Takao Kozakai

The formation of “γ′ precipitate doublets” in NiAl alloys and their energetic stability

Abstract The formation of doublets of γ′(Ni 3 Al) precipitates, i.e. two paired plates whose faces were mutually parallel, was found in NiAl alloys. The doublets seem to be produced by a widening and a subsequent splitting of the cuboidal γ′ precipitate. This process occurs as follows: first a rod-shaped matrix is formed in the centre of the cuboid, then the rod widens (i.e. increases in size) along the {100} planes, and finally the structure reaches the side surfaces. Such extraordinary behaviour can be quantitatively rationalized by assuming that the increment in the surface energy caused by the splitting is compensated for by the elastic interaction energy between the paired γ′ plates.

The phase decomposition of iron-molybdenum binary alloys by spinodal mechanism

Abstract Structural changes during ageing in a range of Fe-Mo binary alloys (Fe-13 ~ 20at.%Mo) were investigated by means of TEM and X-ray diffraction. In an early stage of ageing, a typical modulated structure along the orthogonal 〈100〉 directions which had not been reported for the Fe-Mo alloy system in the literatures was found in Fe−17 and −20 Mo alloys aged at suitable temperature. These modulated structures were theoretically proved to be produced through a mechanism of coherent spinodal decomposition, and the critical temperature (spinodal temperature) was estimated as 860 K for Fe-20Mo alloy. With further ageing, coherency between the Mo-rich and -poor regions was broken down when the wavelength of the modulated structure reached about 25 unit cells. Finally, a stable λ-phase (Fe 2 Mo) nucleated on the periodic structure. On the other hand, when the alloys were held at temperatures over the spinodal line, plate-shaped zones nucleated on the {100} planes of the matrix, and then λ-phase was produced. The latter structural change is identical with previously reported one for the Fe-Mo alloy system.

Stability bifurcations in the coarsening of precipitates in elastically constrained systems

Abstract Characteristic coarsening behaviours of precipitates in elastically constrained alloy systems are comprehensively represented. The important results obtained are as follows: (a) splitting of a precipitate into small particles during coarsening, (b) extremely slow coarsening of precipitates in high solute content alloys, (c) deceleration of the coarsening rate for large precipitates and (d) changes in the distribution function of particle size with progress of aging. These phenomena cannot be explained by the conventional Lifshitz-Slyozov-Wagner or modified Lifshitz-Slyozov-Wagner theories of Ostwald ripening, where only the interfacial energy between the particle and matrix is taken into account. The bifurcation diagrams of the particle stability can explain such extraordinary phenomena observed in elastically constrained alloy systems.

Computer simulations of the phase transformation in real alloy systems based on the phase field method

Abstract The kinetic simulation based on the non-linear diffusion equation becomes a very powerful method in fundamental understanding of the dynamics of phase transformation with recent remarkable development of the computer. In the present study, we calculate the dynamics of microstructure changes in real alloy systems, i.e. Fe–Mo, Al–Zn and Fe–Al–Co based on the phase field method. The composition dependencies of atomic interchange energy are taken into account so as to be applicable for the phase diagram of the real alloy systems. The elasticity and mobility of atoms are assumed to depend on the local order parameters such as the composition, the degree of order, etc. Time dependent morphological changes of the microstructure such as formation of modulated structure by spinodal decomposition, strain induced morphological changes of precipitates, the order–disorder phase transition with phase decomposition will be demonstrated. The results simulated are quantitatively in good agreement with the experimental results in the real alloy systems.

Phase decompositions of Fe-Si-Al ordered alloys

Stable structures of Fe-Si-Al ternary alloys and Fe-Si and Fe-Al binary alloys containing up to about 40 at% solute atoms were investigated by means of transmission electron microscopy. The following results were obtained. Two types of phase separation, B2+DO3 andα + DO3 were observed in the alloys whose compositions lie in a narrow band connecting Fe-10 to 14 at% Si with Fe-20 to 25 at % Al and also in the neighbourhood of a Fe-30 at%Al-3 at% Si alloy. Such compositions of the alloys are located in the phase boundary of B2 and DO3 single phases orα and DO3 single phases. The phase separation in the Fe-Si-Al and Fe-Si alloys produce the 〈100〉 modulated structure which differs from the morphology formed by the phase separation of the Fe-Al system.

Phase equilibria in iron-rich Fe - Al - V ternary alloy system

Abstract Phase separation and phase diagram in the Fe-rich corner of the Fe–Al–V alloy system were investigated by means of transmission electron microscopy, scanning electron microscopy and energy-dispersive X-ray spectroscopy. Phase separation into disordered A2 and ordered L21 phases was found in a wide composition range in the Fe-rich corner at 923 K, 973 K and 1023 K, and their tie-lines were almost parallel to the direction connecting the Fe corner with the center of the Gibbs triangle. In addition to the A2 + L21 phase separation, phase separation into B2 and L21 phases was also observed at 1023 K at the side of Fe–Al binary system.

The phase separations of Fe-Al-Co ordering alloys

Abstract The phase diagram for the Fe-rich Fe-Al-Co ordering system has been determined by observing phase separation by means of electron microscopy and magnetization-measurements. Two types of phase separation, into B2 and B2∗ (Al-rich B2 and Co-rich B2∗) and into α-Fe (A2) and B2, exist. The A2 + B2 phase separation occurs in a band shaped region connecting Fe-22–25 at.% Al with Fe-25–30 at.% Co. The B2 + B2∗ separation is localized in a wide island shaped region, which centres approximately at 25 at.% Al and 25 at.% Co. This island has a steep slope with temperature on the Co side. The microstructure of a B2 + B2∗ mixture is finely periodic along the 〈100〉 directions, i.e. a so-called [100] modulated structure. The B2 + B2∗ phase separation has been calculated thermodynamically by taking into account the ferro-magnetic and ordering excess free energies. The magnetic effect accounts for the shape of the B2 + B2∗ phase separation island.

Experimental and theoretical studies on phase separations in the Fe-Al-Co ordering alloy system

Phase separations in iron-rich Fe-Al-Co ternary alloys were investigated by means of transmission electron microscopy, differential thermal analysis and magnetization measurement. Two kinds of phase separations have been found at 923 K; A2+B2 and B2+B2*. The former occurs in a tongue-shaped composition region ranging from Fe-12 at % Al-15 at % Co to Fe-35 at % Al-45 at % Co and the latter appears in the small region bordering on the Fe-Al binary side of the A2+B2 field. These two-phase fields have theoretically been evaluated on the basis of the so-called Bragg-Williams-Gorsky model taking account of not only chemical but also magnetic interactions. The magnetic ordering accounts for the expansion of the A2+B2 coexistent region. In the measurement of magnetic properties, large increases in coercive force, Hc, and residual magnetic flux density, Br, were observed in the two-phase microstructures.

Phase diagrams calculated for Fe-rich Fe-Si-Co and Fe-Si-Al ordering alloy systems

Theoretical analysis based on the calculation of phase diagrams was employed for Fe-Si-Co and Fe-Si-Al ordering systems to clarify the necessity for the occurrence of phase separation in Fe-base ternary ordering systems. The free energy of Fe-base ternary ordering alloys where B2 and D03 ordered structures are formed is evaluated statistically using a pairwise interaction approximation up to second nearest neighbours, taking into account not only the atomic interaction but also the magnetic interaction, based on the Bragg-Williams-Gorsky model. The calculated phase diagrams are consistent with the experimentally obtained ones. The phase diagram calculation in this work is useful to predict the equilibrium states of the ternary ordering systems. The phase separation in ordering alloys is caused by the contribution of excess free energies due to ordering. The influences of ferromagnetism on the two-phase regions are also demonstrated.

A statistical evaluation of the free energy of Fe-base ternary ordering alloys

The free energy of the Fe-base ternary ordering alloys whereB2 andD03 ordered structures are formed is evaluated. The statistical theory is employed using a pairwise interaction approximation taking into account not only the atomic interaction but also the magnetic interaction, based upon the Bragg-Williams-Gorsky model. The application of this model on Fe-Si-Co ordering alloys are demonstrated. The propriety of the calculation results are performed by comparing the experimental results. The influences of the magnetic energy to the stability of ordered structures are also demonstrated.