Materials Science Forum Vols. 561-565

Abstract: Recently, Mg-Zn-Y alloys with superior performance, which have a long period stacking order (LPSO) phase, have been developed. Therefore, it is important to understand fundamental fatigue properties in such materials. In this study, the fatigue fracture behavior of the Mg96Zn2Y2 alloy has been investigated with a plain bending testing machine, which was originally developed for thin sheet specimen at room temperature and 523K. One end of the sheet specimen is fixed at a voice coil of the loudspeaker and the other end is set free. A bending mode resonance occurs in the specimen due to forced vibration at the fixed end. To estimate stress amplitude of bending, deflections at the free end of the specimen oscillating at a frequency of about 200~500Hz was measured by a laser displacement gauge. For comparison, AZ31B alloy also has been investigated. S-N curve for the Mg96Zn2Y2 alloy was obtained using a stress ratio of R=-1, and the fatigue strengths were estimated as 200MPa at room temperature and 120MPa at 523K at 106~107 cycles. These values correspond to about 50% of 0.2% proof strengths of the Mg96Zn2Y2 alloy. Two types of fatigue surface were observed in the Mg96Zn2Y2 alloy. One was striation-like-pattern and the other was relatively flat surface. Striation-like-pattern was similar to fatigue surface of AZ31B. Therefore, these two types of fatigue surface correspond to crack passing through α-Mg phase and LPSO phase, respectively. The feature of fracture surface at 523K was almost the same as that at room temperature.

Abstract: For development of high strength magnesium alloys, a melt stirring method was employed to Mg-Al and Mg-RE (rare earth metal) alloys. As reinforcement particle, B2O3 powder was added into melts and stirred at 953K in 10 or 20 minutes with argon atmosphere. The grain sizes of both alloys were decreased by 3wt% B2O3 addition. These results show that B2O3 have grain refinement effects to magnesium alloys. Micro Vickers hardness of Mg-Al alloy was increased by 3wt% B2O3 addition. On the other hand, the hardness of Mg-RE alloy was decreased by B2O3. Though addition of B2O3 into Mg-Al and Mg-RE systems make grain size to fine, the hardness of Mg-RE alloys decreased. The tensile properties of Mg-RE alloys with B2O3 were extremely improved by extrusion process.

Abstract: By means of high resolution transmission electron microscopy (HREM) and high-angle annular dark-field image technique (HAADF), morphological, structural and compositional characteristics of the precipitates in the Mg-4Y-3Nd alloy aged at 200°C for different periods of time have been studied. On the basis of HREM observations, an atomic structural model for the β’-precipitate with an orthorhombic unit cell has been proposed. The characteristic distribution of the precipitates which are rich in rare-earth elements (Y, Nd) has been clearly revealed by the HAADF imaging technique.

Abstract: Microstructural characterization and temperature analysis have been performed in friction stir welding (FSW) of A383 and 5052 dissimilar aluminum alloys. Marked difference in microstructure was observed between joints with different arrangements of materials. The temperature at four points on each side of the joint line was measured during FSW in various conditions. In addition, an analytical model assumed that the work generated by the rotation of the tool led to the work for stirring materials and heat generation of the material and the tool. The temperature of the retreating side (RS) for the joint of the advancing side (AS):A383/RS:5052 was about 50K higher than that of AS, while the temperatures of AS and RS for the joint of AS:5052/RS:A383 were almost the same. The experimental temperature could be calculated reasonably by using the model with assumption of the work for stirring the material.

Abstract: The atomistic behaviors of microalloying elements during phase decomposition of Al- Cu-Mg, Al-Zn-Mg and Al-Mg-Si alloys have been systematically predicted in terms of two-body interaction energies between solutes and/or vacancies. The utilized first-principles calculation based on generalized gradient approximation (GGA) and full-potential Korringa-Kohn-Rostoker (FPKKR) Green’s function method accurately estimated such fundamental energies in good agreement with experimentally reported behaviors: e.g. vacancy-trapping model, vacancy-sink model and nanocluster assist processing. The proposed interaction energy maps (IE maps), in which the estimated interaction energies are plotted along the rows of the periodic table, are quite useful for designing new aluminum alloys with microalloying elements.

Abstract: It was reported that the cold and warm press formability of the magnesium alloy was improved by the application of a differential speed rolling (DSR). However, it can be considered that the microstructure and the texture of the DSR processed sheets greatly change with the rolling conditions. In this study, commercial AZ31B magnesium alloy extrusions were processed by DSR at a differential speed ratio of 1.167 and a reduction per pass of 10% or less, and the effects of the rolling temperature, the number of rolling passes and reversal of the rolling direction on texture and mechanical properties were examined. As a result, it was found that the optimal rolling temperature in terms of the workability and formability of the material was 573 K. And the elongation and formability were maximal in sheets processed by 4–6 passes of DSR. Moreover, reversing the shear direction made the microstructure more homogeneous and finer than unidirectional shear, and improved the mechanical properties and formability. This improvement was greater in samples where the shear direction was reversed once in the middle than where it was reversed for each pass.

Abstract: Rheo-extrusions of A7075 aluminum alloy were carried out utilizing semi-solid slurries with different solid granule sizes, which were made by a simple method combined a thin upright tube with a water-cooled tube. Every structure of slurries was granular and average solid granule sizes could be controlled by 0.05 to 0.11mm. These slurries were extruded to round bars at extrusion ratio of 36 and press ram speed of 10mm/s mainly, just after cooling to 833K ( fs > 0.9 ). All of the slurries could easily be extruded to bars with smooth surfaces at much low extrusion forces than those of hot-extrusions. Tensile strength of rheo-extruded bars after solution treatment increased with decreasing of the solid granule size. Peak hardness level at T6 condition equivalent to that of hot-extrusion could be obtained at the finest solid granule size.

Abstract: The microstructures of Ti6Al4V alloy after hydrogenation have been investigated and analysed by optical microscopy (OM), X-ray diffraction (XRD) and transmission electron microscopy (TEM), and the influence of hydrogenation on the hardness of α and β phases has been analysed by microhardness testing. The microstructure observation revealed that δ hydride (fcc structure) precipitated in the specimens with 0.278 wt.% and 0.514 wt.% hydrogen, and a lot of dislocations and twins have been found simultaneously. The diffraction peaks moved to the lower angles because of the lattice expansion of β phase with the solution of hydrogen atoms. The result of microhardness testing shows that the hardness of α and β phases increases synchronously with increasing of hydrogen and the increment of β is larger than that of α. It is considered that the formation of δ hydrides, lattice defects and alloying element diffusion are the major factors leading to the microhardness change.

Abstract: The structure, morphology and size distribution of phases in the Mg-Zn-Y magnesium alloys have very important effects on their mechanical properties. In this paper, the compound powders of Mg-Zn-Y phases were obtained from as cast and as aged Mg-6.0Zn-1.2Y and Mg-6.0Zn-0.6Zr-1.0Y magnesium alloys respectively by using of electrolysis extraction technology. Then several tests, such as X-ray diffraction (XRD), scanning electron microscopy (SEM) and energy dispersion X-ray detector (EDX), were performed to investigate the structure and morphology of these compound powders. The results proved that picric acid - acetic acid - ethanol electrolyte was useful than DDTC-CAS-carbinol electrolyte in the electron extraction for Mg-Zn-Y magnesium alloys. The XRD results showed that the Mg3Y2Zn3, Mg10Y2Zn, Mg3YZn6, Mg-Y-Zn quasicrystal phases were obtained from Mg-6.0Zn-1.2Y magnesium alloy, and Mg3Y2Zn3, Mg10Y2Zn, ZrO2 phases were obtained from Mg-6.0Zn-0.6Zr-1.0Y magnesium alloy. The analyses of morphology and size distribution of these phases were put forth in this paper.

Abstract: High resolution transmission electron microscopy（HRTEM）observation was performed to clarify morphology of precipitates at early stage of aging in Mg-Gd-Zr alloy. In the specimen aged at 423K, monolayer was observed on {1100}Mg planes. With increasing aging time, the monolayer and β” phase grew along the <1120>Mg directions. Arrangement of bright dots corresponding to {020}β’ except for the monolayer and β” phase was observed before peak aging stage. β’ phases formed at the same aging stage. Thus, these precipitates co-existed before peak aging stage. The monolayer, β” phase and β’ phase still co-existed in the peak aged specimen. The β’ phase was dominated in the peak hardness. It is revealed that the monolayer lying on {1100}Mg act as precursor of β” phase and β’ phase.