Self-propagating Combustion Synthesis via an Msr

9 The synthesis of (Ti1-xZrx)B2-Al2O3, (Ti1-xHfx)B2-Al2O3 and (Zr1-xHfx)B2-Al2O3 10 (x=0, 0.5 and 1) powder nanocomposites via a mechanochemical method using TiO2, ZrO2, 11 HfO2, HBO2 and Al as the raw materials was investigated. The formation of the 12 nanocomposites proceeds via a mechanically-induced self-sustaining reaction (MSR) process 13 that involves several simultaneous reactions. The aluminothermic reductions of the TMO2 and 14 HBO2 produce Al2O3 and transition metal and boron elements, which in turn react to yield the 15 diboride phase. The ignition of the complex combustion reaction occurred after a short 16 milling time (15-30 min), instantly transforming most of the reactants into products. The 17 sample composition was marked by the stoichiometry of the combustion reaction, and the 18 resulting nanocomposites were analysed using XRD, ED, SEM, TEM and EDX techniques. 19 The X-ray results confirmed the biphasic character of the prepared composite powder (TMB2 20 and Al2O3 structures); minor amounts of the Zr and Hf oxides were also observed. The 21 achieved microstructure was characterised by the agglomeration of Al2O3 nanocrystallites and 22 diborides crystals with a diffraction domain size ranging between 100 and 300 nm. 23