Forged microstructures of nickel-based superalloys are characterized by the presence of thermal twins (Figure 1). These twins are formed mainly during recrystallization [1,2] and impact the in-service mechanical properties of these materials [3]. There is therefore a strong academic and industrial interest in predicting their appearance and evolution. However, although thermal twinning is a widely studied phenomenon, the criteria of twins’ formation are not yet clearly identified [4]. The ultimate objective of this work is to propose a predictive mesoscopic model enabling to be predictive regarding the thermal twin’s formation mechanisms observed during recrystallization as a function of thermomechanical parameters. To achieve this goal, the criteria for the formation of the twin boundary while a grain is developing must first be clarified, this is the topic that will be addressed in the presentation.

The grain boundary migration rate proposed in the literature [5] does not explain the influence of all the thermomechanical parameters on the resulting thermal twin density. Indeed, it is not able to account for the fact that for a given deformation, a smaller initial grain size leads to a higher twin density [6]. Another explanation based on topological consideration and involving the tortuosity of the recrystallization front has been proposed but needs further confirmation [6].

Pure nickel (99.6 %) is considered as model material in this work. Samples deformed to different strain levels are then annealed at different temperatures and times to obtain partially recrystallized microstructures. The twins formed are quantified and then correlated to various indicators linked to the tortuosity of the recrystallization front. The obtained trends are analysed and discussed.