Fcc Phase Transition Research Articles

Studies in molecular dynamics. XVIII. The square-well phase diagram

Accurate molecular dynamics calculations have been made for the thermodynamic properties of the square-well potential in the solid phase. Hard-sphere perturbation terms for fcc and hcp phases have been computed over the solid range, and a phase diagram including solid–solid and solid–liquid phase boundaries has been constructed. The discontinuous square-well potential gives rise to an fcc–fcc phase transition which ends in a critical point. Both fcc and hcp phases have regions of stability, but calculations show that bcc is unstable.

Sputter etching studies of Fe–Ni and Fe–Pd films

Sputtering studies of Fe1−x–Nix (x=0.05 to 0.95) films were made and compared with those of Fe1−x–Pdx (x=0.06 to 0.7) films. Plots of steady-state AES and ESCA signal intensity ratios versus bulk composition ratios for the Fe–Pd system gave two straight lines intersecting near x=0.38, where a bcc to fcc phase transition occurs. Similar plots for the Fe–Ni system showed a single straight line across the whole composition range although a similar phase transition occurs at x∠0.4. Values of slopes obtained from normalized AES and ESCA data were similar for the Fe–Ni but differed by a factor of 3 for the Fe–Pd system. Since ESCA has a larger sampling depth than AES for both alloy systems, these differences are attributed to the presence of a significant altered layer on the sputtered Fe–Pd surface, which is absent on sputtered surfaces of Fe–Ni films. This result is compared to the different diffusion rates of these two systems: (1) Depth profiles for air-exposed and deliberately oxidized (160°–250 °C) films indicated that diffusion is much faster in the Fe–Ni than the Fe–Pd systems, and (2) direct measurement of interdiffusion in Fe–Ni and Fe–Pd couples demonstrated much faster interdiffusion in the Fe–Ni system.

The nucleation of magnetic domains during the hcp to fcc phase transition in cobalt

Recent experiments have reported difficulties in nucleating the transformation of domains during the hcp to fcc phase transition in cobalt at 400 °C. By writing the energy associated with these domain patterns explicitly in terms of anisotropy, magnetostatic pole, and domain-wall energies, in addition to the energies associated with an applied external field and domain-wall pinning, a condition of instability, i.e., a free transformation of the domain structure, is obtained. It was found that the field needed to nucleate this domain transformation is increased by the effects of magnetostatic energies and wall pinning, but decreased by the effects of anisotropy. Furthermore, it was found that for an internal planar defect, such as a stacking fault, the transformation between the domain structures indigenous to each crystal phase occurs spontaneously in agreement with experimental observations.

Study of the polymorphic hexagonal-face-centered-cubic transformation in Cu2S

The results of a dynamic high-temperature x-ray diffraction investigation into the special characteristics of the hexagonal-fcc polymorphic transformation in Cu2S are presented. Close to the phase-transformation temperature, two groups of superstructural reflections appear on the x-ray diffraction patterns, some of these being indexed in the 12H hexagonal lattice (Ramsdel) and others in the 14H lattice. The mechanism of the hexagonal — fcc phase transition in Cu2S is considered as passing through the intermediate 12H and 14H polytypic structures by way of the successive accumulation of periodically disposed stacking faults. The effect of thermal cycling within the phase-transformation temperature range on the course of the actual transformation is verified.