THE EFFECT OF NITROGEN AND OXYGEN ON THE SYNTHESIS OF B2 NIAL BY CRYOMILLING

Cyvomilling of elemental Ni-50 at.%Al powders in liquid nitrogen resulted in phase transformation through Mo opposing mechanisms: the formation of B2 NiAl and the elimination of NiAl by displacement reactions between NiAl and the liquid nitrogen and oxygen to form AlN and gamma-Al2O3. During annealing, the NiAl reacts to form AlN and gamma-Al2O3, thereby moving the composition of the balance of the mixture toward the Ni-rich end of the phase diagram. The resultant nanoscale AlN and gamma-Al2O3 particles were found to impede grain growth of the nanocrystalline structure at high temperatures.