Precipitation of Non-spherical Particles in Aluminum Alloys Part II: Numerical Simulation and Experimental Characterizat
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INTRODUCTION
THERE have been industrial and academic interests to understand precipitation kinetics during aging treatment of Al-Mg-Si alloys as this alloy system accounts for a large percentage of the total aluminum production in the world. It is well established from the transmission electron microscopy (TEM) investigations[1] that the most effective hardening phase in these alloys is the needle-shaped metastable b¢¢ phase, which has been targeted by the most recent precipitation kinetics modeling efforts.[2–4] As discussed in References 2,3,5 and paper I,[6] precipitate’s non-spherical geometry shape has great influences on growth kinetics. A precipitation kinetics model’s prediction power can be substantially improved if the spherical shape assumption could be relaxed. In addition, it is also clear that this spherical assumption has become a major error source in state-of-the-art Integrated Computational Modeling Engineering (ICME) frameworks, from which predictions on final mechanical properties are expected to be made by tracking microstructure evolution through the whole fabrication route. This is due to the fact that at the desired precision level the mechanical properties are dependent not only on the volume fraction and number density of precipitates, but also strongly on their morphology, i.e., aspect ratio, and particle size distribution. As suggested in Reference 7 that plate- and rodQIANG DU, JESPER FRIIS, and CALIN D. MARIOARA, Research Scientists, are with SINTEF Materials and Chemistry, Trondheim, Norway. Contact e-mail: [email protected] BJØRN HOLMEDAL, Professor, is with the Norwegian University of Science and Technology, Trondheim, Norway. Manuscript submitted May 15, 2015. METALLURGICAL AND MATERIALS TRANSACTIONS A
shaped precipitates result in approximately 2 and 1.75 times higher strengths, respectively, than an equal number of spherical precipitates with equal volumes. The ICME framework’s applicability could be substantially extended if they were able to account for precipitate geometrical shape. Indeed these considerations might have motivated the research efforts reported in References 2,3,8,9 applying the physical-based state variables model to predict the precipitation kinetics of needle-shaped particles. The reported models are based on or similar to the modeling framework proposed by Myhr and Grong[10] and by Kampmann and Wagner.[11] The modeling framework, hereafter referred to as the KWN model, are able to predict the evolutions of precipitates’ volume fraction, average size, and size distribution. The methodology of the type of models is that the precipitate size distribution curve is subdivided into size classes, each of which is associated with a number of identical precipitates. The temporal evolution of the size distribution is then tracked by following the size evolution of each discrete size class. This modeling framework and its most recent CALPHAD-coupled multi-component extensions have been applied to optimize alloy chemistry and heat treatment parameters for many industrial
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