Metastable Dendrite Morphologies in Aluminum Alloys
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Metastable Dendrite Morphologies in Aluminum Alloys H. Henein1 • A.-A. Bogno1 • W. Hearn1 • J. Valloton1
Submitted: 14 May 2020 / in revised form: 20 July 2020 Ó ASM International 2020
Abstract Cubic metallic alloys generally grow along h100i directions due to the anisotropy of the solid–liquid interfacial energy. Under rapid solidification conditions, dendrites may deviate from h100i and develop unusual morphologies. Here, Al-alloy droplets (Al-4.5Cu, Al-10Si, Al-1.9Fe, Al-33Cu, all in wt.%) were rapidly solidified using Impulse Atomization to study the microstructures forming at different cooling rates and undercoolings. Growth morphologies of Al-4.5Cu droplets were characterized using x-ray micro-tomography and EBSD. Aldendrites were found to grow along either h100i or a more unusual h111i depending on the solidification conditions. Also, a transition from h111i to h100i in the same droplet was observed. These uncommon growth directions were also observed in other Al-alloys. In Al-1.9Fe droplets, a change in dendrite growth direction from h100i to h111i was observed, while h110i growth directions were detected in Al-10Si samples. These experimental observations will be related to their solidification conditions using Solidification Continuous Cooling Transformation diagrams.
This article is an invited paper selected from presentations at ‘‘PSDK XIV: Phase Stability and Diffusion Kinetics—Gibbs: Phase Equilibria, Diffusion and Materials Design’’ held during MS&T’19, September 29–October 3, 2019, in Portland, Oregon. The special sessions were dedicated to honor Dr. Patrice Turchi, recipient of the ASM International 2019 J. Willard Gibbs Phase Equilibria Award ‘‘for outstanding and pioneering contributions in the application of first-principles, quantum-mechanical calculations to the modeling of phase equilibria and thermodynamic behavior of alloys’’. It has been expanded from its original presentation. & J. Valloton [email protected] 1
Department of Chemical and Materials Engineering, University of Alberta, Edmonton T6G 1H9, Canada
Keywords aluminum alloys growth directions impulse atomization metastable
1 Introduction Solidification is a complex phenomenon arising in many modern experimental techniques and industrial technologies related to casting, joining and surfaces processing. Rapid solidification of metallic alloys is an ongoing research interest in the metallurgical sphere. Such nonequilibrium processing conditions can give rise to solubility extension or the formation of metastable phases due to nucleation and/or growth kinetics, where the interface is still at local equilibrium. True departure from equilibrium of the solid–liquid interface can also be achieved, such as solute trapping where the phase diagram no longer applies for the interface and the chemical potentials are no longer equal at the interface. The variation of different conditions of solidification (such as undercooling or cooling rate) gives a possibility to control the morphology and size of crystal structure, which substa
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