Phase equilibria and stability of Heusler-type aluminides in the NiAl-Ni₂AlTi-Ni₂AlY (Y: V, Cr or Mn) systems

Phase equilibria between the B2 and L2₁ phases and critical boundaries of the B2/L2₁ continuous ordering in the NiAl-Ni ₂AlTi-Ni₂AlY (Y: V, Cr or Mn) concentration triangles were determined using diffusion couple technique in the temperature range of 1273-1573K. The B2+L2₁ two-phase region in the NiAl (B2)-Ni ₂AlTi (L2₁) pseudo-binary section narrows and then disappears due to substitution of the Y element for Ti. The critical temperatures of the B2/L2₁ ordering at the stoichiometric Ni ₂AlY, which were estimated from the experimental data on the basis of the Bragg-Williams-Gorsky approximation, decrease linearly with increasing average of the 3d+4s electron concentration of the elements occupying the Y site. Furthermore, the critical composition boundary of the B2/L2₁ ordering in the concentration triangle was also well described by the electron concentration of the Y atoms. It can be concluded that the electron concentration of the Y site atoms strongly influences the stability of the Ni₂AlY Heusler phase.