Effect of transition metal addition on microstructure and hardening behavior of two-phase Ni₃Al-Ni₃V intermetallic alloys

The comprehensive mechanisms for the hardening of two-phase Ni₃Al-Ni₃V intermetallic alloys by transition metal addition were presented, based on the microstructural observation, alloying behavior and lattice properties of the additives in the constituent phases. The alloys to which 2 at.% Ti, Nb and Ta were added in different substitution manners for Ni, Al and V, respectively were studied by scanning electron microscopy, electron probe microscopic analyzer, X-ray diffraction and hardness test. The alloys in which the additives were substituted for Al exhibited microstructures with finer and cuboidal Ni₃Al precipitates. Nb and Ta addition resulted in larger lattice expansion in the two constituent phases than Ti addition. The hardness increase by the Nb and Ta addition was larger than that by the Ti addition. Also, the hardness increase was larger in the substitution manner of the additives for Al than in those for Ni and V. The hardening behavior was explained in terms of solid solution hardening mechanism in which the mixture rule in the volume fraction of the two constituent phases and the atomic size misfit evaluated from the changes of the lattice parameters were incorporated. Consequently, the hardening for the alloys in which the additives were substituted for Ni and V was wholly attributed to solid solution hardening. On the other hand, the hardening for the alloys in which the additives were substituted for Al was attributed to the hardening due to microstructural refining in addition to the solid solution hardening.