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Annealing twin formation mechanism

Abstract : Annealing twin is a crystallographic defect that is largely reported in F.C.C. metals especially those with low stacking fault energy. Despite the amount of work dedicated to the subject, the understanding of annealing twin formation mechansims is not complete in the literature. In the present work, by applying both experimental and numerical tools, we tried to have a more profound understanding of this phenomenon, which is essential to Physical Metallurgy. For this purpose, different F.C.C. Materials including 304L stainless steel, commercially pure nickel and nickel based superalloy Inconel 718 are investigated. We confirmed that annealing twins are mainly formed in the recrystallization regime, especially driven by the migration of recrystallization front into deformed regions by using in situ EBSD technique. In addition, we found in the in situ observations that there are almost no twins generated in the grain growth regime. This observation is confirmed by another grain growth experiment performed on Inconel 718. Therefore, curvature driven grain boundary migration by itself is not sufficient to generate annealing twins. A new atomistic model to explain annealing twin formation mechanism, in which the effect of migrating boundary curvature is considered, is proposed. The effects of different thermo-mechanical factors, including prior deformation level, initial grain size, annealing temperature and the heating velocity, on annealing twin formation are determined via two experiments performed on commercially pure nickel. Based on the idea of grain boundary curvature, we proposed a method to quantify recrsytallization front tortuosity. In the present study, we show evidence that this quantity is positively correlated with the twin density at the end of the recrystallization regime. In addition to experimental studies, numerical tools including both mean field and full field approaches are applied to model annealing twin evolution during grain growth by taking into account the revealed mechanisms. A basis of a new mean field model is proposed to model annealing twin density evolution during grain growth. This model, which has only one parameter to be identified, provides a better consistency with the experimental data of Inconel 718 compared to the Pande's model. Besides, full field approaches are also applied to simulate the overall microstructure evolution during grain growth. Two implicit methods i.e. the level set and the multi-phase-field methods are compared in terms of their formulations and their numerical performance in anisotropic grain growth simulations. It is the first time that these two methods are compared in the finite element context with non-structural mesh. In the present numerical context, the level set method is more suitable to describe strong anisotropy in grain boundary energy. A new methodology is thus developed in the level set framework to simulate annealing twin evolution during grain growth. This methodology, in which we can insert annealing twin boundaries into synthetic microstructures and distinguish coherent and incoherent twin boundaries, is proven to be able to counting for the strong anisotropy introduced by coherent annealing twin boundaries.
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Submitted on : Thursday, March 26, 2015 - 5:33:06 PM
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  • HAL Id : tel-01136225, version 1


Yuan Jin. Annealing twin formation mechanism. Materials. Ecole Nationale Supérieure des Mines de Paris, 2014. English. ⟨NNT : 2014ENMP0030⟩. ⟨tel-01136225⟩



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