Order-disorder Reaction Research Articles

Ni8X phases in the systems Ni-V, Ni-V-Nb, and Ni-V-Ta

In nickel-base alloys at the composition Ni8X, where “X” may be combinations of V, Nb, and Ta, the ordering reactions have been investigated by employing electrical resistivity and electron diffraction techniques. The critical temperature for the order-disorder reaction is estimated to be 405° C for Ni8V and is shown to decrease linearly with increasing vanadium content across the pseudo-binary sections Ni8Nb-Ni8V and Ni8Ta-Ni8V, in agreement with calculations based upon the Bragg-Williams model. A secondary ordering reaction between V and Nb atoms has been observed in nickel-base alloys at the composition Ni8(V0.5Nb0.5).

On the elastic free energy of solid solutions—II. Influence of the effective modulus on precipitation from solution and the order-disorder reaction

The reciprocal space representation of the microscopic elasticity theory described in part I is used in reconsidering the role that lattice strain energy plays in precipitation and order-disorder transformations. The results are stated in terms of the effective elastic modulus M( h) where h denotes the reciprocal space variable. The elastic energy of a single defect is shown to be proportional to the average value of the effective modulus, M̄, and it follows that the energy of a random distribution of solute atoms is also proportional to M̄. Neither of these energies are found to differ significantly from those obtained from continuum elasticity theory. The change in elastic energy in going from a random solute distribution to an ordered structure is shown to be proportional to the difference, M(h′) − M ̄ , where ( h′) denotes the position of the superlattice reflection. Only a slight change in elastic energy occurs on forming the ordered Cu 3Au structure contrasting with the result, presented in I, of a complete removal of elastic energy on forming the ordered β-brass structure. The method of Cahn and Hilliard for calculating the “chemical” gradient energy coefficients is extended to solid solutions in which there is a significant contribution to the heat of mixing from the elastic energy. The “elastic” gradient energy coefficient, however, is found to contribute significantly to the total gradient energy only for large size differences, greater than 10 per cent. The microscopic theory is also used to explore the paradoxical role that the elastic energy has in precipitation in which it contributes to the formation of a miscibility gap but inhibits coherent phase separation by lowering the spinodal.

Single Crystal Anisotropy and Magnetostriction Constants of Several Ferromagnetic Materials Including Alloys of NiFe, SiFe, AlFe, CoNi, and CoFe

The anisotropy and magnetostriction of single crystals of several ferromagnetic materials have been determined experimentally. The materials include the metals iron and nickel, binary alloys of nickel iron, silicon iron, aluminum iron, cobalt nickel, and cobalt iron, ternary alloys of molybdenum nickel iron, nickel cobalt iron, and molybdenum aluminum iron, and magnetite. The effect of the order-disorder reaction on these properties was measured for several of the alloys. The present data for the nickel iron and silicon iron systems agree well with recently published values. Ordering generally raises the magnetostriction and lowers the anisotropy of the aluminum iron alloys near the Fe3Al composition. The first anisotropy constant, K1, for the cobalt nickel system as derived from torque curves is similar to old data derived from magnetization curves. However, K1 for cobalt iron (30, 35, and 45% cobalt) appears to be considerably larger than previously reported. In general, the addition of cobalt to nickel and to iron changes the magnetostriction constants (λ100 and λ111) toward large positive values. Thus, λ100 increases to 100×10−6 for 50% cobalt nickel and to 130×10−6 for 45% cobalt iron; λ111 increases to 30×10−6 for 45% cobalt iron but remains approximately constant for the cobalt nickel alloys.