Validation of AlN precipitation modelling results by in-situ austenite grain growth observations

Translated title of the contribution: Validierung der Ergebnisse aus AlN Ausscheidungsmodellierungen durch in-situ Austenitkornwachstumsuntersuchungen

Miriam Schnalzger

Research output: ThesisMaster's Thesis


The control of grain size has become a key aspect in material science and engineering. However, precipitation mechanisms can take complex pathways regarding thermodynamic and kinetic factors. These have driven not only experimental, but also extensive computational studies. In particular, research has increasingly focused on the prediction of grain growth establishing empirical and thermodynamic simulation models offering the possibility to answer open-ended questions arising from experimental observations. However, the calculation of the pinning force incorporated in these models has always been challenging due to its dependence on numerous factors of influence. For this reason, this thesis focuses on the validation of simulated pinning forces of aluminium nitride precipitates using the advanced thermodynamics software MatCalc, in which the chemical composition of experimental alloys with varied aluminium and nitrogen content is defined. The pinning forces obtained from numerical simulations are then implemented in a proposed grain growth model from where the mean grain size for different holding temperatures is calculated. In the framework of performed precipitation calculations, the influence of specific parameters on phase fraction, number density and mean precipitate radius is demonstrated helping to provide a better understanding of how the simulation responds to changes of certain settings. Based on these results, the default settings for the pinning force calculation with MatCalc are defined. The activation energy underlying the model is determined with a non-linear solver fitting in Excel using a reference alloy. For the final validation of the received pinning forces, the calculated grain sizes are compared to experimentally determined grain sizes. Therefore, six different Al and N alloyed samples are prepared by means of a High-Frequency Re-Melter. For the in-situ observation of grain growth, the High-Temperature Laser Scanning Confocal Microscope (HT-LSCM) is employed, which allows a targeted evaluation of grain size at holding temperatures of 950, 1050, 1150 and 1250 °C. These results are thereby represented by the calculation of the mean value of measured grain sizes. In consideration of final adjustments, a direct comparison of modelled and measured grain sizes at respective holding temperatures is given. With reference to the results for samples A (0.026 % Al, 50 ppm N) and D (0.005 % Al, 460 ppm N), it is shown that the calculated pinning forces at 950 up to 1150 °C excellently match with experimental observations. For sample A at 1250 °C, MatCalc does not predict any retarding pressures but nonetheless the grain size prediction model delivers smaller grain sizes compared to the measurement results. For this reason grain size distributions are generated, which indicate the occurrence of abnormal grain growth. It is shown that the larger grain population causes a misleading increase of the mean value of measured grains. In terms of abnormal grain growth, conclusions from the comparison between both results must be therefore drawn carefully. For samples G (0.03 % Al, 194 ppm N), E (0.028 % Al, 250 ppm N) and F (0.025 % Al, 310 ppm N) at 950 °C and 1050 °C, MatCalc underestimates the presence of pinning forces. At 1150 °C and 1250 °C, the retarding pressures are overestimated. However, the comparison between modelled and measured grain size turns out to be no longer representative as soon as abnormal grain growth occurs. Nevertheless, MatCalc offers together with the grain size prediction model a possibility for quick assessment of pinning forces especially for alloys with lower Al and N content. With respect to the measurement results, it is demonstrated how sensitive the HT-LSCM reacts towards small nitrogen variations. Therefore, it proved to be a valuable tool ensuring excellent reproducibility for the evaluation of precipitation kinetics.
Translated title of the contributionValidierung der Ergebnisse aus AlN Ausscheidungsmodellierungen durch in-situ Austenitkornwachstumsuntersuchungen
Original languageGerman
Awarding Institution
  • Montanuniversität
  • Fuchs, Nora, Co-Supervisor (internal)
  • Bernhard, Christian, Supervisor (internal)
Award date20 Dec 2019
Publication statusPublished - 2019

Bibliographical note

embargoed until 02-12-2024


  • AlN precipitation
  • Pinning force
  • Austenite grain growth

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