SYNTHESIS AND CHARACTERIZATION INVESTIGATIONS OF Al-7 WT.% Si COMPOSITES HYBRIDIZED WITH LABORATORY-SYNTHESIZED NbB-NbB2-Nb3B4 PARTICLES VIA MECHANICAL ALLOYING AND PRESSURELESS SINTERING

In this study, Al-7 wt.% Si powders and composites hybridized with niobium boride powders were prepared by a combined method of mechanical alloying (MA) and pressureless sintering. Niobium boride powders containing NbB-NbB2-Nb3B4 phases were mechano-chemically synthesized (for 5 h) from Nb2O5-B2O3-Mg blends and purified with HCl leaching treatment in our laboratory facilities. Laboratory-synthesized NbB-NbB2Nb3B4 powders were incorporated into the Al -7 wt.% Si matrix powders with the amount of 2 wt.% via mechanical alloying for different durations (1, 4 and 8 h) in a Spex T Mixer/Mill using hardened steel vial/balls with a ball-to-powder weight ratio of 7/1. Mechanically alloyed Al-7 wt.% Si-2 wt.% NbB-NbB2-Nb3B4 powders were compacted using a uniaxial hydraulic press with a pressure of 400 MPa and green bodies were sintered at 570 degrees C for 2 h under Ar gas flowing conditions. Characterization investigations of the hybrid powders and composites were performed using X-ray diffractometer (XRD), optical microscope (OM), scanning electron microscope (SEM), particle size analyzer (PSA) and differential scanning calorimeter (DSC). Sintered samples were also characterized in terms of density, Vickers microhardness and wear volume loss. MA has positive contribution on the density and microhardness values of the sintered samples. Amongst allthe samples, a full densification was not achieved and hence the highest relative density value of 96 % and the highest microhardness value of 94 +/- 9 N/mm(2) were obtained by the utilized powder metallurgy techniques.