Grinding mechanism and mechanical properties of the in-situ synthesized Al2O3/TiAl composites

In-situ synthesis of Al2O3/TiAl composite was carried out by mechanical milling of the pre-alloy Ti-45Al-7Nb-0.3W and TiO2 powder mixture and subsequent sintering by spark plasma sintering (SPS) technique. The mixed powders experienced the initial destructive breakage stage, cold welding package stage and breaking up of boulders stage during mechanical milling. The particle size obviously declined with the mechanical milling time increasing. The number-average diameter (D(1,0)) of as-milled powders rapidly reduced from 75.0 mu m to about 15.5 mu m, and its distribution uniformity (U) decreased to 0.55 when milling time increased to 20 h. Nano Al2O3 particles had been formed during milling. The as-SPSed compacts were mainly composed of gamma-TiAl phases, alpha(2)-Ti3Al phases and nano-Al2O3 particles that tended to disperse both on the grain boundaries and inside the lamellar microstructure of the matrix. The contents of Al2O3 increased from 4.3% to 8.9 vol% as the milling time increased from 10 to 20 h. The increasing concentration led to the increase in Vickers hardness of Al2O3/TiAl composites, and a maximum value of 6.2 GPa was achieved. A corrected mixture rule was constructed which can predict the hardness of the composites accurately. The ultimate compression strength (UCS) increased firstly and then decreased with prolonging milling time. The highest value of 1848 MPa was obtained when the mixed powders milled for 15 h, i.e., the fraction of Al2O3 in the TiAl matrix was 7.6 vol%