Sintering behaviour and microstructures of Ti(Al,O)/Al2O3, Ti3Al(O)/Al2O3 and TiAl(O)/Al2O3 in situ composites

The sintering behaviour and microstructures of Ti(Al,O)/Al2O3, Ti3Al(O)/Al2O3 and TiAl(O)/Al2O3 in situ composites produced using Al/TiO2 composite powders have been studied. Ti(Al,O), Ti3Al(O) and TiAl(O) represent α-Ti phase with dissolved Al and O, and Ti3Al and TiAl phases with dissolved O, respectively. The Al/TiO2 composite powders were produced by high energy mechanical milling using Al and TiO2 powders as the starting materials. It was found that the matrices of the composites became fully dense after sintering at 1550 °C, and the Al2O3 particles became fully sintered when the sintering temperature was increased to 1650 °C. The reactions between Al and TiO2 which occur during heating the powder compacts to 1400 °C have little effect on the sintering of the composite powders. The Al2O3 particles in the composites become significantly coarsened during sintering at 1550 °C or above, and the coarsening rate of the Al2O3 particles increases in the order of Ti(Al,O)/Al2O3, Ti3Al(O)/Al2O3 and TiAl/Al2O3 composites. This can be attributed to the decrease of the solidus temperature or melting points of the corresponding matrices. It is evident that in the initial stage of sintering, coarsening of the Al2O3 particles occurs through coalescence. Hipping of the pre-sintered composites at 1300 °C with a pressure of 200 MPa without canning causes significant increase of the density and substantial further coarsening of the Al2O3 particles.