Effect of Mechanical Alloying on Combustion Synthesis in the Ni-Al-B System

Mechanical alloying (MA) of powder mixtures of Ni-33 mol%Al-x mol%B(x=0−2.0) was performed for various milling times, and the effect of MA on combustion synthesis under air and vacuum heating conditions was investigated for the MA powders. The nature of combustion synthesis changed dramatically with MA conditions (milling duration, quantity of B-doping, etc.), and suitably produced MA powders induced combustion synthesis in air (room temperature) and vacuum heating (near 673 K). The addition of B inhibited the combustion of the MA powders. By X-ray diffraction (XRD) analysis, it was found that intermetallic compounds such as NiAl, Ni3Al and Al3Ni2 were formed. The increase in relative intensity of XRD peaks for the intermetallic compounds was greater for vacuum heating than for air. It is suggested that combustion in air for the MA powders in the present study is led by the heat of formation from the oxidation reaction between mechano-chemically activated fine Al powders and oxygen. That is, the heat becomes a trigger for combustion synthesis between reactant species of Ni and Al. The above mechanism in air differs from that in vacuum heating where the MA powders themselves directly cause the exothermic reaction and combustion synthesis.