Abstract
The effect of atmosphere on microstructure and formation of new phases during the reaction synthesis in Ni-Al green compacts was studied. The green bodies were prepared by uniaxial double action compression of Ni + 34 vol.% Al powders. The reactions in compacts during their heating to 660 °C and isothermal exposure for 8 h at 800 °C in air and argon atmospheres were monitored by DTA. Weight increase of oxidized samples determined by TG curves followed parabolic behaviour. This agrees with a diffusive character of thermal oxidation of metals. Heating in argon revealed the strong exothermic reaction represented by a sharp peak in DTA curve close to the melting temperature of Al. The exothermic reaction in the air takes place as well, however, it is spread over a wider temperature interval. Microstructures of samples were analysed by SEM, EDS and XRD. After 8 h of isothermal exposure at 800 °C in argon Ni₃Al and NiAl intermetallic phases appeared whereas after the thermal treatment in air NiO and NiAl phases were predominantly formed. The unreacted Ni was determined after both types of treatments as well. The analysis of the compacts revealed that the applied thermal treatment in the air significantly improved the interparticle bonding providing thus the required structural integrity of Ni-Al compacts.

Key words
oxidation, nickel aluminides, metal matrix composites, reactive synthesis, differential thermal analysis, thermogravimetric analysis

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