Mechanical alloying of the NiAl(M) (MTi,Fe) system

The mechanical alloying of intermetallic compounds has attracted much attention in recent years. The formation of NiAl with B2 structure by mechanical alloying of Ni and Al powder mixture has been found to be an explosive exothermic reaction. In this paper, Ti and Fe elements were substituted for Al and their effects on the mechanical alloying process and the final products of the NiAl(M) (MTi, Fe) system were studied in detail by in situ thermal analysis, X-ray diffraction and scanning electron microscopy. The results showed that the addition of as much as 15 at.% Ti or 5 at.% Fe for Al prolonged the milling time prior to the explosive reaction, which indicated the abrupt formation of β phase NiAl(Ti) or NiAl(Fe). The addition of 20 at.% Ti eliminated the abrupt explosive formation of β phase NiAl(Ti) completely. The addition of 25 at.% Ti induced an NiAlTi amorphous-like phase. The prolongation of the milling time prior to the explosive reaction might be attributed to the addition of Ti or Fe preventing the formation of a fine homogeneous layered structure of Ni, Al and Ti or Fe, thus enhancing the energy threshold for the formation of the β NiAl(M) (MTi,Fe) compound. With an increase in Ti content the degree of ordering of the β phase NiAl(Ti) compounds decreased, whereas the crystal lattices expanded and the particle size increased. The particle size of the amorphous-like phase was much larger than that of the explosive-formed β phase NiAl(Ti) compounds.