Materials Science Forum Vols. 747-748

Abstract: The multilayered materials with different combinations of Ti, Al and Ti-Al intermetallics were prepared by heat treatment and hot pressing (HP) with elemental foils. The microstructures and phase formation of the obtained samples were detected by scanning electron microscopy (SEM), electron probe microanalysis (EPMA), energy dispersive spectroscopy (EDS) and X-ray diffractometer (XRD). When the HP is applied under the melt point of aluminum, aluminum is the only diffusive element across the oxide films on the surface of the initial foils; however, some unusual TiAl₃ particles are found in the multilayered structure due to the broken of oxide films; after hot pressing for 4 hours, all the aluminum was consumed; many voids exist at the centerline of TiAl₃ layers, which are mainly caused by Kirkendall effect and the difference of molar volumes between reactants and products; before the aluminum is completely consumed, TiAl₃ is the only product in the solid reaction under the melting temperature of aluminum; however, other Ti-Al intermetallics like Ti₃Al and TiAl are formed in the updated temperature diffusion after aluminum is consumed.

Abstract: The structures of AlSc with 50at.% and 55at.% Sc were investigated. It was shown that orthorhombic AlSc is formed at Sc 50at.%, and B2 AlSc at Sc 55at.%. After anealing above 1100 °C, the orthorhombic AlSc transited into B2 structure, which indicated that B2 AlSc can also be a metalstable phase in the alloy with Sc less than 50at.%. The effects of these two structures on the phases formation via inter-diffusion were also studied using diffusion couple with titanium and γ-TiAl. It was found that the interdiffusion coefficients are the main factors for Ti/AlSc and structures are the main factors for γ-TiAl/AlSc.

Abstract: The microstructure evolution of TiAl intermetallics with different microstructures loaded under different strain rates and temperatures was investigated. The results showed that the deformation twinning dominated the deformation process under high strain rate, while dislocation slip was another dominating deformation mode under quasi-static loadings. The proportion of twinned grain increased with the increased strain rate. In Duplex TiAl, the plastic deformation was mainly found in equaxied grains and seldom found in lamellar grains.

Abstract: The effects of Fe stoichiometry and degree of order on mechanical properties of intermetallics Ni₃Fe were investigated. The experimental results showed that the phase composition and the grain size of Ni₃Fe alloy was independent on Fe stoichiometry. The Ni₃Fe alloy with the normal stoichiometry had the lowest elongation in the ordered and disordered alloys. The tensile strength of Ni₃Fe alloy depended on Fe stoichiometry of alloy. The effect of Fe stoichiometry on the mechanical properties of the ordered alloy was larger than that of disordered alloy. There were more point defects in lattice when the alloy stoichiometry deviating from the normal stoichiometry in the Ni₃Fe alloy. The changes of the concentration of the point defects and degree of order altered the deformation behavior of the Ni₃Fe alloy.

Abstract: Fe-6.5wt.%Si alloy is one excellent soft magnetic material. In the present study, rapid quenching was applied to fabricate ultra-thin ribbons of Fe-6.5wt.%Si alloy by melt spinning. Continuous ribbons of 20 mm wide, 35-45 μm thick were fabricated with optimized experimental parameters. The ribbons exhibited metallic luster appearance, and can be wound without cracks. Microstructure, texture and mechanical properties of the ribbons were investigated. It was found that the rapidly quenched ribbons had intense {100} plane texture, and exhibited some extent of bending capability. Finally the ribbons were cold rolled and the surface smoothness was improved for stacking purpose.

Abstract: Fe-6.5%Si alloy is an excellent soft magnetic material. However, the industrial production of this sheet was difficult, due to its room temperature brittleness. In order to improve its deformation performance, the directional solidification and inhibitor of Nb were investigated in this study. Rod samples of Φ6.8 mm×100 mm were used both for modeling and experiments. ProCAST-finite element analysis was employed for the modeling with a temperature gradient of 1150K/cm (from 1450°C to 300°C) for directional solidification and the drawing velocities of 10, 35, 60, and 100μm/s. Calculations were carried out on the concerning temperature gradient, solidification rate and grain growth. The results showed that small amount of Nb, with optimized drawing velocity of 60μm/s was helpful for the homogeneity of grain size, which could improve the ductility for subsequent deformation processes.

Abstract: High Nb containing TiAl alloys have been investigated traditionally as potential high temperature structural materials because of their high strength, good oxidation and creep resistance. However, the poor ductility and fracture toughness at room temperature limit their application, which could be improved by controlling microstructure to get refine and homogeneous fully lamellar structure. In this study, a high Nb containing TiAl alloy alloying Mn, B and Y with refined microstructure was produced. The solidification path was analyzed by DSC and SEM microstructure of the alloy was observed, after heating at a certain temperature for 1-24hrs and then quenching in water. The dissolution of β phase was also investigated. The results showed that the β phase could decompose only by heating in single β or near α phase field.

Abstract: In the present paper, the necessity of strictly controlling Al content is demonstrated for Ti-45Al-8.5Nb-0.2B-0.2W alloy by analyzing solidification microstructure, compressive properties at room temperature and α phase transformation point of alloys containing 43-47at.%Al. The results show that the alloy has an optimal combination of mechanical properties and microstructure. Increasing Al content leads to increase in lamellar colony size and decrease in B2 phase for the solidification microstructure. Yield strength decreases linearly with Al content, but total strain increases linearly with Al content. The phase transformation temperature of β + αα rises more than 20 °C when Al increases to 46at. %, and drops about 20 °C when Al decreases to 44at %. Therefore, Al deviation should be as small as possible in producing technical alloy to ensure the stability in terms of properties, microstructure and processing route.

Abstract: The superplastic bulging process of TiAl sheet was simulated by coupling superplastic constitutive equation to finite element model. Based on this model, the effect of coefficient of friction between the sheet and mold and the size of the mold fillet on the superplastic bulging (SPB) performance of TiAl alloys sheet were studied by analyzing the evolution of equivalent plastic strain and thickness distribution in the sheet. The results showed that friction was the dominant factor of the nonuniform thickness of the sheet, while higher friction and smaller radius of mold fillet inhibited the over-thinning of sheet on the entry of mold cavity. The simulation results were in good agreement with the experimental results. Therefore, the present model could be used for optimizing the selction of the deformation parameters and the design of the structures.