Micromechanical study of recrystallization in aluminium alloys

Abstract

An in-depth understanding of the recrystallization (RX) process in alloys to achieve an optimised microstructure is critical to manufacturing metal parts with superior properties. However, the prediction of the RX process under various processing conditions is still in its early research stage and becoming an urgent demand for both the manufacturing industry and scientific research.

Regarding this long-standing microstructure formation problem, deformation bands (DBs) formed in metals are known to contribute to the unsolved problems in the RX process by giving rise to the microstructural heterogeneities. Previous experimental transmission electron microscope (TEM) work has identified the type of DBs at the microscopic scale, showing the importance of understanding the slip activation for DBs. However, the exact mechanisms of how DBs are formed and lead the RX during the subsequent annealing still remain unclear.

Firstly, to clarify the mechanism of DBs formation, single crystal, multi-crystal, polycrystalline pure aluminium (Al) and commercial Al alloys, as well as their corresponding crystal plasticity finite element (CPFE) models, were deformed to explore the effect of grain orientation, strain level and neighbouring grains on the formation of DBs regarding the orientations formed in DBs and the distribution of DBs. It is demonstrated that slip band intersection of primary and secondary slip can constrain the lattice sliding but facilitate the lattice rotation for the formation of DBs including the boundary of DBs and its orientation. It is found that the impact of the above factors on the formation of DBs is caused by the slip field of primary slip. The activation of a sufficient amount of primary slip in grains would be crucial to the formation of a large amount of distinct DBs.

Next, to explore the effects of DBs on the grain nucleation and the subsequent grain growth, specimens were annealed to observe the RX process. Regarding the recrystallized (RXed) texture, it is noticeable that the orientations of nucleated grains nearby DB are originated from the orientation in DB. Regarding the nucleated positions, it is demonstrated that potential nucleation sites are more likely located in DBs in the comparison with the initial grain boundary. Regarding the rate of RX, the number of nucleated grains is found to have a strong positive correlation with the area fraction of DBs which would consequently affect the kinetics of the grain growth in the deformed microstructure. All the above observations imply that the RX process is strongly controlled by the ensemble characteristics of DBs rather than the initial grain boundaries.

On the basis of the above experimental observations, an improved and validated numerical model that is capable of predicting the RX process was developed using a Kobayashi, Warren and Carter (KWC) phase-field model coupled with CPFE analysis. It has been validated by the Electron Backscatter Diffraction (EBSD) mapping that this model enables a reliable and accurate prediction of RX in terms of the incubation time for grain nucleation and the evolution of RXed grain structure and texture. The model provided by this work offers a versatile tool for the manufacturing industry to solve many long-standing problems in the microstructure formation.