The Heterogeneous Nucleation Sequence at the Interface of TiB₂ in Al Alloys

Abstract

Refinement of Al alloys by TiB 2 has been extensively studied in both industry and academia for several decades [1][2][3][4][5][6][7][8][9][10]. The presence of solutes (e.g. Ti) enhances the TiB 2 grain refinement potency by tailoring the heterogeneous nucleation interface of TiB 2 [10]. To date, there is still a lack of atomic scale experimental investigations of the heterogeneous nucleation interface of TiB 2 under industrial solidification conditions (e.g. conventional die casting). In order to address these open issues, a combination of atomic scale high angle annular dark field (HAADF) imaging and electron energy loss spectroscopy (EELS) in the dedicated aberration corrected scanning transmission electron microscope (STEM) is used for the characterization of the heterogeneous nucleation interface of TiB 2 in Al alloys, with a special focus on the partitioning of non-interacting solute elements (e.g. Ti and Cu) to TiB 2 . Experiments were carried out on two distinct material systems in order to more accurately elucidate the multiple roles of Ti in Al alloys and to determine the dominant mechanism for grain refinement in Al alloys. Firstly, as a reference, the heterogeneous nucleation interface of TiB 2 in a commercial Al-Ti-B grain refiner (i.e. Al-5Ti-1B) was studied. Secondly, the grain refinement of Al-Cu based alloys with varying Ti content (but with identical growth restrictions to the reference case) was investigated. The heterogeneous nucleation interface of TiB 2 in Al-Cu based alloys was also studied. The effect of the solidification path (i.e. a peritectic reaction and a subsequent peritectic transformation) on the observed nucleation phenomena is also discussed. Figure 1 shows a significant partitioning of Ti to the surface of TiB 2 in the commercial grain refiner (Al5Ti-1B). During solidification, the presence of a Ti-rich monolayer is strongly dependent on the solidification path. This is because the Ti-rich monolayer can potentially be consumed by a peritectic reaction. Figure 2 shows that Ti partitioning is present together with a significant Cu-rich monolayer (most likely Al 2 Cu) on the surface of the TiB 2 particle, in an Al-Cu based alloy. The above discussed Ti and Cu rich monolayers were observed over the whole surface of the respective TiB 2 particles. A nucleation sequence on the basal plane of TiB 2 is proposed. Firstly, Al nucleation occurs on an Al 3 Ti monolayer. Secondly, the Al 3 Ti monolayer is preserved from the subsequent peritectic transformation by a surrounding eutectic reaction layer that forms Al 2 Cu on Al. Based on this nucleation sequence, the absence or presence of an Al 3 Ti monolayer on the basal plane of TiB 2 can be interpreted. The present work provides a clearer picture of the heterogeneous nucleation interface between TiB 2 and Al and demonstrates that an Al 3 Ti monolayer must be present if the grain refinement is growth-limited. Only then can growth restriction and concentration undercooling play their full part.