Papers by Keyword: Dendritic Segregation

Abstract: The microstructure and segregation of 7N01 aluminum alloy microalloyed with Er and Zr before and after homogenization process at 470 °C for 24 h have been investigated using transmission electron microscope (TEM), and scanning electron microscope (SEM) equipped with energy dispersive X-ray spectroscope (EDS) and transmission electron detecter (STEM). SEM images and EDS line scan results showed that there were dendritic segregation of Mg and Zn and micron-sized primary phases along dendrites in the as-cast alloy. The primary phases were mainly Al₂Mg₃Zn₃ containing Al, Zn, Mg and Mn. And a small amount of primary phases were the particles containing Al, Zn, Mg, Mn, Cr and Er and the particles containing Al, Zn, Mg, Er, Mn, Fe and Cr. After homogenization at 470°C for 24 h, dendritic segregation of Mg and Zn was eliminated, Al₂Mg₃Zn₃ and the particles containing Al, Zn, Mg and Mn were basically dissolved. The particles containing Al, Zn, Mg, Mn, Cr and Er and the particles containing Al, Zn, Mg, Er, Mn, Fe and Cr remained. Meanwhile, TEM and STEM images showed that a large number of secondary phases appeared after the homogenization, which were mainly distributed in the inter-dendrite area. EDS results showed that the block-shaped and rod-shaped phases with the width of 10-70 nm and length of 100-500 nm were the particles containing Al, Cr and Mn or Al, Mg, Cr and Mn, and the ellipsoid secondary phase with the size of 50-100 nm was MgZn₂.

Abstract: This paper explains the type of heat treatments used to improve the bulk hardness and tensile properties of Tungsten Inert Gas (TIG) welded Al6061-SiC reinforced composites. Composites with 8, 10 and 12 wt% of SiC (25 μm) are TIG welded using ER5356 filler material and the hardness, tensile property and microstructural analysis in as weld condition is carried out. Age hardening heat treatment is performed and peak hardness values are noted at different aging temperatures and subjected to characterization. Al6061-12 wt% SiC shows reduction in tensile strength in comparison with the other two in spite of improvement in hardness. Since hardness is more, there arise ambiguity in knowing the actual cause of decrease in the tensile strength. Optical micrographs of the samples are taken and identified the presence of dendritic segregation as the actual cause for the (UTS) reduction. To remove dendritic segregation, all the as weld samples are subjected to 5h homogenizing at 500°C, age hardened and characterized. The result showed substantial hike in UTS with substantial improvement in hardness, homogeneous distribution of hardness over the cross-section in the weld bead. The depth and distribution of hardness and bulk property improvement are observed by the combined effect of homogenizing and low temperature age hardening.

Abstract: The phase transformations of the alloy Ti-40 mas % Nb after tempering and severe plastic deformation are studied. The phase transformations of the alloy according to the type and conditions of external influences are analyzed using methods of XRD, SEM and optical metallography. It is determined that inverse phase transformation of the metastable α''-phase to equilibrium β-phase is carried out after severe plastic deformation. Complete phase transformation α'' → β is typical for the mode, which consists of three pressing operation with the change of the loading axis in cramped conditions, followed by a multi-pass rolling in grooved rolls.

Abstract: The microstructure and properties of Cu-7.5Ni-5Sn alloy after homogenization treatment was investigated. The research results show that homogenization treatment can obviously eliminate dendritic segregation in Cu-7.5Ni-5Sn alloy. The temperature of homogenization annealing has a great influence than the effect of holding time. The ingots of Cu-7.5Ni-5Sn alloy which were homogenization treated at 780°C for 24 hours can be rolled up to 30% deformation by cold-rolling. Some tiny white matter is still remaining in dendrite boundary, but atom fraction of Ni and Sn of the dendritic segregation is decreased by 12.04% and 4.73% respectively compare with casting state. The electrical conductivity and Brinell hardness of Cu-7.5Ni-5Sn alloy homogenization treated at 800°C for 24 hours is 12.8%IACS and 132Hv, and increases 18.5% and 29.9% respectively.

Abstract: The serious dendritic segregation occurs in Cu-20Ni-5Sn alloy. The microstructure of Cu-20Ni-5Sn alloy as-cast is composed of white light dendrite (Cu-Ni solid solution), Sn-rich solid solution (Cu₂Ni₃Sn₃ phase) and block light precipitation (Ni₁₇Sn₃ phase). The microstructure of Cu-20Ni-5Sn alloy affected by alterative and homogenization temperature was studied. The results show that the alterative can refine the microstructures of the Cu-20Ni-5Sn alloy and reduce the segregation of Sn. The grains are quite fine with 0.9%Wt alterative in the alloy. The homogenization temperature of the Cu-20Ni-5Sn alloy decreases with increase of alterative content in the alloy. Grains enlarged with the increase of homogenization temperature, and the secondary phase distributes became more evenly. The optimization condition of homogenization treatment is at 800 in 24h.

Abstract: The effects of hot isostatic pressing on the microstructures of a third generation single crystal Ni-based superalloy DD10 were investigated by using optical microscope (OM), scanning electron microscope (SEM), electron microprobe analyzer (EPMA). The results showed that the micropores in the interdendritic region were eliminated completely after hot isostatic pressing at 1320 and 150MPa. Meanwhile, the morphology of γ precipitates changed to be more cuboidal and the distribution of γ precipitates in both dendrite core and interdendritic region became more uniform after hot isostatic pressing. Hot isostatic pressing also promoted the homogenization of the composition between dendrite core and interdendritic region and the dendritic segregation of Re, W, Al and Ta was decreased.

Abstract: Based on the phenomenon of fracture lamination of 10CrNiCu steel, micro-analysis and Scanning Electron Microscope are applied, and results show that there is a lot of MnS inclusions near the fractures of 10CrNiCu steel，and the banded Structure caused by continuous cast is serious. The improve means is mentioned.

Abstract: The cosine solution of the Fick-2 Law, which was considered as the mathematical model of homogenization of dendritic segregation, has always been used to analyze high temperature diffusion. Cosine solution can not exactly describe the actual distribution of alloying elements in dendrite, as the slope of cosine curve is quite gentle. Based on segregation ratio SR, a new mathematical model was established by using Gauss solution of the Fick-2 Law. The new model could describe the evolution of homogenization of alloying elements during the process of high temperature, which was proved by some specific examples.

Abstract: The effect and the ways of removal of the segregation of alloying elements within the dendrite cell in single-crystal nickel-base superalloys are discussed. The peculiar features of Re and Ru behavior are underlined.