Powder metallurgy routes toward aluminum boron nitride nanotube composites, their morphologies, structures and mechanical properties

Aluminum/boron nitride nanotube (BNNT) composites with up to 5 wt% (i.e., 9.7 vol%) nanotube fractions were prepared via spark plasma sintering (SPS) and high-pressure torsion (HPT) methods. Various microscopy techniques, X-ray diffraction, and energy dispersive X-ray analysis confirmed the integration of the two phases into decently dense and compact composites. No other phases, like Al borides or nitrides, formed in the Al-BNNTs macrocomposites of the two series. The BNNTs were found to be preferentially located along Al grain boundaries in SPS samples (grain size was 10-20 μm) creating micro-discontinuities and pores which were found to be detrimental for the sample hardness, whereas in HPT samples, the tubes were rather evenly distributed within a fine-grained Al matrix (grain size of several hundred nm). Therefore, the hardness of HPT samples was drastically increased with increasing BNNTs content in Al pellets. The value for Al-BNNT 3.0 wt% sample was more than doubled (190 MPa) compared to a pure Al-HPT compact (90 MPa). And the room temperature ultimate tensile strength of Al-BNNTs HPT samples containing 3.0 wt% BNNT (~300 MPa) became ~1.5 times larger than that of a BNNT-free HPT-Al compact (~200 MPa).