Shumpei Ozawa

On determining the phase-selection principle in solidification from undercooled melts—competitive nucleation or competitive growth?

A survey of the published literature on undercooled metallic and oxide melts suggests that phase selection during solidification can be categorized as nucleation controlled or growth controlled. Common characteristics governing the phase-selection pathway have been identified for various alloy systems. It is recognized that when competing stable and metastable phases share the same crystalline characteristics and have comparable interface kinetic coefficients, the principle of nucleation control applies for primary phase formation in a deeply undercooled melt. However, there can be a difference of two or three orders of magnitude in the interface kinetic coefficients for competing phases, either between an ordered intermetallic compound and a disordered solid solution, or between a crystalline phase with a high level of complexity and a simple crystal. In such cases, the principle of growth control will apply; more specifically, the phase with the faster growth kinetics should be favoured and the competing counterpart with sluggish interface kinetics should be suppressed at high undercoolings. Some simple predictions are suggested on the basis of this principle when considering stable and metastable phase diagrams. The specific conditions under which the present categorization is applicable are outlined. Future work is required to elucidate phase competition under conditions of very rapid solidification.

Magnetic properties of metastable phase from undercooled Nd-Fe-B melts

Nd14Fe79B7 alloys were solidified from the undercooled melts by containerless processing. The resultant alloys contained a large amount of a metastable phase with a chemical composition near that of the bulk alloy and a crystal structure similar to that of the Nd2Fe17 phase. The metastable phase had low magnetic properties, a saturation magnetization of 86.7 emu/g, a remanence of 4.5 emu/g and a coercivity of 0.14 kOe. The magnetic properties of the sample annealed at 1020 K were markedly improved due to the increase in the amount of the Nd2Fe14B phase.

Heat treatment of metastable Nd2Fe17Bx phase formed from undercooled melt of Nd–Fe–B alloys

Alloys of Nd10Fe85B5, Nd12Fe82B6, and Nd14Fe79B7 were melted and then solidified without a container using a drop tube. In the as-dropped sample, a large amount of the metastable Nd2Fe17Bx phase was contained. Particularly in relatively large samples, the metastable phase, which was partially decomposed into the Nd2Fe14B and α-Fe phases, shows a microstructure similar to that of normalized low-carbon steel. The heat treatment of the as-dropped samples induces a dual-stage phase transformation. The first stage of the phase transformation, which is dominant in the Nd14Fe79B7 alloy, is the diffusive phase transformation from Nd2Fe17Bx and Nd-rich phases into the Nd2Fe14B phase at 950K, and the second stage of the transformation, which is dominant in the Nd10Fe85B5 alloy, is the decomposition of the metastable phase into the Nd2Fe14B and α-Fe phases at 1100K. Although the rate of the phase transformation in either stage is controlled by the diffusion of Nd atoms, the diffusivity at the first stage of the phas...

Phase selection in containerless solidification of FeSi2 during free fall in drop tube

The melts of the Fe-66.7%Si alloy ejected into a drop tube was solidified during its free fall. The spherical samples collected at the bottom of the drop tube were classified into several groups according to their diameters from 300 μm to 1000 μm. The microstructures of the samples were examined and analyzed by scanning electron microscopy (SEM), X-ray diffraction (XRD) and differential thermal analysis (DTA). In addition to the grain refinement of the constituent phases, it was observed that the primary phase changed from the equilibrium FeSi (ε) phase to the metastable Fe2Si5 (α) phase and then returned to the ε phase again with decrease in the sample diameter. This result indicates that the microstructure of the sample solidified from the melt during free fall is controlled by the phase competition between α and ε depending on the degree of the undercooling. If it is assumed that the phase having the highest growth rate is selected as the primary phase, the dendrite growth model proposed by Boettinger, Coriell and Trivedi predicts the changes of the primary phase from ε to α and then to ε with increase in the undercooling. This means that the metastable eutectic point as a function of undercooling is expressed by the curve just like a character, C, in the Fe-Si binary phase diagram.

Microstructure and magnetic properties of rare earth Nd-Fe system alloys produced by containerless processing

NdxFe100−1.5xB0.5x alloys (x=10, 11.8, and 14) were solidified from the undercooled melts by drop tube processing. The samples contained a large amount of the metastable phase with the composition near to their bulk composition in each alloy. The volume fraction of the metastable phase increased as the sample diameter decreased. The metastable phase was partially decomposed into very fine lamellar grains of the Nd2Fe14B and α-Fe or Nd2Fe14B and some Nd-rich phase by solid-state reaction in the samples with a diameter of 1200μm. The solidification behavior of the metastable phase was explained in terms of a hypothetic scheme of the pseudobinary Nd-Fe-B phase diagram