Al_2O_3-Ni_3Al Composites Obtained by Reactive Milling and Reactive Sintering

摘要

Ceramic-intermetallic composites, especially Al_2O_3-Ni_3Al, can exhibit good values of mechanical properties, mainly associated with wear resistance. Fine and homogeneous precursors powders to produce Al_2O_3-Ni_3Al can be obtained through the reaction 3NiO + 3Al → Ni_3Al + Al_2O_3 during high-energy ball milling processing. If there is no control, the reaction can occur in a SHS (self-sustaining high temperature synthesis) type. In this work, we report on the effect of the amount of Al_2O_3 which was added as diluent in the mixture of the reactants, and also on the effect of milling time, to result in the final products. We have observed that there is a critical amount of diluent that inhibits the reaction to proceed in the SHS manner. For this reaction, the limit amount of alumina which keeps the reaction still in the SHS type was observed to be 0.7 moles. Using concentrations of alumina above this critical value, the reaction proceeds gradually, and if desired it may be further completed during sintering of the milled powders. The phase evolution during milling and sintering was studied by X-ray diffraction, differential scanning calorimetry, scanning electron microscopy and transmission electron microscopy. The results are discussed in terms of the occurrence or not of the SHS reaction during milling, the fine structure of the precursors powders when the reaction is gradual and the microstructure of the sintered samples.