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B. Miesenberger, A. Pasching, E. Kozeschnik, E. Povoden-Karadeniz:
"Characterization and modelling of interactions between Mg-Si precipitates and AlFeSi-based dispersoids during homogenization of 6xxx aluminium alloy";
Talk: Euromat 2019, Stockholm; 2019-09-01 - 2019-09-05.

Introduction/Purpose
Homogenization of 6xxx alloys defines the material state for further processing. Therefore, a deep
understanding of the evolution and interactions of occurring phases is essential. It is certain that the
transformation behavior of elongated, brittle AlFeSi-based β-phase to cubic, spheroidal α-phase is an
important parameter for the process optimization. Still, Cr- and Mn-partitioning among β and α and its
contribution to the transformation behavior is a remaining issue of debate. Recently, interrelations
between Mg-Si type particles and AlFeSi-based dispersoids have been reported in addition to the
conventional β-α transformation, i.e. AlFeSi-based dispersoids seem to represent preferred sites for Mg-
Si precipitates. Such alternative phase changes may either directly influence the subsequent processing
or alter the β-α transformation kinetics.
Methods
In this study, phase changes during different stages of homogenization of 6xxx Al-alloy including Mn, Cr
and Fe are examined by combined dilatometry and differential scanning calorimetry. Chemical changes
are investigated by means of electron probe microanalysis (EPMA), including distributions of Mn, Cr and
Si in the matrix, in the grain boundary region and in second phases. Identification of various precipitates
and transformation products is realized by transmission electron microscopy (TEM).
Results
EPMA reveal surprisingly high mobility of Mn and Cr at grain boundaries leading to dispersoid free
zones and incorporated Mn in the needle-like β-AlFeSi structure. TEM investigations confirm close
spatial relations among Mg-Si- and AlFeSi-based precipitates. Trends of these phenomena are
discussed referring to individual heat treatment steps and their variations.
Conclusions
Characterisation served as validation for thermokinetic precipitation and second phase transformation
simulations with MatCalc, employing tailored thermodynamic and diffusion databases for Al-alloys.
Agreement between experimental results and theoretic predictions is discussed in the light of the
physical meaningfulness of the employed kinetic parametrisation.

Project Head Erwin Povoden-Karadeniz:
CDL-IPE