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David H. Stjohn - One of the best experts on this subject based on the ideXlab platform.

  • the effect of alloy content on the grain refinement of aluminium alloys
    Light Metals, 2013
    Co-Authors: Mark Alan Easton, David H. Stjohn
    Abstract:

    A recent model that predicts the effect of solute content on grain size was shown to predict the effect of titanium additions on the grain size of pure aluminium and an AlSi7Mg0.3 alloy. The model assumes that nucleation on substrates is facilitated by Constitutional Undercooling in front of a growing grain. To determine how generally applicable the model is to a broad range of aluminium alloys, titanium additions were made to five wrought alloys containing the same level of TiB2. It was found that the grain size obtained is a function of the reciprocal of mc0(k-1), the growth restriction factor, and that there is a lower limit to the grain size that can be achieved for the casting conditions used in the experiments. It was also found that at least stoichiometric levels of titanium are required for TiB2 particles to be effective nucleants.

  • Grain Morphology of As-Cast Wrought Aluminium Alloys
    Materials Transactions, 2011
    Co-Authors: Mark Easton, Cameron J. Davidson, David H. Stjohn
    Abstract:

    Two of the most important microstructural features of alloys are the grain size and the secondary dendrite arm spacing (SDAS) and these two factors are shown to combine together to describe the grain morphology. Both grain refinement and the SDAS depend upon alloy composition through Constitutional Undercooling, but in different ways. It is shown that there is a ‘characteristic’ SDAS for each alloy at a particular cooling rate, and reducing the grain size causes the grain morphology to change from dendritic to cellular/rosette-like to globular or spherical. Increasing the cooling rate refines both the SDAS and the grain size, but reduces the SDAS more rapidly leading to finer, more dendritic grain structures. Particular ratios of grain size to SDAS are used to define each morphology and it is shown how these can assist with obtaining a required grain size and morphology through the use of solidification conditions and alloy chemistry.

  • Improved prediction of the grain size of aluminum alloys that includes the effect of cooling rate
    Materials Science and Engineering A-structural Materials Properties Microstructure and Processing, 2008
    Co-Authors: Mark Easton, David H. Stjohn
    Abstract:

    A comprehensive study of the effect of cooling rate on the grain size of a range of grain refined wrought aluminum alloys was carried out under quiescent solidification conditions where nucleation occurs predominantly by a Constitutional Undercooling mechanism. Increasing the cooling rate reduced the grain size by increasing the number of particles that nucleate grains and by affecting the development of Constitutional Undercooling. Both effects are represented using simple analytical relationships. By coupling these results with earlier work, an empirical relationship is developed between grain size, density of nucleant particles, cooling rate, nucleant potency and alloy composition that allows prediction of grain size across a wide range of alloys and cooling rates.

  • Modeling of grain refinement: Part I. Effect of the solute titanium for aluminum
    Journal of Materials Research, 2008
    Co-Authors: X. Yao, C J Davidson, A K Dahle, S.d. Mcdonald, David H. Stjohn
    Abstract:

    Over the past few decades, the grain refinement of Al alloys has been extensively investigated theoretically and experimentally. However, the relative importance of the parameters that contribute to grain refinement still remains unclear and is likely to be dependent on specific solidification conditions. This paper aims to investigate the mechanisms by which Ti, a common grain-refining addition in commercial-purity aluminum (CP), contributes to grain refinement using a cellular automaton—finite control volume method (CAFVM). CAFVM is used to model the grain formation and microstructure morphology under different conditions, e.g., with and without refiners, for Al alloys. In this Part I, the effect of adding solute of Ti on grain formation through its effect on growth restriction, Constitutional Undercooling, and the formation of extra-potential particles are taken into account in the calculations. It is shown that the calculated results are in reasonable agreement with the experimental data.

  • Modeling of grain refinement: Part I. Effect of the solute titanium for aluminum
    'Cambridge University Press (CUP)', 2008
    Co-Authors: Yao X., C J Davidson, A K Dahle, S.d. Mcdonald, David H. Stjohn
    Abstract:

    Over the past few decades, the grain refinement of Al alloys has been extensively investigated theoretically and experimentally. However, the relative importance of the parameters that contributes to grain refinement still remains unclear and is likely to be dependent on specific solidification conditions. This paper aims to investigate the mechanisms by which Ti, a common grain-refining addition in commercial-purity aluminum (CP), contributes to grain refinement using a cellular automaton-finite control volume method (CAFVM). CAFVM is used to model the grain formation and microstructure morphology under different conditions, e.g., with and without refiners, for Al alloys. In this Part I, the effect of adding solute of Ti on grain formation through its effect on growth restriction, Constitutional Undercooling, and the formation of extra-potential particles are taken into account in the calculations. It is shown that the calculated results are in reasonable agreement with the experimental data.No Full Tex

Xi Li - One of the best experts on this subject based on the ideXlab platform.

Arne K. Dahle - One of the best experts on this subject based on the ideXlab platform.

  • Modelling of grain size transition with alloy concentration in solidified Al–Si alloys
    Journal of Materials Science, 2007
    Co-Authors: Xiangdong Yao, Cameron J. Davidson, Arne K. Dahle, David H. Stjohn
    Abstract:

    The transition in grain size with Si content in Al–Si alloys has been systematically investigated by the Cellular Automaton-Finite control Volume Method (CAFVM) to understand the operating mechanisms for this behavior. Three aspects: growth restriction factor (GRF), the chemical driving force (CDF) and the Constitutional Undercooling (Δ T _ C ) have been demonstrated to affect the microstructure formation, and among them the Δ T _ C plays the most important role. Furthermore, it is also shown that the surface modification of the nucleant particles by silicon significantly influences the grain formation. However, the combined effects of the investigated factors on the grain size were not sufficiently strong to cause a grain size change similar to that observed experimentally. This implies that there could be other mechanisms that control the transition.

  • Modelling of grain size transition with alloy concentration in solidified Al–Si alloys
    Journal of Materials Science, 2007
    Co-Authors: Arne K. Dahle, Cameron J. Davidson, David H. Stjohn
    Abstract:

    The transition in grain size with Si content in Al-Si alloys has been systematically investigated by the Cellular Automaton-Finite control Volume Method (CAFVM) to understand the operating mechanisms for this behavior. Three aspects: growth restriction factor (GRF), the chemical driving force (CDF) and the Constitutional Undercooling (Delta T-C) have been demonstrated to affect the microstructure formation, and among them the Delta T-C plays the most important role. Furthermore, it is also shown that the surface modification of the nucleant particles by silicon significantly influences the grain formation. However, the combined effects of the investigated factors on the grain size were not sufficiently strong to cause a grain size change similar to that observed experimentally. This implies that there could be other mechanisms that control the transition.

  • Equiaxed solidification of Al–Si alloys
    Materials Science and Technology, 1999
    Co-Authors: J.e.c. Hutt, L. Hogan, David H. Stjohn, Arne K. Dahle
    Abstract:

    An experimental programme has been undertaken to determine which of the grain formation mechanisms of equiaxed crystals are dominant in the solidification of Al-Si foundry alloys. Small ingots were cast from alloys of varying silicon concentration with and without gauze barriers, using different types of mould materials and different mould preheats. The results show that two mechanisms of grain nucleation are operating. The first is a wall mechanism where crystals are nucleated either on or near the mould wall owing to thermal Undercooling. The second is a Constitutional supercooling mechanism where nucleants are activated in the Constitutionally undercooled zone ahead of the advancing interface. As a consequence, the grain size decreases with increasing silicon content. However a transition in the growth mode occurs once a critical degree of Constitutional Undercooling is exceeded. This change in growth is accompanied by an increase in grain size. The transition point can be shifted with respect to solute content by changing the casting conditions, and a mechanism is proposed to explain this effect. MST/4109

R Srinivasan - One of the best experts on this subject based on the ideXlab platform.

  • development of solidification microstructure in boron modified alloy ti 6al 4v 0 1b
    Acta Materialia, 2011
    Co-Authors: Shibayan Roy, Satyam Suwas, Seshacharyulu Tamirisakandala, D B Miracle, R Srinivasan
    Abstract:

    Hypoeutectic boron addition (0.1 wt.%) to Ti-6Al-4V is known to cause significant refinement of the cast microstructure. In the present investigation, it has been observed that trace boron addition to Ti-6Al-4V alloy also ensures excellent microstructural homogeneity throughout the ingot. A subdued thermal gradient, related to the basic grain refinement mechanism by Constitutional Undercooling, persists during solidification for the boron-containing alloy and maintains equivalent beta grain growth kinetics at different locations in the ingot. The Ti-6Al-4V alloy shows relatively strong texture with preferred components (e.g. ingot axis parallel to[0 0 0 1] or [1 0 (1) over bar 0]) over the entire ingot and gradual transition of texture components along the radius. For Ti-6Al-4V-0.1B alloy, significant weakening characterizes both the high-temperature beta and room-temperature a texture. In addition to solidification factors that are responsible for weak beta texture development, microstructural differences due to boron addition, e.g. the absence of grain boundary alpha phase and presence of TiB particles, strongly affects the mechanism of beta -> alpha phase transformation and consequently weakens the alpha phase texture. Based on the understanding developed for the boron-modified alloy, a novel mechanism has been proposed for the microstructure and texture formation during solidification and phase transformation. (C) 2011 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

B Billia - One of the best experts on this subject based on the ideXlab platform.

  • simulation of directional solidification of refined al 7wt si alloys comparison with benchmark microgravity experiments
    Acta Materialia, 2015
    Co-Authors: Dong Rong Liu, Gerhard Zimmermann, Nathalie Mangelincknoel, Charlesandre Gandin, Laszlo Sturz, Henri Nguyenthi, B Billia
    Abstract:

    Formation of grain structure and segregation during solidification of refined Al–7 wt.%Si alloys under microgravity is simulated using a two-dimensional axi-symmetrical Cellular Automaton-Finite Element (CAFE) solidification model. This fully integrated model resolves a complex interplay between heat transfer at the macroscale and grain structure evolution and microsegregation formation at the mesoscale. Direct comparison with benchmark experiments performed under microgravity conditions of the International Space Station is carried out. Boundary conditions used in the simulations are deduced from the experimental measurements of temperature. Qualitative agreement is achieved concerning CET (columnar-to-equiaxed transition) position, CET transition mode (sharp or progressive), distributions of grain elongation factor and equivalent diameter, and distribution of eutectic fraction. The influences of pulling velocity and thermal gradient on grain structure and intergranular segregation are correctly reflected and are discussed. Moreover, we take advantage of the simulation results to study the effect of otherwise inaccessible parameters such as the degree of Constitutional Undercooling and the amplitude of the undercooled region. Another originality of this paper is to conduct quantitative comparisons between experiments and simulations which allows discussion of the improvements needed in the simulation.