Interfacial bonding is an important factor affecting the mechanical properties of composites. The interface between the in-situ synthesized ceramic phase and the metal matrix is considered ideal, so little attention is paid to the interfacial modification. In this work, we found that interfacial segregation affects the growth of the MgAlB 4 whisker and the interface of reinforcement and matrix in the composites. Systematic experiments and theoretical calculations indicate Fe solute prefer to segregate at the MgAlB 4 - { 10 1 ‾ 0 } /Al interfaces in in-situ synthesized MgAlB 4 /Al composites, which is helpful to reduce system free energy. On the one hand, Fe segregation induces the whisker coarsening and the formation of planar defects with a sandwich structure formed by a layer of Fe atoms and two layers of Al atoms in coarse-whiskers, on the other hand, it improves the interfacial strength . Although extra Fe aggravates the whisker coarsening, the synergistic reinforcing effect of the whiskers and the extra Fe is greater than the sum of their individual reinforcing effects. This work demonstrates that interfacial segregation can be used to modulate the size of reinforcements and improve interfacial strength.
Metal boride whiskers were successfully synthesized in Al matrix by ball-milling combining with vacuum hot pressing process. Composition and structural analysis confirm that the metal boride is hexagonal crystal MgAlB 4 . First-principles calculations indicate that Mg doping can promote the growth of AlB 2 in [0001] orientation and inhibit the growth in [ 21 3 ‾ 0 ] orientation. Based on the experiment and calculation results, we proposed a layer growth model with tip preferential growing in [0001] direction to obtain MgAlB 4 whiskers. Tensile tests show that the in-situ synthesized MgAlB 4 whiskers reinforced Al matrix composites have excellent ultimate tensile strength with an acceptable elongation to break. Electron backscatter diffraction results show that the whiskers can significantly refine the grain of the matrix, resulting in more retention of low angle grain boundaries in the matrix. Fracture analysis reveals that strong interface bonding was achieved between the whisker and the Al matrix, which benefits the load transfer between the matrix and the whiskers and results in crack initiation preferentially in whisker.
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