Fe-Ti(O,N) composites produced from mechanically alloyed powders

Spark plasma sintering of mechanically alloyed powders was used to obtain Fe-Ti(O,N) microcomposites, with nominal contents of 25 and 40 vol% of the ceramic phase. Sintering temperatures up to 900°C together with a pressure of 50 MPa were used in the a sintering process, resulting in densification levels of about 91%. The microstructure of the sintered microcomposites was characterized by x-ray diffraction, scanning electron microscopy, transmission electron microscopy and microanalysis techniques; additionally, Young's modulus, flexural strength and microhardness were also determined. In the bulk of sintered specimens, a dispersion of nanometric Ti(O,N) particles within alpha-iron grains was observed. A surface layer containing cementite was also found on the surfaces of specimens sintered at 900°C. As a result of this, a hardness of about 715-765 kg/mm² was measured on the specimen surface, while the value determined in the bulk was between 540 and 580 kg/mm². The calculated values of Young's modulus were close to estimates made by the law of mixtures and proper consideration of the residual porosity. On the other hand, despite the similarities in microstructures and other characteristics (densification, hardness, Young's modulus) in specimens of nominally different ceramic content, the measured flexural strength changed from about 765 MPa for Fe-25vol%Ti(O,N) to about 225 MPa for Fe-40vol%Ti(O,N) composites. The surface layer rich in cementite increased the hardness and toughness of the composites.