Materials Science Forum Vols. 558-559

Abstract: The local-area analyses of the primary recrystallization sheets were carried out in Fe-3%Si to explain the reason why only Goss appears during the secondary recrystallization. It was found in the present study that Goss grains have the higher Σ9 frequency in their nearest neighbors than {311}<011> grains, both of which were attached to the neighbors having the texture with high Σ9 frequency to the texture of the matrix. Another analysis showed that Σ1 and Σ3 frequencies were higher around {311}<011> grains than around Goss. It is concluded that Goss grains possess the satisfactory criteria to be the secondary recrystallization nuclei from the following viewpoints of high Σ9 frequency and low Σ1 and Σ3 frequencies.

Abstract: Friction stir processing (FSP) is the effective method of the grain refinement for light metals. The aim of this study is to acquire the fine grained bulk Mg-Y-Zn alloy by ingot metallurgy route much lower in cost. Such bulk alloy can be formed by the superplastic forging. The microstructure of as-cast Mg-Y-Zn alloy was dendrite. The dendrite arm spacing was 72.5 [(m], and there are the lamellar structures in it. FSP was conducted on allover the plate of Mg-Y-Zn alloy for both surfaces by the rotational tool with FSW machine. The stirring passes were shifted half of the probe diameter every execution. The dendrite structures disappeared after FSP, but the lamellar structure could be observed by TEM. The matrix became recrystallized fine grain, and interdendritic second phase particles were dispersed in the grain boundaries. By using FSP, cast Mg-Y-Zn alloy could have fine-grained. This result compared to this material produced by equal channel angular extrusion (ECAE) or rapid-solidified powder metallurgy (RS P/M). As the result, as-FSPed material has the higher hardness than materials produced by the other processes at the similar grain size.

Abstract: High-strain conditions as a means of microstructure control have recently been investigated to improve the ductility and enhance the strength of magnesium alloys. The level of superplastic deformation and the fatigue properties of the wrought materials have also been studied. In comparison, only a small number of such reports are available on cast materials. As a part of the search for applications of magnesium alloys, comparisons of structural changes and mechanical properties should be made between wrought and cast materials. In the present study, the grain refinement of cast and extruded materials made from commercially available AZ31 magnesium alloy was conducted using a multi-axial alternative forging method. The relationships between the structural changes and working processes and the relationships between changes in the mechanical properties as well as grain sizes and fatigue properties are discussed. Both the cast and the extruded materials tended to exhibit uniform crystalline structures with an increasing number of working cycles. Dynamic recrystallization was observed during both working and static recrystallization during both reheating and holding. When an equivalent strain of 0.6 was applied, the localized formation of ultra-fine grains of 0.5 μm was observed. The tensile strength and yield stress had maximum values in the initial stage of the multi-axial alternative forging. Although ductility improved with higher numbers of working cycles, the strength decreased. This can be explained by the dynamic and static recrystallization processes and work softening.

Abstract: Strength of Al5083-O FSW joints was evaluated, in relation to FSW conditions. Static strength of the joints was found to be enough because base metal fracture occurred in the tensile tests of joints for some FSW conditions. However, the fatigue strength of those joints with fine static strength varied significantly. For example, in tensile strength of joints with the FSW condition with tool rotation: 800 rpm- welding speed: 100 mm/min (800-100), 800 rpm-200 mm/min (800-200) and 500 rpm- 100 mm/min (500-100), each condition of FSW joint efficiency were all hundred percent. In contrast, joint efficiency for fatigue strength varied from 75% for 500-100 to 31% for 800-200. Crack path in fatigue test was always initiated at the center of back surface of FSW weld zone, and propagated through stir zone. The variance of fatigue strength is believed to be attributed to the stirred condition at back surface.

Abstract: The effect of deformation on the microstructure of a Al-4wt.%Cu alloy has been investigated. Evaluation of the microstructural changes was made by comparing results after both static annealing and tensile testing (deformed and non-deformed regions) at 450 °C. Uniaxial perturbed-rate tests showed that the Al-4wt.%Cu has a low value of the strain rate sensitivity index (m ~ 0.22) and cannot be considered as a superplastic material. It was found that in the deformed regions, specimens showed a significant increase in the grain and particle size. These changes were accompanied by an increase in the aspect ratio of the matrix grains. Tensile tests carried out at constant strain rates and stopped at intermediate strains helped to estimate the rate of the grain and particle growth and the contribution of deformation to it. To examine in detail the mechanism of the particle coarsening during deformation, additional tensile tests were made using the Al-4wt.%Cu alloy annealed at temperature conditions leading to abnormal grain size. Results of these tests also showed strain-induced particle coarsening, so that dynamic particle coarsening was not simply caused by boundary migration effects.

Abstract: The effect of copper content on dynamic grain growth in Al-Cu-Zr system was investigated by studying the microstructural development and texture evolution during uniaxial tensile deformation of Al-2wt%Cu-0.3wt.%Zr and Al-4wt%Cu-0.4wt.%Zr alloys at 450°C with a strain rate of 10-3s-1, with a similar initial microstructure in both materials. The initial microstructure consisted of layers of different orientations, the layers being separated by high-angle grain boundaries with low-angle boundaries separating grains within the layers. The initial grain spacing was about 5
m and the texture was typical of rolled aluminium alloys. The 4wt.%Cu alloy gave a higher strain rate sensitivity index, m, and a greater ductility compared to the low copper content alloy. An increase in grain size occurred in both materials due to deformation, but this dynamic grain growth (DGG) was much greater in the material with the higher copper content. This was associated with a more rapid conversion of low-angle boundaries to high angle ones in the 4wt%Cu material which is consistent with changes in crystallographic texture occurring during deformation.

Abstract: The microstructure of Fe-3 mass% Si alloys before secondary recrystallization has been characterized by analyzing precipitates and grain boundary segregated elements. The samples used were mainly sheets of Fe-3%Si alloys containing manganese, sulfur, aluminum, nitrogen and tin, which were decarburized and annealed up to secondary recrystallization. Grain boundary segregation in primarily recrystallized samples was studied using Auger electron spectroscopy (AES), and precipitates were analyzed using transmission electron microscopy (TEM) with an energy dispersive X-ray spectrometer (EDX). AES spectra showed that tin and nitrogen were enriched on grain boundaries in the Fe-3 mass% Si alloys. TEM/EDX analysis showed that the morphology and distribution of the fine precipitates such as manganese sulfide and aluminum nitride were influenced by addition of tin. The characteristic structure formed by secondary recrystallization of grain oriented silicon steel is considered to be influenced by the fine precipitates and segregation of a small amount of elements, as the abnormal motion of grain boundaries of the silicon steel was correlated with the precipitation and segregation of the alloying elements.

Abstract: High-strength Mg-Y-Zn alloy plate was obtained by friction stir processing (FSP) after casting. In this study, the effect of processing order on the strength of processed specimens was discussed. It was revealed that the microstructures and strength of doubly stirred zone depended on the direction of overlapping pass. In the area of the doubly stirred zone that was affected by the thermal history of Retreating-side, the hardness was 121Hv. It was twice as hard as Base Metal, and harder than the doubly stirred zone that was affected by thermal history of Advancing-side.