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It has been previously established on the basis of an Ising model of alloy ordering (part I of this series) that the observed maxima of short-range-order diffuse scattering in disordered alloys mark the positions of the minima of V(k), the Fourier transform of the pairwise interatomic potential V(r). It was suggested in I that the superlattice spots of the ordered state should occur at these same positions. This would establish a consistency requirement connecting the ordered and disordered configurations. We prove here that this is a sufficient but not a necessary condition, and we derive the full set of necessary conditions. We also show that for most of the diffuse maxima locations observed in bcc and fee alloys, it is possible to establish the ordered configuration required by the disordered phase, or conversely to use a knowledge of the ordered configuration to restrict the possible choices of V(r) in fitting the disordered-phase scattering data. The linear approximation for correlation functions in binary alloys presented by the authors in I is compared quantitatively with the more exact but less general formula of Fisher and Burford. Except at temperatures very close to T c , it is found that the theoretical shape of the short-range order diffuse scattering is in agreement but the temperature dependencies differ. We note also that all the currently known approximate calculations of a(k) for arbitrary V(r) can be represented in the same functional form, a(k)=G2(r)/[l+Gi(r)F(k)], where the temperature-dependent factors G\ and G2 vary according to the method of calculation used, but that V(k), the Fourier transform of V(r), is the same for all. This leads to the conclusion that the ratios V(r n +i)/V(r n ) determined by fitting the theoretical form to experimental data are insensitive to the particular choice of G\ and G2, but that the magnitudes of the V (r n ) so determined vary according to this choice. As a consequence, the values obtained for V(r n +i)/V(r n ) are to be regarded as more accurate than those for V(r n ).

Anharmonic terms of the elast,ic free energy of a solid are used to locate a surface of instability or "strain spinodal". The Rtrains near lattice defects bring some regions close to the strain spinodal causing a rapid change in local elastic properties and resulting in localized "soft mode" centers. These regions are proposed as the primary nucleation sites for martensitic transformations. A number of experiments are proposed to test the physical aspects of the theory and some recent evidence is cited supporting several features of the model.Les termeR an harmoniques de 1'8nergie libre 6lastique d'un solide sont employb pour localizer une surface instable de ,,contrainte spinodale". Les contraintes pres des defauts du reseau amenent quelqnes regions pr6s de la contrainte spinodale, et causent un changement rapide dans les proprietes elarctiques locaux, resultant dans des centres localizes ,,mode douce". Ces regions sont proposees comme les sites primaires de nucleation pour les transformations martensitiques. Un nombre d'experiences sont proposees pour verifier les aspects physiques de la thCorie et quelque evidence recente est citee pour supporter plusieurs traits du modele.

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