of agigng at 423 K, the intensity of the diffuse spots become higher at the 1/2 distance of 1100 or 2110 corresponding to magnesium matrix spots with aging time. Contrasts of mono layers on {100}_Mg planes, and the feature of D0₁₉ type structure, precipitates with five layers corresponding the feature of (020)_b' plane and b' phase co-existed and b' phase formed predominantly in the peak aged Mg-Gd (-Zr) alloy at 423 K, wheres in the peak aged Mg-Y(-Zr) alloy at 423 K, monolayer and precursor of precipitates with five layers corresponding the feature of (020)_b' plane formed predominantly.

Abstract: A permanent mould cast creep resistant MRI 230D Mg alloy was laser surface alloyed (LSA) with Al and Al₂O₃ in order to improve its wear and corrosion resistance. However, this treatment was successful only in improving wear resistance but not corrosion resistance due to the presence of micro−cracks in the coated layer, which has been discussed in an earlier paper. The LSA coated Mg alloy has been further subjected to plasma electrolytic oxidation (PEO) treatment in alkaline silicate electrolyte in order to cover those micro−cracks and improve corrosion resistance, which is discussed in the present manuscript. For comparison, the PEO coating has also been applied on the as−cast MRI 230D Mg alloy. The microstructural characterization of coatings and corroded surfaces was carried out by scanning electron microscope and X−ray diffraction. Electrochemical corrosion tests were conducted in 3.5 wt% NaCl solution having neutral pH to investigate the corrosion behavior. The LSA coatings consisted mainly of β (Mg₁₇Al₁₂) phase, the coatings produced by PEO treatment on MRI 230D Mg alloy consisted mainly of Mg₂SiO₄ phase, and hybrid coatings of PEO on LSA consisted of Mg₂SiO₄ and MgAl₂O₄ phases in the PEO layer. Scanning electron micrographs of the cross−section revealed that the PEO treatment covered the micro−cracks present in the LSA and corrosion tests revealed that it improved the corrosion resistance, though not to the extent of the corrosion resistance of the PEO coated MRI 230D Mg alloy. All the samples exhibited localized form of corrosion.

Abstract: The Mg-Cu-Y system has been experimentally investigated using differential scanning calorimetry (DSC). Vertical sections and phase assemblage diagrams are calculated using thermodynamic modeling. Solidification behavior of the key alloys was discussed in light of the thermodynamic calculation. Melting temperatures of two of the ternary compounds; Mg₁₈CuY and Mg₄CuY, are predicted using the modified thermodynamic database of this system. Key words: Mg alloys, Bulk metallic glass, Differential scanning calorimetry, Thermodynamic modeling.