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

  • promoting the columnar to equiaxed transition and grain refinement of titanium alloys during additive manufacturing
    Acta Materialia, 2019
    Co-Authors: M J Bermingham, D H Stjohn, J Krynen, S N Tedmanjones, Matthew S Dargusch
    Abstract:

    Abstract Preventing columnar grain formation during additive manufacturing has become a significant challenge. Columnar grains are generally regarded as unfavourable as their presence can impart solidification defects and mechanical property anisotropy, however, the thermal conditions experienced during additive manufacturing make columnar grains difficult to avoid. In this work the thermal conditions during solidification (cooling rate, temperature gradients) are characterised during wire based additive manufacturing. For the selection of deposition conditions that favour equiaxed grain formation, the role of alloy constitution is explored in three classical alloy design regimes: an alloy containing no grain refiners (Ti 6Al 4V); an alloy only containing grain refining solutes (Ti 3Al 8V 6Cr 4Mo 4Zr); and an alloy containing both grain refining solute and nucleant particles (Ti 3Al 8V 6Cr 4Mo 4Zr + La2O3). Substantial refinement and equiaxed grain formation is achieved in the latter case which is attributed to β-Ti nucleation on La2O3. However, the thermal environment is dynamic during additive manufacturing and equiaxed grain formation is only achievable when temperature gradients decrease sufficiently to permit Constitutional Supercooling.

  • grain refinement of wire arc additively manufactured titanium by the addition of silicon
    Journal of Alloys and Compounds, 2017
    Co-Authors: S Mereddy, M J Bermingham, D H Stjohn, Matthew S Dargusch
    Abstract:

    Abstract This study demonstrates that silicon additions are effective in refining the microstructure of additive layer manufactured (ALM) titanium components. The addition of up to 0.75 wt% silicon to commercially pure titanium manufactured by wire arc ALM results in a significant reduction of the prior-β grain size. It is observed that silicon also reduces the width of the columnar grains and allows for the nucleation of some equiaxed grains through the development of Constitutional Supercooling and growth restriction. The grain size of the ALM components is compared to a casting process and it is found that the as-deposited microstructure produced during ALM exhibits larger average grain sizes. Using the Interdependence model for predicting grain size, it was determined that the population of nucleant particles that naturally occur in titanium, has comparable potency (i.e. ability to activate at a similar undercooling) regardless of the processing method, however, the ALM process contains fewer, sufficiently potent, nucleant particles than for the casting process due to the effect of subsequent cycles of remelting and heating.

  • The Contribution of Constitutional Supercooling to Nucleation and Grain Formation
    Metallurgical and Materials Transactions A, 2015
    Co-Authors: D H Stjohn, A. Prasad, M. A. Easton, Ma Qian
    Abstract:

    The concept of Constitutional Supercooling (CS) including the term itself was first described and discussed qualitatively by Rutter and Chalmers in order to understand the formation of cellular structures during the solidification of tin, and then quantified by Tiller et al . On that basis, Winegard and Chalmers further considered ‘Supercooling and dendritic freezing of alloys’ where they described how CS promotes the heterogeneous nucleation of new crystals and the formation of an equiaxed zone. Since then the importance of CS in promoting the formation of equiaxed microstructures in both grain refined and unrefined alloys has been clearly revealed and quantified. This paper describes our current understanding of the role of CS in promoting nucleation and grain formation. It also highlights that CS, on the one hand, develops a nucleation-free zone surrounding each nucleated and growing grain and, on the other hand, protects this grain from readily remelting when temperature fluctuations occur due to convection. Further, due to the importance of the diffusion field that generates CS, recent analytical models are evaluated and compared with a numerical model. A comprehensive description of the mechanisms affecting nucleation and grain formation and the prediction of grain size is presented with reference to the influence of the casting conditions applied during the practical casting of an alloy.

  • the influence of ternary cu additions on the nucleation of eutectic grains in a hypoeutectic al 10 wt si alloy
    Journal of Alloys and Compounds, 2015
    Co-Authors: A Darlapudi, S D Mcdonald, D H Stjohn
    Abstract:

    Abstract The Influence of the ternary alloying element Cu on eutectic nucleation in an Al-10 wt.%Si alloy in unmodified and Sr-modified conditions was studied. Cu additions had a relatively minor effect on the unmodified eutectic nucleation frequency. In Sr-modified Al–Si alloys where the nucleation frequency of the eutectic grains is very low compared to the unmodified alloys, the addition of Cu significantly increased the nucleation frequency. Further increases in the Cu concentration resulted in a continuous increase in the number of eutectic grains and an associated decrease in their size. It is proposed that Constitutional Supercooling plays an important role in promoting the nucleation of eutectic grains ahead of the solidifying interface especially in the case of Sr-modified Al–Si alloys.

  • an analytical model for Constitutional Supercooling driven grain formation and grain size prediction
    Acta Materialia, 2010
    Co-Authors: Ma Qian, M. A. Easton, Peng Cao, S D Mcdonald, D H Stjohn
    Abstract:

    Being able to predict the grain formation process and attendant grain size has been a central topic in solidification. Such an analytical model is presented for Constitutional Supercooling (CS)-driven grain formation with several simplifications. The model links the nucleation of new grains to the growth of a larger neighbouring grain. The average grain size ((d) over bar) is thus determined by two components: the minimum growth (r(cs)) necessary to establish sufficient CS (Delta T(n)) for nucleating new grains, and the spatial mean distance ((a) over bar) to the most potent available nucleants. Both spherical and planar growth fronts are considered, covering growth curvatures from small to infinite. Two distinct fundamental approaches are used, which result in identical descriptions of (d) over bar, where (d) over bar = (a) over bar + D . Delta T(n)(v . Q) (D is the diffusion coefficient, v is the growth velocity, Q is the growth restriction factor). The model is compared with literature data produced under various conditions and demonstrated on aluminium alloys as an example. (C) 2010 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

S D Mcdonald - One of the best experts on this subject based on the ideXlab platform.

  • the influence of ternary cu additions on the nucleation of eutectic grains in a hypoeutectic al 10 wt si alloy
    Journal of Alloys and Compounds, 2015
    Co-Authors: A Darlapudi, S D Mcdonald, D H Stjohn
    Abstract:

    Abstract The Influence of the ternary alloying element Cu on eutectic nucleation in an Al-10 wt.%Si alloy in unmodified and Sr-modified conditions was studied. Cu additions had a relatively minor effect on the unmodified eutectic nucleation frequency. In Sr-modified Al–Si alloys where the nucleation frequency of the eutectic grains is very low compared to the unmodified alloys, the addition of Cu significantly increased the nucleation frequency. Further increases in the Cu concentration resulted in a continuous increase in the number of eutectic grains and an associated decrease in their size. It is proposed that Constitutional Supercooling plays an important role in promoting the nucleation of eutectic grains ahead of the solidifying interface especially in the case of Sr-modified Al–Si alloys.

  • an analytical model for Constitutional Supercooling driven grain formation and grain size prediction
    Acta Materialia, 2010
    Co-Authors: Ma Qian, M. A. Easton, Peng Cao, S D Mcdonald, D H Stjohn
    Abstract:

    Being able to predict the grain formation process and attendant grain size has been a central topic in solidification. Such an analytical model is presented for Constitutional Supercooling (CS)-driven grain formation with several simplifications. The model links the nucleation of new grains to the growth of a larger neighbouring grain. The average grain size ((d) over bar) is thus determined by two components: the minimum growth (r(cs)) necessary to establish sufficient CS (Delta T(n)) for nucleating new grains, and the spatial mean distance ((a) over bar) to the most potent available nucleants. Both spherical and planar growth fronts are considered, covering growth curvatures from small to infinite. Two distinct fundamental approaches are used, which result in identical descriptions of (d) over bar, where (d) over bar = (a) over bar + D . Delta T(n)(v . Q) (D is the diffusion coefficient, v is the growth velocity, Q is the growth restriction factor). The model is compared with literature data produced under various conditions and demonstrated on aluminium alloys as an example. (C) 2010 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

Matthew S Dargusch - One of the best experts on this subject based on the ideXlab platform.

  • promoting the columnar to equiaxed transition and grain refinement of titanium alloys during additive manufacturing
    Acta Materialia, 2019
    Co-Authors: M J Bermingham, D H Stjohn, J Krynen, S N Tedmanjones, Matthew S Dargusch
    Abstract:

    Abstract Preventing columnar grain formation during additive manufacturing has become a significant challenge. Columnar grains are generally regarded as unfavourable as their presence can impart solidification defects and mechanical property anisotropy, however, the thermal conditions experienced during additive manufacturing make columnar grains difficult to avoid. In this work the thermal conditions during solidification (cooling rate, temperature gradients) are characterised during wire based additive manufacturing. For the selection of deposition conditions that favour equiaxed grain formation, the role of alloy constitution is explored in three classical alloy design regimes: an alloy containing no grain refiners (Ti 6Al 4V); an alloy only containing grain refining solutes (Ti 3Al 8V 6Cr 4Mo 4Zr); and an alloy containing both grain refining solute and nucleant particles (Ti 3Al 8V 6Cr 4Mo 4Zr + La2O3). Substantial refinement and equiaxed grain formation is achieved in the latter case which is attributed to β-Ti nucleation on La2O3. However, the thermal environment is dynamic during additive manufacturing and equiaxed grain formation is only achievable when temperature gradients decrease sufficiently to permit Constitutional Supercooling.

  • grain refinement of wire arc additively manufactured titanium by the addition of silicon
    Journal of Alloys and Compounds, 2017
    Co-Authors: S Mereddy, M J Bermingham, D H Stjohn, Matthew S Dargusch
    Abstract:

    Abstract This study demonstrates that silicon additions are effective in refining the microstructure of additive layer manufactured (ALM) titanium components. The addition of up to 0.75 wt% silicon to commercially pure titanium manufactured by wire arc ALM results in a significant reduction of the prior-β grain size. It is observed that silicon also reduces the width of the columnar grains and allows for the nucleation of some equiaxed grains through the development of Constitutional Supercooling and growth restriction. The grain size of the ALM components is compared to a casting process and it is found that the as-deposited microstructure produced during ALM exhibits larger average grain sizes. Using the Interdependence model for predicting grain size, it was determined that the population of nucleant particles that naturally occur in titanium, has comparable potency (i.e. ability to activate at a similar undercooling) regardless of the processing method, however, the ALM process contains fewer, sufficiently potent, nucleant particles than for the casting process due to the effect of subsequent cycles of remelting and heating.

A Darlapudi - One of the best experts on this subject based on the ideXlab platform.

  • the influence of ternary cu additions on the nucleation of eutectic grains in a hypoeutectic al 10 wt si alloy
    Journal of Alloys and Compounds, 2015
    Co-Authors: A Darlapudi, S D Mcdonald, D H Stjohn
    Abstract:

    Abstract The Influence of the ternary alloying element Cu on eutectic nucleation in an Al-10 wt.%Si alloy in unmodified and Sr-modified conditions was studied. Cu additions had a relatively minor effect on the unmodified eutectic nucleation frequency. In Sr-modified Al–Si alloys where the nucleation frequency of the eutectic grains is very low compared to the unmodified alloys, the addition of Cu significantly increased the nucleation frequency. Further increases in the Cu concentration resulted in a continuous increase in the number of eutectic grains and an associated decrease in their size. It is proposed that Constitutional Supercooling plays an important role in promoting the nucleation of eutectic grains ahead of the solidifying interface especially in the case of Sr-modified Al–Si alloys.

J S Wettlaufer - One of the best experts on this subject based on the ideXlab platform.

  • morphological instability in freezing colloidal suspensions
    Proceedings of The Royal Society A: Mathematical Physical and Engineering Sciences, 2007
    Co-Authors: S S L Peppin, Grae M Worster, J S Wettlaufer
    Abstract:

    We present a linear stability analysis of a planar ice interface during unidirectional solidification of a hard-sphere colloidal suspension. We find that the interface can become unstable due to Constitutional Supercooling, yielding a new mechanism for pattern formation in colloidal systems. The interfacial stability is shown to depend strongly on the size and concentration of the particles. Increasing the particle radius tends to stabilize the interface, while increasing the concentration has a destabilizing effect. Additional effects that may influence the stability and morphology of such a system are described.

  • Morphological instability in freezing colloidal suspensions
    'The Royal Society', 2007
    Co-Authors: Peppin Ssl, Mg Worster, J S Wettlaufer
    Abstract:

    We present a linear stability analysis of a planar ice interface during unidirectional solidification of a hard-sphere colloidal suspension. We find that the interface can become unstable due to Constitutional Supercooling, yielding a new mechanism for pattern formation in colloidal systems. The interfacial stability is shown to depend strongly on the size and concentration of the particles. Increasing the particle radius tends to stabilize the interface, while increasing the concentration has a destabilizing effect. Additional effects that may influence the stability and morphology of such a system are described. © 2006 The Royal Society