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α-Mg primary phase formation and dendritic morphology transition in solidification of a Mg-Nd-Gd-Zn-Zr casting alloy

Abstract

Microstructure evolution in the commercial Mg-Nd-Gd-Zn-Zr alloy Elektron 21, solidified under nearly isothermal conditions, has been studied via in-situ X-ray radiography. For cooling rates View the MathML source≤0.075 K/s, primary equiaxed α-Mg dendrites undergo a morphological transition after nucleation and an initial stage of growth. The growth regime is observed to change abruptly from a 3D to a more pronounced anisotropic sheet-like growth occurring predominantly along View the MathML source direction, with a 4–5 times increase in the growth velocity. The experimental results together with thermodynamic calculations and density functional theory simulations give support to relate the morphology transition to the formation of ordered rare earth-zinc dimers in the {0001} basal plane and View the MathML source pyramidal plane of α-Mg lattice. At the temperature where the morphological transition occurs, it is found that both the solute concentration and zinc diffusivity in α-Mg are high enough for dimer formation to occur within a diffusive layer extending a few micrometres from the solid-liquid interface into α-Mg, and thereby open for increased solute-partitioning at the growth front.

Category

Academic article

Language

English

Author(s)

  • Daniele Casari
  • Wajira Mirihanage
  • Ken Vidar Falch
  • Inga Gudem Ringdalen
  • Jesper Friis
  • Rainer Schmid-Fetzer
  • Dongdong Zhao
  • Yanjun Li
  • Wim H. Sillekens
  • Ragnvald Mathiesen

Affiliation

  • SINTEF Industry / Materials and Nanotechnology
  • ESA Research and Scientific Support Department
  • Technical University of Clausthal
  • Norwegian University of Science and Technology

Year

2016

Published in

Acta Materialia

ISSN

1359-6454

Volume

116

Page(s)

177 - 187

View this publication at Norwegian Research Information Repository