Materials Science Forum Vols. 706-709

Abstract: Mg-Al-Zn alloys have been reinforced with carbon fibres using either the liquid state process of squeeze casting (SC), or friction stir processing (FSP), a solid state process developed more recently and that appears as a promising alternative for the large-scale production of C-Mg composites. Both processes have shown their ability to produce sound composites with enhanced strength compared to the non-reinforced alloys. In SC composites, the unsized woven C fabric remains intact while in the FSP composites the sized C fabric is fragmented in short fibres, with an aspect ratio typically equal to 4, homogenously distributed in the Mg alloy matrix.

Abstract: A commercial AZ61 magnesium (Mg) alloy composed of coarse initial grains was multi-directionally forged (MDFed) under decreasing temperature conditions from 673 K to 463 K up to a cumulative strain of ΣΔε = 6.4 at a true strain rate of 3 × 10^-3 s^-1. A pass strain of Δε = 0.8 was employed. The average grain size decreased gradually with an increase in the cumulative strain. After straining to ΣΔε = 6.4 (i.e., after 8 passes of MDF), equiaxed ultrafine grains (UFGs) with an average size of approximately 0.8 μm were uniformly produced. These grains were relatively coarse as compared with MDFed Mg alloys having initially finer grains. The hardness of the AZ61 Mg alloy increased monotonically up to 910 MPa with decreasing grain size. The Hall-Petch relationship held within this experimental condition.

Abstract: This paper describes the fatigue crack propagation behavior of extruded AZ31B magnesium alloys (average grain size: approximately 15 and 119 μm, respectively). Fatigue crack propagation tests were performed on center cracked tension (CCT) specimens at a stress ratio of R=0.1 and a frequency of 10 Hz at room temperature. Loading axis was parallel to the extrusion direction; crack face was perpendicular to basal plane of each grain. The crack growth rate (da/dN) of the coarse-grained specimen was approximately 5 times higher than that of the fine-grained specimen. Fracture surfaces of the fine-grained and coarse-grained specimens showed various directional steps independent of macroscopic crack growth direction.

Abstract: Mg alloys are lightweight structural alloys that normally have a good castability and machinability as well as an excellent specific strength and rigidity. However, the mechanical properties of Mg alloys are inferior to those of Al alloys, and their range of industrial applications is limited. Recently, Mg–Zn–Y alloy has been found to show a high tensile yield strength with a good elongation. The alloy has a long-period stacking order (LPSO) phase as the secondary phase in an α-Mg phase. In general, the tensile yield strengths of LPSO-type Mg alloy are known to be markedly enhanced by the formation of kink bands in the LPSO phase and by microstructural refinement of the α-Mg phase during plastic deformation. The separate roles of the LPSO phase and the α-Mg phase in relation to the mechanical properties of high-strength LPSO-type Mg alloy were investigated at ambient and high temperatures. For high strengths at ambient and high temperatures, it was important that the α-Mg phase consisted of a fine-grain region and a nonrecrystallized region, and that the LPSO phase remained as a block-type phase. On the other hands, it was necessary to change the LPSO phase from a block-type phase into a plate-type phase by heat treatment before tensile testing to improve the ductility of the alloy while maintaining its tensile yield strength. Microstructural control of the LPSO phase and the α-Mg phase is necessary to obtained Mg–Zn–Y alloy with superior mechanical properties at ambient-to-high temperatures.

Abstract: Magnesium alloys are known for its light weight. Due to the desirable properties,magnesium is required in the fields such as transportation from the point of significant energy savings.Magnesium alloys are also being expected as an alternative for the next generation materials in fieldsof variety. At present, casting and thixomolding process are the main manufacturing methods formagnesium alloy parts. The major problem for forging of magnesium alloy is the lack of basicprocess data, such as strength and ductility.Mg96Zn2Y2 is a high strength magnesium alloy material newly developed by Dr. Y. Kawamurain Japan. It has attracted much attention recently. The name of this alloy is called as 'Kumadai Goukin'.'Kumadai Goukin' of high strength magnesium alloy material is expected as forged parts of theautomobile and materials for aerospace applications. It is necessary to clarify forging processcharacteristics of this material.The purpose of the present study is to evaluate the processing property of Mg96Zn2Y2 material.In this study, cylinder upsetting tests were performed under various deformation temperatures andspeeds for 'Kumadai Goukin'. Furthermore, the dependence of flow stress and ductility of thesematerials to processing temperature and speed was evaluated. We performed microstructureobservation to examine mechanism. We were analyzed that ductility of this material had improved.

Abstract: The microstructures of thixomolded^® (TM) Mg-Al-Ca alloys consist of α-Mg and eutectic compounds along grain boundaries. Misch metal (Mm) addition to TM Mg-Al-Ca alloys makes precipitates within α-Mg matrix and their number density and size depend on heat-treatment conditions. The small addition of Mm can keep the network-like grain boundary covering and the improvement of microstructure stability during creep. On the other hand, excessive Mm addition causes the deterioration of creep strength. The grain boundary coverage decreases with increasing Mm content due to the formation of coarse spherical Al-Mm based intermetallic compounds. Creep strength is significantly affected by both of the grain boundary coverage and the morphology of eutectic compounds along grain boundaries.

Abstract: The high strain rate behaviour of magnesium alloys is of great interest to automotive,aerospace and/or defence industries because some critical components should have the propermechanical properties to work under crash or impact conditions. In the current study, resultsfrom an extensive experimental program are presented. The uniaxial mechanical behaviour ofAZ31 sheet under dynamic conditions (ε=103 s^-1) is analyzed and compared with that observedat low strain rates. AZ31 sheets have been tested in tension and compression using Hopkinsonbar apparatus at 25°C and 250°C. Moreover, interrupted tests were also performed in orderto relate the evolution of deformation mechanisms with strain. Finally, detailed microstructureand texture examination by electron backscatter di raction (EBSD) and neutron di ractionhas been carried out in order to elucidate the predominant deformation and recrystallizationmechanisms.

Abstract: Repetitive upsetting (RU) was applied to a commercial AZ31 Mg alloy. The samples were processed at temperatures of 230 °C, 250 °C and 300 °C up to 3 passes. Effects of processing temperature on the microstructure and mechanical properties were investigated. The results indicate that the microstructure was effectively refined by RU and an average grain size of ~1.9 μm was obtained at 250 °C. Increasing the temperature resulted in larger mean grain size and higher microstructural homogeneity. Both the strength and hardness were significantly improved. It was also found that increasing the processing temperature led to increase in the strength but decrease in the ductility. The sample after RU 3 passes at 230 °C had tensile strength of 330 MPa compared with 173 MPa prior to the processing.

Abstract: Grain refinements during hot compression of continuous casting AZ80 alloy bars and extrusive AZ61, AZ80 alloy bars were observed. The hot compression true stress-true strain curves of extrusive AZ61 and AZ80 alloy had similar tendencies, which were that the true stress increased and had a peak around 0.2 true strain, and then the stress decreased and was almost even from about 0.4 true strain. The stress peaks shifted to the high strain side as the compression speed, true strain speed, increased. However, the stress peaks did not shift at different compression temperatures. Fine grains were obtained under the condition of high strain speed and low temperature, but the grain sizes were uneven at low temperature compression. The stress peaks in the hot compression true stress-true strain curves of continuous casting AZ80 did not shift at different compression temperatures. AZ80 forging products with fine grains are expected to be formed under various conditions by using continuous casting AZ80.

Abstract: Gravity step-casting experiments were performed to investigate process-structure-property relationships in three different die-cast magnesium alloys – AM60, AZ91 and AE44. The step-cast mold was instrumented to capture temperature profiles of the solidification of molten magnesium. This paper investigates the structure-property relationships of these magnesium alloys, specifically the dependence of the fracture properties upon the porosity that forms during the casting process. Sixteen tensile specimens were cut from the step-casting perpendicular to the solidification front, for each alloy examined. Correlations from X-ray tomography data were used to estimate the maximum area fraction of porosity from the average volumetric porosity in the specimens, assuming a typical size and spatial distribution of porosity. This relationship can be used in the absence of more accurate measure of porosity (i.e. serial sectioning, computed x-ray tomography). A failure model for die-cast alloys – which depends upon the strain-hardening coefficient and the maximum area fraction of porosity in the specimen – was used to predict fracture strains for each specimen. The experimental tensile elongation of each specimen was compared with predicted values. The resulting mechanical properties determined from these cast magnesium alloys will be used to develop process-structure-property relationships.