Reaction sintering of NbNiAl intermetallic alloys

Abstract

Homogeneous NbNiAl powder mixtures with nominal compositions Nb-10Ni-70Al, Nb-20Ni-65Al and Nb-30Ni-60Al (at.%) were pressed into compacts under an applied stress of 300 MPa and then reaction sintered. The resulting pellets were heat treated at 1140 °C for 6 h. Optical metallography, energy-dispersive spectroscopy and X-ray diffraction were used to characterize the microstructure of the as-reacted and heat treated pellets. Reaction sintering occurred for all the compositions investigated. The high nickel containing compacts (30 and 20 at.% Ni) reacted at a temperature of 500 °C and for the other compacts (10 at.% Ni) the reaction occurred at 850 °C. NbAl 3 and unreacted niobium were present, in different amounts, in the microstructure of all specimens containing nickel. NiAl and NbNiAl were only observed in the 30 at.% Ni containing pellet. Ni 2Al 3 was formed in the other compositions. The results indicate that in 20 and 30 at.% Ni compacts the reaction is initiated at NiAl powder interfaces, whereas for 10 at.% Ni compacts the dominant reaction occurs at the NbAl interfaces. The heat treatment at 1140 °C for 6 h induces a decrease in the amount of unreacted niobium and NbNiAl, and the elimination of Ni 2Al 3 with an increase in NbAl 3 and NiAl.