Nanoscale engineering of low-misfit TiB2/Al3(Sc,Zr)/-Al multi-interface to improve strength-ductility synergy for direct energy deposited aluminum alloy

The nature of the interface between ceramic particle (CP) and metal matrix is critical to obtain solidification microstructures with optimal mechanical properties of CP-reinforced Al alloys in additive manufacturing. Generally, when the lattice misfit between the CP (e.g., TiB2) and the alpha-Al matrix is higher than 4%, the interfacial coherency reduces, lowering the effectiveness of CPs. Here, we demonstrate that an L12 three-dimensional compound (3DC), which possesses lattice misfit with alpha-Al lower than 1%, can be introduced in between TiB2 and alpha-Al via properly regulating the solidification cooling rate. In the 5TiB2/Al-4.5Mg-0.7Sc-0.2Zr (wt%) as a model system, the lattice coherence of the TiB2/alpha-Al interface is tailored by introducing a 10-30 nm thick Al3(Sc,Zr) 3DC interphase when the cooling rate is increased to-1000 degrees C/s. But, further increasing the cooling rate to-6800 degrees C/s, only Al3(Sc,Zr) two-dimensional compound (2DC) with an apparent lattice strain forms at the interface. Taking advantage of the cooling rate (102-103 oC/s) provided by laser direct energy deposition (L-DED), the low-misfit "TiB2/Al3(Sc,Zr) 3DC/alpha-Al" multi-structural interfaces are acquired, enabling the 3.56TiB2/Al-4.36Mg-0.72Sc-0.22Zr alloy as fabricated with L-DED to achieved improved strength-ductility synergy (yield strength 257 MPa, elongation 13.8%) due to the isotropically fine Al grain structure and the homogenous TiB2 particle dispersion. The outcome of this study provides fundamental knowledge on designing "ceramic/coherent primary interphase/metal matrix" multi-structural interfaces to improve the mechanical properties of engineering metallic materials manufactured by rapid solidification techniques, such as DED additive manufacturing (AM).