DO3 Phase Research Articles

Ordering With Phase Separation in an Fe-Al-Si Alloy

ABSTRACTTwo kinds of process of ordering with phase separation, B2 or DO3 → (A2 + DO3), on annealing at a temperature of 873 K in an alloy of Fe-6 at%Al-9 at%Si were investigated by means of transmission electron microscopy and X-ray diffraction. The decomposition of DO3 phase proceeds by the development of periodic fluctuations along <100> directions in both B2 and DO3 types of degree of order. Newly born DO3 domains take the form of rectangular prism and align along <100> directions. In the transition from B2, on the contrary, there is a marked difference between the morphologies of the fluctuations in the two types of degree of order at the early stage of annealing. The degree of B2 type order fluctuates along <100> directions, while the fluctuation in the degree of DO3 type order does not form the <100> modulated structure sat the early stage of annealing. The morphology after a prolonged annealing is similar to that in the case of DO3 → (A2 + DO3).

Magnetic behavior in the Fe3Al1 system

The magnetic properties of the Fe3Al1 order-disorder system have been investigated. Magnetization results of a rapildy cooled sample and susceptibility measurements show that the Curie temperature of the partially ordered B2 phase is 590 °C. Slow cooling experiments indicate the the Curie temperature of the fully ordered DO3 phase is about 520 °C. These temperatures are not consistent with either previously determined values or with the predictions of a phenomenological model. The magnetization value at 25 °C in high fields of the DO3 phase is significantly larger than previously observed values and is also larger than the magnetization of the B2 phase as expected from past investigations. The possible existence of a previously undetected antiferromagnetic phase seems to be indicated by the magnetization data at low temperatures and fields of the DO3 sample. The prominent anomaly in the temperature dependence of the magnetization at the B2-DO3 order-disorder temperature has been investigated and the results are explained by the magnetic properties proposed for the B2 and the DO3 phases.

Fe<SUB>3</SUB>Al規則格子合金におけるドメイン構造の観察

A sequence of structural changes and the antiphase boundary (APB) contrast associated with order-disorder transitions in Fe3Al alloy were investigated by means of transmission electron microscopy, and the results are discussed in combination with those of our previous X-ray diffraction study. In the transitions B2→DO3 and α+B2→DO3, the ordering was observed to take place by nucleation and growth of the DO3 domains in the B2 and α+B2 matrix, respectively. In the latter transition the difference in size of the DO3 domains which have nucleated in the α and B2 matrices suggests that the ordering from B2 to DO3 phase is easier than that from α to DO3. In the transition B2→α+DO3, the DO3 domains nucleate at the early stage, and in their growing process the α phase precipitates in both the B2 and DO3 type APB’s. The 111 dark field image shows that the B2 type APB’s with phase angle ±π⁄2 do not produce such a clear contrast as is exhibited by the DO3 type APB’s with phase angle ±π, and the fact is qualitatively explained by the two-beam dynamical theory.

Electron Microscopic Study of Domain Structure in Iron-Aluminum Alloys

The change in the micro-domain structure of Fe-Al alloys during the isothermal annealing has been investigated by means of transmission electron microscopy. The domain structure shows very complicated features in each transition, but the results of the observation are consistent with those of our previous work using X-ray diffraction. In the transitions B2→α+B2 and α+B2→α+DO3 the decomposition of the ordered phase into a mixture of the ordered and disordered phases takes place. The transition α+DO3→DO3 is dominated by the nucleation of DO3 domains in the disordered phase. In the reverse process, from DO3 to α+DO3, the disordered phase precipitates in two steps from the DO3 phase.