In-situ synthesis of MgAlB4 whiskers as a promising reinforcement for aluminum matrix composites

Abstract

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 MgAlB4. First-principles calculations indicate that Mg doping can promote the growth of AlB2 in [0001] orientation and inhibit the growth in [213‾0] orientation. Based on the experiment and calculation results, we proposed a layer growth model with tip preferential growing in [0001] direction to obtain MgAlB4 whiskers. Tensile tests show that the in-situ synthesized MgAlB4 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.