Isamu Yamauchi

Undercooling in Co–Cu alloys and its effect on solidification structure

Undercooling behaviour and solidification morphology change of various Co–Cu alloys were examined. Each alloy was melted in an alumina crucible under an argon atmosphere by high-frequency induction, and the cyclic heating and cooling was repeated several times in the temperature range between 1300 and 1850 K. The temperature change during the experiment was analysed under the Newtonian cooling assumption. The temperature curve showed that the undercooling in a first few cycles was negligibly small but it increased remarkably. The alloy was undercooled below the metastable liquid miscibility gap after the next several cycles. In these samples, liquid separation was observed. The homogeneously mixed spherical grains of copper-enriched phase were observed in cobalt-enriched matrix for the samples solidified immediately after the liquid separation. The two melts became coarser after the separation by mutual coalescence. In the case of the slow start of the solidification after the separation, they formed a clear interface between the upper cobalt-enriched layer and the lower copper-enriched layer located in the lower part according to the density difference between the two melts. It depended on the cooling rate after liquid separation. The very fine duplex structure can be obtained by the rapid cooling of the melt at the initial stage of the separation.

Effect of copper addition on the β-phase formation rate in FeSi2 thermoelectric materials

The transformation kinetics of the β-phase from an as-solidified structure composed of α and ε in the Fe–Si system was investigated by using rapidly, unidirectionally or conventionally solidified FeSi2 alloys containing a small amount of Cu (0.1–1 at%). The addition of Cu decreased the size of primary ε and slightly changed the solidified eutectic morphology. The solubility of Cu in the α-Fe2Si5 phase was estimated to be less than 0.2 at%. A needle-like Cu enriched phase was newly formed in the conventionally solidified alloys containing more than 0.2 at % Cu. Microdifferential thermal analysis (DTA) clearly showed that the addition of Cu drastically accelerated β-phase formation. X-ray diffraction analysis and microstructural observation of the isothermally heat-treated specimens showed that Cu addition was effective in increasing the rate of eutectoid decomposition (α → β + Si) and the initial stage of the peritectoid reaction (α + ε → β). For complete β formation, heat treatment for a long time was still required because it took a long time for the coarse ε-phase in the slowly solidified alloy to be eliminated by peritectoid reaction. The effect of Cu depended on the annealing temperature. The decomposition rate of α in the Cu-added cast specimen was about 15 times higher at 1073 K than that of the binary cast specimen and exceeded more than 30 times at 873 K.

Development of high performance Raney Cu-based catalysts for methanol synthesis from CO2 and H2

Catalytic hydrogenation of CO2 into methanol has been investigated over Raney Cu-based catalysts. The Raney catalysts leached in NaOH/ZnO solutions showed high activities and selectivities for methanol synthesis. The deposition of Zn on the surface of Cu particles increased the surface area and the specific activity of Raney Cu-M. Raney Cu-Zr developed was significantly more active than a ommercial catalyst.

β-phase transformation and thermoelectric power in FeSi2 and Fe2Si5 based alloys containing small amounts of Cu

Abstract The effects of Cu addition on the β phase formation rate and the thermoelectric power in various FeSi 2 and Fe 2 Si 5 based alloys was examined. The peritectoid reaction ( a + e → β ) in FeSi 2 alloys was initially enhanced by the addition of Cu but it became slower for longer annealing times. The retained metallic ϵ was harmful for the thermoelectric power. The inherent thermoelectric properties of (FeSi 2 ) 99− X Mn 1 Cu X ( X =0–1. O at.%), (FeSi 2 ) 99− X Co 1 Cu X ( X =0–1.0 at.%) alloys were attained after the elimination of ϵ. In the case of eutectoid reaction ( α → β +Si). Differential thermal analysis, X-ray diffraction and microscopic observation clearly confirmed that the eutectoid reaction rate was drastically enhanced by the addition of a small amount of Cu and its rate decreased with decreasing Cu content. The rate also depends on the annealing temperature and reached a maximum at about 1073 K for most alloys. The addition of only 0.1 at.% Cu was still very effective even in Mn or Co doped alloys. The thermoelectric power of these alloys increased very quickly with annealing time. Their final values decreased with Cu content and saturated at 0.2 at.% Cu. The value of the 0.1 at.% Cu added alloy was higher than that of both the conventional p- and a-type FeSi 2 based alloys. These results suggest that the Fe 2 Si 5 alloys with a small amount of Cu may be attractive as new thermoelectric materials.

Temperature dependence of β-phase transformation in Cu added Fe2Si5 thermoelectric material

Abstract The time–temperature–transformation diagrams for the eutectoid decomposition (α→β+Si) in slowly solidified Fe 2 Si 5 alloys with small amounts of Mn and Cu were obtained in the temperature range 873 and 1173 K. The shape of the diagrams can typically be described by the character C. The nose temperature in a binary Fe 2 Si 5 alloy was about 1023 K and increased by about 50 K in the Cu added alloy. The addition of small amounts of Cu drastically shifted the diagram to shorter times. The start of the eutectoid decomposition in the Cu added alloy was more than 240 times faster than that in the Cu free alloy at the nose temperature. The shape of the Si dispersoids formed by the eutectoid decomposition changed from lamella in the Cu free alloy to granular in the Cu added alloy. In the FeSi 2 alloy, the peritectoid reaction (α+e→β) was also enhanced by the addition of Cu. These results suggest that the existence of Si is not significant for the acceleration of the β formation. The kinetic analysis of the β-phase transformation based on the Johnson–Mehl–Avrami equation suggests that the addition of Cu does not change the transformation mechanism but changes the growth kinetics. The size of the Si dispersoids was quite fine when heating below 1023 K.

Effects of Cu addition on the β-phase formation rate in Fe2Si5 thermoelectric materials

We proposed Fe2Si5 based alloys with a small amount of Cu as new Fe–Si thermoelectric materials. A few acicular structures enriched in Cu were newly formed in slowly solidified alloys containing Cu above 0.2 at%. α single phase structure was formed by a conventional solidification process in alloys below 0.2 at% Cu. β phase was only formed by the eutectoid reaction (α→β+Si). Differential thermal analysis, X-ray diffraction and structure observation clearly confirmed that the eutectoid reaction rate was drastically enhanced by the addition of a small amount of Cu and its rate decreased with decrease of Cu content. Its rate also depended on the annealing temperature and it was maximum at about 1073 K for most alloys. The addition of only 0.1 at% Cu was still very effective in Mn or Co doped alloys. The final structure after the eutectoid reaction in these alloys was duplex composed of β and Si. The size of Si decreased with decrease of Cu content and the annealing temperature. Transmission electron microscope observation showed that β was transformed from α with many planar faults (stacking fault) that will act as a drag resistance for the transformation. We speculated that the addition of Cu probably decreased the stacking fault energy so as to decrease the drag force and to enhance the formation rate.

Effect of rapid solidification on microstructure of various Fe29.5-xSi70.5-x (0.0≤x≤3.7) alloys

Abstract Two rapid solidification processes were applied to various Fe 2 Si 5 based alloys. The rapidly solidified structure was examined as a function of Si content by TEM, SEM and XRD. The results were compared with those of slowly solidified alloys. Some metastable phases were formed by rapid solidification and they were dependent on the composition of alloys. A few α + ϵ eutectic was newly formed with primary α phase in 70.5 at.% Si alloy where no eutectic was expected from the equilibrium phase diagram. The amount of metastable eutectic decreased with increasing Si content. The lattice constants of the rapidly solidified α phase were different from those of the slowly solidified α phase due to the increase of the solubility of Si in α. A supersaturated α single phase was formed in 71.5–72.0 at.% Si alloys.

β-FeSi2 Phase formation from a unidirectionally solidified rod-type eutectic structure composed of both α and ϵ phases

Abstract Kinetics of the transformation from the α and ϵ phases to the β phase in an iron silicide thermoelectric material was examined. The specimens with a rod-type eutectic structure composed of the α and ϵ phases, prepared by a unidirectional solidification technique, were used as a diffusion couple. The β phase formation from α and ϵ consisted of two processes, depending on temperature. The first process which occurred below 1143 K proceeded in the following three stages: (1) the ring-like β phase was formed on the interface between the matrix α and rod shaped ϵ by the conventional peritectoid reaction, (2) the lamellar structure composed of the β and Si phases was formed by the eutectoid reaction of the remaining α phase after an incubation period, (3) finally, the β phase was formed by the subsequent reaction between the remaining ϵ and Si phases formed by the eutectoid reaction described in stage (2). In the second process above 1153 K, where most of the β phase was formed by the peritectoid reaction, the eutectoid decomposed slowly. A rapidly solidified specimen has a very fine eutectic structure and the β phase was mostly formed by the peritectoid reaction, irrespective of temperature. These two apparently different processes were explained by the combination of the simultaneous progress of the peritectoid and eutectoid reactions of which the kinetics depended on temperature. It was also found that a small addition of Mn remarkably decreased the β phase formation rate.

Computer Simulation of Solidification of Nodular Cast Iron

Solidification of a nodular iron casting was simulated by an undercooling nucleation and diffusion controlled growth model. Simulated cooling curves were similar to measured ones but estimating nodular size distribution requires improving the model.

Effects of Mn and Co addition on morphology of unidirectionally solidified FeSi2 eutectic alloys

Abstract Solidification structure was examined, which was obtained by the unidirectional solidification of various (FeSi 2 ) 100 x (M) x , ( M = Mn or Co, x = 1–3at.%) alloys. The transition of the solidification structure from irregular eutectic to well-aligned rod-like eutectic alloys occurred at a critical growth rate depending on alloys. Its value was 25 μm s −1 for the binary Fe-Si alloy and decreased by the small addition of Mn or Co. The effect of these elements on the transition was explained by the constitutional undercooling criterion in the ternary eutectic alloys. The rod spacing (λ) of the eutectic structure varied with the growth rate ( R ) and the value of λ 2 R was constant for a given alloy depending on the third element. The values of λ 2 R for the ternary alloys were larger than those for the binary alloy. The minimum undercooling assumption proposed by Jackson and Hunt can be applied in this case.